The Chesapeake Bay Crater: Geology and Geophysics of a Late Eocene Submarine Impact Structure

Impact Studies Series Editor: Christian Koeberl Editorial Board Eric Buffetaut (CNRS, Paris , France) lain Gilmour (Op...

Author: Wylie Poag | Christian Koeberl | Wolf Uwe Reimold

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Impact Studies Series Editor: Christian Koeberl

Editorial Board Eric Buffetaut (CNRS, Paris , France) lain Gilmour (Open University, Milton Keynes, UK) Boris Ivanov (Russian Academy of Sciences, Moscow, Russia) Wolf Uwe Reimold (University of the Witwatersrand, Johannesburg, South Africa) Virgil 1. Sharpton (University of Alaska, Fairbanks, USA)

Springer Berlin Heidelberg New York Hong Kong London Milan Paris Tokyo

C. Wylie Poag Christian Koeberl Wolf Uwe Reimold

The Chesapeake Bay Crater Geology and Geophysics of a Late Eocene Submarine Impact Structure

With

i

207

Figures, 42 Tables and a CD-ROM

Springer

C. WYLIE POAG U.S. Geological Survey 384 Woods Hole Road Woods Hole, MA 02543-1598 USA Email: [email protected] DR.

DR. CHRISTIAN KOEBERL

DR. WOLF UWE REIMOLD

Department of Geological Sciences University of Vienna Althanstrasse 14 1090 Vienna Austria Email:

School of Geosciences [email protected] University of the Witwatersrand P.O. Wits 2050 Johannesburg, South Africa Email: [email protected]

Additional material to this book can be downloaded from http://cxtras.s pringer.com.

ISBN 3-540-40441-4 Springer-Verlag Berlin Heidelberg New York

Cataloging-in-Publication Data applied for Bibliographic information published by die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliographie; detailed bibliographic data is available in the Internet at . This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitations, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. Violations are liable for prosecution under the German Copyright Law. Springer-Verlag Berlin Heidelberg New York a member of BertelsmannSpringer Science+ Business Media GmbH http://www.springer.de © Springer-Verlag Berlin Heidelberg 2004

The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover Design: Kirchner, Heidelberg Typesetting: Camera-ready by the authors Printed on acid free paper

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AO - 54 3 2 10

We dedicate this book to David J. Roddy (1932-2002), one of the pioneers of impact cratering studies. For 40 years, Dave was a driving force in the analysis of natural hypervelocity impact structures and the mechanics of nuclear explosion cratering. The wealth of data and observations he contributed remains fundamental to our understanding of the physics of impact cratering and the shock-wave deformation of the Earth's crustal materials .

Preface

"... bangs have replaced whimpers and the geological record has become much more exciting than it was thought to be." Derek Ager (1993) The New Catastrophism. Cambridge University Press, Cambridge, p xix

Scientific and public interest in asteroids, comets, and meteorite impacts has never been more intense than right now. Much of this interest stems from the fervent debates surrounding the causes of the Cretaceous-Tertiary mass extinctions and their possible relationships to a giant bolide impact in Mexico's Yucatan Peninsula. Recent spectacular impacts on Jupiter, and several near misses of our own planet by Near-Earth Objects have intensified professional and popular discussion of society's imperative need to understand the process and effects of bolide impacts. In the United States, the scientific community and the public, as well, were startled to learn, in 1994, that the largest impact structure in this country had been detected beneath Virginia's portion of the Chesapeake Bay. Seismic surveys and deep coring revealed a huge crater, 85 kilometers in diameter and more than a kilometer deep, stretching from Yorktown, Virginia, to 15 kilometers out onto the shallow continental shelf. Several of Virginia's major population centers, including Norfolk, Hampton, and Newport News, are located on the western rim of the crater, and still experience residual effects of the original collision, 36 million years after the impact took place. Exploration and documentation of the Chesapeake Bay impact structure has proceeded in three phases. Phase one was characterized by mainly serendipitous discoveries. Initial clues to its presence came from deep-sea cores collected by scientists aboard the drillship Glomar Challenger, during a coring cruise off the coast of Atlantic City, New Jersey, in 1983. Diagnostic evidence of an impact, in the form of microtektites and impact-shocked minerals, showed up in a few centimeters of late Eocene chalk, dated at - 35 million years old. The thickness and coarse-grained nature of the impact debris indicated that the impact site must have been relatively close to the core site. Three years later (1986), the first of four stratigraphic coreholes in southeastern Virginia recovered additional impactgenerated debris, containing diagnostic shock-metamorphosed minerals. The geologic age of the debris was identical to the microtektite-bearing debris cored off New Jersey. In 1994, acquisition of multichannel seismic reflection data from commercial oil companies revealed that two of the Virginia coreholes had penetrated the massive body of impact breccia that fills an enormous impact crater buried beneath Chesapeake Bay. Each of these three milestone events was the result

VIII

Preface

of chance - surprise discoveries made during geologic investigations of unrelated phenomena. Phase two of the crater documentat ion was marked by the acquisition of additional seismic reflection data to clarify the detailed structure and morphology of the impact structure. When added to the previous data set, the new surveys yielded a database of >2000 kilometers of seismic reflection profiles. These seismic data clearly revealed that the Chesapeake Bay structure is a complex, peakring/central-peak structure, with many features similar to those of other large terrestrial and planetary impact structures, but which displays several unique aspects, as well. Firm knowledge of the crater's structure and morphology allowed the third phase of exploration to begin in 2000. Phase three emphasized the careful selection of new core sites to answer specific questions regarding impact processes and resultant impact-generated deposits. Now, 20 years after the New Jersey core discoveries, researchers have established the principal structural, morphological, depositional , and paleoenvironmental features of the Chesapeake Bay impact and its resultant structure. This volume contains the first synthesis of our current knowledge of this fascinating cosmic event and its aftermath. It is our hope that the broad spectrum of data and interpretations we offer herein will enhance the understanding and appreciation of bolide impacts as crucial events in the geologic and biologic evolution of our planet.

C. Wylie Poag US Geological Survey Woods Hole, Massachussetts, USA

Christian Koeberl University of Vienna Vienna, Austria

W. Uwe Reimold University of the Witwatersrand Johannesburg, South Africa

Acknowledgments

We are indebted to a host of colleagues who contributed data, analyses, interpretations, and advice, during our roughly 12-year study of the Chesapeake Bay impact crater. The list is headed by Debbie Hutchinson, Steve Colman, Tommy O'Brien, Barry Irwin, Dave Nichols, Jeremy Loss, John Evans, and Nancy Soderberg, who constituted the shipboard science party that collected seismic reflection data aboard the RlV Seaward Explorer (1996). Texaco, Inc. (particularly Parish Erwin) contributed the multichannel seismic reflection profiles that originally defined the major features of the crater. Rusty Tirey, John Grow, and Pete Popenoe collected the early USGS seismic reflection data before we knew the crater was there. Dave Foster and John Diebold helped to acquire and process the seismic data collected by the RlV Maurice Ewing (1998). Dave Powars, Bob Mixon, Scott Bruce, and Don Queen carried out the initial coring programs that provided ground truth for the seismic reflection analyses . Marie-Pierre Aubry provided critical analyses of calcareous nannofossils. Gene Shoemaker, Jens Ormo, Filippos Tsikalas, Henning Dypvik, Richard Grieve, Peter Schultz, Kevin Pope, Bill Glass, Ron Stanton, Jeff Williams, Dave Folger, Glen Izett, and Michael Rampino provided expert advice and much needed encouragement during this project. Tom Aldrich, Joe Newell, and the crews of the RlV Seaward Explorer and RlV Maurice Ewing facilitated collection of seismic data in Chesapeake Bay. John Costain, Carl Bowin, Larry Poppe, Warren Agena, Myung Lee, Dann Blackwood, Dick Norris, Ed Mankinen, Judy Commeau, Louie Kerr, and Jeff Plescia provided critical data, data analysis or processing, scientific advice, and technical assistance. Chuck Pillmore provided the Manson seismic profile; Lubomir Jansa provided the Montagnais seismic line. The Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) provided offshore cores. The National Geographic Research Committee provided funding to the senior author for the single-channel survey of the crater. Steve Curtin provided downhole logs. Core drilling in the Chesapeake Bay impact crater has been a cooperative effort among the Hampton Roads Planning District Commission, the NASA Langley Research Center, the Virginia Department of Environmental Quality, the Geology Department of the College of William and Mary, and the USGS . The Chesapeake Coring Team (Greg Gohn, Dave Powars, Scott Bruce, Laurel Bybell, Tom Cronin, Lucy Edwards, Norm Frederiksen, Wright Horton, Glen Izett, Gerry Johnson, Joel Levine, Randy McFarland, Jim Quick, Steve Schindler, Jean Self-Trail, Matt Smith, Bob Stamm, Rob Weems, Art Clark, and Don Queen) acquired and described the NASA Langley, North, and Bayside cores. Becca Drury, Michael Taylor, Andy McIntire, Laura Hayes, Philip Moizer, Emily Denham , Daniel Boamah and Kassa Amare provided computer skills and laboratory assistance. Philip Moizer also collected a new set of gravity

X

Acknowledgments

data on the Delmarva Peninsula and carried out the gravity modeling of the crater. VeeAnn Cross constructed the 3-D structural model of the crater . We are especially indebted to the reviewers, David Crawford, Henning Dypvik, David Foster, Jens Orrno, Larry Poppe, Scott Snyder, Filippos Tsikalas, and Buck Ward, for significant improvements to the manuscript. The USGS Coastal and Marine Geology Program and Earth Surface Processes Program supported Poag's crater research. Koeberl's geochemical and petrographic studies were supported by the Austrian Science Foundation (FWF) project Y58-GEO . Reimold's research was supported by the National Research Foundation of South Africa and a grant from the University of the Witwatersrand to the Impact Cratering Research Group. This is IeRG Contribution No. 45.

Contents

1 Introduction 2 Geological Framework of Impact Site 2.1 Crystalline Basement Rocks 2.1 .1 Regional Tectonostratigraphy 2.1.2 Crystalline BasementRocks in Boreholes 2.1.3 Regional Configuration of Crystalline BasementSurface 2.2 Coastal Plain Sedimentary Rocks 2.2.1 General Stratigraphic Framework 2.2.2 Preimpact Deposits 2.2.2.1 Potomac Formation 2.2.2.2 Unnamed Upper Cretaceous Beds 2.2.2.3 BrightseatFormation 2.2.2.4 Aquia Formation 2.2.2.5 Marlboro Clay 2.2.2.6 Nanjemoy Formation 2.2.2.7 Piney Point Formation 2.2.2.8 Unnamed Upper Eocene Deposits 2.2.3 PostimpactDeposits 2.2.3.1 Chickahominy Formation 2.2.3.2 DelmarvaBeds 2.2.3.3 Old Church Formation 2.2.3.4 Calvert Formation 2.2.3.5 Choptank Formation 2.2.3.6 St. MarysFormation 2.2.3.7 EastoverFormation 2.2.3.8 Yorktown Formation 2.2.3.9 Chowan River Formation 2.2.3.10 Quaternary Formations 2.3 Sequence Stratigraphy 2.4 Paleogeography of Impact Site 2.5 Subsidence of VirginiaContinental Margin 2.6 Initial Evidence of East Coast Impact... 2.7 Onshore Borehole Evidence 2.7.1 Noncored Boreholes 2.7.2 Cored Boreholes

.1 ..41 .41 .41 .43 :45 47 .47 ,48 ,48 50 50 .50 50 51 51 5I 51 52 52 52 .54 54 54 54 S4 55 55 .57 57 64 64 69 69 69

XII

Contents

3 Geophysical Framework of ImpactSite 3.1 Seismic Investigations of VirginiaCoastalPlain 3.2 Seismic Signature of Crystalline BasementRocks 3.3 Chesapeake Bay Seismic Reflection Profiles 3.4 Depth Conversion of Seismic Two-way Traveltimes 3.5 GravityEvidence 3.5.1 Database 3.5.2 Interpretation

.73 .73 73 77 85 86 86 87

4 The Primary Crater 4.1 Crater Structure and Morphology 4.1.1 SeismicInterpretation 4.1.1.1 Outer Rim 4.1.1.2 AnnularTrough 4.1.1.3 Peak Ring 4.1.1.4 Inner Basin 4.1.1 .5 Central Peak 4.1.2 GravityInterpretation

91 91 91 91 .120 .120 .139 ..140 146

5 Secondary Craters 5.1 Location and Identification 5.2 Secondary Craterson Profile T-I-CB. 5.3 Secondary Craterson Profile T-II-PR 5.4 Implications of Secondary Crater Record

..153 .153 .155 .158 .163

6 Synimpact Crater-Fill Deposits 6.1 Oldest BrecciaUnit. 6.2 Displaced Megablocks 6.2.1 Seismic Signature and GeneralLithic Composition 6.2.2 Expression on Downhole Geophysical Logs 6.3 The Exmore Breccia 6.3.1 Seismic Signature and General Geometry 6.3.2 Distribution and Thickness 6.3.3 General Lithology 6.3.4 Sedimentary Structures 6.3.5 Expression on Downhole Geophysical Logs 6.3.5.1 Windmill Point Corehole 6.3.5.2NewportNews Corehole 6.3.5.3 NASA Langley Corehole 6.3.5.4 Exmore Corehole 6.3.5.5 North Corehole 6.3.5.6 Bayside Corehole 6.3.5.7 Kiptopeke Corehole 6.3.6 Petrography 6.3.6.1 ShockFractures 6.3.6.2 PlanarDeformation Features (PDFs)

.171 171 .171 .171 .184 .185 185 188 193 204 .212 212 213 .213 .214 215 .215 .216 216 216 2 16

Contents 6.3.6.3 Impact Melt Rocks 6.3.6.4 Glassy Microspherules 6.3.7 Geochemistry

7 Initial Postimpact Deposits 7.1 Depositional Setting 7.2 Dead Zone 7.3 Chickahominy Formation 7.3.1 Lithology of Cores 7.3.2 Expression on Downhole Geophysical Logs 7.3.3 Seismic Signature 7.3.4 Geometry 7.3.5 Faults and Fault Systems

8 Age of Chesapeake Bay Impact Crater 8.1 Biochronology 8.2 Radiometric Chronology 8.3 Magnetochronology 8.4 Correlation with Other Craters and Impactites

9 Geological Consequences of Chesapeake Bay Impact...

XIII 224 224 233 255 255 .255 259 .259 .259 266 266 270 279 279 283 .283 .283

9.1 General Nature ofConsequences 9.2 Reconfigured Basement Structure and Morphology 9.2.1 Central Peak. 9.2.2 Inner Basin 9.2.3 Peak Ring 9.2.4 Normal Faults 9.2.5 Reverse Faults 9.2.6 Compression Ridges 9.3 Disruption of Preimpact Sedimentary Column 9.4 Source of North AmericanTektite Strewn Field 9.4.1 General Distribution of Distal Ejecta 9.4.2 Correlation Problems 9.5 Far-Field Seismic Effects

287 287 .287 289 289 290 290 291 .291 .292 294 294 .297 .298

10 Comparisons with Other Impact Craters 10.1 Terrestrial Craters 10.1.1 Subaerial Craters 10.1.2 Submarine Craters 10.2 Extraterrestrial Craters 10.3 Comparison with Chicxulub Multiring Impact Basin

301 301 30 1 .307 326 .332

11 Comparisons Between Impactites 11.1 Terrestrial Impactites 11.1.1 Ries Breccias 11.1.2 Manson Breccias

343 343 .343 348

XN

Contents

11 .1.3 Lockne Breccias 11.1.4 Popigai Breccias 11 .1 .5 Montagnais Breccias 11 .1.6 Sudbury Breccias 11 .1.7 Chesapeake Bay Breccias 11 .2 Flowin, Fallout, and Dead Zone 11.3 Other Intrabreccia Bodies 11.4Continuous EjectaBlankets 11.5 Secondary Breccias 11 .6 StrewnFields 11.7 Impact Melt Rocks 12 Implications for Impact Models

12.1 General Conceptual Models and ScalingRelations 12.1 .1 Subaerial Cratering 12.1 .2 Submarine Cratering 12.2Conceptual Model for Chesapeake Bay Crater.. 12.2.1 Stage 1- Contactand Compression 12.2.2 Stage2 - Excavation 12.2.3 Stage 3 - Modification 12.3 GeneralConceptual Model of Crater-Fill Deposition 12.3.1 Intracrater Regimes and Lithofacies 12.3.2 ExtracraterRegimes and Lithofacies 12.4Differentiating Crater-Fill Lithofacies at Chesapeake Bay 12.5 Comparison of Models 13 Biospheric Effects of Chesapeake Bay Impact...

13.1 Local Paleoenvironmental Effects 13.1.1 Sediment Accumulation Rates 13.1.2 Stratigraphic Attributes of Benthic Foraminiferal Community 13.1.2.1 Preimpact BenthicForaminiferal Community 13.1.2.2 Postimpact BenthicForaminiferal Community 13.1.2.3 Bathysiphon Subassemblage 13.1.3 Community Structure of BenthicForaminiferal Associations 13.1.3.1 Predominance and Equitability 13.1.3.2 SpeciesRichness 13.1.3.3 Paleoenvironmental Interpretations 13.2 Possible Global Paleoenvironmental Effects 13.2.1 Hypothetical Short-Term Effects 13.2.2 Possible Long-Term Effects 13.2.3 Implications OfO l 80 Data 13.2.4Implications ofo l3C Data 14 Residual Effects of Chesapeake Bay Impact...

14.1 Hypersaline Groundwater.. 14.2 Near-Surface Compaction Faults

350 .351 354 .354 .357 361 .361 ,362 362 363 363 .365 365 .365 .368 .372 373 373 376 .377 .377 .381 .381 384 387 .387 .387 389 389 .390 .401 402 .402 :407 407 .419 .421 :423 ..424 :B I .433

.433 440

Contents 14.3 Surface Expression of Crater.. 14.4 Altered River Courses 14.5 Relative Change of Sea LeveI...

XV 440 444 A44

15 Summary and C onclusions

.447

Appe nd ix

A53

References

A61

Index

489

CD-ROM Contents Read Me File

Maps and Charts I. Borehole Location Map (Color) 2. Seismic Track line Map (Color) 3. Crater Structure Map (Color) 4. 3-D Crater Structure Model (Color) 5. Basement Structure Map (Color) 6. Depth Sections Along Selected Seismic Profiles (Black and White) 7. Stratigraphic Depth Section From Cores and Downhole Geophysical Logs (Black and White)

Selected Seismic Reflection Profiles 8. SEAX 16-4a-4 9. SEAX 8-7-6 10. SEAX 9-10 11. Texaco I-CB 12. Texaco 8-S-CB-E 13. Texaco 9-CB-F 14a-
Selected Figures from Text (in color) 2.13 4.36 6.14 6. 16 6. 17

6.18 6.19 6.20 6.21 6.22

6.23 6.25 6.26 6.27 6.28

6.29 6.31 6.32 7.3 7.4

7.5 7.6 7.10 9.5 12.4

12.5 13.2 14.1 14.7

1 Introduction

The list of impact craters documented on Earth is short. Only about 165 genuine impact structures have been identified to date (Table 1.1). Even so, the number is steadily increasing at the rate of - 3-5 per year (Grieve et al. 1995; Earth Impact Database at http://www.unb.ca/passc/Impact/Database/). In stark contrast, most other rocky planets and satellites of our solar system are pockmarked by thousands to hundreds of thousands of impact features (Beatty et al. 1999). Nevertheless, impact specialists acknowledge that Earth, too, has undergone billions of years of bolide bombardment (Melosh 1989; Schoenberg et al. 2002). The most intense bombardment, however, took place during Earth's earliest history (-3 .8-4 Ga; Ryder 1990; Cohen et al. 2000; Ryder et al. 2000). Traces of most terrestrial impacts have been completely erased or strongly altered by the dynamic processes of a thick atmosphere, deep ocean, and mobile crust, a combination unique to our planet. Planetary geologists now recognize that processes associated with bolide impacts are fundamental to planetary accretion and surface modification (Melosh 1989; Peucker-Ehrenbrink and Schmitz 2001). Incoming meteorites may have been primary sources for Earth's water, and, perhaps, even organic life as we know it (Thomas et al. 1997; Kring 2000). There is little doubt that impacts played a major role in the evolution of Earth's biota (Ryder et al. 1996; Hart 1996). Only eight impact structures with diameters of 80 km or larger have been discovered on Earth (Fig. 1.1; Table 1.1). Three of these, Vredefort, Sudbury, and Chicxulub, are classified as multiring basins, a structural/morphological category encompassing craters generally 200 km or more in diameter and characterized by three or more structural rings (Spudis 1993; Melosh 1989; Morgan et al. 1997). That leaves only the Chesapeake Bay crater and five other structures (Acraman, Manicouagan, Morokweng, Popigai, Puchezh-Katunki) as representatives of the 80-100-km-diameter class of complex impact craters (having two structural rings). The Chesapeake Bay crater (along with the Popigai crater, virtually a twin with regard to size and age; Masaitis et al. 1999; Poag et al. 1999; Whitehead et al. 2002) is the largest known complex impact structure created since the end of the Jurassic Period (145 Ma). Moreover, the Chesapeake Bay crater is the largest of 30 craters documented in the United States (Koeberl and Anderson 1996b), surpassing the second largest (Iowa's Manson crater) by more than a factor of two. Among Earth's complex craters, only a handful are relatively undeformed and accessible. Examples prior to this study include the Manson, Mjelnir, Lockne, Popigai, and Ries craters. If one focuses only on submarine impact structures, the list is even shorter. Though several structures currently exposed on continental C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

7 Kara/Ust Kara 8 Kamensk

3 Montagna is 4 Mj"lnir

9 Lockne 10 Granby 11 Tvaren 12 Kardla

14 Morokweng 15 Vrede fort 16 Sudbury

13 Kaluga

17 Brent 19 Ames 20 Manson 21 Chicxulub

18 Barringer (Meteor)

Fig. 1.1. Geographic distribution of21 terrestrial impact structures (craters and basins) discussed in this volume. See Table 1.1 for additi onal information about individual structures.

2 Toms Canyon

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Fig. 1.2. Computer-generated 3-D perspective of Chesapeake Bay impact crater, showing location beneath lower part of Chesapeake Bay, its surrounding peninsulas, and inner part of adjacent Atlantic Continental Shelf. Six principal cities shown on southwest margin of crater encompass densest human population in Virginia.

platforms are thought to have been submarine in origin, only four impact craters are still wholly or partly covered by oceanic waters. In order of decreasing diameter, these four are Chesapeake Bay (85-km diameter), Montagnais (45-km diameter), Mjelnir (40-km diameter), and Ust Kara (25-km diameter). A fifth submarine structure, Toms Canyon crater (22-km diameter), is considered by us to be also of impact origin, though that conclusion requires additional confirmation (Poag and Poppe 1998; Fig. 1.1;Table 1.1). The Chesapeake Bay structure (Fig. 1.2) is unique among both subaerial and submarine impact craters on Earth by virtue of the following combination of features: (I) its location on a passive continental margin has preempted the kinds of tectonic or orogenic disruption or distortion typical of many large terrestrial era-

4

Introduction

ters; (2) its original location on a relatively deep continental shelf allowed marine deposition to resume immediately following the impact, which buried it rapidly and completely, thereby preventing subsequent erosion of any principal feature except the distal margins of the surrounding apron of impact debris; (3) the upper part of the breccia body inside the crater was derived from the washback of impact-generated tsunami waves; (4) that same breccia body encompasses a large volume of impact-generated brine; (5) numerous smaller structures, which appear to be secondary craters, are present within a few tens of kilometers of the primary crater, a phenomenon that, in itself, sets the Chesapeake Bay crater apart from all other known impact structures on our planet; and (6) the crater underlies a densely populated urban corridor, whose two million citizens are still affected by craterrelated phenomena, 36 million years after the impact. In several earlier reports, Poag and his collaborators have established the general aspects of the Chesapeake Bay crater's structure and morphology, as well as the large-scale characteristics of the crater-filling impact breccia (Poag et al. 1992, 1994b, 1999; Koeberl et al. 1996; Poag 1996, 1997a; Poag and Aubry 1995; Poag and Foster 2000; Poag et al. 2001). The Chesapeake Bay structure is a complex, peak-ring/central-peak crater, 85 km in average diameter, and ~ 1.3-2 . 0 km deep at maximum estimated depth. The crater interior features a low-relief peak ring (-300 m maximum height) and a rugged central peak (-1,000 m maximum height). At twice the area of the State of Rhode Island and as deep as the Grand Canyon, the Chesapeake Bay crater (along with Popigai) is the sixth largest impact crater currently known on the globe. In hindsight, it is clear that telltale sedimentary and structural evidence of a buried, giant impact structure in southeastern Virginia first came to light in the 1940s through geohydrological studies (Poag 1996, 1997a, 1999c). These studies, mainly sediment and ground-water analyses from shallow boreholes, were carried out by the US Geological Survey (USGS) (Cederstrom 1945a,b,c, 1957). At that time, however, the extraterrestrial implications of the evidence were not appreciated. More than 50 years passed before this early evidence could be unequivocally linked to a late Eocene bolide impact (Poag et al. 1992, 1994b; Koeberl et al. 1996; Poag 1999c). In the interim, however, several authors (USGS scientists in particular) published a large database of subsurface stratigraphic analyses derived mainly from >200 uncored boreholes (Cederstrom I945a,b,c; Richards 1945, 1967; Cushman and Cederstrom 1949; Maher 1965, 1971; Brown et al. 1972; Teitke 1973; Gibson 1983; Gohn 1988; Poag and Ward 1993; Fig. 1.3; COROM.1; Table 1.2). These subsurface studies, along with extensive studies of outcrop stratigraphy (Ward and Krafft 1984; Owens and Gohn 1985; Ward and Strickland 1985; Mixon et al. 1989), firmly established a regional structural and stratigraphic framework for the sedimentary rocks of southeastern Virginia outside the crater rim. Regional surveys of gravity and magnetics, coupled with sparse deep well data and a few onshore seismic reflection surveys, provided a complementary geological framework of crystalline basement rocks beneath the Virginia Coastal Plain (Ewing et al. 1937; Woollard et al. 1957; LeVan and Pharr 1963; Taylor et al. 1968; Sabet 1973; Johnson 1977; Hawarth et al. 1980; Costain and

Introduction

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6

Introduction

Glover 1976-1982; Lyons and O'Hara 1982; Dysart et al. 1983; Thomas et al. 1989). While documenting the Chesapeake Bay structure and establishing its impact origin, Poag and his colleagues published data from five continuously cored boreholes and ~300 km of multichannel seismic reflection profiles (Poag et al. 1992, I994b, 1999; Poag 1996, 1997a; Powars and Bruce 1999; Powars et al. 200 I; Gohn in press). Definition and understanding of the structure and morphology of the crater and its associated features have improved progressively with the acquisition of each new data set (Figs. lA, 1.5). This book affords an opportunity to synthesize the large body of geological data (including sedimentological, paleontological, geochemical, paleomagnetic, and petrographic analyses) and geophysical data (including seismic reflection surveys, gravity surveys , and downhole logging) amassed over the past 16 years (1986-2002). In doing so, we analyze in greater detail much of the old data, refine and(or) reinterpret previously published inferences, and present new interpretations based on abundant new (unpubl ished) data. Among the latter, in particular, we have analyzed approximately 1,700 km of new seismic reflection profiles, 63 new gravity stations on the Delmarva Peninsula, a 90-km-long continuous marine gravity survey over the crater center (1,587 measurements), and >1,780 m (>5,840 ft) of core from three new deep, continuously cored boreholes . We have obtained petrographic analyses of > 100 new samples from the cored sections of crater-fill breccia, have analyzed several hundred micropaleontological samples from the thick marine clay bed (Chickahominy Formation) that caps the breccia, and we provide new descriptions and illustrations of whole and split core sections. We document a fallout layer inside the crater, and recognize a < 1- 3-kyr-long dead zone immediately above the fallout layer. We also offer new interpretations of stable isotopic and paleomagnetic data, and initial interpretations of the impact's short-term and long-term effects on postimpact paleoenvironments at the crater and at other sites around the globe. In addition, we present a comprehensive numerical and field-based conceptual model for cratering processes and the resultant impact-generated depositional regimes at Chesapeake Bay. As a supplement to this volume, we include a CD-ROM, which incorporates color versions of 29 selected figures from the main text and items too large or complex for page-sized illustrations . These items include: color maps for borehole locations, seismic survey tracklines, crater structure, and basement structure ; a 3-D computer model of crater structure ; scaled depth-sections along selected seismic profiles; a depth-scaled stratigraphic section showing core lithologies and downhole geophysical logs; and 18 selected seismic reflection profiles.

Introduction

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Fig. 1.4. Locations of outer rim of Chesapeake Bay impact crater published in previous articles by Poag et al. (1994b), Poag (1997a), and Poag et al. (1999). Note increased detail in peripheral geometry as additional data were acquired. See Chapter 4 for discussion of current interpretation of crater outline.

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Longitude 135°27' E 98 °12' W 04 °23'E 19°15'E 12°4 1' W 52 °59'W 156°38' W 00 055'W 24 °20'E Ill oOI'W 113°00' W 121°40' E 82 °01"E 32 °10' E 0 1°25' W 135°12' E 78 °29'W 85 °57' W 61°42" W 109°30' W 70 °18' W 76 °0 1' W 89 °30' W 5r51 'E 9r48' E

Latitude 32 °01' S 36 °15' N 26 °05 ' N 19° 15" N 20 015'N 16°47' S 71 °15 'N 4 1°10'N 25 °19'N 35 °02'N 44 °36' N 71 °00' N 48 °34 ' N 48 °45' N 06 °30' N 22 °37' S 46°05' N 41 °50 N 27 °38'S 58 °27' N 4r32N 37 °17'N 2 1°20' N 49 °10'N 75 °42'N

Location

South Australia Oklahoma, USA Algeria Chad Mauritania Brazil Alaska, USA Spain Libya Arizona, USA Montana, USA Russia Kazakhstan Ukraine Ghana Northern Territory, Australia Ontario, Canada Michigan USA Argentina Saskatchewan, Canada Quebec, Canada Virginia, USA Yucat an, Mexico Kazakhstan Russia

Name

Acraman Ames Amguid Aorounga Aouelloul Araguainha Dome Avak Azuara* B.P. Structure Barringer (Meteor) Beaverhead Beenchime-Salaaty Bigach Boltysh Bosumtwi Boxhole Brent Calvin* Campo del Cielo Carswell Charlevoix Chesapeake Bay Chicxu lub Chiyli Chukcha

*Impact origin doubtful

12 13 14 15 16 17 18 19 20 21 22 23 24 25

II

2 3 4 5 6 7 8 9 10

I

No . 586 470 ±30 <0.1 <345 3.1 ± 0.3 247 ± 5.5 >95 -40 < 120 0.049 ± 0.003 - 600 40 ±20 6±3 65.17 ± 0.64 1.03 ± 0.02 0.03 ± 0.0005 450 ±30 450 ± 10 <0.004 115 ± 10 357 ± 15 - 35.6 64.98 ± 0.05 46±7 <70

Age [Ma) 90 16 0.45 12.6 0.39 40 12 30 2 1.186 60? 8 5±3 24 10.5 0.17 3.8 8.5 0.05 39 54 85 195 5.5 6

Diam .[km]

Table 1.1. Impact structures (simple and compl ex craters ; multiring basins) documented on Earth . List modified from Grieve et al. ( 1995). Type complex buried, complex simple complex simple complex buried, complex complex complex simple complex complex complex buried, complex complex simple simple buried, comp lex simp le complex complex buried, complex buried, multiring complex comp lex

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Clearwater East Clearwater West Cloud Creek Conno lly Basin Couture Croo ked Creek Dalgaranga Decaturville Deep Bay Dellen Des Plaine s Dobele Eagle Butte Elbow EI'gygytgyn Flynn Creek Gardnos Glasford Glover Bluff Goat Paddock Gosses Bluff Gow Goyder Granby Gusev

No.

26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

Table 1.1. (cont.)

Location

Quebec, Canada Quebec, Canada Wyoming, USA Western Australia Quebec, Canada Missouri, USA Western Australia Missouri, USA Saskatchewan, Canada Sweden Illinois, USA Latvia Alberta , Canada Saskatchewan , Canada Russia Tennessee, USA Norway Illinois, USA Wisconsin, USA Western Australia Northern Territory, Australia Saskatchewan, Canada Northern Territory , Australia Sweden Russia

Latitude 56 °05' N 56 °13' N 43°7' N 23 °32'S 60 °08' N 37 °50' N 27 °38'S 3r54'N 56 °24' N 6 1°48' N 42 °03' N 56 °35 ' N 49 °42' N 50°59' N 6r30'N 36 °17' N 60 0 3 9 'N 40 °36' N 43 °58'N 18°20'S 23 °49'S 56 °27' N 13°29' S 58 °25' N 48 °26' N

Longitude 74 °07' W 74 °30' W 106°45' W 124 °45' E 75 °20'W 91 °23' W Il rl7' E 92 °43'W 102°59' W 16°48'E 87 °52'W 23 °15' E 110 °30' W 106°43' W 172°05' E 85 °40'W 09 °00'E 89 °47' W 89 °32'W 126°40' E 132°19' E 104°29' W 135 °02' E 14°56' E 40 °32' E

Age [Ma] 290 ± 20 290 ± 20 190 ±30 <60 430 ±25 320 ± 80 0.027 <300 100 ± 50 89.0 ±2.7 <280 290 ± 35 <65 395 ± 25 3.5 ± 0.5 360 ± 20 500 ± 10 <430 <500 <50 142.5 ± 0.8 <250 > 136 470 49 .15 ±0.18

Diam.[km] 26 36 7 9 8 7 0.021 6 13 19 8 4.5 10 8 18 3.55 5 4 8 5.1 22 5 3 3 3

Type complex comp lex buried, complex complex ? complex simple comp lex complex complex buried, comp lex buried, complex complex complex comp lex comp lex complex buried , complex comp lex comp lex complex comp lex simple simple twin, buried, simple

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Kaluga Kam ensk Kara Kara-Kul Kilrdla Karikkoselk li Kar la Kelly West Kent land Kgago di Kursk La Moinerie

64 65 66 67 68 69 70 71 72 73 74 75

53 54

55 56 57 58 59 60 61 62 63

Name

Gweni-Fada Haught on Havil and Henb ury Highbury Holle ford lie Roul eau Ilumetsa Ilyinets lso-Naakkima Janisjarvi Kaa lijlirvi Kalkkop

51 52

No.

Table I.I. (cont.)

Estonia Finla nd Russia Northern Territo ry, Australia Indi ana, USA Botswana Russia Quebec, Canada

Zimbab we Ontario, Canada Quebe c, Canada Esto nia Ukraine Finland Ru ssia Estonia South Africa Russia Russia Ru ssia Tajikistan

17" 25' N 75 °22' N

Chad Northwe st Territories, Canada Kansas, USA Northern Territory, Aust ralia

4 8 °2 1' N 69 °06 ' N 39 °0 1' N 59 °0 1' N 63 ° 13' N 54 °55' N 19 °56' S 40 °4 5' N 22°29'S 5 1°42' N 57 °26 ' N

37 °3 5' N 24 °34'S 17° 04' S 44 °28' N 50 041 ' N 57 °58' N 49 °07' N 62 ° 11' N 6 1°58' N 58 °24' N 32 °43'S 54 °30' N

Latitude

Location

27"3 4' E 36 °00'E 66 °37' W

40 0 30' E 64 °09'E 73 °27' E 22 °46'E 25 ° 15'E 48 °02' E 133 °57' E 87 °24' W

2 1°45' E 89 °4 1' W 99 ° 10' N l 33 °0 8' E 30°07' E 76 °38 ' W 73 °53' W 27°25'E 29 °06'E 27°09'E 30 05 5'E 22 °40'E 24 °34'E 36 ° 12' E

5± I >550 <97 <60? 250 ± 80 400 ± 50

455 ± I < 1.88

49. 15 ± 0.18 70.3 ± 2.2 <5

700 ± 5 0.004 ± 0.00 I 0.25 380

1034? 550 ± 100 <300 >0.002 400 > 1000

23 ± I <0.00 1 <0.005

Age [Ma] <345

Long itude

25 65 52 7 1.5 10 10 13 3.4 6 8

14 24 0.01 5 0.157 20 2.3 5 15 0.08 8 3 14 0.1 I 0.64 15

Diam .[km]

buried, co mplex bur ied, complex co mplex buried, complex simple buri ed, co mplex compl ex co mplex simple buried, complex complex

comp lex simple? complex cluster, simple bur ied, complex simp le complex cluste r, simple simple bu ried, co mplex

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Type

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Lappajarvi Lawn Hill Liverpool Lockne Logancha Logoisk Lonar Lumparn Macha Manicouagan Man son Maple Creek Marquez Middlesboro Mien Mishina Gora Mistastin Mizarai Mjelnir Montagnais Monturaqui Morasko Morokweng Mo unt Toondina Neugrund

No .

76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

Table 1.1. (cont.)

Finland Queensland, Australia Northern Territory , Au stral ia Sweden Russia Belarus India Finland Russia Quebec, Canada Iowa, USA Saskatchewan, Canada Texas, USA Kentucky, USA Sweden Russia Newfou nd/Lab rador, Canada Lithuania Barents Sea, Norway Scotian Shelf, Canada Chile Poland South Africa South Australia Estonia

Location Latitude 63 °12'N 18 °40' S 1:0 2 4 'S 63 °00' N 65 °31'N 54 °12'N 19 °58' N 60 °09' N 60 °06' N 5I 0 2 3 ' N 42 °35' N 49°48' N 31 01 7'N 36 °37' N 56 °25' N 58 °43' N 55 °53' N 54 °01'N 73 °48'N 42 °53' N 23 °56' S 52 °29' N 26 °28'S 27 °57' S 59 °20'N

Longitude 23 °42' E 138°39 ' E 134°03'E 14°48'E 95 °56' E 2r48'E 76 °3 1'E 20 0 0 6 ' E Il r35'E 68 °42 'W 94 °35' W 109°6'W 96 °18'W 83 °44' W 14°52' E 28 °03 ' E 63 °18' W 23 °54'E 29 °40'E 64 °I3'W 68 °17' W 16°54 'E 23 °32' E 135 °22' E 23 °40'E 145.0 ± 0.8 < 110 ?

om

121.0 ± 2.3 300 ± 50 38±4 500 ±20 143.1 ± 0.8 50.50 ± 0.76
214 ± I 74.1 ± O.l <75 58.3 ±2 <300

77.3 ± 0.4 >5 15 150 ± 70 >455 40 ±20 30±5 0.052 ± 0.006 - 1000 <0.007

Age [Ma]

Diam.[km] 23 18 1.6 7.5 20 15 1.83 9 0.3 100 37 6 13 6 9 2.5 28 5 40 45 0.46 0.1 70-80 4 6-8

Type comp lex compl ex simple complex complex buried, complex simple complex simple complex buried, comp lex complex comp lex complex simple simple comp lex buried, simple buried, comp lex buried, comp lex simple cluster, simp le buried, complex simp le? comp lex?

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Quebec, Canada North Dakota, USA Northwest Territories, Canada Libya Ukraine Texas, USA Algeria Western Australia Northwest Territor ies, Canada Russia Quebec , Canada Russia Russia North Dakota, USA Brazil Germany Argentina France Wisconsin, USA Namibia Ukraine Finland Manitoba, Canada Ohio, USA Brazi l

Name

New Quebec Newporte Nicho lson Oasis Obolon' Odessa Ouarkz iz Picaninny Pilot Popiga i Presqu'Ile Puchezh -Katunki Ragozink a Red Wing Riachao Ring Ries Rio Cuarto Rochechouart Rock Elm Roter Kamm Rotmistrovka Saaksjarvi Saint Martin Serpent Mound Serra da Cangalha

No .

101 102 103 104 105 106 107 108 109 110 111 112 113 114 lIS 116 117 118 119 120 121 122 123 124 125

Table 1.1. (cont.) Longitude 73 °40' W 101°58' W 102°41' W 24 °24'E 32 °55'E 102°29'W Or33' W 128°25'E Ill oOI"W III ° I l ' E 74 °48' W 43 °43'E 61 °48' E 103°33' W 46 °39' W 10°37' E 64 °14' W 00 056'E 92°14' W 16° 18' E 32 °45'E 22 °24'E 98 °32'W 83 °24'W 46 °52' W

Latitude 61 °17'N 48 °58' N 62 °40' N 24 °35' N 49 °35'N 31 °45'N 29 °00' N 17°32'S 60 017'N 71 °38' N 49 °43' N 56 °58' N 58 °44' N 47 °36' N Or43'S 48 °53' N 32 °52'S 45 °50' N 44°43' N 27 °46'S 49 °II 'N 61 °24' N 51 °7' N 39 °02' N 08 °05'S 1.4 ± 0.1 <500 <400 <120 169± 7 <0.05 <70 <360 445 ±2 35.7 ± 0.2 <500 167 ±3 46 ±3 200 ±25 <200 15 ± I <0.1 214±8 <505 3.7 ± 0.3 120 ± 10 - 560 220 ±32 <320 <300

Age [Ma] 3.44 3.2 12.5 ::::18 20 0.168 3.5 7 6 85 24 80 9 9 4.5 24 4.5 23 6 2.5 2.7 6 40 8 12

Diam .[km] simple buried, simple complex complex buried , complex simple complex simple simple ? complex complex buried , complex buried, complex buried, complex complex buried, comp lex cluster, simple highly eroded simple simple buried, simple simp le complex complex complex

Type

I

g. ::l

c, n

ae

5"

.j::>.

-

Name

Shunak Sierra Madera Sikhote Alin Siljan Slate Islands Sobolev Soderfjarden Spider Steen River Steinheim Strangways Suavjarvi Sudbury SuvasvesiN Tabun-Khara-Obo Ta lemzane Teag ue Tenoumer Temy Tin Bider Tookoonooka Tswaing (Saltpan) Tvaren Upheaval Dome

No.

126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149

Table I.I. (cont.)

Kazakh stan Texas, USA Russia Sweden Ontario, Canada Russia Finland Western Australia Alberta, Canada Germany Northern Territory, Australia Russia Ontario, Canada Finla nd Mongolia Algeria Western Australia Mauritani a Ukrai ne Algeria Queensland, Australia South Africa Sweden Utah , USA

Location

2r36'N 2r07' S 25 °24 'S 58 °46' N 38 °26' N

47 °12 'N 30 °36 ' N 46 °07' N 6 1°02' N 48 °40' N 46 ° 12'N 63 °02' N 16°44'S 59 °30' N 48 °41' N 15° 12' S 63 °07' N 46 °36' N 62 °42' N 44 °06'N 33 °19 ' N 25 °02'S 22 °55' N 48 ° 15' N

Latitude

128 ±5 0 .220 ± 0.052 >455 <65

1630 ± 5 2.5 ± 0.5 280 ± 10 <70

150 ± 20 <3

1850 ± 3 < 1000

95 ± 7 15± 1 <470 2400

368 ± 1.1 - 450 <0.00 1 - 600 >570

45± 10 < 100 fell in 1947

102 °55'W 134 °40 ' E 14°52 'E 8rOO'W 138 °54' E 21 °35' E 125 °05' E 117 °38 ' W 10°04 ' E 133 °35'E 33 °23'E 8 1°II'W 28 °00 ' E 109 °36' E 04 °02' E 120 °53' E 10°24' W 33 °30'E 05 °07' E 142 °50 ' E 28 °05' E I r25' E 109 °54 'W

Age [Ma]

Long itude

n 042' E

3.8 25 16 250 4 1.3 1.75 30 1.9 II 6 55 1.13 2 10

13 25

13 0.027 52 30 0.053 5.5

2.8

D iam .[km] simple complex cluster, simple complex complex simple buried, complex complex buried, complex complex comp lex complex multiring complex? simp le simple complex simple complex complex complex simp le simple complex

Type

~ 0

V;

::s

0

a

e

0-

Russia Brazil Western Australia Lithuania Saskatc hewan, Canada South Africa Saudi Arabia Ontario, Canada Tennessee, USA Manitoba, Canada Alabama, USA Western Australia Australia Ukraine Ukraine Kazakhstan

Ust-Kara" Vargeao Dome Veevers Vepriai Viewfield Vredefort Wabar Wanapitei Wells Creek West Hawk Wetumpka Wolfe Creek Woodleigh Zapadnaya Zeleny Gai Zhamanshin

150 151 152 153 154 155 156 157 158 159 160 16 1 162 163 164 165

*Uncertain See also, Earth Impact Database at http://www .unb.ca/passc/lmpact/Database/

Latitude 69 °15'N 26 °50'S 22 °58'S 55 °05' N 49°35' N 27 °00'S 21 °30' N 46 °45' N 36 °23' N 49 °46' N 32°31' N 19°1O'S 26°03' S 49 °44' N 48 °04' N 48 °24' N

Location

Name

No.

Table 1.1. (cont.)

Age [Ma] 70.3 ±2.2 <70 160 ± 5 190 ± 20 2023 ± 4 0.006 ± 0.002 37 ±2 200 ± 100 100 ± 50 81.0 ± 1.5 <0.3 <300 165 ±5 80±20 1.0 ± 0.1

Longitude 65 °23'E 52 °07' W 125°22'E 24 °34'E 103°4' W 27 °30'E 50 °28' E 80 0 4 5 'W 87 °40'W 95 ° 11' W 86°10' W 127"48' E 114°39' E 29 °00'E 32 °54'E 60 °58' E

Diam .[km] 25 12 0.08 7.5 2.5 300 0.1 16 7.5 12 2.44 6.5 0.875 40 3.2 3.5 13

Type comp lex comp lex simple buried, complex simple multiring simple complex buried, complex simple complex simple complex buried, complex buried, complex complex

,

==

o'

~

s:::

0-

0

:::t

5'

0'\

USGS Corehole, Solomons Island, MD

USGS Oak Grove Corehole

St.M. - Df84 Lexington Park, MD Oh io Oil, L.G. Hammond, Salisbury, MD

J &J Enterprises, E.G. Taylor, Atlantic, VA

5

6

7

9

8

4

Va. Dept. Env. Qual. Newport News Park II Corehole USGS Windmill Point Corehole

USGS Exmore Corehole Va. Dept. Env. Qual. Kiptopeke Corehole

Name/ Location

3

2

#

1971

1944

1982

1976

1986

1992

1990

1989

1986

Absent

Absent

38" 19' 49" 76°27' 52"

38"10' 54" 76"09' 50" 38°15' 48" 76°27' 21"

37"53' 18" 75°30' 54"

38"20' 48" 75"21' 54"

-439' -133.8 m

37"36' 50" 76°16' 55"

Absent

Absent

Absent

-240' -73.2 m

-1082' -329.8 m -1073 ' -327.1 m

Absent

Absent

Absent

Absent

Absent

-530.5' -161.7 m

Not reached

Not reached -2514.6' ·766.5 m diabase -5429.8' -1655 m gneiss

+15' +4.6 m +180' +54.9 m +108 .39' +33 m +54' +16.5m

·6 230' -1911.7m

-5498' -1675.8 m

-1200' -365.7 m -1569.6' -783 .3 m

-621' -189.3 m

-6144' ·1872.7 m gneiss

D &C

Not reached

+4' +1.2 m

-743.8 ' -226 .7 m

+42 ', +12.8 m

C

Not reached

+52' +15.9 m

-570' -173.7 m

-339.3' -103.4 m

Distal breccia Outside crater

0

Outside crater Outside cra ter

Outside crater

8

7

6

5

4

2,3

2,3 Washback channel?

Breccia apron

2,3

1, 2, 3

Reference

Inner basin

Annular through

Relative Location

D&C

C

C

C

C to -1323' -403.3 m

Not reached

C

Cored or Drilled

+7' +2.1 m

-1366' -416.4 m -1993' -607.5 m

-1283' -391.1 m

Basement Elevation! Lithology Not reached

Well Head Elevation +30' +9.1 m

Total Depth

-1180.6' -359.9 m

Top Top Chickahominy Exmore/ Mattaponi

37"12' 08" 76"34' 11"

37"08' 07" 75"57' 08"

37" 35' 08" 75" 49' 09"

Latitiude/ Year Drilled Longitude

Table 1.2. Borehole dat a used to interpret the stratigraphic framework of the Chesapeake Bay impact crater and surrounding coastal-plain sect ions of Virginia and southern Maryland, and , in part icula r, to calibrate the network of seismic reflection profiles. Depths in feet (') and meters (m) .

I

-.J

:::s

g.

<>

l::

0-

0

sa.....

......

VA· VB·T- 4

VA· MID-T· 7

VA·MID ·P -9

VA· MID - p. 10

VA- MID -P· 3

VA -MID -P·8

VA- R1- p . 6

VA · WES - T - 8

VA · WES ·T ·7

VA·KG ·P-IS

VA· KG- P -16

12

13

14

15

16

17

IS

19

20

21

22

II

Som . Ce 9S, near Princess Anne . MD VA·VB ·P·3

Name / Location

10

#

Table 1.2. (cont.)

pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972

1989

71°03' 4S"

76°45' 00" 38°05' 30" 76°49' 15" 38' 12' 00" 76' 55' 40" 3s 020' 00" 77°02' IS" 3s002' 10"

3S'57' 50"

36°57' 40" 76'07' 20" 37'32' 20" 76'19' 50" 37' 35' 45" 76'26 ' 20" 37°36' 00" 76'29' 25" 37'36' 20" 76°31' 30" 37'3S ' 30" 76°34' 30"

75 °58' 3S"

Absent Absent Possibly present Absent

Absent Possibly present Absent

Absent

·479' ·146 m -404' · 123.1 m ·332' . 1Ol.2 m ·330' -100.6 m Absent

·26 S.2 m

·870'

Absent

Absent

-830' ·253 m -439' -133.8 m ·364' ·110.9 m -306' -93.3 m ·300 ' -91.4 m Absent

Top Top Chickah ominy Exmore/ Mattaponi 38°09' 21" Absent Absent 7s042' 32" 36°51' 20" Absent Absent

Year Latitiude/ Drilled Longitude

-4S7.8 m ·1583' ·482.5 m ·1500' -457.2 m ·785' ·239.3 m ·539' · 164.3 m -49S' · 151.S m ·710' -216.4 m -535' -163.1 m ·570' -173.7 m ·521' · ISS.8 m -63S' -194. S m ·990 ' ·301.S m ·565' · 172.2 m

· IS02'

Total Depth +13' +4m +5' +1.5 m +35' +10.7 m +5' +I.S m +15' +4.6 m +30' +9.1 m +40' +12.2 m +7' +2.1 m +130' +39.6 m + 120' +36.6 m +15' +4.6 m +20' +6.1 m +11 0' +33.5 m

Well Head Elevation

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Basement Elevation! Lithology Not reached

D

D

D

D

D

D

D

D

D

D

D

C ·78 to · 1104 m D

Cored or Drilled Outside crater Breccia apron Annular trough Breccia apron Breccia apron Breccia apron Breccia apron Outside crater Out side crater Outsid e crate r Out side crater Second ary crate r? Outside crater

Relat ive Location

7

7

7

7

7

7

7

7

7

7

7

7

9

Reference

;:l

o'

~

c

0..

S

a

00

pre 1972 pre 1972 pre 1972

VA- NAN -P- 8

VA-NAN-P-14

VA-JW -P- 8

VA-SO-T-6

VA-SO-P-3

NC·NOR-T-S

NC-NOR-T·9

NC· NOR-T - 10

VA-CRE-P- 5

VA·POR·P·1O

24

25

26

27

28

29

30

31

32

33

pre 1972 pre

pre 1972

pre 1972

pre 1972

pre 1972 pre 1972

pre 1972

VA-NOR-T-12

23

Top Top Chickahominy Exmore/ Mattaponi

Total Depth

Well Head Elevation

Basement Elevation! Lithology

Cored or Drilled

..,£<>"'.."

..,1\"

36'43' 30" 76'20' 15" 36'4S' 00"

36'30' 05" 77'24' 55"

36'31' 20" 77"22' 30"

36'32' 30" 77'19' 00"

36'38' 05" 77'05' 00" 36'3S' OS" 77"02' 00"

36'50' 30" 76'27' 00" 36'42' 45" 76'39' 35" 36'41' 15" 76'04' 30"

36°52' 20" 76°12' 00"

Absent

Absent

Absent

Absent

Absent

Absent

Absent

Absent

Absent

Absent

-610' -185.9 m

Absent

Absent

Absent

Absent

Absent

Absent

Absent

-340' -103_6 m -185' -56.4 m Absent

-667' -203_3 m

·806' ·24S.7 m ·638'

·144' -43.9 m

·IS5' -56.4m

·255' ·77.7 m

+14 +4.3 m +15'

+100 +30.S m

+100' +30.S m

+50' +15.2 m

+85' +25.9 m +40' +12.2 m

+20 +6.1 m +65' +19.8 m +40' +12.2 m

-634' -193.2 m -652' -198.7 m -880' -268.2 m -290' -88.4m -428' ·130.5 m

+15 +4.6 m

-2567' -782.4 m

D D

Not reached

D

D

D

D

-235' -71.6 m ? -ISO' -4S.7 m ? ·123' ·37.5 m ? Not reached

-340' ·103.6 m

D

D

-828' -252.4 m Not reached

D

D

D

Not reached

-2567' -782.4 m granite? Not reached

~<----~.-~<~~-~-->-=,~-.----~---~."'~~-----=~---------.--------

Year Latitiude/ Drilled Longitude

Name/ Location

#

Table 1.2. (cont.)

Outside crater Breccia

Outside crater

Outside crater

Outside crater

Outside crater Outside crater

7

Breccia apron Breccia apron Breccia apron

7

7

7

7

7

7

7

7

7

7

Reference

Breccia apron

Relative Location

~

'D

......

~

0

C.

(')

~

0 p..

I"t

Name/ Location

VA·CHE ·P·IJ

VA · NAN· P -13

VA·IW · P· 13

VA·SUR· P · 1

VA·1C ·T -1O

VA ·CC ·T · 4

VA ·CC·P ·3

VA·NK·P·6

VA · HEN · P · 12

VA·HAN·P-12

VA ·PG- P-1

VA·PG-P-3

VA ·PG -T·8

#

34

35

36

37

38

39

40

41

42

43

44

45

46

Table 1.2. (cont.)

pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972

36°50' 30" 76°24' 00" 36°02' 30" 76°28' 25" 37°00' 05" 76°07' 50' 37°10' 00" 76°41' 55" 37"13' 00" 76°45' 30" 37°22' OS" 77°02' 35" 37°28' 00" 77°11' 00" 37°34' 30" 77°09' 00" 37°31' 30" 77°18' 00" 37"37' 30" 77°21' IS" 37" 16' IS" 77°11' IS" 37°13' 20" 77°16' 45" 37°11' 00" 77°19' 00"

Latitiude/ Year Drille d Longitude

Absent Absent ·212' ·64.6 m -166' ·50.6 m O.O'? 0.0 m? Absent Absent Absent Absent Absent Absent Absent

Absent

Absent

·201' -61.3 m Absent

Absent

Absent

Absent

Absent

Absent

Absent

Absent

Absent

Absent

Absent

Top Top Chickahominy Exmore / Mattaponi

·633 ' · 192.9 m -414' · 126.2 m ·379' ·115.5 m . ·355' ·108.2 m ·90' ·27.4 m ·244 ' ·74.4 m ·350' -106.7 m · 130' -39.6 m -157' -47.9 m -127' ·38.7 m · 135' -41.1 m +40' +12.2 m

·5 97' · 182 m

Total Depth +3' +0.9 m +22' +6.7 m +40' +12.2 m +15' +4.6 m +10' +3.1 m +70' +21.3 m +120' +36.6 m +150' + 45 .7 m +160' +48.8 m +190' +57.9 m +125' +38.1 m + 120' +36.6 m +140' +42.7 m

Well Head Elevation

D D D

Not reached Not reached Not reached

D D D

Not reach ed Not reached

D Not reached

Not reached

D

D

Not reac hed

Not reached

D

Not reac hed

D

D

Not reached

Not reac hed

D

Cored or Drilled

Not reached

Basement Elevation! Lithology Breccia apron Brecc ia apro n Brecc ia apron Washback channel? Washback channel? Washback channel? Outside crater Outside crater Outside crater Outside crater Outside crater Outs ide crater Outside crater

Relat ive Location

7

7

7

7

7

7

7

7

7

7

7

7

7

Reference

::l

g.

n

c

0.

5'

a

N 0

MD - SO M - T · IO

VA - NOD -T -8

VA -NOD-T·7

VA -LAN·P·I

VA·GLO ·P -1

VA ·YK ·T ·9

VA · SUS -T · 2

VA - YK -T · 6

VA -KW-T -4

VA· KQ· P-2

VA · KQ ·T·S

VA·CHE- P-6

VA - NON-T -3

48

49

50

51

52

S3

54

55

56

57

58

59

Name/ Location

47

#

Table 1.2. (cont.)

pre 1972 pre 1972 pre 1972 pre 1972

pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972

7s 057' 15"

37°48' 10" 77°03' 10" 37°54' 10" 77°06' 20" 36°38' 30" 76°22' 00" 37°12' 10"

37"49' 50" 76°17' 15" 37°43' 30" 76°19' 30" 37"37' 45" 76°20' 30" 37"21' 10" 76°36' 35" 37°19' IS" 76°39' 30" 36°09' 20" 77°01' 30" 37"14' 00" 76°33' 00" 37°32' 40" 76°48' 20"

75 °50' 4S "

38°00' 20"

Year Latitiude/ Drilled Longitude

Absent

Absent

Absent

Absent

-264' -80.5 m Absent

-440' -134 m ·269' ·82 m · 197' ·60 m Absent

Absent

Absent

Absent

Absent

Absent

Abse nt

Absent

-310' ·94.5 m -126' ·38.4 m

-SOO' ·152. 4 m -30S' -93 m ·213' -64.9 m Absent

Absent

Absent

Abse nt

Top Top Chickahominy Exmore/ Mattaponi

-278' ·84.7 m · 11 ' -3.4 m ·701' -213.7 m -441' -134.4 m

-120.1 m ·90' ·27.4 m -458' · 139.6 m -1659' ·505.7 m

-39S '

· 1505' ·458.7 m ·650' · 198.1 m -424' -129.2 m -734' ·223.7 m -417' · 127.1 m

Tota l Depth

+ 192 ' +58.5 m + 17S' +53.3 m + IS' +4.6 m +20' +6.1 m

+2' +0.6 m +3' +0.9 m +7' +2.1 m +75' +22.9 m + 100' +30.S m +50' +IS.2 m +30' +9.1 m

+ I.S m

+5'

+ I.S m

+5'

Well Head Elevation

Not reached

Not reached

Not reached

-382.5 m No t reached

-125S'

Top Trias.

No t reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not Reached

Basement Elevation! Lithology

D

D

D

D

D

D

D

D

D

D

D

D

D

Cored or Drilled

7

7

7

7

7

7

7

7

7

7

7

7

7

Reference

Outside crater Outside crate r Wash back channel ? Inner ba sin

Ou tside crater Outsi de crater Outside crater Breccia apron Wash back channel? Washback channe l? Washback channel? Wash back channel? Washback channel?

Relative Location

,

N

='

0'

~

e

Q.

5'

a

VA-IW·P -7

VA-IW-C-Il

VA-SO-C-8

VA -SO -T -9

VA-SUS -T-3

VA-SUS -T-4

VA-PG -T-7

VA -PG-T- 9

USGS Corehole Haynesville No .2 #82 Cape Charles NY, Phil., Norfolk RR #183 Tangier Island #180 Cobbs Island

61

62

63

64

65

66

67

68

69

72

71

70

VA- NAN - C - 21

Name / Location

60

#

Table 1.2. (cont.)

1925 ? ?

1910

pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 pre 1972 1985

Not reached

Not reached

-858' -261.5 m -1001' -305.1 m

Absent

Absent

37"50' 76°00 37°20' 75°44

-ns:

-228' -69.5 m -371' ·113.1 m -190' -57.9 m -148' -45.1 m -35.1 m -50' -15.2 m -35' -10.7 m -10' -3.1 m -25' -7.6 m -556' -169.5 m -1790' -545.6 m

Total Depth

Top Top Chickahominy Exmore/ Mattaponi 36°45' 40" Absent Absent 76°40' 30" 36°48' 00" Absent Absent 76°45' 00" 36°51' 00" Absent Absent 76°48' 45" 36°54' 20" Absent Absent 76°53' 30" 36°56' 20" Absent Absent 76°57' 00" 37"02' IS" Absent Absent 77°05' 50" 37°05' 40" Absent Absent 77°11' 00" 37"08' IS" Absent Absent 77°14' 45" 37"17' 30" Absent Absent 77°18' 00" 37°57' 13" Absent Absent 76°40' 26" 37°16' -335' -355' 76°01' -102.1m -108.2 m

Year Latitiude/ Drilled Longitude

+2' ? +0.6 m +9' +2.7 m

+60' +18.3 m +82' +25 m +50 +15.2 m +80 +24.4 m +75' +22.9 m +100' +30.5 m +105' +32 m +100' +30.5 m +90' +27.4 m +87' +26.5 m +20' +6.1 m

Well Head Elevation

D

Not reached

D

Not reached

Not reached

Not reached

Not reached

D

D

D

C

D

Not reached

Not reached

D

D

Not reached

Not reached

D

D

Not reached

Not reached

D

D

Cored or Drilled

Not reached

Basement Elevation! Lithology Not reached

Outside crater Annular trough

11

11

10

7

7

7

7

7

7

7

7

7

7

Reference

Outside crater Outside crater Outside crater Outside crater Outside crater Outside crater Outside crater Outside crater Outside crater Outside crater Inner basin

Relative Location

::l

o'

;::: ~

a 0-

......

a

N N

# 17 Ocran Dyrner Fish Co .

# 18 Byrdton, Estate East Rich lands

# 20 Reedville Blunden & Hinton

#21 Reedville McNeal & Dodson

# 42 West Point Chesapeake Camp Co .

# 43 West Point Chesapeake Camp Co.

# 44 G loucester

# 45 Severn J.N . Shakelford # 46 Mathews Elk ins Oil & Gas Co.

# 58 (56) Williamsburg Dunbar Farm

73

74

75

76

77

78

79

80

82

81

Name/ Location

#

Table 1.2, (cont.)

1937

1929

37'25' 76°45'

37°25' 76°18'

37' 19' 76°25'

37'29' 76'30'

pre-

1945 pre 1945

37'32' 76'48'

37'32' 76'48'

37'49' 76' 16'

37'51' 76'15'

37'42' 76'20'

36'39' 76' 20'

1944

1944

pre1945

pre1945

pre 1945

pre1945

Year Latitiude/ Drilled Longitude

·230' ? ·70.1 m

-588' ? -179.2 m

·432' ? · 131.7 m

-347' ? -105.8 m

-178' ? -54.3 m

-183' ? -55.8 m

-275' ? ·83.8 m

Abse nt?

-210' ? ·64.0m

-243' ? ·74.1 m

-280 ? -85.3 m

-803 ? -244.8 m

-567 ? ·172.8 m

-528' ? · 160.9 m

·378' ? -115.2 m

-395' ? · 120.4 m

-345' ? · 105.2 m

-370' ? -112.8 m

·335 ' ? -102.1 m

·375' ? ·114.2 m

Top Top Chickahominy Exmore/ Mattaponi

-610' -185.9 m

-2318' ·706.5 m

·60 2' ·183.5 m

-745' -227.lm

-741' -225 .9 m

·657' -200 .3 m

·86 5' -263.7 m

>·785 ' >-239.3 m

·800' ·243.8 m

-497.5' ·151.6 m

Total Depth

+60' + 18.3 m

+7' +2.1 m

Not reached

+70 ' +21.3 m +8' +2.4 m

Not reached

-2318' ·706.5 m granite

Not reached

Not reached

Not reached

No t reached

Not reac hed

Not reached

No t reached

Basement Elevation! Litho logy

+22' +6.7 m

+10' +3.1 m

+5' ? + 1.5 m

0.0' O.Om

+ 10' +3.1 m

+IO'? +3.1 m

Well Head Elevation

D

D

D

D

D

D

D

D

D

D

Cored or Drilled

Was hback chan nel?

Annular tro ugh

Washback channel?

Washback channel?

Washback channel?

Washback channel?

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Re lative Location

10, 14

10

12

10

10

10

10

10

10

10

Reference

w

tv

o' ::l

~

c

0..

0

:::;

S'

-256' ? ·78 .0 m -286' ? -87.2 m ·242' ? -73.8 m -300' ? -9\. 4 m -281.5' ? -85.8 m ·307' ? -93.6 m -262' ? -79.9 m -300 '? -91.4 m

37° 18' 76°39' 37°19 ' 76°39' 37°17' 76°37' 37 °12' 76°30 '

37°12 ' 76°34 '

37"10' 76°31'

37"10' 76°34' 37°08' 76°33'

1943

1916

1941

1941

# 62 (13) Camp Peary # 63 (7) Camp Peary

86

93

92

91

90

89

88

87

# 67 (8) Lee Hall, Newport News Waterworks # 68 (21)Ft. Eustis 2 # 69 (22) Ft. Eustis 3

# 64 (24) Penniman Fuel Depot # 65 (35) Yorktown Navy Mine Depot # 66 (3) Lee Hall, Newport News Water Co .

-210' ? -64.0 m

37"16' 76°4 3'

1943

# 61 (22) Williamsburg Waller Pond

85

194 1

1942

1942

1942

-177' ? -53.9 m

37°16' 76°40 '

1940

84

37°15' 76°49'

1944

# 59 (49) Williamsburg J . Levinson # 60 (23) Williamsburg RW. Mahone

83

-516.5' -157.4 m ·455' -138.7 m -658 ' -200 .6 m ·519' -158.2 m

-429' ? · 130.8 m -44 3' ? -135.0 m -409' ? -124.7 m

-359' -109 .4 m -390' -118.9 m -430' -131.1 m -540 ' · 164.6 m

-330' ? -100 .6 m -360' ? -109.7 m -362' ? -110 .3 m -380' ? · 115.8 m -457' ? -139.3 m

·457.5' ·139 .5 m

·340' · 103.6 m

·345' -105 .2 m

Total De pth

·34 5' ? · 10 5.2 m

-300' ? -9\. 4 m

Top Top Chickahominy Exmore! Mattaponi -290 ' ? · 180 ? -54.9 m -88.4 m

Latitiude! Lo ngitude

Year Drill ed

Name! Location

#

Table 1.2. (cont.)

Not reached

+84' +25 .6 +74' +22.6 +83' +25.3 +80 ' +24.4

D D

Not reache d

D

D

D

D

D

D

D

D

D

Cored or D rill ed

Not reached

Not rea ched

+ 15' +4.6 m +34' +10.4 m +31' +9.5 m

Not reached

Not reached

Not reac he d

+ IO'? +3.1 m

m

m

m

No t reached

N ot reached

+ 10' ? +3.1 m

m

Not reached

B as em ent Elevation! Lithology N ot reached

+25' +7.6 m

+90' ? +27.4 m

Well Head Elevation

Washback channel? Washback channel?

Washback channel?

Wash back channel?

Wa shback channel? Wash back channel? Washback cha n nel? Annular trough

Wash back channel?

Washback channel?

Washback channel?

Relative Location

10. 14 10. 14

10. 14

10, 14

10, 14 10, 14 10, 14 10, 14

10, 14

10, 14

10, 14

Reference

tv

::s

o'

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e

5"

0-

a

.j:>.

1942

1942

# 72 (23) Mulberry Island

# 73 (30) Camp Patrick Henry # 74 (29) Camp Patrick Henry # 78 (46) Newport News Levinson Mea t Packing

96

97

# 79 (44) Newport News Va. Public Servo Co .

# 80 (13) Newport News Buxton Hospital

# 81 (8c) Fort Monroe

100

101

102

99

98

95

1902

pre 1945

1944

1940

1942

1941

1941

# 70 (20)Ft. Eustis 4 # 71 (17) Ft . Eustis Dozier I

94

37'00' 76'18'

36'59' 76'24'

36'59' 76'04'

37'00' 76'25' 37'00' 76'24' 36'58' 76'26'

37'08' 76'36 '

37"08' 76'33 37'08' 76' 33'

Year Latitiude/ Drilled Longitude

Name/ Location

#

Table 1.2. (cont.)

-630' ? -192.0m

-250' ? ·76.2 m

-413' ? -125.9 m

-314' ? -95.7 m -400' ? -121.9 m ·415' ? -126.5 m -405' ? -123.4 m

-269' ? -82.0 m ?

-830' ? -253.0 m

-440' ? · 134.1 m

-592' ? -180.4m

-396' ? -120.7 m -524' ? -159.7 m -457' ? -139.3 m -485' ? -147.8 m

-431' ? -131.4 -430' -131.1 m

Top Top Chickahominy Exmore/ Mattaponi

-2244' -684.0 m

-810' -246.9 m

-1070' -326.1 m

-455'+ -138.7 m -524' -159.7 m -457' -139.3 m -890' -271.3 m

-512' -156.1 m -440' -134.1 m

Total Depth

-2244' -684 m crystalline rock

Not reached

+10' +3.1 m +10' +3.1 m

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Basement Elevation! Lithology

+12' +3.7 m

+10' +3.1 m +30' +9.1 m +35' +10.7 m +10' +3.1 m

+31' +9.5 m +7' +2.1 m

Well Head Elevation

D

D

D

D

D

D

D

D

D

Cored or Drilled

Annular trough

Washback channel?

Washback channel?

Washback channel? Washback channel? Washback channel? Washback channel?

Washback channel?

Washback channel?

Relative Location

10, 14

10, 14

10, 14

10, 14

10, 14 10, 14 10

10, 14 10, 14

Reference

I~

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g.

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I:

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a

19761982

1976· 1982

C2S Va. Tech

III

Geothermal Portsmouth

19761982

C24 Va. Tech Geothermal Willoughby Bay

110

Geothermal Eastville

CS6 Va. Tech

109

-101I.5'? ·308.3 m?

-916.3' ·279.3 m

-716.3' -218.3 m

36°S3' 76°28'

-293.S m

-897.3' ·273.5 m

36°S8' 76°16'

-962.9'

-1011.5' -308.3 m Not reached

Not reached

-1982.6' -604.3 m

·1024.9' ·312.4 m

-308.3 m

-ion:

-1011.5' ·308.3 m

+22' +6.7 m

+S' +I.S m

+12' +3.7 m

metavo lcanics

·SSO.3 m

-1805.4'

Not reached

Not reached

Not reached

Not reached

-634' -193.2 m

-20S' -62.S m +20' +6.1 m +12' +3.7 m

Not reached

+20' +6.1 m

-565' -172.2 m

·736' ·224.33 m

·224.0 m

·2080' ·634 m gneiss?

-400' ·121.9 m gneiss?

?

-S48.6 m

+110' +33.5 m

· 1800'

+4.6 m

Baseme nt Elevation! Lithology

+IS'

Well Head Elevation

+9' +2.7 m

-400' ·121.9m

· 1990' -606.6 m

Total Depth

·2084' -635.2 m

-73S'

Absent

?

37"21' 76°00'

7soS6'

37°17'

19761982

C28 Va. Tech Geothermal Oyster

108

·177

·S4.0m

38° IS'

1946

Colonial Beach

107

76"S9'

·210' -64m

38°11' 76°S6'

1946

Washington 's Birthplace

106

-S85'

·178.3 m

37"OS' 38"

76°22' 43"

1964

S9E-S NASA Research Center. Langley AFB

lOS

Absent

?

W6839

104

36°S6' 77°10'

?

?

Top Top Chickahominy Exmore/ Mattaponi

36°SI ' 76°29'

Year Latit iude/ Drilled Longitude

WSS21

Name/ Location

103

#

Table 1.2. (cont.)

D& C

D

D

D

D

D

D

D

D

Cored or Drilled

Breccia apron

Annular trough

Inner basin

Secondary crater? Inner basin

Secondary crater?

Annular trough

Outside crater

Breccia apron

Re lative Locat ion

IS

IS

IS

15

14

14

3, 13

13

13

Reference

o' ::l

s:;.

0.. l':

S

a

N 0\

19761982

1976· 1982

C60Va. Tech Geothermal Bun ny's Bar

C55 Va. Tech Geothermal Tasley

C59 Va. Tech Geothermal Smith Point Fentress Ccrehole

Dismal Swamp Corehole

Jenkins Bridge Corehole

114

115

116

118

119

117

113

1980s

1980s

1980s

19761982

1976· 1982

19761982

C26 Va. Tech Geothermal Isle of Wight C27 Va Tech Geothermal Langley AFB

112

Absent

37"53' 76°15'

Absent

Absent

36°37' 76°44'

37°57' 75°35'

Absent

Absent

37"43' 75°43'

36°43' 76°09'

-790.4' -240.9 m

-795.6' -242.5 m

37"06' 76°22'

37"03' 76°18'

-941.9' -287.1 m?

Absent

Absent

Absent

Absent

Absen t

-885.5' -269.9 m?

-995.7' -303.5 m

-1112.5' -339.1 m?

Top Top Chickahominy Exmore! Mattaponi

36°56' 76°36'

Year Latitiude! Drilled Longitude

Name! Location

#

Table 1.2. (cont.)

-1314' -400.5 m

-1857' -566.0 m

·2005' -61J.l m

-934.7' -284.9 m

-753.9' -229.8 m

-951.1 ' -289.9 m

-1473.4' -449.1 m

-1473.4' -449.1 m

Tota l Depth

Not reached

Not reached

Not reached

· 1850' -563.9 m arkose

+40' +12.2 m +10' +3.1 m +15' +4.6m +33' +10.1 m

Not reached

Not reached

+10' +3.1 m

+6' +1.8 m

Not reached

·1319.2' -402.1 m granite

Basement Elevation! Lithology

+5' +1.5 m

+75' +22.9 m

Well Head Elevation

C to · 1100' -335.3 m Cto -1080' ·329 .2 m C except -484 to -1194' -146 to -364m

D

D

D

D

D&C

Cored or Drilled

3

3 Outside crater Outside crater

3

15

15

IS

Outside crater

Outside crater

Outside crater

Annular trough

15

15

Breccia apron Annular trough

Reference

Relat ive Locat ion

~

c:

c.

l'j

I:'

g.

n

Absent

Absent

Absent

38"06' 76"25'

38°02' 76"19'

38"16' 77"07'

38"15' 77"01'

pre 1978 pre 1978

Texaco Wilkins et ux.No. I

128

127

126

St.M .- Ff36 Kitts Point, MD St.M.-Gg 14 Point Lookout, MD JSC Drilling Thompson No. I

125

1989

1968

Absent

38"03' 00" 76"19' 30"

pre 1978

Absent

Absent

38"14' 30" 76"29' 15"

pre 1978

St. M-T-27 Point Lookout, MD St. M-P-22 Kitts Point, MD

123

124

Absent

38°30' 76'59'

Absent

38"18' 15" 75"16' 30"

1940s

pre 1978

Absent

38°00' 58" 75°49' 34"

+ 5' +1.5 m +7' +2.1m + 10' +3.1 m

-676' -206.0m

Absent

Absent

Absent

Absent

Absent

-10115' -3083.1 m in granite

?

-721' -219.8 m

-+20' -+6.1 m

-+180' -+54.9 m

Not reached

+ 100' +30.1 m

-1409' -429.5 m

Absent

-933' -284.4 m

-1765' -538.0 m arkose Not reached

+193' +58.8 m

- -1765' --538.0 m

Absent

--1312' - -400 m arkose - -1457' - -444 m arkose

Not reached

Not reached

-7070' -2154.9 m gabbro

+45 ' +13.7 m

-7116' -2169 m

Absent

-4080.7' -1243.8 m metavolcanics

Basement Elevation! Lithology

Absent

Well Head Elevation +4' +1.2 m

Total Depth -5573.5' -1698.8 m

Top Top Chickahominy Exmore/ Mattaponi

19761982

Latitiude/ Year Drilled Longitude

CH-Ce 37 Charles Co, MD

DGT-I Va. Tech Geothermal. near airport Crisfield, MD Socony Vacuum Bethards Berlin, MD

Name/ Location

122

121

120

#

Table 1.2. (cont.)

D

D

D

D

D

D

D

D

D&C

Cored or Drilled

Outside crater

Outside crater

Outside crater Outside crater

Outside crater

Outside crater

Outside crater

Outside crater

Outside crater

Relative Location

17

17

6

6

16

16

16

7

15

Reference

tv

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0-

e

a

:=

00

C-22 Va.Tech Geo thermal Creeds Fie ld

C-23 Va. Tech Geothermal, Oceana Md # I near Marbury. MD

130

131

# 8 Chuckatuck-

134

# 6 City of Chesapeake Bowers Hill • Production

137

136

# 37 Dr iversMonogram Farm # 5 Virginia Division of Forestry

135

Cedarbrook Farm

Md # 2 Prince Georges Co.• MD

133

132

Texaco P.H. Gouldman No . I

Name/ Location

129

#

Table 1.2. (cont.)

?

?

1943

pre 1945

36"47' 02" 76"24' 55"

36"49' 04" 76"32' 50" 36"48' 08" 76"23' IS"

36"51' 16" 76"33' 26"

38"38' 76"42'

38"35' 77"09'

pre 1978

pre 1978

36"48' 09" 76"02' 30"

36"06' 23" 76"00' 26"

38"07' 76"55'

1980s

1980s

1991

Year Latitiude! Drilled Long itude

?

?

-285' -86.9 m

?

Absent

Absent

Absent

Absent

Absent

?

?

-295' -89.5 m

?

Absent

Absent

Absent

Absent

Absen t

Top Top Chickahominy Exmore! Mattaponi

-979' -298.4 m

-520' -158.5 m -633' -192.9 m

+21' +6.4 m

+20' +6.1 m +20' +6.1 m

Not reached

Not reached

Not reached

Not reached

?

- +100' -+30.1 m

- -2439' --743.4 m +15' +4.6m

-560' -170.7 m schist -2439' -743.4 m

- +90' - +27.4 m

-·560' - -170.7 m

-535' -163.1 m

Not reached

Basement Elevation! Lithology - -1630' -- 497 m arkose Not reached

+10' +3.1 m

+5' +1.5 m

- +10' -+3.1 m

Well Head Elevation

-974.1' -296.9 m

-8015' -2443.0 m in sch ist -969.5' -295.5 m

Total Depth

D

D

D

D

D

D

D

D

D

Cored or Drilled

Breccia apron

Breccia apro n

Breccia apro n

Breccia apron

Outside crate r

Outside crater

Outside crate r

Outside crater

Outside crater

Relative Location

3

3

12

12

16

16

15

15

17

Reference

I

IV '-0

::l

g.

~

Co

eo

? ?

36°48' 53" 76°17' 09"

36°51' 15" W19'17"

?

?

# 11 Lone Star Cement Corp.

# 12 City of

139

140

?

36°59' 39"

?

?

# 19 Rescue Water Co .

# 20 Town of Smithfield Red Point Heights

# 21 Nimmo Well Chuckatuck

142

143

144

145

?

? ?

-247' -75.3 m ? ?

36°59' 12" 76°36' 50"

36°52' 18" 76°31' 30"

36°57' 27" 76°31' 39"

36°59' 32" 76°29' 44"

36°58' 02" 76°34' 48"

36°52' 55" 76°23' II"

?

?

1908

1915

?

# 22 Tidewater Virginia PropertiesGraymore Estate

# 54 Battery Park Water Co.

# 108 Carrolton

# 25 Tidewater Water Co .

146

147

148

W33' 30"

?

36°59' 05" 76°37' 21"

1924

# 81 Smithfield Ice Plant

141

Portsmoun

Absent

36°47' 10" 76°26' 52"

Not reached Not reached Not reached

Not reached Not reached

Not reached Not reached Not reached

+10' +3.1 m +22' +6.7 m +35' +10.7 m +35' +10.7 m +15' +4.6 m

+13' +4.0m +8' +2.4 m +15' +4.6m

-311' -94.8 m -528' ·160 .9 m -477' -145.4 m -543' · 165.5 m

?

?

·382' -116.4 m ·573' -174.7 m

-333' -1Ol.5 m

Not reached

+10' +3.1 m

-1144' -348.7 m

-272' -82.9 m

Not reached

+5' +1.5 m

-790' -240.8 m

·541' -164.9m

Not reached

Basement Elevation! Lithology

+17' +5.2m

Well Head Elevation

-983' -299.6 m

Total Depth

?

?

?

?

?

?

?

Absent

Top Top Chickahominy Exmore/ Mattaponi

1980s

Latitiude/ Year Drilled Longitude

# IOMW4 Corehole

Name/ Location

138

#

Table 1.2. (cont.)

D

D

D

D

D

D

D

D

D

D

C

Cored or Drilled

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Breccia apron

Relative Location

3

12

12

3

3

3

12

3

3

Reference

w

==

o'

....n

c

0-

-aa

0

1908

1942

1896

# 7 Bu rwells Bay

# 25 Lone Star Cement Co. near Mogarts Beach

# 42 Bacon s Castle Test Well

# 24 North End

153

154

ISS

156

37"13' 21" 76°57' 06"

1917

1917

?

# 2 Town of C laremont

# 4 Claremont O .E. Belding

# 50 First Colony

157

158

159

37°14' 34" 76°48 ' IS"

37°14' 20" 76°58' 32"

76"17' 25"

37"06' 30"

37"06' 10" 76"44' 13"

37"00' 29" 76°36' 24"

37"03' 23 " 76"40' 13"

37"04' 34" 76"40 ' OS"

37"02' 36" 76"42' 59 "

36°52 ' 26" 76°18' 56"

36°58 ' 40 " 76°25' 50"

Point

1917

1939

?

# 3a Rushmere

Department of Environme ntal Quality

# 36 Va.

pre 1945

?

Latitiude/ Year Drilled Longitude

152

lSI

# 9 Lamberts

ISO

Point-No rfolk & Western RR

# 26 Newport News- City Hall Complex

Name/ Location

149

#

Table 1.2. (cont.)

?

Absent

Absent

-737' -224.6 m

Absent

?

?

?

Absent

·387' -118.0 m

4 10' -125 m

?

Absent

Absent

-9 17' -279.5 m

Absent

?

?

?

Absent

-397' · 121.1 m

·574' -175.0m

Top Top Chickahominy Exmore/ Mattaponi

-270 ' -82 .3 m -464 ' -141.4 m

-313' -95.4 m

-1169' -356 .3 m

·985' -300.2 m

-324' -98.8 m

-306 ' -93.3 m

-381' -116.1 m

-6 15' -187.5 m

-606' -184.7 m

-870' -265 .2 m

Total Depth

+17' +5.2 m +30' +9.1 m

+90' +27.4 m

+3' +0 .9 m

+70' +21.3 m

+12' +3 .7 m

+15 ' +4.6 m

+5' +1.5 m

+85' +25 .9 m

+10' +3.1 m

+30' +9.1 m

Well Head E levation

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Basement Elevation! Lithology

D

D

D

D

D

D

D

D

D

D

D

Cored or Drilled

Washback channel?

Outside crater

Outside crater

Annular trough

Out side crater

Breccia apron

Breccia apron

Breccia apron

Outside crater

Breccia apron

Breccia apron

Relative Location

3

12

3

14

12

12

12

12

3

12

3

Reference

I

w

::l

g.

()

s::

0-

0

::::t

S

170

169

168

167

166

165

164

# 61 Virgi nia Electric Power Company

# 58 Busch Gardens # 59 Busch Gardens # 60 Hog Island

? ?

37"13' 43" 76"40' 08"

37"14' 06" 76"38' 43" 37"11' 33" 76'40' 53" 37'09' 51" 76"41' 57"

?

?

?

?

?

?

?

?

?

37'09' 50" 76"41' 52"

?

?

?

?

37' 14' 21" 76"38' 28"

?

?

?

?

37"13' 05" 76"46' 37"

?

·180' ·54.9 m ·218' -66.5 m Not reached

?

?

·207' -63.1 m

37' 17' 13" 76°43' 22"

# 51 Williamsburg Carolyn Tourist Court # 55 Jamestown Corehole # 56 York Public Utilities # 57 Hog Island Nuclear Plant

163

37"13' 57" 76'47' 32"

1940

1906

·135' 41.2 m · 130' ·39.6 m

37' 13' 41" 76'47' 28"

1946

# 27a Jamestown

161

162

?

37'08' 32" 76°50' 27"

Top Top Chickahominy Exmore! Mat tapon i

?

Year Latitiude! Drilled Longitude

# 5I Surry Court House #2 # 26 Jamestown 4-H Club

Name! Location

160

#

Table 1.2. (cont.)

·38 6' -117.7 m 457' · 139.3 m ·435' ·132.6 m · 1235' ·376.4 m ·385' · 117.3 m

·272' ·82.9 m ·586' · 178.6 m

·265 ' ·80.7 m ·287' ·87.4 m ·258' ·78 .7 m

·375' ·114.3 m

Total Depth

+53' +16.2 m +85' +25 9 m +5' +1.5 m +35' +10.7 m

D D D

Not reached Not reached

D

D

D

C

Not reached

Not reached

Not reached

Not reached

+80' +24.4 m +34' +10.4 m

Not reached

+1' +0.3 m

D

Not reached

D D

Not reached

+10' +3.1 m +33' +10.1 m +90' +27.4 m

D

Cored or Drill ed

Not reached

Not reached

Basement Elevation! Lithology

+103' +31.4 m

Well Head Elevation

Washback chan nel? Washback channel? Washback channel? Washback channel? Washb ack channel?

Washb ack channel? Washback channel?

Wash back channel? Washb ack ch annel?

Washback channel?

Washback channel?

Relative Location

3

3

3

3

3

3

3

14

14

14

3

Reference

S

o' ::I

~

~

0-

0

::t

w tv

180

179

178

177

176

175

174

# 67 City of

173

1931

?

?

?

?

?

?

?

?

172

Newport News Golf Course # 68 Lee Ha ll Treatment Plan t # 69 Upper Potomac Monitor Well # 70 Upper Potomac Productirn Well # 71 Middle Potomac Product irn Well # 72 Upper Potomac Monitor Well # 73 Va.. Penins ula Eco n. Development Co uncil # 39 Yorktown Colonial National Monument

?

Badische # 64 Grove

37° 13' 36" 76"30' 33"

37"11' 49" 76"35' 34"

37°10' 41" 76"35' IT'

37"11' 12" 76"34' 13"

37" 10' 41" 76"35' IT'

37"10' 01" 76"33' 16" 37"11' 29" 76"30' 38"

37"II ' 20" 76°36' 54" 37"12' 50" 76"36' 52" 37" II ' 14" 76"31' 21"

Year Latitiude/ Drilled Long itude

# 62 Dow

Name / Location

171

#

Tab le 1.2. (con t.)

-375' -114.3 m

-293' -89.3 m

?

-30 8' -93.9 m

-312' -95.1 m

-30 3' -92.4 m -408' -124.4 m

-291' -86.7 m -279' -85.0 m -404' -123.1 m

-524' -159.7 m

-332 ' -101.2 m

?

-369' -112.5 m

-381' -116.1 m

-395' -\20.4 m -588' -179.2 m

-344' -104.8 m -320' -97.5 m 460' -140.2 m

Top Top Chickahominy Exmore! Mattaponi

-72 2' -220.1 m

-530' -161.5 m

-560 ' -170.7 m

-1113' -339.2 m

-11 20' -341.4 m

-1315' 400.8 m -1244' -379.2 m

-148.4 m

481'

-1540' 469.4 m 445' -135.7 m

Total Depth

+50' +15.2 m

+50 ' + 15.2 m

+40' +12.2 m

+35' +10.7 m

+30' +9.1 m

+35' +10.7 m +56' + 17.1 m

+20' +6.1 m +40' +12.2 m +20' +6.1 m

Well Head Elevation

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Basement Elevation! Litho logy

D

D

D

D

D

D

D

D

D

D

Cored or Drilled

An nular trough

Washback channel?

Washback channel?

Washback channel?

Washback channel?

Washback channel? Washback channel?

Washback channel? Washback channel? Washback chan nel?

Relative Location

14

3

3

3

3

3

3

3

3

3

Reference

I

v.> v.>

o' ::l

$4.

~

0-

5"

a

-427' · 130.1 m -410' -125.0 m

37"12'51" 76°27' 08" 37° 1\' 58" 76°28' 13" 37" 18' 45" 76"56' 13"

?

?

?

?

?

37" 16' 25" 76°46' 20"

37" 16' 04" 76"52' 24"

37°21' 45" 76°49' 32"

37°21' 48" 76°46' 10"

37°18' 37" 76°47' 41"

?

?

?

?

?

# 96 James City Water Service Authority

190

?

?

?

Not reached?

37"13' 04" 76°29' 19"

?

?

?

?

?

?

Not reached

Not reached

Not reached

Not reached

+84' +25.6 m +100' +30.1 m +109' +33.2 m

-736' -224.3 m -742' -226.2 m

·726' -221.3 m

-414' -126.2 m

-220' -67.1 m +32' +97.5 m

Not reached

Not reached

D

D

D

D

D

D

D

D

D

Not reached Not reac hed

D

Cored or Drilled

Not reached

Basement Elevation! Lithology

Not reached

+51' +15.5 m +35' +10.7 m +90' +27.4 m

+50' +15.2 m +10' +3.1 m

Well Head Elevation

+90' +27.4 m

-307' -93.6 m

-303' -92.4 m

Not reac hed? ?

-431' -13\.4 m

Not reached?

Total Depth -396' -120.7 m -440' -134.1 m

Not reached?

Top Top Chickahominy Exmore! Mattaponi

37"16' 10" 76°45' 43"

?

# 95 James City Water Service Authority

# 92 James City Water Service Authority # 93 Powhatan Village Corp., E. of Chick. R. # 94 Powhatan Village Corporation

# 90 Charles City County # 91 James City Water Service Authority

189

188

187

186

185

184

1943

?

# 84 U.S. Navy Tank Farm # 41 York County

182

183

?

Year Latitiude/ Drilled Longitude

# 83 U.S. Naval Supply Center

Name/ Location

181

#

Table 1.2, (cont.)

Washback channel?

Washback channel?

Washback channel?

Washback channel?

Wash back channel?

Washback cha nnel?

Washback channel?

Annular trough

Annular tro ugh

Annular trou gh

Relative Location

3

3

3

3

3

3

3

14

3

3

Reference

w

:3

o'

~

=

0..

0

S' q

.j::.

201

200

199

198

197

196

19S

194

193

192

191

#

?

# 103

Fue l Depot U.S. Navy # 114 City of Newport News

# 26 Pennimen

Williams burg Lodge # 104 Williamsburg Motor House # 20 Camp Peary

?

?

1918

1942

?

?

?

?

?

?

37'24' 28" 76'S6' IS"

37' 19' 2S" 76'39' 13" 37"16' S8" 76"36' 33"

37' IS' S6" 76'41' SI"

?

37' IT 49" 76'44' 18" 37' IS' 12" 76'39' 24" 37' IS' 38" 76'40' 06" 36"19' 34" 76"44' 14" 37"16' OS" 76'42' 03"

?

-264' -80.Sm ?

?

?

?

?

?

?

?

-30S' -93.0m -410' -12S.0m

?

?

?

?

?

?

?

Top Top Chickahominy Exmore! Mattaponi ? ?

37'21' 48" 76'46' 10"

76'46' IT'

37"22' 01"

Year Latitiude! Drilled Longitude

Water Service Authority # 98 James City Water Service Authority # 99 Eastern State Hospital # 100 Carven Gard ens # 101 James River Estat es # 102 Ewell

# 97 James City

Name! Location

Table 1.2. (cont.)

·768' -234.1 m

-393 -119.8 m -SIS' -IS7.0 m

-44S' -13S.6 m

-SOl ' -IS2.7 m -SOl' -IS2.7 m -422' · 128.6 m -330' -100.6 m ·430' · 131.1 m

·200' -61.0 m

-188' -S7.3 m

Total Depth

D D

Not reached Not reached

D

Not reache d

Not reached

D

Not reached

+41' +124.S m +20' +6.1 m +10' +3.1 m

D

Not reached

D

D

D

D

D

D

Cored or Drilled

Not reached

Not reac hed

Not reached

Not reached

Basement Elevation! Lithology Not reached

+SS' +16.8m

+90' +27.4 m +90' +27.4 m +80' +24.4 m +100' +30.1 m +70' +21.3 m

+100' +30.1 m

+112.S' +34.3 m

Well Head Elevation

Washback channel?

Washback channel? Washback channel?

Washback channel?

3

Washback channel? Washback channel? Washback channel? Washback chan nel? Washback channel?

3

14

14

3

3

3

3

3

3

Reference

Wash back channel?

Washback channel?

Relative Location

I

v.

<.;.l

o' ::s

s:;

~

0I::

211

210

209

208

207

206

205

# 122 West PointChesapeake Corporation # 123 West PointChesapeake Corporation # 124Chesapeake Corporation # 125 Barnhardt Farms

?

?

?

?

?

?

37"30'08" 76'42' 56" 37'36' 30" 76'31' 26" ?

Absent

?

?

?

?

?

?

Absent

?

?

?

?

?

?

?

?

?

?

Top Top Chickahominy Exmore/ Mattaponi

37'31' 26" 76'45' 41"

37'23' 10" 76'41' 14" 37'26' 21 " 76'04' 42" 37"23' 31" 76°31' 26" 37'32 ' 46" 76'48 ' 30"

?

?

37"23' 12" 76°48'06"

37'23' 59" 76"54'04" 37"24' 51" 76"51' 33"

?

?

# 116 James City County Research Station # 117 James City Water Service Authority # 118 Yorkview Plantation # 119 West End Station # 120Gloucester

203

204

?

# liS Southern Properties

Name/ Location

Year Latitiude/ Drilled Longitude

202

#

Table 1.2. (cont.)

· 100' ·30.1 m ·702' -214.0m

·1255' ·382.5 m

-503' ·153.3 m ·914' -278.6 m · 1715' ·541.0 m ·1252' ·381.6 m

·701' ·231.7 m

· 183' -55.8 m ·905' ·275.8 m

Total Depth

Not reached

+50' +15.2 m +10' +3.1 m +75' +22.9 m +27' +8.2 m

Not reached

+51 ' +15.5 m +40' +12.2m

Not reached

Not reached

+15' +4.6 m

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Basement Elevation! Lithology

+106' +32.3 m

+95' +29.0 m +90' +27.4 m

Well Head Elevation

D

D

D

D

D

D

D

D

D

D

Cored or Drilled

Washback channel? Breccia apron

Outside crater

Washback channel?

appron

Washback channel? Washback channel? Breccia

Washback channel?

Washback channel? Washback channel?

Relative Location

3

3

3

3

3

3

3

3

3

3

Reference

w

::s

o'

~

e

0-

::s

1-g

'"

?

# 129 West

213

# 9 King William

219

220

Count y Aylett Mill

# 40 Bacons Castle Estate # 8 King William County Aylett

218

# 137 Waller Mill

217

Park

# 134 USGS C larks Mill Pond Corehole

216

pre 1957

pre 1957

1917

?

?

?

# 131 USGS

215

Essex Mill Pond Corehole

?

# 130 Town of Kilmarnock Well # 3

214

Irvington Well # 2

?

-40' -12.2 m -125' -38.1 m

- 37'46 ' - 77'06'

?

?

Absent

Absent

Absent

Absent

Absent

-193' -58.8 m

·296' -90.2 m -136' -41.5 m

?

Absent

Absent

Absent

Absent

Absent

Top Top Chickahominy Exmore/ Mattaponi

37'06' 33" 76'43' 22" -37'37' -77'06'

37' 18' 59" 76'42' 04"

37"55' 52" 76'28' 05"

37'52' 30" 76' 51' 04"

37'42' 12" 76'23' 09"

37"39' 41" 76'25' 48"

37"33' 52" 76"37' 28"

Year Latit iude/ Drilled Longi tude

# 126 Rappahannock Community College

Name/ Location

212

#

Table 1.2. (cont.)

-278' -84.7 m

·350' -106.7 m

-348' -106.1 m

-435' -132.6 m

·299' -91.1 m

-214' -65.2 m

·707' -215.5 m

-655' - 199.6 m

-590' -197.8 m

Total Depth

+22 +6.7m

+35' +10.7 m

+70' +21.3 m +19' +5.8 m

Not reac he d

Not reached

Not reac hed

No t reac hed

Not reached

Not reached

+11' +3.4 m +46' + 14.0m

Not reac hed

Not reached

Not reached

Basement Elevation! Lithology

+65' +19.8 m

+15' +4.6 m

+110' +33.5 m

Well Head Elevation

D

D

D

D

D

D

D

D

D

Cored or Drilled

Washback channel?

Washback channel?

Washback channel? Washback channel?

Outside crater

Outside crater

Outside crater

Outside crater

Outside crater

Re lative Location

14

14

12

3

3

3

3

3

3

Reference

I

-.J

w

::s

0

a.

t=

0-

0

S q

229

228

227

226

225

224

223

222

221

#

William County Court Hou se # 34 King William County, Cohoke # 48 King William County, Grimes Landing # 49 King William County, Rumford # 59 King William County, Manquin # 66 King William County, Aylett # I King William County Walkerton # 12 New Kent County Cumberland Landing # 40 New Kent County Providence Forge

# 14 King

Name/ Location

Ta ble 1.2. (eont.)

-50' -15.2 m -114.5' -34.9 m -105' -32.0 m -72' -22.0 m +10' +3.1 m ?

-110' -33.5 m -60' -18.3 m

·42' · 12.8 m

-37'41' -77'O\'

- 37'35' - 76°57'

- 37'39' - 1''' 06'

- 37'44' - 77'05'

-37'43' - 77'09'

- 37'47' - 77'06'

- 37'44' - 77'01'

- 37'32' - 76°58'

- 37'27' - 77'03'

1946

1950

1950

pre 1957

1951

pre 1957

1946

1943

-172' -52.4 m

-160' -48.8 m

-182' -55.5 m

-140' -42.7 m

-130' -39.6 m

-127' -38.7 m

-124' -37.8 m

-212' -64.6 m

-161' -49.1 m

Top Top Chickahominy Exmore/ Mattaponi

pre 1957

Latitiude/ Year Drilled Longitude

Not reached

Not reached

+28' +8.5 m -253' -77.1 m

Not reached

+10' +3.1 m +10' +3.1 m

Not reached

Not reached

Not reached

Not reached

Not reached

Not reached

Basement Elevation! Litho logy

+20' +6.1 m

+ 130' +39.6 m

+128' +39.0 m

+40' +12.2 m

+6' +1.8 m

+ 140' +42.7 m

Well Head Elevation

·290 ' -88.4 m

-365' -11 I.3 m

-350' -106.7 m

-378' -115.2 m

-199' -60.6 m

-190' -57.9 m

-569' -173.4 m

-289' -88.1 m

Total Depth

0

D

0

0

D

0

D

D

D

Cor ed or Drilled

Washback channel?

Washback channel?

Washback channel?

Washback channel?

Washback channel?

Washback channel?

Wash back channel?

Washback channel?

Washback channel?

Relative Location

14

14

14

14

14

14

14

14

14

Referen ce

0-

::s

o'

~

~

e

00

w

# 34 Charles City County Charles City School # 75 Charles City County C harles City

USGS-NASA Langley Corehole USGS North Coreho le USG S Bayside Corehole

230

232

200 1

2001

2000

1933

1948

-593.4' -180.87 m

37°05'44" 76°23'09"

37"19'34" 76°11' 33"

-495.3' -150.97 m -698' -212.75 m

-20' -6.1 m

-37'21' - 77'04'

37°26'41" 76"24'0 2"

-20' ·6. 1 m

I Powars et al. ( 1992) 2 Poag (l997a) 3 Powars and Bruce (1999) 4 Gibson and Bybell ( 1994)

5 Reinhardt et al. (1980) 6 Hansen and Wilson ( 1984) 7 Brown et al. (1972) 8 Robbins et al. (197 5)

-1412.5 -430.53 m -2389' ·728 .17 m

-2075.9' -632.73 m

-175' -53.3 m

-175' -53.3 m

Tota l Depth

·2323.7' -708.26 m gra nite

-2046.8' -623.87 m granite Not reac hed

Not reached

Not reach ed

Basement Elevation! Lithology

C

C

C

D

D

Cored or Drilled

13 Mixon et al. ( 1989) 14 Cederstrom (1957) IS Virginia Tech Geothermal Web Site

+15' +4.57 m +2' +0.61 m

+7.89' +2.41 m

+60' +18.3 m

+45' +13.7 m

Well Head Elevation

9 Hansen and Wilson (1990) 10 Cederstrom ( 1945a ) II Sinnott and Tibbits (1968) 12 Cederstrom (l945b)

-724.3' -220.77 m -913' ·278.28 m

-736.4' ·224.46 m

-90' -27.4 m

-ISO' -45.7 m

Top Top Chickahominy Exmore/ Mattaponi

- 37'2 1' -77'04'

Year Lati tiude / Dri lled Longitude

All bore holes are in Virginia, unless designated MD (Maryland) In depth columns 5·9, a number such as 20', indicates 20 feet Elevations are measured above (+) or below (-) mean sea level

234

233

231

Name! Location

#

Table 1.2. (cont.)

19

19

18

14

14

Reference

16 Hansen (1978) 17 Milici et al. (1995) 18 Powars et al. (2 00 1) 19 Gohn (in press)

Annular tro ugh Annular trough

Annular tro ugh

Washback channel?

Wash back channel?

Relative Location

I

'"

w

o' ::s

~

e

5'

a c..

2 Geological Framework of Impact Site

2.1 Crystalline Basement Rocks 2.1.1 Regional Tectonostratigraphy Crystalline basement rocks beneath the Virginia Coastal Plain include a variety of plutonic, volcanic, and metamorphic rocks that constitute distal parts of the Appalachian orogen (Thomas et al. 1989; Fig. 2.1). The inner edge of the coastal plain sedimentary wedge laps westward onto greenschist-facies rocks of the Eastern Slate Belt, which, in outcrop, form the Virginia segment of the Appalachian Piedmont Province. Composition of slate-belt rocks beneath the inner Virginia Coastal Plain ranges chiefly from muscovite-biotite-quartz-albite schist to fine-grained phyllitic quartzite and phyllitic biotite-muscovite-quartz rock. These metamorphic bodies are intruded by plutons of fine-grained granodiorite , diorite, gabbro, tonalite, and monzogranite (Gleason 1982). Fine-grained felsic volcanic rocks, along with thin-bedded, calcareous, distal turbidites and massive-to-Iaminated, very fine-grained tuff, are part of the subsurface slate-belt complex in easternmost Virginia (Robbins et al. 1975; Gleason 1979). Rocks of the Goochland terrane (Fig. 2.1), an internal basement massif of Proterozoic age, project under the Virginia Coastal Plain, and have been generally called granite by drillers. Most of the Goochland rocks, however, are metamorphic bodies, which include the State Farm Gneiss (granodioritic to tonalitic orthogneiss; Glover et al. 1978), the Sabot Amphibolite, the Maidens Gneiss (garnet-biotite-quartz-plagioclase), and the Montpelier Metanorthosite (Clement and Bice 1982; Farrar 1984). Principal granitic plutons drilled beneath the Virginia Coastal Plain are the Petersburg (biotite-quartz-microcline-plagioclase granite) and Portsmouth (postmetamorphic quartz-microcline-plagioclase-biotite granite) Granites. Petersburg Granite has a zircon U-Pb age of ~3 3 0 Ma (Wright et al. 1975), and Portsmouth Granite has a whole-rock Rb-Sr isochron age of ~263 Ma (Russell and Russell 1980). Both of these intrusive units are marked by conspicuous negative gravity anomalies, and several other similar negative anomalies are interpreted to represent additional granitic plutons (Thomas et al. 1989). Horton et al. (1991) analyzed the tectonostratigraphic terranes accreted during the Paleozoic, which form the east-central sector of the Appalachian orogen (Fig. 2.1). The chief terrane in southeastern Virginia is the Chesapeake Block, a broad region represented by only sparse borehole data from beneath coastal plain sedimentary beds . Drilling in the Chesapeake Block has yielded mainly metamorphic C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

42

Geological Framework of Impact Site

CJ Late Paleozoic

CJ Other Plutons

After Horton et al. (1991)

Granitoids

Fig. 2.1. Tectonostratigraphic terranes recognized within crystalline basement rocks underlying the Coastal Plain and Piedmont Provinces of southeastern Virginia (modified from Horton et al. 1991). Note that Chesapeake Bay impact crater occupies broad Chesapeake Block and western sector of annular trough is underlain by two prominent granitoid intrusions. Culpeper, Taylorsville, Richmond, Petersburg-Studley, Delmarva, and Norfolk basins are sediment-filled Triassic-Jurassic rift grabens (or half grabens) formed by extensional stress in Proterozoic-Paleozoic igneous and metasedimentary basement rocks.

rocks of greenschist facies (argillite, chloritic schist, phyllitic metavolcanic rock, and serpentinized gabbro). It is the Chesapeake Block into which the Chesapeake Bay impact crater was excavated. The Chesapeake Block has been intruded by several post-accretion granitoid plutons, two of which underlie the western sector

Crystalline Basement Rocks

43

of the crater (Fig. 2.1), and have been sampled by drilling (boreholes 81, 105, 232, 234; Figs. 1.3, CD-ROM.1; Table 2.1). The assemblage of Appalachian crystalline terranes was disrupted during the Late Triassic and Early Jurassic by continental rifting, which preceded opening of the Atlantic Ocean basin (Manspeizer 1988). Four half-graben rift basins, filled with coarse siliciclastic sequences assigned to the Newark Supergroup, can be identified on seismic reflection profiles used in this study: Taylorsville basin; Queen Anne basin; Norfolk basin; and an unnamed (also not yet mapped) rift basin seen on line 11 A of Milici et al. (1995), on the Northern Neck Peninsula (see Chapter 3). Table 2.1. Nineteen most significant boreholes in southeastern Virginia and southern Maryland that penetrated crystalline basement rocks. See Fig. 2.2,Table 1.2, CD-ROM. I . No. Name 7 St.M. -Df 84 8 Ohio Oil-Larry G. Hammond 9 1&1 Enterprises-E.G. Taylor 24 VA-NOR-T-1 2 26 VA-IW-P-8 28 VA-SO-P-3 81 #46 Mathews, ElkinsOil & Gas 102 #81 Ft. Monroe 103 W552 1 104 W6839 105 59-E-5 III C-25 112 C-26 120 DGT-I , Crisfield Airport 121 Socony Vacuum-Bethards 127 Md. No. I near Marbury 128 Md. No. 2 Prince Georges Co. 232 USGS-NASALangley 234 USGS Bayside

Elevation -766.5 m -1655 m -1 872.7 m -782.4 m -252.4 m -103.6 m -706.5 m -684 m -548.6 m -1 21.9 m -634 m -550.3 m -402. 1m -1243.8 m -2154.9 m -170.7 m -743.4m -634 m -728.1 7 m

Lithology Diabase Gneiss Gneiss Granite? ? ?

Granite? Granite? ? ?

Gneiss? Metavolcanics Granite Metavolcanics Gabbro Schist ?

Granite Granite

Inside Crater No No No No No No Yes Yes No No Yes No No No No No No Yes Yes

2.1.2 Crystalline Basement Rocks in Boreholes

A total of 25 boreholes have sampled the crystalline basement rocks in or near the study area, five of which are located inside the crater rim (Fig. 2.2; Table 2.1). Granitoid rocks are present in all five of these basement sections. Lithologic descriptions of crystalline basement from boreholes 81, 102, and 105 came from very old driller's logs, however, and their accuracy is questionable. The only reliable samples of in situ basement rocks derived from inside the crater come from the NASA Langley corehole (borehole 232; Fig. 2.2; Table 1.2) and the Bayside corehole (borehole 234; Fig. 2.2; Table 2.1). At the NASA Langley site, basement rocks from the outer edge of the crater' s annular trough are composed of Proterozoic metagranite (-0.6 Ga; Horton et al. 2001). Crystalline basement from near

44

GeologicalFramework of Impact Site

127

•

.128 POlornacR

./VT6 120 9·

.104

• 103

.111

.24

.26

o,

6,0 km

.28

7TOO'

76·00'

Fig. 2.2. Geographic distribution of 17 boreholes in which crystalline basement has been penetrated in southeastern Virginia and southern Maryland, and location of onshore seismic reflection profiles NAB-II and VT-6. Boreholes 81, 105, 232, and 234 were drilled on or close to the granitoid intrusions shown in Fig. 2.1. See Table 2.1 for more information about these boreholes.

Crystalline Basement Rocks

45

the inner edge of the annular trough at Bayside is of similar granitic composition and age as that in the NASA Langley core (Horton et al. 2002).

2.1.3 Regional Configuration of Crystalline Basement Surface

The oldest published structure map of the crystalline basement's upper surface (Brown et al. 1972; Fig. 2.3A) was contoured on the basis of only ten wells widely scattered through the study area (plus several others outside the study area). No seismic reflection profiles were available in 1972. The most prominent feature of the 1972 map is a west-trending embayment north of the Potomac River, defined by the 1000-7000-ft contours. The embayment is bounded to the south by a southeast-trending structural nose, for which there is little control. Farther south, the basement surface is essentially monoc1inal. A minor structural nose defined by the 2000-2500-ft contours south of Norfolk (based on sparse well data outside the study area) was designated the Ft. Monroe high by Richards and Straley (1953). Gibson (1967) called this structure the Norfolk arch. In the northeast corner of the study area, the basement gradient steepens into the deepest part of the Salisbury embayment, but little was known at the time about the offshore geology. By 1978, II more basement wells had been drilled in the study area (total of 21; Fig. 2.3B), and several additional ones were drilled outside the area. The additional wells, supplemented by eight composite seismic reflection profiles and one seismic refraction station, allowed Hansen (1978) to revise the structure map (Fig. 2.3B). The revision steepened the gradient of the basement slightly. For example, the northern segment of the 3000-ft contour was shifted westward, so that it crosses the western bayshore north of the Potomac River, but the major features are virtually unchanged from Brown et al. ' s (1972) version. The northern embayment is nearly identical, but the adjacent nose is less prominent , and its axis is shifted southward by 10-15 km. The "Norfolk arch" is hardly detectable , indicated only by a small eastward deflection of the 2000-ft contour. The steepened gradient in the northeast corner of the study area is indicated by much straighter contours than on the Brown et al. (1972) map, though based on the same well data. Twenty years after Brown et al.'s (1972) map was published , Powars et al. (1992) used a fourfold increase in well control (total of 43 wells in the study area and several more outside it) to upgrade the basement map (Fig. 2.3C). Powars et al. flattened the gradient in the north, as seen in the eastward shift in the 3000-ft contour. The axis of the northern embayment is moved somewhat farther north, and the adjacent nose is shifted another ~5 km to the south from Hansen's (1978) placement. The steep-gradient contours in the northeast corner have been almost completely straightened and are extended southwestward onto the continental shelf. Powars et al. (1992) used the increased well control south of Norfolk to accentuate the "Norfolk arch" into a distinct east-trending structural nose defined by the 2000-3000-ft contours. Accentuation of the "arch " produced a small, adjacent, parallel embayment to the north.

71

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2.2 Coastal Plain Sedimentary Rocks 2.2.1 General Stratigraphic Framework Sedimentary deposits of the Atlantic Coastal Plain constitute a seaward-thickening wedge of dominantly unconsolidated to poorly consolidated siliciclastic sands, silts, and clays of both marine and nonmarine origin. The deposits range in age from Early Cretaceous to Holocene, and generally comprise the updip lithofacies of an - 18-km-thick column of sediments that fills the southern end of an elongate offshore basin known as the Baltimore Canyon trough (Maher 1965; Poag 1985; Grow et al. 1988; Poag and Ward 1993). In Virginia , a hierarchy of 27 formal formations has been developed for these deposits over a period of more than 100

48

Geological Framework of Impact Site

years (Darton 1891; Clark 1896; Cederstrom I945a,b,c, 1957; Richards 1945, 1967; Cushman and Cederstrom 1949; Bennett and Collins 1952; Orton 1955; Murray 1961; Brown et al. 1972; Oaks and Coch 1973; Tiefke 1973; Ward and Blackwelder 1980; Gibson 1983; Ward and Krafft 1984; Mixon 1985; Owens and Gohn 1985; Ward and Strickland 1985; Meng and Harsh 1988; Powars et al. 1992; Poag and Ward 1993; Powers and Bruce 1999; Powars 2000; Figs. 2.4-2.7). Modem emphasis on unconformity-bounded depositional units (Vail et al. 1977; North American Commission on Stratigraphic Nomenclature 1983) has resulted in additional formalization of alloformations, which can be traced across the entire US Atlantic margin north of Cape Hatteras (onshore and offshore), including the outcrops and subsurface beds of southeastern Virginia (Poag and Ward 1993; Poag and Commeau 1995; Figs. 2.4, 2.5). Almost no sequence-stratigraphic analyses exist, however, for the Virginia Coastal Plain. The single published study identified 15 depositional units that correlate with third-order sequences of the Exxon model (Poag and Commeau 1995; Fig. 2.6). Virginia systems tracts are dominated by transgressive and highstand types, with little or no record of lowstand deposits.

2.2.2 Preimpact Deposits

Preimpact sediments of the Virginia Coastal Plain constitute an ~ 1 to 1.5-km-thick interval of Lower Cretaceous to lowest upper Eocene deposits. Seven preimpact coastal plain formations are recognized on the composite basis of lithology and biozonation (Fig. 2.4). 2.2.2.1 Potomac Formation

Oldest outcropping sedimentary strata in the Virginia Coastal Plain are mainly nonmarine, quartz sand- and silt-dominated lithofacies of the Potomac Formation (Ward and Kraft 1984; Mixon et al. 1989; Powars and Bruce 1999; Powars 2000; Fig. 2.4). Potomac beds rest unconformably on metasedimentary and metaigneous rocks of the crystalline basement, and can be examined in outcrop along their updip contact with the Appalachian Piedmont. The Potomac Formation extends into the subsurface, and underlies the entire study area, except where excavated by the Chesapeake Bay bolide impact. Potomac strata constitute the oldest sediments above crystalline basement rocks, except for a possibly Triassic or Jurassic subsurface unit (Crisfield unit) inferred by some investigators (Hansen 1978; Dysart et al. 1983; Hansen and Wilson 1984) from seismic profiles. The Potomac Formation is by far the thickest sedimentary unit recognized on the Virginia Coastal Plain, and reaches as much as 1.3 km in the J&J Enterprises, Taylor # 1 well on the Delmarva Peninsula (borehole 9; Figs. 1.3, CD-ROM.1; Table 1.2). From a thickness of ~600 m at the western margin of the crater, the Potomac thickens to > I km at the eastern (downdip) margin (Grow et al. 1988). No allostratigraphic units have yet been proposed for US East Coast Lower Cretaceous units.

Coastal Plain Sedimentary Rocks

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QUATERNARY TO OLIGOCENE

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Fig. 2.4. General stratigraphic succession of crystalline and mainly preimpact sedimentary formations in southeastern Virginia, showing their relation to each other and to impact deposits. Undulating horizontal lines indicate unconformities.

50

GeologicalFramework of Impact Site

2.2.2.2 Unnamed Upper Cretaceous Beds

Upper Cretaceous beds have not been found cropping out in the Virginia Coastal Plain. A few downdip wells have encountered Late Cretaceou s (Cenomanian and Campanian) megafossils and microfossils, however, in nonmarine (red beds), deltaic (micaceous, lignitic, glauconit ic, quartz sand), and marine beds (shelly, glauconitic silt, clay, and quartz sand) ranging from 40 to 110m in thickness (Clark and Miller 1912; Powars et al. 1992; Powars and Bruce 1999; Powars 2000) . Poag and Ward (1993) included some beds of this interval in the Sixtwelve Alloformation (Fig. 2.4). 2.2.2.3 Brightseat Formation

Oldest Cenozoic deposits of the Virginia Coastal Plain belong to the lower Paleocene Brightseat Formation (Bennett and Collins 1952), a dominantly subsurface unit consisting of mainly clayey, sparsely glauconitic , quartz sand (Fig. 2.4). In outcrop, Brightseat beds are confined to the northeastern part of the state, and are known only along the Potomac and Rappahannock Rivers (Ward 1984), but equivalent strata have been reported in the subsurface from the Oak Grove corehole (borehole 6; Figs. 1.3, CD-ROM. 1; Table 1.2; Reinhardt et al. 1980) and the Dismal Swamp corehole (borehole 118, Figs. 1.3, CD-ROM . I; Table 1.2; Powars et al. 1992; Powars and Bruce 1999; Powars 2000) . These two coreholes are separated by a distance of 180 km. The Brightseat is included in the Island Beach AIloformation ofPoag and Ward (1993) (Fig. 2.4). 2.2.2.4 Aquia Formation

Clayey, silty, shell-rich , glaucon itic, quartz sands of the upper Paleocene Aquia Formation (Clark and Martin 1901; Figs. 2.4, 2.6) crop out in river banks in a continuous arc from north of Baltimore on upper Chesapeake Bay to around Hopewell, Virginia, on the James River (Ward 1984). Equivalent beds are present throughout the subsurface (Powars et al. 1992; Powars and Bruce 1999; Powars 2000), and are included in the Island Beach Alloformation (Poag and Ward 1993; Fig. 2.4). 2.2.2.5 Marlboro Clay

A second upper Paleocene unit is the Marlboro Clay (Clark and Martin 1901; Glaser 1971; Fig. 2.4). The Marlboro Clay is present at scattered outcrops on the western side of Chesapeake Bay from southern Maryland to the James River in Virginia (Ward 1984), and is widespread in the subsurface (Powars et al. 1992; Powars and Bruce 1999; Powars 2000). The thin, silver-gray to pale red plastic clays, interbedded with yellowish-gray to reddish silts of the Marlboro, are part of the Island Beach Alloformation of Poag and Ward (1993) (Fig. 2.4).

Coastal Plain Sedimentary Rocks

51

2.2.2.6 Nanjemoy Formation Oldest Eocene strata in Virginia belong to the lower Eocene Nanjemoy Formation (Clark and Martin 190I; Figs. 2.4, 2.6). Glauconitic sands with variable amounts of clay and silt characterize the Nanjemoy in outcrop (Ward 1984) and subsurface occurrences (Powars et al. 1992; Powars and Bruce 1999; Powars 2000), which extend throughout the Virginia Coastal Plain. The Nanjemoy Formation was included by Poag and Ward (1993) in the Carteret Alloformation (Fig. 2.4). 2.2.2.7 Piney Point Formation The olive gray, clayey, poorly sorted, glauconitic, fossil-rich sand of the Piney Point Formation represents middle Eocene deposition . This formation was originally described from rotary cuttings derived from a well on Piney Point, Maryland, on the Potomac River (Otton 1955). The Piney Point characteristically contains thick beds and lenses dominated by rich accumulations of oyster shells [Cubitostrea sellaeformis (Conrad 1832)], which are cemented into concrete-hard layers of glauconitic , bioclastic limestone. The Piney Point is best exposed on the Pamunkey River, a tributary of the York River (Virginia), but also occurs along the James River and in the subsurface (Ward 1984; Powars et al. 1992; Powars and Bruce 1999; Powars 2000). The Piney Point Formation is included in the Lindenkohl Alloformation of Poag and Ward (1993) (Fig. 2.4). 2.2.2.8 Unnamed Upper Eocene Deposits The bulk of late Eocene deposition in the crater is represented by the Exmore breccia and the postimpact Chickahominy Formation (Fig. 2.4). The presence of late Eocene foraminifers and calcareous nannofossils within the Exmore breccia , however , indicate s that some late Eocene deposits of unknown lithology were already present in the target area prior to impact. We speculate that these early late Eocene deposits were marine clays similar to those of the basal Chickahominy Formation.

2.2.3 Postimpact Deposits Marine sedimentation resumed at the impact site immediately following deposition of the impact-generated Exmore breccia and its capping layer of fallout debris (see Chapter 6 for detailed discussion of Exmore breccia and fallout layer). The crater is now covered by 200-550 m of postimpact sediments, principally siliciclastic silts and sands of marine origin. The postimpact sedimentary column is thickest over the crater, because of increased accommodation space produced by compaction and subsidence of the water-saturated breccia (Poag I 997a) . Twenty postimpact formations are formally recognized in southeastern Virginia (Figs. 2.4-2.6).

52

Geological Framework of Impact Site

2.2.3.1 Chickahominy Formation The upper Eocene Chickahominy Formation (Figs. 2.4-7) lies above the Exmore breccia at all the sites cored within the crater. The Chickahominy is an entirely subsurface unit, which extends only a few kilometers outside the crater. Using cable-tool cuttings from the type well, in York County, Virginia (borehole 89; Figs. 1.3, CD-ROM. I; Table 1.2), Cushman and Cederstrom (1949) originally described the Chickahominy as dominantly blue, brown, and dull gray clays with abundant glauconite. In the seven continuous coreholes drilled in and near the crater, however, the Chickahominy Formation is composed mainly of hard, massive to laminated, silty to sandy, highly fossiliferous, greenish-gray marine clay, containing variable amounts of finely comminuted glauconite and mica . Silt-filled, sand-filled, and pyrite-filled burrows are common in the upper and lower few meters of the formation. Cores and seismic reflection profiles indicate that the thickness of the Chickahominy varies considerably (20-220 m) over the crater, compared to the more uniform thickness of most other postimpact units, and represents a deep-water (outer neritic to upper bathyal) basin-fill deposit. The presence of relatively deep-water microfaunas in its updip occurrences indicates that the Chickahominy Formation may have been originally much more widespread, and that its landward margin has been partly eroded during repeated Cenozoic lowstands of sea level (see Chapter 7 for more details regarding the Chickahominy Formation). Poag and Ward (1993) included the Chickahominy Formation as part of the Baltimore Canyon Alloformation (Fig. 2.4).

2.2.3.2 Delmarva Beds The name Delmarva beds is an informal designation for lower Oligocene, micaceous, clayey, silty, glauconitic sands, which have been cored inside the Chesapeake Bay crater (boreholes 1,2,232,233,234; Figs. 1.3, CD-ROM.l; Table 1.2) and in the Fentress corehole, south of the crater (borehole 117; Figs. 1.3, CDROM . I; Table 1.2; Powars et al. 1992; Powars and Bruce 1999; Powars 2000; Powars et al. 2001; Gohn in press). The Delmarva unit is the only lower Oligocene deposit known in Virginia, and is known only in the subsurface, in and near the crater. The Delmarva beds represent the first down lapping, coarse -grained siliciclastic deposits that began to fill bathymetric lows created by the crater depression and its subsiding breccia fill. Because of this infilling, the Delmarva beds vary widely in thickness. Maximum cored thickness is 7.28 m (23.9 ft), but the unit reaches an estimated maximum of -20 m over the inner basin of the crater . The Delmarva beds are part of the Baltimore Canyon Alloformation (Poag and Ward 1993; Figs. 2.5,2.6).

2.2.3.3 Old Church Formation Upper Oligocene glauconitic sands of the Old Church Formation crop out in Virginia along the Pamunkey River (its type section), a tributary to the York River , and at two other quarry locations (Ward 1984, 1985; Fig. 2.2; CD-ROM. 1). The

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Old Church also has been sampled from the subsurface in the Exmore, Kiptopeke, NASA Langley, North, and Bayside coreholes (boreholes I, 2, 232, 233, 234; Figs. 1.3, CD-ROM. I; Table 1.2) and at a few other sites (Powars et al. 1992; Powars and Bruce 1999; Powars 2000; Powars et al. 2001; Gohn in press). Poag and Ward (1993) included the Old Church as part of the Babylon Alloformation (Figs. 2.5, 2.6).

54

Geological Framework of Impact Site

2.2.3.4 Calvert Formation

Oldest Miocene strata in Virginia are included in the thick, richly fossiliferous, silty, fine sands of the Calvert Formation (Shattuck 1902, 1904; Clark and Miller 1912; Ward and Blackwelder 1976; Ward 1984). Calvert strata crop out widely in Virginia riverbanks, and are well known from many subsurface locations, as well (Ward 1992; Powars and Bruce 1999; Powars 2000). At least three different members are recognized from outcrops, and can be distinguished as distinct depositional units on seismic reflection profiles. The lower part of the Calvert is of early Miocene age, and has been informally designated the Newport News unit in its subsurface expression (Powars and Bruce 1999; Powars et al. 200 I; Gohn in press). The Newport News unit is included in the Berkeley Alloformation of Poag and Ward (1993; Fig. 2.5). The upper part of the Calvert Formation, on the other hand, is of middle Miocene age, and has been assigned to the Phoenix Canyon AIloformation (Poag and Ward 1993; Fig. 2.5). 2.2.3.5 Choptank Formation Middle Miocene strata also are represented by the sandy, shell-rich Choptank Formation (Shattuck 1902, 1904; Ward 1984). The Choptank crops out in a more restricted region than the Calvert Formation, mainly from the Rappahannock River northward, and is poorly known in the subsurface (Powars et al. 1992; Powars and Bruce 1999; Powars et al. 2001). Poag and Ward (1993) included the Choptank Formation in the Phoenix Canyon Alloformation (Fig. 2.5). 2.2.3.6 St. Marys Formation

Upper Miocene strata referable to the St. Marys Formation crop out in Virginia from the Mattaponi River northward (Ward 1984, 1992). The St. Marys, represented by dominantly silty clays, silty shelly clays, and shelly sands, also is widespread in the subsurface (Powars et al. 1992; Powars and Bruce 1999; Powars 2000; Powars et al. 2001). Poag and Ward (1993) included the St. Marys Formation in the Mey Alloformation (Fig. 2.5). 2.2.3.7 Eastover Formation

Additional upper Miocene strata are included in the sandy Eastover Formation (Ward and Blackwelder 1980), which crops out widely over the Virginia Coastal Plain (Ward 1984, 1992), and also is well known from the subsurface (Powars et al. 1992; Powars and Bruce 1999; Powars 2000). The Eastover Formation was included in the Mey Alloformation by Poag and Ward (1993) (Fig. 2.5). 2.2.3.8 Yorktown Formation

Early and early late Pliocene deposition in Virginia is represented by shelly, clayey, phosphatic sands and silty, clayey, very fine sands assigned to the York-

Coastal Plain Sedimentary Rocks

55

town Formation (Clark and Miller 1906, 1912; Mansfield 1928; Johnson and Goodwin 1969; Ward and Blackwelder 1980; Ward 1984), which crops out wide ly south of the Rappahannock River, and is widely distributed in the subsurface (Powars et al. 1992; Powars and Bruce 1999; Powars 2000; Powars et al. 2001) . Nonmarine equivalents of the Yorktown are present as far north as the Potomac River. The Yorktown Formation is included in the lower part of the Toms Canyon Alloformation of Poag and Ward (1993 ; Fig. 2.5). 2.2.3.9 Chowan River Formation Shelly, silty sands and crossbedded sands and silts of late Pliocene age are exposed in borrow pits in Newport News, Norfo lk, and Chesapeake in southern Virginia, and also are known from the subsurface in that region (Powa rs et aI. 1992; Powars and Bruce 1999; Powars 2000) . These strata are assigne d to the Chowan River Formation (Blackwe lder 1981). Poag and Ward (1993) included the Chowan River Formation in the upper part of the Toms Canyon Alloformation (Fig. 2.5). 2.2.3.10 Quaternary Formations A variety of alluvial, estuarine , and back-barrier deposits of Quaternary age constitute the surficial and shallow subsurface strata of the Virginia Coastal Plain (Coch 1968; Bick and Coch 1969; Oaks and Coch 1973; Johnson 1976; Mixon 1985; Mixon et aI. 1989; Powars et al. 1992; Powars and Bruce 1999; Powars 2000) . Crossbed ded sands, gravels, cobbles , silty sands, shelly sands, and sands rich in organic matter are widespread around the bay margin, but good expos ures are limited mainly to borrow pits. Eleven formation s of Quaternary age in Virginia (Oma r Formation , Joynes Neck Sand, Nassawadox Formation , Wachapreague Formation, Kent Island Formation, Windsor Formation , Charles City Formation, Chuckatuck Formation , Shirley Formation , Norfolk Formation , Tabb Formation) are included in the Hudson Canyon Alloformation of Poag and Ward ( 1993) (Fig. 2.5).

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Fig. 2.6. Interpreted sequence stratigraphy for Paleogene formations of southeastern Virginia and southern Maryland (modified from Poag and Commeau 1995). Numbers in parentheses under core labels refer to boreholes shown in Fig. 1.3 and Table 1.2. See Fig. 2.7B for location of section.

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2.3 Sequence Stratigraphy Poag and Commeau (1995) showed that each normal Cenozoic depositional sequence in the Virginia Coastal Plain is bounded by an erosional unconformity (see also Ward and Strickland 1985; Figs. 2.4-6). During each hiatus, erosion removed all (or most) alluvial and fluvial sediments that may have accumulated during sea-level lowstands (Ward and Krafft 1984). In combination with subsequent shoreface erosion and ravinement during the following sea-level rise, each sequence boundary has become a composite surface, which separates marine facies below from marine facies above (Darby 1984; Kidwell 1984). In other words, only highstand and transgressive systems tracts are represented in the preserved record, and lowstand systems tracts are missing. Sea level underwent a series of short-lived eustatic fluctuations during the late Eocene, but on the U.S. Atlantic margin, late Eocene sea levels were higher than at any other time in the Cenozoic (Poag and Ward 1993; Poag and Sevon 1989; Fig. 2.6). When the Chesapeake Bay bolide struck, during the middle part of supercycle TA3 of the Exxon sequence stratigraphic model (Posamentier and Vail 1988; Fig. 2.6), a marine transgression was underway, and relatively deep water (-300 m) covered the impact site (see Chapter 13 for further documentation of target-site paleodepths). Though the Exmore breccia is a major depositional unit, bounded below by a remarkable unconformity (maximum hiatus of >500 myr), that unconformity is a local feature, which does not constitute a sequence boundary in the sense of the Exxon model. In a broad sense, the Exmore breccia is merely an unusual lithofacies deposited in the initial stages of an early Eocene transgressive systems tract. Following emplacement of impact-related deposits and accumulation of a thin (19 em) dead zone (see Chapters 7, 13), normal marine deposition resumed at the impact site, and the silty clays of the Chickahominy Formation began to accumulate in deep water as the marine transgression continued. The upper part of the Chickahominy Formation represents the subsequent highstand systems tract. The lower boundary of the Chickahominy Formation appears to be a conformable surface in some parts of the crater, but the upper boundary is an unconformable sequence boundary. The unconformity formed during an early Oligocene regression, the subsequent lowstand, and the following marine transgression.

2.4 Paleogeography of Impact Site The general paleogeographic-paleoceanographic setting of ground zero at the time of impact can be estimated from the fossil record (mainly microfossils) contained in the Exmore breccia and in the overlying Chickahominy Formation. The oldest planktonic and benthic foraminifera and calcareous nannofossils in the Chickahominy Formation, which began to collect on the seafloor <1- 3 kyr after impact, indicate marine paleodepths of - 300 m (see Chapter 13 for discussion). Subsequent Chickahominy microfaunas and nannofloras show that similar outer neritic

58

Geological Framework of Impact Site

to upper bathyal biotopes persisted at the impact site until the end of the Eocene (Poag and Aubry 1995). If the water depth today were - 300 m deep over the crater center, marine waters would flood the entire coastal plain and the adjoining Piedmont, pushing the shoreline against the flanks of the Appalachian Mountains, > I00 km west of the crater (Figs. 2.7, 2.8). Poag and Sevon (1989) and Poag (1993) showed that late Eocene deposition rates along the Atlantic margin were among the lowest of the Cenozoic (Fig. 2.9). These unusually low deposition rates were chiefly responsible for minimal late Eocene flexural subsidence rates in southeastern Virginia (see discussion under subheading 2.5). The slow subsidence of this continental margin would have created a late Eocene coastal plain and continental shelf with a gentler slope than at any time following the impact (including the present). Therefore, the late Eocene shoreline would have extended farther westward onto the Piedmont than the current position of the 300-m elevation. The late Eocene shelf break, as indicated on seismic profiles, would have been approximately at the present shelfbreak location (Poag and Ward 1993). This puts the impact site on the inner third of the continental shelf, even though the high late Eocene sea level combined with minimal deposition and subsidence rates to create outer neritic to upper bathyal depths at the site (Figs. 2.7,2.8). In addition, late Eocene paleoclimate was much warmer and wetter than modern climate at the impact site (Wolfe 1978, 1992; Prothero 1994; Bestland et al. 1996; Poag 1997b). Tropical rainforests covered the slopes of the Appalachians, and carbonate ramp deposits (bioclastic limestones of the Piney Point Formation and chalks and micritic limestones of the Lindenkohl and Baltimore Canyon Alloformations) had accumulated across the broad continental shelf since the middle Eocene (Poag and Ward 1993; Fig. 2.5).

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Paleogeography ofImpact Site

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2.5 Subsidence of Virginia Continental Margin Variation in postrift (Mesozoic-Cenozoic) sediment thickness across the US Atlantic margin has allowed recognition of three major structural-subsidence regimes (Watts and Steckler 1979; Steckler et at. 1988). On the landward side is a seaward-thickening wedge of Upper Jurassic to Holocene sediments. In Virginia , this wedge consists mainly of sand and silt deposits, which feather out near the Piedmont Fall Line and reach a maximum total thickness of -1 -1.5 km near the present shoreline (Fig. 2.10). This landward sedimentary wedge rests unconformably on continental crust, whose subsidence is attributed almost wholly to flexural downwarping caused by sediment loading (Watts and Steckler 1979). Beneath the inner continental shelf, the sediment thickness increases rapidly just seaward of the shoreline, across a hinge zone comprised of a series of faults and flexures. The hinge zone marks a major transition between undeformed continental crust landward, and rifted crust (thinned, heated), whose relatively rapid subsidence is attributed to thermotectonic processes that formed the offshore Baltimore Canyon trough. Maximum postrift sediment thickness in this trough (11-12 km) is reached between this hinge zone and the present shelf edge, which approximates the western edge of true oceanic crust. The Chesapeake Bay bolide struck approximately on the subsidence hinge zone in Virginia. This location gives a decided eastward tilt to the crater, due to 36 myr of postimpact flexural subsidence , which was driven almost entirel y by postimpact sediment loading (Watts and Steckler 1979).

2.6 Initial Evidence of East Coast Impact Initial evidence of a bolide impact in the western North Atlantic region came from distal ejecta that constitute part of the North American tektite strewn field. This ejecta field includes tektites, microtektites, coesite, stishovite, and shocked grains of several minerals, which display well-documented planar deformation features (PDFs; Glass 1989; Kocber! 1989; Glass and Wu 1993; see Chapter 6 for further discussion of shock deformation features) . The strewn field, as known at that time, was documented from deep-sea cores and surface outcrops, and was extrapolated to cover a -9-million-km2 area of the western North Atlantic, Caribbean Sea, Gulf of Mexico, Barbados , Cuba, and the coastal plains of Texas (bediasites) and Georgia (georgiaites) (Fig. 2.IIA). As an alternative , Koeber! (1989) suggested that the North American tektites might be distributed in irregular rays, extending radially outward from the impact site, rather than forming a continuous thin blanket (Fig. 2.11 B).

After Glass (1989)

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Fig. 2.11. Geog raphic distribution of North Ameri can tektit e strewn field in western North Atlantic, Caribbean Sea, and Gul f of Mex ico regions. A Continuous ejecta distribution inferred by Glass (1989); B Distribution in radially oriented ejecta rays inferred by Koeberl ( 1989). See Chapter 9 for further discussion of North American tektite strewn field .

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Fig. 2.12. Location of Toms Canyon impact crater (irregular shaded area), two exploratory wells drilled into crater (l = Tenneco; 2 = Houston Oil and Minerals), and three nearby coreholes (DSDP Site 612; ODP Sites 903, 904) from which North American tektite debris has been documented, and distribution of seismic reflection profiles crossing and surrounding crater (modified from Poag and Poppe 1998). ODP = Ocean Drilling Program; DSDP = Deep Sea Drilling Project.

First attempts to determine the location of a source crater for the North American ejecta relied on the physical properties of the ejecta layer at Deep Sea Drilling Project (DSDP) Site 612 (Figs. 2.12, 2.13). Site 612 was drilled on the continental slope, 120 km east of Atlantic City, New Jersey by scientists of DSDP Leg 95 (Poag, Watts, et al. 1985; Thein 1985). Because the Site 612 ejecta were coarser and occurred in a thicker layer than at other sites in the North American strewn field, Glass (1989) and Koeberl (1989) concluded that the responsible impact must have taken place within a few hundred kilometers of Site 612. The size and loca-

Initial Evidence of East CoastImpact

67

tion of the Chesapeake Bay crater, discovered five years later, made it the prime candidate for the source of the North American tektite strewn field (Poag et al. 1994). A preliminary geochemical analysis of clasts from the Exmore breccia led Koeberl et al. (1996) to confirm this likelihood. An additional possible source for the New Jersey ejecta was suggested by Poag and Aubry (1995) and Poag and Poppe (1998). These authors proposed that the Toms Canyon structure, a small, elliptical (22 km maximum diameter), possible impact crater, may have supplied all or part of the ejecta cored at DSDP Site 612 and at nearby ODP Sites 903 and 904, cored in 1992 (McHugh et al. 1996; Glass et al. 1998; Figs. 2.12,2.13). The Toms Canyon structure is only 35 km north of the three deep-sea core sites (Fig. 2.12). The Toms Canyon connection has been disputed by McHugh et al. (1996) and Glass et al. (1998), who considered the Chesapeake Bay crater as the most likely source of all the ejecta known from offshore New Jersey.

68

Geological Framework of Impact Site

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Fig. 2.13. Photographs of archive half of cores from DSDP Site 612 and ODP Site 904, showing stratigraphic position and visual expression of two ejecta layers (M = microtektite layer derived from Chesapeake Bay impact; K = microkrystite layer derived from Popigai impact). Depth scale in ern below top of core section. See CD-ROM for color version of this figure.

Onshore Borehole Evidence

69

2.7 Onshore Borehole Evidence 2.7.1 Noncored Boreholes

Ejecta and impact breccia from the Chesapeake Bay impact were recovered from boreholes in southeastern Virginia more than fifty years ago, though their impact origin was not suspected until 1992 (Poag et al. 1992). Early publications by Cederstrom (l945a,b, 1957) described a persistent subsurface interval of highly diverse, brightly colored, variegated rock types containing a mixture of Cretaceous, Paleocene, and Eocene foraminifera . To this interval, which he identified in more than fifty wells, Cederstrom assigned the name Mattaponi Formation (Figs. 2.4, 2.14; Table 1.2). Stratigraphic and geographic inconsistencies and ambiguities in its definition have caused most subsequent authors to ignore or misuse the term Mattaponi (Brown et al. 1972; Tiefke 1973; Ward 1984; Hansen 1978; Meng and Harsh 1988), and it is now seldom used (Poag 1997a; Powars and Bruce 1999). By correlating the Mattaponi with more recently drilled sections, we can now extrapolate the presence of the Exmore breccia in many of Cederstrom's well logs (e.g., Poag 1997a; Powars and Bruce 1999). A particularly widely cited and comprehensive borehole study was published by Brown et al. (1972). These authors constructed subsurface cross sections, which extrapolated the onshore stratigraphy to the undrilled strata underneath Chesapeake Bay prior to discovery of the impact crater or its breccia fill. Figure 2.15 compares a Brown et al. (1972) cross section with our reinterpretation, which acknowledges the presence of the impact crater (see also Powars and Bruce 1999).

2.7.2 Cored Boreholes

Modern sampling of the Exmore breccia was initiated inadvertently through continuous coring carried out collaboratively by the USGS and the Virginia State Water Control Board (now the Virginia Department of Environmental Quality; Powars et al. 1990, 1991, 1992; Powars and Bruce 1999; Powars 2000; Fig. 1.3; Table 1.2). Though originally interpreted as a debriite or channel-fill deposit (Powars et al. 1992), biostratigraphic and petrographic analyses later confirmed the late Eocene age and impact origin of the breccia (Poag et al. 1992; Koeberl et al. 1996). It was not until the acquisition of multichannel seismic reflection data from Texaco, Inc., however, that investigators fully realized that two of the coreholes had been drilled into a giant impact crater (Poag et al. 1994; Poag 1999c).

70

Geological Framework of Impact Site

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Fig. 2.158. Brown et al.'s ( 1972) stratigraphic cross section across southeastern Vir ginia rev ised by us to show presence of crater. Compare also with Powars and Bruce ( 1999). See Fig. 2.14 for profile location .

~

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3 Geophysical Framework of Impact Site

3.1 Seismic Investigations of Virginia Coastal Plain Surprisingly few seismic reflection profiles have been collected during the > 100year history of geological and geophysical studies of southeastern Virginia. A few scattered onshore surveys have been published (Dysart et al. 1983; Bayer and Milici 1987; Milici et al. 1995), but otherwise, the seismic architecture of this coastal province has virtually been ignored. It was not until Poag et al. (1994) began to publish marine seismic profiles from Chesapeake Bay, that the seismostratigraphic and seismostructural framework of the impact crater could be constructed. In this book, we draw liberally from the database of marine seismic reflection profiles, collected mainly by the US oil industry and the USGS , to document the structure and morphology of this submarine impact structure.

3.2 Seismic Signature of Crystalline Basement Rocks The seismic expression of the crystalline basement surface in most of the study area is the upper member of a couplet of high-amplitude, nearl y continuous reflections, clearly manifest on most seismic profiles , and nearly evenly spaced apart over the entire region . We refer to this reflection as AB (acoustic basement; Fig. 3.1A) . Hansen (1978) first identified this couplet signature on profiles collected for a subsurface study of southern Maryland. The profiles were collected by the Vibroseis method, in which a truck-mounted vibrator provides the acoustic source. Hansen used a natural -amplitude synthetic seismogram from a well 45 km northwest (updip) of his Vibroseis trackline to construct a best-fit stratigraphic model. From the model he inferred that the upper member of the reflection couplet represented the top surface of a high-velocity, indurated, sedimentary layer (interval velocity of 2.5 km/s). Hansen inferred that the subjacent lower-velocity interval (interval velocity of 1.9 kmIs) represented a saprolite bed. The lower reflection of the high-amplitude couplet, Hansen interpreted to represent the upper surface of fresh crystalline basement rocks (interval velocity of 4.5 km/s) . Hansen (1978) calculated individual thicknesses of 15-30 m for both the indurated sedimentary layer and the saprolite bed. Hansen noted that the indurated layer was not omnipresent, but patchil y distributed in southern Maryland. He attributed the patchiness to erosion. C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

23!>O

2300

22!>O

______1

Shot Points

Potomac River Profile 11- PR (T-2) I

2200

N

f

~

~.

en

~

.[

~

o ....,

~

[

[

~

~

-..J

.j:>

:§:

Fig. 3.1. Comparison of seismic reflection signatures between Potomac River dip profile Il-PR (A; labeled T-2 on Fig. 3.3) and published strike profile of Dysart et al. (1983) from Smith Point, Virginia (B; labeled SP-l on Fig. 3.3). Profiles are - 8 km apart, as measured along structural and depositional strike. Note much better definition of basement reflection couplet (AB) on profile II-PR. See text for explanation of labeled primary reflections PS, AB, P, K, B.

A

Seismic Signature of Crystalline Basement Rocks

75

Five years later, Dysart et al. (1983) identified a similar couplet of reflections on two Vibroseis profiles collected in southeastern Virginia (Fig. 3.1B). Dysart et al. (1983), however, interpreted the two reflections to be the boundaries of two, rather than three, different rock units. They termed the upper reflection K, and ascribed it to the top of an indurated sedimentary unit of unknown (possibly Triassic) age, whose estimated thickness was 72-111 m. The indurated unit was not cored in the vicinity of their Vibroseis study, but Dysart et al. (1983) correlated it with a 75-m indurated layer drilled 125 km to the south, at Portsmouth, Virginia, and with the top of the Wastegate Formation drilled 40 km east at Crisfield, Maryland (Hansen 1978; Costain et al. 1981 ). Dysart et al. (1983) called the lower reflection of the basement couplet, B. They ascribed it to the top of crystalline basement, and correlated it with metavolcanic basement rocks drilled in the Crisfield, Maryland borehole (Hansen 1978; Costain 1979). Dysart et al. (1983) calculated interval velocities of 4.1--4.7 km/s for the indurated layer at Smith Point, similar to the velocities Hansen (1978) estimated for the crystalline basement in southern Maryland. Dysart et al. (1983) calculated an interval velocity of 6.4 km/s for the upper part of the Smith Point crystalline basement, about 50% higher than Hansen's (1978) estimate for southern Maryland. In 1984, Hansen and Wilson published more Vibroseis data from southern Maryland, and correlated their results with drilling data from a deep basementpenetrating borehole St.M. Df 84 (borehole 7 herein; Fig. 2.2; Table 1.2). In their study, Hansen and Wilson (1984) modeled the basement surface as a single reflection, although the familiar couplet is clearly present on the published Vibroseis profile (Hansen 1978; Fig. 3.2A herein), which is only 4 km east of the DF 84 well. They correlated the lower basement reflection with a diabase section, 16 m of which was drilled in the St.M. Df 84 well. The upper reflection of the couplet they correlated with the top of a sedimentary unit correlative with the Wastegate Formation, of possible Triassic or Jurassic age. In a fourth Vibroseis study, Milici et al. (1995) published additional profiles from southeastern Virginia and southern Maryland (Fig. 3.2), and correlated them with the profiles of Hansen (1978) and Dysart et al. (1983). Milici et al. (1995) also recognized a basement reflection couplet, or more precisely, a variety of different but correlative reflection couplet(s), which, in their view, represented different "basement" events and lithologies. On their profile NAB-II A, for example (Fig. 3.2B), which crosses the coarse, siliciclastic rift deposits of the Taylorsville basin, a reflection couplet marks the boundary between rift deposits and postrift deposits. Both of these latter deposits, however, are composed of sedimentary strata, as sampled by numerous deep wells in the area. Milici et al. (1995) assigned the interval encompassed by the basement couplet to an indurated zone of Lower Cretaceous sedimentary rocks, and projected the interval 45 km downdip (eastward) to correlate with Dysart et al.'s (1983) unit K. However, Milici et al. (1995) believed the indurated zone on NAB-II A to be considerably younger than the Wastegate Formation in the Crisfield, Maryland, borehole, with which Dysart et al. (1983) had originally correlated their reflection K.

60

70

60

50

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102 ,

Fig. 3.2. Segments of onshore seismic reflection profiles : A Profile VT-6, near Crisfield, Maryland (Table 3.2); B Profile NAB-II (Table 3.2). Figure shows seismic signatures and correlation of principal reflections from east (A) and west (B) sides of Chesapeake Bay (both profiles outside impact crater) . See Fig. 2.2 for profile locations and text for further explanation.

A-

---'-

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Chesapeake Bay Seismic Reflection Profiles

77

On the Delmarva Peninsula, Milici et al. (1995) modeled crystalline basement as a single high-amplitude reflection on the basis of a synthetic seismogram from the Crisfield borehole DGT-I (Fig. 3.2A). They correlated this reflection with the top of metavolcanic rocks in the bottom of the borehole , and designated the reflect ion as Bb (Fig. 3.2A), inferring that Band b are coincident in this area , though they are separate reflect ions elsewhere. The stratigraphically next highest highamplitude reflection in Milici et al.' s (1995) analysis marks the top of the inferred Jurassic section, and was referred to by those authors as reflection Mark 3 (M3, Fig. 3.2A). Mark 3 can be distinguished on the Delmarva Peninsula Vibroseis profiles, but the unit whose top it represents pinches out updip; thus Mark 3 is not equivalent to K as Dysart et al. (1983) had inferred . Clearly, the near-basement reflection couplets in different parts of the study area cannot be assumed to represent coeval units or equivalent lithologic units from profile to profile. In areas underlain by rift basins, for example , the crystalline basement surface (b) can be kilometers deeper than our AB horizon (Fig. 3.2B). Our interpretation of the Maryland Vibroseis profiles calibrated with seismic and lithologic data from the Crisfield borehole DGT-I (designated borehole 120, herein; Fig. 1.3) indicates that reflection Bb of Milici et al. (1995) is equivalent to our AB, and represents the top of the metavolcan ic section drilled in the DGT-l well at 1245 m depth (Fig. 3.2A). Thus, except where rift basins are present, we interpret reflection AB to represent the top of crystalline basement.

3.3 Chesapeake Bay Seismic Reflection Profiles For this study, we used 2,0 18 km of onshore and offshore seismic reflection profiles, derived from 11 different sources, and collected over a 25-year period (1975-2000; Fig.3.3; CD-ROM.2; Tables 3.1, 3.2). For seismostratigraphic analysis, we used standard interpretation techniques described by Vail et al. (1977) and Wilgus et al. ( 1988). Four principal sets of seismic reflection data document the structure and morphology of Chesapeake Bay impact crater (Fig. 3.3; CD-ROM.2 ; Tables 3.1, 3.2). The key data set was collected in 1986, by Teledyne Exploration Company, for a partnership whose principal members were Texaco, Inc. and Exxon Exploration Co. Texaco donated the data to the USGS, and we refer to these data as the Texaco profiles (designated by T on Fig. 3.3). The Texaco profiles constitute 310 km of migrated, 48-fold , multichannel data collected within the southern part of Chesapeake Bay and the lower parts of the James, York, Rappahannock, and Potomac Rivers. The Texaco profiles give an unambiguous record of the crystalline basement configuration (faults, peak ring, central peak), the displaced sedimentary megablocks that overlie crystalline basement , the Exmore breccia that fills the crater, preimpact sedimenta ry strata surrounding the crater, and all overlying postimpact sediments except the uppermost 80 m (Poag 1996, 1997a; Poag et al. 1999; Powars and Bruce 1999). All the principal structural, morphological, and gross depositional features of a peak-ring/central-peak impact crater are displayed on the

78

Geophysical Framework of Impact Site

37°30'

.

~ ~

.

"'''Il.

~

e Borehole

.-.

~,

76°40'

~

76°20'

Fig. 3.3. Geographic distribution of onshore and offshore seismic reflection tracklines showing positions at which seismic profiles cross outer rim and inner edge of peak ring of Chesapeake Bay crater. Solid dots indicate key boreholes for calibrating seismic stratigraphy. See Table 3.2, CD-ROM.2, and CD-ROM.8-16 for more detail.

Texaco profiles. A second set of multichannel profiles was collected in 1982 by the USGS. They are called the Neecho profiles, after the research vessel used to collect them, and are designated N (Fig. 3.3; CD-ROM.2; Tables 3.1, 3.2; Poag 1996). Three Neecho profiles (68 line km) in the lower bay and the mouth of the James River intersect the Texaco profiles and provide further documentation of all the impact features except the displaced megablocks. These Neecho profiles also record reflections from the uppermost 80 meters of postimpact sediments, which the Texaco profiles lack. A third set of multichannel profiles (220 line km) was collected in 1998, by the USGS and Lamont-Doherty Earth Observatory, using the R/V Maurice Ewing. These profiles are herein designated E (Figs. 3.3, 3.4; CD-ROM.2; Tables 3.1, 3.2). This set of profiles images the postimpact sediments, preimpact sediments,

Chesapeake Bay Seismic Reflection Profiles

Chesapeake Bay

79

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3730'

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Fig. 3.4. Trackline map for seismic reflection survey of Chesapeake Bay impact crater carried out by USGS and Lamont-Doherty Earth Observatory in 1998 using R/V Maurice Ewing (£-1, £-2, and £-3 of Fig. 3.3; see Table 3.2 and CD-ROM.2, 15, and 16).

and the upper surface of crystalline basement, including the peak ring and a clear expression of the centra l peak. A fourth set of reflection data comprises 875 km of single-channel profiles collected in 1996 by the USGS in collaboration with the National Geographic Society (NGS). These profiles are designated S (Fig. 3.3; Tables 3.1, 3.2), after the Seaward Explorer, the vessel used for the survey (Poag et al. 1999). The USGS-NGS profiles emphasize details of the postimpact sedimentary beds, such as sequence boundaries and compaction faults, but give little information about the displaced megablocks. They also clearly delineate the configuration of the crystalline base-

80

Geophysical Framework of ImpactSite

ment inside the bay, but failed to image the basement surface beneath the thicker sediments of the inner continental shelf east of the Delmarva Peninsula. Five additional sets of offshore profiles do not cross the crater, but are near enough to help constrain its boundaries and(or) to document the depth to basement (Fig. 3.3; CD-ROM.2; Tables 3.1, 3.2). Offshore, the USGS collected 140 line km of multichannel data just seaward of the crater rim , which reveal the depth to crystalline basement and the stratigraphy of overlying sediments (Grow et al. 1988). These profiles are designated U. The US Minerals Management Service (MMS) also contracted for several multichannel surveys east of the crater rim (65 line km), and these profiles are designated M. Three earlier sets of single-channel data collected by the USGS (1975, 1976) also constrain the eastern rim of the crater, and are designated A (after the ship Atlantis II; 140 line km), G (after the ship Gyre; 60 line km), and F (after the ship Fay; 25 line km; Fig. 3.3; CD-ROM.2; Tables 3.1, 3.2). These early single-channel profiles provide reflections from the sedimentary section only; they do not image the crystalline basement.

Table 3.1. Trackline lengths for seismic reflection profiles collected over and near Chesapeake Bay impact crater. Source [Ship* or Organization]

Designated Prefix

Total Trackline Length [km]

Texaco

T

310

USGS-OCS

U

140

Atlantis 11*

A

140

Fay *

F

25

MMS-OCS

M

65

Gyre*

G

60

Neecho*

N

68

Seaward Explorer *

S

875

MauriceEwing*

E

220

Vibroseis (on land)

D,P, SP

100

USGS (on land)

NL

- 15

TOTAL

2018

Multichannel 48-fold COP

Multichanne l 48-fold COP

Multichannel 48-fold COP

Multichannel 6-fold COP

Multichannel 6-fo ld COP

Texaco II-PR

Texaco 10-RR

Texaco 8-S -CB-E

Texaco 7-CB-H

Texaco 13-YR

Texaco 9-CB-F

Neecho I

Neecho 2

Neecho 3

SEAX 8-7-6

SEAX 9-10

SEAX 16-4a-4

SEAX 17-9

SEAX 13- 14-15- 4a-16

T-2·

T-3

T-4·

T-5

T-6

T-7·

N-I

N-2

N-3

Sol·

S-2·

S-3·

S-4

S-5

Single channel

Single channel

Single channel

Single channel

Single channel

Multichannel 6-fold COP

Cro sses crater

Crosses crater

Crosses crater

Crosses crater

Crosses crater

Cro sses crater

Crosses crater

Cro sses crater

Crosses crater

Crosses crater

Cro sses crater

Crosses crater

Cro sses crater

Multichannel 48-fold COP

Multichannel 48-fold COP

Crosses crater

Crosses crater

Contents

Multic hann el 48-fold COP

Multichannel 48-fold COP

Texaco I-CB

T-I·

Data Type

Original Designation

Number

This volume

This volume

This volume

This volume

Thi s volume

This volume

Thi s volume

Thi s volume

Poag (1996, 1997)

Poag ( 199 6, 1997)

Poag ( 1996, 1997)

Poag ( 1996, 1997)

Poag ( 1996, 1997)

This volume

Poag ( 1996, 1997)

Reference

Table 3.2. Seismic reflection profiles that either cross Che sapeake Bay impact crater or help constrain location of outer rim and( or) depth to cry sta lline basement"

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This volume This volume

This volume Poag (1996)

Crosses crater Crosses crater Crosses crater Crosses cra ter Crosses crater Crosses inner basin Crosses inner basin Crosses inner basin & outer rim Crosses crater Constrains crater Constrains crater Constrains crater Constrains crater

Single channel Single channel Single channel Single channel Single channel Multichannel 3-fold CDP Multichannel 3-fold CDP Multichannel 3-fold CDP Single-channel

Multichannel 48-fold CDP Multichannel 48-fold CDP Multichannel 48-fold CDP Multichannel 48-fold CDP

SEAX 22-23-24

SEAX 1-2-18

SEAX 20-21

SEAX5

SEAX 11-12

Ewing I

Ewing 2

Ewing 3

Fay 19

USGS II

USGS 28

USGS 3

USGS 12

S-9

S-IO

S-II

S-12

S-13

E-I

E-2*

E-3*

F-I

U-I

U-2

U-3

U-4

Klitgord et al. (1994)

Klitgord et al. (1994)

Grow and Klitgord (1988)

Klitgord et al. ( 1994)

This volume

This volume

This volume

This volume

This volume

This volume

Crosses crater

Single channel

SEAX 25-26

S-8

This volume

Crosses crater

Single channel

SEAX 3-15-(N-I)-7

S-7

Reference This volume

Single channel

SEAX 13-14-6-27

S-6

Contents Crosses crater

Data Type

Original Designation

Number

Table 3.2 . (cont.)

0
I ~

~

I~

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0

I *~

I[ I 3~

~

::r

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0

00 N

Multichannel 48·fold CDP Single-ehanne l Single-channel

VibroSeis VibroSei s

MMS

Atlantis II

Atlantis II

Gy re 81

Smith Point I

Portsm outh I

Portsmouth 2, 3, 4, 5, 6. 7

Interstate Highway 1-64

Delmarva I

Delmarva 2

Delmarva 3

Delmarva 4

Delmarva 5

M-2

A-I

A-2

G-(

SP-I

P-I

P2-7

1-64

D-I

D-2

D-3

D-4

D-5 VibroSe is

VibroSei s

VibroSe is

VibroSeis?

VibroSeis

VibroS eis

VibroSeis

Single-ehannel

Multich annel 48-fo1dCDP

MMS

M-I

Data Type

Or iginal Designat ion

Number

Table 3.2. (cont. )

Documents basement

Documents basement

Documents basement

Doc uments basement

Documents basement

Documents basement& outer rim

Documents basement

Documents basement

Documents basement

Constrains crater

Constrains crater

Constrains crate r

Constrains crater

Constrains crater

Content s

Hansen (1978)

Han sen (1978)

Hansen (1978)

Hansen (1978)

Han sen (1978)

Bayer and Milici (19 87)

Costain and Glover (1976- 1982)

Co stain and G lover (1976-198 2)

Dysart et al. ( 1989)

Poag (1996 )

Poag (1996)

Poag (1996)

This volume

This volume

Reference

'J>

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VibroSeis VibroSeis VibroSeis VibroSeis

Virginia Tech 6

Virginia Tech 7

Virginia Tech 13

Virginia Tech NAB IlA

USGS·5

VT·6

VT·7

VT·13

NAB IlA

NL

* Indicates profile included on accompanying CD·ROM

High-resolution Multichannel

Data Type

Original Designation

Number

Table 3.2. (cont.)

Documents basement

Documents basement

Documents basement

Documents basement

Documents basement

Contents

Gohn (in press)

Milici et al. (1995)

Milici et aI. (1995)

Milici et al. (1995)

Milici et aI. (1995)

Reference

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Depth Conversion of Seismic Two-way Traveltimes

85

Onshore, four sets of Vibroseis profiles have been collected outside the crater rim (total of 100 line km) , and provide depths to crystalline basement and reflections from the overlying sedimentary section. The oldest set of onshore profiles was published by Hansen (1978), and is designated D (Fig . 3.3; Tables 3.1, 3.2). The second set was published by Dysart et al. (1983), and is designated SP (Fig . 3.3; Tables 3.1, 3.2) . The third set of onshore profiles was published by Milici et al. (1995), and is designated VT or NAB (Fig. 3.3; Tables 3.1, 3.2). The fourth set of profiles onshore was collected by John Costain (Virginia Tech) for a regional geothermal study (see http ://rglsun1.geol.vt.edu/geothermal.html). These profiles are designated P (Fig . 3.3; Tables 3.1, 3.2). An additional USGS high-resolution, multichannel, seismic reflection profile (designated NL) was constructed from data collected along an - I5-kIn transect from the NASA Langley corehole northwestward to a point -10 km outside the outer rim of the crater (Gohn in press) . This profile clearly images the basement reflection couplet (AB), the pre impact stratal reflections outside the crater, and postimpact stratal reflections inside and outside the crater. The profile also resolved the Exmore breccia and the displaced megablocks in the annular trough, which were cored at the NASA Langley site.

3.4 Depth Conversion of Seismic Two-way Traveltimes We used three types of data to help convert the two-way traveltimes to depth : (I) stacking velocities [root-mean-square (RMS) values] from Vibroseis and marine seismic reflection surveys; (2) velocity profiles derived from interval transit-time logs run in the NASA Langley borehole; and (3) especially the subsurface elevations of key stratigraphic boundaries determined from boreholes drilled near the seismic tracklines. We focused on two key horizons for the conversion: (1) the top of acoustic basement, AB (Fig . 3.lA,B) (acoustic basement is composed of varying crystalline lithologies, as noted above); and (2) the base of the postimpact sediments, PS (Fig . 3.1A,B) ; inside and near the crater, this horizon corresponds to the upper surface of the Exmore breccia. Dysart et al. (1983) derived RMS velocities in the sedimentary section from the ground surface to reflection B at Smith Point, -30 km north of the crater rim on the west side of Chesapeake Bay; there, B is approximately 0.9 s deep (2-way traveltime; Fig. 3.1B). The RMS values range from 2175 to 2350 mis, with an average of approximately 2350 mls. This would yield a depth conversion factor of 0.1 s = 113 m. Dysart et al. (1983) did not publish RMS velocities related to reflection K, however. Our correlation of borehole stratigraphy with the seismic profiles indicates average velocities of - 2000 m/s to K. Therefore, we calculated the depth to equivalent horizon A B using the relationship 0.1 s = 100 m. Klitgord and Schneider (1994) provided an unusually large database of offshore seismic velocities calculated from marine reflection profiles collected east of the crater rim (USGS-OCS, MMS-OCS ; Fig. 3.3; Tables 3.1, 3.2) . They derived velocity values from normal moveout analysis of these offshore profiles, combined

86

Geophysical Framework of Impact Site

with sonic logs and velocity checkshot studies in numerous industry boreholes, and wide-angle data from two-ship seismic experiments. On the shallow ends of offshore profiles nearest the crater (profiles U-l, U-2, U-3; Fig. 3.3), where horizon AB is -2.0 s below sea level, RMS values average ~5000 mis, which yields a depth conversion of 0.1 s = 125 m. The RMS velocities derived by Dysart et al. (1983) for the interval from the ground surface to horizon PS (Fig. 3.18) range from 1550 to 1625 mis, averaging 1592 mls. This yields a depth conversion of 0.1 s = 796 m. Stratigraphic correlation of horizon PS between the boreholes and the seismic profiles indicates a nearly identical depth conversion of 0.1 s = 800 m. For convenience, we used the latter value. We have interpolated RMS values (assuming linear variation with depth) above and below horizons AB and PS, and between their onshore and offshore endmember values, to produce the depth sections and structure maps illustrated herein.

3.5 Gravity Evidence 3.5.1 Database

To analyze gravity anomaly data in the vicinity of the impact crater, we combined four sources of data (Fig. 3.5): (1) land and marine gravity anomalies compiled by Carl Bowin (Woods Hole Oceanographic Institution; 3,941 stations; personal communication, 1998); (2) land and marine gravity anomalies compiled by John Costain (Virginia Tech; 14,240 stations; personal communication, 1998); (3) marine gravity collected by R1V Maurice Ewing in Chesapeake Bay (1,587 stations; USGS cruise EW9809, Oct 15-16, 1998; Fig. 3.4); and (4) land gravity data collected by Phillip Moizer (USGS) on the Delmarva Peninsula and its southeastern islands in September, 1998 (63 stations). Data from Bowin and Costain were provided as Bouguer anomalies, compiled from a variety of sources, and apparently generated with the same crustal density of 2.67g/cm 3• The R1VEwing data were measured with a Bell gravimeter at one sample/second, post-processed with a 6minute and 8-minute gaussian filter, and averaged to one-minute intervals. Navigation positioning was measured with three GPS transceivers . Data collected on the Delmarva Peninsula were measured with a Lacoste and Romberg gravity meter and tied to gravity points from the Bowin and Costain data sets. The position of each gravity station on the Delmarva survey was measured with an Ashtech GPS receiver . Positions were corrected in post-processing with reference to the National Geodetic Survey's fixed GPS station CHRI located at Cape Henry, Virginia (Fig. 3.5). We merged data sets from Bowin and Costain, and removed duplicate points. R1VEwing data are referenced to a gravity station at the dock in Portsmouth, Virginia, which is in the Bowin-Costain data set, thus aligning the Ewing measurements with the older data . Data collected on and about the Delmarva Peninsula in

Gravity Evidence

87

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Fig. 3.5. Geographic distribution of gravity stations collected previous to and expressly for this study. 1998 also were tied to the Bowin-Costain data set at the time of the survey.

3.5.2 Interpretation

Visual examination of a gridded image of these data reveals a general eastward decline in gravity anomalies, probably associated with the subsidence of the basement surface beneath the thick sedimentary column on the continental margin (Fig. 3.6A). To remove this regional trend, we fitted a planar surface to the data

88

Geophysical Framework of ImpactSite

-20

-10

0 10 Gravity Anomaly (mGal)

20

Fig. 3.6A. Simple Bouguer gravity anomaly map of the study area. Distribution of relative gravity highs (+) reflects presence of subcircular crystalline peak ring encircling gravity low (-) of the inner basin.

by least squares, with iterative data reweighting (using trend2d software of Generic Mapping Tool; Wessel and Smith 1991). Subtracting the fitted surface from the input data produced a set of residual gravity anomalies (Fig. 3.6B). We gridded the residual data set at an increment of 0.001 degrees for further analysis. The spatial distribution of residual gravity anomalies supports the structural and morphological interpretations derived from our seismostratigraphic analyses. The principal features identified from the gravity surveys are (Fig. 3.6B): (I) a subcircular bull's-eye negative anomaly correspondent with the seismically defined inner

Gravity Evidence

89

37"15'

76°30'

76°15' -20

-10

0

,

r

76°00'

75°45'

10

20

Gravity Anoma ly Residual (mGal)

Fig. 3.68. Residual gravity anomaly map of the study area. Distribution of relative gravity highs (+) reflects presence of subcircular crystalline peak ring encircling gravity low (-) of the inner basin.

basin; (2) a ring of positive anomalies correlative with the peak ring inferred from the seismic profiles (see Chapter 4 for further discussion and illustration of gravity data).

4 The Primary Crater

4.1 Crater Structure and Morphology 4.1.1 Seismic Interpretation

Our extensive network of seismic reflection profiles clearly documents that the structural-morphological features of the outer rim, annular trough, and displaced megablocks are expressed principally by preimpact sedimentary rocks, and to a much lesser degree, by crystalline basement rocks (Figs. 1.5, 4.1, 4.2; CDROM.3-6). In contrast , the peak ring, inner basin, and central peak of the Chesapeake Bay crater are strongly developed within rocks of the crystalline basement (Figs. 1.5,4.1,4.2; CD-ROM .3-6). 4.1.1.1 Outer Rim

The outer rim of the Chesapeake Bay crater is a steep, roughly circular fault scarp constructed almost entirely of sedimentary rocks. On all seismic profiles, the outer rim is manifest as an abrupt loss of coherent, continuous to subcontinuous, moderate- to high-amplitude horizontal reflections, which characterize the preimpact sedimentary section outside the rim (Figs. 4.3--4.19). On typical profiles, the coherency loss marks the steep normal fault scarp, formed by massive failure , slumping, and sliding of the sedimentary section near the maximum lateral limit of strongest ground-shock effects and of surgeback effects from subsequent watercolumn collapse . The rim scarp appears to extend all the way to the crystalline basement surface on most profiles, at which level the failed sediments become detached along a surface or zone of decollement. At or near the base of the outer rim scarp are huge megaslump and megaslide blocks, kilometers long, some of which have been horizontally displaced for short distances toward the crater center. Some of these displaced megablocks have rotated several hundred meters from their original near-horizontal positions (Figs. 4.3, 4.7B). Other megablocks appear to have simply dropped vertically downward , as their basal strata were disrupted by the impact; these blocks display little or no evidence of horizontal displacement (Fig. 4.3B) . The crater outer rim is crossed by seismic profiles at 61 locations (Figs. 3.3, 4.3--4.19; Table 4.1), providing good structural and morphologic control around the full 3600 of the crater circumference, although there are wide gaps between some profiles . General features are similar on each profile, but in detail, morphologic variability is marked . The most extensively imaged part of the outer rim is C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

92

The Primary Crater

Fig. 4.1. Structural map of Chesapeake Bay impact crater constructed from seismic reflection and borehole data. Shaded area represents top of crystalline basement. Boreholes shown encountered either crystalline basement (inside crater; drill depth shown), or Exmore breccia (inside or outside crater), or unconformable surface correlative to Exmore breccia (outside crater). Contour interval 50 m. See CD-ROM.3 for sheet-sized color version.

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Fig. 4.2A. Depth-scaled cross section (derived from composited seismic reflection profiles) through lower part of Potomac River and Chesapeake Bay (A-A '), showi ng location and principal features of Chesapeake Bay primary and secondary impact craters. Exmore brecc ia, displaced megablocks, and postimpact deposits have been removed. See also CD-ROM.6.

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The Primary Crater

Fig. 4.28. Location map for cross section A-A ' shown in Fig. 4.2A.

the section within and directly east of the Rappahannock River. In this area are 21 rim crossings, most of which are diagonal to the rim as seen in map view (Fig. 4.4). These data clearly indicate that the outer rim is not a smooth, circular escarpment , but is broken up into blocky segments (Figs. 4. I, 4.4, 4.5; CD-ROM .3, 4). These segments are roughly 2-4 km on a side, though their precise geometrie s cannot be determined from the spacing of our tracklines . Two elongate narrow blocks form slender promontories that jut 2-3 km inward toward the center of the crater (Figs. 4.1, 4.4; CD-ROM .3, 4). Of particular interest on the northwest segment of the crater rim is a 15-kmlong, 1- to 3-km-wide, canyon-like feature, which extends up the Rappahannock River (Figs. 4.1,4.6,4.7; CD-ROM.3). Here, three subparallel and partly overlapping seismic profiles (SEAX- 12, SEAX-13, lO-RR; Fig. 4.6) show the geometric complexity of the canyon, as the seismic trackline s cross in and out of the canyon in very short lateral distances (0.25- 6 km; average 1.9 km; Fig. 4.7). We infer that the canyon is a collapse zone related to a radial basement fault that appears to extend beneath the Rappahannock River (see Chapter 9 for further discussion of the radial faults).

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Fig. 4.3A. Interpreted segment of seismic reflection profile II-PR crossing northern part of outer rim of Chesapeake Bay impact crater; see Fig. 4.4 for location, CD-ROM .14 for full-scale profile, and Table 3.2 for further information about seismic profile. PS designates base of postimpact sedimentary section (approximate boundary between Exmore breccia and Chickahominy Formation); AB (acoustic basement) designates upper surface of crystalline basement.

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ste ' Fig. 4.38 . Interpreted segment of seismic reflection profile T-9-C B-F crossing northern part of outer rim of Chesapeake Bay impact crater; see Fig. 4.4 for location, CD-ROM. 13 for full-scale profil e, and Table 3.2 for further information. PS designates base of postimpact sedime nts (approximate boundary between Exmore breccia and Chickahominy Formatio n); AB (aco ustic basement) designates upper surface of crystalline basement.

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Fig. 4.5A. Interp reted seg ment of seismic reflection profile SEAX- l0 crossing northern part of outer rim of Chesapeake Bay impact crater. Profile crosses prominent blocks of preimpact sediment, which project out into the crater as part of irregularl y faulted and slumped outer rim. See Fig. 4.4 for precise location and CD-ROM. I0 for full-scale profile; reflection abbreviations as in Fig. 4.3.

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Fig. 4.58. Interpreted seg me nt of seismi c reflection profile SEAX -4 crossing northern part of outer rim of Chesapeake Bay impact cra ter. Profile crosses prominent blocks of preimpact sediments, which project out into the crat er as part of irregularl y fault ed and slumped outer rim. See Fig. 4.4 for precise locat ion and CD- ROM.8 for full-sca le profil e; refl ection abbrev iations as in Fig. 4.3.

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The PrimaryCrater

Table 4. 1. Vertical structural I relief at outer rim of Chesapeake Bay impactcrater.

Seismic Trackline

Shot Point

Relief[m]

Seismic Trackline

Shot Point

Relief[m]

S-I

460

>1200

S-12

597

570

S-2

502

420

S-12

545

530

S-3

137

420

S-12

515

580

S-4

1625

610

S-12

435

470

S-4

1505

540

S-12

220

470

S-4

1450

470

S-13

65

480

S-5

110

700

S-13

240

430

S-5

580

700

S-13

390

480

S-5

768

730

S-13

460

440

S-5

1255

710

S-13

520

450

S-6

155

760

S-16

435

450

S-8

665

660

S-17

050

530

S-9

370

620

S-19

165

>1200

2598

740

S-22

560

>1200 >1200

S-IO S-IO

2501

700

S-22

940

S-IO

2442

700

S-25

455

>1200

S-II

385

570

S-27

455

>1200

S-II

360

600

N-3

S-II

180

570

I-CB

340

630

S-II

148

580

II-PR

440

700

S-II

060

630

9-CB-F

340

700

S-II

030

650

10-RR

1150

550

S-II

005

720

10-RR

1400

520

S-12

870

460

10-RR

1430

450

S-12

830

440

10-RR

1480

430

S-12

755

480

10-RR

1560

470

S-12

750

470

I3-YR

2140

420

S-12

710

500

1-64

4075

300

S-12

635

520

E-3

2430

1250

21940 hrs

420

I Reliefmeasured from sedimentary lip to crystalline floor on seismic reflection profiles 2hrs indicates location on seismic trackline is measured in clock time rather than in shot points

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Fig. 4.6. Detailed tracklin e map, showing shot points for profil es within lower reach of Rappahannock River, which image Rappahannock Canyon. Shaded lines indicate segments of profiles SEAX- 12 (S- 12) and T-IO-RR (l O-RR) shown in Fig. 4.7A,B.

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Fig. 4.7A. Interpreted segment of seismic reflection profile SEAX-12 , which crosses in and out of Rappahannock Canyon . See Fig. 4.6 for precise location; reflection abbreviations as in Fig. 4.3.

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Fig . 4.78. Interpreted segment of seismic reflection profile T-I O-RR, which crosses in and out of Rappahannock Canyon . See Fig. 4.6 for precise location; reflection abbreviations as in Fig. 4.3.

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104

The Primary Crater

The Rappahannock Canyon is a unique feature in our data set; nothing similar has been observed on profiles in other parts of the crater. In contrast, for example, three subparallel profiles in the York River (SEAX-2, SEAX-3 , 13-YR; 0.25 km apart; Figs. 4 .8, 4.9A,B) cross the outer rim of the crater at approximately the same relative position, evidence that this segment of the outer rim is nearl y perpendicular to the river channel. In the James River, three profiles (SEAX- 16, SEAX-17, Neecho-2 ; appro ximately 0.2-1.3 km apart; Figs. 4.10, 4.11) also cross the crater outer rim at the same relative positions. We infer that this segment of the outer rim also is perpendicular to the river channel, and extends southeastward beneath the Norfolk Naval Base and northwestward under the city of Hampton. Poag (1997a) reported that the southeast segment of the crater's outer rim (diagonall y southeast from the Rappahannock Canyon) appeared to have massively collapsed (based on single-channel profile F-I; Fig. 3.3; Table 3.2), which carved out a broad embayment in this segment of the crater rim. One of the new Ewing multichannel profiles (E-3; Figs. 4.12, 4.13) substantiates Poag's original interpretation. On a few seismic profiles, particularly to the north, west, and southeast, mass failure of the outer rim is limited to the upper few tens of meters of the preimpact sedimentary section. This shallow mass failure has formed narrow (0.5-4 km) terraces concentric to the outer rim in these areas (Figs. 4.3A ,B, 4.4) . The outer edge s of the terraces are marked by auxiliary normal faults approximately concentric to the primary rim fault. Auxiliary concentric normal faults also are present in some location s where the preimpact sediments displa y only incipient disruption. The most notable example of this type of fault parallels the rim in the crater's southeast quadrant (Fig . 4.1 ; CD-ROM.3). These auxiliary concentric faults indicate that the impact shock and subsequent water-column collapse were strong enough to initiate additional sedimentary failure outside the crater rim, but that failure was incomplete, resulting in only minor vertical offsets. Evidence of auxiliary concentric faults at nearl y every rim crossing suggests that these feature s probably encircle the entire crater rim . Structural relief at the crater's outer rim, as measured from the sedim entary lip of the crater to the crater floor (crystalline basement surface) varies from a minimum - 300-420 m on the updip (west) side (profiles along Interstate Highway 64 and in the Rappahannock, York, and James Rivers) to a measured maximum of - 760 m on the northern rim (profile S-6; Table 4.1). Extrapolated maximum relief of - 1,200 m occurs on the downdip (east) side where the basement is too deep to have been imaged by our single-channel seismic data. An important and diagnostic feature of the outer rim is the marked thickening and structural sagging that takes place within the postimpact sedimentary units, as they cross into the crater (Fig. 4.14; Table 4.2). This cross-rim thickening varies from 80 m beneath the York River (profile S-3 ; Fig. 4.9A) to 190 m near the mouth of the Rappahannock River (profile S-13; Fig. 4.6) . Average increa se in postimpact sediment thickness at the rim is 119 m. This represents an into-thecrater thickness increase of 24-130 percent; average increa se is 59 percent (Table 4 .2). This change takes place within a lateral distance of 0.7-10 km, averaging 3.0 km.

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_--L~'0L

seve'" RiVer 80

_,~.~, / , ..."

_

I

zo

•

234

~

,#

"

I

76°15'

", ",

. . .........

- 37°15'

~~\Z~~~'<

~\l ,~

.}

N

C~~S~?

<5'~

:r~C'+

~6 '

Fig. 4.8. Detailed trackline map, showing shot points for profiles within lower reach of York River. Shaded lines designate segments of profiles SEAX-3 (S-3) and 13-YR shown in Fig. 4.9A,B. Reflection abbreviations as in Fig. 4.3.

\

,...

/...

/ f",()~

~c.

~o~~11

11,

I I I

oVl

~

o

-a::r'

o

~

'"0::3

~

2'

n

a-

C/l

~

;;l

o

200

250

300

I

9

E

Fig. 4.9A. Interpreted segment of seism ic reflection profile SEAX-3 (S-3) crossing outer rim of Chesapeake Bay impact crater within the lower reach of York River. See Fig. 4.8 for precise location. Reflection abbreviations as in Fig. 4.3.

km

I

I

3 6

I

Vertical Exaggeration =-5:1

Shot Points

0

Basement Downwarp

Outer Rim

'"

~

n

]

:::J .

~

0'\

o

2250

I

I '

w

I f ' : ' J ___ !

Shot Points

,

1950

,

1900

_

'lhil.t

......... .,. ....

13-YR_

1850 E

Displaced Megablocks

Fig. 4.9B. Interpreted segment of seismic reflection profile 13-YR crossing outer rim of Chesapeake Bay impact crater within the lower reach of York River. See Fig. 4.8 for precise location. Reflection abbreviations as in Fig. 4.3.

Cl> 02 . ,'In ·,~._~=~. .5 .>. ~.!:i. = ~t"" ,, _ _... '~ ""'i~~~ . ,f!:.~

~

0.0

~

n

o

--.J

~

o

i

~

0-

~

~

'2"'

2"

tr:

....

(;

...'Ii

"

\

.....

•

\

\

•

.-.

-r

t!!Ei:irJ

'"

I

kin

10

~

1?

'i>",,

<;'G> .>

o

. '

»:::

76'15'

•

.-

--' 20 I

500

400

300

;;;l

.

"

...

n

l

~

-e

l\~

~I

• ~ - !-

~:. ~/

o

00

N

~~otttt·

• • • • •• _

,,- -

§ ~~

1.1.0

';

r -,

A

1

•• •• , • . NORFOLK

" "

- t

11"':rm· ttt1 '" ' "

---

156

•

~~ !4 ." 102 ,

~ ~i'lef

6 3 C"

Hampto n Roads

101

-,

... . . 0

lOS'

98 ;'-. \

. " "

_ _

/

99

•

\

.'1~·.~

.

\

97 ...

"

.-.

\

I

I ~'

1 .'?

_ .'

r-

. :'/.:

113

"A' =:'32 '"tMt' ~

-.

y" •

NASA)'\..:

, ._ ~.

Fig. 4.10. Detailed trackline map, showing shot points for profiles within lower reach of James River (Hampton Roads) and along Interstate Highway 64 (I-64) , and trackline for USGS-NL from NASA Langley corehole to James River. Shaded line indicates segment of profile SEAX-16 (S16) shown in Fig. 4.11 . Numbered solid black dots represent boreholes (see Table 1.2 and CD-ROM. I) .

76~30'

I

i ~

~ ~"e ?

(!:l ....

~i.

IS'

'?(!:l

~

/%

.:

••••\

~

1;

O"?.>-

~..e

~

\

•

•• • •

1.2

1.0

0.8

'

Basement Downwarp

I

AB-r::-.•. • - '-"

0.6

"

~_ . . ....... .;~.;.:~~::;~'!.~ . ~:.1_-_ !. !'". ;_ ..J';

:-"'~ ~.

+-~7"",",-mv~~1'~~.l...~A' ~'~L _ ..._~

..

'O

o!

. ,I•. ,f .....

- , r,

. . . ...

• } o·.·M ..

::'C ·.... .. · 'a· ,·..

d'

.

'*

~

4' ....

. _ ~ _,

'- - - -

i~~:~:~ ~ ~ ~. ~:.~:t·~~: ~~~:2;~::~=;~: __

•: . ", • •

-.

:.::~ .. < •

"'-

400

0:1):

~

. _-

!:, ,~._ ~'.~!""_~~':'I

200

.... ""~--:' 1z' "",lI;' ·,"' ·1Jo;- 3i·#J'4' ~ir"

-__. -!!~_

Fort Monroe 300 Borehole

NE

.;;.-','"" ",',

, 8

.'

,.:--;......"

J ,

."

'j

~.~::;:~.

, >~ ·t

-

Chickahominy . -"

.-,:;:;;;~ -:-_:~

•

100 _

Fig. 4.11. Interpreted segment of seismic reflection profile SEAX-16 (S-16) crossing outer rim of Chesapeake Bay impact crater in lower reach of James River (Hampton Roads) . See Fig. 4.10 for precise location and CD-ROM.8 for full-scale profile . Reflection abbreviations as in Fig. 4.3.

N

I

s

co

>-

g

>

:f: OJ

E

OJ

~

~

0 .2

M

SW Outer Rim

~

~

'-0

o

~

S

-a

o

::l 0-

$lO

~

(')

2

C/'J.

o

110

The Primary Crater

76'00' I

75'00' I

Fig. 4.12. Detailed trackline map, showing shot points for seismic reflection profiles crossing outer rim of Chesapeake Bay crater on inner continental shelf and in mouth of Chesapeake Bay. Shaded lines indicate segments of profiles shown in Fig. 4.13 and Figs. 4.154.19. Solid black dots represent boreholes (see Table 1.2 and CD-ROM . I).

2300

~""'-:-~' :~.~_-"'~

I

o

Outer Rim

~ _ _-"'"" ....:.~.••!..;':....~. __ ....,-.- ..... •J~.....

km

~~~••~,-~;,;:~;,::.=._~~ :::.:~ ~~ ~~,

SIloI PO,"IS

_...iI,~_ - .........~••.~-;. .~~ ~ .;,~~..

2200

I

13

_. ,~~ "::';;"):'e,,'" ·-·1 .".;I. • •=-=..",.~ ~~l':"I ::;~,~_ _~~ "' i:,,_~"!"""1 .. ~ __~._ ~ . ~

Z:""7:' :.-".:: r ~~:~.~.:~~~:!

2600 SE

Fig . 4.13. Interp reted segment of seismic reflection profile E-3 (Ewing-3) crossing outer rim of Chesapeake Bay impact crater on inner continental shelf adjacent to mouth of Chesapeake Bay. See Fig. 4.12 for precise location and CD-ROM. 16 for full-sca le profil e Reflection abbrevia tions as in Fig. 4.3.

18

18

1.4

':": 10 N

>-

~

~ 0 .8

~

4l

~

~ 08

0.4

02

NW

~

-a 5

o

~

::l 0-

III

n

a~

[/j

...,
~

n

0.2

Q)

0.8

--..,:

~;=f~~-

f:;§

Outer Rim

100

Shot Points

.,_ .1,

--~i~~

__

N 000

Preimpact Sediments

Fig. 4.14. Interpreted segment of seismic reflection profile SEAX-6 (S-6) crossing northern part of outer rim of crater near western shore of Delmarva Peninsula. See Fig. 4.4 for precise location and CD-ROM.9 for full-scale profile. Reflection abbreviations as in Fig. 4.3.

A~~:~~

N

f

~

~

,§

0.6

~0 .4

.e

Exmore Corehole (12.5 km E)200

O'0l§ >==-;; _.:.; : : :! , . ,. .,!.;.;_= , ~ IJ~-~~

S

~

n

]

:¥.

;l o

N

0.20

~ 0 30

I

o

600

Outer Rim Shot Points

km

Vertical Exaggeration = -9:1

500

400

I

8

NE

Fig. 4.15. Interpreted segment of seismic reflection profile SEAX- I (S- I) crossing outer rim in eastern sector of Chesapeake Bay impact crater. See Fig. 4. 12 for precise location . Reflection abbreviations as in Fig. 4.3.

0.70

PS

060

~ N Oso

ee

>0-

~

~ 0.40

,§

Q)

s:

SW

0.10

000

w

--

~

g

-a

3:: o

=' 0-

~

~

2"n

VJ

0....

o

;;l

0.9

NO

NO

1.3

70

?

70

NONE

T-IO-RR

S-5

S-6

S-8

240

280

230

200

36 42

100 140

380

380

110

50 48

100

340

130

300

190

340

150

1.3

60

S-13

88

140

NO

30

S-12

100 85

ISO

67

85

62

91

Percent Thickness Increase [m]

110

300

240

130

NO

100

110

80

160

300

150

NO

?

N-3

50

S-17

250

150

50

S- 16

210

100

210

NO

NO

40

T-13-YR

130

Total Thickness Increase [m]

Postimpact Sediment Thickn ess Inside [m]

240

0.8

40

S-3

11 0

Postimpact Sediment Thicknes s Outsid e [m]

130

NO

40

S-2

Rim Terrace [km]

Basement Sag [m]

Profile # I

10.0

2.7

1.0

1.6

0.8

3.4

1.7

3.2

3.2

3.0

2.5

2.5

Lateral Distance [km]

YES

YES

YES

YES

YES

YES

YES

YES

YES

YES

YES

?

' Concentric Fault

10

NO

NO

NO

NO

NO

NO

NO

NO

NO

NO

NO

Raised Rim [m]

Table 4.2. Morphom etric data for structural and depositional features of outer rim of Chesapeake Bay impact crater, as determined from seismic profile s .

<1>

*

.n., ..,

~

::I.

-e S Il>

-'l :r

:i:

1

1

NO

40

NONE

50

?

?

?

?

?

46

S-10

T-I -CB

S-II

S-19

S-22

S-25

S-27

S-I

Avera ge

234

390

380

380

420

410

190

200

190

290

352

550

470

540

530

540

290

300

YES YES

2.0 6.0 1.5 6.4

26 42 24

110

119

160

90

160

41 59

3.0

YES

YES

YES

YES

NO

YES

YES

3.5

32

130 YES

NO

NO

0.7

53

100

2.2

NO

50

100

63

?

NO

YES

YES

4.0 ?

120

310

31

1.5

90

380

Indicates lateral distance between thinner postimpaet section outside rim and thicker postimp act section inside crater, Indicates presenc e of a concentric normal fault or faults outside the rim scarp.

1.0

?

?

?

?

?

NO

NO

0.7

10

S-9

Table 4.2. (co nt.)

~

Vl

~

o

-a0-

o

3:

0..

:3

~

2'
n

2

[/)

o

,

o

Outer Rim Shot Points

.. 0

km

Vertical Exaggeration =-7 :1

400

•• "

;;.:} :.~

...

..

SE

,

8

L' . ~ " ~ :-_-. ~

Raised Rim

:";~~ ~::'."':". _ '~

500

Fig. 4.16. Interpreted segment of seismic reflection profile SEAX-27 (S-2 7) crossing outer rim in eastern sector of Che sapeake Bay impact crater. See Fig. 4.12 for precise location . Reflection abbrevi ations as in Fig. 4.3.

~ N

>-

~

>

Qj

~

Q)

~

300

~

o

Q

~

a

::1.

-e

;l

0\

0.4

1.0

AS

0.6

L;

I

o

I

1 I

2 I

3 km

I

4

Vertical Exaggeration

700 Outer Rim

" -,.~ . ~ . "

::::;;::- :: :::::"i "I·~:~'7=7:..~ ..:._. ___.....

..

~ =- --_._

.2.;;:%1 ... :t~

Shot Points

~=~i£:.g~;;~~·~~~~~~g~~~;:~;;,

600

=-4:1

I

5

600

I

6

I

Basement Downwarp?

N

Fig. 4.17. Interpreted segment of seismic reflection profile SEAX-8 (S-8) crossing outer rim in southeastern sector of Chesapeake Bay impact crater. See Fig. 4.12 for precise location and CD-ROM .9 for full-scale profile . Reflection abbreviations as in Fig. 4.3.

N

ra ~

>-

~ 0.6

Qi

,§

Q)

~

0.2

0.0 •

s

~

-.J

~

:r o

.a

z

c..

§

I

C/J

n

::;.:

- ::

~~::~~::~.

-. . ~ .

r -: _

-_. ~.-

_---_......._.... .......

,..

soo

..

_. ~

;;;::;=---~~-,... ....." .•...

. ..

- -_

...... .... _. ..- .. .

_

-

........

- '-~- .

.•._-

._

- ' ': _ ~

Outer Rim

~~_ ... .-_ . _... ...-

""-'--

..

-- - l"'l".' "': _

600

-

",

.. _'';'''

...........-

_._~

~

. :....: ~_. .:.:." .- ...:.:;::== ...

_ _ .. :'Io_ • .,. _

.. ... _ .. ' e-.--.,.

j~~·: ~;i~~~

..

."--_...'. -:~ :.>. ~~~~~~:.~:~::~

I

_

SE

..:"-~:::=. SEAX - 25=

. .. . .. JIo&..

- "'-:. _ 1' ;. ~ ~.:::.~

• -.. ;"~ .. - -••,. '"'!>.: .•-

.., ..

400

~~:;~~;~ -:'S-. ~~~.: u ··· .~ ~.

_ - :. :.: .

.'. .... _.

...

~ - ' - . - --- ~-~. -

Shot Points

Fig. 4.18. Interpreted segment of seismic reflection profile SEAX-25 (S-25) crossing outer rim in eastern sector of Chesapeake Bay impact crater. See Fig. 4.12 for precise location . Reflect ion abbreviations as in Fig. 4.3.

N

~

>-

~

>

Qi

,§

C1l

:§:

O.l C

NW

~

()

1

oJ "'C

."

00

600

j

..: .:..""" -:.,

I

o

.:=:'; r: ::: : . .:: .1': ..=:.1 - - _ _

. I ;~~- ~~

I

Raised Outer Rim

500

,...~

<

'... . .

~-;;

---

--==

S 22

-~-:---..

1000

.,,-- .;) C fV\ -

I

Raised Outer Rim

9?0 -- _.

~..;,....- . . - · -.:.-f ~"': ·- c:

Shot Po ints

km

Vertical Exaggeration = -13:1

I

13

~Chickahominv ...· ~""!'• . .;..:. ~,~;:'''';';' ~-~

----

800

Fig. 4.19. Interpreted segment of seism ic reflect ion profile SEAX-22 (S-22) cros sing outer rim in eastern sector of Chesapeake Bay impact crater. See Fig. 4.12 for precise location. Reflect ion abbreviations as in Fig. 4.3.

0 .70

0.60

0.50

0.20

N

\0

--

~

::r o

!

5-

~

~

(')

2'

(/)

~

o

120

The Primary Crater

The point at which sagging and thickening takes place provides a means to recognize the outer rim where other sedimentary features are difficult to discern on seismic profiles and where basement reflections are not visible. The eastern sector of the crater is a good example of the latter, and we have used the saggingthickening relationship to identify the position of the outer rim in that part of the crater (Figs. 4.12, 4.14-4.19). In several places, the crater's outer rim also is manifested on the surface of the crystalline basement. We have relatively closely spaced seismic control on the basement structure and morphology at the outer rim covering 2300 (about 2/3) of the crater circumference - essentially the western and southern sectors. On most seismic profiles that cross the outer rim, the basement surface is offset by a marked downwarp into the crater and by extensive normal faulting. The lateral position of the downwarp varies from approximately 1 km outside the sedimentary rim (SEAX- I6 in the James River; Fig. 4. I I) to directly at the toe of the outer-rim escarpment (SEAX-3 in the York River; Fig. 4.9A), to ~ 1 km inside the crater rim (SEAX-8 ; Fig. 4.17). The relief across the downwarp varies from 10m (SEAX-9) to 70 m (T-1O-RR), and averages 48 m (Table 4.2). Six profiles on the eastern side of the crater did not image the basement surface (Figs. 4.15, 4.16, 4.18, 4.19). 4.1.1.2 Annular Trough

The floor of the annular trough laterally separates the toe of the outer rim scarp from the elevated outer flank of the peak ring (Fig. 4.2). As seen on most multichannel profiles in the western half of the crater, the floor of the trough is formed by a broad, wrinkled , faulted, eastward-dipping surface of crystalline basement. The width of the annular trough is highly variable (15-28 km), and averages -21.4 km (Figs. 4.1, 4.2; CD-ROM .3, 5; Table 4.3). Extensional features dominate the floor of the trough, which is riddled with small-offset «25 m) normal faults (Figs. 4.1, 4.13, 4.14), several normal faults with >25-m offsets (see Chapter 9), a few reverse faults, three principal radial fault systems, diverse linear depressions, and scattered concentric, low-relief, sometimes faulted, compression ridges (Figs. 4.7, 4.11, 4.20; Table 4.4). Many of the normal fault blocks form small grabens and horsts in the basement surface . The most prominent compression ridges are elevated 30-80 m above the floor of the annular trough, and are 0.9-3.5 km wide (Figs. 4.11, 4.20). Some compression ridges also are present beneath the outer rim (Fig. 4.7) and beneath the Rappahannock Canyon (Figs. 4.7,4.20). 4.1.1.3 Peak Ring

The boundary between the annular trough and the peak ring is defined by a point at which the crystalline basement surface begins to rise gently in a radial direction toward the crater center. The surface gradually reaches an apex at the crest of an irregular, fractured, blocky ridge, which encircles the deep inner basin of the crater (Figs. 1.5,4.1,4.2,4.21-29; CD-ROM.3-6). This subcircular ridge, or peak

1.0

0.8

0.6

~

;~

~~

E

...- .....---. ~' ~ __.1.X-1 2 ':'-:-

~-r ..;. , ~ ~---:=."

..~AB

l!~~-::::I: ~:fi'rc·~;:· :-::r····"':·:;7'i' !:I~·:~,,~~·:f':: E:::~~pS

.••_......, , ~ : :.:~-" I~'":L.,- ..: _ _ ~ "":

700

_

Fig. 4.20A. Interpreted segment of seismic reflection profile SEAX-12 (S-12), showing compression ridge on basement surface in annular trough of Chesapeake Bay impact crater. See Fig. 4.6 for precise location. Reflection abbreviations as in Fig. 4.3.

N

~

>.

~

~

.§

-; 0.4

li)

0.2

"~ . ~

Shot Points

800 900 0.0.:_=.!3~::mti~~;;~:

W

--..:..-

Rappahannock Canyon - -

~

I N

~

:r o

.a

~

o,

~

~ ~

o:

n

=...

1.0

0.8

o

I

,- ~ . ...

....,.•...

I

.• :"

_

..

.. _

. - , .. , ..... ; . .

_

:.=. ..!

. .... '

-

"_,,

,

',,,-

verucai exaqqerauon

km

= -4:1

Compression Ridge

... ,-

,.

'j .

~-..:....

II .

._

'·h·'

,_:: .:'

~

'.

'-.. .~.:. ;:*·~~ ·0§·!:J,=-·-+2G=; '-"'0;

... : : . . . . . ; "., •• ' .., ;~

, . . . . . . . . .,.....

§§

==~~~~~#£..i..

~~ "

::::=:r:;:;-=~:~. :>~;:.:=

~: .~~~\-~ .~: !. _ _:.

X

N

,!,:j·::·t·"-·-"~:-':'-:i;";:-"-':;';;;;:;;;;:;~··· - ""'- _•• ' ''' ''~~l;..=~~:;;::r~

_ _ _ ' .. _ .

1_"

I

..

300

I

7

AS

Fig . 4.208. Interpreted segment of seismic reflect ion profile SEAX-4 (S-4), showing compression ridge on basement surface in annular trough of Chesapeake Bay impact crater. See Fig. 4.21 for precise location and CD-ROM .8 for fulI-scale profile . Reflection abbreviations as in Fig. 4.3.

N

~ 0.6

>-

~

a;

.§

T

Shot Points 400

. . ._. _.. == :

-:==:1

0.21' :

'; 0.4

lil

500

o.OM ::2-",:'.:

S

~

~ o

3

::1 .

"'t:I

;l

tv tv

Crater Structureand Morphology

123

Table 4.3. Morphometric data for annular trough of Chesapeake Bay impact crater. Profile #

Width [km]

Shot Point [outer edge]

Shot Point [inner edge]

T-I-CB

27.5

2650

1550

S-4

28

1635

805

S-6

22

145

880

S-8

15

668

218

S-IO

20

2590

1800

S-14

21

- 800

640

S-3 (dogleg)

16

135

720

T-13-YR (dogleg)

16

2125

1375

S-16/S-14A Composite(dogleg)

24

435

55

E-3/S-8 Composite

24

668

1261

Average

2 \.4

Table 4.4. Morphometric data for compression ridges in the crystalline basement surface of the Chesapeake Bay impact crater. Profile #

Shot Point

Structural Relief [m]

Width [km]

Position

S-4A

360

40

0.9

Annular trough

S-12

810

50

1.8

RappahannockCanyon

S-13

820

40

1.9

Annular trough

S-16

280

30

1.5

Annular trough

S-15

825

70

1.2

Annular trough

1200

50

1.8

Outer rim

150

80

3.5

Annular trough

51

1.8

T-IO-RR E-l Average

124

The Primary Crater

••••••••••• $ubpeaks on Cenlml Peak <1000 m deep • - - - - - Cenlml Pea -1600 m oeep ____ Inner Rim of Peak Ring

- 37' 30'

2000

- - - Outer Rom of Peak Rong

-

,

. ..

N

JII?

...

I

•

234

/

-?,

- 37' 15'

59 .

- ..... _-", •

156 200

o I

10

20

I

I

km

l 76'15'

I 76"'00'

.300

Fig. 4.21. Detailed map, showing shot points of seismic reflection profiles crossing peak ring, inner basin, and central peak of Chesapeake Bay impact crater. Shaded lines indicate segments of profiles shown in Figs. 4.22--4.30 and 4.32--4.34. See CD-ROM.2 for more details of seismic tracklines.

.

\.If'''

12

10

o,

_.

-

u.

"

-9,$. .

r

.•

\l

.,..

_&

600

,

..~

km

Vertical Exaggeration

n __ I ...

JIl '"

.&,.

=-5:1

.. - ------.

~

~,,*.

(record not avaitabln)

$V

l' --- ~

... _.. ._---.:;;r- -.

~

08

.-

Shot POints

.....;.V ....- ....,....'lIIII:~-

500 N

T-8-S-CB-E

700

I

10

Displaced Crystalline Megablocks

Fig. 4.22. Interpreted segment of seismic reflecti on profile T-8-S-CB- E cross ing peak ring in southern sector of Che sapeake Bay impact crater. See Fig. 4.21 for precise location and CD-ROM .12 for full- scale profil e. Reflection abbreviations as in Fig. 4.3.

AS

N

~

ro

>-

~ 06

>

Qj

~

Ql

~

0.4

02

s

~

Vl

IV

~

o

-a:::-

o

~

0..

:::

~

n

2' ~

[/)

o

.

0.40

.

100

:. ~'\,,~.OO:-_'\"2,...

1930

• • __ '

.

"• •

Hours

... ,~.,. ,'.i-~ .~'" --t

1845

-1_1- -

~~

E

-.....

....

~I ¥I'.

·/L'· .... w ~ ~ .,..,~I1:.""' ." .

C

-... : . ~, ... -~

,

,"'.;YO.,

--i....

' ~I

,,.,i:"v , . i· ~ "' V" ::\ ~. .

' · ~; t t~1

wt\"r">,:" ..~~~h ",;,;;,: " '.. . :. ,v; :"': ~ ~;'~ " '~~\ I~ ...:::'t;';(n..t-'~" . \It'~(, =' -Wb X '" " 'lo~\r: ....,.~~':JIr:::;.,~~,",. J.l\~~,

" . , ~ ;....; ;' -t. ".""'~""':.~";.-X" ~ -.:-' '',,~ ''. ''''''':-' '''j

tI.,

.~: ~

.... .. . .... ,'i- ~~~ ...~ .:. "~' ., ~..~~:. ... . AS ~'~.::;C;~ ~'~" "'".:'-~""""""_".:"",_~ ~~~~,'~:\:~~,:,:,,,':':"~ '" .. ~. :. ' ...~ ..-....:."", ~~ ••~~_ ._~ ~,,;;

.........

-: ' - - -r;:e.J ~

1830

~~-- k,..;..~~ ~'[T'-r;::::.._.:~~~.~~~~.~~~~:.......~~I;•.\t~:~.~?;!.~~~~~ ~.~ ~~~~~~ _...:,... w_-·;.- ••_ _ ..:';'~ ""'.="":.. ~.....~ ~-"' ,. , .'-:~ ' .";'""":·,~""':':~l l.'~:-: ;' ..,,:\,,.:-, - . ~ ..-;"\~ "' " ...

--- -.

1900

-::~;;;::~:~~.~~~~=~~~--e: ;.~u:' _

1915

.,":'~':':: ~'~~~, ::.w~~. . :-~~~,;;;=-~,,~~~,\~

0"

...

_

. ,,~.:~~. "~~:~...;t&Zr~~..

W

F ig. 4.23A. Interpreted segment of seismic reflection profile Neecho- I (N-I) crossing peak ring in western sector of Chesapeake Bay impact crater. See Fig. 4.2 I for precise location. Reflection abbreviations as in Fig. 4.3.

N

~

ro

>- 060

~

>

Q)

,§

OJ

:§:

0.20

1945

...~

n

Q

g

:I .

'"t:l

<1l

;J

tv

0\

I

o

..

200

.

_-

-.--

300

N

~-=-=-

~_

km

Vertica l Exaggeration = -4 :1

-_--~

_.

I

7

,, ~. - -~~~t_::[~

Shot Points

·!'"·~:'~ ~ " --""·"': ' ''' ··'' · ·:;:' - ·:i!~ ~·:

100

See Fig. 4.21 for precise location and CD-ROM .8 for full-scale profile. Reflection abbreviations as in Fig. 4.3.

Fig. 4.23B. Interpreted segment of seismic reflection profile SEAX-4 (S-3) crossing peak ring in western sector of Chesapeake Bay impact crater.

AS

N

~ 0.6

>.

g

>

Qj

§ 0.4

:§:

s

tv -.J

~

i

z

c..

§

()

2' ~

[/J.

'"'

~

n

800

.. ::0;;:'

..-•.;.., ,,,.....~:;;:s...~

I

o

1 .0....L.,....;...~.. ~~~

I i··

0 .8 -f .::~,·

km

Vertical Exaggeration

=-4:1

::~~:.-.' ::::--~-:::::~'-;::-

Shot Points

"' ; :;;

N

I

7

F ig. 4.24A. Interpreted segment of seismic reflection profiles SEAX-7 and 8 (part of S- I) cross ing peak ring in southern sector of Chesap eake Bay crater. See Fig. 4.2 I for precise location and CD-ROM.9 for full-scale profil e. Reflection abbreviations as in Fig. 4.3.

AS

100

0.2 4"=\"..,,'t':..I·--"'......··~ :::~· ,_ ···: ..:1~ , ;' , '.;'·;:'ij:;;~,::;;:~~~

S

(12 km E) 700

Kiplopeke Corehole

~

iil

(")

~

3

::l .

""C

<1l

;J

tv

00

1.2

1.0

0.8

I

o

~'~~"~'

-~ ..

«

- _:.

._'

-',an

~~,

•

I

,

600

.. • ...."

'

,~

_._~.~~---.~.,:-~

km

Vertical Exaggeration = - 4:1

~~;"~ ~ ;.-:-::::.. :-f _

Shot Points

-. ;r -c, ;:'. ~'~

500

b

.

700

~'~ I' ::~~~~~~:~

.

800

I

10

N

Fig. 4.248. Interpreted segment of seismic reflection profile SEAX-IO (S-2) crossing peak ring in southern sector of Chesapeake Bay crater. See Fig. 4.21 for preci se location and CD-ROM . I0 for full-scale profile. Reflection abbreviations as in Fig. 4.3.

AS

s

~

\0

tv

-

~

g-

~

~

0-

~ §

()

2

VJ

n

06

;"'' ' :,...-:r7.. . "': ."

-~-,

i ..--·~··.,

..

I

....

. ...

......~·i ~ ........ ~

f km

Vertical Exaggeration

=-5:1

(record not available)

I

10

AS

N 2000 I

-,-:'...~.,.,..ri·- ~lt ............ _ ,.."

1900

&.. ...-....~•.'"~"C...-..!'_V'!'4~~. ..,.rl.~-..,.,'b:,...,,..iAi

' ~ --: ":~ .

,.1~"'~: }!...-.:

Shot Points

. " .... ~iift.~.• · , ~ I'~~- .. ,_ · ••• '-:;;;.;~rlfl!!~.~~; ~~"r~"'- ' ;~~ ~ '~~:.:" ~~_ ~ ~ '~i.lo ,. ~. r~ .... - ; '.J,4 , l '''','''' 1'1!r:' '_ ,1 vr: ";r:~* ~",."'.'t. \.'1"'" - ',:ii. .,. , ••• ~~ ;I '~ _ :,,', ,. "t'l .~ ._..; :; ....~ .....V"'...IIIIr1' ''' · ,....... ... :- . .. .1..:1._,........ • • " , , \ ",'-1-/ ...1.• ..- - 1.':..... » :» ' . . ., . ",~"""" . .. .. . . ,;I ' Exmore brecclak '.''''''', .",:" .,..,.... , .."..._~........ , ... ~°'1 ....~:-~.~~;.;.~.;;:: .;t..... I c;;:;u. " .01>__ 11 l ,w'I "- .~ . ~ ~ ~.. .... 6 --it;

~

I

I

J~':~;'-,~1r-:~~~~2'1 ~

I

1600

1700

PS

See Fig. 4.2 1 for precise location. and CD-ROM .12 for full-scale profile. Reflection abbreviations as in Fig. 4.3.

Fig. 4.25A. Inter preted segme nt of seismic reflection profile T-8- S-CB-E cro ssing peak ring in northern sector of Chesapeake Bay impact crater.

Displaced Crystalline Megablocks

>

a;

,§

Q)

~

00

S

I

I

t.;J

<1l

.n., III ..,~

~

:J.

-e 3 III

-1 :r

0

1500

I

o

,.o(~~, r

'"';''-::'d>

I

I

" :" ; "':

km

. .. __

~....

=-7:1

.~W~N

Shot Points

Vertical Exaggeration

~ ~\}.l d..-~ ~ .

I " ~'I '

1600

-Ao'

..... . . .

~~ -

N

10 I

'4?:::=~~

_.~::::::::::I;;:;;.;;::;:;;:::;;;;;;:;::

W...,J:.. ,. ·--

- - _...~.. .

1700

AS

Fig. 4.258. Interpreted segment of seismic reflection profile SEAX- IO (S- I0) crossi ng peak ring in northern sector of Chesapeake Bay impact crater. See Fig. 4.2 1 for precise location and CD-ROM . I0 for full-scale profile. Reflection abbreviations as in Fig. 4.3.

1.2

1.0

0.2

s

I

~

W

~

=o

iI

zo

::l 0.

'"

2' til

n

2'

en

o

'

02

2.0

1.8

1.8

14

,nn 'j-

I

o

?~

--

''I!". ,~:-'.

-,.!..._ !Y:. -

~ ..,, - ~-

I

-

~L

500 -

-

I

•

700

~.

Vertical Exaggeration

••

Chickahominy

_....-.. - ,

.

~;_-, '

, "-0 ' _

M I

20 km

!!

•

Ewing-2

11.00 SE

'. ,__.

,

NE

so-rom

":t'tf"""--;:. :;...;~ ,~_; ...J"~.~~:'~;':''''' " ~-, -'t., ...(: .L-:;'"'_-- ''>-_~. _

~""~~J. ~~~ ...;_'..:,~.~-.~"",~•.'__~~~

= -13:1

\0

'.

,

•

800

-:-.,"- ._.....-:0.---.

.. ..

I

600 ---

- .. ~._::r'"I.-': _ •• -~~-_.___

-==

4nn

Sho t Poonls '\M

-,-

N 18' Turn 15' Turn

40 ,

30 ,

,~~~~-_. ~'r; ~~- = ~ ~~ ~~ ~-~~.~

••. _ . .~.

5

NW 15' Turn

Fig. 4.26A. Interpreted segme nt of seismic reflection profil e Ewing-2 (E-2) crossing peak ring in northern sector of Chesapeake Bay impact crater. See Fig. 4.21 for precise location and CD-ROM.15 for full-scale profile. M = mult iple; other refl ection abbreviations as in Fig. 4.3.

N

:t

lO'M

1,2

I

00

1

N 30· Turn

'"

~..,

('J

~

"::J• -e a

;J

w

tv

Ql

1.2

----- - ~ .

.

2200

2

• 2500 ,

2400 I

..

- ~ - . . .

12 I

18 ,

24 ,

t3

::r

.a

3:

& en 2' n c@ ::3 '" c..

n

w w

~

-

=-10:1

W

Fig. 4.268. Interpreted segment of seismic reflection profil e Ewing-T (E-2) cross ing peak ring in southern sector of Chesa peake Bay impact crater. See Fig. 4.2 1 for precise location and CD· ROM. IS for full-scale profil e. Reflection abbrev iations as in Fig. 4.3.

6,

Vertical Exaggeration

.'

o 0-

,

o

2800

I

EWJng-2 '~7':~'~ -"'''-..-.-:: -

2700

-· :3~~ :J'~-;'" ~

I

2600

+

N 90' Turn

km

2.0

18

1.6

Displaced Crystalline ~" Megablocks

N

.

Shot Points

2100

'~4;"-" '~ . ·, , :;"~~d_~- :_ ~'::* . _ - .~-~ jJ~ "'1

fO

~

:al> 0 8

E

2000

02~ "~t~·~~~:-:-2±%l~tJ~~~I~ -...;,....-

SW

25' Turn

0

I

2.0

o

10

20 .

Subparallel crossings of same subpeak 30 40 . 50 t km

60

70

80 I

90

I

N r_ . ~ u ~

Bay impact crater. See Fig. 4.21 for precise location, Fig. 4.26A,B for navigation details, and CD-ROM. IS for full-scale profile. Reflection abbreviations as in Fig. 4.3.

Fig. 4.27. Interpreted segment of seismic reflection profile Ewing-2 (E-2) and corresponding gravity profile crossing inner basin of Chesapeake

N

~

>.

~

~1 > .0~1.w: --- . •, .

E

Q)

~

0.0

-2 0

a::

.~

- 15

•

-5 -10

-15

s

Continuous Marine Gravity Profile

o

iii Q; - 2 0

Q)

oS! - 5 en~ - 1 0

~E

~

n

Q

8

~

;l

~

w

2.00

180

180

eoo

\ 1_,-, : __ 1

•.

.:__

700 SlloI"-s 800

...... ....

-"'--.~~~~- -~ ~

900

1000

,..~

0 -.. ,~.

Ewing-3

1100

N

~

~.r:;;r4·W·'i,;jJ;S:;:\~~&\~Jt.i\;>·~"c.~;;\':;.4,3;~;;4t'~'i·~·-·~""'{J)·." ? : (~::-:·r-;'\j4::':~·"J~~~~""'r·~1~.:: "/~·"

l "0 j ;;'~·'~'~

500

ment of peak ring juts out into inner basin from western wall. See Fig. 4.21 for precise location and CD-ROM .16 for full-scale profil e. Reflection abbr eviations as in Fig. 4.3 .

Fig. 4.28. Interpreted segment of seismic reflection profile Ewing -3 (E-3) crossing inner basin of Chesapeake Bay impact crater. A Narr ow seg-

N

~

>, 1 20

0 .20

400

<:l ....

Vl

w

~

go

.:a

zo

c..

:l

~

@

2'

n

a-

[/)

r;

n

1 60

1 40

1.20

1.00

0

0600

km

Shot POInts

Displaced Crystall ine Megablocks

Vertical Exag geration = - 4:1

(record not available)

0500

0800

D900

1000

1100

T-8-S-CB-E

1200

Fig. 4.29A. Interpreted southern segment of seismic reflection profile T-8-S-CB-E cross ing peak ring, inner basin, and central peak of Chesapeake Bay impact crater. Arrows indicate strong (high-amplitude) reflections within Exmore breccia, which may represent megablock clasts or melt bodies. See Fig. 4.21 for precise location and CD-ROM .12 for full-scale profile . Reflection abbreviations as in Fig. 4.3.

N

'f

CO

>.

~

S 0700

~

( ")

l

-e

~

W 0\

•

1400

_.=

1500 1600

1700

= -4:1

_ .\'..

~~

. .-0; ::

~

. '. '.

-~ ~.:~:- ~~ ...~.:-,- _

--

km

!

10

--

- '0

-

.

•

•

.•

'-'

~

••

Cil

AS

0.60

PS

0.40

020 .

(record not available)

20

1.60

1.40

1.20

N

~

10

,::,

l'O

1 .00 ~ >

~~:~~~~~Eo80i._ , ::r~ r: .

_._ .;<.:"~:::-,,:~...,,,. ,,:.~'-t "-<,..~_., ..... . .•: _~::-::....: --~~~ ~:..~.~~~:2.~~, ~~~~~::~. ,

_

.. ~.. c~~

I -tl-~-I.. ~-t:.i

Fig. 4.29B. Interpreted northern segment of seismic reflection profile T-8-S-CB -E crossing inner basin, crystalline displaced megablocks, and peak ring of Chesapeake Bay impact crater. Arrows indicate strong (high-amplitude) reflections within Exmore breccia , which may represent megablock clasts or melt bodies. See Fig. 4.21 for precise location and CD-ROM . 12 for full-scale profile. Reflect ion abbreviations as in Fig. 4.3.

!

o

Vert ical Exaggeration

.., .. _;.:....,.:~~~' .'- _ -. -

Chickahomln

~.

1900

~ ~7_

.~

reoo

--~--::::=='.' ' .. _ . _'--

--::-. ... -

Shot Points

=-_2 _ ,-....-. --- i}/il-..,.....2,....2.._ .. ,",...,.._A -_ _ ....~ -.~-~ ~ . .-~- ~. --- . ~ -., . . :::-~ ~: .~-.~.~. ~ .~.~_:-~;~.~= : . ~

1300

-...l

w

z ~o ~

0-

::l

~

(')

2' ~

[fJ

~...

o

138

The Primary Crater

Table 4.5. Morphometric data for peak ring of Chesapeake Bay impactcrater. Profile

Width [km]

Relief [m]

IS_4 south

6.5

65

55

275

S-4 north

10.8

60

835

435

S-IOsouth

8.4

140

429

705

S-IO north

6.0

100

S-6 north

11.0

100

680

1010

S-8 south

8.5

140

218

740

S-3 west [1]

4.5

150

720

not crossed

S-3 west [2]

4.5

150

852

not crossed

11.0

50

T-8-S-CB-E south

5.7

175

430

680

T-8-S-CB-E north

11.0

80

2020

1700

#

S-15 west

Shot Point [outeredge]

1809

not crossed

ShotPoint [inneredge]

1605

430

T-7-CB-H west

?

-50

not crossed

1100

N-I west

9.75

150

22050 hrs

21833hrs

N-2 south

8.0

200

21400 hrs

21537 hrs

Ewing-2 north

14.5

220

430

800

Ewing-2 south

>7.5

>100

2600

2350

T-I-CB north

11.5

140

1775

1335

T-I-CB south

12.5

300

450

925

Ewing-3 north

14

120

095

501

Ewing-3 south

22

220

1410

761

T-13-YR west (I)

4.25

40

1225

not crossed

T-I3-YR west (2)

4.25

40

1375

not crossed

S-14

6.7

50

635

Average

9.2

124

775

IDirection indicates side of peak ring shownon profile; no data for easternhalf of crater 2Location on seismic trackline measured in clocktime ratherthan in shot points ring, varies considerably in all morphological and struct ural characteristics. Measured width is 4.25-22 km; the interpolated average is -10 km (Tab le 4.5) . The ring is widest on the western side, near the mouth of the York River, and narrowest about 20 km southeast of the York River mouth (Fig. 4.21; Table 4.5). The diameter of the peak ring, as measured along its crest, varies from 35 to 45 km,

CraterStructure and Morphology

139

and averages 40 km. The elevation of the ring crest varies from 500 to 950 m below sea level; relief is 40-300 m, with an average of 123 m, where measured at 17 locations on seismic profiles (Table 4.5). Four individual peaks and a small sinuous ridge segment are prominent features of the southwest sector of the peak ring, which attains the highest ring elevations (Figs. 4.1, 4.21; CD-ROM.3-5). The largest of the four peaks is a triangular feature, whose eastern flank slopes down to the inner edge of the peak ring. This peak displays the maximum relief (300 m) of any structural or morphological feature so far identified on the peak ring or in the annular trough . The three smaller peaks jut out into the western inner edge of the inner basin and produce an irregular outline in map view (Fig. 4.1). The geometry of the peak ring in the eastern side of the crater is not as precisely known as elsewhere, because we lack basement images in that area. We have assumed that the peak-ring structure is roughly symmetrical, as indicated by the gravity anomaly data (Fig. 3.6), and have extrapolated the morphology of the peak ring on that basis. The smoother surface of the basement shown on the eastern side of the crater (Figs. 4.1; CD-ROM .3,4) is a result of the lack of control there.

4.1.1.4/nner Basin The inner edge of the peak ring is a steep cliff of crystalline basement rock, roughly circular in outline, which forms the outer wall of the inner basin (Figs. 1.5,4.1,4.21-4.29; CD-ROM .3--6; Table 4.6) . On most seismic profiles (western half of crater only), the position of the crystalline wall is indicated by the abrupt truncation of the basement reflection couplet (Figs. 4.21-4.29). The exact diameter of the inner basin has not been measured, because no profile traverses the entire crater passing through its center. However, profiles SEAX-IO, E-3, T-8-SCB-E, and E-2 approximate the diameter at 29.75 km, 31 km, and 41.5 km, respectively (Fig. 4.21; Table 4.6). The inner basin radius, as measured from the approximate center along seven different profiles, ranges from 10 to 18.3 km, averaging 14.2 km, which would yield an average diameter of 28.3 km (assuming symmetry; Table 4.6). From the structure map, we estimated an average diameter of ~30 km, and used this value to extrapolate the geometry of the inner basin wall to the eastern half of the crater. The best seismic data available to us prior to 1989 (when we acquired the Texaco multichannel profiles) were stratigraphically too shallow «1 km) to image the floor of the inner basin. The Ewing profiles provide deeper data, but we observed no obvious reflections that could be interpreted as a distinct floor of the inner basin. From this we conclude that either the seismic system was not sufficient to resolve this contact through more than a kilometer of breccia, or the impedance contrast between the basal part of the crater-fill breccia and the fractured upper part of crystalline basement is not significant. Seismic studies of several other craters also have failed to distinguish a prominent reflection that would identify the floor of the inner basin (e.g., Manson crater, Schultz and Anderson 1994; Chicxulub crater, Christeson et al. 1999; Houghton crater, Scott and Hajnal 1988), which supports the hypothesis that impedance contrasts are negligible at that level. However, Poag (I 997a), Poag et al. (1999), and we (herein) estimate the minimum

140

The Primal)' Crater

depth of the floor of the inner basin to be at - 1.6 s 2-way traveltime (-1.6 km; Figs. 4.26--4.29), using the scaling relationship proposed by Grieve and Robertson (1979) [edt = 0.52Do.2) , in which d, is true depth, and D is the final diameter of the crater (85 km)). Table 4.6. Morphometric data forthe inner basin of the Chesapeake Bay impact crater. Profile #

I

Apparent Diameter [km]

Shot Point Interval [wall to wall]

S-4

5

275-440(south to north)

S-IO

29.75

709-1609 (south to north)

S-6-7-8

237

730-1000 (southto north)

T-I-CB

12

925-1365 (southto north)

T-8-S-CB-E

31

670-1625 (south to north)

7-CB-H

>26

101-1 100 (southeast to northwest)

Ewing 2

241.5

800-2350 (south to north)

Average

28.3

Profile

Radius

#

[km]

Shot Point [Outer wall]

N-I + S-7

11.75

31835 hrs

N-2 + S-7

10.00

31550 hrs

S-15 + S-7

218.30

430

S-7

16

730

S-14

14.75

775

Average

14.2

I Noprofile transects entire inner basin directly through crater center 2Not a straight-line profile 3Location on seismic trackline measured in clock time rather thanin shotpoints

4.1.1.5 CentralPeak Several seismic profiles offer subtle evidence that a rugged, irregular central peak is present near the center of the inner basin (Fig. 4.21; Table 4.7; CD-ROM.3-6). Profile SEAX-7, for example, located a few kilometers south of the mouth of Cape Charles harbor (Figs. 4.21, 4.30), displays a series of persistent, though faint, diagonal reflections, which Poag et al. (1999) and we interpret to be side reflections from a central peak. The top of the peak on profile SEAX-7 is not plainly obvious, but Poag et al. (1999) and we (herein) place it at roughly 575-600 m

Crater Structure and Morphology

w

160

60

Shot Points

14 1

E

0.2

0.4 ---. (f)

....... Q)

E :;:; a;

> 0.6

11l ....

>.

11l

~I

N

0.8

1.2 km

Fig. 4.30. Interpreted segment of seismic reflection profile SEAX-7 (at entrance to Cape Charles harbor, Delmarva Peninsula) crossing central peak of Chesapeake Bay impact crater. Arrow indicates inclined parallel reflections interpreted to be side echoes from central peak. See Fig. 4.21 for precise profile location.

142

The Primary Crater

Borehole 2 Kiptopeke

Borehole 70 (Well No. 82) Cape Charles 0 -,-- - - - - - - - --, (Sampled by drill cuttings )

(Sampled by continuous coring)

100

200

~ >

..'" j 300

.!1 oil

.s

Green day with b1acl< specks

a.

-

-

-

-

-

-

-

-

-

•...

Gray day. hard 8. soft layers

e

"0

.......

1 ' - - - -- - - - - - - - - 1 355

'C

375

Greenisll sand . compact

0

.

MIXed brown & gray sandy day . hard

400

& son layers

MoWed clay . sand . grave l. hard and

so~ layers

483

1----'- -'--- - - - - - ---1 530

600

~ : .

...J.;5 ~ 5< 2

.

crystalline basement Mixed

•

microfoss. assemblages range from Early Cretaceous to late Eocene (P15i NP19·20) .

...

• Exmore • breccia . "

IIll....

•

•

• ..

TO

• •

394

Matnx of greenlsll b
••

Gray clay with crusts of sandstone

Cederstrom ( 1945a)

".

~ ~ 490 512 ,..-:•

Roddish-bro-..n day. sUcky. no sand

_ ....... -. dry. Stltyday. abundant - -. microfossjls. solitarycorals (age P15-P17 ; NP19-20)

_ :: -

~

Green sandy clay . hard Pale ptnk sandy clay

.,.

-

•

:~

Mixed sand . gravel . day. hard and son layers

500

-

C""'" ~ hiCk- --_- -_ - _ - - _ahOfl1' - -1 335 .:: - _ _- _ ~Y _ Greenisll gray to b<""n. dense .

Ught1Jreen clay . son and hard layers - -_

1- -

£;

-

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TO

Fig. 4.31. Stratigraphic correlation between two boreholes drilled on Delmarva Peninsula into inner basin of Chesapeake Bay impact crater. See Figs. 1.2, 4.12, and CD-ROM .I for borehole locations.

08

I

o

NW

I

5

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I

10

km

1600

1500

I

15

1700

1800

I

20

+

1900

NW 48° turn

NE

I

25

2000

Fig. 4.32. Interpreted segment of seismic reflection profile Ewing-2 (E-2), showing subparallel crossings of southwestern flank of central peak of Chesapeake Bay impact crater. See Fig. 4.2 I for precise location and CD-ROM . IS for full-sca le profile. Reflection abbreviations as in Fig. 4.3.

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1.8

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Fig. 4.33. Interpreted segment of seismic reflection profile SEAX-6 (S-6), showing inclined, para llel, side echoes from flank of central peak of Chesapeake Bay impact crater. See Fig. 4.21 for precise location and CD-ROM .9 for full-scale profile. Reflection abbreviations as in Fig. 4.3 .

N

~

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1100 N

Fig. 4.34. Interpreted segment of seismic reflection profile SEAX-IO (S- IO), showing inclined, paralle l, side echoes from flank of central peak of Chesapeake Bay impact crater. See Fig. 4.21 for precise location and CD-ROM .10 for full-scale profile . Reflection abbreviations as in Fig. 4.3.

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146

The Primary Crater

depth (below sea level) between shot points 0 and 31. Onshore, about 0.3 km to the east, a deep well (borehole 70, not cored) terminated in Exmore breccia at 552 m without having encountered crystalline basement (Figs. 4.21, 4.31). This provides some evidence that the peak is not more than 1 km in diameter at this location. The best evidence for the central peak, however, is provided by profile E-2 (Figs. 4.21, 4.32). This profile images the southwestern flank of the central peak on two subparallel crossings between shot points 1280 and 1850. The peak massif is approximately 12 krn in diameter where intersected by E-2, and expresses irregular relief of 200-400 m. Three prominent knobs or subpeaks are shown on this profile, each approximately 2.5-3 krn in diameter, and their crests vary from 950 to 1050 m below sea level. Maximum subpeak relief is 500-600 m above the inferred floor of the inner basin (-1.6 krn depth). Distinctive diagonal and hyperbolic reflections at roughly I-krn depth on profiles SEAX-6 (Figs. 4.21, 4.33; shot points 1430-1730), SEAX-IO (Figs. 4.21,4.34; shot point 1109), SEAX-15 (Fig. 4.21; shot points 1200-1350), T- 8-S-CB-E (Fig. 4.21; shot points 1025-1175), and at the intersection of N-l and N-2 (Fig. 4.21; 1675-1710 hrs) provide evidence that, on its lower flanks, the central peak broadens out to 12 krn or more in diameter, and has an average vertical relief of 620 m and an average crestal elevation of 890 m (Figs. 4.1,4.21; Table 4.7; CD-ROM.3--6). Table 4.7. Morphometric data for central peak of Chesapeake Bay impact crater. Width [km]

Bounding Shot Points

0.60

4.5

1010-1180

1.00

0.60

5.5

600-825

S-6

1.00

0.60

8.0

1400-1650

S-7

0.60

1.00

1.0

0-61

S-lO

0.95

0.65

3.0

910-1000

S-14

0.75

0.20

2.5

1200-1350

E-2

0.90

0.70

12.0

1240-1951

S-15

Not distinguishable

?

?

Neecho

Not distinguishable

?

?

Average

0.89

0.62

Profile #

Crest Elevation [km]

Approximate Vertical Relief [km]

T-8-S-CB-E

1.00

T-7-CB-H

4.8

4.1.2 Gravity Interpretation To test the seismically interpreted geometry of the impact structure, we compared simple Bouguer gravity (Fig. 4.35) and residual gravity (Fig. 4.36) anomaly maps (see Chapter 3), and applied 2-D geologic modeling along transects through the

Crater Structure and Morphology

147

Fig. 4.35. Bouguer gravity anomaly map over Chesapeake Bay impact crater (onshore contours from simple Bouguer values; offshore contours from free air values). Contour interval l mGaI. Modified from Poag (1997a).

148

The Primary Crater

-30 -25 -20 -15 -10

-5 0 5 10 15 20 Residual gravity anomaly (mGaI)

o

25

30

60 kin

Fig. 4.36. Residual Bouguer gravity anomaly map with superimposed outlines of principal structural features derived from seismic reflection profiles. White dashed line represents outer rim of crater; two solid black lines represent outer and inner boundaries of peak ring; dashed black line represents outline of central peak. Solid black circles represent corehole locations. See text for further explanation and CD-ROM for color version of this figure.

CraterStructure and Morphology

149

gridded residual gravity data (Fig. 4.37A,B). In each of the three modeled sections (Fig. 4.37A), placing a low density (2.57 g/cm') body below the basement/sedimentary rock interface provides a large improvement from the starting model in fitting the observed residual gravity anomaly. The bodies had bottom elevations varying from -2.67 to -3.21 km, for an approximate thickness of 2 km. The positions of these bodies correlate well with seismic interpretations of the morphology of the peak ring and inner basin. An even better fit results when the basement surface is elevated by an average of 500 m at the edges of the lowdensity body in the basement (Fig. 4.37B). These elevated areas lie within the bounds of the zone interpreted from seismic reflection profiles to be the peak ring. Quantification of the depth of the crater and height of the peak ring depend on the assumptions made about the relative densities of the basement and crater-fill material. Although this modeling demonstrates the likely presence of an inner basin surrounded by a peak ring, their absolute elevations cannot be uniquely determined with the available data. Horizontal limits of each body are plotted in Fig. 4.37A,B. If one assumes that the residual gravity map represents a qualitative image of the inner structure of the crater , it can be used to interpret the geometry of the eastern half of the peak ring. In general , the gravity data support the extrapolation of the seismic data across the Delmarva Peninsula . On the residual anomaly map (Fig. 4.36), a relatively symmetrical ring of positive anomalies coincides with our placement of the seismically-derived peak ring. Two broad, elongate positive anomalies on the southwest sector of the peak ring correlate with the highest peak-ring relief noted on the seismic profiles (Fig. 4.1; CD-ROM.3-6). The highest gravity-anomaly values associated with the seismically extrapolated peak ring occur on the eastern side of the crater (under the shallow eastern bays of the Delmarva Peninsula) where we lack seismic control. This suggests that the crest of the peak ring may attain its highest elevation in this area. A broad, irregularly circular gravity low is present over the seismically imaged inner basin (Fig. 4.36). This negative depression is interrupted by several small , irregular gravity highs that suggest the presence of several individual knobs on the central peak. Furthermore , the gravity signature suggests that the highest elevation of the central peak may be - 10 km north of the Kiptopeke corehole, a location for which we have no seismic data.

150

The Primary Crater

-,

'-' \\

)

!

(

j

./ ,/

.

~

:'

./

.......... ......................................

7630 '

76'00'

Fig. 4.37A. Location map, showing three lines of transect across Chesapeake Bay impact crater, for which gravity models are shown in Fig. 4.378. Thickness changes along each line correspond to inner and outer boundaries of the peak ring as indicated by the gravity models. Seismic boundaries of the peak ring are indicated by dotted lines.

Crater Structure and Morphology

NE 23g1em' sw ~ 'l,__::"__'"""'I~=:;:~;-7-"---1

g.2

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E

151

2 67 em '

o

80

80

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NE

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SW

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60

80

80

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S

S

N

N

2 3 40

60

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4

80

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0

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80

40

Error

Fig. 4.378. Two-dimensional gravity models (A,B) along three transects (see Fig. 4.37A) across Chesapeake Bay impact crater (constructed by P. Moizer). See text and Chapter 3 for further explanation.

5 Secondary Craters

5.1 Location and Identification Telescope and satellite images of the moons and planets of our solar system reveal that large primary impact craters frequently are accompanied by smaller secondary craters of variable size, shape, and distribution (Shoemaker 1962; Melosh 1989; Spudis 1993; Greeley 1994; Fig. 5.1). Roddy (1977) showed that secondary craters also are commonly produced by large man-made explosions. The projectiles that produce secondary craters are inferred to be mainly blocks and clods derived from the target rocks, which are ejected into ballistic trajectories by the primary impact. Planetary secondaries usually are first recognizable beyond the edge of the continuous ejecta blanket , and their geographic range can extend many crater diameters from the primary crater (Melosh 1989). The maximum diameters of secondaries are roughly proportional to the diameter of their primary craters (e.g., lunar secondaries are - 4% as wide as their primaries). Besides isolated individual secondary craters, clusters (open or closed) , chains , loops, gouges, and rays of secondaries are common on large and small planetary bodies. Secondary craters nearest to the rim of the primary crater may have irregular shapes because their impactors interfere with one another, and because their impact velocities are low relative to the velocit y of the primary impactor. Distal secondaries usually have more regular shapes , but tend to be asymmetrical in cross section ; the crater walls tend to be steepest in the direction toward the primary crater. Ejecta blocks that produce secondary craters may reach several kilometers in diameter; fragment size is inversely proportional to the ejection velocity . The extent of a secondary crater field away from the primary crater is evidently strongly controlled by gravity. Secondary craters tend to cluster closer to their primaries on larger planetary bodies than on smaller ones. According to Melosh (1989), the quantity of ejecta that produces secondary craters is small, typically one to three percent of the total ejecta derived from the primary impact. Thus, most of the large blocks and smaller clasts composing a continuous ejecta blanket do not produce well-defined secondary craters . Despite the apparent near-ubiquity of secondary craters on other planetary bodies, secondary craters have rarely been documented on Earth. For example, though intact l-km-long megablocks of Maim limestone have been ejected as far away as 7 km from the 24-km-diameter Ries peak-ring crater of southern Germany, no specific secondary craters have been found associated with this extensively studied primary crater (Pohl et al. 1977; Harz et al. 1983). On the other hand , Sturkell C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

154

Secondary Craters

Fig. 5.1. Satellite image showing secondary craters associated with lunar crater Euler (27km diameter; Apollo 17 image; from Greeley 1994). (1998) reported that a ISO-m-thick boulder was ejected from the Lockne crater and had excavated a 40-m-deep secondary crater. Among the large number of small terrestrial craters «5 km in diameter) known on our planet (Table 1.1), nearly all are interpreted to be primary craters. No secondaries have been previously reported to be associated with the larger submarine craters (Montagnais, Jansa and Pe-Piper 1987; Mjelnir, Gudlaugsson 1993; Manson, Koeberl and Anderson 1996a) or with the only known submarine multiring impact basin (Chicxulub, Hildebrand et al. 1991). Terrestrial secondary craters, like terrestrial primaries, are subject to alteration and removal by weathering, ero-

Secondary Craters on Profile T-I-CB

155

sion, and the effects of plate subduction. At least one primary terrestria l crater, however, appea rs to have produced secondaries. Small secondary craters have been reported associated with the Bigach crater in Kazakhstan (Table 1.1; Kiselev and Korotuschenko 1986; Masaitis 1999). The 85-km-diameter Chesapeake Bay primary crater is unusually well preserved, because it is relatively young (late Eocene ; - 35.8 Ma; see age discussion in Chapter 8), it formed in a relativel y deep marine setting (Poag et al. 1994; see also Chapt er 13), and it occupies a basin characterized by relatively rapid postimpact marine sedimentation and no post impact tecton ism (Poag 1996). This advantageou s setting appe ars also to account for the presence nearb y of at least 23 smaller fault-bounded excavations, which we interpret to be secondary craters (Poag I997a, 1998, 1999a; Fig. 5.2). These 23 secondary craters appear north and northwe st of the crater rim on two multichannel seismic profiles collected by Texaco and Exxon Exploration Company. Other segments of these same two profiles also help to defin e the Chesapeake Bay primary impact crater (Fig. 5.2; Poag et al. 1994; Poag 1996, I997a, 1998). We performed a seismostratigraphic analysis of these profile s, and calibrated the profile s with lithostratigraphy and biostratigraphy from nearb y outcrops and boreholes (Poag et al. 1994; Poag 1996, I997a ). Our analysis highlights the structural and stratigraphic contrasts between the normal success ion of flat-lying sedimentary coastal plain rocks and the stratal disruptions char acteri stic of the secondary craters. Apparent diameters (apparent becaus e most secondaries are crossed by only a single profile) of the secondary craters range from 0.4 km to 4.7 km, and average 1.9 km; only four have apparent diameters greater than 3 km. Apparent depths of the secondaries (mea sured from sedimentary lip to crater floor) range from 50 to 710 m, averaging 370 m. In six of the secondaries, the entire preimpact sedimentary section appears to have been exca vated and replaced by impact breccia (Table 5.1). The breccia fill ranges from 30 to 680 m in apparent thickness, and average s 266m.

5.2 Secondary Craters on Profile T·1·CB Five secondary craters (C-I to C-5) are imaged by north-south seismic profile T-I CB, where it crosses Chesapeake Bay near the mouth of the Potomac River (Fig. 5.2; Table 5.1). Crater C-3 was illustrated by Poag ( 1997a; his Fig. 31). The C-I structure is the southernmost secondary crater, located at the junction ofT-I -CB and T-II -PR, appro ximately 8 km east of Smith Point, Virginia, and 35 km north of the northern rim of the Chesapeake Bay primary crater. This location is outside the seism ically identifiable periphery of the brecc ia apron of the primary crater (Figs. 2.14, 5.2). Undisturbed, flat-lying, coastal plain formation s extend continuously (interrupted only by a few collapse structures and scattered clusters of small -offset normal faults; Fig. 5.3) for the entire 35-km distance between the

Fig. 5.2. Geogra phic distribution of secondary craters along seismic reflection profiles I- CB (C-I - C-5) and II-PR (P- I- P-1 8), north and northwest, respectively, of Chesapeake Bay primary crater. Solid dots indicate borehole locations; onshore seismic tracklines shown in southern Maryland (St.M.-2, St.M.-3) and at Smith Point, Virginia. Md = Maryland; Va = Virginia. See CD-ROM. 14a-d for profil e II-PR.

Secondary Craters on Profile T-I-CB

157

Table 5.1. Morphometric data for 23 secondary craters inferred from reflection characteristics on seismic profiles T- I-CB (5 Chesapeake Bay secondaries) and T-II-PR (\8 Potomac River secondaries). Secondary Crater Designation

Shot Point Location

Appare nt Diameter [km]

Apparent Disruption Depth ' [m]

Maximum Apparent Crater Depth 2 [m]

Maximum Apparent Breccia Thickness [m]

Raised Rim Present

CB I

3840-3885

1.1

32

150

80

No

CB2

4560-4660

2.7

44

310

180

No

CB 3

4720-4750

1.0

56

440

330

No

CB4

4785-4865

2.2

74

710

3640

No

CB5

4935-5085

4.2

68

700

3520

Yes

Potomac I

1780- 1797

0.4

18

50

30

No

Potomac 2

1808-1840

0.7

20

80

50

No

Potomac 3

1890- 1945

1.5

41

340

250

No

Potomac 4

2130-2 170

1.1

28

140

100

No

Potomac 5

2230-23 10

2.1

60

450

270

Yes

Potomac 6

2385-2535

4.2

40

300

280

No

Potomac 7

2580-2600

0.6

40

300

120

Yes

Potomac 8

2720-2890

4.7

51

440

360

No

Potomac 9

2960-3048

2.4

66

710

3680

Yes

Potomac 10

3335-3425

2.4

59

640

3610

Yes

Potomac II

3475-3495

0.4

22

350

110

Yes

Potomac 12

3663-3725

1.8

52

580

3540

No

Potomac 13

3775-3850

2. 1

40

130

90

No

Potomac 14

4005-4 115

3.0

48

400

300

No

Potomac 15

4165-4 185

0.6

14

70

50

Yes

Potomac 16

4240-4255

0.4

20

80

50

Yes

Potomac 17

4480-4535

1.5

52

550

120

No

Potomac 18

4580-4655

2.0

48

600

3350

Yes

1.9

43

37

266

Average

' Maximum depth of sediment disruption below sea level 2Depth of crater floor below crater rim 3Entire preimpact sedimentary section excavated and replaced by impact breccia

northern rim of the primary crater and the C-I secondary crater. All five secondaries on profile T- I-CB have similar general characteristics. The principal differences are in their respective apparent diameters and depths. Each crater is marked by clearly expressed rim escarpments constructed by en echelon (presumably concentric) down-to-the-basin normal faults (Figs. 5.3, 5.4).

158

Secondary Craters

The rim faults truncate horizontal, parallel, continuous to subcontinuous reflections, which represent the same preimpact target rocks disrupted by the Chesapeake Bay primary impact (Lower Cretaceous to lower Eocene siliciclastic sediments and middle Eocene bioclastic limestone; Fig. 2.4). Inside each secondary crater, the seismic reflections are chaotic or incoherent. We interpret these signatures to represent impact breccia equivalent to the Exmore breccia. A key seismic reflection horizon on profile T-l-CB is the top of the middle Eocene Piney Point Formation, represented by an essentially horizontal reflection at -0.25 s (two-way traveltime; -160 m) north and south of the group of secondary craters. The type section for the Piney Point Formation is -19 km west of the profile, in a borehole drilled at Piney Point on the north bank of the Potomac River (Fig. 5.2). The Piney Point Formation also is present at 64 m below sea level in the Baltimore Gas and Electric No.1 borehole, 48 km northwest (and updip) from the T-I-CB secondary craters (Ward 1984; Gibson 1989). The same key horizon can be identified on seismic profile T-11-PR, which passes within 5 km of the Piney Point borehole at shot point 3200. The floors of secondary craters C-I and C-2 appear to be formed wholly within the fractured and faulted sedimentary rocks of Early Cretaceous (Potomac Formation) to Paleocene (Aquia Formation) age (Fig. 5.4), but C-3, C-4, and C-5 appear to have been excavated down to the surface of the crystalline basement (Fig. 5.5). The overlying postimpact formations (mainly middle Miocene to Quaternary sediments) thicken and sag into all five secondary craters on profile T-1-CB, just as they do where they cross the outer rim of the primary crater. Along profile T-1-CB, the relatively flat basement reflection rises gradually northward (updip) from 0.9 s (-900 m) at the northern rim of the primary crater to - 0.925 s (-925 m) at secondary C-1. The basement reflection rises to 0.875 s (875 m) beneath C-2, then dips to 0.9 s beneath C-3 and >0.9 s (>900 m) below C4; it then rises to 0.83 s (830 m) north of C-5. Along this profile segment, the basement reflection is gently warped and cut by numerous individual and clustered normal faults, having small vertical offsets. These faults bound scattered grabens and horsts interspersed between wide intervals that contain few or no faults (Fig. 5.6). Only a few of the crater-bounding faults can be traced into the crystalline basement. Most appear to be detached from the basement surface, similar to those at the outer rim of the primary crater (Figs. 5.4, 5.5).

5.3 Secondary Craters on Profile T-11-PR Eighteen similar, small, collapse or excavation structures are distributed along a IlO-km segment of profile T-11-PR (P-1-P-18; Figs. 5.2, CD-ROM.14a--d; Table 5.1), which extends from the northern rim of the primary crater to a location in the Potomac River, - 5 km east of the town of Colonial Beach. We infer that these structures, too, are secondary impact craters created by blocks ejected from the primary crater. Though most features of the Potomac River (P) secondaries are

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Raised Rim

Fig. 5.4. Interpreted segment of seismic reflection profile I-CB crossing Chesapeake Bay secondary crater C-2, show ing fault traces, crater-fill breccia (lighter shading) , and crystalline basement (AB ; darker shading) . See Fig. 5.2 for location.

N

~

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SE

Top of Preimpact Sediments

I

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g

g

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Top of Preimpact Sediments

-

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I

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Fig. 5.5. Interpreted seg ment of seism ic reflection profile I-CB cross ing Chesapeake Bay secondary craters C- 3, C-4, and C- 5, showing fault traces, crater-fi ll breccia (lighter shading), and crystalline basement (A B; darker shading). See Fig. 5.2 for locations.

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Implications of Secondary Crater Record

163

similar or identical to those of the C secondaries, some diffe rences can be noted (Figs. 5.7-5.12). For example, eight of the P secondaries have well -developed, raised, sedimentary rims, almost identical to the few raised rim segments preserved on the primary crater. Perhaps the most important difference, however, is that some of the normal faults on most of the P secondaries appear also to disrupt the basement reflec tion. This baseme nt disruption is especially evident for P-5, P-7-P-I O, P-13, P-14, P17, and P-18 (Figs. 5.7-5 .12). In this region, the basement rocks have had a particularly comp lex history. Along part of the same profi le, the basement rocks consist of metasedimentary units within the Sussex terrane and Chesapeake block, and, in part, constitute the upper lithified sedimentary deposits of the Taylorsville rift basin (Fig . 2. 1). The basement surface in the Taylorsville segment (particularly beneath P- 13 and P-18) has been offset by eight reverse faults of 100 m or more vertical displacement (latera l displacement is unknown), seven of which display underthrusting to the west (Figs. 5. 11, 5. 12). The basement beneath P-1 8 (Fig. 5.12) has been thrust into two high-angle ant iclinal folds. The thrust folds extend to the top of the preimpact target rocks . Thus, the thrusting coinci ded with the primary impact (i.e., late Eocene). We interp ret these thrusts as distal products of the compressive shock wave that radiated from the primary impact site. Some of the reverse faults , however, appear to have been reactivated as normal faults, probably during the late stages of crater deformation. The original fault planes acted as zones of crustal weakness, along which normal displacement took place during the ejecta bombardment that formed the secondary craters. All this wou ld have taken place with in a few minutes of the prima ry impact (see Chapter 12). The C- I crater is the only secondary for which we can determine a true diameter, because all the other secondaries are crosse d by only a single seismic profile. It is highly unlike ly, of course, that a sing le profile wou ld symmetrically bisect any secondary crater. Secondary C-I , however, is crossed by both profiles T- 1CB and T-II-PR (Fig. 5.2), which allows us to estimate the true diameter to be I.I km. The position and size of the secondaries illustrated in Figure 5.2 are diagrammatic, and intended only to show : (I) the locations where they intersect the profiles; (2) that they are not all centered on the profiles; and (3) that their diameters vary.

5.4 Implications of Secondary Crater Record The stratigraphic and structural characteristics of the 23 structures we interpret as secondary craters match the general morphologic and structural features of simple crate rs (as opposed to complex craters ; e.g., Melosh 1989), though the Chesapeake secondaries presumably were not formed by hypervelocity impacts. The principal difference is the apparent lack of overturned flaps (though any flap present may be too sma ll to be resolved on our profiles). This is not surprising, however, because the impacts took place in the late Eocene ocean. There is amp le evidence that eve n small ocea nic impact craters lack these features, probably as a res ult of ex -

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Fig. 5.7. Interpreted seg ment of seismic reflection profile ll-PR cross ing Chesapeake Bay secondary crater P-8, showing fault traces, crater-fill breccia (lighter shading), and crystallin e basement (AS ; darker shading). See Fig. 5.2 for locat ion and CD-ROM.14b for full-sca le profile.

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Fig. 5.9. Interpreted segment of seismic reflection profile ll-PR crossing Chesapeake Bay secondary craters P-1O and P-Il , showing fault traces, crater-fill breccia (lighter shading), and crystalline basement (AB ; darker shading). See Fig. 5.2 for locations and CD-ROM. 14b for full-size profile.

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Fig. 5.12. Interpret ed segment of seismic reflection profile I I-PR crossing Chesapeake Bay secondary craters P-I? and P-18, showing fault traces, crater-fill breccia (lighter shading), and crysta lline basement (AS ; darker shading). Note thrust faults reactivated as normal faults and note associated imbricated chevron folds. See Fig. 5.2 for locations and CD-ROM .14a for full-size profile.

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Secondary Craters

tensive slumping of their unconsolidated, water-saturated walls, and of hydraulic erosion resulting from both surgeback of the oceanic water column, and washback from tsunami wavetrains (Higgins and Butkovich 1967; Kieffer and Simonds 1980; McKinnon and Goetz 1981; Silver 1982; Roddy et al. 1987; Jansa 1993; Poag and Poppe 1998; Ormo and Lindstrom 2000; see Chapter 12). Ours is the first report of possible buried secondary craters associated with a buried primary source crater. On other planetary bodies, of course, only exposed secondaries can be identified because of limitations imposed by observation instruments currently available. The characteristics of the buried Chesapeake secondary craters are in general agreement with the features of exposed planetary secondaries. In both cases, the secondaries appear to occur in distinct clusters, or perhaps chains, and their apparent diameters range from 0.05 to 0.1 of the diameters of their respective primaries . Most of the Chesapeake secondaries, though, unlike many planetary secondaries, appear to be symmetrical rather than asymmetrical in cross section. Additional seismic surveys are necessary, however, to confirm this. Dip reversal is notable within the postimpact sediments overlying the secondary craters. This phenomenon caused Poag (l997a) to speculate that reversed dips in outcropping postimpact coastal-plain sediments of southeastern Virginia (Ward and Strickland 1985; Johnson et al. 1998) might be caused by faulting and differential subsidence (due to breccia compaction) associated with underlying secondary craters . If this hypothesis proves to be accurate, we can expect to find numerous additional secondary craters scattered around the perimeter of the Chesapeake Bay primary crater at locations of dip reversal. The presence of secondary craters below the bed of the Potomac River may shed light on one of the problems regarding recognition of the Mattaponi Formation as a formalized lithostratigraphic unit. As pointed out by Poag (1997a) and Powars and Bruce (1999), the presence of the Mattaponi Formation in the two boreholes that Cederstrom (1957) chose as the Mattaponi co-type sections, has been in doubt for nearly 50 years. This doubt stems from two principal sources. First, the type boreholes (106 and 107; Figs. 1.3,2.14,5.2; Table 1.2) are located near Colonial Beach and Washington's Birthplace, on the south bank of the Potomac River, more than 50 km northwest of the main depocenter for the Mattaponi Formation. Second, several additional boreholes, drilled within a few kilometers of the co-type boreholes, have not yielded sediments attributable to the Mattaponi Formation. If, however, the Mattaponi Formation consists of impact breccia, as we infer (see also Poag 1997a), and in the vicinity of Colonial Beach is confined to secondary impact craters, it would not be surprising to find it this far away from the main crater. It also would be normal not to find the Mattaponi in other boreholes near Colonial Beach that happen not to penetrate a secondary crater. Our inference that secondary crater P-18 extends under the south bank of the Potomac River (Fig. 5.2) rests on our assumption that Cederstrom (1957) correctly identified the Mattaponi Formation in borehole 106.

6 Synimpact Crater-Fill Deposits

6.1 Oldest Breccia Unit Five coreholes (Exmore, Kiptopeke, NASA Langley, North, Bayside) have sampled crater-fill deposits of the Chesapeake Bay crater, and two others (Windmill Point, Newport News) have sampled partly equivalent deposits outside the outer rim (Figs . 6.1-6.3, CO-ROM .7; Table 1.2). The crater-fill deposits can be subdivided into several distinctive units, which we will discuss in chronostratigraphic order from oldest to youngest. The stratigraphically oldest crater-fill debris, which is inferred from seismic reflection profiles to be present only in the inner basin, has not yet been sampled by drilling. On the seismic profiles, this deepest crater-fill is expressed by chaotic or incoherent reflections (Figs. 4.2, 4.26, 4.28, 4.29), which hypothetically represent impact breccia dominated by large blocks of shock-metamorphosed crystalline basement rocks (megabreccia). The oldest crater-fill unit sampled to date was encountered just above crystalline basement in the Bayside corehole . The unit is 20.33 m thick (66.7 ft), and consists of polymict, sediment- and crystalline-clast, matrix-supported brecc ia. The number of pebble- and cobble-sized crystalline clasts (mainly weathered granite) is significantly greater than in any other cored section . In addition, this unit contains numerous pebbles and cobbles of hard, cemented red (Triassic?) sandstone, which are unique to this core segment. The red sandstone clasts display white, millimeter-scale rinds, which may represent shock alteration, though no petrographic analyses have been completed at this writing . We interpret this breccia unit to represent debris ejected from the inner basin (Figs. 6.2, 6.4, COROM .7).

6.2 Displaced Megablocks 6.2.1 Seismic Signature and General Lithic Composition The stratigraphically next-to-deepest crater-fill deposits sampled to date come from a layer composed of 0.5- to 2-km-sized sedimentary megaslump blocks (with extensive internal deformation) and megaslide blocks (with little or no internal deformation) (Figs. 4.3A,B, 4.7B, 4.9A,B, 6.1-6.3, CO-ROM.7). As seen on the multichannel profiles , these displaced sedimentary megablocks are extensively C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

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174

Synimpact Crater-Fill Deposits

NE'M'ORT NEWS Land Surface +50 ft (.15 2 m)

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Displaced Megablocks

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Fig. 6.38. Sections of downhole SP logs from North corehole (in outer part of annular trough) of Chesapeake Bay impact crater, Bayside corehole (near outer flank of peak ring), and Kiptopeke corehole (inside peak ring), showing stratigraphic succession from crystalline basement to Chickahominy Formation. Vertical scale given in feet below land surface to match original logs and coring record. See CD-ROM .7 for more details.

176

Synimpact Crater-Fill Deposits

fractured and cut by myriad compressional and extensional faults. On the singleand two-channel profiles, however, ringing interference (repetition of shallower reflections) between the basement and postimpact sediments prevents clear definition of the displaced megablocks (Figs. 4.11, 4.13, 4.14, 4.17, 4.20B). On seismic profiles, most displaced megablocks appear to be translational or collapse features that are detached from the crystalline basement surface by a zone of decollement (Fig. 4.3A,B). However, excellent examples of detached rotational megablocks also are prominent on several multichannel profiles. The best expressions of rotational megablocks are on profiles Il-PR and 9-CB-F, near the toe of the outer rim scarp (Fig. 4.3A,B). At both of these locations, the preimpact sedimentary section has been broken into megaslide blocks at least 0.75-1 km long (as measured on the seismic profiles). The blocks have rotated so that their toes are now 100-150 m lower than their heads. Additional sets of parallel, inclined reflections can be seen at numerous other locations on the multichannel profiles (at shot point 1100 on profile T-IO-RR, for example; Fig. 4.7B), which indicates the widespread occurrence of both small and large rotational blocks. The layer of displaced sedimentary megablocks averages 200-300 m in thickness over most of the annular trough. Our seismic data indicate that, in contrast, sedimentary megablocks are absent over most (perhaps all) of the peak ring (Figs. 4.22--4.26), and are totally lacking in the inner basin and over the central peak (Figs. 4.32--4.34). The upper surface of the megablock layer is highly irregular, commonly with hundreds of meters of relief (Figs. 4.3A,B, 4.7B, 4.9A,B, 6.2). Many megablocks appear to retain the seismic signature (vertical succession of reflection packages) of their undisturbed stratigraphic equivalents outside the crater. By correlating these reflection packages, we estimate that the highest points of the displaced megablocks are approximately 100-150 m lower than their stratigraphic equivalents outside the crater. In other words, the minimal vertical displacement of individual blocks within the megablock layer is ~ 100 m; maximum vertical displacement is ~3 5 0 m. Displaced sedimentary megablocks have been sampled in three coreholes, NASA Langley, North, and Bayside (Figs. 6.2, 6.3, CD-ROM.7). As a rule, the general sedimentary compositions and structures of the three cored megablock sections are similar. Each is composed of a variable succession of massive nonmarine sands (1-20 m thick), coarsely to finely stratified sands (2-10 m thick), shattered dark clays (0.3-1 m thick), and sticky varicolored clays (1-7 m thick). Some of the sandy strata are inclined at 15-30 degrees from horizontal, some are cross-bedded, and others are virtually horizontal. Relatively thin intervals «I m) of chaotically mixed clasts, clay-injected clasts, and(or) other expressions of plastic, soft-sediment deformation punctuate the thicker stratified sections. Sediments at the top and base of the displaced megablocks tend to be massive sands, which appear to have been destabilized or fluidized by the impact shock wave. In detail, however, each cored section of displaced megablocks is unique. The most complete stratigraphic sections through the megablock interval were cored at NASA Langley and Bayside, where the sections are 177 and 152 m thick, respectively (Figs. 6.2, 6.3, CD-ROM.7). The base of the megablock section at NASA Langley is a coarse, matrix-supported breccia, 1.4 m thick (Fig. 6.5A-C), which

Displaced Megablocks

2280 .4 ft (695 .07 m)

A

2282 .2 ft (695.61 m)

2291 .9 It (698.57 m)

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c

177

2293 .9 It (699.18 m)

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Fig. 6.4. Photographs of four segments of Bayside core, showing polymict, sediment- and crystalline-clast, matrix-supported fallback breccia. Note white (fusion?) rinds on several clasts (R) and sulfide splotches due to possible hydrothermal mineralization (H). Numbers at top of each segment indicate drill depth at top of segment.

178

Synimpact Crater-Fill Deposits

2052 .1 ft (625.48 m)

2050 .5 ft (625.00 m)

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Displaced Megablocks

1980 .5 It (603.66 m)

1982.4 It (604.24 m)

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179

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Fig. 6.6. Photographs of four segments of NASA Langley core, showing different aspects of soft-sediment deformation within basal zone of decollement (possible shock fluidization): clay injection; brecciation; scaly clay; and possible hydrothermal mineralization. Numbers at top of each segment indicate drill depth at top of segment.

180

Synimpact Crater-Fill Deposits

1947.0 It (593.45 m)

1722 .85 It (525.13 m)

1560 .0 It (455.49 m)

1865 .8 It (568.70 rn)

o

1868.15 It (569.41 m)

E

Fig. 6.7. Photographs of five segments of NASA Langley core, showing four different manifestations of stratified sands (A-D) and two examples of brecciation (B, E) within displaced sedimentary megablocks. Arrows indicate crossbeds in segment A; brecciation in segment B; ripple cross-lamination in segment C. Numbers at top of each segment indicate drill depth at top of segment.

Displaced Megablocks

1581.4 It (482.01 m)

A

1583.4 It (482.62 m)

B

1342.5 It (409.19 m)

o,

6 I

em

c

181

1345.6 It (410.14 m)

o

Fig. 6.8. Photographs of four segments of NASA Langley core, composed of brightly colored sticky clays of Lower Cretaceous paleosols found within displaced sedimentary megablocks. Numbers at top of each segment indicate drill depth at top of segment.

182

Synimpact Crater-Fill Deposits

1872.1 ft (570.62 m)

1579.75 ft (481.51 m)

1852.3 ft (564.58 m)

6

A

I

em

c

1634.55 ft (498.21 m)

o

Fig. 6.9. Photographs of four segments of NASA Langley core, showing three different manifestations of sedimentary breccias (A, B, C) and three examples of soft-sediment deformation (arrows on B, C, D) within displaced sedimentary megablocks. Numbers at top of each segment indicate drill depth at top of segment.

Displaced Megablocks

1478.25 It (450.57 m)

1481 .35 It (451.52 m)

183

1472.25 It (448.78 m)

/

A

oI

6

I

em

B

c

Fig. 6.10. Photographs of three segments of the NASA Langley core, showing steeply inclined stratification (dashed lines on A) in sand section and complexly distorted contact geometry (arrows on B, C) in upper part of displaced sedimentary megablock. Numbers at top of each segment indicate drill depth at top of segment.

184

Synimpact Crater-Fill Deposits

rests directly on the weathered surface of crystalline basement (metagranite; Fig 6.5C). Angular to rounded pebbles and cobbles of clay dominate the clasts, but several cobbles of relatively fresh granite also are present. The matrix is gray, clayey, quartz sand. Resting above the basal breccia at NASA Langley is a 32-m interval of shattered scaly clays and massive sands (no clear evidence of stratification; Fig. 6.6B,D). Several short «I m thick) sections within this interval display chaotic sedimentary features attributable to plastic deformation of soft sediment (Fig. 6.6A). This interval and the basal breccia also are notable for patches or splotches (2-10 em in diameter) of brightly colored red, purple, and greenish-yellow mineralization, which appear to be sulfide deposits resulting from hydrothermal activity (Fig. 6.5A, 6.6A,D). Some sands in this basal megablock interval and its equivalent at Bayside are more indurated (cemented) than any other sediment yet encountered in the coreholes. The middle section of the displaced sedimentary megablock at NASA Langley contains several excellent examples of finely and coarsely stratified sands (Fig. 6.7A-D) interspersed with sticky, varicolored, paleosol clays (Fig. 6.8A-D) and short sections «1 m thick) of chaotically mixed sediment breccias (Fig. 6.7E, 6.9A,B). The upper 4 m of the displaced megablock interval at NASA Langley consists of medium to coarse-grained, moderately stratified sand, with bedding planes inclined at 20-30 degrees from horizontal (Fig. 6.l0A). Intense internal disruption in this upper section is particularly well displayed, however, by a 60-degree inclined contact (Fig. 6.10C). The upper boundary of this megablock interval is a nearly vertical contact with an overlying clay-clast breccia (Fig. 6.l0B). Similar lithic and structural attributes characterize the megablock section at North and Bayside, but their stratigraphic distribution and lithic composition are notably less similar. Displaced megablocks of crystalline basement rocks also can be distinguished along the walls of the inner basin of Chesapeake Bay crater on some seismic profiles (Figs. 4.22, 4.25A, 4.26B, 4.29A,B). No crystalline megablock, however, has yet been sampled by drilling.

6.2.2 Expression on Downhole Geophysical Logs

As part of our analysis of sedimentological properties of the crater-fill deposits at Chesapeake Bay, we have used the spontaneous potential (SP) logs, a standard method for identifying the lithic boundaries between subsurface units and for determining their relative permeabilities (Asquith 1982). The general expression of the displaced megablock sections on downhole spontaneous potential (SP) logs is a predominance of thick, blocky, permeable units (sands, silts) separated by distinctly less permeable intervals (mostly sticky clay paleosols; Figs. 6.3A,B, CDROM.7). The less permeable sections are notably thinner than the more permeable sections in the outer crater at NASA Langley, but at North and Bayside they are equal in thickness (9-18 m) to the more permeable sections. Several upwardfining sequences can be distinguished at each of these three sites (Figs. 6.3A,B,

The Exmore Breccia

185

CD-ROM.7). No correlation is obvious between individual SP units in the megablock sections of these coreholes.

6.3 The Exmore Breccia 6.3.1 Seismic Signature and General Geometry

The central peak, inner basin, peak ring, annular trough, and outer rim of the Chesapeake Bay impact crater are totally buried by an unusually thick deposit of impact breccia (Figs. 6.1, 6.2). This breccia is informally named the Exmore breccia, after the town of Exmore, Virginia, where the deposit was first cored (Powars et al. 1992; Poag 1997a). We infer that the bulk of the breccia formed from fragments (sand-sized particles to kilometer-sized megablocks) of sedimentary rocks and crystalline basement rocks violently disrupted initially by the impact and further deformed and turbulently mixed together by subsequent watercolumn collapse and surgeback, followed by runup and washback of the resultant tsunami wave train (see Chapters II and 12 for further discussion). Gohn (in press) refers to the breccia by the more general, nongenetic, term diamicton, in deference to the abundance of both rounded and angular clasts within the debris. The upper surface of the Exmore breccia is manifest on most seismic reflection profiles as a distinctive high-amplitude, nearly continuous reflection (reflection PS; Figs. 4.3A,B, 4.5A,B, 4.7B, 4.9A,B, 4. 13, 4.15, 4.16, 4.18-4.20, 4.22, 4.244.26, 4.28, 4.29A,B, 4.32-4.34). This upper surface reflection is horizontal to subhorizontal and relatively smooth on a kilometer scale, but is quite irregular on a scale of tens to hundreds of meters. The upper surface of the breccia is easily traceable over the entire crater, and characteristically sags into the crater along radial transects across the outer rim (Figs. 6.2, 6.11). Relief on the breccia surface at this outer-rim sag varies from 30 to 190 m, averaging 110m (Table 6.1). Where measured inside the crater, the upper surface of the breccia varies from 210 m below sea level under the York River (profile S-3), to more than 550 m below sea level in the eastern sector of the crater (profile S-I ; Fig. 6.11). Average elevation of the breccia inside the rim (relative to mean sea level) is -347 m, versus -238 m, on average, outside the rim. The upper surface of the breccia also sags as it crosses the peak ring into the inner basin (Figs. 4.22-29; Table 6.2). The sag relief ranges from 40 to 230 m and averages 117 m. Average elevation outside the inner basin is -298 m, versus -416 m inside the basin. An equivalent sag takes place across the central peak (Figs. 4.27-30; Table 6.3). Sag there ranges from 30 to 90 m and averages 47 m. Average elevation of the breccia on the central peak is -372 m, whereas it averages -4 17 m in the inner basin.

186

Synimpact Crater-Fill Deposits

.I. · I

·I· · .I •

37'00'

60 ,

Fig. 6.11. Structural map of upper surface of Exmore breccia (depths in meters below sea level; contour interval 50 m). NN, Newport News corehole; L, NASA Langley corehole; K, Kiptopeke corehole; "E, Exmore corehole; W. Windmill Point corehole, N, North corehole; B, Bayside corehole. Heavy dashed line approximates seaward extent of breccia distribution. In contrast, the lower surface of the Exmore breccia is difficult to determine with precision, except where it rests directly on crystalline basement (on the peak ring, for example). Over most of the annular trough, the Exmore breccia lies upon the kilometer-scale sedimentarymegablocks (Figs. 1.5, 4.3A,B, 4.7B, 4.9A,B, 6.1, 6.2). The breccia-megablock contact can be approximated as an irregular boundary of high relief (200-300 m) on some of the Texaco multichannel profiles (e.g., Fig. 4.3A,B). Seismic ringing and the presence of extensive intrabed multiples, however, obscure the megablock morphology and structure on nearly all singlechannel and 2-channel profiles (e.g., Figs. 4.11, 4.13). Between its upper and lower surfaces, the internal seismic signature of the Exmore breccia is variable, depending on the type of data analyzed (multichannel,

The Exmore Breccia

187

Table 6.1. Vertical sag of upper surface of Exmore breccia across outer rim of Chesapeake Bay crater as measured on 22 seismic profiles. Profile #

Elevation Outside Rim 2[mbsl]

Elevation Inside Rim 2[mbsl]

S-2

110

210

1Sag

[m] 100

S-3

130

210

80

T-13-YR

130

240

110

S-16

150

250

100

S-17 N-3 S-12 S-13

150

300

150

130

240

110

160

220

60

150

340

190 100

T-IO-RR

200

300

T-I-CB

200

280

80

S-5

230

340

110

S-6

280

380

100

S-8

240

380

140

S-9

290

380

90

S-IO

190

310

120

S-II

290

90

540

130

S-22

200 410 420

530

110

S-25

380

540

160

S-27

380

470

90

S-I

390

550

160

E-3

310 238

340

30

347

110

S-19

Average

I Sag is elevation difference from outside to inside crater 2meters below sea level

single-, or two-channel), and on the thickness of the breccia and its overlying sedimentary cover. In its clearest expression, the breccia appears on many of the Texaco multichannel profiles as a zone of incoherent hyperbolic reflections, which contrasts markedly with the high-amplitude, horizontal reflections of the crystalline basement surface and of the postimpact sedimentary section (Figs. 4.3A,B, 4.7A,B, 4.9A,B, 4.22, 4.25A). On almost all single-channel and two-channel profiles (and older multichannel profiles, e.g., Neecho), however, acoustic ringing and intrabed multiples completely mask the internal breccia signature (Figs. 4.5B, 4.11,4.13,4.14, 4.20B, 4.23A,B, 4.24A,B, 4.26A,B, 4.32).

188

Synimpact Crater-Fill Deposits

Table 6.2. Vertical sagof upper surface of Exmore breccia across peak ring of Chesapeake

Bay impact crater as measured at 18 crossings byseismic reflection profiles. Profile #

S-6 3 S_1O

(N)

4S_ 10 (S)

S-4 (N) S-4 (S) S-7 S-14 S-15 T-I-CB (N) T-I-CB (S) T-8-S-CB-E (N) T-8-S-CB-E (S) 7-CB-H E-2 (N) E-2 (S) E-3 N-I N-2 Average

Elevation Outside Basin [2 mbsl] 400 330 270 300 290 340 330 360 300 250 300 280 220 320 280 220 280 300 298

Elevation Inside Basin embsl] 440 420 370 350 350 400 430

'Sag [m]

40 90 100

50 60 60 100

440

80

475 475 420 380 450 460 420 380 420 400 416

175

225 120 100 230 140 140 160 140 100

117

'sag is elevation difference from outside to inside crater 2meters below sealevel 3north end of given profile "south end of given profile

6.3.2 Distribution and Thickness

We determined the structure, distribution, and thickness of the Exmore breccia (Figs. 6.11, 6.12) within the crater by integrating multichannel, single-, and twochannel seismic profiles, with seven continuously cored boreholes. We supplemented these data with 124 additional non-cored boreholes (Figs. 2.12, CDROM.l ; Table 1.2). The Exmore breccia covers the entire crater, completely fills the inner basin and the annular trough, and overlaps the outer-rim escarpment. Lithostratigraphic successions in the Exmore breccia sections cored at NASA Langley, Exmore, Bayside, and Kiptopeke are easily correlatable with seismostratigraphic interpretations of the nearest reflection profiles (SEAX-16, S-6, S4, and S-7, respectively; Figs. 4.10, 4.21). The thickest section of Exmore breccia cored inside the crater was 249 m at the Bayside site (Figs. 1.3,2.14,6.2,6.3, CD-

The Exmore Breccia

189

Table 6.3. Vertical sag of upper surface of Exmore breccia across central peak of Chesapeake Bay impact crater as measured on 10 seismic reflection profiles.

Profile #

S-6 S-7 S-1O

S-14 S-15 T-8-S-CB-E T-7-CB-H N-l

N-2

E-2 Average

Elevation above peak crest 2[mbsl] 290 330 370 420 410

390 370 380 380 380 372

Elevation in adjacent inner basin 2[mbsl] 330 390 400 460 440 440 420 410 410

470 417

ISag

[m]

40 60 30 40 50 50 50 30 30 90 47

'sag is elevation difference fromoutside to inside crater 2meters below sea level ROM.7; Table 1.2). At the Kiptopeke site, the projected thickness of this breccia (derived from the seismic profiles) is on the order of 1.2 km, but only the upper 17.7 m was cored there (prior to identification of the crater). Over the annular trough, the Exmore breccia is generally - 200 m thick in the western half of the crater, but thickens eastward to - 400 m (rough estimate) in the eastern half, where seismic evidence is poorest. The breccia thickens markedly to more than a kilometer (-1.2 km) in the inner basin (Figs. 4.27-4.29, 6.12), but the elevation of its basal surface there cannot be determined directly from our current seismic data. Just outside the crater rim, the Exmore breccia makes up an irregularly distributed band of impact debris, averaging - 30 km in width (Figs. 6.2, 6.12). At most impact craters, such a debris band would be referred to as an ejecta blanket. However, though the debris band at Chesapeake Bay contains some ejected materials, it cannot be treated as an ejecta blanket if, as we propose, it was deposited in large part by washback from the impact-generated tsunami wave train. We prefer to call this band a breccia apron, a neutral term that does not imply a particular depositional origin. The breccia apron has been cored at Newport News (9.4 m thick) and at Windmill Point (12.7 m thick; Figs. 6.2, 6.3, CD-ROM.7). At Windmill Point, the breccia apron is too thin to distinguish on the nearby seismic profiles, but the rest of the lithostratigraphic succession is unambiguously correlative with the seismostratigraphy displayed on profile 8-12, which is only 1.5 km south of the core site (Figs. 4.6, 4.7A,B). One hundred-twenty four additional boreholes have sampled the breccia apron on the west side of the crater (Figs. 2.14, CD-ROM.I). Many of these additional boreholes are quite old, having been completed in the late 1800s and early 1900s,

190

Synimpact Crater-Fill Deposits

,, ,, ,

,,

/

/

37 O'

/

I

· I· · I·

37'00'

I ,,

/ 60

.

km

76'00'

7540'

.

\ 75'20 '

Fig. 6.12. Isopach map of Exmore breccia (thickness measured in meters; contour interval irregular). Abbreviations as in Fig. 6.11.

and were sampled from cable-tool cuttings or rotary cuttings, but no cores were taken . Our interpretation of the lithostratigraphy from these cuttings depends, in part, on 50-year-old drillers ' logs, and upon interpretations of the distribution of the Mattapon i Formation (Cederstrom 1945a,b , 1957), which we infer to be equivalent to the Exmore breccia. Close to the crater (within - 10 km), there is good agreement between our interpretation and those of Cederstrom (1957) and Powars and Bruce (1999), regarding the distribution of the breccia apron. Farther west on the York-James Peninsula, however , our viewpo int diverges from that of Powars and Bruce (1999). We accept Cederstrom's contention that the Mattaponi Formation spreads westward of Williamsburg, which creates a broad tongue of breccia between the Mattaponi and Pamunkey Rivers (Fig. 2.14) . Using Cederstrom's interpretations, we constructed three structural cross sections perpendicular to the inferred direction of tsunami washback (Fig. 6.13) . The cross sections show that the breccia fills a distinct, broad channel. Because we tentatively infer that this channel was eroded by the runup and washback of the impact tsunamis , we tentatively call it a washback channel.

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The Exmore Breccia

193

The position of the limit of the breccia apron around the eastern half of the crater (Fig. 6.12) is a rough estimate. The ejecta layer here is too thin to identify on the seismic profiles. Poag (I997a), however, reported evidence of correlative impact ejecta from coastal wells in North Carolina, Maryland, and New Jersey. Miller et al. (1998) also reported evidence of late Eocene impact-generated subsurface deposits in the New Jersey Coastal Plain.

6.3.3 General Lithology Several lithic units, or lithofacies, can be recognized within the Exmore breccia on the basis of cores and downhole geophysical logs. These finer subdivisions are discussed in detail in Chapters II and 12. In this section, we limit our descriptions to the broader lithic aspects of the breccia body. In a general sense, the Exmore breccia is polymictic, variably matrix-supported to clast-supported , and upward fining, and the entrained clasts consist mainly of poorly consolidated sediments. At the six sites where core recovery was most complete (NASA Langley, North, Bayside, Exmore, Windmill Point, Newport News; CD-ROM .7), the upward-fining trend resembles that of a coarse-grained megaturbidite or debris-flow deposit (Powars et al. 1992; Poag and Aubry 1995; Poag 1997a; Fig. 6.14). Microfossils generally are common to dominant within the Exmore matrix and constitute a stratigraphically mixed assemblage of marine and nonmarine taxa (ostracodes, planktonic and benthic foraminifera, calcareous nannofossils, dinoflagellate cysts, spores, and pollen) ranging in age from Albian to late Eocene (Poag and Aubry 1995; Poag 1997a; Gohn in press; Fig. 6.15A,B; see also Chapter 13). The mixed assemblages are derived from the entire succession of eight preimpact sedimentary formations present in the target area (Fig. 2.4). Individual lithic fragments ripped from each of these formations make up the large majority of clasts within the breccia, as determined by their characteristic lithologies and individual microfossil suites (Poag et al. 1992; Poag and Aubry 1995 ; Gohn in press). In the lower part of each cored breccia section, the percentage of matrix in the breccia averages - 10% (by volume; Fig. 6.14F), and clasts (mainly of sedimentary rocks) reach cored thicknesses of up to 20 m (CD-ROM .7). Small quantities of crystalline basement clasts also are present in the lower sections, but dominantly in millimeter to centimeter sizes, with occasional lengths of 10-30 em. Seismic Fig. 6.14. (Opposite page) Photographs of ten core segments recovered from NASA Langley corehole showing: A, slightly weathered crystalline basement rocks; B, strongly weathered crystalline basement rocks overlain by basal breccia; C- E, silts and sands of displaced sedimentary megablocks (C, massive, fluidized? sand from decollement zone; D, horizontal stratification; E, inclined stratification); F-J, succession of upward-fining breccias (F, clastsupported sediment-clast breccia with no matrix; G, matrix-supported breccia with moderately large sedimentary clasts; H, matrix-supported breccia with small sedimentary clasts; I, nearly 100% matrix of glauconitic quartz sand; J, continuation of matrix interval, overlain in succession by flowin unit, fallout layer and laminated dead zone, and capped by Chickahominy Formation (marine clay). Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for color version of this figure.

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Synimpact Crater-Fill Deposits 1215.33 ft (370.43 m)

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1217.75 ft (371.17 m)

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Fig. 6.16. Photographs of three segments of split cores from matrix of Exmore breccia (from Exmore corehole). Note scattered dark glauconite grains (G), variable orientations of mollusc shells (M; white streaks), and dominance of clayey quartz sand (clasts rarely larger than a few millimeters). Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for colorversion of this figure.

The Exmore Breccia

A

_

1254 .9ft _ .,.;. (3.8 2.49 m)

197

1261 .0 It (384 .35 m)

B

1361.0 It (414 .83 m)

1354.4 ft (412 .82 m)

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1288 .3 ft (392 .67 m)

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Fig. 6.17. Photographs of whole (A, C, D) and split (E, E, F) sections of core from Exmore corehole, showing angular clasts (AC) and rounded clasts (RC) within glauconite/quartz matrix (GM) of Exmore breccia. Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for color version of this figure.

198

A

Synimpact Crater-Fill Deposits

1348 .2 ft (410 .93 m)

B

1374 .5ft (418 .95 m)

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Fig. 6.18. Photographs of split sections of core from Exmore corehole, showing inclined contacts between clasts and matrix within Exmore breccia and mud rims on clasts (arrows on C. D. E). Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for color version of this figure.

The Exmore Breccia

1361.0 ft (418.83 m)

199

1367.4 It (416.78 m)

c

A 1377.3 It (419.8 m)

D

848 .2 It (258.53 m)

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o

Fig. 6.19. Photographs of split sections of core from Exmore corehole (A, B, C, D) and whole section from North corehole (E), showing complex plastic deformation of softsediment clasts within Exmore breccia. Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for color version of this figure.

200

A

Synimpact Crater-Fill Deposits

1326.8 ft (404.41 m)

B

1344.0 ft (409.65 m) --~-.

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Fig. 6.20. Photographs of split sections of core from Exmore corehole, showing squeezeouts (arrows) in clay clasts within Exmore breccia. Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for color version of this figure.

The Exmore Breccia

1433.7 ft (437.0 m)

928 .2 ft (282.93 m)

943 .23 ft (287.50 m)

946.8 ft (288.59 m)

201

A

c .....

.......-... em

Fig. 6.21. Photographs of whole sections of core from Exmore breccia showing flame structures (arrows), in which one lithic unit has been injected into another lithic unit. A Bayside core; B, C, D North core. Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for color version of this figure.

202

Synimpact Crater-Fill Deposits

data suggest that kilometer-sized blocks also may be supported by this matrix at some uncored locations (Fig. 4.29A,B). In the middle part of the Exmore breccia, the size and frequency of clasts (mostly sedimentary pebbles, cobbles, and boulders) decreases, and the percentage of matrix consequently increases (Fig. 6.14G). The upper part of the Exmore breccia consists of 50-95 percent matrix (Fig. 6.14H,I). Macroscopically, the matrix is a medium gray to brownish gray, calcareous, fossiliferous (mainly microfossils), clayey, glauconitic, medium- to coarse-grained, angular to rounded, quartz sand (Figs. 6.16-6.22). In the upper 14 m, few clasts are larger than 1-2 cm. At the very top of the breccia, the glauconite-quartz sand is replaced by dark greenish-gray, clayey silt, with .centimeter-scale laminae of fine to very fine sand. This silty interval also contains numerous nodular concentrations of framboidal pyrite (Figs. 6.14, 6.22). This silt-rich facies contains no indigenous foraminifera, but foraminifera reworked from deeper in the Exmore breccia (chalky, leached specimens) are concentrated in the thin, white, horizontal sand laminae, which also contain concentrations of muscovite flakes (Fig. 6.22C). At NASA Langley, this silt-rich layer is in sharp contact with the underlying glauconitic quartz sand of the Exmore breccia (Fig. 6.22A, D). At Bayside and North, however, the silt layer is in transitional contact with the underlying sandy breccia matrix. Also, the upper part of the silty layer at Bayside and North is more obviously stratified with white sandy, micaceous laminae and burrow casts, whose spatial orientations change markedly along the core (Fig. 6.23; flowin lithofacies of Chapter 11). Some laminae are horizontal, but others are inclined at variable angles and in different directions. A few rippled laminae and occasional centimeter-sized clasts are present in the silt at Bayside. In the upper - 3 em of the laminated silt-rich interval at NASA Langley, is a concentration of millimeter-sized, porous lattices of framboidal pyrite (Figs. 6.22A, 6.24A,B). The key impact-related feature of the pyrite lattices is their pore structure. Each pore is nearly perfectly spherical, of uniform - T-mm diameter, and spatially arranged as if the lattice originally had enveloped a layer of microFig. 6.22. (Opposite page) A whole segment of NASA Langley core, showing lithic transition from Exmore breccia to dead zone. Dashed line is contact between sand matrix of Exmore breccia and silt-rich layer, which contains coarse-grained burrow-fills, pyrite lattices with spherical pores, nodular concentrations of framboidal pyrite, and reworked specimens of foraminifera. Dotted straight lines are boundaries of falIout layer. Black rectangles are sample locations. B vertically split segment of NASA Langley core, showing concentrations of framboidal pyrite in upper part of silt-rich layer (arrows indicate position of this segment in A). C horizontally split segment of NASA Langley core showing complex microlithologies (micaceous silt, dark clay band, coarse-grained burrow-filI, nodular pyrite concentrations) of silt-rich layer just above contact with Exmore breccia; thin, micaceous, white laminae contain benthic foraminifera reworked from Exmore breccia. D verticalIy split segment of NASA Langley core, showing irregular contact between sand matrix of Exmore breccia and overlying silt-rich layer. Datum for vertical scale is top of fallout layer. See Chapters 5, 13 for further discussion of dead zone; see CD-ROM for color version of this figure.

The Exmore Breccia

A Fallout Layer Pyrite lallices

773.65 It (235.81 m)

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204

Synimpact Crater-Fill Deposits

spherules at least three-microspherules-thick (~3--4 mm). These properties are quite similar to those of impact-derived layers of glass (or glass altered to clay) microspherules (microtektites) reported from other fallout ejecta deposits (Bohor 1990; Olsson et al. 1997; French 1998). Poag (2000, 2002b), Poag et al. (2001), and Poag and Norris (in press) inferred that the pores in the pyrite lattices originally contained glass microspherules ejected from the Chesapeake Bay crater. They speculated that after the spherules settled out as sedimentary particles, the framboidal pyrite encompassed them, like foam rubber around ball bearings. Over time, the microspherule glass dissolved, or altered to clay, which was inadvertently washed away during sample preparation. (See Chapter 13 for further discussion of this layer of pyrite lattices). Though a stratigraphically equivalent silt-rich interval is present at the top of the Exmore breccia at the other core sites (where it is inferred to be the final impact-generated deposit), the pyrite lattices have been found only at NASA Langley.

6.3.4 Sedimentary Structures

Whole-core segments and split-core sections of the Exmore breccia reveal an immense variety of excellently preserved sedimentary flow structures and softsediment deformation features indicative of deposition in a turbulent, even violent, fluid milieu. Clear examples of brittle deformation also are present, but are relatively scarce (Figs. 6.14, 6.18-6.21). Both rounded and angular clast boundaries are common (Fig. 6.14, 6.21). Inclined clast-to-matrix contacts dominate (Figs. 6.18-6.21). Squeezing, folding, stretching, twisting, rotation, truncation, shearing, faulting, and fracturing are manifest in various clasts ranging from massive clay to laminated silt and clay, to fine, medium, and coarse sand, to indurated bioclastic limestone, and granitic crystalline basement (Figs. 6.14, 6.16-6.21). On both a geological and historical time scale, all these deformational processes operated essentially simultaneously (within a few minutes to hours) in various parts of the extremely turbulent incipient breccia body, but the detailed succession of deformation (on a scale of hours) depended upon distance from the point of impact and the progression of shock compression, rarefaction, ejection, fallback, slumping, sliding, surgeback, washback, flowin turbulence, and fallout, produced by the impact event. Perhaps the most noticeable sedimentological feature of the Exmore breccia, on a macroscopic scale, is the scarcity of horizontal contacts, horizontal laminae, or horizontal bedding planes. Nearly all clast-to-matrix contacts (for pebbles and larger clasts) are inclined, often at 45 degrees or more from horizontal (Figs. 6.186.21). Sixteen of the most pronounced inclinations in the Exmore core ranged from 18° to 90°, and averaged 43°. Drag and shear folds, fractures, faults, truncated laminae, and complexly convoluted flow bands give evidence of diverse differential motions having taken place along these inclined contacts. Laminations and(or) bedding planes within individual clasts also are frequently inclined, attesting to rotational motion during the depositional process (Figs. 6.18-6.21). Many

The Exmore Breccia

940.6 ft (286.7 m)

915.7ft (279 .11 m)

205

916 .8 ft (279.44 m)

c 6 I

cm

A

B

Fig. 6.23. Photographs of one whole-core section from North corehole (A), showing uphole changes in stratal geometry in silt-rich layer and two whole-core sections from Bayside corehole (B, C), showing multidirectional, inclined stratification in silt-rich layer. Sparsely scattered clasts shown by arrows . These silt-rich layers represent flowin depositional facies discussed in Chapter II . See CD-ROM for color version of this figure.

206

Synimpact Crater-Fill Deposits

Fig. 6.24A. Scanning electron micrographs showing fragments of framboidal pyrite lattices extracted from fallout silt layer at NASA Langley core site. Note similar diameters of hemispherical depressions in lattice fragments. Arrows indicate smoothly concave depressions in which glass microspherules are inferred to have originally rested. l-AB and 2-AB are stereopairs.

The Exmore Breccia

207

Fig. 6.24B. Conceptual reconstruction of pyrite lattices with glass microspherules restored. Numbered clusters refer to pyrite-lattice fragments shown in Fig. 6.24A. Modified from Poag (2002b).

208

Synimpact Crater-Fill Deposits

clasts are surrounded by thin (millimeter-scale) rims of clay (Fig. 6.18), from which we infer that these clasts moved through the fluid as individual fragments, rather than being merely entrained within a larger slump block or debris flow. Folds are frequent among the less consolidated silt, sand, and clay clasts. The folds vary from gentle, open folds, to tight, 1800 recumbent folds (Figs. 6.186.21). The limbs of some primary folds have undergone additional wavy or crenulate folding (Fig. 6.25). Fold axes display a wide variety of spatial orientations. Small faults and fractures are present in abundance, even in some of the less competent silt and clay clasts (Figs. 6.18-Q.21, 6.25-Q.27). Many of the faults and fractures display clay fillings, even though the lateral offsets are measured only in millimeters (Fig. 6.18). Some clasts, clays in particular, have been stretched or squeezed into thin necks that connect adjacent thick blebs of the same lithology (Figs. 6.19-Q.21). The margins of some clasts are drawn out into fine, wispy peaks or "tails" (Figs. 6.20, 6.21). Less frequently, clasts have been stretched by shear forces, so that wispy tails extend in opposite directions away from the parent clast. A few spectacular examples of flame structures are present, in which one lithic component has been injected into another. It is not uncommon to observe flame structures in which the compressive forces that formed them appear to have been directed laterally rather than vertically (Fig. 6.21). In some cores, centimeter-scale mollusk fragments are abundant within the matrix of the Exmore breccia, and stand out as thin white streaks (Fig. 6.16). The shell fragments appear to have no preferred orientations, though no statistical measurements of them have yet been carried out. Angular clasts are abundant in the breccia, and are composed of lithified and unlithified sands and clays, as well as tightly cemented bioclastic limestones and fragments of crystalline basement rocks (Figs. 6.17-6.20, 6.27). Cement-filled fractures are common in the more competent clasts, but macroscopic examination does not reveal whether or not the fractures formed as a result of impact shock (Fig. 6.27). However, petrographic studies of basement clasts (see section 6.3.6) reveal microscopic shock-deformation features and incipient shock melting produced at impact pressures as high as 60 GPa (Koeberl et al. 1996). One of the most interesting lithologic expressions of high shear strain in sedimentary rocks is scaly clay (Maltman 1994a,b; Lash 1989; Moore et al. 1986). This unusual fabric has been described as "... subparallel undulations of shiny surfaces anastomosing around narrow lenticles of less fissile material" (Maltman I994b, p 275). In macroscopic perspective, scaly clay appears to be intensely fractured on a centimeter to millimeter scale. This "shattered" fabric has been reported from deep-sea accretionary prisms, in stratal disruption zones of folded mountains, and even in drumlins and landslides. The highly fractured fabric of large Paleocene clay clasts incorporated within the Exmore breccia (Fig. 6.26) is similar to scaly clay, and may represent a similar physical response to impact shock. However, Moore et al. (1986) concluded that scaly clays in the Nankai Trough formed at much lower strain rates (1O·I3 S· I) than those typically developed in bolide impacts (104 S·1 to 106 S· I; French 1998). This difference may indicate that the scaly clays sampled at Exmore formed at localities distal from ground zero.

1377.3 ft (419.80 m)

A'

S'

Fig. 6.25. Split (A) and whole (B) core sections from Exmore corehole, showing vertical squeeze deformation and drag folds in clast of Lower Cretaceous laminated, organic-rich silt within Exmore breccia. Tracings (A', B1 show interpreted sense of motion on principal faults (solid lines) and trace of main fold axis (dashed lines). Numbers at top of each segment indicate drill depth at top of segment. See CD-ROM for color version.

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210

Synimpact Crater-Fill Deposits

548.8 ft

2012 .2 ft (613 .32 m)

(167 .27 m)

A'

A

o

4

I

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em

Fig. 6.26. Photograph s of whole core section s, showing boulders (continuous core) of scaly clay within Exmore breccia in Windmill Point core [A 0.76 m (2.5 ft) thick] and NASA Langley core [B 0.37 m (1.2 ft) thick]. Number s at top of segments indicate drill depth at top of segments . See CD-ROM for color version of this figure.

The Exmore Breccia

A

Ex 1361.0ft (414 .83 m)

B

Ex 1346 .8 It (410 .51 m)

c

211

Ex 1196 .8 It (364 .79 m)

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F

o

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WP 554.3 ft (168.95 m)

Ex 1354.4 ft

. (412.82 m

-

em

em

em

Fig. 6.27. Photographs of five whole sections (B, C, D, E, F) and one split section (A) of core, showing variety of small, angular, crystalline basement clasts within Exmore breccia from Exmore corehole (Ex) and Windmill Point corehole (WP). Numbers at top of segments indicate drill depth at top of segments. See CD-ROM for color version of this figure.

212

Synimpact Crater-FillDeposits

6.3.5 Expression on Downhole Geophysical Logs

A comparison of downhole geophysical logs from each of the coreholes reveals considerable variability in the expression of various matrix-rich and boulder-rich sections. For example, though some of the matrix-rich intervals produce strong positive deflections in the SP curve, indicative of high permeability, other matrixrich intervals produce negative deflections, which indicate low permeability (Figs. 6.3A,B, CD-ROM.7). Likewise, in some cases, boulders produce more positive SP deflections than matrix-rich intervals, but other boulders produce more negative deflections than matrix-rich intervals (Figs. 6.3A,B, CD-ROM.7). In other words, because of the random spatial distribution of different clasts within the Exmore breccia, and their great lithic variability, one cannot confidently use the SP curves to differentiate sands from clays in the traditional manner of electric-log interpretation. These relationships create severe difficulties in trying to correlate different parts of the breccia column from corehole to corehole. It also means that any attempt to use downhole logs to interpret lithologic successions in drilled (but not cored) sections of crater-fill deposits is likely to be unreliable. On the other hand, the SP logs are quite useful in understanding the general stratigraphic succession relative to overlying and underlying strata. The logs also provide important information regarding the relative permeability of units and the nature of their interstitial fluids. In this chapter we show the principal general relationships between the rock types and their log expressions in individual coreholes. We have found it more instructive in this section to discuss the log characteristics from the top to the base of the breccia, rather than in stratigraphic order of deposition. 6.3.5.1 Windmill Point Coreho/e

In six of the seven coreholes (NASA Langley is the exception), in the upper part of the Exmore breccia, the SP curve on downhole geophysical logs deflects to more positive values relative to those of the overlying Chickahominy Formation, though the relative increase is variable (Figs. 6.3A,B, CD-ROM.7). The Windmill Point log (location outside the crater) is a particularly good example of this positive deflection (Figs. 6.3A, CD-ROM.7). In this corehole, the Exmore breccia displays three intervals of positive SP (upper, middle, basal), each of whose values are higher (greater permeability) than any other in this corehole, except for the l.5-m-thick (5-ft-thick) Potomac sand recorded at total depth. The upper penneable section is 2.7 m thick (9 ft), and corresponds to a matrix-dominated (>95% by volume), friable, glauconitic, quartz sand. Clasts larger than small pebbles are rare. The next lower lithic unit displays the lowest SP values in the Windmill Point corehole (Figs. 6.3A; CD-ROM.7). This interval corresponds to a 1.5-m-thick (5 ft) boulder of scaly clay. Below the scaly clay boulder, SP values increase gradually downward in a 1.8m-thick (6 ft) interval, which contains three ~O.3-m-thick (1 ft), silty, Cretaceous boulders encompassed by glauconitic quartz-sand matrix (Fig. CD-ROM.7).

The Exmore Breccia

213

Next downhole is a 1.5-m-thick (5 ft) interval that displays the highest SP values recorded in the borehole. But rather than representing a matrix-ric h interval, this deflection corresponds to a boulder of indurated, bioclastic, middle Eocene limestone. The limestone must be highly fractured to produce such high SP values (Fig. CD-ROM .7). In normally stratified sediments, this log pattern would indicate a sand, but in this case, we drilled a boulder, not a stratified bed. Next below the limestone interval is a 0.3-m-thick (I-ft-thick) interval of low SP, which corresponds to a hard, silty, Cretaceous boulder. The basal interval displaying high SP values in the Exmore breccia at this site is 3.4 m thick (11 ft; Fig. CD-ROM.7). The SP increases gradually downward through a section of mainly glauconitic quartz-sand matrix, which encompasses a few cobbles of silt and limestone. No core was recovered in the lower 1.5-m (5-ft) interval, which displays the next-to-highest SP in the corehole, so its lithology is not definitely known. By analogy with the overlying log-core relationships, this basal breccia interval could represent either a permeable, matrix-dominated core section, or a fractured limestone (Fig. CD-ROM.7). 6.3.5.2 Newport News Coreho/e

On the Newport News geophysical log (this core site also is outside the crater), the Exmore breccia again shows a marked positive shift of SP values relative to those of the Chickahominy Formation. Here, the breccia displays particularly high SP values in the highest interval (4.9 m thick; 16 ft) and lowest interval (3.1 m thick; (10 ft)(Fig. 6.3A). The upper interval corresponds to a sandy matrix-dominated core section containing abundant pebbles and cobbles of hard, white limestone (Fig. CD-ROM.7). The upper 2.6 m (8.5 £1) of the basal 3.1-m (10-£1) interval corresponds to a highly fragmented section of core, composed of short sections (3.75-5 em; 1.5-2 in) of sandy matrix alternating with similar thicknesses of small glauconite-sand cobbles and pebbles (Fig. CD-ROM.7). The lower 0.5 m (1.5 £1) of this cored interval was not recovered, but a collection of loose quartz and limestone pebbles at the bottom of the core is consistent with good permeabi lity indicated by high SP values (Fig. 6.3A). Sandwiched between the two permeable sections is a 2.7-m (9 £1) interval of low SP values, which corresponds to a core section dominated by boulders of 0.50.6 m (1.5-2 ft) apparent thickness (Fig. CD-ROM.7). 6.3.5.3 NASA Langley Coreho/e

Geophysical logs from the NASA Langley corehole (-5 km inside outer rim of crater) and the North corehole (also - 5 km inside outer rim) are the only downhole records of the complete synimpact crater-fill succession documented in the outer part of the crater's annular trough (the Exmore core recovered only the top of the Exmore breccia). Each of these two coreholes records a different manifestation of the transition from the Exmore breccia to the Chickahominy Formation (Figs. 6.3A, CD-ROM.7). In particular , the SP values of the uppermost 38.7 m (127 ft) of the Exmore breccia at NASA Langley are lower than those of the overlying

214

Synimpact Crater-Fill Deposits

Chickahominy Formation (rather than higher, as at other sites). In fact, with the exception of ~ 1 0 . 7 m (35 ft), SP values in the entire upper 105.5 m (346 ft) of Exmore breccia at NASA Langley are more negative (lower permeability) than those of the Chickahominy Formation . These relatively low values arise from a section of highly variable lithologies, ranging from nearly 100% clayey sand matrix in the upper 38.7 m (127 ft) to a 16.5 m-thick (54 ft) clast of highly fractured sandy, silty clay near the base of this section. The most permeable interval in the upper 105.5 m (346 ft) of breccia at NASA Langley, as indicated by high SP values at 275.8-278.3 m (905-913 ft), is only 5.5 m (18 ft) thick. This permeable interval is not a cohesive sand body, as the log signature might suggest, but is composed of numerous matrix-supported, 15-20em-thick (6-8 in) clasts, which vary in composition from weathered granite to silty clay (Figs. 6.3A, CD-ROM .7). The NASA Langley SP log from 341.4 to 442 m (1120-1450 ft) shows a series of alternating permeable and impermeable intervals, each ~9 .2-24.4 m thick (3080 ft). Below 442 m (1450 ft), which approximates the boundary between the Exmore breccia and the displaced megablocks, permeable sand intervals dominate (Figs. 6.3A, CD-ROM .7). The unconsolidated nature of these sands is reflected in a significant decrease in core-recovery in the megablock interval. 6.3.5.4 Exmore Coreho/e

At Exmore, ~5 km inside the outer rim of the crater, 54.2 m (177.8 ft) of Exmore breccia was cored and logged (Figs. 6.3A, CD-ROM .7). The SP differential between the lower Chickahominy Formation and the upper Exmore breccia is not nearly as marked at this locality as at those outside the crater rim (Windmill Point and Newport News) . Moreover, the SP values at Exmore are higher in the upper Chickahominy than in the upper breccia . The upper log unit at Exmore is a 4.0-m-thick (13 ft) interval of intermediate SP values, which corresponds to a cored section almost entirely composed of clayey, glauconitic, quartz-sand matrix (clasts larger than small pebbles are rare; Fig. CD-ROM .7). The lowest 0.6 m (2 ft) of core in this section contains increased glauconite and a few cobbles of crystalline basement. This basal section corresponds to a modest increase in SP. Below this upper permeable unit, the SP decreases in a l.8-m-thick (6 ft) interval, which correlates with a boulder of scaly clay in the core (note that a similar scaly clay boulder is present at about the same distance below the top of the Exmore breccia at Windmill Point). At 375.2-m depth (1231 ft), a 1.2-m-thick (4 ft) interval of higher SP values corresponds to another section of glauconitic quartz-sand matrix in the core (Fig. CD-ROM.7). The 16.2-m-thick (53 ft) interval from 377 to 393.2 m (1237-1290 ft) displays relatively low SP. This interval corresponds to another core section dominated by glauconitic quartz-sand matrix. Two boulders [~0.3-m (1 ft) apparent thickness] are present in the core, but are too small to record a recognizable log signature .

The Exmore Breccia

215

The 13.7-m-thick (45 ft) section from 393.2 to 406.9 m (1290-1335 ft) displays slightly elevated SP values. This section maintains the matrix -dominated lithology. The SP values decrease from 406.9 m (1335 ft) to total depth. Lowes t values occur between 416.7 m (1367 ft) and TD (Fig. CD-ROM .7). This interval correlates with a core section in which sedimentary boulders dominate over matrix . Some boulders reach - I m (3 ft) in apparent thickness . 6.3.5.5 North Corehole The position of the North core hole relative to the morpho logy of Chesapeake Bay crater is analogous to that of the NASA Langley and Exmore coreholes - it is situated in the annular trough - 5 km inside the outer rim (Fig. 6.2). The North site, however, is about half way between the NASA Langley and Exmore core holes, as measured along the circ umference of the crater, and is farther updip (west) than any of the other intracrater coreho les. The upper - 10.7 m (35 ft) of breccia at North displays elevated permeabilities, but the next deeper - 84.4 m (277 ft) between 236.2 and 320.7 m (775-1052 ft) are notably impermeable (Figs. 6.3B, CD-ROM .7). The section consists of a variety of rotated, para llel-bedded blocks of sand and clay (-19.8-m-section; 65 ft), plus intervals of highly fractured, sticky, clay-rich paleosols (also - 19.8-m section; 65ft). The basa l 28 m (92 ft) of the Exmore breccia at North (320.7-348.7 m; 10521144 ft) regains significantly more permeabi lity than most of the overlying section, before decreasing again near the top of the underlying displaced megablocks . This basal section consists mainly of tilted blocks of sand and silt. 6.3.5.6 Bayside Corehole The Bayside corehole is located in the outer part of the annu lar trough, a few kilometers from the outer flank of the peak ring (Fig. 6.2). At Bayside, the upper -9.8 m (32 ft) of section shows the typical increased permeability relative to the overlying Chickahominy Formation. The next lower 51.8 m (170 ft), from 292.0 to 343.8 m (958- 1128 ft), shows moderate , but rapid downhole shifts in permeability, in a section dominated by cobb le-size sedimentary clasts (Fig. 6.3B, CDROM.7). Between 292 .0 and 460.3 m (1128- 1510 ft) is a 116.4-m (382 ft) section of thick (6. 1- 15.2 m; 20-50 ft), blocky, high-permeability interva ls, separated by equally thick low-permeability interva ls. Most of the high-permeability intervals consist of bedded and massive sands, whereas the low-permeability intervals comprise highly fractured , clay-rich paleosols, with lesser amounts of matrixsupported , cobble-rich breccias (Figs. 6.38, CD-ROM.7). The 45 .7-m (150 ft) interval from 460.3 to 506 m (1510-1660 ft) is dominated by low permea bility arising from a succession of highly fractured, clay-rich paleoso ls. A 30.5-m (100 ft) basal section of dominantly high permeability (506-536.5 m; 1660- 1760 ft) separates the Exmore breccia from the underlying section of displaced megablocks (Figs. 6.3B, CD-ROM .7). Tilted blocks of bedded sand with internal softsediment deformation characterize this basal section.

216

Synimpact Crater-FillDeposits

6.3.5.7 Kiptopeke Corehole The Kiptopeke corehole is the only site drilled to date inside the peak ring (Fig. 6.2). It is particularly unfortunate that only a l7.7-m (58 ft) interval was cored there (394.1-411.8 m; 1293-1351 ft). Moreover, core recovery was poor in the breccia interval, which precludes direct correlation of the logs with downhole rock types. Based on analogies with the other logged coreholes, however, we have interpreted the two uppermost intervals of relatively high SP values (combined thickness of 12.2 m; 40 ft) to be permeable sands of the Exmore matrix, whereas the intervening low SP values we interpret to be a less permeable interval of undetermined lithic composition (Figs. 6.3B, CD-ROM.7). We have not attempted to interpret the detailed succession of lithic units below the cored interval. We note, however, that the upper 12.2 m (40 ft) of relatively permeable section is underlain by 54.9 m (180 ft) of section (413-467.9 m; 1355-1535 ft) that displays relatively low permeabilities, like the upper ~ 100-m sections at NASA Langley, North, and Bayside.

6.3.6 Petrography In order to firmly establish the impact origin of the Exmore breccia, and to assess the types of impact metamorphism brought about by the impact, we performed petrographic analyses on individual quartz grains and on small clasts (mm- to emsized) of crystalline basement rocks extracted from the breccia (Tables 6.4-6.7). We examined samples mainly from the two core sites outside the crater (Windmill Point and Newport News), two sites drilled in the outer part of the annular trough (Exmore and NASA Langley), and from the only site drilled inside the peak ring (Kiptopeke). We also examined thin sections from basement cores outside the crater (Table 6.8) for comparison with future analyses of basement rocks inside the crater at NASA Langley and Bayside. Our analyses corroborate the findings of Poag et al. (1992), Koeberl et al. (1996), Powars et al. (2001), and Horton et al (2001, 2002), that the Exmore breccia contains abundant evidence of shock metamorphism. The shock-metamorphic features most common in the Exmore breccia samples fall into four categories: (1) shock fractures; (2) multiple sets of PDFs (planar deformation features); (3) shock melt; and (4) glass microspherules. We also documented the lack of shock metamorphic features in basement rocks outside the crater.

6.3.6.1 Shock Fractures Typical shock fractures in quartz, indicative of relatively low shock pressures (::0:8 GPa), are common in clasts of crystalline basement extracted from the Exmore breccia (Fig. 6.28).

The Exmore Breccia

217

6.3.6.2 Planar Deformation Features (PDFs)

Higher shock pressures produce planar deformation features, and these are common in crystalline basement fragments from the Exmore breccia, where they are expressed mainly in quartz and feldspar grains (Fig. 6.29). On the other hand, PDFs in individual quartz grains of the breccia matrix (i.e., grains not incorporated in basement clasts; Fig. 6.29) are quite rare, constituting <1% (by volume) of the matrix grains (Poag et al. 1992). In the basement clasts, multiple sets of PDFs (as many as six sets) are common in both quartz and feldspars. In order to verify the shock origin of the PDFs, Koeberl et al. (1996) measured their crystallographic orientations jn selected qua!lz grains (Fig._6.30). Jhe resultant orientations [(0001), {1013}, {1012}, {1122} , {lOll}, {1121} , {5161}] (c, to, 7I:,~, r, z, s, x, respectively) are characteristic of impact shock. The relative frequencies of these crystallographic orientations indicate initial shock pressures of 20-30 OPa (Koeberl et al. 1996). We have measured the crystallographic orientations of PDFs in quartz within an additional eight thin sections of Exmore breccia from the Exmore corehole (Table 6.4; samples 1235.43-.67 ft, 1237 ft, 1280.78-1291.0 ft, 1290.6--.76 ft, 1312.0-.14 ft, 1329.20 ft, 1341.5- .67 ft, 1356.8 ft). A total of 22 quartz grains with 45 sets of PDFs could be detected in these eight sections (Table 6.9). Two grains contained a single set of PDFs each. Seventeen grains exhibited two sets of different crystallographic orientations, and in 3 cases, three sets of PDFs per grain could be measured. Thus, a total of 45 orientation measurements were carried out (Table 6.9; Fig. 6.30). Measurements in 14 grains (63.6% of all data) could be properly indexed with {hkI} . The results clearly show that most ~easurements correspond to the orientation (angle relative to the c-axis) of the {1013} zone, but also, a few measurements correspond to other shock-characteristic crystallographic orientations, including the {1Ol2}, {lOll}, and {2131} zones. This predominance of the {1013} orientation can be interpreted to indicate that most of the shock-metamorphosed quartz grains we identified originated from a moderately shocked crater regime (-16 OPa shock pressure). However, after comparing these results with those of Koeberl et al. (1996; Figure 6.30), derived from a different set of Exmore breccia samples, we conclude that the individual statistics, though independently confirming the presence of impact-characteristic shockmetamorphic effects in quartz grains, do not allow us to determine a statistically valid average shock pressure for the combined sets of data. Instead, we infer that these shocked clastic components were derived from different parts of the evolving impact crater and were subsequently mixed together by turbulent depositional processes that produced the Exmore breccia.

Coherent sample of interbanded (at several mm spacing) medium-grained sand/silt; fossiliferous . No shock deformation in quartz or other minerals.

Particulate sample: chert with partially shock-melted quartz; also fragment of reddish breccia that contains several shocked quartz clasts (shock fractures , partial isotropization). Fig.6.3lA shows a nearly completely melted and annealed quartz grain . Fig. 6.28A shows an example of typical shock fracturing (product of relatively low shock pressure, ca. 8 GPa) in quartz of granitoid or vein quartz origin.

1210.575 (369 .0)

1220.0 (371.9)

Coherent sample of medium- to fine-grained silt; larger grains consist of glauconite, calcite, quartz, muscovite, and magnetite. Grains well-rounded to angular; generally cemented by calcite. Calcite also is dominant mineral in matrix; no foliation . No evidence of shock deformation.

Coherent sample of glauconite cemented by phyllosilicate matrix; also a few spherulitic grains of quartz and carbonate ooliths; limited fracturing is not shock -specific .

Coherent sample of glauconitic sand ; almost entirely composed of well-rounded, moderately sorted, coarse-grained glauconite grains; some quartz in interstices . Most glauconite grains have brownish rims - presumably oflimonite. The cement is formed mainly by limonite, besides some minor silica . No shock deformation.

Particulate sample; glauconite in carbonate matrix ; contains a cryptocrystalline (devitrified glass?) particle, shale, shocked quartz fragments with fracture patterns typical of Hospital Hill quartzite shocked to 8 GPa (Reimold 1988; Huffman and Reimold 1996) or single sets of PDFs ; silt, granite-derived fragments with shocked quartz; shocked quartz (I set of PDFs) in a carbonate clast adhering to strongly shocked K-feldspar.

Grain mount; I clay, I silt, I em-sized K-feldspar grain with small quartz poikiloblasts (possibly derived from vein quartz or pegmatoid) . Both K-feldspar and quartz contain up to two sets of PDFs per crystal. Fig. 6.29B shows K-feldspar grain with PDFs) .

1228.0 (374.3)

1231.35 (375 .3)

1232.1 (375 .5)

1235.43 (376.6)

1237.0 (377.0)

1220.625 (372.05) Breccia pocket in granitoid fragment, with some evidence of melting (isotropic matrix ; Fig. 6.3IB).

Coherent silt with abundant fine-grained quartz , calcite, glauconite, magnetite, biotite, feldspar, and muscovite. Matrix consists of calcite and silica; no foliation present. No shock deformation noted in fine-grained minerals.

Description

1210.2 (368 .9)

Sample Depth [ft (m)]

Table 6.4. Petrographic analyses of individual clasts in Exmore breccia, taken from Exmore borehole inside Chesapeake Bay impact crater .

~.

o

.g

t:l

:=

~::n

o

l

en

'§

ClO

N

Coherent sample of fine-grained, slightly defo rmed glauco nitic san d (elongation of nodul es).

Cohere nt sample of clay with very fine-grained quartz fragments; no shock defo rmation detected.

Part iculate of granite fragments, silt. microc line, sand. a shocke d granite frag men t containing quartz with shock fractures and one set ofPDFs, a piece ofa reddish breccia with annealed quartz clasts and shocked granite-derived clasts (quart z + feldspar with shock fracturing and/or one set of PDFs).

Coherent medium-grained feldspat hic litharenite; poorly sorted and imm atur e (> 5 vol% phyllosilicates). In order of decreas ing abundance, this sample consists of qua rtz. glauco nite, microcline, some other feldspar, hematite and magnet ite. and musc ovite. G lauconite is genera lly we ll-rounded ; other min erals also angular. Cement mainly quart z and calcite, besides phyllosilicates. No foliation; no shock deformation.

Coherent sample of medium - to coarse-grained litharenite; poorly sorted, ro unded to angular grains. Quartz is mos t abundant, besides glauconite, muscovite, magnetite, feldspar, calcit e. carbon ate-shell fragments (brachiopods?), and chert. All these phas es are set into a fine- grained matr ix of predomi nantl y calc ite; locall y staine d by hematite. No evidence of shock deformation.

Coherent sample; matrix-do minated (ca . 50 vo l%), medium- grained, glauco nitic sand; mostly angular grains, exce pt glauco nite pellets mostly round ed. A significa nt feldspar clas t component (do minantly K-feld spar). No shoc k deformation; only limited intra gra nular fracturin g.

Particulate sample comprisi ng clay, silt, a sericite-sc his t clast, medium -grain ed arkose (uns hoc ked), medium-grained and stra ined carbonate, mylonite, a few granite-derive d part icles, part ially melted san dstone (or quartzite); also one fragmen t of fine-grai ned melt breccia (containing partially annea led quartz clasts).

Cohere nt sample of glauco nitic sand; has significa nt feldspar component and sma ll compo nent of carbonate and pyroxene (presumably volcanic-derived). No shock deform ation .

Cohere nt glauconitic sand; medium-grai ned, unshocked.

Particulate sample. One strongly a ltered, heavil y fractured, locally brecciated, granitoid fragment, with partia l annea ling of brecc ia zones, as well as shock features (planar fractures in feld spar, shock fracturing, mosaic ism, and local isotropization in quartz); one clay particle and one 0.5-cm microclin e fragment (unshoc ked).

Coherent, medium -grained glauco nitic sand; no visible shock effec ts.

1238.0 (377.3)

1239.98 (377.9)

1240.85 (378 .2)

1244 .01 (379 .2)

1248.0 (380.4)

1249.5 (380.8)

1250.85 (381.3)

126 1.05 (384.4)

1262.87 (384.9)

1263.0 (385 .0)

1264.48 (385.4)

Tabl e 6.4. (co nt.)

'D

N

iii'

o

~

to

~

S

~

tTl

Particul ate sample: a breccia particle containing shocked quart z, unshocked granite , and granite-derived fragments, pegmat ite- or vein quartz-derived quartz (this seems to be the precursor material for many of the shocked quartz particles ; also observed in bands across granite-fragments), clay, sand, and a single shocked gran ite clast. Generally, the shocked mineral and lithic fragments are derived from crystalline basement lithologies ; no bona fide shocked sediment particles were observed. Some rare shocked quartz grain s are present in carbonate, but there are carbonate veins cutting across granite as well.

Coherent, medium-grained arkose (abundant microcline, some orthoclase and plagioclase). No shock effects, though some frayed and kinked muscov ite.

Coarse -grained vein quartz clast in glauconitic sand (ca. 25 vol% matrix); chert, carbonate, clay, and oxide components in clast population. Much granitoid-derived feldspar (similar to the other sandstone samples) ; also an iron-ox ide-cemented nodule . No shock deformation.

1290.6 (393.4)

1291.37 (393 .6)

1294.6 (394 .6)

Polymict lithic and mineral breccia . Contains red breccia clasts in contact with glauconitic sand . Medium-grained, angular mineral and lithic clasts often show minerals with reduced birefringence (mostly quartz and feldspar) . A part of two sections is hematitestained. The contact to this zone is irregular, but well-defined. Large mica clast s are kinkbanded. Enhanced cleavage present in some feldspar grains, but no unambiguous PDFs or other shock deformation. This sample represents either a clast of brecciated granite in the sandstone or a thin injection vein ofpolymict allogenic breccia.

1286.7 (392 .2)

Glauconitic sand, similar to sample at 1349.5 ft. Angular clasts, some carbonate, and a lot of feldspar. No shock deformation.

Particulate sample comprising silt, shocked quartz , unshocked microcline , clay, and glauconitic sand fragments .

1280.78 (390.4)

Particulate sample : one microcline fragment, one quartz fragment with possible shock fractures (Fig. 6.28C) , four clay particles, two silt fragments, and one fragment ofK-feldspar with a dense PDF pattern (multiple sets of different crystallographic orientation (Fig . 6.298).

Particulate sample : nine silt fragments, one clay (all 10 are well-rounded), and one quartz fragment with shock-derived planar fracturing . Fig . 6.29A shows densely spaced PDFs in K-feldspar. Typical , low-shock "shock fracturing" visible in Fig. 6.28.D.

1280.78 (390.4)

1290.6 (393.4)

Glauconitic sand, very similar to material from 1349.5 ft depth, but clast size, on average , somewhat larger. Matrix is dominantly sericitelbrownish phyllosilicate. Clasts mostly angular; strong feldspar clast component (ca. 15 vol%), mainly K-feldspar; some carbonate clasts . No shock deformation . A second specimen from this depth has up to 3-mm-wide pellets ofglauconite. Carbonate could be of fossil origin. No shock evidence.

1272.6 (387 .9)

1288.36 (392.7)

A l-cm quart z pebble , unshocked and barely fractured , in glauconitic sand similar to that from 1249.5 ft depth . Unshocked.

1269.11 (386.8)

Table 6.4. (cont.)

~.

o

.g

tl

E1

~

O

P-

)-

en

o

N N

Particulate containing clay, un shocked microcline, partiall y alte red impact melt (aphanitic, with K-feld spar clas ts; Fig. 6.31 .0), silt, sand, sho cked (shock fracturi ng, rare POFs; local melt ing along frac tures and grain bound aries) quartzite (annea led granite- or veinderived?), shocked sand (local isotropization in quartz).

Particulate of carbonate, chert , granite with fractures and undul ous extinc tion in quartz; shocke d grani te with mosaicism and numerous sets of POFs; unshoc ked granite frag ments.

1329.20 (405. 1)

1341. 5 (408 .9)

Particulate of silt, shocke d (mo saicis m and plan ar fractures, or shock fracturing) and unshocked granitic fragments , shale, clay with a shocked quartz grain (mosaic ism), partiall y shoc ked (shock fracturing or one set ofPOF s in quartz) and locall y melted granite, unshocked microcline, weak ly shoc ked (frac tured) and fine-grained chert, unshocked sandstone, a carbonate plu s opaque mineral fragme nt. Fig. 6.29C shows POFs in K-feldspar.

1323.82 (40 3.5)

Sand (app arently layered); locall y glauco nitic, locally shea red. Some large (up to 1 em) quartz clasts. Varia ble grain si ze in different band s ranging from fine- to medium-grained. No shoc k deformation .

Coherent sample of we ll-banded clay/sa nd (interlaye red quart z-rich/qu artz-po or layers in pattern remini scent of cross-banding). Abundant medium-grained quartz; no shock deform ation.

1314 .75 (400 .7)

1339.03 (408 . 1)

Coherent silt. Abundant, fine-grained quartz grains, less calci te and muscovite, and minor feldspar and magnetit e; set into a ph yl losilicate-d ominated, locally ferruginous matri x, and cemented by calcite. No foliation, no shock deformation .

1313.0 (400 .2)

Four clay and two quartzite frag ments; one quartzite fragment has shoc k fractures .

Particulate of unshoc ked quartz, brecciated quartz with hemat ite-rich material adhering to it (equiva lent of reddis h brecc ia"), clay, weakly shoc ked/strained sand (undulous extinction of quartz grains) , a shoc ked granit e fragme nt with quartz showing local shock fracturin g, brecciation, and melt ing (strong shoc k heterogeneit y at the grain sca le), silt, and band ed carbonate.

Particulate containing silt, brecciated leucogr anit e, strongly altered and unshocked granite, and sand with car bonate lens es. Besides local cataclasis in a leucogranite fragment, no likely shock met amorphi c effects.

Coherent sample of interbedded (at a 3-5 mm scale) clay/g lauco nitic clay, which cont ains some carbonate frag ments.

1307.95 (398 .7)

3 12.0 (399 .9)

1332.0 (406 .0)

Silt comprised of quartz, magnetite, calcite, glauconit e, mu scovite, and biotite fragm ents, cemented by calcite. Seco ndary quartz and calcite form veins and fill interstices. No foliation ; no shoc k deform ation .

1302.0 I (396 .9)

1332.35 (406 . 1)

Cohere nt, fine-grai ned c1ayfbiotite-schis t; band ed. Too fine to recog nize shoc k effects.

1298.54 (395 .8)

Table 6.4. (co nt.)

-

tv tv

Pl '

go

tIl

I

~

tTl

Particulate of sand, arkose, clay, and schist fragments .

Silt, banded with Fe-oxide-rich and Fe-oxide-poor layers.

Glauconitic sand with 1.5-cm nodule of proto mylonitic, arkosic sand (hardly any fine-grained matrix) . Large (several mm) concentric nodules of glauconite . No shock deformation.

Coherent sample of fine-grained silt. Some opaque globular fragments in clastic matrix (fragmental impact breccia?) composed of mostly feldspar grains.

Arkosic sand; poorly sorted with > 15 vol% fine-grained matrix . Clasts include (in order of decreasing abundance) quartz (angular to subangular), glauconite (rounded grains), magnetite, calcite , and muscovite. Many glauconite pellets have brownish margins, apparently as a result of oxidation. Feldspar grains also appear brownish due to oxidation of Fe. Brown staining in the matrix can be attributed to oxidation of fine-grained glauconite particles.

Glauconitic sand with largely sericitic matrix ; a significant component of feldspar (ca. 10 vol% ; mostly microcline) clasts . Most clasts are angular . No shock deformation. A second section contains a few (up to 0.5 em, ovoid) silt clasts .

Shocked quartz (Fig. 6.29.D) in a granitoid fragment.

Completely weathered (oxides , chert, some carbonate) granitoid clast. Seems to have mafic patches that could be relics of primary mafic minerals .

Silt, grad ing into clay ; sheared; tiny flakes of mica.

Similar to sample B. Possibly containing some pollen . None of these three samples shows shock deformation.

Silt layer in slightly coarser (still fine-grained) sand . No glauconite.

Section of the sand only. Rather mature «20 vol% matrix , rather quartz -rich). Contains some feldspar clasts (about 5 vol%). No shock deformation.

1345.0 (410.0) B

1346.0 (410 .3) A

1346.0 (410 .3) B

1346.15 (410.3)

1347.0 (410 .6)

1349.5 (411.1)

1356.8 (413.6)

1358.0 (413.9) A

1358.0 (413 .9) B

1358.0 (413.9) C

1359.4 (414 .3) A

1359.4 (414.3) B

1362.18 (404 .2) A Particulate with many clay and silt clasts . One silt fragment with a relatively large, unshocked granitoid fragment; two perthitic, unshocked K-feldspar particles .

Seven clay and four silt fragments (too fine-grained to identify possible shock deformation).

1345.0 (410 .0) A

Table 6.4. (cont.)

o ~.

.g

== t::l

.::n~

o

p.

~.

~

N N N

Fine-grained sand, dom inated by subangular quartz and chert fragments, with minor glauc onit e, muscovite, biotite, and magnetite. The cement consists of microcrystall ine ca lcite and silica. Co herent sample; no shock deform ation .

1377. 6 (4 19.9) 8

Particulate. One fine-grained meta-quartzite particl e (in places chert-lik e), attached to which is a glass sphe rule (Figs . 6.32 D,E) .

Medium- to coarse-grained litharenit e. Poorly sorted, with rounded to angular fragments . Ca lcite and quartz are the most abundant cla st phases, besides glauconite, muscovite, microclin e and plagi ocla se. Fragments within a fine-grained matri x of similar mine ralogical composition, but with ca lcite as most abundant mineral. In places, magneti te alteration has cause d Fe-ox ide staining of the matrix . No foliation ; no shock deformation.

1377. 6 (4 19.9) A

1388.2 (423 . 1)

Folded and miero-b oudin aged ferrugi nous clay band in sheare d sand. Abundant tiny mica flakes. Some quart zitic nodul es. The sand is very fine-gr ained, nearly a silt. No shoc k deform at ion .

1377.3 (41 9.8)

Particulate of one fine-grained sand particle, one clay fragment, and one silt fragment.

Silt consis ting ofquartz, calcite, muscovite, fe ldspar, biotite, minor magnetite, and glauconit e; all within a matri x of phyllosi licates , claeite, and minor quartz. No foliation; no shock deform ation .

1375.3 (41 9.2)

Medium-grained glauconitic sand. In order of decreasing abundance, the constituents are: glauconite, quartz, ca lcite, carbonate fragments (including intracl asts, brach iopod shell fragme nts, and mollu sc shell fragment s), magnet ite, and limonit e. The gro undmass is almo st entirely opaque, but also contains some carbon ate. No foliation ; no shock deform ation .

Co ntac t betwe en fine-grained, glauco nitic sa nd (ca . 35 vol% phyllosilicate in the matrix ) and clay, containing s and lenses. Few feldspar clasts. No shock deform ation .

1378.33 (420. 1)

Medium-grained glauco nitic sand; unshocked .

1371.2 (417 .9) 8

1374 .0 (41 8.8)

1387.4 (422 .9)

Well-laminated (on a rnm-scalc) silt to sand sequence; slightly sheare d. Contains a few feld spar clast s. No shock deform ation .

Particulate of two quart zite fragments, one spherule remnant , and one weathered, monomict granitic breccia (cataclas tite). Th e latter two of likely impact origin.

1366.6 (41 6.5)

Coherent, fine-grained clay.

1365 .9 (4 16.3)

137 1.2 (41 7.9) A

Particulate composed of schist; shocked and unshocked, fine- grained sand; clay; sho cked (shoc k fracturing, mosaicism) and unshocked quart z particles.

1362.18 (404.2) 8

Table 6.4 . (co nt.)

W

tv tv

S;.

n

g

to

3o
;><

rn

-l ::r (1l

224

Synimpact Crater-Fill Deposits

Table 6.5. Petrographic analyses of bulk samples of Exmore breccia taken from Kiptopeke corehole, inside peak ring of Chesapeake Bay impact crater. Description

Sample Depth [ft (m)] 1329.2 (405.1)

Quartz, microcline, some plagioclase fragments, some mudstone and siltstone, orthoclase, glauconite, glauconitic sand, some micro-oolitic (foraminifera?) carbonate, some other carbonate fossils; >90 vol% granitederived material, - 10 vol% sediment. Some of the fine-grained quartzitic fragments could be annealed granite-derived clasts. No shock deformation.

1331.0 (405.7)

Three sections from this sample. Quartz, microcline, some plagioclase fragments, some mudstone and siltstone, orthoclase, glauconite, glauconitic sand, some micro-oolitic (foraminifera?) carbonate, some other carbonate fossils; somewhat lower proportion of sediment than sample at 1331.0 ft, especially less fine-grained sand/quartzite. No shock deformation.

1331.2 (405.8)

Clay, aplite, and quartz fragments. No shock deformation.

1332.25 (406.1)

Two sections of this material, which is similar to sample at 1331ft. Some quartz crystals contain subplanar fluid inclusion trails, as well as some undulatory extinction, but no definitive shock deformation.

6.3.6.3 Impact Melt Rocks

At impact pressures greater than 45 GPa, target rocks begin to melt. Typical shock-melted minerals or mineral assemblages are present in many crystalline basement clasts within the Exmore breccia (Fig. 6.31). We observed partly or wholly melted fragments of granite and feldspar, breccia clasts enclosed by possible suevite, melted/annealed quartz grains, annealed melt veins in granite, aphanitic impact melt with K-feldspar clasts, and local melt zones around fractures and grain boundaries. This group of melt products indicates that shock pressures as high as 55-60 GPa are recorded in basement clasts within the Exmore breccia (see also Koeber! et al. 1996). 6.3.6.4 Glassy Microspherules

Spherical bodies of impact melt in the I-mm or smaller size range, are found in proximal impact deposits of only a few terrestrial impact craters [Barringer (Meteor Crater, Arizona; Mark, 1987), Wabar (Saudi Arabia; Krinov 1966; See et al. 1989), Lonar (India; Murali et al. 1987), Ries (Engelhardt 1997)]. In contrast, glass microspherules (microtektites) are widely distributed in the four documented distal ejecta (tektite) strewn fields (Koeber! 1994). The Chesapeake Bay crater contains the first known occurrence of proximal, glassy, impact-melt microspherules in a submarine impact crater (Poag 2002b; Fig. 6.32). Three microspherules

The Exmore Breccia

225

Table 6.6. Petrographic analyses of bulk samples and individual clasts from Exmore breccia, taken from Newport News corehole, outside Chesapeake Bay impact crater. Sample Depth [ft (m)]

Description

426.0 (129.8)

One apparent breccia fragment composed of medium-grained quartz fragments in a silica-phyllosilicate matrix; quartz clasts show undulatory extinction and local annealing, but no unequivocal shock deformation ; three granitic clasts are altered but apparently unshocked.

427.15 (130.2)

One quartz-rich schist particle (greywacke?), one silt fragment , one large quartz grain with shock fracturing (~8 GPa) and strong undulatory extinction, but no PDFs.

431 .2 (131.4)

One altered (much secondary carbonate) microgranite fragment, one granitic fragment (quartz plus altered perthitic K-feldspar), two chert fragments , and one metaquartzite fragment. No shock deformation .

432.25 (131.7)

A coherent piece of unshocked silt/clay in contact with greywacke. Minerals present include quartz, glauconite, magnetite, calcite, feldspar, and muscovite . Poorly sorted sample with grain shapes ranging from angular to well rounded ; matrix consists mainly of calcite but also some fine-grained fragments of the other listed mineral phases. Slightly foliated rock with banding on a 0.5-cm scale.

433.8 (132.2)

One piece of fractured and locally brecciated granite, one fragment of a fine-grained melt rock that could represent impact melt rock (its clast population comprises a number of unshocked feldspar clasts), one unshocked granite clast , and a piece of fine-grained melt rock with angular as well as well-rounded quartz clasts and heavily altered matrix . Whether this melt rock represents impact melting or endogenous deformation is not clear.

438.2 (133.6) A

One glauconite-quartz fragment with some carbonate clasts, one piece offossiliferous carbonate, and one piece of fossiliferous carbonate breccia.

438.2

Three fragments of cherty breccia with granite-derived clasts, two fresh, unshocked granite fragments, one of which has a granophyric component.

(133.6) B 441.95 (134.7)

One fragment of fine-grained melt rock with an angular, unshocked plagioclase clast, two ca. 0.5-cm unshocked pieces of perthitic K-feldspar, and one brecciated and locally melted granitoid .

444 .72 (135.6)

One glass spherule attached to a strongly altered fragment with silicic matrix and granite-derived clasts (Fig. 6.32C); one chert fragment with a spherule indenting this chert and a second, chloritized fragment (Figs. 6.32A,B); one chert particle with a single, angular , quartzitic clast.

446.7 (136.2)

Several completely altered granitoid fragments ; one piece of a silicic breccia after granitoid-derived material.

449.25 (136.9)

One chert fragment, one medium-grained and unshocked granite fragment, one fragment of metasediment with a cherty matrix and several granite-derived clasts , and one fragment of metasediment with phyllosilicate matrix and very small granitoid-derived clasts .

226

Synimpact Crater-Fill Deposits

Table 6.7. Petrographic analyses of bulk samples and individual clasts from Exmore breccia, taken from Windmill Point corehole, outside Chesapeake Bay impact crater. Sample Depth [ft (m)]

Descript ion

539.8 (164.5)

Particulate sample: three silt fragments and remn ants of a numb er of clay fragments; one plagioclase fragment nearly isotropi c, heavily fractured and show ing, in places, mosaic extinctio n.

544.03 (165.8)

Glauconitic sand with probably shell-derived fragments of carbonate. Generally similar to the other Windm ill Point samples in this series. Coherent; no shock deformati on.

552.11 (168.3)

Medium- grained glauconitic sand with ca. 20 vol% matrix. Many internally very fine-grained carbon ate clasts (possibly shell-d erived) show a slight alignment; some folded, phyllonitic (white mica) clasts . Fossiliferous carbonate is prominent , in contrast to most of the glauconitic sands from the Exmore corehole. Coherent; no shock deformation.

553.7 (168 .8)

Particulate: One coarse-grained, strongly altered granitoid fragment, fractured but lacking characteristic shock effects; one fine-gra ined fragment of a feldspath ic melt rock containing unshocked quart z and K-feldspar clasts; one piece of metasediment with cherty matrix and granite-derived minerals (feldspar and quart z) and lithic clasts.

555.6 (169 .3) A

Matrix-dominated glauconitic sand with silty matrix. A few sand clasts in the glauconitic sand and a significant carbonate component. Coherent; no shock deformation.

555 .6 ( 169.3) B

Straight contact between glauconitic sand (as in 555.6 A) and a relatively finergrained, clast-dominated sand with distinct micro-laths of muscovite. Coherent; no shock deformation.

555 .6 ( 169.3)C

Glauconite-rich sand layer grading into a less-glauconitic variety that is similar to 555.6 A and B, and then grading into a thin, dark-brown clay layer and silt (finergrained than the fine-grained sand in 555.6 B). Coherent; no shock deformation.

563.7 ( 171.8)

Medium- grained litharenite; poorly sorted and immature sand compose d of angular quartz, rounded glauconite, >5 vol% of phyllosilicate. Other grains include muscovite, magn etite, feldspar, and calcite. Several carbonate clasts are present, including intraclasts (fragments of sediment eroded from older strata and redeposited ) and some bioclasts (bryozoa ns?). The cement consists largely of carbonate. Coherent; no shock deformat ion.

564.55 (172 .1)

Particulate : one fragment of vein quartz (or quart z-pegmat oidj) , partially annealed, especially along grain boundaries and fractures; only irregular fracturing noted. Two unshock ed fragments of metaquart zite.

565.05 (172.2)

Similar to sample 552. 11, but with prominent clay nodules. Coherent; no shock deformation.

566.4 (172 .6)

Particulate: one chert particle with small, angular quart z and feldspar clasts; one fragment of fine-grained quartzite; one piece of silicic breccia of granitic material (could represent a monomict fragmental breccia), and I fragment, ca. 0.5 em wide, of quartz with irregu lar fracturing and undul atory extinction.

The Exmore Breccia

227

Table 6.8. Petrographic analyses of crystalline basement samples derived from coreholes outside Chesapeake Bay impact crater.

Description

Corehole [Name/ Number]

Sample Depth [ft (m)]

C251111

1869.75 (569.9)

Coarse-grained microcline-granite with chloritized amphibole and sericitized as well as saussuritized plagioclase. No shock deformation.

C251111

1960.4 (597.5)

Magnetite-bearing, muscovite-plagioclase granite with a few irregular fractures. No shock deformation.

C26111 2

1498.6 (454.0)

Coarse-grained microcline-granite, similar to C25/1869.75 ft, but more strongly altered. No shock deformation.

C261112

1500.0 (457.2)

Similar to C2611489.6 ft, but more strongly altered; quartz strongly annealed; contains secondary biotite; plagioclase completely altered. No shock deformation.

C261112

1524.0 (464.5)

Biotite-granite; some annealing and deformation in the form of relatively large subgrain domains, the formation of which would have required a significant time. No shock deformation.

Table 6.9. New measurements of planar deformation features in grains from Exmore breccia of Chesapeake Bay impact crater.

Sets of planes

1 2 2 2

2 2 2 2 2 2 2 2 2 3 3 3

Orientations 1121,(0001-1013) 5261,1012 (0001-1013 ), iou 1122,2131 1013, 1013 2131,2131 1013,(1012-1122) 1013, iou 1121,2241 (0001-1013) ,1013 1013, 1012 1013, 1122 1013, 1121 2131, 1012, 1013 (0001- 1013), (0001-1013 ), ioh 1011, 1122, 1012

(0001 -1013) means between0° and 23° (1012-1122) means between24° and 48° Of22 grains examined. data for 14(63.6%) could be indexed (see Grieveet al. 1996)

Number of grains

2 I I I

2 I

4 2 I I I I

I I I I

~

;"

I

~:..~~~r,!·,··~·~

Ex 1290.6 ft (393.4 m)

D

B

Ex 1280.78 ft (390.4 m)

Ex 1235.43 ft (376.6 m)

Fig. 6.28. Photomicrographs of thin sections (A, D plane-polarized light; B, C crosse d polarizers), showing typical fracture patterns res ulting from low-pressure (-8 GPa) shock metamorphism in clasts of crystalline basement extracted from Exmore breccia (Exmore corehole). A typica l shock fractures in quartz (shocked to :0;8 GPa) within granitoid fragment or vein, width of field 1.1 mm; B shocked quartz frag ment with fracture patterns similar to those of Hospital Hill quartzite (South Africa; shocked to :0;8 GPa; see Reimold 1988; Huffm an and Reim old 1996), width of field 3.4 mm; C quartz with possible shock fractures, width of field 2.75 mm; D typical shock fracture s in feldspar, width of field 2.2 mm. See CD-ROM for color version of this figur e.

c

*

-:Al'>."1/'

'

., .." ~:"lr;'4"'.,

.. ,,' . . ,

..

':~~~~.~~;~::~" ., ,~~~

.

Ex 1220.625 ft (372.05 m)

,

.

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~.

oen

.g

o

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~

n

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§.

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00

N N

- . .-

Ex 1323.82 ft (403.5 m)

-

Ex 1280.78 ft (390.4 m)

. .

o

Ex 1356.8 ft (413 .6 rn)

NL 820.6 ft (250.12 m)

K 1332.25 ft (406. 10 m)

Fig. 6.29. Photomicrographs of thin sections (plane-polarized light), showing POFs (planar deformation features) resulting from shock metamorphism (20-30 GPa) in clasts of crystalline basement (A -D) and in individual quartz grains (E. F) extracted from Exmore breccia. A densely spaced multiple sets of POFs in K-feldspar grain , width of field 220 urn ; B K-feldspar grain with dense pattern of multiple POFs, width of field 355 urn ; C multiple sets of POFs in K-feldspar grain , width of field 335 urn ; D multiple sets of POFs in quartz grain from granitoid fragment, width of field 565 urn; E and F individual quart z grains from matrix of Exmore breccia (cross-polarized light) , each showing two sets of POFs , width of field - 0 .2 mm. Ex = Exmore corehole; NL = NASA Langley corehole; K = Kiptopeke corehole. See CO-ROM for color version of this figure .

c

A

tTl

N N '-0

p;'

(')

g

tl:I

~

S

~

230

Synimpact Crater-Fill Deposits

10

,M ....

A

32 planes in 24 grains

....

0

;= N

I.... 0

-.. ....

....

;:::::

1(0

0

e-

:--

o ;e....: 50

B

o

~

10

20

30

J

40

~40

........

n 50

60

70

80

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"0

~30

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--....

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0

--....

.(0

;e.

0 0

>.

0

c

~ 10

90

M ..... 0

a.

.S '020

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32 planes in 24 grains ; 9% of unindexed planes

l/l

CIl "0

;=:.

:--

IT 0

U;

-.. .... ..... -..0 N .... IN .... - -

;;

e-

CT

~

U.

0

10

0

20

30

40

50

60

70

80

90

20

C

{1013}

'" 15 C1J

c

til

0::

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'0 10 Q;

N

.0

~

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I~

E

0

::>

~

~

z5

0 10

20

30

40

50

60

70

80

90

Angle between pole to plane and C axis

Fig. 6.30. Crystallographic orientation of PDFs in quartz from clasts of crystalline basement extracted from Exmore breccia. A standard histogram plot (after Engelhardt and Bertsch 1969) showing all measured data; B histogram showing frequency of indexed PDFs (after Grieve and Steffler 1996; Grieve et aI. 1996) versus angle between c axis and the poles of PDFs after transformation of the optic axis into the center of a standard stereographic projection; from Koeberl et aI. (1996). C additional measurements made by us for this volume.

Ex 1323.82 ft (403.5 m)

c

.: •

"' p;' J} "

I .'..

.' . • ..

-s- • . .

••

~

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.

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r

•

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r· J "

""

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.. ., .,'.~

.

Ex 1341.5 ft (408.9 m)

~

'0' ~ .~ ~_. . '\ ' "~) / _ -:t,,~

.

~.:~. ""

... ; ,-,' . . ...• ~ .,. ' < 1~;" ; .." . ' ". ~I '•~." • ~ ' .: . ,~ . .. • .......' . ~,4 .:,.. : '' h"~''.'.>t""... "... , " ..•/ ~..III' . '1iI. • •~

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: '"1 '".l . '"I..t.•..~!\~: ...-..~,;{.(t,~ J ,~ .·" f~. ~

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,

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.-, . . .. ..... ...1'": 'K"'; \ .

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Ex 1220.625 ft (372 .05 m)

, ~ r "·J-~ · · . ·.·.;r\' ··, . ~ . "...

. . ....

•._...:~:. •',' .

1": •, ..

B

Fig. 6.31. Photom icrographs of thin sections (all with crossed polarizers), showing melt features resulting from shock metamorph ism (35-60 O Pa) in clasts of crystalline basement extract ed from Exmore breccia. A nearly compl etely melte d/annealed quartz grain; width of fie ld 3 .4 mm ; B breccia pocket with apparent presence of melt matrix ; width of fie ld 3.6 mm ; C granite fragment with annealed melt vein; width of field 3.4 mm ; D aphanitic impact melt with K-feld spar clasts; width of field 3.4 mm . Ex = from Exmore corehole. See CD-ROM for color version of this fig ure .

Ex 1220.0 ft (371 .9 m)

A

....,

~

N

j;;'

() ()

@

to

~

;><

rn

:r (1)

Ex 1388.2 ft (423.1 m)

E

Ex 1388.2 ft m(423 .1 m)

NN 444.72 ft (135.55 m)

NN 444 .72 ft (135.55 m)

. . -.;~_Jt.~,"'j)t·~

Fig. 6.32. Photomicrographs of thin sections, showing glass microspherules from Exmore breccia. Microspherules indicate shock-melting of sedimentary target rocks at Chesapeake Bay crater. A chert fragment and chloritized fragment, both indented by glass microspherule; parallel polarizers; width of field 2.75 mm; B same glass microspherule as in A (crossed polarizers); C glass microspherule attached to and indenting strongly altered fragment with silicic matrix and granite-derived clasts ; parallel polarizers ; height of field 2.5 mm ; D glass microspherule firmly attached to glass splash on clas t of fine-grained particle of meta-quartzite; parallel polarizers ; width of field 4.5 mm ; E Same glass microspherule as in D (crossed polarizers). NN = Newport News corehole; Ex = Exmore coreho le. See CD-ROM for color version of this figure .

D

B

~.

,]

rtl

t:l

::::

::n

~';'

n

~

~

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I;Il

N

w

N

The Exmore Breccia

233

have been identified to date, one from the Exmore corehole inside the crater , and two from the Newport News corehole , outside the crater. Current geochemical evidence indicates that such microspherules form when near-surface target rocks (in this case, unconsolidated, siliciclastic, shallow-marine sediments) are melted prior to the impactor's contact with the crystalline basement rocks (Koeberl 1994; Engelhardt 1997; French 1998). Our petrographic observations can be summarized as follows: (1) The suites of particulate samples we investigated contain populations of sedimentary clasts, as well as a range of granitoid materials . Clast populations in the Exmore, Kiptopeke, and Newport News samples are not distinctly different from each other. Some individual samples are dominated by sedimentary clasts, whereas others have a significant granitoid portion. Generally, however, all samples have an important or dominant glauconite-rich fraction. In contrast, all the Windmill Point samples we studied are sediment-dominated; (2) All granitoid samples we studied from outside the crater are unshocked; (3) The proportion of shockmetamorphosed clasts is consistently small « 1%), and most are derived from the crystalline basement; (4) Mafic minerals other than biotite are rare, and we encountered no obvious volcanic rock fragments; (5) We detected no systematic variation in clast population within the entire suite of samples we analyzed. Nor was there any stratigraphically significant variation in the mean degree of shock metamorphism; clasts with low and high grades of shock metamorphism appear to have been thoroughly mixed; (6) Shock effects cover the entire range of shock metamorphism from <5 GPa to >45 (-60) GPa. This involves (in order of increasing shock pressure) shock fracturing, single and multiple sets of PDFs per grain, beginning isotropization of quartz or feldspar, diaplectic glass of quartz or feldspar, mineral melting, and, finally, bulk rock melting (particles of impact melt rock); (7) We found impact glass microspherules in two samples from the lower part of the Exmore breccia, as sampled in the Exmore corehole, and in one sample from the Newport News corehole; (8) Clasts of suspected impact melt breccia are present in three different samples from Newport News and Windmill Point; (9) The PDF orientation statistics of Koeberl et al. (1966) are significantly different from our data, because of the different numbers of grains with only one set, or more than one set, of PDFs that could be measured ; (10) Considering the fact that we measured grains with one to >3 sets of PDFs, one could conclude that these grains were shocked at pressures of - I0-30 GPa. However, we discourage calculation of average shock pressures on the basis of relative abundance of specific orientations, because different results would be obtained for these different samples (suites of measurements).

6.3.7 Geochemistry For this study, we carried out a large number of new chemical analyses for 40 clasts (85 samples and subsamples) of different lithic types (quartz sand, glauconitic sand, clayey sand, silt, clay, cherty breccia ,) separated from the Exmore breccia (Tables 6.10, 6.11, 6.12A-e, 6.13, 6.14) together with corresponding CIPW normative mineral compositions. The first objective of these analyses was

234

Synimpact Crater-Fill Deposits

to investigate whether it was possible to chemically discriminate the different lithological components that constitute the Exmore breccia. The second objective was to carry out mixing calculations , in order to better characterize the indigenous siderophile element s in the breccia . The third objective was to determine whether the Exmore breccia carries a meteoritic component. In addition, we attempted to determine the likely protolith lithologies for the sedimentary components in the Exmore breccia . We analyzed the samples in two different forms. Some fragments were quite small (millimeter-sized) and were analyzed as they are. For most samples, however, several grams of drillcore material were available. These samples were carefully broken (in a few cases with the help of a tabletop-siz ed alumina (ceramic) ja w crusher), and then powdered in either a boron carbide rapid mill (Retsch) in the case of small samples (~2 g), or in an automatic alumina ball mill. Up to 5 g of the sample powder were then used for major and trace element analysis by xray fluorescence (XRF) analysis. Trace elements analyzed by XRF are V, Cr, Co, Ni, Cu, Rb, Sr, Y, Zr, Nb, and Sa. Details (especially on precision and accuracy) are reported by Reimold et al. (1994). Some major and minor elements , and the trace elements were analyzed by instrumental neutron activation analysis (INAA). Of the trace elements analyzed by XRF, Cr, Co, Ni, Rb, Sr, Zr, and Sa were also analyzed by INAA, and, if the results agreed within errors, they were averaged; if not, the data with the better precision were selected. Information on irradiation , counting, instrumentation, standards, accuracy, and precision is given by Koeber! (1993). The only differences between our methods and those described by Koeber! (1993) are that we used a larger volume (more sensitive) germanium detector with 48% relative efficiency for all counts of the sedimentary samples, and we used four international standard reference mater ials (granites AC-E and G2; Govindaraju 1993; WMG 1, and Allende meteorite ; Jarosewich et al. 1987) for standardization and internal accuracy check in all counting batches (instead of the synthetic standards described by Koeber! 1993). The results of the major and trace element analyses of all Exmore breccia samples are given in Tables 6.10 and 6.11. The samples were all of sedimentary origin and included mainly glauconitic sands, and some silt and clay. The petrographic descriptions, including the results of our search for evidence of shock metamorphic effects , were given in Tables 6.4 to 6.7. Samples are ordered by borehole and depth within the boreholes. A large compositional range for the major and trace element contents of the various samples is evident from the data in Tables 6.10 and 6.11. For example , the SiO z content of the Exmore breccia in the Exmore core samples varies from 32.6 to 85.3 wt%, and the other major elements show similar or even wider ranges. No systematic compositional difference between the different sediment types (i.e., silt versus glauconitic sandstone) is evident from the data in these tables. To check for any systematic changes with depth in the borehole, we plotted the contents of some of the major elements with depth in Fig. 6.33. None of the three oxides plotted there (CaO, Al z03, Fez03) show any such systematic variation. In fact, the diagram clearly shows a fairly constant average content for at least two of

The Exmore Breccia

235

Table 6.10. Chemical composition of samples from Exmore breccia (Exmore corehole).

Depth [It] 120 8.2 Depth [m] 368 .3 Silt

1210.2 368.9

121 5.4 370.5

121 7.5 3 71.1

1228.0 3 74.3

1232.1 375 .5

1234 .1 37 6 .2

1234.5 376.3

Sd

Glc.Sd

Glc.Sd

Silt

Sd

Glc.Sd

Glc.Sd

- - - -- - - - - -- - - - - - --SiO, TiO, AI,O, Fe,O, MnO MgO CaO Na,O K,O P,O, LOI Total

-

49.12 0.47 8.82 6.4 1 0.028 1.67 12.19 1.17 2.28 0.21 17.42

55.43 0.62 9.63 4.55 0.032 1.28 8.86 1.76 2.38 0.13 14.0 1

64 .04 0.61 7.85 5.88 0.050 1.31 6.70 1.20 2.64 0.27 8.54

60.53 0.51 7.74 6.10 0.060 1.25 8.56 1.04 2.66 0.30 10.09

46.98 0.60 9.58 4.21 0.Q35 2.12 14.06 1.25 2.21 0.13 18.75

43.11 0.14 5.96 18.61 0.022 3.82 6.60 0.85 7.24 2.93 9.95

68.36 0.02 6.54 8.61 0.010 1.45 3.97 0.12 3.23 0.63 5.45

68.82 0.57 8.96 5. 19 0.070 1.04 3 .81 1.18 3.06 0.29 5.86 98 .85

99 .78

98.67

99 .09

98.84

99.92

99.22

98.39

Sc

8.87

9.2 1

4.99

n.d.

n.d.

133 7.11 18 <2 116 15 21.8 0.96 1.23 87.1 5 13

96.4 11.9 18 <2 118 13 42.3 0.61 0.45 83.2 365

7.35 138 88.7 8.22 25 <2 72 9 10.4 0.5 1.1 84.2 382 19 129 10 0.02 0.42 3.22 259 23.1 53.5 26.3 4.6 1 1.05 4.5 0.65 4.2 0.37 2.38 0.31 6.52 0.65 3.1 < 1.3 0.3

10.6

V

6.43 105 81.3 6.97 18 <2 53 8 8.57 0.3 1.5 90.3 326 19

5.44 20 101 5.95 8 <2 90 11 2 1.2 0.1 0.8 105 430 5 80 4 0.02 0.74 2.23 250 22.4 53.1 25.1 4.49 1.12 5.1 0.68 4 0.31 1.71 0.25 4.93 0.45 1.3 <0.8 0.6

Cr Co Ni Cu Zn Ga As Se Br Rb Sr Y

Zr Nb Ag Sb Cs Ba La Ce Nd Sm Eu Gd

n.d. 235

n.d. 322

n.d.

n.d.

0.04 0.83 4.19 211 24.9 52.2 21.9 4.94 0.95 4.35 Tb 0.61 Dy 3.8 Tm 0.29 Vb 2.07 Lu 0.29 Hf 3.59 0.67 Ta W 0.4 Ir (ppb) <0.8 Au(ppb) 1.4 Hg 0.12 Th 7.26 4.42 U

0.04 1.3 3.33 299 28.6 60.0 26.1 5.62 0.27 4.72 0.74 4.5 0.39 2.74 0.36 5.95 0.85 0.87 <0.7 0.9 0.11 8.5 1 2.80

KJU

5152

8493

122 II 0.01 0.45 2.45 249 2 1.9 50.2 25.1 4.77 1.02 5.2 0.68 3.9 0.33 1.81 0.27 5.97 0.73 5.2
n.d.

n.d.

5.86 1.85

6.66 2.12

11892

10456

n.d.

n.d.

128 13.1 33 <2 335 14 5.95 1.01 0.45 98 .1 414

147 16.5 21 <2 125 21 13.5 0.26 0.26 207 247

n.d. 286

n.d. 0.06 0.48 5.94 116 27.2 55.6 24.4 4.74 0.9 1 4.81 0.64 4.1 0.35 2.32 0.31 6.05 0.78 0.97 <0.7 1.1 0.13 9.28 2.64 837 1

n.d. 27 1

n.d. 0.05 0.96 2.89 51 81.8 22 1 76.7 16.5 3.4 14.3 2.21 9.3 0.69 4.88 0.60 0.89 0.17 0.37 <0.7 1.8 0.04 1 2.62 5.04 14365

n.d. 4.01 1.51

n.d. 83 79 II 17 <2 44 8

n.d. n.d. n.d. 83 196 20 189 II

n.d. n.d. n.d. 305

n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

17826

65.5 54.1 ZrlHf 20.4 19.8 47.3 304 16.2 LaiTh 3.43 3.36 3.74 3.47 2.93 3 1.22 5.59 Hfrr a 5.36 7.00 8.18 10.0 7.76 5.24 11.0 Th/U 1.64 3.04 3.17 3.14 3.52 0.52 2.66 8.13 LaNIYb N 7.05 8.18 6.56 7.92 11.3 8.85 Eu/Eu' 0.63 0.16 0.63 0.70 0.58 0.68 0.72 Major clements in wt%, trace elements in ppm, except as noted. All Fe as Fcz03. See Table 6.4 for petrography of samples. Glc. = glauconite; Sd = sand ; n.d. = not determined .

236

Synimpact Crater-Fill Deposits

Table 6.10 . (cont). Depth 1ft] 1248.0 Depth 1m] 380.4 Arenite Si0 2 Ti0 2 AhO ) Fe20 ) MnO MgO CaO Na20 K20 LOI

64.66 0.44 7.35 6.02 0.027 1.03 6.49 1.44 2.99 0.22 8.92

Total

99.58

P20 S

1249.5 380 .8 GIc.S
4.81 0.016

1.13 3.0\

1254.2 382.3 GIc.Sd

126 1.2 384.4 GIc.S
1269.5 386.9 GIc.S
1272.5 387.9 GIc.S
1279.4 390.0 Silt

1283.1 39 1.1 GIc.Sd

64.77 0.53 7.24 5.89 0.050 1.12 7.27 0.99 2.72 0.33 8.52

67.12 0.50 8.0 \ 4.99 0.035 0.96 5.79 1.11 2.79 0.30 7.49

64.54 0.52 8.19 4.4 2 0.028 0.90 7.67 1.08 2.52 0.24 8.73

69.05 0.55 8.92 6.67 0.040 1.16 2.65 1.11 3.10 0.36 5.48

70.1\ 0.78 13.02 4.21 0.034 0.76 1.26 2.05 2.6 \ 0.05 4.32

67 .78 0.55 8.19 7.81 0.020 1.30 3.42 1.01 3.30 0.44 5.94

99.43

99.10

98.84

99.09

99.21

99.76

6.34 Sc V n.d. 101 Cr 8.95 Co Ni 15 <2 Cu 87 Zn Ga 20 As 12.8 0.28 Se 0.23 Br Rb 85.5 276 Sr n.d. Y Zr 236 Nb n.d. Ag 0.07 Sb 0.69 2.14 Cs Ba 363 24.9 La 54.3 Ce Nd 25.5 Sm 5.08 0.75 Eu 4.64 Gd 0.70 Tb 4.\ Dy Tm 0.29 Yb 2.05 Lu 0.25 4.32 Hf 0.51 Ta W 0.77 1r(ppb) <0.7 Au (Ppb) 1.2 0.04 Hg Th 6.33 1.94 U

7.29 n.d. 61.3 8.22 45 <2 63 9 5.69 0.2 0.4 89.1 150 n.d. 195 n.d. 0.Q2 0.41 2.59 400 22.7 53.2 22.8 4.\ 5 1.15 4.9 0.7\ 4.3 0.36 2.02 0.28 4.05 0.68 1.7
6.41 119 77. 1 7.32 19 <2 59 10 10.4 0.1 1.3 88.6 250 19 162 II 0.02 0.51 2.65 247 22.5 50.7 25.2 4.42 1.11 4.8 0.69 4.2 0.38 2.11 0.31 5.45 0.63 1.2 <0.7 0.4 0.03 4.98 1.61

7.31 90 68.3 7.36 15 <2 54 12 5.44 0.2 0.9 78.3 2 14 2\ 149 II 0.02 0.28 2.26 296 22.3 50.5 20.3 4.23 1.09 4.1 0.67 3.9 0.33 2.07 0.3 1 6.31 0.58 1.8 <0.5 0.7 0.04 5.39 1.78

7.37 72 62.1 7.68 \4 <2 55 9 5.54 0.1 0.7 73.4 200 21 159 10 0.0 1 0.36 2.24 284 21.5 48.2 23.3 3.82 1.11 4.2 0.71 4.1 0.34 2.04 0.31 5.72 0.62 1.3 <0.6 0.3 0.02 5.\5 1.61

7.60 116 93 .7 8.46 18 <2 70 11 10.1 0.3 1.1 105 177 24 118 10 0.02 0.46 3.18 292 28.6 62.5 30.7 5.63 1.34 5.8 0.8 \ 4.9 0.41 2.3 1 0.34 4.34 0.63 0.4 <0.8 0.8 0.05 6.13 1.77

11.9 n.d. 67 .6 23.2 21 <2 367 22 2.15 0.31 0.15 77. 1 20\ n.d. 333 n.d. 0.04 0.39 2.14 403 26.1 53.1 25.7 5.\6 0.5 4.32 0.63 4.0 0.37 2.30 0.32 7.11 0.75 0.58 <0. 7 1.2 0.Q28 7.37 2.20

7.3 1 125 102 8.51 26 <2 63 8 14.1 0.3 1.5 106 200 22 135 9 0.03 0.5 \ 3.09 273 24.8 54.6 27.2 4.79 1.09 4.7 0.75 4.1 0.34 1.94 0.31 4.77 0.58 1.5
15412 54.6 3.93 8.47 3.26 ThIU 8.21 LaNlYb N 0.47 EulEu'

22803 48.1 4.29 5.96 4.01 7.59 0.78

14079 29.7 4.52 8.65 3.09 7.21 0.74

13062 23.6 4.14 10.9 3.03 7.28 0.80

13043 27.8 4.17 9.23 3.20 7.12 0.85

14595 27.2 4.67 6.89 3.46 8.37 0.72

KIU ZrfHf Laffh Hfrra

11864 46 .8 3.54 9 .48 3.35 7.67 0.32

1395 9 28.3 4.28 8.22 2.94 8.64 0.70

Major elements in wt%, trace elements in ppm, except as noted . All Fe as Fe20 ). See Table 6.4 for petrography of samples. Glc. = glauconite; Sd = sand; n.d. = not determined.

The Exmore Breccia

237

Table 6.10. (cont.)

Depth [ft] 1284.0 Depth [m] 391.4 Arenite

1288.4 392.7 Glc.Sd

Si0 2

68.06 0.54 8.85 7.17 0.048 1.28 3.10 1.08 3032 0.41 5.62 99.48

-

- - -- - --

no, Ah03 Fe203 MnO MgO CaO Na20 KzO P20S LOI Total

67.95 0.55 9.99 6.59 0.049 1.28 2.35 1.45 3.26 0.25 6.00 99.71

Sc

8.52 n.d. Cr 108 Co 11.7 Ni 17 <2 Cu Zn 510 Ga 9 As 8.59 Se 0.23 Br 0.19 Rb 91.4 Sr 197 Y n.d. Zr 275 Nb n.d. Ag 0.09 Sb 0.59 2.41 Cs Ba 296 La 22.7 48.9 Ce Nd 23.1 Sm 4.49 Eu 0.97 Gd 3.84 Tb 0.62 Dy 3.6 Tm 0.29 Yb 2.06 Lu 0.28 Hf 5.10 Ta 0.65 W 0.69 Ir(ppb) 0.7 Au (Ppb) 0.8 Hg 0.071 Th 5.38 U 1.94 V

KIU 16804 Zr/1If 53.9 LaiTh 4.22 Hf/Ta 7.85 Th/U 2.77 LaNlYbN 7.45 EulEu* 0.71

- - -

1294.5 394.6 Glc.Sd

1302.0 396.8 Silt

1305.3 397.9 Glc.Sd

68.52 0.57 8.98 6.78 0.020 1.23 2.69 1.22 3.28 0037 5.28 98.94

56.93 0.91 15.43 5.51 0.034 1.88 2.86 1033 2.54 0.06 12.01 99.50

85031 0.40 6.78 0.76 0.090 0.06 0.58 2.14 2.62 0.06 0.43 99.23

- --

-

7.22 122 87.1 8.11 22 <2 71 11 11.1 0.1 1.2 11 6 203 21 142 11 0.02 0.43 2.84 300 20.9 49.2 22.5 4037 1.08 4.4 0.63 3.8 0.31 1.76 0.28 4.05 0.57 2.2 0.2 OJ 0.03 5.28 1.95

6.67 123 79.2 7.84 21 <2 64 14 1.03 0.2 1.1 83.2 192 21 131 11 0.03 0.41 2.81 312 21.9 50.6 24.6 4.21 1.15 4.4 0.65 3.9 0031 1.91 0.28 4.81 0.57 1.7 <0.8 0.7 0.05 4.56 1.54

15.6 n.d. 151 17.5 21 <2 110 21 8.97 0.75 0.52 135 176 n.d. 338 n.d. 0.06 0.64 7.84 269 36.8 71.4 27.3 5.79 0.26 4.95 0.68 4.5 0.43 2.95 0038 7.27 1.13 0.49 <0.6 0.7 0.13 13.2 2039

2.02 <15 11.4 1.75 8 <2 11 9 10.9 0.03 0.1 60.9 168 8 134 9 0.02 0.29 0.81 554 11.3 20.2 10.2 1.78 0.47 1.9 0.27 1.7 0.16 0.99 0.14 4.59 0.56 3.0
\4188 35.1 3.96 7.11 2.71 8.02 0.75

17749 27.2 4.80 8.44 2.96 7.75 0.82

10628 46.5 2.79 6.43 5.52 8.43 0.15

25388 29.2 3.52 8.20 3.73 7.71 0.78

1313.0 400.2 Gr.Breccia 69.82 0.81 12.02 5.88 0.021 0.76 0.47 1.65 1.95 0.09 5.91 99038 13.6 n.d. 65.2 10.8 18 <2 272 14 1.81 0031 0.14 57.5 104 n.d. 401 n.d. 0.06 0.53 3.52 239 28.6 61.5 25.3 5.28 1.03 4.86 0.62 3.8 0036 2.65 0036 9.02 0.72 0.82 <0.7 0.6 0.031 8.61 1.65 1\818 44.5 3.32 12.5 5.22 7.29 0.62

1317.5 401.6 Glc.Sd

1330.5 405.5 Glc.Sd

70.69 0.57 8.51 6036 0.010 1.04 2.73 1.12 3.11 0034 5.20 99.68

69.84 0.55 8.73 6.41 0.019 1.06 2.47 1.24 3.27 0032 5.09 99.00

6.79 119 76.8 7.86 22 <2 54 11 8.7 0.2 0.7 89.1 174 21 130 12 0.03 0038 2.72 315 21.9 50.7 24.4 4.11 1.09 4.1 0.64 3.9 0.29 1.79 0.27 4.21 0.64 1.1
7.42 11 7 80.7 7.77 23 <2 52 12 8.72 OJ 1.1 100 185 21 135 12 0.02 0.46 2.75 312 26.1 58.9 28.5 5.25 1.22 5.1 0.69 4.0 0032 2.0\ 0.29 4.8\ 0.66 0.1 <0.8 OJ 0.05 9.21 1.88

14895 30.9 4.58 6.58 2.75 8.27 0.8\

14495 28.1 2.83 7.29 4.90 8.77 0.72

Major elements in wt%, trace elements in ppm, except as noted. All Fe as Fez03. See Table 6.4 for petrography of samples. Glc. = glauconite; Sd = sand ; n.d. = not determined .

238

Synimpact Crater-Fill Deposits

Table 6.10. (cont.) Depth [ft] 1334.4 Depth [m] 406.7 Glc.sd

1337.1 407.5 Glc.Sd

sto, no,

72.05 0.61 9.17 4.99 0.027 0.95 2.23 1.32 3.09 0.35 4.27 99.06

AIz0 3 Fez0 3 MnO MgO CaO NazO KzO PzOj WI Total

69.97 0.60 9.11 6.35 0.020 1.08 2.21 1.27 3.39 0.36 4.77 99.13

1347.0 1361.9 410.6 415.1 Greywacke Schist,SS 69.51 0.90 12.74 4.12 0.044 0.92 0.95 1.87 2.54 0.05 5.76 99.41

64.37 0.88 16.13 4.75 0.017 1.03 0.34 1.65 2.20 0.09 7.80 99.25

1366.5 416.5 SiltiSd 71.43 0.85 13.00 3.07 0.029 0.65 0.43 1.64 1.65 0.04 6.23 99.02

!375.3 419.2 Silt

1377.6 419.9 Silt

!387.4 422.9 Glc.Sd

63.56 0.92 14.98 5.54 0.056 1.35 1.04 1.78 2.52 0.12 7.82 99.67

73.57 0.77 9.10 4.42 0.043 0.92 1.73 1.29 2.87 0.22 4.40 99.34

32.65 0.15 3.12 6.52 0.087 2.02 26.78 0.44 2.95 1.67 22.78 99.17

8.34 n.d. 79.7 10.9 23 <2 453 8 4.67 0.29 0.11 73.4 250 n.d. 4! 7 n.d. 0.04 0.39 1.83 383 32.7 68.5 33.8 7.14 1.44 6.61 1.08 6.5 0.49 3.23 0.40 8.13 0.8! 0.42 <0.7 0.9 0.045 8.62 2.08

7.32 n.d. 180 4.93 33 <2 397 6 6.01 0.12 0.15 81.4 244 n.d. 275 n.d. 0.05 0.77 1.14 92 47.5 95.9 51.8 12.2 2.77 12.4 1.93 10.9 0.68 4.77 0.55 1.51 0.13 0.53 <0.7 1 0.057 2.68 4.34

6.83 113 72.9 7.91 24 <2 59 12 10.3 0.1 1.3 88.9 199 22 152 11 0.02 0.39 2.71 334 22.2 51.7 24.4 4.26 1.21 4.7 Tb 0.7! 4.1 Dy 0.35 Tm Yb 2.08 0.31 Lu 3.43 Hf Ta 0.47 W 1.3 1r(ppb) <0.8 Au (Ppb) 0.1 0.08 Hg 4.37 Th 1.61 U

6.89 109 68.8 7.81 16 <2 49 9 6.59 0.2 0.7 90.1 152 17 121 10 0.02 0.36 2.41 365 30.6 73.9 35.8 5.74 1.27 5.4 0.73 4.2 0.33 1.98 0.26 3.36 0.56 0.2 0.2 0.3 0.03 21.8 2.04

13.9 n.d. 76.6 12 19 <2 321 11 1.58 0.73 0.24 75.4 189 n.d. 542 n.d. 0.05 0.51 2.92 353 36.9 76.6 32.8 6.88 1.33 6.59 0.89 5.3 0.46 3.28 0.45 12.2 0.89 0.8 <0.7 0.4 0.11 10.6 2.28

17.3 n.d. 77.5 14 56 <2 193 12 0.93 0.23 0.02 98.1 152 n.d. 776 n.d. 0.08 0.34 4.81 284 173 527 212 33.5 8.17 28.1 3.85 20.9 0.79 5.21 0.59 6.74 0.85 0.78 <0.7 2.4 0.36 10.1 2.24

12.2 109 56.3 14.2 22 <2 90 8 1.96 0.2 0.3 101 149 24 309 15 0.01 0.39 3.65 250 29.7 64.5 29.9 5.32 1.24 5.4 0.85 5.4 0.48 3.22 0.48 8.31 1.09 1.8 0.1 0.3 0.03 9.11 1.92

16.5 n.d. 88.7 21.9 2! <2 124 18 5.95 0.51 0.19 93 180 n.d. 4 18 n.d. 0.04 0.59 3.63 367 35.4 69.1 35.3 7.02 1.4 6.26 1.04 6.2 0.49 3.57 0.48 8.2 0.81 1.24 0.7 1.1 0.09 10.1 2.23

K/U 17547 44.3 Zr/Hf 5.08 LaITh IIfffa 7.30 2.71 ThIU LaNlYb N 7.21 0.83 EulEu'

12623 36.0 1.40 6.00 10.7 10.4 0.70

11140 44.4 3.48 13.7 4.65 7.60 0.60

9830 115 17.1 7.93 4.51 22.4 0.81

7161 37.2 3.26 7.62 4.74 6.23 0.71

11278 51.0 3.50 10.1 4.53 6.70 0.65

Sc V Cr Co Ni Cu Zn Ga As Se Br Rb Sr Y Zr Nb Ag Sb Cs Ba La Ce Nd Sm Eu Gd

13808 51.3 3.79 10.0 4.14 6.84 0.64

6797 182 17.7 11.6 0.62 6.73 0.69

Major elemen ts in wt%, trace elements in ppm , except as noted . All Fe as FeZ0 3. See Table 6.4 for petrography of samples. G1c. = glauconite ; Sd = sand; SS = sandstone; n.d. = not determin ed .

The Exmore Breccia

239

Table 6.11. Chemical compositio n of Exmore breccia samples from Newport News (NN) and Windmill Point (WP) coreholes. [ft] [m] SiO, TiO, AI,O , Fe,O, MnO MgO CaO Na,O K,O P,O,

NN 432.3 131.7 Shale

LOI

55.09 0 .58 11.62 10.23 0.021 2.35 3.24 0.58 4.35 0.36 10.79

Total

99 .20

Sc V Cr Co Ni Cu Zn Ga As Se Sr Rb Sr Y Zr Nb Ag Sb Cs Sa La Ce Nd Sm Eu Gd Tb

10.5 n.d. 207 11.9 15 <2 239 16 16.2 0.92 0.29 140 158 n.d. 266 n.d. 0.09 1.10 4.56 153 32.1 76.1 30.3 6.8 1.28 9.14 0.84 Dy 4.9 Tm 0.45 Yb 2.4 1 Lu 0.32 Hf 4.9 Ta 0.72 W 0.79 Ir (Ppb) <0.8 Au (ppb) 0.9 Hg 0.072 Th 8.55 2.87 U

KJU

1515 7 Zr/H f 54.3 LalTh 3.75 Hfrra 6.81 Th/U 2.98 9.00 LaNlYb N EulE u* 0.50

NN 438.2 WP 544 .1 133.6 165.8 Calc .lChert Glc.S
WP 563.7 WP 565.2 WP 566.5 171.8 172.3 172.7 Sd GIc.Sd GIc.Sd

WP 570. 7 173.9 Ca lc. Sd

WP 544.5 166 .0 GIc.Sd

WP 552.5 168.4 GIc.Sd

59.03 0.54 7.36 7.08 0.030 1.37 10.02 0.90 3.07 0.42 9.32

64 .10 0.45 7.43 6.66 0.040 1.38 6.82 0.79 3.25 0.49 8.12

59.87 0.52 7.35 6.96 0.060 1.41 8.59 0.79 3.15 0.64 9.69

53.87 0.43 7.95 8.89 0.063 1.85 8.87 0.76 3.35 0.64 12.51

58.14 0.44 7.27 7.0 1 0.090 1.44 10.0 1 0.77 3.0 1 0.47 10.59

55.14 0.45 8.41 11.50 0.0 10 2.43 6.27 0.69 4.33 0.84 9.53

30 .85 0.10 2.82 8.29 0. 157 2.33 26 .49 0.35 3.6 1 1.59 22.85

99. 14

99 .53

99 .03

99 .18

99.24

99 .60

99.42

7.64 n.d. 145 16.3 22 <2 235 25 19.1 0.46 0.22 110 293 n.d. 253 n.d. 0.09 1.10 2.65 228 30.2 69.3 28 .2 6.72 1.31 5.96 0.98 5.9 0.4 1 2.52 0.32 3.64 0.52 0.47
6.44 121 104 7.64 25 <2 74 8 12.6 0.05 1.1 86. 1 325 21 109 9 0.03 0.55 2.23 274 22 .8 53.5 23.4 4.49 1.16 4 .\ 0.66 3.8 0.29 1.64 0.25 3.18 0.42 0.9
10.7 141 145 10.5 38 <2 88 9 15.2 0.4 0.6 152 268 46 133 10 0.02 1.34 5. 12 258 39.3 105 56.4 9.98 2.51 10.2 1.46 7.3 0.59 3.37 0.47 3.27 0.48 2.1 < 1.2 4 0.03 5.89 2.24

3.53 n.d . 184 3.52 19 <2 99 .7 4 4.12 0 .067 0.077 95.4 185 n.d. 165 n.d. 0.07 0.67 0.97 74 37.5 68.3 34.3 8.39 1.69 9.85 1.32 7.2 0.45 2.73 0.36 1.06 0.11 0.08 <0.7 0.7 0.02 1.35 3.98

10030 69.5 6.48 7.00 1.40 8.10 0.63

17790 34 .3 4 .68 7.57 3.45 9.39 0.83

16109 40 .7 6.67 6.81 2.63 7.88 0.76

6.58 136 97.6 7.05 17 <2 56 9 14.9 0.2 1.5 103 2 11 16 77 8 0.04 0 .88 2.34 300 24.7 60.3 30.1 5.49 1.40 5.3 0.78 4.7 0.39 2.23 0.32 3.52 0.42 2.2 <1 0.3 0.02 4.52 1.98

7.19 123 101 7.62 26 <2 61 12 9.95 0.3 0.7 101 270 95 9 0.02 0.42 2.33 288 2 1.8 54 .3 26 .8 4 .81 1.28 4.5 0.66 3.9 0.35 1.88 0.28 2.4 1 0.51 1.3 <0.8 0.4 0.04 4 .18 1.51

7.09 120 102 8.14 26 <2 71 8 12.9 0 .1 1.3 101 299 24 98 9 0.03 0.65 2.54 280 33.8 65.6 28.1 5.27 1.22 4.8 0.72 4.3 0.36 1.93 0.28 2.38 0.59 I
12921 21.9 5.46 8.38 2.28 7.48 0.79

17936 39 .4 5.22 4 .73 2.77 7.84 0.84

13672 41.2 6.08 4.03 2.90 11.83 0.74

22

907 0 156 27 .8 9.64 0.34 9.28 0.57

Major elements in wt%, trace elements in ppm, except as noted. All Fe as Fe,03' See Table 6.6 for petrography ofNN samples; Table 6.7 for WP samples. Glc. = glauconit e; Sd = sand; Calc = calcareous .

240

Synimpact Crater-Fill Deposits

30 ,.--

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

-

---,

-.-AI,O.

-D-Fe,o, . . /1 .. CaO

25

.

20 ......................................................................................................................................•................

;i

~

C 15 Q) C o

'.. ...

o

10

5

.

370

380

390 400 410 Depth Exmore Corehole (m)

420

430

Fig. 6.33. Content (wt%) of major elements in samples of Exmore breccia plotted against depth (m) in Exmore corehole.

the oxides, with occasional wide variations. The only oxide that shows a hint of a decrease in content with depth is Fez03' but the wide variations make this trend not very clear. Similar plots (not shown here) were constructed for the other major element oxides and for a number of trace elements (Sc, Cr, Co, Rb, Sr, the REE, Hf, Th), but they expressed the same behavior (no clear systematic trend). One of the most important groups of trace elements for provenance studies is the rare earth elements (REE) . For comparison with each other, and with a variety of other reference materials, the abundances of the REEs are normalized to the abundances in Cl-type carbonaceous chondrites (normalization values from Taylor and McLennan 1985). The normalized abundance patterns of four samples that represent some of the widely different patterns are shown in Fig. 6.34, and the total range of all Exmore breccia samples , as well as the overall average, are shown in Fig. 6.35. The average REE composition is similar to that of typical post-Archean upper crustal composition (cf. Taylor and McLennan 1985), with a modest slope of the light REE (LREE) pattern, a moderate negative Eu anomaly, and an almost flat heavy REE (HREE) pattern. The range mostly reflects this pattern, just with higher and lower absolute abundances. The REE patterns of individual samples show some more variety, as is indicated in Fig. 6.34. Europium anomalies of all samples are negative (see Tables 6.10 and 6.11) ; no positive Eu anomalies or values close to unity are recorded in our samples. Again , this is typical of sedimentary samples. The extent of the Eu anomaly (Eu/Eu*, where Eu

The Exmore Breccia

1000 . - - - - - - --

-

-

-

-

-

-

-

-

-

241

----------, ..... ExI208 .2 Sand -a- Ex1232.1 Sand -o- E. 1305.25 Sand ....... Ex1361.9 SChist

8c: co -g

100

:>

.c

QI

.!::! 'iij

E (;

z &> ·"C c

10

c:

o

J::.

U

La

Ce

Pr

Nd

Fig. 6.34. Abundances of rare earth elements in samples of Exmore breccia from four different depths in Exmore corehole. Normalization values from Taylor and McLennan (1985).

1000 ....--

-

-

-

- - - - --

-

-

-

- - - - -_ _ Average

-

-

-

---,

...... Maximum _ .. - Minimum

'" 8 c:

co

"C

c:

:> 100

.0

-c al

.!::!

iij

E o

z

2

'C

10

"C

c:

r .

o

J::.

_

.

0

-

_

•

•

_

••

_

.

.

_

••

_.

_

••

_

U

La

Ce

Pr

Nd

Fig. 6.35. Abundances (average, range) of rare earth elements in samples of Exmore breccia from Exmore, Windmill Point, and Newport News coreholes. Normalization values from Taylor and McLennan (1985).

242

Synimpact Crater-Fill Deposits

is the measured value and Eu* is the calculated value if there were no Eu anomaly) ranges from 0.15 (strong negative anomaly) to 0.85 (very small negative Eu anomaly). Some of the REE patterns are much steeper than that of the average Exmore breccia. In most cases, the slope of the LREE part of the patterns is much steeper than that of the HREE part of the pattern . A number of samples show distinct positive Ce anomalies, especially those samples that have high absolute REE contents. For some samples this could be an indication of the inclusion of a minor marine sedimentary component. For example, sample ExI232.1, with a pronounced Ce anomaly, also has a very high phosphorus content, which might be indicative of the presence of some organic marine detritus . The situation for sample Ex1387.4 is similar. In the case ofa few other samples (e.g., ExI208.2) the Ce anomaly might simply indicate an advanced state of weathering of the sediment. The diagram of the extent of the Eu anomaly versus the slope of the REE pattern (Eu/Eu* vs. LaN/Yb N; Fig. 6.36) shows no correlation between these two values, in agreement with the sedimentary origin of the materials. Any differentiation due to melting would appear as a correlation in this diagram . There is also no correlation between the absolute amount (sum) of the REE and the degree of the Eu anomaly. In addition to using the elemental and oxide abundances of the Exmore breccia samples, we also calculated CIPW normative compositions (using the MinPet program, version 2.0; see Rollinson 1993 for details on the computation of the CIPW norm and for components), as well as the chemical index of weathering (CIW) and the chemical index of alteration (CIA) (Rollinson 1993). The results of the calculations are reported in Table 6.12 for the same sample suite for which the compositions are given in Tables 6.10 and 6.1 1. Samples are identified by their number (= depth in the core in feet). The CIW and CIA values vary widely, but, on average, are on the order of 30 to 50, which indicates fairly weathered sediments. Some of the CIPW normative data, together with the major element compositions, are plotted in ternary diagrams (Fig. 6.37). The ternary diagrams in Figure 6.37 represent some of the multiple attempts to use major-element chemical data to determine the sedimentary lithologies forming most of the clast components in the Exmore breccia. It is obvious that neither absolute abundances nor CIPW normative abundances allow unambiguous discrimination of these rock types. On the other hand, Discriminant Function analysis (Table 6.13, Fig. 6.38), based on the equations given by Rollinson (1993) provides a strong indication that the vast majority of analyzed sedimentary clasts are derived from felsic to intermediate igneous rocks that are known to form the crystalline basement below the sedimentary section in the Chesapeake Bay region. However, a quartzose sedimentary provenance also must be considered (Fig. 6.38B). Due to the lack of obvious macroscopic impact breccia and impact melt rock samples among our suite of Exmore breccia samples, and due to the small size of basement rock clasts among these breccia samples, we measured no chemical compositions of such clasts . We used all of those clast samples for petrographic

The Exmore Breccia

243

1 0.9

o

0.8 0.7

..

0.6

::J

ill

:; 0.5

o

ill

0.4

o

0.3 0.2

o

0

0.1 0 0

5

10

15

20

25

Fig. 6.36. Plot of EulEu* versus LaN/Ybr; in samples of Exmore breccia from Exmore, Windmill Point, and Newport News coreholes. Solid triangles indicate values for bcdiasite and georgiaite tektites (cf. Table 6. 14). studies (i.e ., mainly the search for shock metamorphic features ; cf. Koeber! et al. 1996). Thu s, it was not possible (nor neces sary) to try to reproduce the composition of the Exmore breccia from target rock compositions in mixing calculations (wh ich, given the weathered state of most of those samples, may not have yielded any meanin gful results anyway). Analysis of the crystalline basement rock types will have to await data from the deep core s that are currently bein g drilled by the USGS (Gohn in pre ss). It is difficult to assess the presence of a meteoritic component in the Exmore breccia samples. Commonly the siderophile trace elements, including Cr, Co, Ni, and the plat inum group elem ents , especially iridium (lr), are used for such identification (cf. Koeber! 1998). This procedure requires a number of prior conditions to be fulfilled. First , brecci as used for such comparison are commonly melt- sample suite. Second, the compo sitions of a complete set of targ et rocks that were in-

24 .9 CIA CIW 26 .8 CIPW no rm 18 .36 Q 16. 99 or 13 .2 5 ab 15 .24 an C 11.6 3 di hy 17. 14 wo ac 0.0 6 iI 5.63 hem I. 15 ti 0 .5 5 ap ru KMS

[ft)

31.6 35.7

8.3 7

5 .6 8

0 .09 4.74 1. 34 0.6 5

8.6 1

10 .1 8

0 .06 3.86 1.4 9 0 .33

25 . 28 39 .0 5 17 . 12 18 .0 5 19.2 5 12.47 13.8 2 9.54

30.9 33.6

21.9 30 .8

37. 5 46 .9

42.4 50.3 29.7 34.2

28 .9 32.8

34 .3 39. 3

3.6 7

6 .90 0 .05 1.5 2

7.13 1.04

0 .12 4 .0 7 1.1 6 0.6 8

4.6 7 0.90

0.05 4. 85 0 .9 8 0 .53

8 .08

6.57 4 .47

3 .98 1.20 0 .72

0.0 9 4.75 1. 15 0.80

6. 0 7

7.99

7.16

11.91 9 .96 51.2444 .69 38 . 11 40 .764 3 .3 8 16.49 38 .86 2 1.9 5 20 .37 20.4 0 18.601 8 .87 1.24 11.94 14 .93 10.2 9 11.4 1 14 .17 17 .69 8 .94 11.40 5. 50 8 .42 9. 8 9

24 .2 25 .8

8. 15 14 .78 12 .61 6 .29 10.28 19 .3 7 7.29 0 .08 0 .04 0.11 5 .02 3.71 13 .67 1.09 1.46 0.29 0 .74 0 .34 7.3 2

34 .84 18 .57 11.03 10 .16

27 .7 30 .9 4 7.1 57 .3

60.5 69.7

41.7 51.0

3.5 8 1.2 6 0 .5 8

0.07 5 .23 0.40 0.85 0 .26 0 .11 0 .57

0 .05 3 .20

0 .04 6.13 1.24 1.04

4 6.31 44 .9 2 44 .38 2 0 .6 0 16.81 2 1.9 5 11.21 20 .0 6 10.21 11.47 6 .46 9 .0 9 5. 53 3.77 5 .77 3.6 0 2 .2 9 2.15 8. 55

40 .0 3 17 .29 11.2 6 11.6 8

30 .7 34. 3

0 .58 0.38

0 .09 5 .13

3.94

4 2.17 2 1.4 9 14.53 11.19 0.49

49 .5 60.0

5 .61 1.27 0 .96

1. 19 3.37

44 .04 22.01 10 .8 8 10 .6 6

44 .6 54.4 60.4 67.6

47 .8 59.7 67 .9 77 .1

0 .04 5 .30 1.07 0 .87 0.07

0 .1 5 0.72

0 .0 6 4 .5 2

0.1 5 0 .5 6 0 .6 6 0 .1 3

0.21 0.6 1

0.04 4 .59

3 3 .4 9 60 .4 4 52 .75 17 .67 16.0 3 12.90 14.06 2 0.2 4 16.59 16. 25 1.22 1.65 6.99 8 .0 1 0 .24 3 .81 6.11 0 .0 5 2 .3 5

44 .3 3 2 1.7 4 12 .2 9 10.48

46.2 56 .6

46.4 57.2

0 .02 4 .94 1.3 0 0 .7 9

0 .33 3.04

5.00 1.05 0 .7 5 0.0 8

3 .28

4 7.8 4 46 .12 20 .4 8 2 1.63 11.21 12.47 10. 05 9 .6 3

45.6 55 .6

Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Ex Sampl e Ex Ex Ex Depth 1208.21 21 0.21215 .3121 7.1122 8.0 1232 .11 234 . 11234.41 24 8.0 1254 .01 26 1.0 1269.11 272 .11 279.41 283 .012 84 .0 128 8.31294.11 30 2.0 1305 .2131 3. 0 131 7.11 330 .1

Table 6.12. Geochemical results from samples of Exmorc breccia from Exmore (Ex), Windmill Point (WP), and Newport News (NN) corehol es.

~.

'"

"0 0

n

t:l

E1

';'

~

(')

~

'ei 3' "0 n

CIl

~

~

tv

48 .2 59. 9

CIA CIW

0.4 7

0.45

ru

KMS NMS COS

0 .81

3.8 4

2.90

0.53

0 .84

0 .03 4 .93

3 .32

0 . 24

4 8.4 6 2 0. 18 13.10 9. 70

48 .9 59 .6

1337 .0

Ex

ap pero

ti

hem

il

ml

ac

01

wo

hy

di

ne kal C

Ie

Q or ab an

54 .6 4 22 .29 12 .69 9 .59

1334 .3

CIPW norm

Ex

Sample

D epth [ft]

Tab le 6.12. (conl.)

0 .21

0.67

0 .66

0.04 3.74

3.21

13.11

4 6.3 7 14. 6 8 16.7 3 1.24

0. 12

0.07 3.1 8

2.81

6 .4 1

46 .75 16 .60 18 .5 7 4.85

73 .9 82.9

1361. 9

134 7.0

62 .8 72.6

Ex

Ex

0.66

0 .09

2.40

2.01

9.57

55 .78 10 .9\ 16 .4 8 2 . 10

7 1.2 78.9

1366.1

Ex

0 .67

0 .28

0 .1\ 4 .34

4.1 9

0. 1 1

4 1.30 16 .74 17 .97 4 .92

66 .5 75 .6

137 5.3

Ex

0.56

0 .5 1

0 .0 7 3.40

2 .80

1.51

51.76 18 .72 12.7 9 7 .89

52. \ 63.3

\ 377 .6

Ex

WI'

0 .1 I 7.53 0.92 1.63

8 .4 6

12.41

2 6.40 24 .0 5 8 .29 10 . 19

27 .5 31.4

0. 17 5 .8 0 0.84 1. 17

13 .28

9 .43

3 1.12 21.10 8. 20 8 .89

24.2 27.2

WI'

0.02 9 .4 1 1.0 8 2.06

0 . 12

15 .7 5

25 .96 30. 03 7.27 8 .29

32.8 40.2

566 .6

WI'

NN NN

4 .54

3.43 0 .23 0 . 19 6 .88

19 .85

17. 56

\ 6 .81

5 .1 5.5

0 .4 6

0. 89

0 .04 8 .4 7

7 .71

1. 20

28 .2 1 3 0.53 6 . 19 16 .30

50. \ 62 .9

1.1 8 13 1. 2

88 .96

0 .07

- 129 .7 5 .39

2 .05 0 .55

0.5 0.5

570.7 432.3 4 38 .2

28 . 10

5 .35 1.0 8 1.18

5 .73

8. 7 8

39 .3 5 22. I 3 8 . 18 8 .2 2

30 .1 35 .2

565 .0

WI'

21. 85

5 .78 1.32 1.03

13 .4 2

8.85

30 .7 4 2 1. 2 3 9.4 6 8. \ 7

24.2 2 7.2

WI' 563.7

WI' 55 4.3

2.4 3

0 .\1 5 .72 1. 11 1.58

9.39

9.18

34 . 12 21. 93 8 .36 8 .49

26 .6 30 .3

WI' 55 4.0

0 . 16

4 .7 6 0.0 9

0.1 9 5 . 12

4 .31

29 .7 1

15 .21

18 .5 8

5.6 5.9

\ 387 .4 552 .6

Ex

e-

-l trl

Ul

~

IV

p;'

(l) (') (')

..,to

@

0

a

;>(

(l)

246

Synimpact Crater-Fill Deposits

or+ab+an

+ Glauconitic sand

o Quartzsand • Clayeysand A

Silt

o Clay & Chertybreccia

Fig. 6.37. Mineralogical and chemical classification of sedimentary clasts from Exmore breccia. A, B compositional variation based on CWW normative proportions for orthoclase (or), anorthite (an), quartz (Q), diopside (di), hypersthene (hy), and olivine (ol), calculated from analyses listed in Tables 6.10-6.12. C-E chemical variation within samples illustrated by ternary diagrams based on different combinations of major elements.

The Exmore Breccia

247

Table 6.13. Samples for Discriminant Function analyses of Exmore breccia in this study.

Sample Drill Depth [ft] lEx 1208.2 Ex 1210.2 Ex 1215.3 Ex 1217.1 Ex 1228.0 Ex 1232.1 Ex 1234.1 Ex 1234.4 Ex 1248.0 Ex 1254.0 Ex 126 \.0 Ex 1269.1 Ex 1272.1 Ex 1279.4 Ex 1283.0 Ex 1284.0 Ex 1288.3 Ex 1294.1 Ex 1302.0 Ex 1305.2 Ex 1313.0 Ex 1317. 1 Ex 1330. 1 Ex 1334.3 Ex 1337.0 Ex 1347.0 Ex 136 \. 9 Ex 1366. 1 Ex 1375.3 Ex 1377.6 Ex 1387.4 2WP 552.6 WP 554.0 WP 554.3 WP 563.7 WP 565.0 WP 566.6 WP 570.7 lNN 432 .3 NN 438 .2

Lithology

Subsamples

Discriminant Function I

Discriminant Function 2

Clay Silt Glauconitic. sand Glauconitic sand Silt Glauconitic sand Quartz sand Quartz sand Quartz sand Glauconitic sand Glauconitic sand Quartz sand Glauconitic sand Silt Glauconitic sand Glauconitic sand Glauconitic sand Glauconitic sand Silt Silt Silt Glauconitic sand Glauconitic sand Glauconitic sand Glauconitic sand Quartz sand Silt Silt Silt Quartz sand Glauconitic sand Clay Glauconitic sand Glauconitic sand Quartz sand Glauconitic sand Glauconitic sand Glauconitic sand Silt and sand Glauconitic sand

3

3. 11088

0.772 16

I I

0.63463 -1.39614

1.3603 -0.53683 0.03169

I Exmore corehole inside crater lNewport News coreholeoutside crater See Rollinson (1993, p. 2 1) for equations

1

0.0114 3

3

2.27303

1.75655

I

-\.67041

-0.92765

1

-2.234

-3.68086

I

-3.07535

5

- 1.30743

-0.69294 0.3984

1

-1.18566

-0.34666

1 I

-2.04536 -0.84147

0.35688

2

-2.9 1852

-1.76552

3

-1.88523

-0.22543

1

-2.5266 1

-1.78654

-0.26981

1

-2.71741

-1.2094

I

-2.75063

-1.57034

1

-3.07865

- \.4 2341

3

-0.64005

-2.06394 0.67974

I

-6.9564 8

6

-1.15862

-2.57676

I

-3.21632

- \. 50673

I 1

-3.33424 -3.5596

-\.24429 - \.101 79

1

-3.97454

-0.94997

6

-2.77325

-0.85295

3 3 3

-0.43784 -2.26781 -0.98072

- \. 98419 -2.12231 -\.655 16

3

-4.7429

- 1.28388

5 1

7.57273 -0.53796

6.0726 5 -0.04575

I 1

0.61864 - \. 76101 0.70502

0.65404 -0.5947 -0.66519

1

0.58422

I

-0.77437

0.259 11 -1.78352

5

7.327 19

5.91427

2

-\.84854

4

22.3283

-2.55298 15.6201

I

2Windmill Point corehole outside crater See Table 6.4 for more complete sample descriptions

248 8

Synimpact Crater-FillDeposits

A

6

Quartzose Sedimentary Provenance

4 N

A

2

C

0

Mafic Igneous Provenance

nc ::;)

u,

c11l

0

.

A

A

0

eH

~.~

Aato

o~ -2

e

-;:; 0

Ul

Intermediate Igneous Provenance

0-4

Felsic Igneous Provenance

•

-6 -8

-1~10

-5

0

5

10

Discriminant Function 1 8

B Felsic Igneous Provenance

4

Intermediate Igneous Provenance

N

02 2 "0 c ::;)

u. C

0

•

11l C

°e -2

°C

o

Ul

0-4

Mafic Igneous Provenance

Quartzose Sedimentary Provenance

• Glauconitic sand o Quartz sand e Clayey sand A Silt DClay

-6 -8

-5

o

Discriminant Function 1

5

10

Fig. 6.38. Discriminant Function analysis for chemical compositions listed in Tables 6.10-6.13. Most samples represent felsic igneous provenance, with minor sources from intermediate igneous and quartzose sedimentary provenances. For details on diagrams see Rollinson (1993, p. 210--211 ).

The Exmore Breccia

249

volved in the production of the breccia need to be known. This condition is not fulfilled either. Thus , the following statements are only of qualitative nature. The values for Ir are not very reliable, because most values that did appear to give a positive signal are right at the detection limit for Ir by INAA (depending on sample type and composition, about 0.1 to 1 ppb for our procedure). A few samples seem to indicate Ir contents of 0.1 to 0.7 ppb, but we believe that these values are not reliable. No correlation between these values and the contents of the other siderophile trace elements, such as Co or Ni, are apparent. Chromium contents are fairly high in most samples, but this seems to be a characteristic of the weathered sediments, and does not indicate any meteoritic component. Cobalt and Ni contents vary somewhat, but the abundances are fairly similar to typical crustal values (Taylor and McLennan 1985). Also, the Ni and Co, or the Ni and Cr and the Co and Cr contents do not correlate with each other, which would be the case if a significant proportion were of meteoritic origin. Thus, we have not yet been able to identify a meteoritic component in these Exmore breccia samples. As most impact-derived breccias have only minor meteoritic contaminations anyway, this result was more or less expected. It will be necessary to identify suevitic-type breccia, with a significant melt rock or glass component, and analyze these , as well as crystalline and sedimentary target rocks, with a more sensitive technique than INAA (e.g., radiochemistry or ICP-MS after preconcentration) for the contents of the platinum group elements at the parts per trillion level. Another interesting topic is the comparison of the compositions of Exmore breccia samples with that of the North American tektites. Based on age and location arguments, Poag et al. (1994) suggested that the Chesapeake Bay crater is the long-sought source crater of the North American tektites, the bediasites, and the georgiaites. In order to determine whether the Chesapeake Bay impact crater is the source of the North American tektite strewn field, Koeberl et al. (1996) analyzed major and trace elements from small samples of the Exmore breccia. They found that most breccia samples contained 32-73 weight % Si0 2 . A few carbonate-rich samples yielded low Si0 2 values. The compositions of some of the highSi02 samples agree well with the compositions of average North American tektites for mostly nonvolatile elements (Fig. 6.39) . To identify the exact source beds for the North American microtektites, Koeberl et al. (2001) analyzed several samples of Cenozoic sedimentary formations (Aquia, Nanjemoy, Piney Point) that would have constituted part of the uppermost target rocks at the Chesapeake Bay crater (Fig. 2.4). They found no geochemical matches, however, which indicates either that the tektites must have been formed from the underlying Cretaceous sediments, or that the North American strewn field was not produced by the Chesapeake Bay impact. We made some further attempts to compare the samples from the present suite of Exmore breccia with North American tektites. Table 6.14 reports the average, minimum, and maximum oxide and element contents in Exmore breccia, compared to average compositions of the bediasite and georgiaite tektites. A variety of differences in composition is clearly apparent, even if the Exmore breccia composition is recalculated on a volatile-free basis . First, silica contents are simply too low, and, especially, CaO contents are too high in the breccia to provide a reas-

250

Synimpact Crater-FillDeposits

(/)1 0 .".------------------------------~

!

- - Breccia (1313.0 ft; 400.20 m)

E

- - -- - - Sand (1347 It; 410.57 m) - - - Sill (1377 6 f1; 419.89 m)

~ ~

o

()

u; . ...... "'.. .............. -.. .. .~ l :r~~~"::"~----;~~'#~-b"......~~~.,....:~:;;.;~':S'~-'=.:~::s;;;...~~~~ Cll

'0 Q) lD Q)

Cl

~

Q)

~ O.I-'-..--"'T""""""T--r-....,...-..--"'T""""""T--r-.......- r -- r - - , -...............- r-"""T""--.-...............-r-.......-J

i Ti AI Fe Mg Ca

a K Sc Co Ga

h U

Fig. 6.39. Average composition of 32 bediasite samples compared with composition of three high-silica samples of Exmore breccia from Exmore corehole. Abundances in core samples recalculated on volatile-free basis. Sample numbers are drill depth (ft and m). From Koeber!et al. (1996).

1000 -r--------------------------~

-+- Exmore breccia (Average) _ .. - Exmore breccia (Minimum) - - - .Exmore breccia (Maximum) .. ~ .. Bediasites (Average) .. A · · Georgiaites (Average)

-
ell

u

C

{l c

100

::l .0

" 1:>. .

(ij

E

- - - ........

o

z

2 -c: -0

......... .. ·X 10

. ....1:>.

C

o

/

L:

.. ..

U

La

Ce

Pr

Nd

Sm

Eu

Gd

1:>.

..

- .. __ .. _.. _.. _ .. _. - .. - .- . . -

Tb

Dy

Ho

Er

Tm

Yb

Lu

Fig. 6.40. Abundances (average, range) of rare earth elements in samples of Exmore breccia plotted against average abundances in North American tektites (bediasites, georgiaites). Normalization values from Taylor and McLennan (1985).

The Exmore Breccia

251

onable match. This could indicate that: (1) a material similar to that which was involved in forming the breccia also was involved in the tektite production , but did not contain any carbonate ; or (2) the rock had to be totally decarbonated and dewatered; or (3) some near-surface rock units, wh ich were not sampled by the breccia, were involved in the tektite production . The third possibility seems to be most likely, based on consideration s from other tektite strewn fields (Koeber! 1994). The interelement ratios shown in Table 6.14 indicate that simple devolatilization would not work, as many of these ratios are simply too different. Some lithophile element distributions, though, such as the REE patterns (Fig. 6.40), indicate a general similarity between the brecc ia and tektite compositions. However, this general similarity is true for almost all upper crustal rocks. The diagram shown in Fig. 6.41, which was used by Albin et al. (2000) to show the relation between North American tektites and various other rock types, can be used to compare these rocks with the Exmore breccia, as well. The ThISc and La/Th ratios of the tektites fit relatively well with the breccia values (cf. Table 6.14), but the La/Sc ratios would be skewed to lower values, and do not provide a good fit. This relationship reinforces the conclusion made above that we have not yet identified the exact rocks from which the tektites were produced. The observations of Poag et al. ( 1994) and Koeber! et al. (1996), however, still stand, and the Chesapeake Bay crater rema ins the most likely source of the North American tektites.

LalTh

AC

Bediasites Barbados DSDP MO

Th/Sc L--- - - - - - - - - - - - - - ---1. La/Sc Fig. 6.41. Plot of ThlSc, La/Th, and La/Sc for North American tektites (georgiaites, bediasites, Barbados, DSDP) compared to Archean (AC) and post-Archean (PA) crust (Taylor and McLennan 1985) and to moldavite (MO) and indochinite (Ie). DSDP = Deep Sea Drilling Project cores. From Albin et al. (2000).

252

Synimpact Crater-FillDeposits

Table 6.14. Averages and ranges of compositions of Exmore breccia samples compared to North American tektites. Average Exmore Min. Max. Average Exmore Std. Exmore Exmore All Breccia Deviation Breccia Breccia Breccias 10.14 60.94 Si02 64.47 32.65 85.31 0.58 0.22 0.02 0.54 Ti0 2 0.92 A1203 9.44 2.86 3.12 16.13 8.83 6.28 2.73 0.76 18.61 Fe203 5.92 MnO 0.04 0.02 0.01 0.09 0.04 MgO 1.25 0.62 0.06 3.82 1.36 4.91 5.33 0.34 26.78 7.09 CaO 0.41 0.12 2.14 1.16 1.28 Na20 0.90 7.24 2.96 K20 2.92 1.65 0.56 0.04 0.43 0.37 2.93 P20 S 9.62 LOI 8.03 4.69 0.43 22.78 99.25 Total 99.21 Sc V Cr Co Ni Cu Zn Ga As Se Br Rb Sr y Zr Nb Ag Sb Cs Ba La Ce Nd Sm Eu Gd Tb Dy Tm

8.76 105 89.0 10.2 21.8 <2 144 12.0 9.28 0.32 0.65 93.0 233 19.1 244 10.5 0.04 0.52 3.01 291 33.0 77.8 34.5 6.46 1.39 6.11 0.90 5.16 0.39 2.52 Vb 0.34 Lu 5.54 Hf 0.66 Ta W 1.25 Ir (Ppb) 0.4 Au (ppb) 0.8 Hg 0.07 7.17 Th 2.17 U

3.54 28.1 31.9 4.60 9.3

2.02 20 11.4 1.75 8.0

17.3 138 180 23.2 56

136 4.3 7.99 0.25 0.48 25.7 94 5.1 148 2.2 0.02 0.22 1.32 94 28.6 89.3 34.8 5.66 1.39 4.69 0.66 3.37 0.13 0.97 0.10 2.24 0.21 1.04 0.3 0.5 0.07 3.71 0.90

II 6.0 0.93 0.03 0.02 57.5 104 5.0 80 4 0.01 0.28 0.81 51 11.3 20.2 10.2 1.78 0.26 1.90 0.27 1.70 0.16 0.99 0.14 0.89 0.13 0.10 0.1 0.1 0.02 2.62 0.86

510 22.0 42.30 1.01 1.50 207 513 24.0 776 15 0.09 1.30 7.84 554 173 527 212 33.5 8.17 28.1 3.85 20.9 0.79 5.21 0.60 12.2 1.13 5.20 0.7 2.4 0.36 21.8 5.04

8.33 115.1 96.1 9.77 22.1 <2 136 11.9 9.89 0.31 0.66 94.4 236 20.8 220 10.2 0.04 0.58 2.90 283 31.9 74.7 33.4 6.35 1.38 6.13 0.89 5.09 0.39 2.44 0.33 4.90 0.60 1.20 0.4 0.8 0.06 6.57 2.18

Std. Bediasite Georgiaite Dev. Average Average All (21) (24) 14.23 76.37 81.8 0.22 0.51 0.76 3.12 13.78 11.2 2.82 4.21 2.79 0.03 0.03 0.03 0.63 0.61 0.63 9.28 0.65 0.45 0.44 0.94 1.54 0.97 2.08 2.44 0.53 7.18 <0.01 <0.01 100.05 100.77 3.46 33.8 40.7 4.62 8.8 125 4.7 7.51 0.26 0.48 29.3 87 7.2 143 2.0 0.02 0.28 1.37 94 25.5 79.1 31.0 5.08 1.25 4.32 0.60 3.08 0.13 0.93 0.10 2.39 0.23 0.97 0.3 0.7 0.06 3.59 0.91

13 49 13.5 8 II I

0.1 66 125 25 230 0.05 1.8 470 35 76 33 7.2 1.58 6.4 0.97

8.7 45 7.5 7.4 9

76 163 18.2 187 8.1 1.74 572 21.1 46.2 20.6 4.07 0.99 3.44 3.22

3 0.47 6.7 0.6 <0.5 <0.5 7.6 2

1.91 0.29 4.64 0.57 <0.5 <0.5 5.81 1.46

The Exmore Breccia

253

Table 6.14. (cont.) Geochemically important ratios for Exmore breccia samples compared to ratios from North American tektites. Average Max. Average Std. Min. Exmore Dev. Exmore Exmore All Breccia Exmore Breccia Breccia Breccias 25388 13132 KIU 13275 4309 5152 ZrlHf 53.9 56.1 16.2 304.5 56.6 La/Th 5.57 5.90 1.40 3 1.22 6.25 8.42 2.05 5.24 13.71 Hfrra 8.04 Th/U 3.59 1.75 0.52 10.69 3.28 2.82 6.23 22.44 LaNN b N 8.32 8.47 Eu/Eu* 0.66 0.17 0.15 0.85 0.67 La/Sc 3.95 2.77 2.10 16.39 4.12 Th/Sc 0.87 0.48 0.37 3.16 0.8 1 Rb/Cs 35.2 14.6 16.3 75.2 37.8

Std. Bediasite Georgiaite Dev. Average Average All (21) (24) 4494 10400 16712 53.9 34.3 40.3 6.32 4.61 3.63 I I. I 7 8.14 2.10 1.69 3.80 3.98 2.57 7.88 7.47 0.16 0.71 0.81 2.68 2.69 2.43 0.44 0.58 0.67 16.7 36.7 43.7

Data for tektites from Koeber! (1986), Koeber! and Glass (1988), Glass et al. (1995), Albin et al. (2000)

In summary, our geochemical studies revealed that: (I) the major element and trace element compositions of representative samples of Exmore brecc ia from three cores (Exmore, Newport News , and Windmill Point) show a wide variation, in agreement with their variable carbonate and clay contents and weathe ring states; (2) crystalline rock samples were not presen t in suffic ient quantity for complete chemical analyses, (the few small clasts recovered from the breccia were used mainly for shock petrographic studies); (3) trace element compositions, especially of the REEs, show some variation, but are in agreement with a sedimentary, upper crustal, post-Archean source for the Exmore breccia; (4) The detection limit of the present analyses was not good enough to clearly discern the presence of a meteoritic component in the Exmore breccia, which would have to be smaller than about 0.5% ; and (5) in order to better constrain the source rocks for the North American tektites, it will be necessary to analyze a wider variety of pre-impact target rocks, especially those that were close to the surface , and, especially, to carry out detailed Sr and Nd isotopic studies. This work is in progress .

7 Initial Postimpact Deposits

7.1 Depositional Setting Postimpact deposits over and near the Chesapeake Bay impact crater consist of a thick (200->500 m) succession of fossiliferous siliciclastic formations, which range stratigraphically from regionally distributed barren silt and richly fossiliferous late Eocene clay deposits, to a variety of locally distributed Quaternary and Holocene sand units (see Chapter 2; Figs. 2.4,2.5, 6.2, 7.1, 7.2). Because the impact took place in moderately deep water (-300 m) on the middle continental shelf (Chapter 13; Poag 1997a), normal marine sedimentation resumed immediately after the bolide-generated atmospheric and oceanic perturbations ceased. The first (oldest) postimpact deposit is a dead zone at the top of the Exmore breccia; a thin (19-49 em) layer of dominantly clayey silt, which appears to lack indigenous microfossils (Poag 2002b) . This dead zone is succeeded conformably by as much as 220 m of typical clay-dominant Chickahominy Formation, which began to fill in the bathymetric lows on the irregular upper surface of the Exmore breccia (Fig.7.1). Chickahominy deposition continued for - 2.1 myr to the end of the Eocene. Thus, the nature and history of the dead zone and the Chickahominy Formation yield crucial information regarding the age of the impact (see Chapter 8) and late Eocene paleoenvironments and biota (see Chapter 13) that evolved in its aftermath.

7.2 Dead Zone The initial postimpact deposit is a 19- to 49-cm-thick dead zone composed mainly of fine, horizontal, parallel laminae of fine to very-fine sand, silt, and clay, which constitutes the stratigraphically youngest unit above the Exmore breccia (Fig. 6.22; Poag 2002b). The dead zone directly underlies the Chickahominy Formation. The dead zone is best represented at the NASA Langley core site (Fig. 7.3), where its lower boundary can be determined by the presence of the presumed fallout layer (Fig. 6.22). There, the base of the dead zone rests conformably on the layer of pyrite lattices (Fig. 6.24A ,B). The fine, horizontal, clay and silt laminae of the dead zone are disturbed in places by horizontal , vertical , and inclined burrows, which are filled with medium to coarse sand and reworked microfossils derived from the underlying Exmore breccia matrix (Fig. 7.3A). Additional reworked microfossils comprise much of the micaceous white sand concentrated in C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

Preimpacl Sediments

Crystall ine Basement

~

~

~

~

••

•

0

•

•

•

•••

•

•

•

•

SEAX6

•

0

••

II SEAX7

i II

km

Exmore Breccia

®&l~ Chickahom iny Formation

~ Postimpaet Sed iments

~

'~" ~~'~~ ' , . .~ .. ~.. !" '."~ .¥ J.U

••

~

§

Displaced Megablocks

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

I

8 Gap ~

f

20

1.5

I

I 'g E 1.0 ..><

~.

CI>

(0

I 05

"S ~ o

-e ~. 3

~

_

-

~

=

U1 0\

~ <

~ § ~>~ ' .. '.. ! .... .. .. . ~ 00

SEAX8

'?

Fig. 7.1. Depth-scaled cross section of Chesapeake Bay primary and secondary craters along composite seismic reflection profile, showing complete stratigraphic succession of preimpact, synimpact, and postimpact deposits. See Figs. 3.3 and 4.21 for profile locations. Compare with Figs. , 2 and 5.1, and CD-ROM .6.

~ ~ ~ ~

TEXACO l lPR

Gap

tv

Dead Zone

257

60, 75-40'

kin,

75 20'

I

I

I

,

Fig. 7.2. Isopach map of postimpact sedimentary deposits associated with Chesapeake Bay impact crater (contour interval 50 m). Note that thickness variations reflect morphology of basement structure under crater (deposits thin over basement highs, such as peak ring and central peak). horizontal laminae and lenses (Fig. 7.3B,C). At NASA Langley, the dead zone differs from the silt-rich layer below the pyrite lattices primarily in the more uniform, horizontal, parallel distribution of laminae, and the lack of nodular concentrations of pyrite (pyrite is concentrated instead in the horizontal laminae). In the Bayside core, the dead zone reaches its maximum known thickness (-49 em), No pyrite-lattice layer has been identified at Bayside, however, so the base of the dead zone can only be approximated by the upward change from multidirectionally inclined, moderately thick lamination to horizontal, thin lamination (see further discussion of the dead zone in Chapter 13).

258

Initial Postimpact Deposits

A

(773.05 It; 235.63 m)

Cll C

0_ N""''O "iji ", -

Cll

c

6 I em

Fig. 7.3. Core segments in dead zone from NASA Langley corehole. A, vertically split segment, showing contact with overlying dense clay of Chickahominy Formation; B, horizontal split of segment A, showing complex laminar lithologies at contact with Chickahominy; C, horizontal split of segment A taken 0.6 em below B, showing laminae and burrow; D, vertically split segment, showing uniform distribution of repetitious, horizontal laminae (sand, silt, clay). Approximate depths shown at tops of segments. See CD-ROM for color version.

Chickahominy Formation

259

7.3 Chickahominy Formation 7.3.1 Lithology of Cores

The most complete lithic records of the Chickahominy Formation come from four sites inside the crater: the North corehole (69.49 m; 228 ft); the Bayside corehole (65.53 m; 215 ft); the Kiptopeke corehole (63.4m; 208 ft); and the NASA Langley corehole (52.70 m; 172.9 ft; Figs. 6.3A,B ; CD-ROM .7). At all core sites, fresh cores of the Chickahominy Formation are typically gray-green clay (Fig. 7.4A,B; see CD-ROM for color version) that weathers to yellowish olive brown, and contains variable amounts of finely comminuted glauconite and muscovite (Fig. 7.4C). The clay is silty to sandy, richly fossiliferous, and commonly displays fine to coarse (often faint) lamination (Fig. 7.4C). The biota are mainly marine microfossils (benthic and planktonic foraminifera , calcareous nannofossils, bolboformids, ostracodes, dinoflagellates, radiolarians) , but also include common to abundant remains or evidence of invertebrates (echinoid spines, solitary corals, thin bivalves, scaphopods , pyritized burrow casts), and vertebrates (fish skeletal debris and teeth; Fig. 7.4A,B) . Sediments subjacent to the upper boundary of the Chickahominy Formation are usually intensely burrowed ; those near the lower boundary are moderately burrowed . Larger burrows are filled with coarser material (sand) than the Chickahominy itself (clay), and can be identified as far as 2 m into the Chickahominy. Burrows at the top of the Chickahominy are filled with glauconitic quartz sand and microfossils reworked downward from the overlying Oligocene Delmarva unit or Old Church Formation (Fig. 7.5A). At the base of the Chickahominy Formation, the smallest, most abundant burrows are filled with framboidal pyrite. The largest burrows in this basal interval are filled with quartz sand and mixed microfossil assemblages reworked upward from the Exmore breccia (Fig. 7.5B,C). The presence of the sand-filled burrows causes the upper and lower sediments in the Chickahominy section to fracture and crumble upon drying (Fig. 7.6C,D), in contrast to the dense, massive character maintained throughout most of the remainder of the unit (Figs. 7.4A,B; 7.5C; 7.6A,B) .

7.3.2 Expression on Downhole Geophysical Logs

Downhole spontaneous-potential (SP) logs from five of the seven coreholes drilled through the Chickahominy Formation (both inside and outside the crater) show that the Chickahominy is notably less permeable (low SP values) compared to the units that bound it (Figs. 7.7A,B; CD-ROM .7). Where both short normal (16") and long normal (64") resistivity curves are available, the two curves track each other closely, indicating no invasion of the mud filtrate, another indication of low permeability (Fig. 7.7B). The Chickahominy sections at NASA Langley and

260

A

Initial Postimpact Deposits N-L (641 fl; 195 .38 m)

c

WP (53 1.33 fl; 161.95 rn)

N-L (644 .2 fl; 196.35 m)

.. oI

- - -5

an

I

Fig. 7.4. Photographs of Chickahominy Formation illustrating lithic characteristics. A, laminar surface of horizontally split fresh core segment showing needle-like echinoid spines distributed parallel to laminae and scattered white spots representing benthic foraminiferal tests (from NASA Langley corehole); B, laminar surface of horizontally split fresh core segment showing echinoid spines, thin-shelled clams, and foraminifera (from NASA Langley corehole); C, segment of vertically split core (photographed after ten years of dry storage) showing laminated interval within Chickahominy Formation (from Windmill Point corehole). See CD-ROM for color version of this figure.

Exmore are exceptions, however. At NASA Langley, the SP log indicates greater permeability than the underlying Exmore breccia, and at Exmore the upper and lower sections of the Chickahominy have SP values essentially identical to their bounding units. At all seven of these core sites, the Chickahominy Formation can be partitioned into four principal lithic subunits on the basis of log-defined relative permeability (Figs.7.7A,B). At each site inside the crater, subunit SP-l (at the base) is characterized by the lowest permeability. At Exmore, North, Bayside, and Kiptopeke, subunit SP-2 is characterized by gradually upward-increasing permeabilities. At Windmill Point and NASA Langley, on the other hand, the increase is abrupt,

Chickahominy Formation

A

B

N·L (601.4 ft: 183.31 m)

261

Ex (1210.2 ft; 368.87 m)

o

5 I

I

em

C

N-L (773.05 ft: 235.63 m)

o I

em

em

Fig. 7.5. Photographs of core segments showing burrows in upper and lower parts of Chickahominy Formation, A, intensely burrowed clay (Iight-cotored) in fresh whole core of upper part of Chickahominy Formation at NASA Langley core site. Burrows filled with greenish-black, glauconitic quartz sand derived from overlying Delmarva beds (lower Oligocene); E, Burrows (arrows) in dried out clay of lower part of Chickahominy Formation (vertically split weathered core) at Windmill Point core site. Sand in burrows derived from underlying Exmore breccia; C, well-defined sand-filled burrow parallel to bedding plane (horizontal) in lower part of Chickahominy Formation (whole fresh core) at NASA Langley core site; burrow-fill derived from underlying Exmore breccia. See CD-ROM for color version of this figure.

262

Initial Postimpact Deposits

A

1203.8 It (366.92 m)

B

1207.0 It (367.89 m)

c

1209.5 It (~68 .6_6 m)

o I

Fig. 7.6. Photographs of weathered split core segments from Chickahominy Formation overlying Exmore breccia at Exmore core site. A, B, segments typical of hard, finely laminated clay that makes up bulk of Chickahominy at this site; C, D, segments displaying sand-filled burrows (arrows) near base of Chickahominy at this site; burrows cause weathered cores to fracture and crumble. See CD-ROM for color version of this figure.

Chickahominy Formation

263

reaches highest values for those two coreholes, and then tapers off upward , before declining steeply toward the top (Figs. 7.7A,B). At Newport News, subunit SP-2 is only weakly developed. Subunit SP-3 is more permeable than SP-2 at Exmore , North, Bayside, and Kiptopeke, but less permeable than subunit SP-2 at NASA Langley, Newport News , and Windmill Point. In subunit SP-4 , permeabilities decrease relative to SP-3 at Newport News , Windmill Point, North, Kiptopeke, and NASA Langley, but show a relative increase at Exmore (Fig. 7.7B) and Bayside (Fig.7.7A). A fifth subunit (SP-5) at the top of the formation can be recognized only at Newport News and Kiptopeke . In subunit SP-5, permeability decreases distinctly at its base , then increases appreciably up-section. At four intracrater sites (NASA Langley, North, Bayside, and Kiptopeke), downhole gamma-ray (GR) logs, which reflect mainly the relative amount of clay and(or) glauconite in the Chickahominy Formation, provide a somewhat stronger definition of downhole lithic changes than do the SP curves (Fig . 7.7A) . The GR curves at all four of these sites indicate a fivefold subdivision (A-E) of the Chickahominy. The upward succession of relative GR values , like that of the SP values, is closely similar at North , Bayside, and Kiptopeke. The basal GR subunit (GR-A) displays lowest values at these three sites, but in stark contrast, GR-A gives highest values at NASA Langley. Subunit GR-B shows upward increasing values at North and Kiptopeke , uniformly slightly higher values than GR-A at Bayside, and uniformly much lower values than GR-A at NASA Langley (Fig. 7.7A). In subunit GR-C , the gamma-ray values continue to increase upward at NASA Langley, North , and Bayside , but decrease slightly before increasing again at Kiptopeke. In subunit GR-D , values decrease at all sites relative to GR-C. Maximum GR values are reached at the top of the Chickahominy Formation in subunit GR-E at all four core sites (Fig. 7.7A). Correlations between SP and GR subunits are complex within individual coreholes, as well as between different coreholes. The most consistent correlation is between SP-3 and GR-C , whose upper and lower boundaries coincide (or nearly coincide) at four sites (Fig. 7.7A). There also is good correlation between SP-I and GR-A and between SP-2 and GR-B at North , Bayside , and Kiptopeke, but these correlations break down at NASA Langley . At the top of the Chickahominy section , SP-4 is equivalent to GR-D and E, with the exception of Kiptopeke, where SP-5 correlates with the top of GR-E. Thus the geophysical logs indicate that though the Chickahominy Formation may appear macroscopically to be a lithically uniform deposit, it is far from it. Shifts in sediment sources, sediment composition, accumulation rates, and variable diagenetic processes have created a much more complex lithic unit than we expected to encounter. For example, the low permeability at the base of the Chickahominy Formation (subunit SP-I) is probably due to extensive diagenesis in this section (pyritization and silicification; see further discussion in Chapter 13), in spite of the prominence of sand-filled burrows , which otherwise should have increased permeability. The unusually low permeability at the top of the two extracrater cores (Windmill Point and Newport News) also may be a response to diagenetic change s.

o

:••••

m

.~... .. ~ ~ •• • 0 °

",

Klptopeke)

. . . i" ",

' .

Sp

BAYSIDE CORE GR

Inner Annulnr Trough

Inner Basin GR

KIPTOPEKE CORE

Fig. 7.7A. Expression of Chickahominy Formation on downhole geophysical logs from four core sites inside Chesapeake Bay impact crater. SP = spontaneous potential; GR = gamma-ray. See text for further discussion and CD-ROM.7 for full-scale logs.

R,m

Cra te, Outer

NOI1h

: 1\ O~

f

I";:::~~r.." :\" '·"'~;::,.

Outer Annular Trough sp GR

NASA LANGLEY CORE

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oen

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t'

d'

§:

[

~

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I

432 ft (131.7 m>?

d

I

~c5

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z

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0

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c

>.

u.

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10

c .Q

Delmarva Beds

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\

Resistivity SP

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Delmarva Beds

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,

Resistivity

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I"~

WINDM ILL POINT CORE Breccia Apron

(342,9 m)

112511 ........

SP Delmarva I Beds

I

EXMORE CORE Outer Annular Trough Resistivity _

Fig. 7.7B. Expression of Chickahominy Formation on downhole geophysical logs from two core sites outside Chesapeake Bay impact crater and one core site inside crater. SP = spontaneous potential; GR = gamma-ray, See text for further discussion and CD-ROM .7 for full-scale logs ,

ti,

\

f~\\

Vertical scale indicates drill depth (tt )

I

,

I

,

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I

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SP

NEWPORT NEWS CORE Breccia Apron

n

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266

Initial Postimpact Deposits

During deposition of subunit SP-2, the sedimentary regime at Windmill Point and NASA Langley provided coarser grains (greater permeability) than elsewhere in the crater, and glauconite was probably the primary sand constituent (indicated by the unusually high gamma-ray values in subunit GR-A at NASA Langley). In contrast, nearer the center of the crater (at North, Bayside, and Kiptopeke), glauconite is a more prominent constituent in the upper part of the Chickahominy Formation.

7.3.3 Seismic Signature

Integration of coring and downhole geophysical records allows us to correlate the Chickahominy Formation precisely with its reflection signature on the seismic profiles. Normally, a significant impedance contrast exists between the relatively dense, weakly permeable Chickahominy clay and the loosely consolidated, strongly permeable sands of the overlying Oligocene Delmarva beds (Powars et al. 1992) and(or) the Miocene Calvert Formation (Fig. 2.5). This impedance contrast produces an easily recognized high-amplitude reflection at the upper boundary of the Chickahominy Formation (Figs. 4.3A,B, 4.5A,B, 4.7A,B, 4.9A,B, 4.13, 4.15, 4.18, 4.19, 4.23-4.26, 4.32), which can be traced over the entire crater and for a short distance outside the crater rim. The lower boundary of the Chickahominy also is characterized by a strong impedance contrast and a resultant highamplitude seismic reflection (PS), where it contacts the underlying unconsolidated sands of the Exmore breccia (Figs. 4.3A,B, 4.9A,B, 4.13, 4.23A,B, 4.25A,B, 4.26A,B, 4.32). Even on profiles where its boundary reflections are weak, the large number of intersections between profiles assures correct stratigraphic placement of both the upper and lower boundaries of the Chickahominy Formation. In the thickest portions of the Chickahominy Formation, as displayed on the seismic profiles, internal reflections indicate the probability of meter-scale bedding. In short, the seismostratigraphic signature of the Chickahominy is easy to recognize and to trace over the crater. Therefore, its present structure (Fig. 7.8) and thickness (Fig. 7.9) can be accurately mapped.

7.3.4 Geometry

The structure, morphology, and distribution of the Chickahominy Formation have been influenced strongly by the original irregular geometry of the upper surface of the Exmore breccia, and by the long-term subsidence differential between the unconsolidated, water-saturated impact breccia inside the crater and the semiconsolidated preimpact sedimentary column outside the crater. Differential subsidence is partly responsible (along with original bathymetric differences between the crater basin and its peripheral lithotopes) for a much thicker section of Chickahominy clay inside the crater than outside the crater (Fig. 7.9). In addition, continued differential subsidence during the roughly 34 million years of Oligocene-Holocene

Chickahominy Formation

267

Rappahannock Canyon

,

,

,, . , ,,

···. . ···, ·,, I

I

I

,, , ,, ,, ,

.

,

o

60 ,

I

76'40'

76'20'

76'00'

7540'

km

75'20'

Fig. 7.8. Structure map of upper surface of Chickahominy Formation (contour interval 20 m). Dashed line marks approximate shoreward limit of formation. time has caused the Chickahominy Formation to sag irregularly over the crater rim in concert with differential compaction of the underlying breccia. That is, the Chickahominy Formation thickens and sags as it crosses into the annular trough and inner basin, just as the Exmore breccia does (Table 7.1; Figs. 4.3A,B, 4.5A,B, 4.9A,B, 4.11, 4.13, 4.14). Likewise, the Chickahominy mimics the geometry of the underlying breccia body by arching up over the peak ring and central peak (Figs. 4.22-4.26, 4.32, 4.34). Inside the crater, the Chickahominy Formation is ~2 o->220 m thick, and averages -100-120 m (Fig. 7.9). The thickness varies greatly, because the unit fills various pits and troughs in the upper surface of the underlying Exmore breccia, which have been accentuated by the postimpact differential compaction (Figs. 4.7A,B, 4.13, 4.22, 4.26A,B, 4.32). In general, the Chickahominy Formation is thickest where the underlying Exmore breccia is thickest (where the basement surface is deepest) and thins where the Exmore breccia is thinnest (where the basement shallows). The Chickahominy Formation thickens from 20 m to >90 m where it crosses the western part of the outer rim; from 20 m to > 150 m across the

268

Initial Postimpact Deposits

Rappahan nock Canyon

37'30'

3700'

o,

60 , 75'40' km

75'20'

Fig. 7.9. Isopach map of Chickahominy Formation (contour interval 20 m). Note that thickness variations reflect morphology of basement structure under crater (formation thins over basement highs such as peak ring and central peak).

northern part of the outer rim; and from 20 m to >160 m across the eastern and southern parts of the rim (Fig. 7.9; Table 7.1). The thickest measurable part of the formation (>220 m) occupies the western sector of the inner basin. We have no seismic data for the eastern sector of the inner basin. The Chickahominy Formation thins over broad areas of the western, northern, and southern sectors of the annular trough, being thinnest over the southwestern crest of the peak ring and over the central peak. More than 120 m of Chickahominy sediments occupy Rappahannock Canyon (Figs. 4.7A,B, 4.20A, 7.9). The Chickahominy thins rapidly to <10 m within a few kilometers outside the crater outer rim, and is too thin to trace beyond that point on the seismic profiles. The formation is < \0 m thick in most of the non-cored boreholes that have penetrated it outside the crater.

Chickahominy Formation

269

Table 7.1. Elevation, sag, and thickness data (from seismic profiles) for Chickahominy Formation where it crosses outer rim of Chesapeake Bay impact crater. Seismic Profile #

Elevation Outside Rim ' [mbsl]

Elevation Inside Rim [mbsl]

S-2

100

175

S-3

85

T-13-YR

Thickness Outside Rim [m]

Percent Thickness Total Inside Thickness Thickness Rim Increase Increase [m]

75

25

110

85

340

120

35

10

90

80

800

Amount of Sag [m]

75

120

45

10

90

80

800

S-16

110

190

80

15

80

65

433

S-17

125

170

45

10

100

90

900

N-3

110

170

60

10

70

60

600

T-II-PR

120

220

100

40

100

60

150

T-9-CB-F

120

210

90

40

100

60

150

S-12

140

180

40

20

140

120

600

S-13

150

200

50

15

110

95

633

T- IO-RR

180

230

50

10

140

130

1300

200

25

10

90

80

800

100

80

400

T-I -CB

175

S-4

150

180

30

20

S- IO

175

220

45

10

130

120

1200

S- I I

160

180

20

10

120

11 0

1100

S-5

220

240

20

10

120

11 0

1100

S-6

220

270

50

15

70

55

367

S-8

255

310

55

15

60

45

300

S-9

280

320

40

15

100

85

567

S-19

365

390

25

10

120

110

1100

S-22

370

390

20

10

100

90

900

S-25

350

380

30

30

150

120

400

S-27

315

320

5

20

130

110

550

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'rnbsl = meters below sea level

270

Initial Postimpact Deposits

7.3.5 Faults and Fault Systems In addition to producing the thickening, thinning, and sagging of the Chickahominy Formation, differential compaction of the Exmore breccia also has created a series of normal-offset growth faults and fault systems within the postimpact sedimentary section. These faults break the Chickahominy Formation into discrete fault blocks (Figs. 4.5A,B, 4.9A,B, 4.11, 4.13, 4.20A,B, 4.22-4.26, 4.32). The NASA Langley borehole crossed a postimpact compaction fault, which slices through the Chickahominy Formation at 229.9 m (754.4 ft; Fig. 7.10). The two most prominent systems of compaction faults are expressed on the seismic profiles as complex intervals of disrupted and offset reflections that form distinct grabens . Because these graben structures are present on almost every seismic profile that crosses the outer rim and(or) the wall of the inner basin, we infer that they represent parts of two nearly continuous concentric graben systems that encircle the crater just inside the outer rim and the peak ring (Fig. 7.11). In addition to the two continuous concentric graben systems, we have documented (on the seismic profiles) more than 700 individual faults and fault clusters (small grabens, horsts, or single normal faults) scattered in mainly concentric orientations throughout the Chickahominy Formation (Fig. 7.11). Most of these compaction faults also extend upward through part or most of the postimpact stratigraphic section (Figs. 4.5A,B, 4.11, 4.13, 4.20A ,B, 4.22-4.26, 4.32). Furthermore , the throw on most faults decreases upsection (Fig. 7. 12A-C), indicating that they are growth faults, along which long-term continuous or intermittent movement has occurred . Most faults die out before they reach the near-surface Quaternary deposits, but some continue into the Quaternary section as well, to within 10-15 m of the modem bay floor (Figs. 7.13A,B; Colman and Hobbs 1987; Colman and Mixon 1988).

Chickahominy Formation

271

754.4 ft (229 .94 m)

o I

em

Fig. 7.10. Photograph of core segment, showing minor branch of postimpact compaction fault system, which cuts Chickahominy Formation at 229.94 m (754.4 ft) in NASA Langley corehole. Hanging wall and footwall separated by 1.5-cm-thick layer of pyrite-rich fault gouge. See CD-ROM for color version of this figure.

272

Initial Postimpact Deposits

37'30 ' Outer Ring Graben

37"00' ,

Peripheral Fault System

75'40'

76'2 0'

o

60 I

I

km

Fig. 7.11. Horizontal distribution of concentric ring-grabens and additional normal faults that offset upper and lower bounding reflections of Chickahominy Formation (mapped on upper surface of Chickahominy Formation), Outer ring-graben marks location of crater outer rim; inner ring-graben marks wall of inner basin. All these faults also extend into postimpact sedimentary section above Chesapeake Bay crater.

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Fig. 7.12A . Interpreted segment of seismic reflection profile SEAX- 14 (single channel) across Chesapeake Bay crater, showing incremental displacement along postimpact growth faults. Displacement due to nonuniform differential compaction ofunderiying Exmore breccia during last 36 myr. Letters at right of each profile segment identify reflections used to demonstrate decreasing upward displacement of strata. Note that some faults nearly reach bay floor.

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Fig. 7.12C. Interpreted segment of seismic reflection profile Neecho-I (multichannel) across Chesapeake Bay crater, showing incremental displacement along postimpact growth faults. Displacement due to nonuniform differential compaction of underlying Exmore breccia during last 36 myr. Letters at right of each profile segment identify reflections used to demon strate decreasing upward displacement of strata. Note that some faults appear to reach bay floor.

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8 Age of Chesapeake Bay Impact Crater

8.1 Biochronology The biochronological age of the Chesapeake Bay impact crater has been determined by documenting the stratigraphic ranges of three groups of microfossils (planktonic foraminifera, bolboformids, and calcareous nannofossils), sampled in two different geographic areas. We compared these ranges to the global geochronostratigraphic framework constructed by Berggren et al. (1995 ; Fig. 8.1). Sample area one is in and near the crater itself, where Poag and Aubry (1995), Poag and Commeau (1995), and Poag (l997a) first established the detailed planktonic biostratigraphic framework of the Exmore breccia and Chickahominy Formation [Fig. 8.2; see also Poag and Norris (in press) for correlative planktonic foraminiferal biostratigraphy of the Chickahominy Formation in the NASA Langley corehole]. The second sample area for micropaleontological analysis is the New Jersey Continental Slope, where several different investigators have documented microfossil assemblages from impact ejecta cored at DSDP (Deep Sea Drilling Project) Site 612 and ODP (Ocean Drilling Program) Sites 903 and 904 (Fig. 8.3; see Poag and Aubry 1995, for the most recent synthesis and references). The youngest diagnostic foraminifera and nannofossils within the Exmore breccia represent an - 0.8 myr interval (35.2-36.0 Ma) in which the upper part of planktonic foraminiferal Biochron P15 overlaps the lower part of calcareous nannofossil Biochron NP 19-20 (Fig. 8.1). This same biochronologic overlap interval is present in the Exmore breccia in all seven coreholes drilled inside and outside the crater, and extends into the lower third of the overlying Chickahominy Formation. The remaining upper two-thirds of the Chickahominy Formation incorporates biochrons P I6-PI8 (forams) and NP 19-20 and NP21 (nannofossils), which represent the final -1.5 myr of the late Eocene. Almost identical biostratigraphic relationships are seen at the offshore New Jersey sites. Both the impact ejecta and the immediately overlying pelagic carbonates there belong to the PI5/NPI9-20 overlap interval (35.2-36.0 Ma), and the remaining offshore upper Eocene section correlates directly with the upper microfossil zones of the Chickahominy Formation (Poag and Aubry 1995; Figs. 8.1-

8.3).

C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

280

Age of Chesapeake Bay Impact Crater

Plankton Zones Time (Ma)

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Chron Berggren & Miller (1988)

Berggren et al. (1995)

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a

Fig. 8.1. Geochronostratigraphic chart for Eocene and Oligocene epochs used for interpretations in this volume (modified from Berggren et al. 1995). Arrow and dotted horizontal line mark stratigraphic level at which Chesapeake Bay impact took place, as determined by microfossils (planktonic foraminifera, bolboformids, calcareous nannofossils) and magnetostratigraphic analyses of samples from Chesapeake Bay impact crater.

Biochronology PLANKTONIC \ ~BOFORr FORAMINIFERA

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282

Age of Chesapeake Bay Impact Crater

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Radiometric Chronology

283

8.2 Radiometric Chronology Radiometric analyses for the Site 612 ejecta (Fig. 2.12), based on 40 Ar/39 Ar plateau values from tektite glass, yield an age range of 35.2 ± 0.3-35.5 ± 0.3 Ma, nearly identical to that obtained from biochronology (Obradovich et al. 1989; Poag and Aubry 1995). We infer that the crater age is equivalent to the ejecta age, but no radioisotopic impact age has yet been obtained from rocks within the Chesapeake Bay crater itself.

8.3 Magnetochronology Poag et al. (2002) reported the results of a magnetochronological analysis of samples taken at approximately 2-m intervals from the continuously cored, 66-m-thick Chickahominy section in the Kiptopeke borehole (Fig. 8.4). If one assumes a uniform rate of sediment accumulation, the average temporal sampling interval would be approximately 25-38 kyr. The Chickahominy biostratigraphic record can be tied to the magnetostratigraphic record by using Berggren et al.'s (1995) geochronological framework. That framework shows that the boundary between planktonic foraminiferal Zones P15 and P16 lies near the middle of Chron C 15r (Figs. 8.1, 8.4). Therefore, by extrapolation, the normally magnetized basal section of the Chickahominy Formation represents the upper part of Chron CI6n.2n. Because the ages at the tops of Chrons C16n.1nand C16n.2n have been determined (Fig. 8.4), we can calculate that the average rate of sediment accumulation for the intervening section was 67 rn/myr. By extrapolating this rate to the top of the Exmore breccia, we estimate the time of impact to have been -35.78 Ma (Fig. 8.4; see Chapter 13 for more thorough discussion of sediment accumulation rates).

8.4 Correlation with Other Craters and Impactites Poag et al. (2002) concluded that the Chesapeake Bay impact, deposition of the Exmore breccia, deposition of the North American tektite strewn field, and deposition of the ejecta-bearing impactite at Massignano, Italy (5.61 m above the base of the outcrop section), all took place within the upper part of Chron C16n.2n (Figs. 8.4, 8.5). This conflicts with initial biostratigraphic assignment at Massignano, in which the impactite at that Italian outcrop was placed within planktonic foraminiferal Biozone P16, rather than PI5 (Coccioni et al. 1988; Montanari et al. 1993). In a more recent study of the Massignano section, however, Spezzaferri et al. (2002) indicated that the impactite there lies within the overlap of Zone PI5 (forams) and NP19-20 (nannofossils), just as the Chesapeake Bay impact ejecta (North American tektites) and the Exmore breccia do.

284

Age of Chesapeake Bay Impact Crater

332.2

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Fig. 8.4. Magnetobiochronology of Chickahominy Formation, which overlies Exmore breccia in Kiptopeke corehole. Age of impact extrapolated from average sediment accumulation rate between magnetochron boundaries at 35.3 and 35.6 Ma. Modified from Poag et al. (2002).

w

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0:

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Fig . 8.5. Geochronological correlations between Chesapeake Bay impact crater and late Eocene distal ejecta deposits at DSDP Site 612, Massignano, Italy, Bath Cliff, Barbados, and Maud Rise in Southern Ocean (ODP Site 689B) . Depth scales (m) are drill depths (Chesapeake Bay, DSDP 612, ODP 689B) or heights above base of outcrops (Massignano, Bath Cliff) . Upper datum is top of Eocene section; lower datum is level of impact debris. Dotted correlation lines based on magnetostratigraphy. Modified from Poag et al. (2002) .

w

o

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w

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Poa9 and Aubry (1995)

Poag and Aubry (1995)

and new data , herein

DSDP Site 612

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Poag et al.'s (2002) paleomagnetic data also have interesting implications regarding late Eocene ejecta cored at Antarctic Site 689B (Fig. 8.5). According to three different authors, Stott and Kennett (1990), Speiss (1990) , and Miller (1992), the Antarctic ejecta correlate with either Chron 16n.ln or 15r, both of which are ~0. I -o. 9 myr younger than all three known late Eocene impacts. If any of the three published magnetochronologies is correct, then the Site 689B ejecta must represent an unidentified late Eocene impact, which took place in the interval between deposition of the Massignano ejecta layer and a younger iridium-enriched layer (within Chron 13r) in the Contessa Highway section of Italy. However, the stratigraphic relationships and compositions of the ejecta at Site 6898 strongly suggest correlation with the Popigai impact (Whitehead et al. 2000; Liu and Glass 2002), and possibly also, in part, with the Chesapeake Bay impact (Glass and Koeberl 1999a). In either case, the ejecta age would cast doubt on the published magnetostratigraphy of Site 6898. Poag et al. (2002) inferred that one or more unconformities have interrupted the stratigraphic succession at Site 689B (Fig. 8.5; see further discussion of the correlations and related features in Chapters 9, 13).

9 Geological Consequences of Chesapeake Bay Impact

9.1 General Nature of Consequences The most obvious consequences of the Chesapeake Bay impact, for which there is direct evidence , involve immediate (nearly instantaneou s) changes in the structure, stratigraphy, and morphology of the sedimentary and crystalline target rocks of southeastern Virginia , and subsequent accumulation of impact-generated deposits (impactites) within and near the primary crater. Consequences of a more regional or hemispheric nature include deposition of distal ejecta and production of farfield seismic effects , such as the possible fracturing of sedimentary beds along the nearby continental shelf edge. Other consequences of a hypothetical nature, for which there is only indirect evidence (or no evidence at all), include possible atmospheric effects (greenhouse warming , climatic cooling), severe stresses imposed upon the global biosphere (extinctions, migrations, faunal or floral turnovers), and, perhap s, even impact-tr iggered changes in relative sea level (Kendall et al. 1995).

9.2 Reconfigured Basement Structure and Morphology Seismic reflection profiles gathered during the study of the Chesapeake Bay impact crater (Chapters 4, 5; CD-ROM.8-18) allow us to thoroughly revise earlier interpretat ions of the configuration of the crystalline basement surface in southeastern Virginia (Figs. 9.1, CD-ROM.3-5). Comparison with three previously published structure maps of the basement surface (Fig. 2.3A,B ,C) shows a 20-year evolution in the perception of regional basement morphology. Basement-imaging marine seismic reflection profiles available from Chesapeake Bay and from the continental shelf east of Delmarva, plus profiles from scattered onshore locations in southeastern Virginia , now total more than 2,000 km of data (Chapter 3; Figs. 3.3, CD-ROM.2 ; Table 3.2). Several coreholes drilled for studies of geothermal resources of the Virginia Coastal Plain (Costain and Glover 1976-1982) document the basement composition and depth outside the crater (Fig. 2.2; Table 2.1). Two recent coreholes (USGS-NASA Langley, October, 2000; USGS Bayside, August, 2001) provide the first direct assessment of basement composition and possible shock-alteration of in situ basement rocks C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

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inside the crater (Horton et al. 200 I, 2002; Powars et al. 200 I; Gohn in press). We have integrated the old and new data to construct a new version of the basement structure map, which gives an entirely different picture from the previously published maps cited earlier herein (Figs. 9.1; CD-ROM .5). On this new structure map, five features stand out (see also Chapter 4): (I) The broad regional sweep of structure contours is dramatically disrupted by a deep, irregularly subcircular depression in the basement surface, centered approximately beneath the town of Cape Charles on the western shore of the Delmarva Peninsula . This depression is the inner basin of the Chesapeake Bay impact crater (Poag 1997a); (2) The inner basin is surrounded by an irregular, knobby, annular ridge, which constitutes the peak ring of the Chesapeake Bay crater (Poag 1997a); (3) An irregular massif of basement rock occupies the center of the inner basin; this is the central peak of the crater (Poag et al. 1999); (4) The basement surface outside the inner basin is marked by extensive systems of normal faults oriented radially and concentrically to the peak ring and inner basin; (5) Interspersed among these faults are numerous small welts and low ridges, many of which also are concentrically oriented around the inner basin and peak ring; and (6) The structural gradient of the eastward-deepening basement surface steepens notably under the continental shelf, beneath the eastern flank of the peak ring, and even more markedly approximately 50 krn east of the center of the inner basin (Figs. 9.1, CD-ROM.5).

9.2.1 Central Peak

Poag et al. (1999) pointed out that several of the seismic reflection profiles near the center of the inner basin display faint arcuate reflections believed to represent side echoes from an irregular central peak. Poag and Foster (2000) showed seismic and gravity data from the Ewing cruise to firmly establish the presence of an irregular central peak, centered approximately beneath the town of Cape Charles , Virginia (Figs. 4.32, 9.1, CD-ROM .5; see also Chapter 4). We have mapped the outline of this uplift to resemble a misshapen dumbbell (Figs. 9.1, CD-ROM.5); its highest elevation is marked by a relative positive gravity anomaly, centered approximately below the town of Cape Charles (Figs. 4.35, 4.36) .

9.2.2 Inner Basin

In its western two-thirds , the irregular periphery of the inner basin is firmly constrained at 20 intersections between seismic reflection profiles (Figs. 4.21, 9.1; CD-ROM.2). In the eastern third, the outline is interpolated to be roughly symmetrical , consistent with the ncar-symmetry of the negative Bouguer gravity anomaly that underlies the inner basin (Poag 1997a; Figs. 4.35, 4.36) and of the positive gravity anomalies that signify the peak ring (Figs. 4.35, 4.36). For mapping purposes , we assumed that the inner basin walls are nearly vertical (at the map scale) down to an estimated depth of 1.6 km. This assumption is based on the Texaco seismic profiles, the Ewing seismic profiles, and Pilkington and Grieve's

290

Geological Consequences of Chesapeake BayImpact

(1992) model calculations. For comparison, Sabet (1973) constructed a gravity model for a single profile across the southern Delmarva Peninsula, which showed a deep depression with a maximum depth of ~ 1.2-2.2 kIn beneath Cape Charles. Our gravity model (Chapter 4) suggests that the structural floor of the crater could be as much as 2 km deeper than the lip of the crater's outer rim (Fig. 4.37B).

9.2.3 Peak Ring The diameter of the crest of the Chesapeake Bay peak ring is ~40 km on average (Table 4.5). Seismic profiles show that the ring displays a series of irregular peaks and ridges, the heights of which vary significantly along the western two-thirds of the ring (Figs. 9.1, CD-ROM.5). Detailed physiography of the eastern third of the peak ring is not known due to a lack of appropriate seismic profiles in that region. The gravity signature, however, suggests that the peak ring is roughly symmetrical (Chapter 4).

9.2.4 Normal Faults The basement surface of southeastern Virginia has heretofore been considered to be relatively fault-free (Poag 1997a). In contrast, we have documented more than 500 individual normal faults, or groups of normal faults, which offset the basement surface along the seismic reflection profiles (Figs. 9.1, CD-ROM.5) . We can correlate a few of the fault traces between two or more seismic profiles, but for the majority of faults, there is too little control to determine their spatial orientations. We mapped the faults mainly as subperpendicular to the seismic lines to give a sense of random orientations, but we cannot substantiate this pattern without more closely spaced seismic profiles. Undoubtedly, many faults with smaller offsets are present, but cannot be distinguished at the resolution of our data. For simplicity, we divided the normal faults into two categories: (1) those with vertical throws of 25 m or less; and (2) those with throws of >25 m. By far the majority of the normal faults have relatively small throws (type 1). Such faults can be recognized on all the seismic profiles, regardless of the seismic collection system used. The type 2 normal faults, on the other hand, appear to have a nonrandom distribution. They occur mainly as concentric faults along the northwest periphery of the inner basin, and as radial fault systems, which originate near the peak ring and extend up the Rappahannock, York, and James Rivers (Figs. 9.1, CD-ROM.5) . We expect to find additional radial fault systems on the eastern side of the crater , as soon as better seismic data are available from that sector. Though the positions of the radial fault systems on the western side of the crater appear to be related to the courses of the rivers (or vice versa), their apparent restriction to these locations may be an artifact of our data distribution. Because the river-mouth profiles are more closely spaced than those anywhere else in the bay, they provide plausibility for our choice of connections between individual segments of the radial faults.

Reconfigured Basement Structure andMorphology

291

9.2.5 Reverse Faults Reverse faults are relatively rare on the seismic profiles , and some appear to have formed prior to the impact (that is, during the Appalachian orogeny in Paleozoic time). The most prominent set of reverse faults in the basement rocks is present on the Potomac River profile (the Cobb Island fault system of Poag I997a), where apparently it was reactivated as a series of normal faults by the Chesapeake Bay impact (Fig. 5.12) .

9.2.6 Compression Ridges On a regional scale, the basement surface of southea stern Virginia is gently warped into a series of broad ridges and noses, with long axes that are approximately parallel or slightly diagonal to the dip of the basement. Where we have closer control near the crater , however , numerous smaller welts and ridges can be distinguished (Figs. 4.7A,B, 4.11, 4.20A,B), and their inferred complexity increase s with increased structural control. Most of these smaller feature s are elongated concentrically to the inner basin and its surrounding peak ring (Figs . 9.1, CD-ROM .5). Farther offshore (outside the crater), where the basement gradient steepen s, the structural grain changes also. Here, the long axes of the ridges and valleys are perpendicular to the regional dip. In summary, our collection of more than 2000 km of seismic profiles in the bay and offshore , and the recent coring of basement rocks, reveal that the basement structure beneath Chesapeake Bay is much more complicated than previously suspected. Of the broad embayments and noses presumed to be regional basement features by earlier authors (Fig. 2.3A-C), only the principal axis of the northern embayment is substantiated by the new data (Figs. 9.1, CD-ROM.5). The other features are replaced by a series of smaller-scale ridges , wrinkle s, and troughs. These long, narrow feature s form concentric adjuncts to the 35-km-diameter basement excavation that forms the inner basin of the impact crater. Because of their structure, geometry , and distribution , we interpret the elevated concentric structures as compre ssion ridges, created directly by the compressional shock wave that radiated from ground zero . As a whole, though, the preponderance of normal faults in the crystalline floor of the annular trough indicates that extensional stresses dominated this area during the impact. The tradit ional concept of the Norfolk arch (Chapter 2; Owens and Gohn 1985; Gibson 1971; Powars et al. 1992) as a positive basement feature, appears not to be substantiated by the new map. Rather , the local elevation differences cited as evidence for this basement arch appear to be due to the subsidence differential between the crater and surrounding rocks.

292

Geological Consequences of Chesapeake BayImpact

9.3 Disruption of Preimpact Sedimentary Column In addition to such dramatic alteration of the basement configuration, the Chesapeake Bay impact created concentric zones of vaporization , excavation, collapse, and fracturing within the preexisting sedimentary rock column of southeastern Virginia. Our structure maps and cross sections show that the geometry of the preimpact sedimentary units is strikingly different from that published by most previous authors (e.g., Brown et al. 1972; notable exceptions are Powars and Bruce 1999, Powars 2000). Traditionally, the onshore structural and stratigraphic trends of these units have been extrapolated, virtually unchanged, beneath Chesapeake Bay and the Delmarva Peninsula (e.g., Brown et al. 1972; Mixon et al. 1989). The contrast is clearly exemplified by comparing the structure map of the Lower Cretaceous Potomac Formation of Brown et al. (1972; their Cretaceous F unit; our Fig. 9.2) and our new map of the same formation (Fig. 9.3). Brown et al. (1972) mapped the top of Cretaceous Unit F in southeastern Virginia as a broad, gently eastward dipping, slightly flattened, structural nose, with embayments to the north and south of it. The structural contours strike nearly north-south beneath Chesapeake Bay, and the dip of the formation increases markedly at about 300 m depth beneath the eastern margin of Chesapeake Bay (Fig. 9.2). The flattening of the nose gives a hint of the effects of the underlying crater displayed graphically on our new map. In contrast to the map of Brown et al. (1972), our new map of the top of the Potomac Formation (Fig. 9.3) shows the 300-m contour following the western margin of the bay in the northern half of the map, but in the southern half, the contour arches strongly westward following the rim of the crater, indicating regional uplift during the cratering event. Seismic profiles from the inner continental shelf, combined with the results of the gravity modeling (Chapter 4), allow us to infer a comparable arching of contours around the eastern rim of the crater . North of the crater, the eastward slope of the surface increases at 200 m depth and again at 400 m, between which there is a broad structural shelf. The surface of the Potomac Formation is depressed and relief is quite irregular where the formation has collapsed to form displaced megablocks in the annular trough, though precise elevations of this surface cannot be determined on all seismic profiles. The Potomac Formation is missing altogether (except as breccia clasts) from the zones of excavation and vaporization in the center of the crater (from the peak ring and inner basin; Fig. 9.3). Structure maps and cross sections of the younger (Paleogene) preimpact sedimentary formations outside the crater show similar regional structural trends to those of the Potomac Formation (Powars and Bruce 1999). Each unit dips eastward outside the crater, and the dip is much steeper east of the crater. However, there is a notable difference in the intracrater distribution of Paleogene strata. Seismostratigraphic analysis and extrapolation of corehole results (Poag et al. 1994; Poag 1996; Poag 1997a) suggest that, whereas thick sections of Potomac Formation are present in the displaced megablocks that occupy the floor of the annular trough, Paleogene sediments (as well as youngest Cretaceous strata) have

Disruption of Preimpact Sedimentary Column

293

Fig. 9.2. Previously published structure map of upper surface of Lower Cretaceous Potomac Formation (Cretaceous unit F of Brown et al. 1972), modified to show metric contour intervals; contour interval 50 m.

been removed from the tops of the megablocks. This differential removal may be due to the weaker consolidation of Paleogene strata compared to that of the Lower Cretaceous units. Thus it appears that the only preimpact Paleogene strata preserved inside the crater occur as clasts within the Exmore breccia, or as comminuted components of the glauconite-quartz sand matrix.

294

Geological Consequences of Chesapeake Bay Impact

Fig. 9.3. New structure map of upper surface of Lower Cretaceous Potomac Formation based on our seismic reflection profiles and borehole data (contour interval 50 m). Hachured area in center indicates absence of Potomac Formation in inner basin and over peak ring. Potomac Formation is present in annular trough in the form of displaced megablocks and breccia clasts.

9.4 Source of North American Tektite Strewn Field 9.4.1 General Distribution of Distal Ejecta

The most significant documented regional effect of the Chesapeake Bay impact was deposition of distal ejecta. Centimeter-scale layers containing tektites, microtektites, and shocked minerals correlative with the Chesapeake Bay and Toms Canyon craters , are widespread in the western North Atlantic, Caribbean, and Gulf of Mexico. Scattered individual tektites of this age also are present in the coastal 2 6 plains of Georgia and Texas. Ejecta from 10 sites in this 9 x 10 km region repre-

Source of North American Tektite Strewn Field

295

sent the bulk of the North American tektite strewn field (NATF; Fig. 2.11; Glass 1989; Koeberl 1989). Vonhof and Smit (1999), Glass and Koeberl (1999a, b), and Liu et al. (2000) recently expanded the possible distribution of the NATF to include ODP Site 689B on the Maud Rise (Leg 113; Barker, Kennett et al. 1990), ODP Site 216 on Ninetyeast Ridge in the eastern Indian Ocean (Leg 22; von der Borch, Sclater et al. 1974), and ODP Site 1090B (Leg 177 Gersonde , Hodell et al. 1999) near the Agulhas Ridge (S. Atlantic) (Fig. 9.4). Latest reconsideration of these distal sites by Liu and Glass (2002), however, casts doubt on their inclusion in the NATF. The radiometric age of the NATF is estimated from 40ArP9Ar stepheating dating of microtektite glass sampled at two locations: (I) A northern location, cored at DSDP Site 612, located 20 km south of the Toms Canyon structure and 330 km northeast of the Chesapeake Bay crater (Figs. 2.12,9.4), yielded ages of 35.2 ± 0.3 to 35.5 ± 0.3 Ma (Obradovich et al. 1989; Poag and Aubry 1995); (2) A southern location, sampled at Bath Cliff, Barbados, 3,500 km southeast of Chesapeake Bay (Fig. 9.4), yielded an essentially identical age of 35.4 ± 0.6 Ma (Glass et al. 1986). On the basis of its geographical proximity, similar age, and similar geochemistry, the North American tektite strewn field is now believed to have been produced mainly by the Chesapeake Bay impact (Poag et al. 1994; Koeberl et al. 1996; Glass et al. 1998; Glass and Koeberl 1999a,b), though some of the Site 612 ejecta may have come from the Toms Canyon crater (Poag and Aubry 1995; Poag and Poppe 1998; Fig. 2.12). At deep-sea sites containing ejecta of the North American strewn field, the deposit is dominated by microtektites and shocked quartz, with minor amounts of stishovite and coesite (Fig. 9.4; Glass and Wu 1993; Glass et al. 1998). The microtektite layer is often underlain by a thin concentration of microkrystites (spherules containing clinopyroxene crystals and(or) nickel-rich spinels) (Glass et al. 1985; Glass and Burns 1987; Pierrard et al. 1998, 1999; Fig. 2.13) associated with a moderate iridium anomaly (Sanfilippo et al. 1985; Montanari et al. 1993; Pierrard et al. 1998). These two layers are generally separated by 2-25 ern of sediment or less (equivalent to 10--20 kyr; Glass et al. 1998; Glass and Koeberl 1999a, b), and both layers reside in the P 15INP19-20 overlap interval, which embraces the Chesapeake Bay impact (Poag 1997a,b; see also Chapters 8, 13). Some sites in the North American strewn field, however, contain only one or the other of these layers, or in some cases, the layers are so closely spaced as to be stratigraphically indistinguishable. In contrast to the distribution of North American microtektites, only the microkrystite layer has been compellingly documented at Pacific sites (Whitehead et al. 2000; Fig. 9.4). Ejecta inferred to have come from the Popigai crater has been reported from the world stratotype of the Eocene-Oligocene boundary at Massignano , Italy (Odin and Montanari 1989; Berggren et al. 1995; Figs. 8.5, 9.4). Geographically, the Massignano section appears to fit within the expanded concept of the NATF (near its northern limit). The composition of the Massignano ejecta , however, resembles a mixture of the microtektite and microkrystite layers. Nickel-rich spinels, shocked quartz, and an Ir anomaly have been documented (Montanari et al. 1993; Clymer et al. 1996; Langenhorst 1996; Pierrard et al. 1998, 1999), and the 3He content is unusually high (Farley et al. 1998) . The presence of the nickel-rich

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spinels at Massignano has been taken as evidence of its correlation with the microkrystite layer and the Popigai impact (Vonhof and Smit 1999; Whitehead et al. 2000). Though the Massignano ejecta appears to occur in a single layer, and microtektites have not yet been identified, it has been speculated that this Italian occurrence may represent a condensation of the microtektite and microkrystite layers, where the separate identities of the two layers have been obscured by bioturbation (Vonhof and Smit 1999; Huber et al. 2002) and low deposition rates in this relatively thin (12 m) upper Eocene section. Liu et al. (200 I) attempted to determine whether or not both ejecta layers are present at Massignano, but their results were not conclusive . Magnetostratigraphic studies of the Massignano section reveal that the ejecta deposit resides within the upper part of Chron 16n.2n (Bice and Montanari 1988; Lanci et al. 1996), and is, therefore, >35.6 myr old. A radiometric age of 35.7 ± 0.4 Ma for the Massignano ejecta has been derived indirectly by complicated interpolations between: (I) a weighted mean age estimate from an ash bed at Massignano (35.4 ± 0.3 Ma); and (2) a similarly derived estimate (36.4 ± 0.4 Ma) from an ash bed located - 80 km away in the Contessa Highway section , which was extrapolated by means of magnetochronology to the Massignano section (summarized by Montanari et al. 1993; see also Chapter 8).

9.4.2 Correlation Problems The biostratigraphic position of the Massignano ejecta was reported by Coccioni et al. (1988) and Montanari et al. (1993) to be within planktonic foraminiferal Biozone P16, whose biochron spans the interval from 34.0 to 35.2 Ma (Berggren et al. 1995). If true, the Massignano ejecta would be biochronologically younger, but radiometrically older, than the Chesapeake Bay and Toms Canyon impacts (and their related impactites). However, Spezzaferri et al. (2002) apparently have resolved this anomaly by demonstrating that the Massignano ejecta layer does belong in Biozone P 15, as defined by Berggren et al. (1995). Nevertheless , it has been convincingly demonstrated that the stratigraphic ranges of key species of planktonic foraminifera and calcareous nannofossils are not isochronous between the Massignano section , DSDP Site 612, and the Chesapeake Bay crater (Exmore breccia and Chickahominy Formation; Miller et al. 1991; Montanari et al. 1993; Poag and Aubry 1995). Biochronological correlations with other pertinent upper Eocene sections are hampered by poor specimen preservation in deep-sea sediments (including Bath Cliff, Barbados) and by significant faunal and floral disparities due to biogeographic differences between tropical micro- and nannofossil assemblages (Caribbean and Gulf of Mexico) , mixed tropical-temperate assemblages (DSDP Site 612, Chesapeake Bay), Tethyan assemblages (Massignano), and Antarctic assemblages (ODP Site 689B; Fig. 9.4) . Inconsistent biostratigraphic interpretations at two additional coreholes near Site 612 (ODP Sites 903, 904; McHugh et al. 1993; Snyder et al. 1993; Aubry 1993) have further confused correlations with the offshore New Jersey sites (Figs. 2.11, 2.12) . For example, at Site 904A, these three

298

GeologicalConsequences of Chesapeake Bay Impact

groups of authors placed the Zone P15-P14 boundary (planktonic foraminifera), the Zone NP 19/20 lower boundary (calcareous nannofossils), and the Zone NP 16 lower boundary (calcareous nannofossils) at significantly different core depths. Although the magnetostratigraphic records at Massignano and Site 689B have helped to resolve some of the biostratigraphic and radiometric correlation problems (Chapter 8), critical inferences, extrapolations, and interpolations are still required, which weakens some of the correlations . The magnetostratigraphic and chemostratigraphic (stable isotopic) data from the Chesapeake Bay impact site further constrain the age of the impact (Chapters 8, 13) and clarify correlations among ejecta sites. The Chesapeake Bay data also provide stratigraphic constraints that will assist in the search for additional occurrences of late Eocene impact ejecta.

9.5 Far-Field Seismic Effects Hypothetically, powerful seismic waves generated by a large bolide impact would produce significant far-field effects, possibly fracturing and faulting preimpact strata at distances of several hundred kilometers from ground zero. In the case of the Chesapeake Bay impact, there is evidence (gathered by deep-diving submersibles, drilling, and swath bathymetric surveys) of extensive fracturing and brecciation (Fig. 9.5) and unusual steep-walled channels (Fig. 9.6) along a broad outcrop band of lower and middle Eocene limestones on the continental slope 120 km east of Atlantic City, New Jersey (Robb et al. 1983; Farre and Ryan 1985; McHugh et al. 1993, 1995). Though this area of unusual fracturing is 300 km northeast of the Chesapeake Bay crater, Poag et al. (1992) and McHugh et al. (1995) have suggested that the fractures may have been generated by the Chesapeake Bay and(or) Toms Canyon impacts. We propose that the enigmatic, steep-walled channels also may be distal erosional products of the Chesapeake Bay impact. Fig. 9.5. (Opposite page) Seafloor photographs of outcrops of intensely fractured middle and lower Eocene pelagic limestones on New Jersey Continental Slope adjacent to Toms Canyon crater and - 300 km northeast of Chesapeake Bay crater. A, fractured limestone beds form near-vertical cliff in upper half of photograph (arrow). B, terrace of highly fractured limestone (arrows mark fractures). C, talus of angular limestone clasts at base of cliff. Photographs from submersible dive courtesy of David C. Twichell. See CD-ROM for color version of this figure.

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Fig. 9.6. Seafloor photographs showing unusual steep-walled channels in Eocene pelagic limestones exposed at foot of New Jersey Continental Slope. A, view down axis of channel (channel ~3 m wide). B, vertical view of channel and nearly vertical channel wall; arrows indicate approximately right-angle intersection between channel wall and fractured flat limestone surface that forms top of channel wall. Photographs from submersible dive courtesy of James M. Robb.

10 Comparisons with Other Impact Craters

10.1 Terrestrial Craters The Chesapeake Bay crater is large and well preserved, and exhibits more impactgenerated features (including secondary craters) than most other terrestrial impact structures yet studied, but it appears not to have caused an immediately subsequent mass extinction. These properties make it an important benchmark for improving our understanding of the dynamics of crater formation , ejecta generation and distribution, breccia origin and deposition, and consequent environmental perturbations, or lack thereof. A few known craters in its size class (75-100 km diameter), and several smaller ones, exhibit some or most features of the Chesapeake Bay crater (Table 10.1), but in several key aspects , the Chesapeake Bay crater does not conform to general conceptual models widely applied to explain the formation of comple x terrestrial and planetary craters. Some of its unusual features (perhaps all) appear to be related to its original submarine location , which presented a three-layered target compo sed of: (I) a moderatel y deep (~3 00 m) water column; (2) a water-saturated , unconsolidated sediment column (300-500 m thick) ; and (3) a basement of consolidated crystalline (granitoid and metasedimentary) rocks. Other considerations, such as impactor size , composition, and trajectory, howe ver, also may be pertinent to explaining some of the differences. In order to identify the principal differences, we compare the attributes of the Chesapeake Bay crater with those of other known subaerial and submarine craters on Earth , as well as with some of those on other planetary bodies.

10.1.1 Subaerial Craters Among the few well-preserved and well-documented impact craters originally formed in subaerial target rocks, the closest analogues for the Chesapeake Bay peak-ringlcentral-peak structure appear to be the Popigai crater (85-km diameter, late Eocene , Anabar Shield , Northern Siberia, Russia; Masaitis 1994; Masaitis et al. 1999; Whitehead et al. 2002) and the Ries crater (24-km diameter, late Miocene , southern Germany; Pohl et al. 1977; Newsom et al. 1990; Figs. 1.1, 10.1). Both of these craters formed in mixed sedimentary-crystalline target rocks, are clearly marked by steep outer-rim escarpments, and have distinct peak rings of upraised crystalline basement rocks. Each crater also is characterized by a broad annular trough outside the peak ring and a deep inner basin inside the peak ring. Some authors have inferred a crystalline central peak for each, though the deep C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

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'1

no

yes

yes

yes

no

no

no

no

no

'1

no

yes

subaerial

subaerial

10·50

500-1000

>200

<200

100·200

300-500

200·600

- 300

Surgeback Tsunami Flowin Fallout Estimated Deposits Washback Depo sits Depo sit s Paleodepth Depo sits [m)

core 7430 probably I IO-V-OI

none

Tand sbyn none breccia

yes

crystalline crystalline crystalline, yes sedimentary, above depression faulted deco llement

Manson [37)

sedimentary, '1 faulted

yes

yes

crysta lline

sedimentary, '1 faulted

Kamensk [38)

eroded

sediments

sedimentary, above sediments decollement faulted

Mja lni r [40)

sedime nts

yes

sedimentary, no crystalline crystalline crystall ine, yes depression eroded decollement

N.Am . tekti tes

Bre ccia Proximal Distal on Breccia Ejecta Central Peak Exmore breccia

Central Peak

crystalline, crysta lline crystallin e yes sedimentary, above decollement breached? faulted

Inner Basin

Chesapeake Bay [85) Mont agnai s [45)

Megablocks Peak (Slumpback Ring Deposits)

Outer Rim

Crater Diam [km)

Table 10.1. Comparison of Chesapeake Bay crater and its impact-generated deposit s with other sele cted submarine and subaerial crat ers and impactites.

n

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Terrestrial Craters

303

interior of neither of these two craters has been fully documented. In terms of age and general morphology, Popigai is practically a twin of the Chesapeake Bay crater. Their ages are indistinguishable [Chesapeake Bay = 35.78 Ma, new date herein (see Chapters 8, 13); Popigai = 35.7 ± 0.4 Ma; Bottomley et al. 1997]. These two craters also appear to be virtually identical in size. Though the diameter of Popigai is commonly listed as - 100 km (e.g., Grieve et al. 1995; Masaitis et al. 1999; Masaitis personal communication 2002), the geologic maps of Masaitis et al. (1999) and Whitehead et al. (2002) indicate an average diameter of - 85 km. Aside from minor irregularities in the peripheral outlines, the outer rims of the two craters can hardly be distinguished from one another when superposed (Poag et al. 1999; Fig. 10.2). Other morphological features are quite similar, but not identical. For example, there is a slight asymmetry to the Popigai structure, in which the peak ring and central peak are offset to the west relative to the center of the outer rim. The diameters of the peak ring and central peak also may be slightly larger at Popigai. The peak ring and central peak are buried (for the most part) by impact-generated deposits at both of these sites (Fig. 10.1; see also Chapter II). However, at Popigai, the crystalline rocks of the peak ring crop out in two narrow strips on the northwest side of the crater. Masaitis et al. (1999) estimated a maximum depth for the Popigai annular trough to be - I km to the northwest, but twice that (-2 km) to the southeast (Fig. 10.1C). At Chesapeake Bay, a similar deepening of the annular trough takes place from northwest (-0.7 km) to southeast (-1.5 km). There is an enormous difference between the two structures, however, in the maximum structural relief from the deepest part of the annular trough to the crest of the peak ring. This relief at Chesapeake Bay is - 200-600 m, whereas at Popigai it is more than 2 km (Fig. 10.1C). Significant structural differences in the annular troughs also can be inferred from the drilling and geophysical data at Popigai and Chesapeake Bay. Masaitis et al. (1999) showed (based partly on drilling) that crystalline basement rocks in the Popigai annular trough are deeply faulted, have a high local relief, and incorporate displaced megablocks composed of sedimentary target rocks. At Chesapeake Bay, in contrast, the crystalline surface of the annular trough exhibits only moderate to low structural relief (see Chapters 4, 9). Furthermore, the crystalline rocks of the annular trough at Chesapeake Bay appear not to enclose any sedimentary blocks. Instead, the surface of the trough acts as a zone of detachment, along which the overlying sedimentary megablocks have slid, slumped, and collapsed. The deeper parts of the inner basin at Popigai, as at Chesapeake Bay, have not yet been drilled, but gravity modeling suggests that the inner basin of each crater may be as deep as 2 km below the lip of the outer rim (Fig. 10.1B,C). As a working hypothesis, however, we use a depth of - 1.6 km for the inner basin at Chesapeake Bay (see Chapter 4). Masaitis et al. (1999) reported a low-relief central peak at Popigai on the basis of three-dimensional computer modeling of gravity data. Cross sections of the Ries crater based on drilling and geophysical surveys (seismic reflection, gravity, geomagnetics) show comparable structure and morphology to Chesapeake Bay and Popigai, except for its smaller size (with con-

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Fig. 10.1. Comparative geologic cross sections of Ries impact crater, southern Germany (A opposite page) modified from Pohl et al. (1977) ; Chesapeake Bay impact crater (B opposite page); and Popigai impact crater, Northern Siberia (C constructed from two different cross sections of Masaitis et al. 1999; central peak and inner basin interpolated). Note contrasting vertical exaggerations.

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306

Comparisons with Other Impact Craters

N

i 37'30'

371>0'

oI 76'40 '

75

50 I

76'20 '

km

100 I

!

761>0'

75 40'

Fig. 10.2. Superposed peripheral outlines of Chesapeake Bay and Popigai impact craters showing similarity in size and geometry of principal features. Solid lines represent Popigai; dashed lines represent Chesapeake Bay. Short concentric and radial fault traces outside craters are from Popigai. Modified from Poag et al. (1999).

sequent smaller values for other properties, such as structural relief, and depth of the annular trough and inner basin) and the undetermined presence of a central peak (Fig. IO.IA). The crest of the crystalline peak ring crops out at Ries, and at least one deep corehole has penetrated the crystalline floor of the inner basin (Steffler 1977; see also Chapter II). Like most other intermediate to large terrestrial craters, however , details of the deeper structure at Ries have not been thoroughly investigated , leaving open such question s as whether the sedimentary megablocks are detached from the crystalline basement , or what is the nature of the central peak.

Terrestrial Craters

307

10.1.2 Submarine Craters

Several authors have addressed the hypothetical aspects of how impacts into the ocean might produce craters that differ from those generated by subaerial collisions (Higgins and Butkovitch 1967; Kieffer and Simonds 1980; McKinnon and Goetz 1981; Melosh 1981; Gault and Sonett 1982; McKinnon 1982; Silver 1982; Roddy et al. 1987; Ormo and Lindstrom 2000; Artemieva and Shuvalov 2002; Ormo and Miyamoto 2002; Shuvalov et al. 2002; Wiinnemann and Lange 2002; Ormo et al. 2002). Unfortunately only about a dozen submarine craters have been identified, and fewer have been carefully studied in the field. Four of these are still wholly or partly submerged and also are completely buried by postimpact sedimentary rocks (Chesapeake Bay, Toms Canyon, Montagnais, and Mjalnir; Figs. 1.1, 10J). Of those remaining, six also are entirely buried by postimpact deposits (Ames, Manson, Granby, Kardla, Kamensk, and Kaluga). Only two submarine craters (Lockne and Brent) are whollyor partly exposed (Fig. 1.1). Among submarine craters, Montagnais (45-km-diameter, early Eocene (-51 Ma), Nova Scotian Shelf, Canada; Jansa et al. 1989; Pilkington et al. 1995; Poag et al. 2002) is next in size to Chesapeake Bay, and was the first submarine crater to be discovered (Figs. 10.3-10.7). This structure is known mainly from 1,000 km of seismic reflection surveys, and a single deep (but uncored) borehole that penetrated its crystalline central peak. We have reinterpreted certain aspects of the Montagnais structure after examining a full-scale version of profile 3203-82 (Fig. 2A of Jansa et al. 1989; Figs. 10.5, 10.6, CD-ROM.17). The outer rim of the Montagnais crater is a steep escarpment cut into sedimentary rocks (Figs. IOJ , 10.5), similar to the outer rim at Chesapeake Bay and Ries. The annular trough, - 7 km wide, has a flat, relatively horizontal floor excavated into lithified Barremian sedimentary rocks known as the Roseway Unit (Fig. 10.5). The excavation surface is parallel to bedding of the Roseway Unit. The crystalline basement surface forms a marked depression beneath the annular trough on the east side of the structure, but descends deeply beneath a thick sedimentary wedge on the western side of the crater (Figs. 10.3, 10.5). A subtle arch on the upper surface of the basement is present on each side of the crater (Figs. 10.3, 10.6), and this may represent a low-relief peak ring. If so, the peak ring's position at 0.5-1.0 km below the surface of the annular trough is quite unusual. A narrow inner basin (3-5 km wide) separates the possible peak ring from a prominent central peak. This central peak is 16 km in diameter at its base and II km across at the top. The central peak is bounded by high-angle normal faults, and rises more than I km above the floor of the inner basin, and -200 m above the outer rim of the crater. A distinct structural depression (200 m deep) occupies the center of the peak. The large diameter and great height of the Montagnais central peak are in stark contrast to the relatively narrow and low central peak at Chesapeake Bay and that inferred at Ries (Fig. 10.1). The central peak at Popigai appears to be of similar height above the inner basin to that of Montagnais, but may be narrower in diameter. Pilkington et al. (1995) found no negative gravity anomaly over the inner basin of the Montagnais crater, and attributed its lack to the unconsolidated nature of the

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Fig. 10.3. Comparative geologic cross sections of Mjelnir crater, Barents Sea (A opposite page) modified from Tsikalas et al. (1998a) ; Chesapeake Bay crater (B opposite page) ; and Montagnais crater, Scotian Shelf (C) modified from Jansa et al. (1989) .

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400 ,

500 ,

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Fig. 10.4. Loca tion map for Montagnais crater, seismic trackline A-B (see Figs. 10.5, 10.6, CD-ROM. I?), and relevant boreholes (solid black dots). Modified from Jansa et at. (19 89). Offshore bathymetry give n in meters.

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Fig. 10.5. Segments of seis mic profile across annular trough of Montagnais cra ter (Poag et al. 2002 ; A, western side of crater; B, eastern side of crater) . Impact breccia shaded; heavy dashed line indicates surface of crystalline basement. Prominent seismic signature of Roseway Unit provides stratigraphic datum for correlation . See Fig. 10.4 for location of protil e and CD-ROM. I7 for full- scale profil e. Profil e courtesy of LF Jansa.

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Fig. 10.6. Segment of seismic profile across central peak of Montagnais crater (interpreted by Poag et al. 2002). Impact breccia shaded ; heavy dashed line indicates surface of crystalline basement. See Fig. lOA for location of seismic profile and CD-ROM.I? for full-scale profile. Profile courtesy of LF Jansa .

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Terrestrial Craters

313

314

Comparisons with OtherImpact Craters

sedimentary target rocks. They inferred that the unconsolidated shelf deposits were compacted, but not fractured by the impact, thus preventing the low-density contrast that causes the gravity low characteristic of many subaerial craters. Pilkington et al. (1995) implied that the lack of a central negative gravity anomaly might be characteristic of submarine impact structures. On the other hand, Poag et al. (2002) superimposed the structure and gravity maps of Montagnais and concluded that a subcircular negative gravity anomaly marks the narrow inner basin of Montagnais (Fig. 10.7). Furthermore, the presence of a distinct central anomaly at Chesapeake Bay (Chapter 4) argues against the general lack of such an anomaly in submarine impact craters . The next smallest of the impact craters still buried by marine waters is the Mjelnir crater (40-km diameter, Late Jurassic, Barents Sea, Norway; Dypvik et al. 1996; Tsikalas 1996 and papers cited therein ; Tsikalas et al. 1998a,b, 1999; Shuvalov et al. 2002; Figs. 10.3A, 10.8-10.1 0). A recently drilled continuous core on the central peak of Mjelnir penetrated 83 m of chaotically deposited sediments (ejecta and slumped debris; Smelror et al. 2001) . The nearest other analyzed deep borehole is 30 km outside the Mjelnir crater rim. Extensive seismic reflection surveys have been carried out over the crater, however (872 krn of high-resolution, single-channel data; 174 krn of shallow, 60-fold, multichannel data; 1081 km of conventional multichannel data; Fig. 10.8). The most obvious difference between the Mjelnir crater and the Chesapeake Bay, Ries, Popigai, and Montagnais craters is the complete lack of crystalline rocks in the Mjelnir structure . Nevertheless, the general morphology of Mjelnir is similar to that of the other structures (Figs. 10.3A-C, 10.9, 10.10). The outer rim of Mjelnir is irregularly ovate, and is expressed as a steep fault scarp with no raised lip (Figs. 10.3A, 10.8-10.10). A 12-krn wide annular trough is shallow, nearly flat, is floored by sedimentary rocks, and forms a decollement (detachment zone) separating the trough floor from the sedimentary megablocks lying above (Figs. IO.3A, 10.9, 10.10). Displaced megablocks present in the outer part of Mjelnir's annular trough are decoupled from the trough's floor by listric normal faults, as at Chesapeake Bay. We should point out that Tsikalas (1996) and Tsikalas et al. (1999) interpreted the top surface of the megablocks to be the floor of the annular trough, whereas we place the trough floor at the decollement beneath the megablocks (as at Chesapeake Bay and Montagnais) . Our placement of the trough floor doubles the height of the outer-rim escarpment (to - 300-500 m) compared to Tsikalas's interpretation (Figs. 10.3A, 10.9, 10.10). On the east side of Mjelnir, a slight arch of the decollement may mark the presence of a subtle, low-relief peak ring. On the west side, the presence of a peak ring cannot be confirmed . A narrow, slightly depressed decollement interval on the east side of the crater could conceivably be considered as an inner basin (Fig. Fig. 10.7. (Previous page) Comparison of structure map and gravity anomaly map for Montagnais crater. A, structure map; contour interval 0.2 km; enclosed depressions of inner basin shaded; A-B is trackline for seismic profile (see Figs. 10.5, 10.6). B, gravity anomaly map with main structural features superposed to show coincidence between inner basin and ringof gravity lows. Modified from Poaget al. (2002).

Terrestrial Craters

25°

• 20'

315

30'

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74' Mj01nir

c.\

74°

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Barents

Sea

72"

72'

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,,

o, 73°30' 29"00'

Fig. 10.8. Location maps showing main structural features of Mjelnir crater, seismic tracklines, and boreholes (modified from Tsikalas et al. 1999). Shaded segments of seismic lines are those interpreted by us in Figs. 10.9, 10.10.

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Fig. 10.9. Segment of single-channel seismic profile across central peak and annular trough of Mjelnir crater. Preimpact sediments in darker shading; impact breccia(?) in lighter shading. Interpretation modified from Tsikalas et al. (2002), who interpreted reflection at "TD" as floor of crater, whereas we consider the decollement as structural floor (see section I0.2). See Fig. I0.8 for location of seismic trackline.

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Fig. 10.10. Segment of shallow multichannel seismic profile across annular trough and outer rim of Mjelnir crater. Preimpact sediments in darker shading ; initial stratified postimpact sediments in lighter shading. Interpretation modified from Tsikalas et aI. (1998a) . Tsikalas et aI. (l998a) interpreted "TD" as floor of crater (= morphological floor in our terminology; see section 10.2) See Fig. 10.8 for location of seismic trackline

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318

Comparisons withOtherImpact Craters

10.9). Mjelnir's prominent central peak (12-km diameter across the base; 8-km diameter at the crest; composed entirely of sedimentary rocks) rises 300 m higher than the lip of the outer rim, and ~500 m above the floor of the inner basin (~250 m higher than the inner basin according to Tsikalas; Figs. 1O.03A, 10.9, 10.10). The elevation of the peak was originally even greater, because its crest has been truncated by Late Cenozoic glacial erosion. The central peak is covered directly by postimpact sediments (no impact breccia indicated over the peak). The Manson impact crater is considered to be a possible shallow-water submarine crater (36-km diameter ; Late Cretaceous; 74 Ma; Manson, Iowa; Koeberl and Anderson 1996a and papers therein; Figs. 10.11-10.13). Though now located in the middle of the North American craton, Manson has been interpreted by some researchers to have originated from an impact into the Late Cretaceous interior seaway (an epicontinental sea; Izett et al. 1993; Anderson and Witzke 1996; Steiner and Shoemaker 1996). Coring (1,283 m recovered from 12 boreholes), combined with seismic reflection, gravity, and geomagnetic surveys, has revealed the following concentric features at Manson (Fig. 10.12): (1) a fault-bounded outer rim of 36-km diameter, constructed within consolidated Paleozoic and Mesozoic sedimentary rocks (sandstone, siltstone, shale, limestone), surrounding; (2) a zone of terraced, folded, and overturned megablocks (annular trough) ~ 7 km wide; we have examined a full-scale copy of the north-south seismic profile across Manson crater (Fig. 3 of Schultz and Anderson 1996; our Figs. 10.12, CDROM.18) and we interpret the slumped megablocks of the annular trough to have dropped down along listric normal faults, which sole out along a decollement at the top of crystalline basement, just as at Chesapeake Bay; (3) a "moat," (inner basin) ~6 km wide, floored by plutonic, and metasedimentary basement rocks; and (4) a crystalline central peak ~ 10 km in diameter at the base and ~8 km across the crest. The central peak, which exhibits a notable central depression, is capped by scattered deposits of four different breccia units (Witzke and Anderson 1996). The upper part of the Manson structure, including the central peak, is truncated by an erosional surface, which was subsequently buried by a layer of Quaternary glacial till. Thus, though the Manson crater lacks a peak ring, in many other respects it resembles a scaled-down version of the Chesapeake Bay crater (Fig. 10.13). Ormo (1998) and Ormo and Lindstrom (2000) summarized most previous studies of submarine impacts (marine-target impacts), presented new data from detailed field studies (particularly of Sweden's 24-km-diameter Lockne crater; Figs. 1.1, 10.14-10.16), and provided a synthesis of those features that appear to separate submarine craters from subaerial ones on Earth and on Mars. Some of the results published by Ormo and his colleagues help to refine our interpretation of the Chesapeake Bay structure. The general stratigraphic framework of preimpact rocks at Lockne, the structure, morphology, and impact deposits associated with the crater, and the disposition of postimpact sedimentary rocks closely resemble those of the Chesapeake Bay crater. However, Ormo (1998) used a different descriptive terminology than ours, which he derived from Quaide and Oberbeck's (1968) study of "concentric" lunar craters. Quaide and Oberbeck (1968) hypothesized that when a bolide strikes a two-layered target in which the lower layer is denser than the upper layer,

Terrestrial Craters

319

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basin at Lockne also contains a low-relief crystalline central peak, though in contrast to Chesapeake Bay, the Lockne peak is not buried by impact breccia (Lindstrom et al. 1996). Autochthonous, monomictic, Tandsbyn Breccia fills the inner basin of Lockne crater (see Chapter I I). The basal contact between Tandsbyn Breccia and crystalline bedrock varies from sharp to gradual. The feature that sets Lockne apart from known subaerial craters on Earth is the presence of four broad, breccia-filled gullies (maximum size is I km wide, 3 km long, 100 m deep), which slice radially through the annular trough and peak ring and extend into the inner basin (Fig. 10.15). Ormo (1998) and von Dalwigk and Ormo (2001) referred to these gullies as resurge features (we prefer surgeback as a parallel construction to the terms fallback, slumpback, and washback; see Chapter 12). In Ormo's hypothesis, the surgeback process involves violent collapse of the ejecta-filled water column and turbulent bottom-flow back toward the center of the crater during the modification stage of crater formation. This process is believed to have hydraulically eroded fracture-weakened zones within the peak ring and crater floor, to produce surgeback gullies. The surgeback gullies subsequently filled with surgeback breccia. Ormo and Miyamoto (2002) applied numerical modeling to estimate the magnitude of surgeback flow at Lockne. They concluded that 1.2 x io'' m3 of sea water would be required to fill in the impact excavation at Lockne. Assuming an initial water depth of 200 m at Lockne, Ormo and Miyamoto (2002) calculated an average surgeback velocity of 27.5 m S-I, which took 2200 s (36.7 min) to refill the excavation cavity. Such a high-velocity flow created a maximum erosive force (unit stream power) of 1.9 x 105 W m". This is the same order of magnitude of erosive force calculated for the glacial Lake Missoula Flood, which produced the highrelief Channeled Scablands of Washington State (Baker and Milton 1974). At Chesapeake Bay, the combination of greater paleodepth, a much larger impact crater, and the poorly consolidated nature of the sedimentary target, would have produced truly prodigious surgeback erosion on that late Eocene seafloor. As at Chesapeake Bay, marine sedimentation resumed at Lockne immediately following the impact. But unlike Chesapeake Bay, the Lockne crater and its breccia deposits are covered by a moderately deep-water (>200 m paleodepth) carbonate deposit (the Dalby Limestone). The biochronological age of the Dalby Limestone is the same (middle Ordovician) as the youngest preimpact sediments in the floor of the crater. Postimpact sedimentary deposits thicken over the Lockne crater, just as they do over the Chesapeake Bay crater. The general structure and morphology of the Chesapeake Bay crater appear to be very similar to those of the Lockne crater, but on a grander scale (Fig. 10.16). Especially notable at Lockne are: (1) the unraised sedimentary outer rim; (2) the flat, shallow, annular trough; (3) the low-relief, crystalline peak ring; (4) the deep inner basin excavated into crystalline basement; and (5) the subtle crystalline central peak buried by postimpact sediments. The two craters differ in detail, however. The Chesapeake Bay crater is, of course, nearly four times larger, the peak ring has greater relief, and the inner basin is much deeper than Lockne . Furthermore, the outer rim at Chesapeake Bay is a fault scarp, rather than an ero-

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sional one; the floor of the annular trough is crystalline basement rather than sedimentary rock; and in the annular trough, kilometer-scale megablocks lie between the impact breccia and the basement. Also, the crater-fill breccia at Chesapeake Bay is much thicker and completely buries the peak ring and central peak. The gravity signature over Chesapeake Bay suggests that the peak ring may be breached in the southeast quadrant, which could indicate the presence of a surgeback gully, but we have no deep seismic data in that area to confirm a gully-like morphology, nor are there core data to determine the possible presence of surgeback breccia at that location. Ormo (1998) identified two additional exposed submarine craters with brecciafilled surgebackgullies. The Kamenskcrater (Fig, 1.1), a structure buried near the Ukraine-Russia border, has been explored by more than 330 boreholes (Movshovich and Milavsky 1990). Kamensk displays 12 branching gullies, some 100 m deep, filled with allogenic breccia and attributed by Orrno (1998) to surge-

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back processes. Like Lockne, the Kamensk crater has an annular trough (9 km wide), the inward-sloping floor of which is eroded into preimpact sedimentary rocks. A 20-km-diameter peak ring is excavated more deeply into the sedimentary strata but does not penetrate crystalline basement rocks. A sedimentary central peak also is present, and is buried by a thick lens (-2 km) of authigenic breccia overlain by -200 m of allogenic, polymict, sedimentary breccia. Kardla is a much smaller, buried, Ordovician submarine crater (4-km diameter) excavated into crystalline rocks of the Baltic Shield in Estonia (Suuroja et al. 2002; Fig. 1.1). The structure and stratigraphy of Kardla are known from 160 boreholes, one single-channel seismic reflection profile, and gravity and magnetic surveys. Ormo (1998) and Suuroja et al. (2002) identified two surgeback gullies with associated surgeback breccia at Kardla. From present evidence, then, it appears that the Chesapeake Bay crater is the only unequivocal peak-ring/central-peak crater among the larger complex submarine structures currently known. Montagnais may have a genuine peak ring, but only one seismic profile across Montagnais has been published, and the ring is difficult to ascertain on the west side of the structure. Mjelnir has been described as having a peak ring, but the supposed ring is a subtle feature, formed entirely by sedimentary rocks; it's precise nature is obscure. Definition of Mjolnir's morphology is complicated by the fact that it has undergone postimpact compaction, which may have altered the original morphology of a peak ring. Kamensk has a sedimentary peak ring; Lockne's central peak is poorly developed; the Toms Canyon structure formed entirely within sedimentary strata and, so far, has given no hint of a peak ring or central peak; the Manson structure has no peak ring.

10.2 Extraterrestrial Craters Geometrical analyses (morphology, structure) of impact craters on other planetary bodies are limited by the inability to image the structural floor of the craters. That is because the craters are partly filled with deposits such as impact breccia, basalt flows, and impact melt sheets, whose upper surfaces form the observable morphological floor. Peak rings and central peaks cannot be recognized on other planets unless they protrude above the crater-fill deposits. In the case of the Chesapeake Bay crater, on the other hand, we have numerous seismic profiles that image the peak ring, and a few that document the central peak, even though these features are deeply buried by Exmore breccia. A datum comparable to the morphological floor of a typical planetary crater would be at a level within the Exmore breccia, above which the peak ring and central peak would protrude (Fig. 10.17). Such a surface, however, is a conceptual feature, which cannot be traced within the chaotic reflections of the Exmore breccia. The best available proxy for that surface is the upper surface of the Exmore breccia. Because of differential breccia compaction, the morphology of this breccia surface mimics the morphology of the under-

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Fig. 10.18. 3-D perspectives comparing morphological features of surface of Exmore breccia (A, this is morphological floor of crater) with those of surface of crystalline rocks (B, this is structural floor of crater). See CD-ROMA for more details of crater morphology.

Extraterrestrial Craters

329

Fig. 10.19. Selected Magellan radar images of Venusian peak-ring/central-peak craters, showing variability in geometry and distribution of peak rings and central peaks. A, Bartan crater; 51-km diameter; type A crater with nearly complete ring of highly irregular ridges and peaks, including several isolated knobs; E, Aglaonica crater; 62-km diameter; type B crater with partial ring of extremely irregular ridges, peaks, and isolated knobs; C, Bonheur crater; I02-km diameter; type C crater displaying continuous knobby ridge with steep scarp facing inner basin; D, Charpentier crater; 48-km diameter; type D crater with closely clustered group of isolated irregular peaks and ridges; E, Lagerof crater; 57-km diameter; type B crater with highly irregular partial ring of isolated peaks surrounding irregular central peak; F, Yablochinka crater; 66-km diameter; type A crater with nearly complete ring of irregular peaks and ridges surrounding small central peak. Images courtesy of WB McKinnon. See Alexopolous and McKinnon (1994) for discussion of craters types A-D.

330

Comparisons withOtherImpact Craters

lying basement surface. Thus, a structure map of the Exmore breccia surface (i.e., the morphological floor of the crater; Figs. 10.18A, CD-ROM A) displays the same principal features (outer rim, annular trough, peak ring, central uplift) as the buried structural floor (Fig. I0.18B), though the vertical relief on the breccia surface is much reduced. Morphometric studies of peak-ring and central-peak craters on the Moon (Hale and Head 1979, 1980), Mercury (Hale and Head 1980; Pike 1988; Neukum et al. 2001), Mars (Wood 1980), and Venus (A1exopolous and McKinnon 1992, 1994; Wood and Tam 1993) indicate that craters displaying both peak rings and central peaks (protobasins of Pike 1988; Type 0 craters of Alexopolous and McKinnon 1994; Fig. 10.19) are comparatively rare in the solar system. Melosh's (1982) hydrodynamic model for ring and central peak formation explains this by treating peak rings as late-stage products of central-peak collapse (see also O'Keefe and Ahrens 1999). Thus peak-ring/central peak craters are envisioned as evolutionary transitions between craters with only central peaks and those with only peak rings. Many field data support this model (Hale and Grieve 1982; Alexopolous and McKinnon 1994), but agreement is not unanimous (Hale and Head 1980). Morphologic and structural variability may depend on such factors as the preimpact geology of target rocks, the velocity, size, and incidence angle of the impactor, and the degree to which crater-fill deposits bury the structural crater floor. The general morphology of the Chesapeake Bay crater agrees well with that of both Martian protobasins and Venusian Type D craters in most respects . Chesapeake Bay differs , however, in having a larger diameter than most Type D craters . Alexopolous and McKinnon (1994) showed that Type D craters cluster between diameters of 40-50 km, though one example was 70 km wide. In contrast, the diameters of protobasins on Mars cluster at 100-150 km with maximum diameters of 215-285 km (Wood 1980). A1exopolous and McKinnon (1994) suggested that the diameter differences might be related to geological differences. This would fit the increasing body of evidence indicating that the geological history of Mars is more comparable to Earth's than to that of Venus. The King crater on the Moon's farside appears to be (morphologically and structurally) an approximate lunar analogue to the Chesapeake Bay crater (e.g., Greeley 1994; Schultz and Anderson 1996; Fig. 10.20). The King crater is approximately the same diameter (75 km) as Chesapeake Bay, and displays a distinctly breached crystalline peak ring, from which Schultz and Anderson (1996) inferred a low-angle impactor trajectory. Missing from King is a visible central uplift, though the possible presence of a buried central peak cannot be ruled out. The King crater also differs from the Chesapeake Bay crater in that the peak ring and displaced megablocks at King are not covered by crater-fill breccia; the outer rim is distinctly raised; the surrounding ejecta blanket is well developed; and the ratio of outer-rim diameter to peak-ring diameter is not proportional between the two craters (Fig. 10.21).

Fig. 10.20. Apollo 16 image of King impact crater (~75-km diameter) on lunar farside , showing principal morphological features . Note general similarity to those of Chesapeake Bay crater , except that here : (1) rim has distinct raised lip; (2) ejecta blanket is strongly developed; and (3) lack of thick crater-fill breccia exposes displaced megablocks , floor of annular trough, and peak ring . Image courtesy of Ronald Greele y.

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10.3 Comparison with Chicxulub Multiring Impact Basin The Chicxulub multiring basin is the largest impact structure to be surveyed with a state-of-the-art multichannel seismic reflection system. The seismic data provide excellent 2-dimensional cross-section profiles of the northern third of Chicxulub's general structure and morphology (Snyder and Hobbs I999b). Though its much greater size (-200 kIn diameter) and more complex structure and morphology place Chicxulub in a different category of impact structures (Morgan et al. 1997, 2002; Morgan and Warner 1999a,b; Grieve and Therriault 2000), it has many features analogous to those of the Chesapeake Bay crater. Like the Chesapeake Bay impactor, the Chicxulub bolide struck a broad continental shelf. The resultant crater was subsequently buried by - I km of postimpact marine sediments, and has

Comparison with Chicxulub Multiring ImpactBasin

333

not been perturbed by any significant tectonism. Several onshore boreholes provide stratigraphic evidence of the impact (crater-fill breccia, impact melt rocks). Offshore seismic surveys, especially the four deep-crustal multichannel profiles acquired by the British Institutions Reflection Profiling Syndicate (BIRPS; Morgan et al. 1997, 2002; Brittan et al. 1999; Morgan and Warner 1999a,b; Snyder and Hobbs 1999a; Figs. 10.22-10.27), provide structural and morphological cross sections of the northern third of the basin. Unfortunately, the borehole stratigraphy must be extrapolated long distances (75-230 km) to the seismic profiles (for example, the recently completed Yaxcopoil corehole is 85 km from its projection on the nearest offshore seismic profile; Smit et al 2002) . Experience at other craters clearly demonstrates that such long-distance extrapolations can be seriously misleading. Nevertheless, a rough stratigraphic calibration of the seismic profiles has been attempted (Brittan et al. 1999). The published seismostratigraphic interpretations agree roughly with P-wave velocity interpretations, but correlations with gravimetric data are ambiguous (Brittan et al. 1999). The outer ring of the Chicxulub basin is recognizable at -80-100 km radial distance from the basin center on the BIRPS profiles (Figs. 10.22-10.26). (We should point out that three different sets of terminology have been applied to the principal topographic and structural features of the Chicxulub basin; we use the terminology of Morgan and Warner 1999b.) The outer ring is manifested by a zone of significant disruption of the preimpact sedimentary section (numerous megablocks bounded by moderate-offset normal faults) located farthest from the center of the basin. Morgan and Warner (1999b) described this feature as a graben linked to a fault or shear zone that dips 30-40 degrees toward the basin center and penetrates to the Moho. Because of the smaller size and different structural style of the Chesapeake Bay structure (it is a complex crater rather than an impact basin), we do not expect to find an analogue of the Chicxulub outer ring at Chesapeake Bay. The next significant feature toward the center of the Chicxulub basin is the inner ring (Morgan and Warner 1999b). Seismic expression of the Chicxulub inner ring is marked by a distinct structural sag and thickening of the postimpact sedimentary section (Figs. 10.23-10.26). A nearly identical analogue is represented by the outer rim of the Chesapeake Bay crater (Fig. 10.27). A stratigraphically deeper structural change also marks the Chicxulub inner ring. This change is most clearly manifested by a series of prominent high-amplitude, continuous, horizontal-to-subhorizontal reflections (inferred from borehole extrapolation to be a succession of carbonate rocks and evaporites), which can be traced laterally from their undisturbed counterparts outside the basin's outer ring (at - 3-3.5 km depth; Figs. 10.23-10.26). Approximately at the Chicxulub outer ring, the section represented by these prominent reflections begins to break up along a series of downto-the-basin normal faults. Farther basinward, at the inner ring, the highamplitude section is abruptly displaced downward by about 1-3 .5 km, and breaks up into numerous tilted megablocks. This structural change is similar to that observed at the outer rim escarpment of the Chesapeake Bay crater (Fig. 10.27). At Chicxulub, these megablocks can be traced inward across the annular trough to approximately beneath a topographic high that has been called a peak ring; this

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distribution also is similar to the megablock distribution at Chesapeake Bay (Fig. 10.27). If one assumes that the entire 1- to 3.5-km-thick preimpact section above the prominent carbonate/evaporite reflections is preserved in each of the displaced megablocks, one can trace the approximate tops of the megablocks across the annular trough (Figs. 10.23-10.27). This exercise shows that there is (at minimum) a 2- to 4-km-thick stratigraphic section between the tops of the megablocks and the base of the postimpact sediments. We infer from the incoherent seismic signature and the moderate P-wave velocities (Brittan et al. 1999) that this 2-4 km section is composed of basin-fill breccia (Morgan and Warner 1999b). The displaced megablocks at Chesapeake Bay are buried in a similar fashion by crater-fill breccia (Fig. 10.27; Chapter 6). The annular trough at Chicxulub is topographically expressed as a depression in the top surface of the basin-fill breccia. Inward toward the basin center, a distinctto-subtle topographic elevation (relief of 400-700 m above the topographic floor of the annular trough) has been defined as a peak ring (Morgan et al. 1997; Brittan et al. 1999; Snyder and Hobbs 1999a; Figs. 10.23-10.27). The seismic signature of the topographic peak ring is expressed as a pair of high-amplitude reflections that represent the earliest postimpact (Cenozoic) sedimentary layer, which is draped over the upper surface of the basin-fill breccia. Thus defined, the peak ring has an apparent diameter of - 80 km along composite seismic profile Chicx-A/AI (this profile does not pass through the center of the basin, however). An irregular circular gravity high appears to only roughly approximate the seismically identified peak ring (Brittan et al. 1999). P-wave velocities indicate that the rocks below the topographic peak ring are comparable in density to sedimentary deposits Fig. 10.23. (Previous page) Interpretation of segment of seismic reflection profile Chicx-A across northwestern rim of Chicxulub impact basin. Profile from Morgan et al. (1997); interpretations modified from Brittan et al. (1999) and Morgan and Warner (I 999b). Shaded interval of high-amplitude reflections in Cretaceous carbonate and evaporite section illustrates impact-generated displacement of preimpact sediments along series of normal downto-the-basin faults. Heavy dashed diagonal line indicates position of inferred differential motion between outward-thrusted material from central-peak collapse and inward-thrusted material from transient-crater collapse, which may have created topographic peak ring (Brittan et al. 1999; Collins et al. 2002). Deep corehole Yaxcopoil-I (completed February, 2002) is projected from - 85 krn southeast of profile. See Fig. 10.22 for location of profile. Ring terminology: crater rim (after Morgan and Warner 1999a); outer ring, inner ring, peak ring (after Morgan and Warner 1999b); R ring, A ring, P ring (after Snyder and Hobbs I999a). Fig. 10.24. (Opposite page) Interpretation of segment of seismic reflection profile ChicxAI across northeastern rim of Chicxulub impact basin. Profile from Snyder and Hobbs (1999b); interpretations modified from Brittan et al. (1999) and Morgan and Warner (I 999b). Shaded interval of high-amplitude reflections in Cretaceous carbonate and evaporite section illustrates impact-generated displacement of preimpact sediments along series of normal down-to-the-basin faults. See Fig. 10.22 for location of profile. Ring terminology: crater rim (after Morgan and Warner 1999a); outer ring, inner ring, peak ring (after Morgan and Warner 1999b); R ring, A ring, P ring (after Snyder and Hobbs 1999a).

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Comparisonswith Other Impact Craters

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Fig. 10.25. Interpretation of segment of seismic reflection profile Chicx-B across northwestern rim of Chicxulub impact basin. Profile from Morgan et al. (1997); interpretations modified from Brittan et al. (1999) and Morgan and Warner (1999b). Shaded interval of high-amplitude reflections in Cretaceous carbonate and evaporite section illustrates impactgenerated displacement of preimpact sediments along series of normal down-to-the-basin faults. See Fig. 10.22 for location of profile. Ring terminology: crater rim (after Morgan and Warner 1999a); outer ring, inner ring, peak ring (after Morgan and Warner 1999b); R ring, A ring, P ring (after Snyder and Hobbs 1999a). in the lowest part of the basin (Brittan et al. 1999). This interpretation is opposed to the interpretation of Sharpton et al. (1993), who speculated that the peak ring consisted of fractured, uplifted, deep, crystalline basement rocks . The interpretation of Brittan et al. (1999) is closer to that of Pilkington et al. (1994), who concluded that the Chicxulub topographic peak ring consists of low-density breccia.

Comparison with Chicxulub Multiring Impact Basin

339

At Chesapeake Bay, the peak ring also can be recognized as a topographic high on the surface of the crater-fill brecc ia (Figs. 4.23 , 4.25-4.29, 1O.18A, 10.27), but there, the topographic peak ring is underlain by a seismically and gravimetrically defined structural high in the crystalline basement. The elevated topographic expression of the peak ring (upper surface of the Exmore breccia) at Chesapeake Bay is the result of differential compaction of the underlying breccia across the underlying structural peak ring in the basement. The seismic profiles at Chicxulub display no prominent high-ampl itude reflections at shallow depth (above 4-km depth) that we could unambiguou sly interpret as a structural peak ring (Brittan et al. 1999; Snyder and Hobbs I999a) . However, on seismic profiles Chicx-A, B, and C, at - 6-7.5 km depth, there are indistinct, arched , or inclined reflections, which could be interpreted as possible manifestations of a structural peak ring in the higher-velocity crystalline basement rocks. Brittan et al. (1999) and Collins et al. (2002) interpreted the topographic peak ring at Chicxulub to be a result of differential motion, in which outwardly-thrusted crystalline breccia from the collapsing central peak overrode inwardly-slumping sedimentary megablocks that were produced by collapse of the transient-crater (Fig. 10.23). Prior to the BIRPS seismic studies, several investigators of the Chicxulub structure speculated that a central peak composed of uplifted crystalline basement rocks explained the structure's positive central gravity anomaly (Pilkington et al. 1994; Sharpton et al. 1996; Hildebrand 1997). More recentl y, however, studies of the deep structure of the central Chicxulub basin using wide-angle ocean-bottom seismometers, have failed to document a central peak (Christeson et al. 1999). On the other hand, Snyder and Hobbs ( 1999) noted an elevated zone of dipping reflections (dips of 15-25 degrees) near the center of composit e profile Chicx-A/AI (reaching from 25 km up to - 15 km depth), which they attributed to shear zones and possible melt intrusions in the crystalline basement rocks. It is not unreasonable to infer that this zone of dipping reflection s may represent the fractured flank of a central peak, whose highest prominence is south of profile Chicx-A/A l. Christeson et al. ( 1999) also concluded that a central peak, if present , must lie south of profile Chicx-A/A I and north of a paralle I onshore refraction profile labeled Chicx-D (Fig. 10.22). Fig. 10.26. (Next page) Interpretation of segment of seismic reflection profile Chicx-C across northeastern rim of Chicxulub impact basin. Profile from Morgan et aI. (1997); interpretations modified from Brittan et at. (1999) and Morgan and Warner (1999b). Shaded interval of high-amplitude reflections in Cretaceous carbonate and evaporite section illustrates impact-generated displacement of preimpact sediments along series of normal downto-the-basin faults. See Fig. 10.22 for location of profile. Ring terminology: crater rim (after Morgan and Warner 1999a); outer ring, inner ring, peak ring (after Morgan and Warner I999b); R ring, A ring, P ring (after Snyder and Hobbs I999a).

340

Comparisons with Other Impact Craters

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11 Comparisons Between Impactites

11.1 Terrestrial Impactites The term impactite has been used in a variety of narrow Iy defined senses (Masaitis 1994; Stoffler and Grieve 1994), but usually applies to shock-metamorphosed rock bodies (Jackson 1997). We use impactite in the broader general sense of French (1998) and Koeberl (1998) to embrace any rock body created directly (e.g., impact breccia) or indirectly (e.g., impact tsunamiite) from processes associated with a bolide impact. The most common and voluminous type of impactite found in and around most terrestrial impact structures is impact breccia. Field studies of these breccia deposits reveal some persistent, broad-scale characteristics, which can serve as general standards for comparing gross composition and stratigraphic relationships, and, thereby , to infer depositional processes. We will examine the breccia records at the Ries crater (southern Germany; Pohl et aI. 1977), the Manson crater (Koeberl and Anderson 1996a), the Lockne crater (Ormo 1998), the Popigai crater (Masaitis 1994; Masaitis et aI. 1999), the Montagnais crater (Jansa et aI. 1989), and the Sudbury multiring basin (Steffler et aI. 1994) for comparison with Chesapeake Bay breccias (Figs. 11.1-11.7). We selected these structures for comparison because their breccias are well studied from outcrops and(or) boreholes.

11.1.1 Ries Breccias Pohl et aI. (1977), Stoffler (1977), Horz and Banholzer (1980), Horz et aI. (1983) , Engelhardt et al. (1995), and Engelhardt (1997), among others, have documented four principal breccia units from the 24-km-diameter Ries crater (Miocene; Figs. 11.1-11.3). These units are separated from each other by stratigraphic position, by general lithic composition, and by degree of shock metamorphism exhibited. The stratigraphically lowest breccia unit documented at Ries is known as dike breccia, and has been sampled in one deep borehole (Figs . 11.2, 11.3). As the name implies , this breccia occurs as narrow dikes or veins, irregularly distributed through the upper 600 m or more of the crystalline basement. Dike breccia contains angular fragments of crystalline and sedimentary rocks and of individual mineral fragments , all supported by a fine-grained matrix . Some of the rock fragments display shock metamorphism, but many have been recrystallized strongly enough to obliterate the shock features . Dike breccias at Ries vary in C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

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color from grayish to red-brown, and vary in thickness from a few millimeters to a meter. Narrow veinlets of pseudotachylitic breccia also have been documented from the zone of dike breccias (Reimold 1995, 1998). Stratigraphically above the dike breccias is a glass-bearing, crystalline-clast breccia unit known as fallback suevite or crater suevite (as much as 400 m thick). Fallback suevite covers the crystalline floor of the inner basin and, presumably, buries the inferred central peak (Fig. 11 .3). Rock clasts in the fallback suevite are derived primarily from crystalline basement. Shock-metamorphic features representing shock pressures of 45-60 GPa and postshock temperatures of 900-1300° C are exhibited by some crystalline clasts in the fallback suevite. Glassy melt rock occurs in the suevite as small inclusions or as isolated larger bodies. In the 8-28 mm size range, glass bodies make up 2-39 vol.% of the fallout suevite . The matrix (particles smaller than I mm) of the fallback suevite typically consists of 3-13 vol.% glass. As indicated by their chemical compositions , glasses in the fallout suevite are believed to represent shock melt formed from a variety of different igneous and metamorphic rocks that constitute the crystalline basement. Stratigraphically above the fallback suevite, and cropping out extensively, especially outside the outer rim, is the Bunte Breccia. The Bunte Breccia is a dominantly sediment-clast breccia, locally as thick as 100 m, containing clasts derived from consolidated limestones, shales, and sandstones of Jurassic and Triassic age (Figs. 11.1 -11.3). Clasts in the Bunte Breccia are poorly sorted and chaotically mixed, ranging from millimeter-sized fragments to blocks tens of meters long, many of which have been strongly distorted by plastic and(or) brittle deformation. These clasts are supported in a fine-grained matrix derived from local unconsolidated sands, silts and clays of Cenozoic age. Only trace quantities of crystalline basement rocks are present in the Bunte Breccia, and they exhibit evidence of only low-level impact shock. Though no obvious vertical sorting has been documented, the Bunte Breccia does exhibit a fining-outward radial grading (Harz et al. 1983). The Bunte Breccia is essentially confined to the annular trough and the surrounding ejecta blanket of Ries crater. It is thought to have been deposited by a combination of ballistic ejection and lateral ground surge. These two processes would have moved large volumes of debris radially outward from the transient crater during the excavation stage of crater formation. The fourth, and stratigraphically highest Ries breccia, termed fallout suevite, is present as small patches (as thick as 30 m) on top of the Bunte Breccia, both in the annular trough and on the ejecta blanket (Figs. 11.1-11.3). Clasts in the fallout suevite consist mainly of ejected glass bombs (60-80 vol.% of clasts) and fragments of crystalline basement enveloped by a matrix (80 vol.% of breccia) of dominantly fine-grained glass particles , montmorillonite , and quartz (Engelhardt 1990; Engelhardt et al. 1995). The greater glass content, presence of aerodynamically shaped bombs, and near lack of sedimentary clasts differentiate fallout suevite from fallback suevite. Fallout suevite is believed to have settled out from a turbulent gas cloud as a mixture of ejected basement melt (melt temperatures of 2000° C; shock pressures of >80 GPa) and unmelted particles of crystalline basement (plus minor amounts of sedimentary debris) .

346

Comparisons Between Impactites

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Comparisons Between lmpactites

In the annular trough of the Ries crater, the Bunte Breccia is separated from the crystalline basement by a fifth crater-fill unit. This unit is a slumpback deposit, composed of sedimentary megablocks, which have been displaced from the outer rim of the crater (Figs . 11.1, 11.3).

11.1.2 Manson Breccias Anderson et a1. (1996) and Witzke and Anderson (1996) identified five principal impact breccia units in cores taken from the annular trough and central peak of the 35-km diameter Manson crater (Late Cretaceous; Figs. 11.1, 11.4). The stratigraphically lowest breccia unit in the Manson crater is a crystalline-clast breccia, termed crystalline-rock megabreccia. This megabreccia includes shocked clasts of crystalline basement rocks, which commonly are penetrated by veins of impact melt rock and fine-grained breccia dikes, similar to the dike breccias of the Ries crater. The matrix is composed of sand- to silt-sized granitic rock fragments and mineral grains, nearly all of which display evidence of shock metamorphism. The crystalline-rock megabreccia is known only on the central peak, where its maximum drilled thickness is 66 m. Anderson et a1. (1996) inferred that the crystalline-rock megabreccia represents impact-brecciated crystalline basement derived from the floor of the transient crater. The stratigraphically next highest breccia unit on Manson's central peak also is a crystalline-clast breccia, termed suevite breccia , which resembles the fallback suevite at Ries (Figs. 11.1, 11.4). Clasts in the suevite breccia are dominantly centimeter- to meter-sized fragments of crystalline basement. Matrix dominates this unit, however, and consists of silt- to sand-sized grains of crystalline basement and minerals derived therefrom, plus minor amounts of sedimentary rocks and impact melt rock. All constituents of the suevite breccia display shock-metamorphic features . Stratigraphically above the suevite breccia on the Manson central peak is impa ct-melt breccia, in which large clasts are conspicuously sparse . The few large clasts present are dominantly composed of melt rock (some angular, others plastically deformed). The matrix is mainly quartz, which displays a wide variety of shock-related features (multiple sets of PDFs, planar fractures, partial to total melting, isotropization, annealing, and recrystallization). The next-to-highest breccia unit on the central peak at Manson is a sedimentclast breccia termed the Keweenawan shale-clast breccia (Figs. 11.1, 11.4). As its name suggests, the principal clasts of this breccia are centimeter- to meter-sized fragments of dark gray shale and mudstone of Keweenawan age (Proterozoic), mixed with devitrified impact melt-rock clasts, and minor amounts of siltstone and sandstone . The matrix of this breccia unit consists of sand- to clay-sized particles of the same constituents. The stratigraphically highest breccia unit at Manson is the Phanerozoic-clast breccia. This breccia unit covers the entire Manson structure (maximum drilled thickness 191 m, but may reach 2 km; Figs. 11.1, 11.4). Clasts are dominantly

Crystalline basement

'

Preimpact sedimentary rocks

"

I>,:':. '::". .",.., Crystallineclast breccia

.. . . .

.......... ~

Sediment -clast breccia

Sediment-clast breccia

Fig. 11.4. Ge neral stratigraphic distribution of impact-generated breccias cored at Manson impact crater. Data from Anderson et al. ( 1996) and Witzke and Anderson (1996) .

NOl io scale

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~ ~.

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350

Comparisons Between Impactites

Cretaceous shales and mudstones mixed with fewer fragments of Paleozoic carbonates and rare Keweenawan Red Clastics. The matrix is composed of sand- and silt-sized grains of the same constituents, plus rare fragments of crystalline basement and impact melt rock. Shock-metamorphic features are rare. The Phanerozoic-clast breccia resembles the Bunte Breccia at Ries in its abundance of shallowtarget rock types and its sparsity of shock features, but differs in its content of rare impact melt-rock clasts. The Phanerozoic-clast breccia is the only breccia unit identified in the annular trough at Manson. Anderson et al. (1996) interpreted the Phanerozoic-clast breccia to be a landslide or debris-flow deposit, which originated at the outer rim of the crater during the late stages of crater modification. Anderson et al. (1996) speculated that the Phanerozoic-clast breccia may have formed in a shallow epicontinental sea, and subsequently was transported into the inner basin by turbulent flow produced by the collapsing water column (analogous to surgeback breccia). So far, no impact breccias have been found outside the crater rim at Manson. As at Ries, there is an additional principal crater-fill unit in the annular trough at Manson. This unit is a slumpback deposit composed of displaced sedimentary megablocks that have collapsed from the outer rim of the crater. These megablocks lie between the crystalline basement and the Phanerozoic-clast breccias of Anderson et al. (1966) and Witzke and Anderson (1966; Figs. 11.1 , 11.4).

11.1.3 Lockne Breccias

The stratigraphically lowest Lockne breccia unit, which fills the inner basin and covers the central peak, is the Tandsbyn Breccia (Lindstrom et al. 1996; Ormo 1998; von Dalwigk and Ormo 2001; Figs. 10.14, 10.16, 11.1). The Tandsbyn Breccia is a clast-supported deposit of angular fragments «Icm to several decimeters in diameter) derived from the Revsund Granite (crystalline basement). Many of the clasts are internally brecciated. The Tandsbyn Breccia matrix is composed of finely crushed Revsund Granite. Tandsbyn Breccia also has been injected as sills and dikes into the flanks of the peak ring, a phenomenon which contributes to the ring's elevation above the floor of the annular trough. Stratigraphically higher than the Tandsbyn Breccia are two different sedimentclast breccias - the Lockne and Loftarsten Breccias (Lindstrom et al. 1996; Figs. 10.14, 10.16, 11.1). The Lockne Breccia is widespread around the crater, and is inferred to be the initial surgeback deposit formed by the collapsing marine water column. Lockne Breccia is dominated by sedimentary clasts, principally Ordovician limestone, with minor amounts of Cambrian shale, Tandsbyn Breccia, and Revsund Granite. Lockne Breccia is poorly sorted and unbedded; clasts range in size from granules to meter-sized blocks. A vertical succession of two lithofacies can be recognized, however, within the Lockne Breccia. The basal lithofacies is a monomictic, clast-supported deposit resembling a debriite, which predominates on the floor of the annular trough. Here, huge blocks of bedded Ordovician limestone are separated from underlying undisturbed equivalents by a zone of shattered limestone. The upper Lockne lithofacies also is widespread in the annular

Terrestrial Impactites

351

trough, but, in addition, fills the surgeback gullies (von Dalwigk and Ormo 2001; see Chapter 10) and forms a thick fill above the Tandsbyn Breccia in the inner basin. The upper Lockne Breccia lithofacies is polymictic and pebble-graded, and contains more crystalline clasts than the basal Lockne lithofacies. Maximum thickness of the Lockne Breccia is 155 m in the inner basin. Loftarsten Breccia represents the final and stratigraphically highest surgeback deposit of the Lockne crater (Figs. 10.14, 10.16, 11.1); sedimentologically it resembles a turbidite. Loftarsten Breccia is distributed mainly in the inner basin, where it attains maximum thickness of 45 m. The lower, coarse-grained part of the deposit is graded, whereas the upper finer-grained part displays current lineations, cross bedding, and dewatering structures (Simon 1987). Lithologically, the Loftarsten is a graywacke-like sandstone, which resembles the upper lithofacies of the Lockne Breccia. Sand grains of the Loftarsten are derived from both the crystalline basement and preimpact sedimentary beds. Simon (1987) reported as much as 20 vo\.% melt lapilli in the Loftarsten, which appear to be fallback (fallout?) particles of impact melt incorporated into the surgeback flow (Sturkell and Ormo 1997).

11.1.4 Popigai Breccias Masaitis (1994) and Masaitis et a\. (1999) have described the impact breccias of the 85-km-diameter Popigai crater (late Eocene subaerial impact; Figs. 11.1, 11.5, 11 .6). The stratigraphically lowest breccia unit at Popigai (Figs. 11.1, 11.6) is allogenic, blocky, polymict, sediment-clast megabreccia containing mixtures of crystalline and sedimentary clasts, supported by either fine-grained clastic matrix (coptoclastite) or impact melt-rock matrix. This megabreccia reaches ~ I km in thickness and rests mainly above the displaced megablocks and crystalline fault blocks of the annular trough. Stratigraphically above the sediment-clast megabreccia is crystalline-clast megabreccia, supported in part by impact melt-rock matrix. This megabreccia occurs in the annular trough (400-500 m thick), on the flanks of the peak ring, and in the inner basin (presumably burying the crystalline central peak; Figs. 11 .1, 11 .5, 11.6). The crystalline-clast megabreccia is now missing from the exposed crest of the peak ring (Fig. 11.6), but appears to have originally covered it. The stratigraphically highest breccia unit at Popigai is fallout suevite (maximum thickness of 193 m), which is separated from the crystalline-clast megabreccia by a thick (>600 m) impact melt sheet (tagamite of the Russian literature; Figs. 11.1, 11.5, 11.6). Fallout suevite at Popigai is composed of fragments and bombs of impact glass and numerous sedimentary and crystalline clasts enclosed by a finely comminuted and partly altered matrix of the same materials. This composition closely resembles that of the fallout suevite at the Ries crater. Investigators at Popigai have not identified a crater-fill unit equivalent to the slumpback megablocks we recognize at Ries, Manson, and Chesapeake Bay. However, the cross section of Masaitis et al. (1999) shows megablocks of upper

352

Comparisons Between Impactites

100 I

D'

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D

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...' \ . '-

..

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D

Impact meitsheet (Tagam,te )

Fig. 11.5. Distribution map of breccia and other impact deposits on modem topographic surface in and near Popigai impact crater. Modified from Masaitis et al. (1999). Cross section X-X ' illustrated in Fig. 11.6.

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?

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Fig. 11.6. Geo logic cross section through outer rim, annu lar trough , and peak ring in southwest sector of Popigai impact crater (modified from Masaiti s et al. 1999). See Figure 11.5 for location of cross section.

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354

Comparisons Between Impactites

Paleozoic sedimentary rocks lying on, or emplaced within, crystalline basement rocks and covered by sediment-clast megabreccias (Fig. 11.6). These megabreccias may be analogous to the displaced sedimentary megablocks of our terminology (Figs. 4.3A,B, 4.7B, 4.9A,B, 6.1,6.2).

11.1.5 Montagnais Breccias

Impact breccias from the Montagnais impact structure are known only from a single borehole (Montagnais 1-94) on the crest of the central peak (Figs. lOA, 10.6, 11.7). Jansa et aI. (I989) identified the following impact deposits in rotary cuttings taken from that borehole, which penetrated to 1,646 m in a structural depression (listed from deepest to shallowest) : (I) a basal interval of autochthonous crystalline-clast breccia (391 m thick), which includes two zones of crystalline impact melt rock; (2) a 131-m-thick interval of allochthonous sediment-clast breccia including clasts of limestone, granite, siliciclastic sediments, and metamorphic basement rocks; and (3) a cap of suevite, 38.5 m thick, composed of dominantly glass fragments (Fig. 11.7). Fewer than 10 vol.% of all these different breccia clasts display shock metamorphic features. Moderate- to high-pressure (35--45 GPa) shock metamorphism is indicated by isotropization of quartz and feldspars, as well as the presence of impact glass (Jansa et al. 1989). In situ basement rocks at Montagnais are part of the lower Paleozoic Meguma Group - low-grade metasedimentary rocks (metasubgraywacke, phyllite, metaquartzite) that exhibit extensive hairline fractures . Weakly developed shock-deformation features in quartz grains in the basement rocks indicate low shock pressures in the 6-8 GPa range. Seismic reflection profiles indicate that crater-fill impact breccia of unknown composition covers the entire Montagnais structure, from outer rim to central peak, but the breccia is not identifiable seismically outside the crater (Figs. 10.5, 10.6). Neither has impact breccia been identified in wells outside the crater. By analogy with the other submarine craters that we have discussed in this volume, we infer that most of the Montagnais crater-fill is sediment-clast breccia. The Montagnais crater-fill breccia is thickest (-2 km) in the inner basin, and is also relatively thick (1-2 km) over the displaced megablocks. The crater-fill breccia thins dramatically in the annular trough, however, and pinches out on the upper part of the erosional scarp that marks the outer rim (Fig. 10.5). We infer from the seismic record that this crater-fill breccia originally was much thicker in the annular trough, but has subsequently been drastically thinned by erosion. Postimpact compaction and erosion have also thinned the crater-fill breccia unit over the displaced megablocks on both sides of the Montagnais structure (on seismic profile).

11.1.6 Sudbury Breccias

Though the Sudbury impact structure (Ontario, Canada; 1850 ± 3 Ma; Table 1.1) is classified as a multiring basin (-200-km diameter ; SHiffler et al. 1994; Deutsch et aI. 1995; Ivanov and Deutsch 1999), its extensively studied succession of crater-

Terrestrial Impactites

355

Montagnais 1-94 Well (Interpreted from rotary cuttings) Vesicular glass fragments 700

Clasts of oolitic and bioclastic limestone. glauconitic mudstone. chalk. rare granite. metamorphics. mixed with quartz sand 800

·•'. .' .>:.>: · . . . .. .. 900

·· .. .

·x· . ..

s=

Clasts of metamorphic basement rocks and melt rock

1000

· ... . . ... .. · .. . ..

· ···

·..

··.. .

. .. Low-grade metamorphic rocks. including metasubgraywacke. phyllite. meta-quartzite

Fig. 11.7. Stratigraphic column of impact-generated breccias inferred from rotary cuttings in 1-94 borehole on central peak of Montagnais impact crater. Data from Jansa et al. (1989).

356

Comparisons Between Impactites

fill breccias is generally comparable to the breccias cited for smaller terrestrial impact structures, though details of origin and emplacement are still vigorously debated. Avermann (1994) and Steffler at aI. (1994), for example, described three principal (thickest) layers of polymict impact breccia, and a fourth, much thinner, breccia layer associated with the Sudburystructure (Fig. 11.1). All four layers are assigned as separate members of the Onaping Formation, which overlies the primary impact melt sheet of the Sudbury Igneous Complex (Brockmeyer 1990; Avermann 1992; Dressler and Reimold 200I). The stratigraphically lowest breccia, the Basal Member of the Onaping Formation, is 300 m thick (Fig. 11 .1). It contains abundant metasedimentary clasts, but only minor amounts of crystalline basement clasts within a crystalline matrix of melt rock. The Basal Member is in sharp to gradational contact with the underlying granophyre of the clast-poor Sudbury impact melt sheet, and is considered by some authors (Brockmeyer 1990; Avermann 1992) to be an integral part of the melt sheet. Next highest is the Gray Member of the Onaping Formation (Fig. 11.1), a 500m-thick metasediment-clast breccia, also containing fragments of crystalline basement and irregular melt inclusions in a clastic matrix. Clasts within the Gray Member display shock-metamorphic features . Scattered within the Gray Member are "breccia bodies," which are differentiated by their clast size and different proportionsof clasts and impact melt inclusions. The stratigraphically highest breccia, the Black Member of the Onaping Formation, is also the thickest (~1 km thick), and can be subdivided into an upper unit and lower unit (Fig. 11 .1). The Black Member as a whole is a sediment-clast breccia. The Black Member incorporates fragments of the underlying breccia layers and displays evidence of an euxinic aquatic paleoenvironment, possibly representing a surgeback deposit (Bunch et aI. 1999). The lower unit of the Black Member differs from the upper unit in containing a few small fragments of shocked crystalline basement,melt inclusions, and chloritized melt particles. At the contact between the Black and Gray Members of the Onaping Formation, is the irregular, relatively thin (5-70 m) Green Member (Fig. 11.1). The Green Member is unique among documented impact breccias; it comprises finegrained, often chloritized fragments and small, highly shocked, polymictic mineral clasts in a microcrystalline matrix. The Green Member appears to contain fallback ejecta, and may represent the collapsed fireball of the Sudbury impact deposit (Avermann 1999). To summarize, this brief review of well-documented impact breccias reveals a generally uniform stratigraphic succession of broadly defined breccia units. Crystalline-clast breccias or megabreccias usually are at the base of the sequence, where they rest on crystalline basement rocks, and are succeeded upward by sediment-clast breccias of variable compositions. These breccia units are capped, in two of the cited cases, by glass-rich suevite. Impact melt sheets occur at different levels in the crater-fill succession.

TerrestrialImpactites

357

11.1.7 Chesapeake Bay Breccias

In broad scope, the seismostratigraphic, lithostratigraphic, and downhole geophysical analyses of the Chesapeake Bay breccia column suggest that here, too, there is a general upward progression from crystalline-clast to sediment-clast breccias. In detail, however, some of the sediment-clast breccias have unique features that have not yet been documented elsewhere . In the inner basin of the Chesapeake Bay crater , the synimpact crater-fill deposits may be divided stratigraphically into six principal subhorizontal units (layers or lithofacies ; Figs. 11.1, 11.8; Poag 2000). The three stratigraphically next-to-highest units are encompassed in the general term Exmore breccia (units 3-5 of Figs. 11 .1,11.8, 11.9), but each may be recognized as a lithically distinct (but locally variable) unit. Here we briefly describe the physical aspects of these units. See Chapter 12 for further discussion of their origins and depositional processes that formed them. Crater-fill unit I (Cfu-I), at the base of the Chesapeake Bay crater-fill, is known mainly from seismic reflection profiles , and appears to be generally confined to the lower part of the inner basin (Figs. 11 .1, 11.8, 11.9). However a 23-m section at the base of the Bayside breccia column might represent part of Cfu-l, because the site is near the inner basin and the core contains an unusual abundance of crystalline and lithified sedimentary clasts that have not been observed at any other core site. This Bayside core section has not yet been analyzed for shockrelated features, however. The relatively high-relief upper surface of Cfu-l is expressed on seismic profiles by a series of high-amplitude , broad-wavelength, parabolic reflections at 1.01.25 s (1250-1500 m below sea level). Maximum estimated thickness of the lowest breccia is 0.65 s (-600 m). Poag (1996) speculated that Cfu-l is composed mostly of fallback breccia, probably represent ing a complex, crystalline-clast breccia, similar in composition and origin to the fallback suevite of the Ries crater, the crystalline -rock megabreccias at Manson, the Tandsbyn Breccia at Lockne, the crystalline megabreccias at Popigai, and the Gray and Basal Members of the Onaping Formation at Sudbury. All these cited breccias are typified by large quantities of crystalline clasts, which display a wide range of shock metamorphism, and are enclosed in a highly variable suite of fine-grained matrices. Crater-fill unit 2 is composed of decimeter-to-kilometer-scale megablocks of mainly Lower Cretaceous stratified sediments , which have slid, slumped, and collapsed from the outer rim of the crater (Figs. 11 .1, 11 .8, 11.9). At Chesapeake Bay, Cfu-2 appears to be confined to the annular trough of the crater. Crater-fill unit 3 (third lowest Cfu and basal part of Exmore breccia; Fig. 11.1) at Chesapeake Bay has been sampled by the five continuously cored intracrater boreholes (Fig. 11.8). As we showed earlier (Chapter 6), this layer is largely a clast-supported breccia, confined to the annular trough and inner basin, where it buries the peak ring and central peak (Figs. 11.1, 11.8, 11.9). Most individual clasts documented in this unit are weakly consolidated sediments (clays, silts, sands), with maximum apparent thicknesses of 15-20 m. We interpret Cfu-3 to have formed through hydraulic processes as the marine water column surged back into the crater cavity following impact of the bolide (see Chapter 12).

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Fig. 11.8. Geologic cross section of western half of Chesapeake Bay impact crater showing relative locations of coreholes and stratigraphic distribution of six principal crater-fill units (Cfu I-Cfu6). Checklist above each core site indicates presence (check) or absence (cross) of inferred lithofacies. See Chapter 12 for further discussion of lithofacies terminology (slumpback, washback, etc.).

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Debris scoured from seafloor and crater walls and floor by collapsing marine water column Coarse debris in megaslides and megaslumps derived from collapse of sedimentary outer walls Coarse debris derived from crystalline crater floor; ejected short distance before falling back

Clast-supported breccia; clasts mainly weakly consolidated sediments with 15-20-m maximum apparent thicknesses; fining upward sequence; cored at 5 sites

Decimeter- to kilometer-scale megablocks of mainly stratified Lower Cretaceous sediments; cored at 3 sites

Inferred to be composed of dominantly crystalline clasts with abundant evidence of shock metamorphism; not yet cored

Cfu-3

Cfu-2

Cfu-1

lower

Debris scoured from seafloor and coastal plain by tsunami waves and washed back out to sea

Debris scoured from seafloor and coastal plain by tsunami waves and washed back out to sea

Dominated by polymictic, centimeter to millimeterscale sedimentary clasts supported by glauconite/ quartz sand matrix; matrix -30% or less by volume; fining upward sequence; cored at all sites

Cfu-4

Dominated by polymictic, meter to centimeterscale sedimentary clasts supported by glauconite/quartz sand matrix; matrix mainly 50-100% by volume; fining upward ; cored at all sites

Fallback

Siumpback

Surgeback

Wash back

Wash back

Flowin

Fine-grained, multidirectional gravity flows from impact-generated storms

Thin bedded , slit-rich unit with multidirectional, inclined stratification ; cored at 3 sites

Cfu-5

Cfu-4 upper

Fallout

Fine atmospheric debris and vapor condensates raining back into crater

Depositional Regime

Centimeter-scale silt laminae, pyrite molds of inferred glassy microspherules; cored at 1 site

Genesis

Cfu-6

Lithic Texture

Fig. 11.9. Relationship between lithic texture of crater-fill units, genesis of units, and depositional regime in which each unit formed in Chesapeake Bay impact crater. See Chapter 12 for more details regarding genesis and depositional regimes.

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360

Comparisons Between Impactites

Crater-fill unit 4 at Chesapeake Bay (the middle unit of the Exmore breccia; sampled by five intracrater boreholes) also is dominated by polymictic sedimentary clasts (Figs. 11.1, 11 .8, 11.9). We subdivide Cfu-4 into a lower and upper part. In contrast to the underlying Cfu-3, the clasts in the lower part of Cfu-4 are supported by the characteristic glauconitic quartz sand matrix. This matrix also contains a small percentage of centimeter-to-millimeter-size fragments of crystalline basement (granitic and metasedimentary rocks). Though sparse, the basement clasts do contain evidence of shock metamorphism, which ranges from lowpressure fracturing to partial and complete melting (Koeberl et al. 1996; Horton et al. 2001; Chapter 6). Seismic profiles indicate that the lower part of Cfu-4 is present throughout the crater The upper part of Cfu-4 at Chesapeake Bay (sampled at all seven core sites) is similar to the lower matrix-supported part, but differs in the strong dominance of matrix over clasts (as measured by the relative thickness of matrix intervals versus clasts in the cores; Chapter 6). The upper part of Cfu-4 covers the entire crater and also extends a few kilometers outside the crater rim to form a thin breccia apron (Figs. 11.1, 11.8, 11.9). We interpret Cfu-4 as a two-part tsunami-washback unit (see Chapter 12). Crater-fill unit 5 at Chesapeake Bay is a thin-bedded clayey silt-rich unit, with distinctly inclined, undulating thin layers and lenses of white, micaceous, fine-tovery fine sand and occasional clasts of medium-to-coarse glauconite-quartz sand derived from the underlying Cfu-4. The direction and angle of bed inclination in Cfu-5 change frequently (commonly reversing) through the section, indicating rapidly changing flow directions during deposition. We interpret this interval to represent the final water-borne synimpact flowin unit, laid down by a series of hypercanes (runaway hurricanes; Emanuel et al. 1995) that developed over the impact region (Poag 2002; see Chapters 6, 12). This flowin unit constitutes the upper part of the Exmore breccia (Figs. 11.1, 11 .8, 11 .9). The stratigraphically highest synimpact layer at Chesapeake Bay (Cfu-6) is an ~5-cm-thick fallout unit containing evidence of stacked glass microspherules (Figs. 6.24A,B), which hypothetically might have originally covered all the underlying crater-fill units (Figs. 11 .1, 11 .8, 11.9). So far, however, this fallout layer has been identified only in the NASA Langley core. The relative stratigraphic position, extensive spatial distribution, and gross composition of the washback (Cfu-4) and surgeback (Cfu-3) lithofacies of the Exmore breccia at Chesapeake Bay (see Chapter 6) resemble those of the Bunte Breccia at Ries, the Phanerozoic-clast megabreccia at Manson, the Lockne and Loftarsten Breccias at Lockne, and the Black Member of the Onaping Formation at Sudbury. The Exmore differs from the polymict megabreccias at Popigai, however, mainly because the latter are overlain by crystalline-clast megabreccias , and are spatially restricted to the annular trough at Popigai. Compelling evidence of subaqueous deposition in the Exmore breccia (Chapter 6) also is shared by the Phanerozoic-clast megabreccia at Manson, the Lockne and Loftarsten Breccias at Lockne, and the Black Member at Sudbury. Though the Bunte Breccia displays similar sedimentary evidence of turbulent flow during its deposition, the depositional processes are inferred to have been quite different.

Flowin, Fallout, and Dead Zone

361

Bunte Breccia presumably was deposited by ballistic ejection and radial ground surge, which were processes of crater excavation . In contrast, the flowin, washback and surgeback facies at Chesapeake Bay, the Phanerozoic-clast megabreccia, the Lockne and Loftarsten Breccias, and the Black Member, formed from turbulent marine processes, which acted to refill the excavated craters .

11.2 Flowin, Fallout, and Dead Zone The inferred synimpact flowin (Cfu-5) and fallout (Cfu-6) layers and the overlying laminated clay-silt-sand of the initial postimpact deposit (dead zone) at Chesapeake Bay (see Chapters 6,7) have no precise equivalents yet documented at any other terrestrial crater. Though the presence of fallout debris inside a crater is unusual, it is not unique . The Ries and Popigai craters, for example , contain fallout deposits (Fig. 11.1), but in both cases the fallout debris is composed of suevite. The suevite at Ries is overlain, moreover, by 17.2 m of coarse-tail-graded, matrixrich breccia, which grades further upward to sand and coarse silt. Fiichtbauer et al. (1977) interpreted this succession to be either a lacustrine turbidite or a fallout deposit. This graded postsuevite section at Ries most closely resembles a combination of breccia-unit 4 (Cfu-4) and the fallout layer (Cfu-6) at Chesapeake Bay, and is suggestive of a turbidite sequence (at the base) and a fallout layer (at the top). At Sudbury, the inferred fallout unit (Green Member) lies beneath an inferred surgeback deposit (Black Member), perhaps indicating that the Black Member is the initial postimpact deposit at Sudbury, equivalent to the dead zone at Chesapeake Bay. At present, Chesapeake Bay is the only known crater in which proximal fallout materials are part of an unconsolidated, intracrater, marine sediment layer. So far, flowin deposits and a laminated dead zone also are unique to Chesapeake Bay.

11.3 Other Intrabreccia Bodies In addition to the crystalline-clast breccia (Cfu-I) and the overlying three principal layers of the Exmore breccia (Cfu-3-5), some smaller, distinctive , seismicreflection features are notable within the inner basin at Chesapeake Bay. For example , a series of high-amplitude , parabolic reflections can be observed within the surgeback breccia between 0.42 and 0.62 s (~525-775 m below the bay floor),just above the inner rim of the peak ring on profile T-8-S-CB-E (Figs. 4.29, CDROM.12). These reflections indicate an irregular body of rock with contrasting velocity and(or) density, ~ 150 m thick and ~ 7 km long, embedded in the breccia. Other features of similar size and seismic signature are present within the surgeback layer on profiles not illustrated herein. These isolated rock bodies may be: (1) suevitic breccias like those at Ries; (2) "breccia bodies" similar to those of the Gray Member at Sudbury; (3) impact melt rocks (tagamites), such as those at

362

Comparisons Between Impactites

Popigai; (4) unusually large blocks of sedimentary or crystalline target rocks; or (5) some combination of these deposits.

11.4 Continuous Ejecta Blankets Grieve (1991) listed only Barringer (a l-krn-wide simple crater of Pleistocene age; also known as Meteor crater), Ries (a 24-km wide complex crater of Miocene age), and Ragozinka (a 9-km-wide complex crater.of early Eocene age) as terrestrial craters having continuous ejecta blankets, whereas discontinuous ejecta blankets have been described around many other craters. Several earlier publications have described a continuous ejecta blanket around the Chesapeake Bay crater as well (Poag et al. 1994; Poag 1997a). As we have pointed out earlier herein, however (Chapter 6), the impact debris encircling the Chesapeake Bay crater cannot be considered a true ejecta blanket. Whatever debris might have composed a possible original ejecta blanket has been remobilized and redistributed as components of the tsunami washback breccia. With regard to the formation and distribution of impact-generated breccia deposits, then, the location of the Chesapeake Bay impact on a broad continental shelf, not far from the shoreline of a continental landmass, was a critical condition. The presence of a marine water column and a cover of water-saturated, unconsolidated sediments above crystalline basement, resulted in a distinctive suite of breccias and related deposits, whose compositions, stratigraphic succession, and depositional processes are not precisely duplicated at any of the other craters yet studied in relative detail.

11.5 Secondary Breccias The Chesapeake Bay impact also appears to have produced additional isolated breccia bodies that are deposited outside the primary crater. Each of the inferred Chesapeake Bay secondary craters described herein (Chapter 5) exhibits its own chaotic seismic reflections, which we interpret to represent small bodies of sediment-clast breccia derived from the local sedimentary target rocks. So far, however, none of these secondaries has been cored, though an early report of breccialike lithologies in a well (borehole 106) near Colonial Beach, Virginia (Cederstrom 1945b; Fig. 5.2), might indicate the presence of secondary breccia. As no other secondary terrestrial craters have been thoroughly documented, we have no basis for comparison with the Chesapeake Bay secondary breccias. There is speculation, however, that some constituents of the Bunte Breccia were derived from isolated local breccias produced by secondary cratering processes (large ejecta blocks might have ploughed through the sedimentary substrate and created an irregular landscape of secondary craters and their respective breccias; Oberbeck 1975; Morrison and Oberbeck 1978).

StrewnFields

363

11.6 Strewn Fields Aside from the Chicxulub multiring basin, the Chesapeake Bay and Popigai impact structures have produced the best documented, most widespread ejecta strewn fields yet identified (hemispheric to global distribution; see discussion in Chapter 9). Well-known other strewn fields include the Australasian tektites (derived from an as yet unidentified source) and the Central European strewn field (known for its moldavites). The Central European strewn field covers parts of the Czech Republic, Austria and Germany, about 450 km east of its source at the Ries crater . Isotopic evidence indicates that the moldavites represent melt products of the Miocene sands that constituted the surface sediments at the Ries impact site. Koeberl et al. (2001) have analyzed preimpact Cenozoic sediments at Chesapeake Bay for possible isotopic correlation with the tektites of the North American strewn field, but, so far, there is no direct evidence of a match beyond the geochemical data reported by Koeberl et al. (1996). (See Chapter 6 for additional discussion of North American strewn field).

11.7 Impact Melt Rocks According to Grieve (1987), complex craters the size of the Chesapeake Bay crater usually are characterized by pockets (a few meters thick) or coherent sheets (several hundred meters thick) of impact melt rock, such as the thick tagamite sheets at Popigai (Masaitis 1994; Masaitis et al. 1999). Impact melt sheets often are rather homogeneous in composition, being mixtures of melted and vaporized target rocks that were driven at high velocities down into the expanding transient cavity, as turbulent flows (Dressler and Reimold 2001) . Impact melt sheets are, on the other hand, heterogeneous in texture, especially near the upper and lower surfaces where they incorporate mixtures of shocked and unshocked lithic and mineral clasts . The impact melts differ from igneous melts in having been superheated and, therefore, may contain ultra-high-temperature mineral phases, such as baddeleyite, in addition to shock-metamorphosed clasts. There is no megascopic evidence of massive iinpact melt rock at Chesapeake Bay. But direct evidence for impact melting is expressed in melt zones and meltrock particles identified in thin section in some of the smaller crystalline clasts (petrographic studies discussed in Chapter 6). Also, as we have indicated above, there is plausible indirect evidence (in the form of distinctive seismic reflection signatures) that large bodies of impact melt rock could be present in the upper levels of the inner-basin fill at Chesapeake Bay (Figs. 4.29A,B, CD-ROM . 12).

12 Implications for Impact Models

12.1 General Conceptual Models and Scaling Relations 12.1.1 Subaerial Cratering

Shoemaker (1963), in his classic study of Meteor (Barringer) crater, divided the impact cratering process into a temporal succession of stages. Shoemaker's pioneering ideas have served as the basis for succeeding conceptual and computergenerated models of the physical processes involved in creating impact structures. Gault et al. (1968) constructed a widely cited model of impact cratering on the basis of extrapolations from nuclear explosions combined with laboratory experiments and computer simulations. Gault et al. (1968) described the impact process in terms of three cratering stages (contact and compression; excavation; modification), while emphasizing that the boundaries between successive stages are not sharply defined. According to the Gault et al. model, the initial few seconds of impact (stage 1) can be viewed as a succession of contact and compressive events. Upon contact, the projectile pushes the target rocks out of its path, thereby compressing and accelerating them, as the target's resistance decelerates the projectile. Resultant shock waves may reach hundreds of GPa, whereupon both the projectile and the target rocks melt or vaporize. Contact and compression constitute the shortest of the three cratering stages (generally < 1 s), and are completed as soon as the shock wave and its companion rarefaction wave pass through the impactor. In the Gault et al. (1968) model, crater excavation (stage 2) lasts for a few seconds to minutes immediately following contact and compression. Excavation flow is promulgated by a hemispherical shock wave and its subsequent rarefaction wave, which set the target rocks in subsonic motion and fragment the near-surface rock layers. Ensuing ejection of this material opens up a transient crater, whose diameter is an order of magnitude greater than that of the impactor. For peak-ring craters, the diameter of the crest of the peak ring approximates the diameter of the transient crater (O'Keefe and Ahrens 1997). The depth of excavation is much shallower than the maximum depth of the crater, being, as a rule of thumb, about one-tenth the diameter of the transient crater (= 0.1 the diameter of the peak ring; Melosh 1989). Applying this estimate to Chesapeake Bay crater, therefore, the predicted maximum depth of excavation would have been about 4 km. Aspects of the excavation stage have recently been reexamined by Melosh and Ivanov (1999), who emphasized the role of acoustic fluidization as a mechanism for temporarily degrading the strength of rocks surrounding the impact site. C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

366

Implications for Impact Models

Modification of the crater (stage 3) follows completion of excavation (Gault et al. 1968; Melosh 1989), and is characterized by gravity-driven collapse (slumping and sliding) of the nascent crater's outer margin and rebound of target rocks in the crater interior to form a peak ring and( or) central peak. For subaerial craters, the modification stage lasts for tens of seconds. In the case of submarine craters, additional modification of the crater's outer margin is brought about by loading and hydraulic effects as the marine water-column collapses and surges back into the crater (Ormo and Lindstrom 2000). Furthermore, the final process of tsunami washback might extend the duration of crater modification to several days or weeks, depending upon the water depth and distance to the shoreline. Kieffer and Simonds (1980) derived a semiquantitative cratering model by applying Shoemaker's (1963) ideas to field data from 32 subaerial terrestrial craters. Kieffer and Simonds (1980) subdivided the cratering process into seven stages. They stressed, in particular, the roles of the composition of target rocks and their contained volatiles in constraining crater development and subsequent impactgenerated deposition. Final structure and morphology of a complex crater can be expressed as, or predicted from, scaled relations between various physical or conceptual features of the crater (such as diameter of outer rim, diameter of central peak, height of central peak, and diameter of transient crater). For example, Melosh (1989) showed that the diameter of the central peak is roughly 0.22 times the outer-rim diameter on all terrestrial planets. For the Chesapeake Bay crater, this relation would predict a central-peak diameter of ~ 19 km. The average diameter measured by us is ~ 15 km (~O .18 times the outer-rim diameter). On the primary basis of field observations, Grieve and Robertson (1979), Grieve (1991), and Pilkington and Grieve (1992) postulated "rule-of-thumb" scaling relations for terrestrial subaerial craters. For example, Grieve (1991) concluded that excavation does not occur all the way out to the outer rim, but is constrained to the central 0.50--0.65 of the outer-rim diameter. Applying this relation to the Chesapeake Bay crater would predict an excavation diameter of 42.5-55.5 km (or ~2l-28 km radial distance from the crater center). Our measurements indicate that the diameter of excavation at Chesapeake Bay averages ~50 km (derived from the average diameter of the outer flank of the peak ring). Outside the peak-ring periphery, only the Cenozoic section seems to have been excavated from the tops of the displaced megablocks, which indicates only weak excavation there in the annular trough. We infer that this "excavation" was achieved mainly through surgeback erosion processes during crater modification. Grieve (1991) also determined that stratigraphic uplift of the central peak is 0.09--0.12 times the diameter of excavation. In the case of the Chesapeake Bay crater, this relation would yield a stratigraphic uplift of 4.5-6.0 km, in general agreement with Melosh's (1989) maximum depth of excavation. Grieve and Robertson (1979) and Pilkington and Grieve (1992) indicated that, for complex craters, the true depth (dt) of the crater is 0.52D°.2 (D = diameter of outer rim). If one applies this relation to the Chesapeake Bay crater, the predicted dt would be 1.2 km. There is no obvious signature of such a structural boundary on our seismic profiles to confirm this prediction. Neither are there any drill data

General Conceptual Models and Scaling Relations

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368

Implications for Impact Models

yet from this part of the crater. Overall, where available, the morphometrics of the Chesapeake Bay crater appear to fit rather well the quantitative predictions of Gault et al. (1968), Melosh (1989), Grieve (1991), and Pilkington and Grieve (1992).

12.1.2 Submarine Cratering It is widely accepted on the basis of field studies and computer modeling, that submarine cratering includes some unique mechanisms or substages due to the presence of a marine water column (Higgins and Butkovitch 1967; Kieffer and Simonds 1980; McKinnon and Goetz 1981; Melosh 1981; Gault and Sonett 1982; McKinnon 1982; Roddy et al. 1987; Ormo and Miyamoto 2002; Shuvalov et al. 2002). Most conceptual and quantitative discussions of submarine cratering, however, have dealt with deep (abyssal) ocean impacts (Ahrens and O'Keefe 1987; Roddy et al. 1987; Nemchinov et al. 1993; Hills et al. 1994; Hills and Goda 1999; Artemieva and Shuvalov 2002; Wiinnemann and Lange 2002). Oberbeck et al. (1993) published one of the earliest and most perceptive conceptual models of an impact into a shallow (neritic) sea (Fig. 12.1). The sevenstep Oberbeck model, which is based on field observations and laboratory experiments, focuses only on the excavation and modification stages of crater development. During excavation, a mixture of water and fragmented target rocks would form a slurry rim, which extends upward from the sea surface and forms a raised lip around the transient crater (Fig. 12.1, step 1). A cone-shaped curtain of ballistically ejected target fragments proceeds radially away from the crater center. Debris trailing from the ejecta curtain's base settles onto the shallow seafloor. The slurry rim then moves landward (and seaward) and initiates a giant tsunami wave, which resuspends and erodes seafloor debris (Fig. 12.1, step 2). Subsequently, the wall of the seawater cavity collapses back into the excavation and rushes toward the center of the crater, where it produces a central slurry spout, whose collapse initiates a second tsunami wave (Fig. 12.1, step 3), followed by additional slurry spouts and tsunami waves (Fig. 12.1, steps 4,5), until wave oscillations over the crater are sufficiently damped (Fig. 12.1, step 6). Oberbeck's model includes a steep-faced delta system at the shoreline (Fig 12.1), which is massively disrupted (slumps, sediment gravity flows) by the impinging train of tsunami waves (Fig. 12.1, steps 3-6). The return flow of the tsunamis produces further nearshore erosion and deposition (Figs. 12.1, steps 4-7). Most effects of the tsunami washback in the Oberbeck model are concentrated in the nearshore region, and have no role in filling the crater, which in this model is a simple crater (no central uplift or peak ring). Tsikalas (1996) and Tsikalas et al. (1998b, 1999) used the Mjolnir crater to construct a conceptual model for submarine impact into entirely sedimentary target rocks (sandstones, shales, claystones, limestones; Fig. 12.2). A subsequent numerical simulation (Shuvalov et al. (2002) supports and refines the main features of the Tsikalas model. In the Tsikalas model, the transient crater of the excavation stage is shown to be ~4 km deep (from sealevel to maximum penetration

General Conceptual Models and Scaling Relations

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370

Implications for Impact Models

into sediments; -6 km in the Shuvalov et al. simulation), and is formed of seawater (Fig. 12.2A). As crater modification begins, the sedimentary floor of the crater rebounds (reaching a height of >5 km above the seafloor in the Shuvalov et al. simulation), and the outer margins of the crater begin to slump and slide along a decollement (Fig. 12.28). This is followed by collapse of the water column, which surges back into the crater, further collapsing the sedimentary walls and eroding the crater floor. Tsunamis were not included in the Tsikalas model, presumably because the Mj0lnir impact was too far from a landmass to have produced tsunami washback deposits inside or near the crater. However, the numerical simulation of Shuvalov et al. (2002) created large-amplitude (200 m) tsunamis, as well as surgeback flows with velocities of 50-70 mls. The final crater at Mj01nir displays a broad, conical central peak, whose relatively low density and distinctive seismic signature are inferred to represent intensely disrupted sediments. Around the periphery of the central peak, the crater floor is inferred to be buried by a relatively thick allogenic breccia (fallback?), which, in turn, is overlain by authigenic breccia (surgeback?). Neither breccia body covers the crest of the central peak. Tsikalas et al. (1999) calculated a collapse factor (ratio of final crater diameter to transient crater diameter) of 2.5 for Mj0lnir crater (2.0---2.5 according to Shuvalov et al. 2002). These authors considered this value to be significantly greater than the collapse factor for terrestrial subaerial craters of equivalent transientcrater diameter, which Gault et al. (1968) and Melosh (1989) indicated should be 1.4-2.0. Tsikalas et al. (1999) and Shuvalov et al. (2002), thereby, implied that for a given set of bolide properties (diameter, velocity, density, trajectory), submarine craters would have a greater collapse factor than subaerial craters. The calculated collapse factor for the Chesapeake Bay crater, on the other hand, is 2.1, more in line with Melosh's (1989) estimate for terrestrial subaerial craters. Ormo (1998) proposed a shallow-water submarine cratering model on the basis of his interpretations of the Lockne crater. Ormo's model was built upon an earlier lunar cratering model of Quaide and Oberbeck (1968), which has no similarities to the more recent model of Oberbeck et al. (1993). Ormo and Lindstrom (2000) updated the submarine cratering model of Ormo (1998) and emphasized the role of a collapsing and "resurging" marine water-column during the modification stage (Fig. 12.3). The transient crater in the Ormo-Lindstrom model, like that of Tsikalas et al. (1999), has a raised lip of seawater, but its floor is formed in crystalline rather than sedimentary rocks (Fig. 12.3A). During excavation, an ejecta curtain moves radially away from the impact site (Fig. 12.38). Crystalline basement is exposed as a flat (perhaps slightly raised) surface around a nested central basin, and the sedimentary cover is further excavated to form the crater's outer rim. At the crater's outer margin, the sedimentary section is folded back and injected with breccia sills and dikes to form a slightly raised lip. Modification is initiated by rebound of the crystalline basement, and the water column collapses and Fig. 12.3. (Opposite page) Conceptual model of excavation and deposition associated with a submarine bolide impact (modified from Ormo and Lindstrom 2000; based on Lockne crater). Compare with Figs. 12.1,12.2.

General Conceptual Models and Scaling Relations

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372

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surges back into the crater (Fig. 12.3C). The inwardly cascading surgeback flow erodes radial channels through the crystalline crater floor (but curiously, not through the sedimentary walls) and fills the channels with surgeback breccia. The crater's central basin and low central peak are also buried by surgeback breccia (any fall-back breccia that may have been present in the central basin is reworked into the surgeback deposit). Ormo et al. (2002) recently simulated the Lockne impact using the SOYA computer hydrocode (Shuvalov 1999). Using an 800-m-diameter asteroid (density 2.63 glcm3) and a paleodepth of 1000 m, the simulation produced impact features similar to those observed or inferred at Lockne. These features included a small, bowl-shaped, central depression, or nested crater, surrounded by a cylindrical water cavity. As in the Tsikalas model, no tsunami washback is included in the OrmoLindstrom (2000) or Ormo et al. (2002) models. However, Sturkell et al. (2000) described some aqueous sedimentary structures related to the Lockne impact, which crop out 45 km from the Lockne crater. Sturkell et al. (2000) interpreted these features to have formed from Lockne surge back processes. In our opinion, however, some of the surge back deposits around the Lockne crater have the same sedimentary features expected of a tsunami or hypercane deposit. In particular, the subaqueously formed sedimentary structures (indicating successive, opposite, flow directions) described from an outcrop 45 km from the Lockne crater (Sturkell et al. 2000) are very close analogues to the silt-banded, multidirectional, flowin facies at the top of the Exmore breccia.

12.2 Conceptual Model for Chesapeake Bay Crater We present a conceptual model for the Chesapeake Bay crater, which includes a two-dimensional computer simulation of the contact and compression stage, the excavation stage, and the early part of the modification stage. We are especially indebted to David Crawford of Sandia National Laboratories for providing this simulation (Fig. 12.4). The simulation was performed with the CTH shock physics hydrocode (McGlaun and Thompson 1990). Tabular equations-of-state, including accurate renditions of the solid/liquid/gas transitions were used. The materials were treated as strengthless, which is appropriate, since the Hugoniotelastic-limit is typically exceeded during the early phases of the impact processes depicted here. We also thank Toshihiro Matsumoto of the Japanese Broadcasting Company (NHK) for providing 3-D snapshots of the impact (Fig. 12.5), derived from a video animation of Crawford's simulations. We have integrated these simulations with our seismic and borehole interpretations (which mainly bear upon the modification stage) to create a holistic conceptual model of the structural, depositional, and morphological aspects of submarine crater formation on the North American Atlantic Continental Shelf ~36 Ma. Nevertheless, we are forced to omit such important aspects as possible suevite and melt-sheet forming processes, because we lack the requisite field data. Moreover, although some of our model concepts may

Conceptual Model for Chesapeake Bay Crater

373

apply to other submarine craters (and to some subaerial ones, as well), each submarine crater identified so far displays enough individual variability to set it apart from all other such craters.

12.2.1 Stage 1 - Contact and Compression

As the impactor for the 2-D simulation, Crawford chose an asteroid that would produce a 40-km-wide transient crater (asteroid diameter 3.3 km; density 3.32 glcm 3; impact velocity 20 km/s; impact trajectory vertical). This transient-crater diameter matches the average diameter of the Chesapeake Bay peak ring (Chapter 4). The simulated target layers consist of 120 km of air, 300 m of seawater, 200 m of sediments, and 100 km of granite. The composite 500 m of water column plus seafloor sediments are so thin as to be barely discemable at the spatial scale of the simulation, but the general aspects (size, depth, height, temporal succession) of early impact processes and features are clearly expressed (Fig. 12.4). We have almost no field data bearing on stage-l processes at Chesapeake Bay, other than the compositions and thicknesses of the target layers, the measured diameter of the peak ring, and an estimate of the pre impact water depth. The diameter of the peak ring probably constrains the diameter of the transient crater, which, in tum, constrains the modeled properties of the impactor. The presence of shockmetamorphosed and melted basement clasts within the Exmore breccia, however, is direct evidence of the high temperatures and pressures produced during stage 1 of the Chesapeake Bay impact, which lasted less than 10 seconds.

12.2.2 Stage 2 - Excavation

At ten seconds after contact (in the Crawford simulation), the Chesapeake Bay transient crater is ~20 km wide, its floor is 15 km below the seabed, and its outer wall (a flask-shaped ejecta curtain composed of a slurry of water and rock debris) rises nearly 40 km above the sea surface. The transient crater depth at this time slice is 18 km below the seafloor (Figs. 12.4, 12.5). The impact fireball is constrained within the crystalline walls of the lower transient crater. A conical shock wave is expanding through the crustal rocks (40 km from ground zero) and the atmosphere, and a hot plume of vaporized bolide, target rocks, and seawater extends ~90 km into the atmosphere. At 30 seconds after impact, the interior fireball has expanded to 20-25 km height, but is still contained within the ejecta curtain, whose top is at ~40 km (Figs. 12.4, 12.5). The transient crater diameter has expanded to 30 km, and its depth is 20 km. Some ejecta fragments have reached nearly 80 km into the atmosphere, and are beginning to fall back, forming incandescent meteors as friction reheats their outer surfaces. The leading edge of the shock wave is now >60 km Fig. 12.4. (Next page) 2-D computer simulation of Chesapeake Bay impact, showing five time slices (10 s, 30 s, 60 s, 120 s, 170 s). Simulation generated by D.A. Crawford, Sandia National Laboratories. See text for further explanation and CD-ROM for color version.

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Fig. 12.5. 3-D rendering of four of the time slices shown in Fig. 12.4 (10 s, 30 s, 60 s, 170 s) derived from computer-assisted video animation prepared by NHK (Japanese Broadcasting Company; courtesy of Toshihiro Matsumoto). See text for further explanation and CD-ROM for color version.

376

Implications for Impact Models

from the point of impact. At 60 seconds after impact, the crater has achieved its maximum transient diameter of 40 km, and the center of the granitic crater floor has rebounded to form a mountainous, lO-km-high, central peak (Figs. 12.4, 12.5). The fireball has begun to collapse and cool within the crater. The ejecta curtain is still well developed up to about 50 km height, but is breaking up above this elevation. Field evidence of initial excavation-stage processes at Chesapeake Bay can be seen in a combination of properties peculiar to the annular trough. There, the crystalline basement rocks are highly fractured and faulted (seen both on seismic profiles and in cores), and the bases of the overlying sedimentary megablocks have been fluidized into massive, structureless units. We infer that these features were caused by impact-generated compression and rarefaction. The degree of shock metamorphism in the breccia clasts (fragments of crystalline basement) indicates that impact pressures reached >60 GPa. Density differences between the porous, water-saturated, unconsolidated sediments and the denser crystalline basement rocks appear to have been particularly important in their respective reactions to the impact shock. The sediments absorbed significantly more shock energy than the basement, which allowed differential motion between the basement and the sedimentary cover, and enabled huge sedimentary megablocks to detach from the basement surface (zone of decollement), and to slump and slide across its surface. In some places, though, the megablocks show little or no rotation or internal deformation that would indicate lateral motion. Instead, these megablocks appear to have dropped vertically downward as their shock-fluidized bases collapsed.

12.2.3 Stage 3 - Modification

At 120 seconds postimpact, the fireball has dissipated (Fig. 12.4). The central peak has reached maximum height of ~30 km, and is 15-20 km in diameter. Sediment collapse along the crater's outer rim has expanded the crater diameter to ~60 km. The ejecta curtain is breaking up in the atmosphere, as its upper diameter has expanded to 120 km and the sedimentary section of the seafloor is folding back upon itself at the crater's outer rim. A significant volume of weakly ejected debris is raining back into the crater above the central peak. At 170 seconds after impact, the central peak has collapsed to <20 km height, a 40-km-wide peak ring is forming, and the crater's outer-rim diameter has expanded to its maximum diameter of 85 km (Figs. 12.4, 12.5). The annular trough can now be differentiated, and its floor is ~1-1.5 km below the lip of the outer rim. The disassociating ejecta curtain is now ~ 180 km in diameter across the top. Figure 12.5 (170s) shows the initial tsunami wave forming and beginning a radial expansion that will take it to the eastern flank of the Appalachian Mountains. The Crawford simulation did not run long enough to show final collapse of the central peak, subsequent gravity collapse of the oceanic water column, or impactgenerated tsunami activity. Other published numerical simulations using different hydrocodes (e.g., Wunnemann and Lange 2002, using a 2-D SALE code;

General Conceptual Model of Crater-Fill Deposition

377

Shuvalov et al. 2002, using the SOYA multimaterial code) indicate that most of these remaining processes of crater modification, except for tsunami washback and hypercane (flowin) deposition, would be completed by 800-1300 s (13-20 min) after impact. Tsunami and hypercane activity might last for days or weeks, depending on many variables, such as bolide diameter, water depth, physiography of the seafloor, and distance to the nearest shoreline.

12.3 General Conceptual Model of Crater-Fill Deposition The modification stage of impact cratering embraces a variety of depositional regimes and their resultant deposits or lithofacies, which in themselves, are sufficiently independent from actual crater-forming processes to deserve separate study. The superb preservation of the Chesapeake Bay crater and the abundance of high quality seismic and borehole data have yielded more evidence of these depositional aspects than any previous study of submarine craters. We propose a two-part general depositional model for Chesapeake Bay, which contrasts regimes and lithofacies inside the crater (intracrater) with those outside the crater (extracrater).

12.3.1 Intracrater Regimes and Lithofacies

Previous field studies (including those at Chesapeake Bay; see Chapters 6, 11) and conceptual models indicate that there is a general temporal succession of six crater-filling depositional regimes inside submarine craters (Fig. 12.6A). This succession of regimes is determined from the spatial distribution of the lithofacies they produce (Fig. 12.6B). The basal lithofacies (on the floor of the inner basin and covering the outer flank of the peak ring) are bodies of fallback breccia. Fallback breccia consists primarily of weakly ejected fragments that fall directly back into the crater, mixed with allochthonous debris that never left the crater. For most large craters, individual clasts in the fallback breccia are mainly remnants of the deepest target rocks, which constitute crystalline basement (Fig. 12.7). Stratigraphically above the fallback breccia is the slumpback lithofacies (slumped megablocks), derived mainly from gravitational collapse of the outer crater walls and of the margins of peak rings and central peaks (Figs. 12.6, 12.7). In cases where crystalline rocks are overlain by sedimentary rocks, the bulk of the slumpback lithofacies is expected to consist of sedimentary megablocks, many of which can be kilometer-sized blocks. Mixing of crystalline and sedimentary clasts is expected to occur at the fallbacklslumpback boundary. The next highest intracrater stratigraphic unit in submarine craters consists of the surgeback lithofacies, derived from loading pressures on, and hydraulic erosion of, the crater walls and surrounding seafloor caused by collapse of the oceanic water column (Fig. 12.6, 12.7). In targets with surficial sedimentary rocks, surgeback lithofacies should be dominated by sedimentary clasts, with a significant component of finer grained matrix.

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Surgeback - Debris scoured from seafloor, crater walls, and crater floor by collapsing water column; complex mixture of mainly sedimentary clasts and matrix; upwardfining sequence

Washback - Debris scoured from seafloor and coastal plain by tsunami and then washed back to sea; dominantly sedimentary matrix; upward-fining sequence

Foldback - Beds at outer rim fold back to produce reversed stratigraphy

Seafloor surge - Debris ejected radially outward from crater as ground-hugging, scouring, flows; laterally graded deposit

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Above the surge back lithofacies, and mixed with it near their mutual boundary, is the washback lithofacies (Figs. 12.6, 12.7). This lithofacies contains principally sedimentary debris scoured from the surrounding seafloor and adjacent coastal plain by the runup and washback of tsunami wavetrains. A major component of fine-grained matrix is expected to be incorporated in washback lithofacies. All coreholes within Chesapeake Bay crater have documented an upward-fining sequence of breccia in the washback lithofacies (Figs. 12.6, 12.7; see also Chapter 6). So far, Chesapeake Bay is the only crater in which a washback lithofacies has been specifically delineated. On the basis of the Chesapeake Bay cores (Chapter 6), we propose a sixth depositional regime for this general conceptual model - the flow in regime. In the uppermost few meters of the Exmore breccia at Chesapeake Bay, the sediments differ from the preceding lithofacies in three important ways. First, this upper interval contains finer sediment - mainly fine sand, silt, and clay; second, the upper section loses the massive character of the underlying units and becomes bedded, or laminated on a centimeter-scale; third, the bedding is dominantly inclined, and the dip direction changes frequently over short vertical distances (centimeterscale). These features indicate a complex regime in which flow direction and velocity changed relatively rapidly through time, in contrast to the generally unidirectional washback inferred for the tsunamis. This depositional style suggests a succession of postimpact storms, which stirred the seafloor around the crater with sufficient energy to send small debris flows or turbidity currents into the crater. The story is complicated, however, by the fact that benthic foraminifera in the initial marine sediments overlying the dead zone at Chesapeake Bay suggest paleodepths of around 300 m (Chapter 13). At such depths, the seafloor is not likely to be stirred by normal storm action, so we speculate that the stirring agent may have been a succession of runaway hurricanes, or hypercanes. The concept of hypercanes was introduced by Emanuel et al. (1995), who described them as runaway hurricanes. These authors proposed that hypercanes could develop when the sea-surface temperature exceeds a critical threshold, such as might occur in the vicinity of a marine bolide impact. Emanuel et al. (1995) focused on the ability of hypercanes to inject massive volumes of water and aerosols into the stratosphere, but here we are more interested in the potential for moving seafloor sediment in deep water. A computer simulation by Emanuel et al. (1995) showed that a bolide that could produce sea-surface temperatures of 50°C over an area of 50 km or more could generate a hypercane with maximum wind velocities of 300 m/s (650 mph), or about three times the maximum horizontal velocity of known hurricanes. Crawford's computer simulations (Fig. 12.4), combined with the shockmetamorphic features documented in the Chesapeake Bay cores (Chapter 6), show that the Chesapeake Bay bolide would have produced even higher temperatures over a larger area, thus we consider the presence of hypercanes to be a plausible inference. Hypercane flowin deposits also might be expected in other wellpreserved submarine craters excavated into shallow continental shelves. The final synimpact, intracrater, depositional regime is that of fallout. Fallout debris is composed of millimeter- to submicrometer-sized particles, some of

General Conceptual Model of Crater-Fill Deposition

381

which could remain entrained in or above the Earth's atmosphere for weeks-tomonths following the impact (Figs. 12.6, 12.7). Microtektites, microkrystites, and shocked minerals are among the components of the fallout lithofacies.

12.3.2 Extracrater Regimes and Lithofacies

Field studies of submarine impact deposits immediately outside the crater rims have documented two distinct depositional regimes (Fig. 12.6). These regimes are expected to produce two of the three stratigraphically highest synimpact lithofacies found inside the crater - washback lithofacies overlain by fallout lithofacies. In subaerial craters, two additional synimpact extracrater regimes have been recognized. The stratigraphically lowest subaerial extracrater depositional regime is that of ground surge, in which a viscous mixture of fragmental target material is ejected radially along the ground surface (Fig. 12.6). Hypothetically, the equivalent regime in submarine craters would be seafloor surge. This regime operates close to the seabed, flushing debris outward from the crater, scouring radial channels or striations, and also leaving a laterally graded deposit (coarser debris closest to the crater). No seafloor-surge lithofacies has been recognized at Chesapeake Bay, presumably, in part, because its constituents would have been incorporated into the washback lithofacies, and thus would be extremely difficult to differentiate. The remaining extracrater depositional regime of subaerial craters is that of ballistic ejection, which produces a conical curtain of debris moving outward in ballistic trajectories, and lays down a laterally graded deposit (Fig. 12.6). When the larger clasts within this ballistic ejecta curtain eventually impact the Earth's surface, they may create additional concussive debris, or even excavate secondary craters. Obviously, in many cases it may be difficult, or impossible, to confidently separate products of ballistic sedimentation from those of the seafloor-surge and wash back regimes. At Chesapeake Bay, any ballistic ejecta would have been stirred into the washback lithofacies. The numerous sand-sized and occasional cobble-sized crystalline clasts within Chesapeake Bay extracrater washback deposits probably represent reworked ballistic ejecta. Some modelers also expect a separate extracrater deposit to be formed from high-velocity jetting during the early stages of impact (Melosh, 1989), but, so far, we have found no field evidence for such a regime related to the Chesapeake Bay impact.

12.4 Differentiating Crater-Fill Lithofacies at Chesapeake Bay On the basis of cores, geophysical logs, and seismic reflection profiles, the broadly defined Exmore breccia can be divided into six stratigraphically distinct synimpact lithofacies (Fig. 12.7), which can be compared to the general model of

382

Implications for Impact Models

crater-fill lithofacies (Fig. 12.6). From the nature of these lithofacies, their respective depositional regimes can be inferred (Table 12.1). In order to apply the crater-fill model to Chesapeake Bay impact lithofacies, we must know: (1) the gross lithologies of the deposits, which can be derived from cores and downhole geophysical logs; (2) the stratigraphic succession (stacking order) of different lithic units within and near the crater, which can be derived from cores, downhole geophysical logs, and seismic reflection profiles; (3) the geometry of the lithic units, which can be determined from seismic profiles; and (4) the location of each core relative to the morphological and structural features of the crater, as determined from seismic profiles. The stratigraphically deepest lithofacies at Chesapeake Bay, assumed to contain fallback breccia, appears to be restricted mainly to the deepest parts of the inner basin (Figs. 11.8, 12.7). However, the deep inner basin has not yet been cored, and none of the currently available cores contain breccia dominated by crystalline clasts. In fact, basement clasts larger than sand size are extremely rare (Chapter 6). Moreover, there is no clear signature of fallback breccia on the seismic profiles, though Poag (1996a) identified a possible upper boundary for a presumed fallback unit. Near the bottom of the Bayside corehole, however, is a 20.33-mthick, crystalline-clast breccia (688.54--708.87 m) unlike any other so far encountered at Chesapeake Bay (see Chapter 6). This breccia might represent fallback debris. This core section is composed of a matrix-supported breccia with abundant, mainly cobble-sized clasts (Fig. 6.4). The matrix in this unit is a white-tolight gray sand, quite distinct from the typical greenish gray, glauconite-rich Exmore matrix; it appears to be a finely comminuted immature granitic sand. Most cobbles in this section are either hard, indurated sandstones or weathered granite, as opposed to the poorly lithified sand, silt, and clay clasts of the typical Exmore breccia. Also, many of the cobbles display thin white rinds, similar to fusion rinds. In terms of volume and lateral distribution, deposits from the washback, surgeback, and slumpback regimes are the dominant lithofacies encompassed by the Exmore breccia (Figs. 11.8, 12.7). Poag (1999b, 2000) attempted to quantify the differences between these lithofacies by calculating the relative thickness of matrix versus boulders (MiB ratio) in the cored sections. Where appropriate, we use the MlB ratio in conjunction with several qualitative lithic properties of the cores to differentiate these three breccia types. Poag (1999b, 2000) also attempted to correlate the spontaneous potential (SP) curve from the downhole geophysical logs with individual lithologies from each corehole, in order to determine whether the SP curve could be used to calculate an MIB ratio in the uncored sections of the Exmore and Kiptopeke boreholes. Our further analyses of the NASA Langley, North, and Bayside logs suggest that the lithic variability of the breccias is too great to yield reliable MIB ratios from the logs alone. Inside the Chesapeake Bay crater, the three cored sections of slumpback lithofacies are characterized by the nearly complete absence of glauconitic quartz sand typical of the Exmore matrix. Neither is there other lithic or microfossil evidence of Cenozoic strata in the displaced megablocks. Instead, the megablocks contain thick sections (as thick as 20 m) of relatively undisturbed (mainly tilted), stratified

Differentiating Crater-Fill Lithofacies at Chesapeake Bay

383

(bedded or laminated), sedimentary beds (mainly nonmarine sands, silts, and clays, including thick paleosol intervals) of Early Cretaceous age (Figs. 6.7, 6.8). These normally stratified Cretaceous intervals are occasionally interrupted by zones of brecciated sediments, which we interpret as evidence of internal deformation within the displaced megablocks. In the deepest cored megablocks, near the basement surface at NASA Langley and at Bayside, 22-32 m of sedimentary section near the basement surface displays intervals of massive stratal disruption (possibly caused by acoustic fluidization), moderate cementation, and splotches of possible hydrothermal mineralization (Fig. 6.6). The stratigraphically next highest depositional lithofacies in the Chesapeake Bay crater is composed of surge back breccia, which was documented at all five intracrater core sites, but not outside the crater (Figs. 12.6, 12.7). Surgeback breccia in most coreholes is clast-supported, having too little matrix to calculate a meaningful MIB ratio. The exception is Bayside, where a greater abundance of matrix allows calculation of an MIB ratio of 1:9. At North and NASA Langley, we calculated instead, a cobble-to-boulder ratio (C/B ratio), which is 2:1 at both sites. Clasts in the surge back breccia are notable for their extreme plastic deformation, inclined interclast contacts, and high-angle to near-vertical stratal orientations (Figs. 6.9, 6.10). Boulder-sized clasts are commonly 12-20 m thick in this lithofacies. The stratigraphically next highest crater-fill lithofacies is notable for the dominance of typical Exmore matrix (medium-to-coarse, glauconitic, quartz sand; Fig. 6. 14H,I). We calculated MIB ratios in this lithofacies ranging from 2:1 (North corehole), to 3:1 (Bayside corehole), to 10:1 (NASA Langley corehole). Largest clasts within this lithofacies are notably smaller (2-4 m) than those in the surgeback breccia. The unit displays a distinct upward-fining sequence, especially notable at the NASA Langley and Bayside sites, where the upper ~30 m of section contains no clasts larger than a few centimeters. Larger clasts in this lithofacies display the same intense plastic deformation features, inclined contacts, and highangle stratal orientations, characteristic of the surgeback breccia. We interpret this lithofacies to be composed of tsunami washback breccia. The washback lithofacies is also present outside the crater at the Windmill Point and Newport News core sites. In fact, it is the only crater-fill lithofacies encountered at sites outside the crater. A small component « 1 vol. %) of impact ejecta (microtektites, sand-sized crystalline clasts, shocked and melted mineral grains) is scattered throughout the washback breccia at all sites. Some of these constituents might represent ballistic ejecta and( or) seafloor-surge deposits, which were remobilized during the washback process. Inside the crater, the washback lithofacies can be further divided into upper and lower sublithofacies. The lower sublithofacies (Fig.6.4G) is coarser (MiB ratios of 0.01:1 to 2:1) and contains larger clasts (5-7 m) than the upper sublithofacies. The upper sublithofacies approaches nearly 100 percent matrix. Clasts larger than 1-2 cm are rare, which gives MIB ratios of essentially 1:0 (Fig. 6.141). Overlying the washback lithofacies at Chesapeake Bay is a flow in deposit, a lithofacies not accounted for in previous models. In sharp contact (at most sites) above the washback lithofacies is a thin layer (~0.6 m) of clayey silt with gener-

384

Implications for Impact Models

ally inclined thin laminae and lenses of white, fine-to-very fine sand (mainly a mixed suite of benthic foraminifera and other microfossils redeposited from washback breccia deposits outside the crater rim, plus abundant muscovite flakes and framboidal pyrite). The azimuth and steepness of inclination among these sandy laminae change repetitiously up the core, indicating that their depositional geometry was controlled by multiple flow directions (Fig. 6.23). We interpret this silt-rich flowin layer to represent a succession of localized turbidites or debrisflow deposits produced by hypercanes (Emanuel et al. 1995), which stirred up the shallow seafloor regimes outside the crater basin. Hypothetically, a fallout lithofacies should be present at the top of the flowin lithofacies. We have found sparse evidence of the fallout regime at Chesapeake Bay, however, because constituents, such as shocked grains and microtektites, are exceedingly rare «1 vol.%). Presence at the NASA Langley site of in situ pyrite lattices containing millimeter-sized spherical cavities (implying a layer of glass microspherules; Poag 2002; Fig. 6.24A,B) is the only evidence that fallout debris accumulated inside the Chesapeake Bay crater (Figs. 12.6, 12.7).

12.5 Comparison of Models Our field evidence supports many aspects of the Oberbeck impact model (Oberbeck et al. 1993; Table 12.1; Fig. 12.1), which emphasized that submarine impacts generate lithofacies that are laterally extensive, with highly complex structure and extreme spatial variability. The Oberbeck model, however, focused mainly on extracrater deposits, characterized by upward- and distally-fining sequences, which contain sorted and unsorted, massive, graded, finely layered, and crossbedded units. Near the impact site (outside the crater), the Oberbeck model predicts that ejecta deposits would be crudely graded. The lower layers would contain megaclasts, the middle layers would be water-sorted, and the upper layers would be argillaceous and finely laminated. In nearshore areas distal to the crater, Oberbeck et al. (1993) inferred chaotic mixtures of ejecta, reworked seafloor sediment, massive slumps, and turbidites. Landward of the shoreline, high-velocity ejecta would plough and brecciate the subaerial substrate, loosening it up for the impinging tsunami waves to entrain sedimentary clasts and wash them back to sea. The Oberbeck model, however, gives no role to the washback or flowin processes in filling the primary crater. The Ormo model (Ormo 1998; Ormo and Lindstrom 2000; Table l2.l; Fig. 12.3) differs considerably from the Chesapeake Bay impact model. In particular, the Ormo model lacks a peak ring, displaced megablocks, fallback breccia, washback breccia, flowin deposits, and a fallout layer. Almost all the impactgenerated deposits in the Ormo model are derived from surge back processes (Ormo and Miyamoto 2002). The thick and widespread nature of breccia deposits in the Tsikalas model (Tsikalas et al. 1999; Table 12.1; Fig. 12.2) is similar to that of the Chesapeake Bay model, except that the crest of the central peak is not covered by breccia. Our inference of similarity is weakened, however, by the sparsity of core documentation

subtle; crest buried by breccia

Surgeback Slumped megablocks breccia at crater none margins and near shoreline

inferred inferred from density from values seismics

Peak Ring Fallback breccia none none

inferred from density values; possibly cored Onnc consolidated subtle; none none none outcrops; (Lockne;20 sediments over crystalline indicated dominant km diameter) crystallines crater-fill deposit extensive in extensive This volume unconsolidated prominent; prominent; possibly (Chesapeake sediments over crystalline; crystalline; cored; annular throughout crater; cored Bay; 85 km crystallines trough; buried by buried by inferred diameter) and inferred breccia breccia from cored and from seismics; inferred seismics; mainly in from inner basin seismics; buries central peak and peak buried by and outer ring surgeback annular trough and washback breccia

Central Peak sediments over none crystallines

Target Rocks

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Oberbeck (Generalhypothetical)

Model

cored none inside documented crater; too thin to image on seismics

cored inside crater; too thin to image on seismics; covers breccia inside crater extensive throughout crater; limited distribution outside crater; cored and inferred from seismics; buries central peak and peak ring

none

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possibly present west of crater

several none documented at outcrops

none

none

none

none

none

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none

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none

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Table 12.1. Comparison of principal structural, depositional, and erosional features among four conceptual models of submarine bolide impacts

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at Mj0lnir. The presence of a peak ring at Mj0lnir also is not strongly documented, due perhaps to its sedimentary composition and unusually low relief. The Tsikalas model also lacks flowin and fallout deposits. Our Chesapeake Bay model differs from all other models in emphasizing a strong crater-filling role for tsunami washback processes, and highlights the intracrater breccias and megabreccias that are so widespread and voluminous at Chesapeake Bay. Complete burial of the peak ring and central peak by surgeback, washback, and flowin deposits appear to set the Chesapeake Bay crater apart from all other known impact craters of its size range. On the other hand, it shares this attribute with the much larger Chicxulub impact basin (Chapter 10), which also is the result of a submarine bolide strike into mixed target rocks on a shallow continental shelf.

13 Biospheric Effects of Chesapeake Bay Impact

13.1 Local Paleoenvironmental Effects The Virginia cores contain an excellent, detailed sedimentological and micropaleontological record of successive postimpact depositional and biotic changes at the Chesapeake Bay impact site. This record is contained mainly in the Chickahominy Formation (Chapter 7), which represents one of the most complete sections of immediately postimpact marine strata yet discovered (apart from deep-sea cores) for the last - 2.1 myr of late Eocene time. The exceptional Chickahominy record is a product of relatively deep-water, fine-grained, microfossiliferous deposition within a closed basin. Deposition took place on a slowly subsiding, passive continental shelf that underwent no syndepositional tectonism or synchronous major eustatic sea-level changes. Subsequent Cenozoic sea-level falls and marine transgressions have eroded the top of the Chickahominy Formation, but it has never been subjected to significanttectonic activity.

13.1.1 Sediment Accumulation Rates

By plotting magnetochronology (Poag et al. 2003) against drill depth in the Kiptopeke corehole, three distinct depositional episodes have been recognized within the late Eocene Chickahominy Formation (Fig. 13.1 ; Table 13.1). The stratigraphically lowest depositional episode is defined by the 11 samples between paleomagnetic boundaries at 384.0 m and 365.0 m (1259.8- 1197.6 ft; 35.60-35.30 Ma), in which sediments accumulated at an average rate of 67 m/myr. This is the most rapid rate of sediment accumulation during Chickahominy time at this site. If one assumes (on the basis of similar lithology) that the II Chickahominy samples below 384.0 m (1259.8 ft) represent accumulation at the same rate of 67 m/myr, then the extrapolated age of the first postimpact sample (and, therefore, the approximate age of the impact) is 35.78 Ma (Fig. 13.1; Table 13.1; Chapter 8). The second Chickahominy depositional episode is represented by sediments between paleomagnetic boundaries at 365.0 m and 354.8 m (1197.6-1166.7 ft; 35.30-34 .60 Ma), which accumulated at an average rate of 10 m/myr (Fig. 13.1; Table 13.1). The third Chickahominy depositional episode is represented by the 20 stratigraphically highest samples (354.8-327.7 m; 1166.7-1077 .6 ft; 34.6- 33.7 Ma), which accumulated at an average rate of 30 m/myr (Fig. 13.1; Table 13.1). C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

388

Biospheric Effects of Chesapeake Bay Impact

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393.2 396.2

Fig. 13.1. Stratigraphic column for Chickahominy Formation at Kiptopeke core site, showing magnetochronology, biochronology, and sediment accumulation rate curve. Average rate = 32 m/myr. Modified from Poag et al. (2003).

Thus, during the first -480 kyr following impact, the average sediment accumulation rate was maximum for the late Eocene at Kiptopeke. For the next 700 kyr, the rate dropped to approximately 15% of the initial rate, but then tripled during the final 900 kyr. We have compared this chronodepositional framework with

Local Paleoenvironmental Effects

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Table 13.1. Sediment accumulation rates calculated from ages and depths of magnetochron boundaries in the Chickahominy Formation at Kiptopeke. Upper Chron Boundary Age [Ma]

Lower Chron Boundary Age [Ma]

Upper Chron Boundary Depth [m]

Lower Chron Boundary Depth [m]

Thickness Between Chron Boundaries [m]

Time Between Chron Boundaries [myr]

Sediment Accumulation Rate [rn/myr]

33.7

34.6

327.7

354.8

27.1

0.9

30.1

34.6

35.3

354.8

365.0

7.27

0.7

10.4

35.3

35.6

365.0

382.3

20.23

0.3

67.4

35.6

35.78 (extrapolated)

382.3

384.0

11.8

0.18

67.4 (assumed)

Average sediment accumulation rate = 31.92 rn/myr @30.1 rn/myr, I m = 33 kyr @10.39 rn/myr, I m = 92 kyr @67.43 rn/myr, I m = 15 kyr specific biotic characteristics of the late Eocene benthic foraminiferal community at the Kiptopeke site to interpret the paleoenvironmental succession following the impact.

13.1.2 Stratigraphic Attributes of Benthic Foraminiferal Community

13.1.2.1 Preimpact BenthicForaminiferal Community To asse ss the general effects of the impact on the local biota , we have examined the excellent record of fossilized late Eocene benthi c foraminifera at and near the impact site. Evidence of the nature of the preimpact foraminiferal community in the target zone was completely destroyed upon impact, through vaporization and excavation. Subsequently, erosion during several succe ssive Cenozoic mar ine regressions and tra nsgressions has virtua lly erased the record of late Eocene deposi tion elsewhere in southeastern Virginia. We know, however, that well -deve loped marine foraminiferal assemblages (planktonic as well as benth ic) were prese nt in the target area prior to impact, because representatives of them are mixed into assemblages of older microfossils within the Exmore breccia (Poag and Aubry 1995; Fig. 8.2). To get a more quantitative estimate of the composition of these pre impact assemblages, howe ver, we have to rely on record s from outs ide the impact area . The closest documentation comes from boreholes in Mary land and New Jersey. Poag and Commeau (1995) reported a diverse late Eocene planktonic foraminiferal assemblage from a corehole in southern Mary land (Ohio Oil-Larry G Hammond # I well, near Salisbury, Maryland) . The planktonic foraminife ral species represented there are typica l of mixe d tropical -temperate planktonic assem-

390

Biospheric Effects of Chesapeake Bay Impact

blages found at many other late Eocene sites around the globe. We have studied the benthic foraminifera from the same Hammond corehole , and found a rich, diverse assemblage (Table 13.2), which includes most of the species represented in the Chickahominy Formation in Virginia (Fig. 13.2A,B). Table 13.2. Species of benthic foraminifera in early late Eocene sediments (preimpact) from Hammond corehole, near Salisbury, Maryland.

Alabamina midwayensis Anomalinoides bilateralis Anomalinoides "kiptopekens is" Bolivina gracilis Bolivina "preavirginiana" Bolivina virginiana Bulim ina jacksonensis Bulimin ellita curta Caucasina marylandica Char/tonina madrugaensis Cibicidina sp. Cibicidoides "chickahominyanus" Cibicidoides cocoaensis Cibicidoides pippeni f. speciosus Cibicidoides renzi Cibicidoides "rugoumbonatus" Cibicidoides sculp turatus Epistominella minuta Gaudryina alazanensis Globobulimina ovata Globocassidulina subglobosa G/obulina gibba Grigelis annulospinosa Grigelis "tubulosa" Guttulina hantkeni

Guttulina irregu /aris Gyroidinoides aequi/atera/is Gyroidinoides byramensis Gyro idinoides p/anatus Hanzawaia blanpiedi Kaleshnikovella sp. Lenticulina virginiana Loxostomina vicksburgensis f. spinosa Marginulina cocoaensis Massillina decorata Melonis planatus Oridorsalis umbonatus Planu/aria sp. Pullenia quinque/oba Pyramidina sp. Spirop/ectinella miss issippiensis Stilostomella "aduncocostata" Stilostomella cocoaensis Stilostomella "exilispinata" Textularia virginiana Turrilina robertsi Uvigerina gardnerae Uvige rina j ackso nensis Vaginulin ops is jacksonens is

Total number of species = 49

13.1.2.2 Postimpact Benthic Foraminiferal Community The initial postimpact response of the foraminiferal community at the impact site is best documented in the NASA Langley core. This corehole , completed in October, 2000, was the first to be drilled after the impact crater' s presence had been convincingly demonstrated (Poag et aI. 1994). As a result, the cores from NASA Langley were described , photographed , and sampled more carefull y and systematically than those from the previous four core sites (Newport News, Windmill Point, Exmore, Kiptopeke). As we showed in Chapter 7, the Chickahominy Formation at the NASA Langley site is overlain by a distinctive 19-cm-thick unit of laminated sand, silt, and clay (top at 235.65 m; 773.12 ft), which is a dead zone, barren of indigenous biota. The lower boundary of this dead zone is interpreted to

Local Paleoenvironmental Effects

391

be a fallout layer, which contains evidence (pyrite lattices) of the prior presence of microspherules (Poag, 2002b; see Chapter 6). This dead zone does contain white laminae, lenses, and burrow casts, however, (Figs. 6.22, 7.3), composed of fine to very-fine-grained quartz sand, glauconite, mica, pyrite, and mixtures of reworked specimens of older (preimpact) foraminifera (middle Eocene, early Eocene , Paleocene, Late Cretaceous, and Early Cretaceous microbiota). Thus, it appears that the dead zone represents an interval of immediately postimpact abiotic deposition . We can roughly estimate its maximum duration to have been ~3 kyr by extrapolating the sediment accumulation rate from the Kiptopeke magnetochronology (Fig. 13.1; Table 13.1). However, the distinctly coarser grain size of the dead zone sediments may indicate considerably more rapid deposition than that of the lower Chickahominy section, possibly representing only -1 kyr or less of accumulation. An equivalent dead zone is present in the North and Bayside coreholes, and is represented by the first sample below the Chickahominy Formation (sample 0 at -394.05-394.23 m; 1292.8-1293.4 ft) in the Kiptopeke corehole . This Kiptopeke sample contains an unusually sparse assemblage made up of single specimens of only 17 different species, all of which appear to have been reworked. As yet, however, no microspherule-bearing interval has been identified at any corehole other than at NASA Langley, so we cannot pinpoint the precise base of the dead zone at the other six sites. The succession of foraminiferal assemblages in the remaining late Eocene section (the Chickahominy Formation) above the dead zone is best documented from the Kiptopeke core, where the formation is 66.4 m thick (217 .8 ft). We have identified 150 species of benthic foraminifera among 47 stratigraphically successive Chickahominy samples taken from Kiptopeke . We have displayed the stratigraphic ranges of the benthic species as a consecutive succession of samples in order of sample depth (Fig. 13.2A,B; CD-ROM.13.2). The first sample above the dead zone at Kiptopeke (sample 1; 393.9 m; 1292.3 ft) contains representatives of 33 species of benthic foraminifera (Fig. 13.2A). Of these , six represent agglutinated species and 27 are calcareous species . Seven of these species are known from the middle Eocene Piney Point Formation in Virginia and Maryland (Jones 1990), and 14 occupied the site prior to impact (by inference from the Hammond core) . The most abundant species in this assemblage are typical of the rest of the Chickahominy samples at this site, and 20 of the species (61%) are represented in nearly every sample to the top of the late Eocene section. We call this the Cibicidoides pippeni Assemblage, after one of the most abundant and persistent species in the assemblage. The sediments that contain the assemblage comprise the Cibicidoides pippeni Zone (Partial-Range Zone; Table Fig. 13.2. (Next two pages) Stratigraphic range chart for benthic foraminifera in Chickahominy Formation at Kiptopeke core site. Broad shading indicates nominate taxa for principal benthic foraminiferal subassemblages that constitute Cibicidoides pippeni Assemblage. Heavy black lines indicate agglutinated species. Small open circles indicate reworked specimens found in dead zone. Complete, full-size version of this figure can be found on CD-ROM.

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13.3; Fig. 13.3). The Cibicidoides pippeni Assemblage also typically includes several species of unusually large lenticulinids (2-4 mm in diameter), and an association of large marginulinids, stilostomellids, and nodosariids, whose representative specimens reach lengthsof 3-7 mm. Table 13.3. Prominent benthic foraminiferal species of Cibicidoides pippeni Assemblage.

Species listed are those whose specimens are persistently present and(or) abundant. Bolivinajacksonensis Buliminajacksonensis * Caucasina marylandica * Charltonina madrugaens is * Cibicidoides pippeni f. speciosus * Epistominella minuta * Globobulimina ovata * Globocassidulina subglobosa * Globulina gibba * Grigelis "e1ongata" Grigelisannulospinosa * Grigelis "tubulosa" * Guttulina hantkeni * Guttulina irregularis * Gyroidinoides aequilateralis * Gyroidinoides byramensis * Gyroidinoides planatus * Lenticulina americana f. typica Lenticulina americana f. spinosa Lenticulina "juvenilis" Lenticulinagutticostata

Lenticulina virginiana * Loxostomina vicksburgensis f. spinosa * Marginulina cocoaensis * Marginulina karreriana Marginulina splendens Melonis planatus * Nodosariacapitata Nodosariasoluta Oridorsalis umbonatus * Parafrondicularia cookei Proxyfrons virginiana Pseudonodosaria virginiana Pullenia bulloides Sigmoidella plummerae Spiroplectinella mississippiensis * Stilostomella "aduncocostata" * Stilostomella cocoaensis * Stilostomella cookei Stilostomella "exilispinata" * Uvigerina gardnerae * Vaginulina "barbulata"

*indicates species also present in earliest late Eocene (preimpact) sample in Hammond corchole near Salisbury, Maryland Moving upsection, samples 2-4 (393.7-391.6 m; 1291.7-1285 .0 ft) contain 25 additional species (Fig. 13.2A), which, along with the 33 species in sample I, constitute most of the long-ranging Chickahominy benthic foraminiferal species. In terms of species composition, then, the typical Cibicidoides pippeni Assemblage of benthic foraminifera, rich both in species and specimens, repopulated the target site at nearly full strength by approximately 36 kyr pti (by extrapolation of the sedimentaccumulation rate; pti = postimpact; Fig. 13.4). On the basis of the stratigraphic ranges of several other distinctive species, the C. pippeni Zone (or Assemblage) can be subdivided into five Subzones (or Subassemblages. The C. pippeni Zone and its five Subzones are defined as follows (Figs. 13.2A,B, 13.3, 13.4): Cibicidoides pippeni Taxon-range Biozone - That part of the Chickahominy Formation embracing the stratigraphic range of the nominate species. Cibicidoides pippeni appears to have a more extensive stratigraphic range in other localities, however, such as the US Gulf Coast and Caribbean (van Morkhoven et al. 1986).

Local Paleoenvironmental Effects

395

Fig. 13.3. Nominate species for benthic foraminiferal zone and subzones recognized in Chickahominy Formation . Scale bar = 100 urn. I Cibicidoides pippeni (Cushman and Garrett), 1938. Exmore corehole ; umbil ical view. 2 Bulimina jacksonensis Cushman, 1925b. Exmore corehole ; lateral view. 3 Lagenoglandulina virginiana (Cushman and Cederstrom), 1949. Newport News corehole ; lateral view. 4 Uvigerina dumblei Cushman and Applin , 1926. Exmore corehole; lateral view. 5 Bolivina tectiformis Cushman, 1926. Exmore corehole; lateral view. 6 Bathysiphon sp. Kiptopeke corehole ; lateral view.

Bulimina jacksonensis Interval Subbiozone - That part of the Chickahominy Formation embracing the partial stratigraphic range of the nominate species (Fig. 13.3) between its lowest occurrence and the lowest occurrence of Lagenoglandulina virginiana. Lagenoglandulina virginiana Interval Subbiozone - That part of the Chickahominy Formation embracing the partial stratigraphic range of the nominate species (Fig. 13.3) between its lowest occurrence and the lowest occurrence of Uvigerina dumblei.

396

Biospheric Effects of Chesapeake Bay Impact

Subzone or Zone

Duration Boundary Depth & (kyr) Postimpact Age (pti) 327.7 m =2.1 myr -

Bolivina tectiformis

270 332.0 m =1.83 myr-

Uvigerina dumb/ei

538 348.2 m =1.29 myr-

Lagenoglandulina virginiana

719 360.8 m =573 kyr -

Bulimina jacksonensis 573 ,. ........ _-_ .. _._--- -_ ........... ·370.3 m =355.5 kyr--:Bathysiphon 353 ~FUII recovery@ -36 kyr pt]) , 394.0 m =3 kyr (max.) Dead Zone <1-3 000.0 m =Impact Fig. 13.4. Benthic foraminiferal subzones in Chickahominy Formation at Kiptopeke site, their duration in thousands of years (kyr) and the depths and postimpact (pti) ages of their boundaries.

Uvigerina dumblei Interval Subbiozone - That part of the Chickahominy Formation embracing the partial range of the nominate species (Fig. 13.3) between its lowest occurrence and the lowest occurrence of Bolivina tectiformis. Bolivina tectiformis Taxon-range Subbiozone - That part of the Chicka hominy Formation embracing the tota l range of the nominate species (Fig. 13.3). Bathysiphon Abundance Subbiozone - That part of the Chic kahominy Formation at the base of the Bulimina jacksonensis Subzone, wh ich contains the peak development of an agglutinated suite of benthic forami nifera (Fig . 13.5), in which Bathysiphon sp. is a notable (persistent and relatively abundan t) constituent. The oldest subasse mblage of calcareous taxa is the Bulimina jacksonensis Subassem blage, which is present in samp les 1-22 at Kiptopeke (Figs. 13.2A,B, 13.4; Table 13.4). Representatives of 42 species in this subassemblage are persistentl y prese nt and(or) abund ant in the Bulimina jacksonensis Subzo ne (Interval Subzone). The Bulimina jacksonensis Subasse mblage is prese nt throughout planktonic foram iniferal Zone PI S, and precedes the appearance of Lagenoglandulina virginiana and Hanzawaia blanpiedi at Kipto peke.

Fig. 13.5. Agglutinated benthic foraminiferal species characteristic of the Bathysiphon Subassemblage of the Chickahominy Formation at the Kiptopeke core site. Scale bar = 200 lim. Note prominent sponge spicu les in specimens 4,9, 17, 18. 1-6 Ammobaculites spp. 7-9 Bathysiphon spp. 10-12 Cribrostomoides spp. 13 Cyclammina cancellata Brady, 1879. 14- 17 Reophax spp. 18 Genus and species indeterminate. 19 Technitella sp.

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Table 13.4. Prominent calcareous benthic foraminiferal species of Bulimina jacksonensis Subassemblage. Species listed (64) are those whose specimens are persistently present and(or) abundant or are restricted (or nearly so) to this subassemblage. Alabamina midwayensis Amphimorphina "planata" Anomalinoides "kiptopekensis" Bolivina gracilis Bolivina jacksonensis Bolivina "praevirginiana" Bolivina striatella Bolivina virginiana Bulimina jacksonensis Caucasina marylandica Charltonina madrugaensis Chilostomella oolina Cibicidoides "chickahominyanus" Cibicidoides cocoaensis Cibicidoides pippeni f. speciosus Discorbitura sp. Dentalina sp. Epistominella minuta Fursenkoina danvillensis Globobulimina ovata Globocassidulina subglobosa Globulina gibba Grigelis "e!ongata" Grigelis annulospinosa Grigelis "tubulosa" Guttulina hantkeni Guttulina irregularis Gyroidinoides aequilateralis Gyroidinoides byramensis Gyroidinoides planatus Hoeglundina elegans Kaleshnikovella sp.

Lenticulina americana f. typica Lenticulina americana f. spinosa Lenticulina "carinata" Lenticulina crassilimbata Lenticulina virginiana Loxostomina vicksburgensis f. spinosa Marginulina cocoaensis Marginulina karreriana Marginulina splendens Massilina decorata Melonis planatus Nodosaria capitata Nodosaria cooperensis Oridorsalis umbonatus Parafrondicularia cookei Proxyfrons virginiana Pseudonodosaria virginiana Pseudouvigerina sp. Pullenia bulloides Pyramidina sp. Saracenaria hantkeni Sigmoidella plummerae Spiroplectinella mississippiensis Stilostomella "aduncocostata" Stilostomella cocoaensis Stilostom ella cookei Stilostomella "exilispinata" Stilostomella 'juvenocostata" Stilostomella "multispiculata" Turrilina robertsi Uvigerina gardnerae Vaginulina "barbulata"

The stratigraphically next highest significant species event is a faunal turnover documented in sample 23 at 361.2 m (1185 .0 ft; Figs. 13.2A,S, 13.4). At this level, eight new calcareous species first appear, and ~ 10 species disappear from the section (some exit only temporarily, however) . The most distinctive new taxa of this group are Lagenoglandulina virginiana (Fig. 13.3) and Hanzawaia blanpiedi, both of which persist in most samples from this stratigraphic level to the top of the Chickahominy Formation (Fig. 13.2 A,B). We call this the Lagenoglandulina virginiana Subassemblage (Table 13.5), because this taxon is represented in more samples than any other species in the subassemblage (Figs. 13.2A,B, 13.4). This faunal turnover took place at ~3 5 .3 Ma, approximately 573 kyr pti (Table 13.1), and coincided with: (I) a turnover in planktonic foraminifera, which defines the P l6/P 15 boundary (Berggren et al. 1995); and (2) the boundary between the Bolboforma latdorfensis and B. spinosa zones (Poag and Aubry 1995). The strati-

Local Paleoenvironmental Effects

399

graphic position of this turnover approximates the level at which the sediment accumulation rate drops from 67 to 10 m/myr (Fig. 13.1).

Table 13.5. Prominent benthic foraminiferal species of Lagenoglandulina virginiana Subassemblage. Species listed (55) are those whose specimens are persistentlypresent and(or) abundantor are restricted (or nearlyso) to this subassemblage. Amphimorphina "fragilicostata" Anomalinoides bilateralis Bolivina gardnerae Bolivina jacksonensis Bolivina "postvirginiana" Bolivina virginiana Bulimina alazanensis Bulimina jacksonensis Caucasina marylandica Ceratobulimina perplexa Charltonina madrugaensis Cibicidoides cocoaensis Cibicidoides mexicanus Cibicidoides ouachitaensis Cibicidoides pippeni f. speciosus Dentalina sp. Epistominella minuta Gaudryina alazanensis Globobulimina ovata Globocassidulina subglobosa Globulina gibba Grigelis annulospinosa Grigelis "tubulosa" Guttulina hantkeni Guttulina irregularis Gyroidinoides aequilateralis Gyroidinoides byramensis Gyroidinoides planatus

Hoeglundina elegans Lagenoglandulina virginiana Lenticulina americana f. typica Lenticulina "carinata" Lenticulina virginiana Loxostomina vicksburgensis f. spinosa Marginulina cocoaensis Marginulina karreriana Melonis planatus Nodosaria capitata Nodosaria cooperensis Nodosaria vertebralis Oridorsalis umbonatus Proxyfrons virginiana Pseudonodosaria virginiana Pullenia bulloides Saracenaria sp. B Sigmoidella jacksonensis Sigmoidella plummerae Siphon ina tenuicarinata Spiroplectinella mississippiensis Stilostomella cocoaensis Stilostomella cookei Stilostomella "exilispinata" Turrilina robertsi Uvigerina gardnerae Vasiglobulina tuberculata

The stratigraphically next highest significant species event takes place in sample 32 (347 .3 m; 1139.4 ft). A faunal turnover at this level features the lowest appearances of Uvigerina cookei, Uvigerina dumblei, and Cibicidoides "rugoumbonatus" (Figs . 13.2A,B, 13.3, 13.4) and the near absence of Bulimina jacksonensis, Caucasina marylandica, and Gyroidinoides byramensis, which otherwise, are present through most of the rest of the Chickahominy section. Uvigerina dumblei is the most persistent species, and gives its name to this Subassemblage (Table 13.6). The U. dumblei Subassemblage appeared at ~ 1.29 myr pti (Fig. 13.4). The stratigraphically highest significant species event takes place in sample 42 (331.6 m; 1087.9 ft), -1.83 myr pti (Figs . 13.2A,B, 13.4). Bolivina tectiformis is

400

Biospheric Effects of Chesapeake Bay Impact

Table 13.6. Prominent benthic foraminiferal species of Uvigerina dumblei Subassemblage. Species listed (52) are those whose specimens are persistently present and(or) abundant or are restricted (or nearly so) to this subassemblage. Amphimorp hina "fragilicostata" Bolivina byramensis Bolivina j acksonensis Bolivina "postvirginiana" Bulimina cooperensis Cibicidoides mexicanus Cibicidoides pipp eni f. speciosus Cibicidoides sculpturatus Dentalina sp. Epistominella minuta Frondovaginulina tenuissima Globobulimina ovata Globocassidulina subglobosa Globulina gibba Grigelis "elongata" Grigelis annulosp inosa Grigelis "tubulosa" Guttulina hantkeni Guttulina irregularis Gyroidinoides aequilateralis Gyroidinoides planatus Hanzawaia blanpiedi Lagenoglandulina virginiana Lenticulina americana f. typica Lenticulina americana f. spinosa Lenticulina "carinata"

Lenticulina crassilimbata Lenticulina virginiana Loxostomina vicksburgensis f. spinosa Marginulina cocoaensis Marginulina karreriana Marginulina sp lendens Melon is planatus Nodosaria capitata Nodosaria coope rensis Nodosaria vertebralis Oridorsalis umbonatus Proxyfrons virginiana Pullenia bulloides Sigmo idella j acksonensis Sigmoidella plummerae Siphon ina jacksonensis Siphonina tenuicarinata Spiroplectinella mississippiensis Stilostomella cocoaensis Stilostomella cookei Stilostomella "exilispinata" Uvigerina dumblei Uvigerina ga rdnerae Uvigerina jacksonensis Vaginulina "barbulata" Vasiglobulina tuberculata

the most prominent new taxon to appear, and gives its name to this Subassemblage (Fig. 13.3; Table 13.7). The appearance of B. tectiformis is accompanied by the first appearance of Loxostomina vicksburgen sis f. reticulata, the persistence of BuIiminellita curta, and the reappearance of Bulim ina jacksonen sis, Caucasina marylandica, and Gyroidinoides byramen sis. In summary, a species -rich, but slight ly underdeveloped Cibicidoides p ippeni Assemblage was reestablished at Kiptopeke, < 1-3 kyr after the impact (Fig. 13.4). This Assemblage reached full postimpact recovery - 36 kyr pti, and subsequently, three faunal turnovers took place among the Chickahominy calcareous benthic foraminifera (spaced - 0.6 myr apart) : at 573 kyr, 1.29 myr, and 1.83 myr pti. Nearly all species that constitute the Cibic idoides pippeni Assemblage disappear from the Virginia Coastal Plain record at the unconformable Eocene/Oligocene bounda ry, to be replaced at the crater core sites by a relat ively specie s-poor assemblage characterized by an abundance of large specimens of Uvi geri na vicks burgens is .

Local Paleoenvironmental Effects

40I

Table 13.7. Prominent benthic foraminiferal species of Bolivina tectiformis Subassemblage. Species listed (61) are those whose specimens are persistently present and(or) abundant or are restricted (or nearlyso) to this subassemblage. Bolivina multicostata Bolivina "postvirginiana" Bolivina regularis Bolivina tectiformis Bulimina jacksonensis Buliminellita curta Cassidulinoides sp. Caucasina marylandica Char/ton ina madrugaensis Cibicidoides pippeni f. speciosus Cibicidoides sculpturatus Dentalina sp. Epistominella minuta Globobulimina ovata Globocassidulina subglobosa Globulina gibba Grigelis "elongata" Grigelis annulospinosa Grigelis "tubulosa" Guttulina hantkeni Guttulina irregularis Gyroidinoides aequilateralis Gyroidinoides byramensis Gyroidinoides octocameratus Gyroidinoides planatus Hanzawaia blanpiedi Hopkinsina danvillen sis Lagenoglandulina virginiana Lenticulina americana f. typica Lenticulina virginiana Loxostomina vicksburgensis f. spinosa

Loxostomina vicksburgensis f. reticulata Marginulina cocoaensis Marginulina karreriana Marginulina splendens Melonis planatus Nodosaria capitata Nodosaria cooperensis Nodosaria vertebralis Nonionella jacksonens is Oridorsalis umbonatus Palmula sp. Parafrondicularia cookei Proxyfrons virginiana Pullenia bulloides Quadrimorphina sp. Saracenaria hantkeni Sigmoidella jacksonensis Sigmoidella plummerae Siphon ina jacksonensis Siphon ina tenuicarinata Spiroplectinella mississippiensis Stilostomella "aduncocostata" Stilostomella cocoaensis Stilostomella cookei Stilostomella "exiiispinata" Stilostomella "multispiculata" Uvigerina gardnerae Uvigerina dumblei Vaginulina "barbulata" Vaginulinopsis jacksonensis

13.1.2.3 Bathysiphon Subassemblage

Poag (1997a) noted that another important stratigraphic feature of the Chickahominy foraminiferal association in the Kiptopeke core is the presence of a distinctive subassemblage of agglutinated benthic species (tests built of sediment grains derived from the seafloor) superimposed on the lower two-thirds of the B. jacksonensis Subassemblage in samples 1-17 (393.9-373 .5 m; 1292.3-1225.3 ft). This agglutinated subassemblage characterized the first ~355.5 kyr of postimpact deposition at the Kiptopeke site (Figs. 13.2A,B, 13.4, 13.5). The agglutinated subassemblage consists of representatives of 11 species, the stratigraphically most persistent of which are Bathysiphon sp., Cribrostomoides sp., and Cyclammina cancellata; we call this the Bathysiphon Subassemblage (Fig. 13.2A,B, 13.4, 13.5;

402

Biospheric Effectsof Chesapeake Bay Impact

Table 13.8. Agglutinated benthic foraminiferal species (II) of Bathysiphon Subassemblage. Ammobaculites sp. Bathysiphon sp. Bermudezina sp. Cribrostomoides sp. Cyclammina cancel/ata Dorothia sp.

Pseudoclavulina sp. Reophaxsp . Sprioplectinel/a mississippiensis Technitel/a sp. Textulariavirginiana

Table 13.8). Of these agglutinated species, only one, Spiroplectinella mississippiens is, is represented consistently above sample 17 (373.5 m; 1225.3 ft). Spiroplectinella mississippiensis, in fact, persists to the top of the Chickahominy Formation. At Kiptopeke, Gaudryina alazanensis (Fig. 13.2A,B) is a distinctive agglutinated form in the middle of the Lagenoglandulina virginiana Subzone , but is absent in stratigraphically higher subassemblages.

13.1.3 Community Structure of Benthic Foraminiferal Associations We have used three attributes of community structure to help interpret the paleoenvironmental implications of the Chickahominy benthic foraminiferal associations: (1) generic predominance; (2) generic equitability; and (3) species richness.

13.1.3.1 Predominance and Equitability Species predominance (the species whose members constitute the greatest relative abundance in an assemblage) and species equitability (a measure of the uniformity of predominance) are properties widely used to evaluate the paleoecology of benthic organisms, including foraminifera (Dodd and Stanton 198 I; Hayek and Buzas 1997). Poag (1981) demonstrated that generic predominance tends to parallel or mimic species predominance among benthic foraminifera and, thus, can be an important foraminiferal community property for assessing the paleoenvironments represented in Cenozoic marine strata. Likewise, generic equitability can be substituted for species equitability. We have, therefore, evaluated generic predominance and equitability among the Chickahominy benthic foraminifera at Kiptopeke. (We use equitability to mean simply the number of predominant or copredominant genera in a given sample.) We designate sole predominance if there is a spread of 4% or more between a given genus and other genera in the sample. A spread of <4% would constitute co-predominance (Fig. 13.6). In samples in which no single species was solely predominant or clearly clustered, we designate co-predominance to those species whose relative abundances were within 2% of each other (e.g., samples 4,8,41,46; Fig. 13.6). The predominant benthic genus in sample I (393.9 m; 1292.3 ft; 3--4.5 kyr pti) is Bolivina (34% ; Figs. 3.6-13.8) . This strong predominance produces a generic

Local Paleoenvironmental Effects

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404

BiosphericEffects of Chesapeake Bay Impact

equitability value of I for this sample. Many modem and fossil species of Bolivina are well known as opportunists - species whose populations have the capacity to rapidly increase in absolute and relative abundance whenever appropriate environmental conditions arise. This opportunism is expressed at the base of the Chickahominy Formation by the fact that Bolivina is so strongly predominant in sample 1 (the next most abundant genus represents only 19% of the sample), yet is quite sparse «1-3%) in samples 2-6 (Figs. 13.6-13.8). The community structure

Local Paleoenvironmental Effects

405

changes dramatically in sample 2 (393.7 m; 1291.7 ft), in which Gy roidin oides assumes sole predominance (20%; Figs. 13.6-13 .8). Generic equitability remains low at a value of 1. Samples 3- 9 (392.2- 387.1 m; 1286.8-1 270.2 ft) represent an interval of marked co-predominance in which Gy roidinoides, Grigelis, Globobulimina, Uvigerina, Bulim ina, Caucasina, and Bolivina generally share similar relative abundances (8- 18%), and generic equitability ranges from I to 5 (Figs. 13.613.8).

In samples 10-25 (384.7-357.8 m; 1262.2-1173.9 ft), generic predominance shifts back and forth between principally Caucasina , Bolivina, and Ep istominella, and generic equitability is mainly I (Figs. 13.6-13.8). Epistominella expresses an opportunistic tendency in sample 23 (361.2 m; 11 85.0 ft), where it is unusually abundant at 39% (the next most abundant genus represents only 12%). Samples 26-28 (356.3- 354.4 m; 1169.0-1162.7) represent a transitional interval: samples 26 and 27 are dominated by Uvigerina (23% each; equitability = I); sample 28 is co-predominated by Grigelis (II %), Stilostomella (10%), and Gy roidinoides (8%) (equitability = 3; Figs. 13.6, 13.8). Samples 29--46 (352.6-328.0 m; 1156.7-1076.1 ft) represent a thick interval in which predominance shifts frequently between mainly Bolivina, Uvigerina, and Epistominella ( 10-38%) and equitability is mainly I or 2 (Figs. 13.6-13.8). Grigelis returns to co-predominance in sample 46 (10%; equitability = 4), and is singularly predominant (20%) in sample 47. Another way to view the stratigraphic changes in generic predominance is to examine the vertical abundance patterns of individual genera (Fig. 13.8). We see clearly the great relative abundance of Globobulimina, Bulim ina, and Caucasina in the Bulimina jacksonensis Subzone versus their scarcity higher in the section. Stilostomella and Globocassidulina have an inverse distribution pattern, being least abundant in the Bulimin a jacksonensis Subzone. Gyroidinoides and Grigelis are moderately abundant throughout the Chickahominy section. Uvigerina, Bolivina, and Epis tominella are generally relatively abundant, but all display one or more isolated peaks of >30% abundance, a reflection of their opportunistic life strategy. Fig. 13.7. (Opposite page) Important species from Chickahominy Formation used for paleoenvironmental interpretations. Scale bar = 100 urn. 1 Epistominella minuta Olsson, 1960. Exmore corehole; umbilical view. 2 Charltonina madrugaensis (Cushman and Bermudez), 1948. Exmore corehole, umbilical view. 3 Gyroidinoides aequilateralis (Plummer), 1927. Exmore corehole; umbilical view. 4 Gyroidinoides byram ensis (Cushman and Todd), 1946. Exmore corehole; umbilical view. 5 Uvigerina gardnerae Cushman, 1926. Exmore corehole; lateral view. 6 Caucasina marylandica (Nogan), 1964. Exmore corehole; lateral view. 7 Bolivina gracilis Cushman and Applin, 1926. Exmore corehole; lateral view. 8 Bolivina virginiana Cushman and Cederstrom, 1949. Exmore corehole; lateral view. 9 Bolivina "praevirginiana." Exmore corehole; lateral view. 10 Globobulimina ovata (D'Orbigny), 1846. Exmore corehole; lateral view. 11 Grigelis cookei (Cushman), 1933. Exmore corehole; lateral view, final two chambers. 12 Grigelis annulospinosa (Bandy), 1949. Exmore corehole, lateral view. 13 Stilostomella cocoaensis (Cushman), 1925b. Exmore corehole; lateral view. 14 Stilostomella "exilispinata." Exmore corehcle; lateral view.

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13.1.3.2 Species Richness

Species richness (the number of species represented in a sample; a measure of diversity; Dodd and Stanton 1981) is another attribute of community structure that helps to interpret the Chickahominy paleoenvironments. Species richness among the Chickahominy benthic foraminifera at Kiptopeke varies from a minimum of 25 in sample 35 (343 m; 1125.3 ft) to a maximum of 60 in sample 23 (base of Zone P16) and in sample 47 (Fig. 13.9). When we plot species richness on a time scale, three major cycles of low-to-high species richness can be recognized at Kiptopeke (Fig. 13.10). Cycle I lasted - 600 kyr, and is nearly coincident with Zone PI5 and the Bulimina jacksonens is Subzone. Species richness Cycle 2 lasted - 800 kyr, and encompasses the Lagenoglandulina virginiana Subzone and the lowest three samples in the Uvigerina dumblei Subzone (Fig. 13.10). Cycle 3 embraces the final - 600 kyr of the late Eocene at Kiptopeke, and includes most of the Uvigerina dum blei Subzone and all of the Bolivina tectiformis Subzone. The boundaries of each of these three richness cycles correlate approximately, but not precisely, with the major shifts in sediment accumulation rate at Kiptopeke (Fig. 13.10). The imprecision of the correlations is most notable for the two major reductions in species richness at the base of richness Cycles 2 and 3. The species richness shifts lagged the changes in sediment accumulation rate by 100-300 kyr. Further study of additional Chickahominy sections in the other Chesapeake Bay coreholes will help to clarify the reasons for this lag effect. 13.1.3.3 Paleoenvironmental Interpretations

Samples from outside the Chesapeake Bay crater show that rich, deep-water benthic foraminiferal assemblages occupied the target site and surrounding seafloor biotopes prior to the bolide impact. As we showed in Chapters 6 and 7, the transition from synimpact to postimpact deposition began with a 1.5-5 cm fallout layer followed by a dead zone, the latter of which represents no more than - 3 kyr of postimpact deposition (Fig. 13.4). Subsequent samples show that initial repopulation of the impact site rapidly renewed the rich preimpact benthic assemblage (Tables 13.1-13.8; Figs. 13.2, 13.4, 13.9). This distinctive assemblage (Cibicidoides pippeni Assemblage) recovered fully by 36 kyr pti, vigorously persisted for the remaining - 2.1 myr of the late Eocene, and underwent three moderate faunal turnovers before experiencing a major turnover at the unconformable EoceneOligocene contact. The persistence of the basal Bathysiphon Subassemblage to 355 kyr pti may reflect lingering effects of the impact on paleoenvironments inside the crater. Fig. 13.8. (Opposite page) Polygon plot of stratigraphic variability in relative abundance for individual predominant genera in benthic foraminiferal communities in Chickahominy Formation at Kiptopeke site. Scale should be read as the sum of percent shown on each side of zero (e.g., in sample I, the percentage of Bolivina is 17 plus 17 = 34). Numbered polygons (31-39) indicate all relative abundances greater than 30, which are values associated with blooms of opportunistic species. Heavy dashed line is P15/P16boundary. Cycles 1- 3 refer tocycles of species richness displayed in Fig. 13.10.

408

Biospheric Effects of Chesapeake Bay Impact

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Local Paleoenvironmental Effects

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410

Biospheric Effects of Chesapeake Bay Impact

The reliability of our assessment of the late Eocene paleoenvironments rests in large part on current knowledge of the environmental limits and preferences of counterpart species and genera in the modern oceans. Several syntheses and reviews of modern foraminiferal ecology provide guidance in this assessment (e.g., Poag 1981; Culver and Buzas 1980, 1981, 1982; Sen Gupta 1999). Our paleoenvironmental interpretations focus mainly on five factors that strongly influence the distribution of modern foraminiferal populations: (I) seafloor physiography; (2) substrate sedimentology; (3) microhabitat; (4) bottom-water chemistry; and (5) nutrient supply. By inference, these factors also were significant for their late Eocene analogues, and, therefore , are reflected in the composition of the Chickahominy assemblages. 13.1.3.3.1 Seafloor Physiography The NASA Langley and Kiptopeke core sites, prior to impact, occupied the middle part of a broad, gently sloping continental shelf (see Chapter I). But after impact, of course, the two sites were inside the crater, a partly filled, subcircular excavation, whose upper surface formed a depression or closed basin in the seafloor. Presumably, the depression was somewhat deeper in the center than along the periphery, but the precise relative relief remains to be determined. The Chickahorniny Formation is also present at sites outside the crater rim, such as Windmill Point and Newport News . Preliminary examination of the extracrater Chickahorniny assemblages indicates slightly shallower paleodepths than inside the crater, but quantitative analyses of the extracrater assemblages have not been completed. Nearly all the Chickahominy species at Kiptopeke have modern counterparts (in fact, some are still extant), which are most abundant (individually and in similar species associations) in outer neritic to upper bathyal marine biotopes (150500 m water depths; Table 13.9; Charietta 1980; Poag 1981; van Morkhoven et al. 1986). Many of these species (such as Bulimina jacksonensis, Cassidulina tenui-

carinata, Hoeglundina elegans, Turrilina robertsi, Bolivina byramensis, Stilostomella spp.) also occur in other Paleogene outer neritic-bathyal deposits (Beckman 1954; Tjalsma and Lohmann 1983; van Morkhoven et al. 1986). We infer a paleodepth of - 300 m for the Chickahominy assemblage at Kiptopeke. Preliminary semiquantitative analyses of the Chickahominy benthic suites at the NASA Langley site have recently been completed (Poag and Norris in press). The benthic foraminiferal suites at NASA Langley closely resemble those at Kiptopeke (both in species composition and relative species abundance), and, thereby, indicate similar paleoenvironments to those documented herein at Kiptopeke. 13.1.3.3.2 Substrate Sedimentology The sediments occupied by the Chickahominy benthic foraminiferal communities were soft, fine-grained muds (mainly micaceous, silty to sandy clay). Chickahorniny fossil suites (mainly microfossils, echinoid spines, thin-shelled clams, and burrow casts of invertebrate organisms) indicate that these substrates and overlying marine watermasses supported abundant populations of benthos (foraminifera, os-

Local Paleoenvironmental Effects

411

tracodes, echinoids, ophiuroids, solitary corals, bivalves, scaphopods, and, occasionally, siliceous sponges), plankton (foraminifera, calcareous nannofossils, radiolarians, diatoms, dinoflagellates, bolboformids), and nekton (fish). The common presence of filamentous organic detritus and pollen grains still in the sediments indicates an abundant supply of terrigenous organic carbon during Chickahominy deposition. The dark color of the Chickahominy clays, and the abundance of pyrite (as framboidal aggregates, burrow casts, thin wafer-like crusts, irregular nodules, and frequent replacements of shell material in many of the fossil groups) indicate that sulfate-reducing conditions commonly existed below the sediment-water interface. 13.1.3.3.3 Microhabitats

Modem benthic foraminifera have been assigned to different microhabitats, mainly according to the depth at which they are most abundant in the substrate (Corliss 1985, 1991; Gooday 1986; Rathburn and Corliss 1994; Jorissen et al. 1995, 1998; Jorissen 1999). Such microhabitats are present consistently in outer neritic, bathyal, and abyssal marine settings, but are not well developed in coarsergrained middle neritic and inner neritic settings (Murosky and Snyder 1994; Lueck and Snyder 1997). These vertically separated deep-water microhabitats are further characterized by their ambient physical, chemical, and biological properties, such as oxygen content, food supply, toxic substances, and potential for interactions with other organisms. The shallowest microhabitat is occupied by epifauna (forms at or protruding above the sediment-water interface) . Next deepest is the shallow infauna, which constitutes the uppermost 2 em of the substrate (Lutze and Thiel 1989; Corliss 1991; Buzas et al. 1993; Gooday 1994; Jorissen 1999). Generally, the epifaunal and shallow infaunal microhabitats are relatively well oxygenated and receive a relatively rich supply of labile, easily metabolizable organic detritus . In the intermediate (2-4 ern depth) and deep (4-10 em depth) foraminiferal microhabitats, oxygen values generally decrease downward as the organic detritus becomes progressively more refractory and difficult to metabolize. 13.1.3.3.4 Bottomwater Chemistry

Most of the predominant genera and species in the Chickahominy benthic foraminiferal assemblages have modem counterparts notable for their opportunistic life strategies, and their tolerance of, or preference for, oxygen-depleted (disoxic, microxic, anoxic) muds rich in organic detritus. Among the best documented of these modem taxa are the calcareous genera that predominate in the Cibicidoides pippeni Assemblage : Epistominella, Boliv ina, Bulimina, Globobulimina, Globocassidulina, Uvigerina , and Buliminella (counterpart to Caucasina) (Phleger and Soutar 1973; Douglas and Heitman 1979; Mackensen and Douglas 1989; Kaminski et al. 1995; Jorissen et al. 1992; Sen Gupta et al. 1996; Bernhard and Sen Gupta 1999; Loubere and Fariduddin 1999; Table 13.9). Most of the members of the Chickahominy Bathysiphon Subassemblage also are typical inhabitants of oxygen-depleted, nutrient-rich substrates (Gooday 1994; Kaminski et al. 1995).

412

Biospheric Effects of Chesapeake Bay Impact

Table 13.9. Benthic foraminiferal species used for interpretation of Chickahominy paleoenvironments at Kiptopeke and NASA Langley core sites . Species

Test Construction

Ammoba culites sp. agglutinated

Microhabitat Oxygen! Nutrient Tolerance infaunal low/high

calcite

infaunal

low/high

calcite

infaunal

low/high

Bathysip hon sp.

agglutinated

epifaunal

low/high

Bolivina byramensis Boliv ina gardnerae Boliv ina gracilis

calcite

IS

infaunal

low/high

calcite

s infaunal

low/high

calcite

2i_J d infaunal low/high

Amphimorphina

"fragilicostata" Amphimorphina

"planata"

Bolivina calcite jacksonensis Bolivina calcite multicostata Bolivina plicatella calcite

i---
low/high

s infaunal

low/high

s infaunal

low/high

calcite

s infaunal

low/high

calcite Bolivina "preavirginiana" Bolivina regularis calcite

s infaunal

low/high

i---
low/high

Bolivina striatella calcite

s infaunal

low/high

calcite

s infaunal

low/high

calcite

s infaunal

low/high

calcite

i---
low/high

calcite

i---
low/high

calcite

i---
low/high

calcite

phytodctrital low/high

calcite

d infaunal

low/high

calcite

epifaunal

high/low

Bolivina

"postvirginiana"

Bolivina tectiformis Bolivina virginiana Bulimina alazanensis Bulimina cooperensis Bulimina jacksonensis Caucasina marylandica Charlton ina madrugaensis Cibicido ides p ippeni

Preferred Paleodepth

Opportunist

outer neritic-upper bathyal outer neritic-uppe r bathyal outer neritic-upper bathyal Bathyal-abyssal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic- upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upp er bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic- upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic- upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal

yes

yes

yes

yes

yes yes yes

Local Paleoenvironmental Effects

413

Table 13.9. (cont.) Species

Cribrostomoides

Test Microhabitat Oxygen! Construction Nutrient Tolerance agglutinated s infaunal low/high

sp.

Preferred Paleodepth outer neritic-upper bathyal

Cyclammina cancellata Dorothia sp.

agglutinated

s infaunal

low/high

outer neritic-upper bathyal

agglutinated

d infaunal

low/high

Epistominella minuta

calcite

epifaunal

low/high

Gaudryina alazanensis Globobulimina ovata

agglutinated

d infaunal

low/high

outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal

aragonite?

i-d infaunal

low/high

Globocass idulina subglobosa Grigelis cookei

calcite

phytodetrital

low/high

calcite

infaunal

low/high

Grigelis

calcite

infaunal

low/high

"curvicostata" Grigelis "elongata"

calcite

infaunal

low/high

Grigelis

calcite

infaunal

low/high

"elongostriata'' Grigelis "gigas"

calcite

infaunal

low/high

Grigelis "tubulosa"

calcite

infaunal

low/high

Grigelis "tumerosa"

calcite

infaunal

low/high

Gyroidinoides aequilateralis Gyroidinoides byramensis Gyroidinoides octocameratus Gyroidinoides planatus Hoeglundina elegans Marginulina cocoaensis Marginulina karreriana Melonis planatus

calcite

s infaunal

low/high

calcite

s infaunal

low/high

calcite

s infaunal

low/high

calcite

s infaunal

low/high

aragonite

epifaunal

low/high

calcite

infaunal

low/high

outer neritic-upper bathyal

calcite

infaunal

?I'!

outer neritic-upper bathyal

calcite

i-d infaunal

low/high

calcite

infaunal

low/high

outer neritic-upper bathyal outer neritic-upper bathyal

Nodosaria capitata

Opportunist

outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic- upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic- upper bathyal outer neritic-upper bathyal

yes

414

Biospheric Effectsof Chesapeake Bay Impact

Table 13.9. (cont.) Species

Test Construction

Microhabitat

Nodosaria pustulosa

calcite

infauna l

Oxygen! Nutrient Tolerance low/h igh

Nodosaria saggitula

calcite

infaunal

low/h igh

Nodosaria soluta

calcite

infaunal

lowlh igh

Nodosaria vertebralis calcite

infaunal

low/high

Oridorsalis umbonatu s Reophax sp.

calcite

epifaunal

low/high

agglutinated

i-d infaunal

low/high

Spirople ctinella mississippiensis Stilostomella "aduncocostata" Stilostomella annulosp inosa Stilostom ella ''bicostatus'' Stilostom ella cocoaensis Stilostomella "exilispinata" Stilostom ella 'juvenocostata" Stilostomella "multispiculata" Technitella sp.

agglutinated

d infaunal

low-high

calcite

infauna l

low/high

outer neritic-upper bathyal

calcite

infaunal

low/high

outer neritic-upper bathyal

calcite

infaunal

low/high

outer neritic-upper bathyal

calcite

infaunal

low/h igh

outer neritic-upper bathyal

calcite

infaunal

low/high

outer neritic-upper bathyal

calcite

infaunal

low/high

outer neritic-upper bathyal

calcite

infaunal

low/high

outer neritic- upper bathyal

agglutinated

infaunal

low/high

Turrilina robertsi

calcite

infaunal

lowlh igh

Uvigerina cookei

calcite

s infaunal

lowlhigh

Uvigerina dumblei

calcite

s infaunal

low/high

Uvigerina gardnerae calcite

s infaunal

low/high

calcite Uvigerina jacksonensis Uvigerina spini costata calcite

s infaunal

low/high

outer neritic- upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal

s infaunal

low/high

's = shallow depth (0-2 em) below sediment-water interface 2i = intermediate depth (2--4 cm) below sediment-water interface 3d = deep (4-10 cm ) below sediment-water interface

Preferred Paleodepth

Opportun ist

outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal outer neritic-upper bathyal inner neritic-upper bathyal outer neritic-upper bathyal

outer neritic-upper bathyal

yes

Local Paleoenvironmental Effects

415

13.1.3.3.5 Nutrient Supply

Outer neritic, bathyal, and abyssal benthic foraminifera, as a whole, depend upon the flux of labile organic carbon for their food source (Gooday 1994; Loubere and Fariduddin 1999). There is considerable evidence from modern oceans that the geographic distribution, test size, and abundance (absolute and relative) of certain benthic foraminiferal species and genera are strongly correlative with the flux of organic detritus to the seafloor (Caralp 1989; Corliss and Fois 1990; Corliss and Silva 1993; Pfannkuche 1993; Linke et al. 1995; Gooday 1996; Morigi et al. 2001). In particular, most of the predominant Chickahominy calcareous genera (and those of the Bathysiphon Subassemblage) have modern counterparts that are most abundant, and often have largest test sizes, in organic-rich muds, which are often also oxygen-depleted. Of special note in the Chickahominy assemblage is an association of small, smooth, thin-walled, hyaline, opportunistic genera, such as Epistominella, which in modern oceans live epifaunally on the seafloor within aggregates of phytodetritus (a gelatinous matrix containing the remains of phyto- and zooplankton; Gooday 1993, 1994). These species have opportunistic feeding strategies, and grow explosively into large concentrations during peak development of phytodetritus. Among the predominant Chickahominy taxa, species of Epistomin ella, Caucasina, and Globocassidulina, are probablyrepresentative of this lifestyle (Table 13.9). 13.1.3.3.6 Paleoenvironmental Summary

Overall, then, the Chickahominy benthic foraminiferal associations documented in the Kiptopeke corehole represent consistently diverse, species-rich communities living within the upper 10 em of fine-grained substrates, in paleodepths of ~3 00 m, generally typified by high flux rates of organic carbon and by oxygen deficiency. The common presence of burrow casts, however, indicates that the bottom waters and surficial sediments were not anoxic. The development of five stratigraphically successive subassemblages and several shifts in generic predominance, equitability, and species richness point to marked temporal changes in environmental properties other than paleodepth. These changing properties we infer to include sediment delivery rates, prominent-to-subtle changes in substrate chemistry and nutrient flux, and broad-scale shifts in climate (from the 8180 record) and the global carbon budget (from the 813C record; Fig. 13.10). At the Kiptopeke site, the initial postimpact benthic foraminiferal association in Chickahominy sample I contains 33 species, is solely dominated numerically by opportunistic Bolivina (34%), and is associated with pyrite crusts and burrow casts, silica diagenesis, and the agglutinant suite that comprises the Bathysiphon Subassemblage. These characteristics distinguish this sample from all subsequent samples (Bolivina reaches >30 % in three other samples, but in none of these is it associated with the Bathysiphon Subassemblage). Thus, the initial postimpact benthic foraminiferal community at Kiptopeke occupied the floor of an enclosed basin rich in organic detritus, which depleted the dissolved oxygen content both at the sediment-water interface (indicated by Bathysiphon) and within the shallow infaunal microhabitat (indicated by Bolivina). In addition, the living Chickahominy

416

Biospheric Effects of Chesapeake Bay Impact

assemblages occupied a basin below which shallowly buried impact breccia contained elevated interstitial salinities. Groundwater salinity as high as 25,000 mg/L characterizes the Exmore breccia at the Kiptopeke site (see Chapter 14 for further discussion). Moreover, this brine probably was warmer than surrounding bottom waters, because of its proximity to superheated crystalline basement rocks, whose elevated temperatures may have endured for 1 myr pti (Chapter 12). These early postimpact conditions lasted only for -2 kyr, because in sample 2, the benthic foraminiferal association changed significantly. Bolivina was reduced to 2%, and generic predominance shifted to Gyroidino ides, probably reflecting a reduction in the flux rate of organic carbon. Gyroidinoides remained one of several co-predominant genera during most of the next -120 kyr, sharing its abundance with Grigelis, Globobulimina, Uvigerina, Bulimina, and eventually joined again by Bolivina (Fig. 13.6). This association represents full recovery of the Chickahominy benthic foraminiferal community following the impact, which was reached by the time the sediments of sample 4 had accumulated. These microfaunal relations were briefly interrupted by the sole predominance of Caucasina (in sample 6), which presumably indicates a short period of increased phytodetritus accumulation. At -122 kyr pti, co-predominance shifted to Bolivina and Caucasina for 131 kyr, and then to Bolivina and Epistominella for 702 kyr. Both associations indicate continued oxygen depletion and abundance of organic carbon, the latter often available through phytodetrital flux. At 955 kyr pti, Uvigerina became the sole predominant form for 196 kyr, followed by one sample (-121 kyr duration) with co-predominant Gyroidinoides, Grigelis, and Stilostomella. These changes probably reflect a shift away from phytodetritus as a major source of nutrients, which 13e, which took place may be associated with initiation of the negative shift in o nearly concomitantly (Fig. 13.10; see section 13.2.4). These changes also may be partly attributable to the low sediment accumulation rate that characterized this time interval. At 1.27 myr pti, coincident with a shift to higher sediment accumulation rates, Bolivina became predominant again, and continued as predominant or copredominant genus for most of the next 800 kyr. In the early part of this interval, Bolivina shared predominance with, or was replaced by, Uvigerina; in the late part of the interval, it shared predominance with Epistominella. Grigelis, Stilostomella, and Globocassidulina joined or replaced these associations in three single-sample exceptions. We interpret the Bolivina-Uvigerina association to indicate continued oxygen depletion, but with a relatively sparsity of phytodetritus. The subsequent increase in Epistominella indicates a return to abundant phytodetritus, which may be related to the beginning of the latest Eocene subpulse of 18 warm climate (indicated by a negative shift in 0 0 ; Fig. 13.10; see section 13.2.3). At the top of the Kiptopeke Eocene section (sample 47), another faunal shift raised Grigelis to sole predominance for the only time during the entire postimpact interval. The significance of this change is not obvious to us, but it may be related 18 to the beginning of a positive shift in both 0 0 and Ol3 C isotopic values, along with a major sea-level fall, accompanied by a change in sediment composition (to

Local Paleoenvironmental Effects

417

coarse, glauconitic, quartz sand) and a shift in accumulation rate, all of which took place at the Eocene-Oligocene contact at this site (Fig. 13.10) . Poag and Norris (in press) have shown that a nearly identical biostratigraphic and paleoenvironmental record is represented in the Chickahominy foraminiferal assemblages at the NASA Langley site (borehole 233), 39 km southwest of Kip topeke (borehole 2), near the outer rim of the Chesapeake Bay crater (Fig. 1.3).

13.1.3.3.7 Comparison with Biotic Changes at K-T Boundary Several studies of benthic foraminiferal assemblages have been carried out across the impact-produced Cretaceous-Tertiary boundary (Dailey 1983; Keller 1988, 1992; Thomas 1990; Nomura 1991; Kaiho 1992; Schmitz et al. 1992; Widmark and Malmgren 1992; Kuhnt and Kam inski 1993; Cocc ioni and Ga leotti 1994; Speijer and van der Zwaan 1996). Most of the K-T study sections, however, are distal ejecta deposits, thousands of kilome ters away from their source, the Chicxulub impact site. In contrast, the Kiptopeke site is inside Chesapeake Bay crater. Three differences stand out between the K-T and Chesapeake Bay scenarios : (1) At neritic to mid-bathyal sites for the K-T boundary, major long-term lineage changes took place between the preimpact and postimpact benthic foram iniferal communities, whereas at Chesapeake Bay, the long-term postimpact comm unity (once it was reestablished) was essentially the same as the preimpact benthic community; (2) The postimpact benthic foraminiferal association at several neritic and upper bathyal K-T boundary sites represents shallower depositional facies than do the pre impact assemblages, whereas at Chesapeake Bay, the paleodepth increased following impact , because the partly filled crater formed a seafloor depression; (3) Benthic forami niferal changes across the K-T boundary are associ ated with major extinctions and turnovers in calcareous plankton and a profound productivity crisis in the pelagic ecosystem. At some K-T sites , the extinction event produced massive fluxes of organic carbon to the seafloor, whereas at other sites, reduced productivity significantly diminished the flux of organic carbon (Cocc ioni and Galeotti 1994; Brinkhuis and Zachariasse 1988; Pospichal 1994; Speijer and van der Zwaan 1996). No analogous ocean-wide pelagic crisis arose from the Chesapeake Bay impact, though there is evidence that plankton comm unities along the U.S. Atlantic margin may have been stressed (MacLeod 1990; MacLeod et al. 1990). On the other hand, it is fair to say that the presence of a dead zone subjacent to the Chickahominy Formation indicates that local bottom and water-colu mn conditions were hostile to benthic as we ll as planktonic organisms for - 1-3 kyr or less. Otherwise, there are some striking simi larities in the immediately postimpact K-T and Chesapeake Bay benthic foraminiferal associations and in the sediments that encompass them (Fig. 13.11) . A good comparative example is the outcrop at EI Kef, Tunisia (the type section for the K-T boundary, which represents outer neritic to upper bathyal paleodepths; Speijer and van der Zwaan 1996; Fig. 13.11) . The postimpact boundary clay at EI Kef is 50 em thick, is laminated, and contains pyritized and hemat itic burrow casts , much like the 19-cm thick, laminated, silty, dead zone at Chesapeake Bay. The lowest postimpact assemblage at EI Kef is

~

300 em

o

(Diagram not to scale)

E

U

~.~ W.<:>

x ~

E

Dead Zone', laminated:

-

0-

.s::U-

-u

_~ ._

<s:

0;
5-

Fig. 13.ll. Comparison of post impact recovery among benthic foraminiferal community at Kiptopeke site inside Chesapeake Bay crater with distal ejecta outcrops correlative to K-T impact at Chicxulub, Yucatan, Mexico .

o

~e

::>

o

Fallout Layer

5

2l j!l

~~-

" ""

>-

_.~

Full recovery at 36 kyr pti

Chesapeake Bay Inside crater

. "ml I - ~ I ~ •••••••••••••••••••••••••••• ~~.:~ .....-........ -_....... <J)

~ - --

Mass mortality at surface, pulse of organic detritus to seafloor, oxygen depletion

","

",

","

? Mass mortality at surface, pulse of organic detritus to seafloor, briny and(or) warm bottom water, oxygen depletion ~"

Caravaca, Spain Distal ejecta

<J)

0..

"iii

Q)

g

c

Q)

EI Kef, Tunisia Distal ejecta

p.

.[

to ~

o

~

~

otIJ

o ...., o :>"

~

:::::l

tTl

::J. n

~ :>" o

o'

to

00

.4

Possible Global Paleoenvironmental Effects

419

notable for relatively low diversity (species and genera) accompanied by a unique spike (sole predominance; 22-32%) in the abundance of Bulimina ovata (synonymous with Globobulimina ovata in the Chickahominy assemblages). This is reminiscent of the low diversity and unusual Bolivina abundance spike (34%) in the first postimpact assemblage at Chesapeake Bay. There also is a distinct increase in the abundance of several agglutinated species in the lower few centimeters of the postimpact section at EI Kef, just as at Chesapeake Bay. It should be noted, however, that the EI Kef study was based on analysis of the >250 11m size fraction, whereas we analyzed the >63 11m size fraction, which encompasses a more complete representation of the Chickahominy benthic association. Indeed, many of the predominant Chickahominy taxa, such as Grigelis, Caucasina, Bolivina, Epistominella, and Stilostomella , would have been acutely underrepresented if we had analyzed a coarser size fraction. Another good example, though representing somewhat deeper paleodepths (middle bathyal) than Kiptopeke, is the K-T boundary section at Caravaca, Spain (also based on analysis of the >250-l1m size fraction; Coccioni and Galeotti 1994; Fig. 13.11). At Caravaca, the initial postimpact benthic foraminiferal assemblage (within a 7-cm-thick laminated clay) contains representatives of only two genera; Bolivina (calcareous) and Spiroplectammina (agglutinated). This low-oxygen, high-nutrient association lasted for an estimated 0.5-0.6 kyr after impact. Following development of this initial low-diversity assemblage, numerous preimpact taxa reappeared progressively upward through the section (commonly known as Lazarus species), until normal polytaxic assemblages regained prominence at ~6.0-6.5 kyr pti. This compares with the ~3 -kyr duration of the initial Bolivina-dominated association at Kiptopeke and full recovery of the Cibicidoides pippeni Assemblage at ~36 kyr pti (Fig. 13.11).

13.2 Possible Global Paleoenvironmental Effects According to some authors, a bolide impact the size of the Chesapeake Bay event, accompanied by atmospheric perturbations such as those cited in Chapter 12, should have produced a mass extinction severe enough to eliminate ~50% of marine species (Raup 1991a,b). To date, however, only sparse evidence of an immediate, widespread, impact-derived perturbation of the late Eocene biosphere has been proffered (Sanfilippo et al. 1985; Keller 1986; MacLeod et al. 1990; Brinkhuis and Coccioni 1995; Vonhof et al. 2000; Spezzaferri et al. 2002). No major extinction event greater than the normal 5% background value is known to have taken place (Poag 1997b). No evidence of mass mortality, pulsed extinctions, or mass extinctions has been found distal to the known late Eocene craters or associated with late Eocene ejecta deposits. This dearth of extinctions has major implications for the kill curve proposed by Raup (1991a,b; Fig. 13.12) to relate impact crater size to the resultant percent of marine species loss due to mass extinction (Jansa et al. 1990; Jansa 1993; Poag 1997b; Fig. 13.12). Jansa et al. (1990), Jansa (1993), and Poag (1997b) showed that the Chesapeake Bay data (along with data from the Montagnais crater) invali-

420

Biospheric Effects of Chesapeake Bay Impact

----

100-

80-

~ ~

~ 1/1 Q)

·0

Q)

a.

C/)

-----

60-=

Rl";'"

~~

~'V

~

---

,

-:

0

'0/

{::

il;'

II

I

I

'\

/

I

-= Montagnais

0...,-

#.

'")'Ii ,

I I I I

20-

Chicxulub

;;;. I

I

I

--

.....

~I

I

40-=

"

""

I

/

I

/

1

1 1 111

300

Fig. 13.12. Kill curve designed by Raup (199Ia) to relate impact crater size to extinction of marine species. See text for further explanation. Modified from Poag (1997b).

date Raup's kill curve for impact craters smaller than - 100 km in diameter (Fig. 13.12). It is well established that the severity of impact effects depends on natural variability in such things as: (I) the size, composition, trajectory, and speed of the impactor; (2) the relative consolidation and composition of the target rocks; (3) the latitudinal and topographic location of the impact site; (4) the nature of and ambient stress regime of the preimpact biota; and (5) the general nature of the existent terrestrial, oceanic, and atmospheric environments. The impact at either Chesa7 peake Bay or Popigai alone, however, would have produced enough energy (_10 Mt equivalents of TNT) to significantly alter atmospheric conditions regardless of extenuating cosmic, geological, or other environmental circumstances (Chapters 9, 12; Table 13.10; Morrison et al. 1994; Toon et al. 1994).

Possible Global Paleoenvironmental Effects

421

Table 13.10. Impact effects as a function of energy yielded and bolide/crater diameter (modified from Morrison et al. 1994). Yield [Mt]

Diameter Diameter of Bolide of Crater

Environmental Consequences

<10

No crater Upper atmosphere detonation of stones and comets; only irons penetrate to surface of Earth

101-102 75 m

1.5km

Irons make craters (like Meteor Crater); Stones produce airbursts (like Tunguska); land impacts destroy area the size of a large city (Paris)

102_103 160m

3km

Irons and stones produce groundbursts; comets produce airbursts; land impacts destroy area the size of a large urban area (New York)

103_104 350m

6km

Impacts on land produce craters; ocean tsunamis become significant; land impacts destroy area the size of a smalI state (Delaware)

104_10 5 700m

12km

Tsunamis reach oceanic scales, exceed damage from land impacts; land impacts destroy area the size of a moderate state (Virginia)

105_10 6 1.7 km

30km

Land impacts raise enough dust to affect climate, freeze crops; oceanic impacts generate hemispheric-scale tsunamis; global destruction of ozone; land impacts destroy area the size of a large state (California) or smalI country (Japan)

106_10 7 3 km

60km

Both land and ocean impacts raise dust, change climate; impact ejecta are global, triggering widespread wildfires; land impacts destroy area the size of a large nation (Mexico)

_ 107

3-5km

85 km

Chesapeake Bay and Popigai impacts

7km

125 km

Prolonged climatic effects, global conflagration, probable mass extinction; direct destruction approaches continental scale (United States)

108_109 16km

250km

Catastrophic mass extinction (Chicxulub impact and K-T extinctions)

> 109

?

Threatens survival of alI advanced life forms

7

10 _10

8

?

13.2.1 Hypothetical Short-Term Effects

Potential short-term global environmental effects of the Chesapeake Bay impact are hypothetical, based primarily on models and predicted atmospheric disruptions extrapolated from the results of nuclear explosions (Tables 13.10, 13.11). The scaling calcu lations for an 85-km-diameter crater predict that the Chesapeake Bay bolide must have been 3-5 km in diameter (Table 13.10). An impactor of this size would produce an explosion equivalent to _ 107 Mt of TNT (Morrison et al. 1994). Several authors (Adus hkin and Nemchinov 1994; Rampino and Haggerty 1994;

422

Biospheric Effectsof Chesapeake Bay Impact

Table 13.11. Estimated environmental damage from Chesapeake Bay-sized bolide impact on deep continental shelf (modified fromToon et al. 1994). Disruptive Agent

Disruptive Mechanism(s)

Duration of Disruption

Geographic Scaleof Disruption

Dust loading

Cooling; cessation of photosynthesis; loss of vision Burning; soot cooling; pyrotoxins; acid rain Ozone loss; acid rain; cooling Mechanical pressure; acoustic fluidization Drowning Poisoning Warming

Years

Global

Months

Global

Months to years

I Regional

Seconds to minutes

I Regional to global

Hours to days Years Decades

'Regional Global Global

Fires NOx generation Shockwave Tsunamis Heavy metals Waterand CO2 injections

to global

'regional meansan areaof 106 km2

Toon et a!. 1994) have concluded that a bolide impact of this magnitude would disperse ejecta , water vapor, and submicrometer dust on a global scale. The resultant atmospheric opacity would appreciably cool the atmosphere and Earth's surface for months to years (Tinus and Roddy 1990; Bailey et a!. 1994; Toon et a!. 1994), and would limit photosynthesis for several months (Gerstl and Zardecki 1982; Grieve and Shoemaker 1994), though Pope (2002) has argued against the submicrometer-dust scenario. Such severe atmospheric deterioration could be exacerbated by an immediate, shock-induced, heat pulse (Rampino and Haggerty 1994), short-term enhanced greenhouse warming (Emiliani et a!. 1982; Toon et a!. 1994), global wildfires (Melosh et a!. 1990), acid rain (Toon et a!. 1994), and dense photochemical fog (Wolbach et a!. 1988). A substantial increase in atmospheric CO 2 derived from carbonate target rocks should, in tum, have created decades of greenhouse warming after the cooling effects of atmospheric dust had dissipated (O'Kee fe and Ahrens 1989; Sigurdsson et a!. 1992; Covey et a!. 1994). So far, though, little compelling evidence of such short-term atmospheric perturbations has been derived from studies of late Eocene impacts. However, Vonhof et a!. (2000) and Spezzaferri et a!. (2002) have noted evidence among dinoflagellates and benthic foraminifera, respectively, for a possible short-term cooling associated with deposition of late Eocene ejecta at Massignano, Italy, and at ODP Site 689B in the Southern Ocean.

Possible Global Paleoenvironmental Effects

423

13.2.2 Possible Long-Term Effects

The weakness or lack of an immediate or short-term atmospheric response to the late Eocene impacts does not, however, preclude a longer-term response. There is ample evidence that the Chesapeake Bay, Popigai, and the Toms Canyon impacts took place during the late stages of a long-term, step-wise climatic cooling event. This cooling event is evidenced by the buildup of Antarctic ice sheets, which culminated in a sharp temperature decline, accompan ied by mass extinction, in the early Oligocene (Keller et al. 1987; Marty et al. 1988; Miller et al. 1991; Wise et al. 1991; Prothero 1994; Miller 1992; Prothero and Berggren 1992; Clymer et al. 1996). Poag (1997b) and Poag et al. (2003) pointed out that marine isotopic signatures and changes in the fossil record of late Eocene terrestrial and marine organisms seem to suggest that the long-term temperature decline was interrupted by a global pulse of atmospheric warmth, which began at approximately the time of the Chesapeake Bay, Popigai, and Toms Canyon impacts (regardless of whether they were simultaneous or sequential; Fig. 13.13). Diverse evidence for this late Eocene warming includes a 6-8 °C temperature increase deduced from leafmargin analysis of North American land plants (Wolfe 1978), a 0.5 %0decrease in b l 80 measured in Southern Ocean cores (Miller 1992), and migration of lowlatitude nannofloras into high latitudes (Haq and Lohmann 1976). Pearson et al. (2001) and Kobashi et al. (2001) have recently strengthened the case for warm late l8 Eocene oceanic surface waters on the basis of b 0 analyses of planktonic foraminifera and shallow-water molluscs, respectively. Poag (1997b) proposed that impact-generated greenhouse warming interrupted the progressive, long-term Eocene cooling, and, ironically thereby, may have postponed a pending late Eocene mass extinction until the early Oligocene. In fact, the particularly large temperature differential between a late Eocene greenhouse and an early Oligocene icehouse may have triggered the mass extinction (Fig. 13.13). The work of Farley et al. (1998), who recorded the relative abundance of extraterrestrial helium isotopes eHe) in late Eocene sediments at Massignano (Fig. 13.14A), supports Poag's (1997b) supposition ofa late Eocene heat pulse. Farley et al. found that the concentration of 3He in the Massignano section increased dramatically in the late Eocene, reached a peak (-5.5 times the baseline value) coincident with the impacts at Chesapeake Bay, Popigai, and Toms Canyon, and gradually declined to near baseline values 1-2 myr later in the early Oligocene. Farley et al. (1998) interpreted this distinctively high 3He concentration to represent accelerated deposition of interplanetary dust particles (by which the 3He was carried) when a comet shower bombarded Earth in the late Eocene. If this interpretation is correct, additional late Eocene impact craters may be expected to be found. If successive impacting continued for 1-2 myr following the Chesapeake Bay, Popigai, and, Toms Canyon collisions, a resultant production of long-term atmospheric warming may have extended into the early Oligocene. When the comet shower abated, global temperatures declined along an unusually steep gradient, accelerated by ice-sheet buildup on Antarctica (Miller 1992).

424

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13.2.3 Implications of 0 0 Data Poag (1997b) used Miller's (1992) isotopic data (benthic foraminifera) from Southern Ocean ODP Sites 689B and 703, along with other evidence ofwann late Eocene paleoclimates (Haq and Lohmann 1976; Wolfe 1978, 1992; Wolfe and Poore 1982; Aubry 1992; Keller et al. 1992; Prothero 1994; Bestland et al. 1996), to support the speculation that the three known late Eocene bolide impacts pro-

Possible Global Paleoenvironmental Effects

425

duced overriding greenhouse effects and triggered a long-term (~2 myr) pulse of late Eocene climatic warmth. Miller (1992) used a few widely spaced samples to infer an interval of relatively negative 0 180 (warm bottom water) between the ejecta layer in ODP core 689B and the Eocene-OIi~ocene boundary at that site (Fig. 13.14B). Zachos et al. (2001) used additional 0 80 data to indicate a similar late Eocene warm pulse. A broad, late Eocene warm pulse also can be inferred from the 0 180 records at numerous other localities in the Atlantic, Pacific, and Indian Oceans (e.g., Miller et al. 1985, 1987; Keigwin and Corliss 1986; Shackleton 1986; Steckler et al. 1999; Zachos et al. 2001; Fig. 13.15), which attests to its near-global effects . The 0 180 record in the Chickahominy Formation comes from a thicker, more rapidly deposited upper Eocene sedimentary section than any other impactite site studied (63.4 m thick at Kiptopeke, 52.4 m at NASA Langley, vs 45 m at DSDP Site 612, 25 m at Bath Cliff, 14 m at ODP Site 689B, and 12 m at Massignano). The expanded record at Kiptopeke and NASA Langley shows a three-fold subdivision of the inferred late Eocene warm pulse (Figs. 13.14A,B 13.15). The oldest warm subdivision, W-I, is expressed by negative 0 180 values in Chron 16n.2n and the lower part of Chron 16r.1 r. Subpulse W-I correlates with the Chesapeake Bay, Toms Canyon, and Popigai impacts, but has not been identified at Massignano or Bath Cliff (insufficient data; Fig. 13.14A). The next youngest Chickahominy warm subpulse, W-2, coincides with Chron 16n.ln and the lower two-thirds ofChron 15r (Figs. 13.14A,B 13.15). Subpulse W-2 correlates with the lower part of the lower warm interval at Massignano, but has not been identified at Bath Cliff (insufficient data; Fig. 13.14A). The youngest Chickahominy warm subpulse, W-3, occupies the middle to upper part of Chron 13r, and correlates with the upper warm interval at Massignano and the single warm interval identified at Bath Cliff (Figs. 13.I4A, 13.15). Relatively positive 0 180 values in the Chickahominy record during Chron 15n and early Chron 13r indicate cooler water, and correlate with a similar interval at Bath Cliff. This interval, however, is difficult to recognize at Massignano (Figs. 13.14A,13.15). Fig. 13.14. (Next two pages) A stable isotope records in cores and outcrops containing late Eocene impact deposits (Chesapeake Bay; Kiptopeke corehole), DSDP Site 612 (deep-sea core), Massignano, Italy (outcrop), and Bath Cliff, Barbados (outcrop). Shaded intervals of 0 180 curve show three negative (warm) excursions (W-l, W-2, W-3); dotted pattern of ol3 C marks two negative excursions. B stable isotope records in cores and outcrops containing late Eocene impact deposits at Chesapeake Bay (Kiptopeke corehole), in Antarctic (deep-sea cores), and at ODP Site 689B (deep-sea core). Symbols as in Fig. 13.14A. Modified from Poag et al. (2003).

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Possible Global Paleoenvironmental Effects

429

Upon further consideration of the published record , we conclude that the presence of at least two to three late Eocene warm pulses also can be inferred from Antarctic cores (Fig. 13.148). We found that the isotopic record at Site 6898 is more complicated than indicated by Miller's (1992) coarse sampling. For example, in a subsequent isotopic study (benthic foraminifera) of the same core, Diester-Haass and Zahn (1996) sampled at 20-cm intervals, and produced a much different 8 180 profile from that of Miller (1992) . Warm subpulses W-I and W-2 appear to be represented (Fig. 13.148), and perhaps the lower part ofW-3 . There is no clear separation of W-2 and the lower part of W-3, however, and the upper part ofW-3 is only faintly represented. 18 Abreu and Anderson (1998) composited and smoothed the late Eocene 8 0 record in 19 Southern Ocean sites (including Site 6898) and presented a third paleoenvironmental interpretation. They recognized two relatively negative intervals (ice retreat; warmer bottom water) separated by a distinct positive excursion (ice advance; colder bottom water) during late Chron 15n and early Chron 13, which they designated as event EPi-2 (Fig. 13.148). The 8 180 correlations between the Kiptopeke core, Massignano, and 6898 suggest that the magnetostratigraphy at 6898 is correct in the interval interpreted to represent Chrons 15r to l3r. If one assumes that the ejecta layer represents the upper part of Chron 16n.2n, however , then Chrons 16n.1nand 16r.1r appear not to be represented at Site 6898 (Fig. 13.148). We infer, then, that the stratigraphically lower warm subpulse of Abreu and Anderson (1998) represents , in reality, two warm subpulses separated by an unconformity. Thus, three warm pulses appear to be represented in the Antarctic region, although not all can be distinguished at every site studied . In summary, there is increasing evidence that the late Eocene warm pulse proposed by Poag (1997b) is composed of at least three subpulses. A plot of the two 18 8 0 records from Chesapeake 8ay and the 3He record from Massignano on a time scale (Fig. 13.16) indicates an especially strong correlation between oxygen and 3 He abundance for subpulse W-I. The 3 He abundance decreases in the lower half of W-2, but reaches a second major ~eak in the upper half of W-2. The correlation is weakest for subpulse W-3. The He abundance begins to decline in the lower half of W-3, and maintains near-baseline values for the rest of the Eocene. We have modified Poag's (l997b) pulse diagram to show the three subpulses of greenhouse warming (Fig. 13.16), which produced a particularly steep cooling gradient in the early Oligocene, as ice sheets built up rapidly on Antarctica (Miller, 1992). It is this steep gradient, which Poag (I 997b) suggested might have triggered threshold events that initiated the early Oligocene mass extinction. McGhee (2001) termed this proposed extinction mechanism, the "lag-time multiple impacts hypothesis ." The significance of this late Eocene warm pulse (a succession of at least three subpulses) has been overshadowed in all previous studies by the spectacular cooling event that followed in the early Oligocene. In contrast to the isotopic studies , Vonhof et al. (2000) speculated (on the basis of an unusual assemblage of dinoflagellates) that a short interval of global cooling Fig. 13.15. (Opposite page) Distribution of 0 180 and sites (deep-sea cores; from Poag et al. 2003).

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might have been associated with deposition of the impact ejecta at Massignano and at Site 689B (though there remains a strong probability that the late Eocene stratigraphic record at Site 689B is incomplete) . Most recently, Spezzaferri et al. (2002) studied foraminiferal trends in a short section that includes the ejecta layer at Massignano (a 4-m section representing -526 kyr). These authors found foraminiferal evidence of an - 20-kyr cool-water interval immediately following deposition of the late Eocene ejecta at that site. Above that 4-m interval, however, the foraminiferal assemblages, as well as the dinoflagellate assemblages, indicate a pulse of warm water that lasted 27-40 kyr, before cooler waters returned . We speculate that this warm pulse may be correlative with W-I at Kiptopeke. Unfortunately, Spezzaferri et al. (2002) did not provide microfossil paleoenvironmental interpretations for the remaining 15 m of upper Eocene section at Massignano.

13.2.4 Implications of 013 C Data 13C

The late Eocene 8 record has not received as much attention as the 8 180 record because of the latter's importance in defining the early Oligocene ice-advance signal and initial development of the oceanic cryosphere. The Chickahominy 8 13C curve contains easily distinguishable subdivisions, however, and holds promise for identifying fundamental late Eocene environmental changes and for improving correlations with other upper Eocene sections and related impactites (Figs. 13.14A,B,13.15). The most obvious characteristic of the late Eocene 8 13C record is the persistence of distinctly negative values in the upper two-thirds of the section at Chesapeake Bay, Massignano and Bath Cliff (encompassing Chron 16n.1n through most of Chron 13r; Fig. l3.l4A). A coincident negative 8 13C excursion characterizes the late Eocene record at many additional deep-sea sites (often the same sites 18 showing the negative 8 0 excursion ; Figs. 13.l4A,B). Zachos et al. (2001) noted this excursion, for example, in their synthesis of the Cenozoic marine stable isotope record, but made no attempt to explain its origin. A second, older, less prominent, negative 8 13C excursion is present at Chesapeake Bay (Kiptopeke and NASA Langley coreholes; upper part of Chron 16n.2n), and can be seen in some (e.g., Shackleton 1986), but not all the other sites discussed herein (Figs. 13.14A,B). Several notable negative excursions of 8 13C have been documented in other parts of the geologic record, also coincident, or nearly so, with negative 8 180 excursions. Perhaps the most dramatic is the Late Paleocene Thermal Maximum (Norris and Rohl 1999). The origins of such rapid 8 13C excursions have been attributed mainly to either a catastrophic release of biogenic methane from global dissociation of submarine gas hydrates (Bains et al. 1999; Norris and Rohl 1999) Figure 13.16. (Opposite page) Chronostratigraphic chart showing correlations of 8 180 at Chesapeake Bay and 3 He at Massignano with proposed subpulses of greenhouse warmth for the late Eocene. At right is revision of Poag's (I 997b) single-pulse diagram to show the three subpulses (white arrows).

432

Biospheric Effects of ChesapeakeBay Impact

or a massive injection of atmospheric CO 2 by sustained, intense , volcanic degassing (Zachos et al. 1993; Thomas and Shackleton 1996). The late Eocene 13 negative excursion of I) C, however, was long lasting (1.7 myr) , and has been interpreted in paleoceanographic contexts to indicate an incursion of colder, more vigorous bottom waters (Miller et al. 1985) and increased paleoproductivity (Diester-Haass and Zahn 1996). Theoretically, atmospheric disturbances due to dust and particulates from a single projectile the size of the Chesapeake Bay impactor (3-5 km) would last, at most, for only a few decades (Table 13.14; Toon et al. 1994). However, individual late Eocene impacts could be expected to also change atmospheric gas composition (e.g., the global carbon budget) by injecting massi ve volumes of CO 2 into the atmosphere from vaporized seawater (e.g., Chesapeake Bay and Toms Canyon) and carbonate-rich target rocks (e.g., Popigai - Masaitis et al. 1975; and Toms Canyon - Poag and Poppe 1998). A succession of surface impacts (accompanied, presumably, by many more atmospheric bursts) during a 2-myr-Iong comet shower, on the other hand, could plausibly have sustained atmospheric perturbations for most of the late Eocene.

14 Residual Effects of Chesapeake Bay Impact

The Chesapeake Bay impact crater is unusual in that more people live above or adjacent to it than at most other large impact craters. The only exception is the Vredefort multiring basin of South Africa, where some eight million people inhabit the deeply eroded crater region. The seven coastal cities of Virginia Beach, Norfolk, Portsmouth, Chesapeake, Suffolk, Hampton, and Newport News incorporate a population of two million people - far more than any other comparable area of Virginia - clustered in an arcuate corridor around the crater rim (Fig. 14.1). Among this urban sprawl is one of the U.S. East Coast's premier seaports (Norfolk), as well as numerous universities, hospitals, historical tourist attractions, and a particularly dense concentration of military bases, airfields, research installations, and weapons depots. Therefore, it is societally relevant to document and evaluate any lingering effects of that ancient impact upon the existing geology, geohydrology, geomorphology, and hydrography of southeastern Virginia.

14.1 Hypersaline Groundwater Excavation of the Chesapeake Bay crater eliminated the antecedent freshwater aquifer system over an area twice the size of the state of Rhode Island (-6,400 krrr'; Poag 1997a). In place of the previous system of vertically alternating aquifers and confining units, deposition of the Exmore breccia within the crater created a huge complex reservoir whose sediment volume (-4,300 knr') is great enough to cover the states of Virginia and Maryland with a layer 30 m thick. Groundwater tests in the main body of the Exmore breccia and in the surrounding breccia apron have revealed that the breccia contains interstitial brine, with existent chlorinities as high as 25,700 mgIL inside the inner basin (at the Kiptopeke core site; Poag 1997a; Powars and Bruce 1999; Fig. 14.2). The presence of shallow brine aquifers around the lower part of Chesapeake Bay was known nearly 100 years ago (Sanford 1911, 1913), but early investigators did not know its detailed distribution and origin. The high-salinity problem was particularly highlighted by the studies of Cederstrom (1943, 1945a,b), who synthesized groundwater data for the Middle Neck and York-James Peninsulas . Additional analyses carried out during the 1980s and 1990s showed that isochlors for shallow aquifers near and inside the crater curve concentrically around the western rim of the buried crater, and that the chlorinity gradient steepens markedly at the crater rim (Larson 1981; Meisler 1981, 1989 ; Trapp 1992; Focazio et al. 1993 ; Poag 1997a ; Powars and Bruce 1999; Fig. 14.2). C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

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Possible explanations for the presence of the brine include the following: (I) short-distance migration of seawater from the adjacent Chesapeake Bay or Atlantic Ocean (Larson 1981; Leahy and Martin 1993); (2) long-distance migration of brines from Jurassic salt beds in the Baltimore Canyon trough, the largest postrift sedimentary basin of the U.S. Atlantic margin (Larson 1981; Meisler 1989); (3) incomplete flushing of ancient seawater (Cederstrom 1946; Powars and Bruce 1992; McFarland 2002); (4) concentration of dissolved salts through reverse chemical osmosis and membrane filtration, enhanced by rapid sedimentation (Larson 1981; Meisler 1989); and (5) presence of evaporite beds in the impact target rocks (Poag 1997a). Only the latter hypothesis explains why the brine is localized

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in southeastern Virginia within the Exmore breccia, but this hypothesis remains unsupported by other evidence. We support an hypothesis that depends upon flash evaporation of a huge volume of seawater at the time of impact followed by thousands of years in which the breccia was heated by the underlying shock-heated basement rocks (Sanford 2002 ; Figs. 14.3A,B). At the center of the bolide explosion, the seawater would have been completely vaporized, but as impact temperatures declined radially away from the center, a broad zone of seawater boil-off (evaporation) would surround the vaporization zone. Salts left behind would elevate salinity in the watersaturated sediments of the Exmore breccia. Preliminary hydrochemical studies indicate that the Exmore brine is depleted in 2H and 180, a condition consistent with a late Eocene origin of the brine (McFarland 2002 ; Sanford 2002) . If one assumes a porosity of - 20% in the breccia, and that it is a single saturated reservoir with a volume of 4,300 knr', then it would require evaporation of -430 km' of seawater to bring the entire breccia reservoir to -1 .5 times the salinity of normal seawater. The Exmore interstitial fluids were subsequently sealed in by postimpact resumption of clay deposition as the Chickahominy Formation began to accumulate. Calculations using Darcy's law (Sanford 2002) indicate flow rates within the breccia of < 1 mm/yr, which would be insufficient to move the brine out of the crater within the 36 myr since the impact took place . Sanford (2002) also applied Fick's law to estimate that at typical molecular diffusion rates, > 100 myr would be required for the brine solutes to escape from the Exmore breccia into shallower aquifers. Initial studies of interstitial fluids in the Exmore breccia from the NASA Langley, North, and Bayside coreholes (McFarland and Bruce 2002; McFarland 2002) indicate a general increase in salinity toward the center of the crater, as previous investigations had predicted (Poag I977a). It is quite clear, at this point, however, that the Exmore breccia is not a single fully saturated reservoir (McFarland 2002) . The downhole geophysical logs and preliminary analyses of pore water squeezed from cores at NASA Langley, North, and Bayside, reveal that salinity values vary too randomly to be stratigraphically correlated from one core hole to the next. Regardless of how it formed, the presence of the shallow Exmore brine reservoir has two significant implications for the citizens living around southern Chesapeake Bay (Fig. 14.4): (1) The shallow stratigraphic position of the top of the breccia and the existence of hundreds of faults within its overlying seal (the Chickahominy Formation), create a contamination hazard for overlying freshwater aquifers; and (2) The shallow depth to the top of the brine reservoir and its great thickness inside the crater limit the availability of freshwater aquifers to communities overlying the crater. It is especially limiting on the Delmarva Peninsula, which extends over the thickest and saltiest parts of the brine system . Here , postimpact sedimentary beds may be the only potential sources of fresh groundwater. If we assume, as current investigations indicate, that the brine occupies the entire lateral extent of the Exmore breccia, including the breccia apron outside the crater, then the brine covers an area the size of the state of Connecticut (-8,000 km"). Our maps of the breccia distribution offer a means of predicting the lateral extent and potential thickness of the brine reservoirs in areas not yet drilled. This know-

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ledge, along with future geochemical and hydrogeological studies, will help guide prudent management of fresh groundwater resources in this region.

14.2 Near-Surface Compaction Faults The presence of an extensive system of impact-related, near-surface, compaction faults also must be considered a potential geohazard for the region around lower Chesapeake Bay (Poag 1997a; Fig. 7.11). Most of the postimpact faults appear to have long histories of dip-slip movement, showing the typical upward decrease in throw attributable to growth faults (Fig. 7.12A,B,C). Most of these faults begin at the base of the Chickahominy Formation and extend upward into the Miocene and Pliocene sections, and some are traceable into the base of the Pleistocene units. High-resolution seismic reflection profiles published by Colman and Mixon (1988) show several faults that can be traced to within 15 m of the bay floor (Figs. 7.13A,B). At shallower depths, the Quaternary sediments are generally too soft and water-saturated to preserve the upper extent of fault planes. Thus, we are not sure that any of the faults actually cut the bay floor or surrounding land surfaces. Nevertheless, the compaction faults have created a zone of structural weakness over the crater, which is more susceptible to earthquake displacement than areas outside the crater. Though historical earthquakes in southeastern Virginia have been rare and relatively mild (Sibol et al. 1996, 1997), Johnson et al. (1998) pointed out that surface projections of the epicenters of all four significant historical earthquakes in this region (1884, 1899, 1918, 1995) were near or inside the trace of the rim of the crater (Fig. 14.5).

14.3 Surface Expression of Crater Though the Chesapeake Bay crater is now buried beneath 300-500 m of postimpact sedimentary formations, its presence is expressed on the modem land surface in the distribution of outcropping sedimentary units and in the configuration of certain topographic features (Poag I997a). In terms of geologic expression, the geologic map of Virginia (Mixon et al. 1989) shows clearly that the sedimentary units inside the perimeter of the crater are of late Pleistocene and Holocene age, whereas most of those outside the crater perimeter are of middle Pleistocene and older ages (Fig. 14.6). Furthermore, on the Middle Neck and York-James Peninsulas, the contact separating the lower and middle Pleistocene units (Shirley, Windsor, and Chuckatuck Formations) from the upper Pleistocene units (Sedgefield, Lynnhaven, and Poquoson Members of the Tabb Formation) arches (convex to the west) around the western perimeter of the crater within approximately one kilometer of where the crater outer rim projects to the surface (Fig. 14.6). Outside the crater rim, in contrast, the contact between these older and younger units trends almost due north-south.

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Similarly, on the Delmarva Peninsula, the contact separating middle Pleistocene sediments (Accomac Member of the Omar Formation) from upper Pleistocene sediments (Occohannock and Butlars Bluff Members of the Nassawadox Formation) marks the surface projection of the crater outer rim (Fig. 14.6). In addition, along the southern shore of Chesapeake Bay between Norfolk and Cape Henry, the surface projection of the outer rim is marked approximately by the contact between the Sedgefield Member of the Tabb Formation (upper Pleistocene) and Holocene shoreline sands. The outer rim of the crater also is expressed in the coastal topography. Peebles (1984) showed that two Pleistocene shoreline erosional features on the Middle Neck and York-James Peninsulas, known as the Suffolk scarp (Harpersville scarp, in part, of Johnson 1976) and the Big Bethel scarp (Johnson 1972), mark the contact between older and younger Pleistocene units on the western side of the bay (Figs. 14.6, 14.7). Thus the positions of these scarps (with topographic relief as

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great as 22 m) also approximate the western boundary of the crater. The buried southern rim of the crater is marked at the surface by the arcuate southern shore of Chesapeake Bay and by the parallel slope of the Diamond Springs scarp (Powars and Bruce 1999). On the Delmarva Peninsula, a landform comparable to the Suffolk scarp is known as the Ames Ridge; relief there is - 5 m (Fig. 14.6). Though these scarps have been interpreted to be erosional features (Peebles, 1984), we infer that longterm differential subsidence across the crater's outer rim played a major role in controlling the maximum westward extent of the two successive Pleistocene transgressions that produced these erosional shoreline features . Further evidence of this westward topographic limitation is the fact that on the Middle Neck and York-James Peninsulas, the spatial positions of the Suffolk and Big Bethel scarps are nearly coincident. That is, the two successive transgressions that eroded them reached the same westward limit in this region. In contrast, south of the James River, the Suffolk shoreline was able to transgress 22 km farther west than the Big Bethel shoreline (Fig. 14.6). Between Norfolk and Cape Henry, the location of an additional landform approximates the southern edge of the crater rim. The northern boundary of the Oceana Ridge essentially mimics the curve of the crater's outer rim in this area (Fig. 14.6).

Fig. 14.7. Photograph of Suffolk scarp, a modem topographic expression of the outer rim of Chesapeake Bay impact crater. Topographic relief at this location is - 14 m (arrow). Location is on west side of Virginia State Highway 14, between towns of Gloucester and James Store, - 0.5 km north of Mt. Zion Church. See CD-ROM for color version of this figure.

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14.4Altered River Courses The lower courses of the four largest modern rivers that cross the surface trace of the crater's outer rim bear signs that long-term differential subsidence also has determined the locations of their channels (Poag 1997a). The James, York, Piankatank, and Rappahannock Rivers all make sharp bends approximately at the outer rim of the crater (Figs. 14.6, 14.8). The upstream courses of the James and York head directly southeastward to the Atlantic , but in each case, the river turns abruptly to the northeast near the crater rim (an acute angle in the case of the James), and its channel heads toward the center of the crater. In the case of the Piankatank, its channel makes a right-angle turn to the northeast and parallels the outer rim for about 5 km before turning 90 degrees back to the southeast toward the crater's center. The channel of the Rappahannock River makes its eastward turn toward the crater farther upstream, approximately 20 km from its mouth at Windmill Point. The location of this bend approximates the position of the northwesternmost extension of the crater rim, which we have termed the Rappahannock Canyon. The lower course of the modern Susquehanna River also apparently has been altered by differential subsidence over the buried crater. The Susquehanna makes a sharp turn to the east where it exits Chesapeake Bay over the southern part of the crater (Fig. 14.8). This bend to the east is a reaction to the eastward tilt of the crater rim. Furthermore, subsurface studies by Colman and Mixon (1988) show that three successive older (buried) channels of the Pleistocene Susquehanna River also responded to differential subsidence in a similar fashion (Fig. 14.8). Each makes a distinct course change toward the lower eastern side of the crater upon entering the boundaries of the crater (Fig. 14.8). Though differential subsidence across the outer rim may be the crater's primary influence on the river courses, it may not be the only influence thereon. The remarkable similarity between the position and geometry of the radial basement faults and the lower courses of the James, York, and Rappahannock Rivers, introduces the possibility that the faults also exert partial control on the courses of these rivers (Figs. 9.1, CD-ROM.5) .

14.5 Relative Change of Sea Level Long-term tide (sea-level) gauges in the Chesapeake Bay region indicate that during the last 70 years, the rate of relative sea-level rise in this area has been among the highest values (4 mmlyr) in the continental United States (Nerem et aI. 1998; Table 14.1). This rate is twice the global average of 1.8 mmlyr (Douglas 1991; Douglas et aI. 2001). Several authors have attributed about half of this high value to structural relaxation and rebound of a large regional periglacial bulge created by Wisconsinan ice sheets during the last glacial maximum in eastern North America (Davis and Mitrovica 1996; Nerem et aI. 1998). In addition, as most of these gauges are near population centers, some of the relative rise may be the result of

Relative Change of Sea Level

445

37-30'

c

8

o

r:

..37'00' 50 I

km

76'00'

75°40'

75"20'

Fig. 14.8. Map showing distinctive bends in lower courses of modern (solid lines) and Pleistocene (dashed lines) river channels. Bends inferred to have been caused by differential subsidence of Exmore breccia within Chesapeake Bay crater.

land subsidence due to groundwater extraction (Gomitz and Seeber 1990; Holdal and Morrison 1974). Nevertheless, when Nerem et al. (1998) corrected for periglacial rebound, they found that sites inside and near the Chesapeake Bay crater still averaged 4 mm/yr relative sea-level rise (Table 14.1). Rates at all four sites inside the crater are higher than the global average, and at the Gloucester Point station, the value is as high as 6.7 mm/yr. We infer that differential subsidence of the Exmore breccia has contributed an additional component to these high relative sea-level rise values, by differentially lowering the ground surface and bay floor over the crater. The rate of only 4 mm/yr in relative sea-level rise seems trivial at face value, but the gentle slope of the coastal plain surface around Chesapeake Bay makes this rate a major threat in the form of wetland loss. Slope values of 1:1000 are applicable to much of the bay margin , which would result in a I-m horizontal loss of wetlands (transgression of the shoreline) for every I-mm rise in relative sea level.

446

Residual Effects of Chesapeake Bay Impact

Thus it is important for planning future land use in the Chesapeake Bay region to understand fully all the causative components of relative sea-level rise and their interrelationships in this region. Table 14.1. Trends of relative sea-level rise measured at tide gauges inside and near buried outer rim of Chesapeake Bay impact crater, 1930-1993. Data from Nerem et al. (1998). Location

Relative Latitude Longitude [degrees N] [degrees W] sea-level trend [mm/yr]

Chesapeake Bay Bridge tunnel

37.000

76.003

Gloucester Point, Va. (inside crater)

37.247

Hampton Roads, Va. (inside crater)

Periglacial rebound trend [mm/yr]

Corrected sea-level trend [mm/yr]

7.5 ± 1.1

0.9

2.5

76.500

6.2±2.2

0.8

6.7

36.927

76.006

4.1 ± 0.2

0.8

3.3

Kiptopeke, Va. (inside crater)

37.167

75.988

3.2 ± 0.3

1.1

2.1

Wachapreague, Va. (near crater)

37.607

75.687

6.7 ± 1.4

I.1

5.6

5.54

0.9

4.0

(inside crater)

Average

15 Summary and Conclusions

We have presented a comprehensive geological and geophysical synthesis of the Chesapeake Bay crater , the largest known bolide impact structure in the United States. The structure, morphology, stratigraphy, and age of the crater and the nature and depositional history of the crater-fill rocks are documented by >2,000 krn of seismic reflection profiles and >2,000 m of continuously cored and logged borehole sections (Chapter I). The Chesapeake Bay bolide struck the ~300-m-deep continental shelf of eastern North America ~35.78 Ma at a site now covered by the lower part of Chesapeake Bay, the low-lying peninsulas of southeastern Virginia, and the shallow marine waters of the inner Atlantic Continental Shelf. The impactor struck a threelayered target (Chapter 2). The upper layer comprised a column of seawater ~300 m deep; the middle layer encompassed 600-1000 m of poorly consolidated , watersaturated, sedimentary rocks (Early Cretaceous to late Eocene strata); the basal layer was a crystalline basement composed of metasedimentary and metaigneous rocks (Proterozoic to Paleozoic in age). The bolide impact created a crater 85 krn wide and 1.3-2.0 krn deep (Chapter 4). Today the crater features a steep sedimentary outer-rim escarpment (300-1200 m high), a relatively flat, crystalline-floored annular trough (15-28 krn wide), a crystalline peak ring (35--45 krn wide; 40-300 m high), a deep, crystalline -floored inner basin (10-18 krn wide; 1.3-2 krn deep), and an irregular crystalline central peak (12 krn wide; 200-600 m high), all attributes typical of other large complex craters found on Earth and its planetary neighbors. The Chesapeake Bay crater is filled with an orderly succession of inferred and documented synimpact deposits (Chapters 6, 11, 12). Filling the lower part of the inner basin is an inferred layer of fallback breccia, dekameters thick, presumably dominated by meter-todekameter-sized clasts of crystalline basement rocks. Such fallback breccia is known from the deep inner basins of other complex craters, but the inner basin at Chesapeake Bay has not yet been cored. One of the Chesapeake Bay coreholes, however, the Bayside corehole, contains ~20 m of matrix-supported breccia above the basement surface, whose abundant crystalline and sandstone lithoclasts appear to represent fragments of rocks from deep within the inner basin, and thus may constitute a modest section of fallback breccia . The basal synimpact deposit in the annular trough at Chesapeake Bay is an ~300-m-thick layer of hectometer-to-kilometer-sized, displaced, sedimentary megablocks (slumpback lithofacies; Chapter 6). These megablocks are derived from the shock-generated collapse and basal fluidization of poorly consolidated, mainly Lower Cretaceous sediments that sloughed off the crater's outer rim. C. W. Poag et al., The Chesapeake Bay Crater © Springer-Verlag Berlin Heidelberg 2004

448

Summary and Conclusions

Seismic reflection profiles indicate also that kilometer-sized megablocks of crystalline basement have slumped from the walls of the inner basin. The next highest crater-fill deposit, 100-200 m thick, is surgeback breccia, a sediment-dominated, subaqueous deposit, which covers the entire crater, burying both the fallback and megablock deposits, as well as the peak ring and central peak. Surgeback breccia was formed by hydraulic erosion and gravity-driven collapse of the sedimentary crater rim and the tops of the displaced megablocks. An enormous hydraulic head developed as the 300-m-thick oceanic water column plunged back into the crater cavity. Above the surgeback deposits is a sediment-dominated, matrix-supported, upward-fining, washback breccia, dekameters thick. The matrix is characteristically a greenish gray to nearly black, glauconite/quartz sand, containing stratigraphically mixed microfossils. This washback breccia not only covers the entire crater, but also extends as a breccia apron a few kilometers outside the crater rim. The washback breccia is a tsunamiite, created by runup and washback processes as impact-generated tsunami wave trains eroded and redistributed shock-weakened sediments from the inner continental shelf and coastal plain. Both the surgeback and washback breccias contain granitoid clasts derived from the crystalline basement, which have been variably shock metamorphosed from <5 to > 45 (~60) GPa (Chapter 6). The geochemistry of these two breccia deposits indicates derivation from a sedimentary, upper crustal, post-Archean source, similar to the source inferred for the North American tektite strewn field (Chapter 6). The antepenultimate synimpact crater-fill deposit is a clayey silt unit, a few meters thick, which displays evidence of multidirectional sediment flow during deposition. This is a flowin unit, attributable to hypercanes that moved across the continental shelf and triggered successions of small debris flows from the crater rim. The final synimpact crater-fill deposit is a thin (1-5 em thick), clayey silt, which contains evidence of impact-derived microspherules (Chapter 6). The 1mm cavities that originally contained the microspherules are preserved in distinctive pyrite lattices, from which glass-derivative clay may have been inadvertently washed away during routine sample preparation. We infer that this microspherule layer is a fallout product of the condensing impact vapor plume. Outside the primary crater, seismic profiles reveal 23 small structures that appear to be secondary craters (3-6-km diameters), because they display characteristic downfaulted sedimentary rims, raised lips, and chaotic crater-fill reflections (Chapter 5). Though no recent coreholes have penetrated any of the secondary craters, there is evidence from older boreholes that at least one of the possible secondaries contains crater-fill deposits lithologically equivalent to the Exmore breccia. Perhaps the most dramatic aspect of the impact process is the enormous speed with which it took place. Computer simulations of the impact indicate that the 85 x 1.3 km excavation (4,300 krrr') was created and refilled within a geological blink-of-the-eye (a few minutes to hours; Chapter 12). The age of the Chesapeake Bay impact structure has been determined indirectly by biochronological and magnetochronological studies of sediments (the Chicka-

Summary and Conclusions

449

hominy Formation) directly overlying the crater-fill (Chapter 7). Microfossil biochronology indicates that the Chesapeake Bay impact took place during a 0.8-myr interval in which the top of planktonic foraminiferal biochron P15 (upper boundary at 35.2 Ma) overlaps the base of calcareous nannofossil biochron NP19-20 (lower boundary at 36.0 Ma; Chapter 8). A similar crater a§e (35.2 ±0.3 to 35.5 ±0.3 Ma) has been derived from radiometric analyses (40 Ar/ 9Ar) of distal ejecta from the North American tektite strewn field (DSDP Site 612 and Bath Cliff, Barbados), currently thought to be a product of the Chesapeake Bay impact. Extrapolation of a magnetochronologically-derived sediment-accumulation rate from the lower part of the Chickahominy Formation at the Kiptopeke site refines the impact age to 35.78 Ma. This age for the Chesapeake Bay impact is statistically indistinguishable from the 35.7 ±OAMa radiometric age of the Popigai crater in Northern Siberia and the 35.7 ±OA age of the distal ejecta that crops out near Massignano, Italy. The stratigraphic separation of microkrystite ejecta (derived from Popigai) from microtektite ejecta (derived from Chesapeake Bay) in deep-sea cores (Atlantic Ocean and Caribbean Sea), however, indicates that the Chesapeake Bay impact is younger than that of Popigai by 10-20 kyr. The Chesapeake Bay crater and its sedimentary fill are buried now by 300-500 m of postimpact (late Eocene to Holocene) siliciclastic, mainly marine, sediments (Chapters 2, 7, 13). The initial postimpact deposit is a 20-cm-thick , laminated silt layer, which contains no indigenous biota, and represents the first - 0- 3 kyr of lifeless marine deposition following the bolide impact (Chapter 7). Thereafter, normal marine deposition resumed and formed the Chickahominy Formation, a sandy-to-silty, massive-to-Iaminated, glauconitic, micaceous, highly microfossiliferous marine clay, of relatively deep-water origin (-300 m paleodepth) . The Chickahominy represents the final 2.1 myr of Eocene sediment accumulation over the crater. Three distinct episodes of sedimentation (distinguished by different rates of accumulation) can be documented within the Chickahominy clay (Chapter 13). These three depositional intervals correspond roughly to three cycles of lowto-high species richness among the benthic foraminiferal community. Culmination of the first cycle represents full recovery of the benthic foraminiferal community -36 kyr following the bolide impact. Superimposed on these three cycles of species richness are five biotic subzones defined by characteristic associations of benthic foraminiferal species. As a whole, the Chickahominy benthic foraminifera record a succession of paleoenvironments characterized by oxygen deficiency and an abundant supply of organic detritus at the seafloor and in shallow interstitial waters. Phytodetrital feeders were prominent members of this benthic community, especially in the upper part of the formation. Though no immediate global loss of marine or terrestrial species comparable to that of the K-T mass extinctions arose from the Chesapeake Bay impact, there is evidence that long-term climatic changes may have resulted from it. The climatic perturbations, in tum, may have triggered a major extinction event in the early Oligocene, -2 myr after the Chesapeake Bay impact (Chapter 13). Stable isotope 18 records (0 0 and 013C) derived from the tests of the benthic foraminifer Cibicidoides pippeni indicate that postimpact climate at the impact site was punctuated by at least three warm pulses. The final pulse was accompanied by a notable

450

Summary and Conclusions

negative excursion in o values. The 0180 results are best understood in the context of a late Eocene comet shower, which produced unusually high concentrations of extraterrestrial 3He at the late Eocene outcrop near Massignano, Italy, which contains 35.7-myr-old impact ejecta. We infer that a succession of impacts during the comet shower (including those at Chesapeake Bay and Popigai) pro18 duced the climatic warming indicated by the 0 0 record. Though buried for the last ~3 6 myr, the Chesapeake Bay crater and its related deposits still have important consequences for the citizens of southeastern Virginia (Chapter 14). The Exmore breccia subsided differentially as it compacted under a load of postimpact sediments, and this subsidence, in tum , produced a vast network of near-surface faults. The pervasive fault systems have destabilized the bayfloor, seafloor, and low-lying wetlands above and near the crater, contributing to rapid rates of relative sea-level rise that characterize the Chesapeake Bay rel3e

gion. The most important modem consequence of the ancient impact may be the presence of high-salinity groundwater (derived from flash-evaporation of huge volumes of seawater during the bolide impact) at shallow depths within the Exmore breccia. This brine limits the quality and availability of potable shallow groundwater for more than two million citizens in the rapidly growing urban corridor surrounding lower Chesapeake Bay (Chapter 14). Comparison of the Chesapeake Bay crater and its associated deposits with other complex craters of comparable submarine origin reveals some significant similarities (Chapters 10,11,12). On the other hand, each known crater has distinct characteristics that set it apart from all the rest. Our analyses lead us to emphasize the following principal points: (1) The succession of marine modification processes associated with surgeback, washback, and flowin depositional regimes appear to be unique to submarine impacts on shallow continental margins. These processes are responsible for the unusually thick body of sediment-clast breccia that fills the Chesapeake Bay crater and several other submarine craters. Surgeback processes may also operate in deeper, open-ocean settings, but washback and flowin processes require a nearby, easily erodable, land surface or shallow continental shelf. (2) The density differential between crystalline basement rocks and overlying sedimentary rocks is important in constraining both the excavation and modification processes of submarine crater development. This differential appears to account for the great structural and morphological disparity in between the Chesapeake Bay and Mjelnir craters, for example. (3) This density differential depends in large part on the degree of water saturation and lithification of the sedimentary target rocks. In the case of Chesapeake Bay, the sedimentary target rocks are primarily loosely consolidated quartz sands and silts, most of which today are (and presumably were in the late Eocene) important freshwater or saline aquifers. Their weak consolidation must have facilitated acoustic fluidization of the basal target sediments by the impact shock wave, thereby promoting widespread sliding and slumping of megablocks along a basement decollement , without producing pervasive brittle deformation features, such as faults, which ordinarily are expected in decollement zones. This displacement of megablocks significantly widened the crater. (4) Though there is scattered evidence of an upturned lip on the

Summary and Conclusions

451

outer rim of the Chesapeake Bay crater, the lip is insubstantial compared to the lips of well-preserved subaerial craters on Earth and other planetary bodies. This appears to be, in part, due to intense modification of the outer rim by surgeback and washback processes. The lack of a well-defined outer-rim lip appears to be common to all known submarine impact craters. The principal structural, morphological, depositional, and paleoenvironmental aspects of the Chesapeake Bay impact crater are now thoroughly documented by borehole, seismic-reflection, and gravimetric data . Acquisition and analysis of new cores and geophysical surveys continue at Chesapeake Bay, however, and, undoubtedly, will help to refine and revise some of the interpretations we have presented. Several critical questions remain to be answered, especially regarding the central features of the structure: (1) What is the nature (composition, shock history) of the crystalline basement that comprises the peak ring, central peak, and floor of the inner basin? (2) Does fallout breccia dominated by large crystalline clasts occupy the floor of the inner basin? (3) Are large melt bodies or melt sheets associated with this structure ? (4) What is the radiometric age of the crater? (5) Is there a breach in the southeastern margin of the peak ring, as suggested by the pattern of gravity anomalies ? (6) What is the configuration of the basement surface in the eastern sector of the crater? and (7) Are displaced sedimentary megablocks, which are common to the western sector, also present in the eastern sector? Questions 1--4 can best be answered by obtaining cores from the central features of the crater. The cores can be obtained from a series of deep coreholes (700-2,000 m deep) drilled on the Delmarva Peninsula near the town of Cape Charles, Virginia. Questions 5-7 require additional deep seismic reflection surveys across the southern part of the Delmarva Peninsula and the inner continental shelf east of Delmarva. The search for these answers will provide stimulating challenges for a new generation of planetary geologists . The answers themselves will contribute significantly to understanding the essential role of bolide impacts in the history of our solar system and their implications for its living species .

Appendix

Data Collection, Processing, Analysis, and Interpretation Seismic Reflection Surveys

We include here only the most recent three marine seismic reflection surveys, which provided digital data, and upon which we relied most heavily for detailed interpretation of the structure and morphology of the Chesapeake Bay impact crater. Seaward Explorer

The Seaward Explorer collected 875 km of seismic reflection data over the Chesapeake Bay impact crater during the interval of April 21-30, 1996. The primary imaging system for the crater survey consisted of single-channel reflection profiling using a GI gun (Generator-Injector airgun). This system provided reflections from the basement surface (at about 1.0 s two-way travel time) and overlying sediments. Components for this system were : (I) GI gun (Seismic Systems Inc.) configured in the harmonic mode with generator and injector chambers both set to 45 in3 . The depth of the gun beneath the surface was fixed at 2.5 m. Firing pressure was maintained at 2000 psi; (2) Teledyne 2-channel streamer reconfigured for one channel. The single active section, 50 m long, was attached to a weighted dead section (25 m long), which, in tum, was attached to a feathered lead-in cable. The front of the dead section was approximately even with the gun. The center of the active trace was approximately 50 m behind the gun. Differential GPS navigation was recorded every lOs. An Ashtech GPS XII receiver accepted the GPS information and a Megapule Accufix Dl 00 receiver provided the differential information. We used a USGS PC-based logger, which also provided real-time line and steering information for the ship's laboratory and a remote display for the ship's bridge. We acquired digital seismic data for the GI-gun profiles within Chesapeake Bay using USGS Masscomp hardware and software . Time of each shot was logged from a True Time GPS Model XL-DC clock. A Digital Delay Generator (Model 70 I0) provided the master trigger to fire the gun. Recording parameters were: firing rate 13 s; filters 20-500 Hz; sample rate 0.5 ms; record length 2s. The Seaward Explorer data were processed by Myung Lee (USGS, Denver, Colorado), in the following steps: (I) Trace DC removal; (2) AGC 200 ms; (3) Direct arrival removal; (4) 2-nd zero-crossing deconvolution filter with length of op-

454

Appendix

erator 240 ms; (5) Wavelet processing; (6) F-X deconvolution; (7) Plot. Filtered by a 12-18-250-300 Hz band-pass filter.

RN Maurice Ewing The R/V Maurice Ewing collected 220 Ian of 2-channel seismic reflection data over the crater on October 15-16, 1998. The seismic source for this crater survey consisted of a tuned array of six airguns with chamber capacities of 80, 120, 145, 200, 305, and 500 in' , at 2000 psi. The airgun array was towed 35-45 m behind the ship at a depth of 6 ± 2 m. The hydrophone streamer was 263 m long, with two 50-m active sections, and was towed 100 m behind the ship at a depth of 5 m. Shooting interval was 12 seconds. Continuous marine gravity data were collected simultaneously over the same tracklines used for the seismic survey. The recording instrument was a Bell gravity meter, with a logging interval of 1 s. The shipboard gravity meter was tied to a reference base station by using a portable Lacoste Model G#70 gravity meter. The base station was a brass plate located at dockside in Norfolk, Virginia, where the ship was docked (at the NOAA Atlantic Marine Center, 439 West York Street, Dock 3, 150 ft south of the SW comer of Warehouse Bollard 2; lat 36° 51.193 N , long 76° 17.896 W). The drift in the Bell gravity meter was determined from the difference between the precruise station tie and the postcruise station tie.

Texaco Data The 310 Ian of seismic data kindly provided to the USGS by Texaco, Inc., were collected in 1986 by Teledyne Exploration Co., using a DFS IV recording instru3 ment and an array of 6 air guns (984 in at 2000 psi) as the energy source . The data were filtered at 8-128 Hz, and recorded on 96 channels and processed digitally to produce 48-fold, migrated CDP profiles. Record length was 6 s, sample rate was 2 ms, and spread array was 125-1312 m. Shot point interval was 25 m, with a group interval of 12 m.

Paleomagnetics We took 32 samples for paleomagnetic study from the 66-m section of the Chickahominy Formation cored continuously at a site near Kiptopeke, Virginia (Poag et al. 1994; Poag and Aubry 1995; Poag 1997a). We sampled the Kiptopeke core at approximately 2-m intervals , representing approximately every 25-38 kyr, with the following results :

Appendix Sample Number 1 2 3 4 5

6 7 8 9 10 II

12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Sample Depth [m] 330.1 332.0 333.8 335.7 338.6 339.9 344.5 346.3 348.1 351.7 353.2 354.6 356.4 358.2 360.1 362.7 364.3 366.8 368.2 370.0 371.9 373.7 375.4 378.3 379.8 381.7 383.6 385.3 387.2 388.6 390.5 392.1

Normal Polarity yes ?

455

Reversed Polarity ? yes yes yes yes yes yes yes

yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes yes

The core was oriented only with respect to top and bottom, so the geomagnetic polarity of each sample is based on magnetic inclinat ion. The natural remanent magnetization (NRM ) of each sample was measured with a superconducting magnetometer housed in a magnetically sheltered room (Ed Mankinen, USGS, Menlo Park, California). The NRM was very weak, with a geometric mean intensity of 0.055 mA2/kg for 31 samples. Progressi ve alternating-field (AF) demagnetization experiments on a few selected samples indicated that this method would be ineffective for recovering the primary remanence within this core. Remaining samples, therefore, were subjected to progressive thermal demagnetization in a partial vacuum (50 urn) to remove any gases emitted during the experiments and to inhibit chemic al alteration. Ovens used for the heating experiments were enclosed in a Rubens coil array, which maintained an ambient field of approximately 40 nT. All samples began to chemically alter after heating to temperatures higher than 350 °C. Isothermal remanent magnetization (lRM) curves were generated for representati ve samples by applying direct magnetic fields varying from 0 to 700 nT. Normali zed magnetic intensities were determined for each sample during thermal demagnetization, and geometric means of the entire sample population were calculated at several temperature steps. The range of I standard deviation about

456

Appendix

each mean is quite large, because of an increased amount of experimental noise due to the very weak magnetic intensity, and because of varying thermal properties of the individual samples. An initial rise in intensity often occurred at low temperatures due to the removal of an anti-directed secondary component of magnetization. Some samples generally decreased in intensity until a sharp rise occurred at about 200°C and formed a peak between 200°C and 260°C, which is characteristic of Fe9SIO (Schwarz 1973). Finally, chemical alteration of the samples occurred at different times during the heating experiments. Despite these complicating factors, the general pattern of behavior for the sample population as a whole is readily apparent. There is an almost monotonic decrease in intensity, characteristic of Fe7Sg (Schwarz 1973), with maximum unblocking temperatures reached in the experiments occurring between 300 350°C . These unblocking temperatures are characteristic of pyrrhotite, which has a Curie temperature of about 325°C . The temperature-step curve also provides a hint of a small intensity rise at 200°C, indicating that both Fe7Sg and Fe9SIO must be present in a significant number of samples. The very strong intensity increase beginning above 350°C also is characteristic as the sulfides begin to oxidize and form magnetite. The oxidation seen here is an actual breakdown of the remanence carriers, because, although pyrite also oxides to form magnetite, it does so at a higher temperature (450°C ; Nguyen Tkhi Kim Tkhoa and Pecherskiy 1984) than was reached in our experiments. We conclude that pyrrhotite is the most likely carrier of remanence in the Chickahominy Formation where sampled in the Kiptopeke corehole. Resistance of the samples to AF demagnetization further indicates that this pyrrhotite must be fine-grained (Clark 1984; Rochette et al. 1990). Because pyrrhotite is capable of carrying a strong remanent magnetization comparable to that of magnetite, the weak intensities encountered in the Chickahominy samples must mean that the concentration of pyrrhotite is very low. Perhaps conditions existing within the crater basin were more favorable for the formation of pyrite rather than pyrrhotite. Alternatively, perhaps the original iron sulfide was greigite, which has long since altered to generally less magnetic materials, including pyrite and marcasite, along with small amounts of pyrrhotite and(or) magnetite, depending on redox conditions (Krs et aI. 1992). A significant lag time in the acquisition of remanent magnetization is not likely, regardless of whether pyrrhotite formed as one of the early authigenic minerals, or resulted from breakdown of greigite, because both usually occur in the early stages of diagenesis. Because of the low signal-to-noise ratio in these samples, it often was difficult to ascertain whether or not magnetization had reached a stable end point, or to allow us to rely entirely upon principal component analysis (Kirschvink 1980). Directional trends during the experiments, however, generally were unambiguous and allowed us to determine the geomagnetic polarity with some degree of confidence. Samples in which the magnetic direction was inclined below the horizontal are considered to be of normal polarity; those with inclinations above the horizontal are interpreted to be of reversed polarity. Because of alteration occurring at relatively low temperatures, it is possible that all secondary components of magnetization were not removed. Interpretation of one-sample reversals, therefore, 0and

Appendix

457

should be avoided, although some of these may represent actual geomagnetic field behavior (e.g., the "tiny wiggles" or "cryptochrons" of Cande and Kent I992a,b). Misinterpretation of a single sample's magnetic polarity also may result from incorrect orientation or physical disturbance of the core segment. The polarity zonation we use emphasizes the predominant polarity of any given interval within the Kiptopeke core.

Stable Isotopes

We performed stable-isotope analyses for oxygen and carbon on 40 samples taken from the same cored interval sampled for the foraminiferal study at Kiptopeke. The raw data are given below. We used monospecific samples [-3-20 individuals of Cibicidoides pipp eni f. speciosus (Cushman and Cederstrom) 1949] in the >63 urn grain-size fraction. Mass spectrometry was achieved using a Finnigan MAT 252 instrument with an on-line automated carbonate reaction Kiel device (Dick Norris, Woods Hole Oceanographic Institution). Analytical precision based on repeated analysis of standards (NBS- I9, Carrara Marble, and B-1 marine carbonate) was better than 0.03 °/0 0 for 0 13C and 0.08 °/0 0 for 0 180 relative to the Peedee belemnite (PDB) standard. Poag and Norris (in press) used identical processing on 66 samples of the Chickahominy Formation from the NASA Langley corehole. Depth [m]

326.3 327.7 328.0 329.6 330.4 330.7 331.6 332.3 333.8 337.5 339.6 341.7 343.2 344.5

/) 13e

/)

0

Depth [m]

s13e

/)

0

Depth [m]

s13e

/)

+0.061 -0.27 -0.34 -0.71 -0.44 -0.75 -0.97 -0.75 -0.53 -0.39 -0.60 -0.56 -0.27 -0.70

+0.808 -0.152 -0.013 +0.201 +0.027 -0.037 +0.036 -0.218 -0.165 -0.182 +0.006 +0.080 -0.089 -0.120

347.4 349.1 352.6 354.4 355.2 365.3 360.3 361.2 363.0 366.7 366.9 369.4 371.2 375.3

-0.43 -0.65 -0.85 -0.83 -0.71 -0.98 -0.72 -0.46 -0.84 -0.85 -0.68 -0.04 -0.16 -0.33

-0.192 +0.046 +0.106 +0.148 +0.071 +0.048 +0.063 -0.040 -0.166 -0.243 -0.127 +0.086 -0.173 +0.252

384.7 387.2 389.2 390.2 376.6 378.6 381.0 383.5 390.7 391.7 392.2 393.7

-0.06 +0.128 +0.21 +0.04 +0.15 +0.147 +0.104 +0.058 -0.22 -0.25 -0.28 -0.68

+0.056 -0.130 -0.011 -0.070 +0.022 +0.029 -0.040 -0.019 -0.020 -0.111 -0.146 -0.275

18

18

18

0

Foraminiferal Assemblages

We extracted foraminiferal assemblages from - 20-cm3 core sections using standard micropaleontological procedures. Core samples were boiled in a solution of water and sodium hexametaphosphate to disperse the clays, and then wet-sieved over a 63-llm screen to separate both planktonic and benthic foraminiferal tests.

458

Appendix

Oven-dried (70°C) specimens were examined and identified to species level, using optical and scanning-electron microscopy. Taxonomic nomenclature employed by us is based on comparison with published literature and the senior author's personal collections of US Gulf and Atlantic Coastal Plain foraminiferal assemblages . Species-level taxonomy is primarily from Cushman (1935) and Cushman and Cederstrom (1949). Generic-level taxonomy is mainly from Loeblich and Tappan (1988). We have used quotation marks (see Chapter 13) to indicate informal species names that have no valid taxonomic status.

Isopach and Structure Maps

We constructed isopach (equal thickness) and structure maps from a 2,000-km network of intersecting seismic reflection profiles, after converting 2-way traveltime scales to depth scales, as described in Chapter 3.

Gravity Modeling

For comparison with the observed Chesapeake Bay gravity anomalies, P. MoIzer (USGS 1998) translated models (blocks of constant-density rocks) into gravity responses with the software program GM-SYS (Northwest Geophysical Associates , Inc. 1999). We illustrate the gravity model with three cross sections through the inner basin, which also pass through, or near, as many data points as possible . Each section is ~90 km long and contains - 500 gridded gravity points. For modeling, Moizer constructed profiles with simple geometries along each line. Each model's upper, topographic surface was extracted from a Defense Mapping Agency 5-second horizontal resolution grid. MoIzer picked flat basement surfaces for each line by connecting basement depth picks at the end of each line, outside the crater's disturbed zone. The depth values come from the structure map of the basement surface defined by scattered well penetrations and numerous seismic reflection profiles (Figs. 4.1, CD-ROM .I,2). Because no direct measurements of rock density were available for this area, MoIzer chose generic density values used in gravity investigations of other impact craters (Plescia 1996; Pohl et al. 1977; crystalline basement rock = 2.67 g/cnr' ; layered siliciclastic sedimentary rock = 2.3 g/crrr'; crater fill = 2.56 g/cm'). Because the regional gravity anomalies are quite heterogeneous, consistent with probable heterogeneity in the basement composition (Lefort and Max 1991), Moizer made no attempt to model the gravity beyond the seismically-defined outer periphery of the peak ring. Cross sections of the starting model conform to observations from nearby seismic reflection profiles, with the exception of disturbances in the basement due to the impact (Fig. 4.36). Seismic profiles through the impact structure show disturbance and excavation of the basement surface inside the peak ring, characterized by loss of the basement surface reflector . The reflection data indicate uplift of the basement surface to form the peak ring. Outside the crater, the basement rock is overlain by an

Appendix

459

undisturbed sedimentary section, which sags into the crater. Continuous reflectors in the postimpact sedimentary section continue relatively undisturbed across the crater, with no indications of significant lateral facies change that could complicate the shallow density structure. Modeling thus focused on variations in basement density and shape of the basement surface, assuming a laterally homogeneous overlying sedimentary section.

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Index

*In order to conserve space, we have used CBC as an abbreviation for "Chesapeake Bay impactcrater" throughout the index. Accomac Member, Omar Formation, 441 accommodation space, 5I accordion folds, 168 accumulation rate curve,388 drop, 399 rapid,387 sediment, 62, 263, 283, 284, 290, 387-391 ,394,399,407,416,449 shift, 417 uniform, 283 acid rain, 422 acoustic fluidization, 365, 384, 422 Acraman crater, I, 10 age *CBC, 279-286 foraminiferal subzoneboundaries, 396 impact, 255, 292, 387 Popigai crater, 303 agglutinated. See benthic foraminifera Aglaonica crater, 329 Agulhas Ridge, 289 airfields, around *CBC, 433 airgun,453, 454 Albian fossils 193 Alloformation Accomac Canyon, 49 Babylon, 53 Baltimore Canyon, 49,52 Berkeley, 53 Carteret, 49, 5 I HudsonCanyon,53, 55 IslandBeach, 49, 50 Lindenkohl, 49, 5 I Mey, 53, 54 Phoenix Canyon, 53, 54

Sixtwelve, 49, 50 Toms Canyon, 53, 55 allostratigraphic unit, 48. See also Alloformation Ames crater, 10, 307 Ridge, 443 Anabar Shield,Siberia, 301 annulartrough *CBC,45, 85, 91 ,120-123,139,175, 185-189,215,267,268,291 Chicxulub crater, 336 Kamensk crater, 326 Kingcrater, 331 Lockne crater, 322, 350 Manson crater, 3I8 Mjalnir crater, 314, 318 Montagnais crater, 307, 354 Popigai crater, 351 Ries crater, 306, 345 Antarctic assemblages, 291 cores, 429 Site 689B, 285, 286 Antarctica, 423, 429 anticlinal folds, 163 Appalachian Mountains, 58, 376 orogen, 41 orogeny, 285 Piedmont Province, 41, 48, 59 Aquia Formation, 50, 158,249 aquifer freshwater, 433, 438 saline, 450 39 40Ar/ Ar plateauvalues, 283 39 40Ar/ Ar step-heating dating, 289

490

Index

Atlantic City, New Jersey, 66, 292 Coastal Plain, 47 Continental Shelf, 3 margin, 57, 58, 64,435 Ocean, 43, 425,435,444

Atlantis II, 80 atmosphere CO 2, 432 disruptions, 421 effects, 287 ejecta, 376 gas, 432 perturbations , 4 19, 432 response, 423 warmth, 423 Austria, 363 Babylon Alloformation, 53 baddeleyite, 363 Baltic Shield, 326 Baltimore Canyon Alloformation, 52, 58 trough, 47, 63,64,435 Baltimore, Maryland, 50 Barbados, 64, 285,295-297,425,43 1 Barents Sea, 13,309,314 Barremian sedimentary rocks, 307 Barringer (Meteor) crater, 10, 224, 362 Bartan crater, 329 Basal Member, Onaping Formation, 356,357 basalt flows, 326 basement acoustic (AB),*CBC, 4, 4 1-45 , 47, 48,73-80,85,91 ,92, 104, 120, 139,1 46,1 58,1 71, 178,1 84, 187, 193,204,208,227,233,281 ,288, 360,362,376,377 Chicxulub basin, 338 cores, 216 couplet, 75 faults, 444 gradient, 285 Lockne crater, 351 Manson crater, 348, 350 map, *CBC, 45, CD-ROM.5 Mje lnir crater, 318 Montagnais crater, 307-309, 311- 313 Popigai crater, 354 reconfigured, 293

reflection, 158, 163 rocks, 438 superheated , 363, 416 upraised, 30 1 basin central, 370, 372 Chicxulub, 1, 139, 154, 336, 338, 339, 387,42 1 closed, 387, 410 Culpeper, 42 Delmarva, 42 multiring, I Norfolk, 43 Queen Anne, 43 salinities, 416 Taylorsville, 42, 43, 75, 163, 167- 169 Bath Cliff, Barbados, 285, 295, 297, 425,431

Bathysiphon Abundance Subbiozone, 396 sp., 395,40 1 Bathysiphon Subassemblage, 401, 402, 407,411,415 bay floor, 277, 361, 440 Bayside breccia, 357 core, 177,201 corehole, 43,45, 53, 171-173 , 175, 184, 186,188,193 ,202,205,2 15, 259, 260, 263,266,383,391 ,438 logs, 383, CD-ROM .7 section, 216 bedding, inclined, 184,38 1 bediasites, 64, 249,250 benthic foraminifera abyssal, 415 agglutinated, 391, 396, 397, 40 1, 402, 419 assemb lages, 407, 411 associations, 4 15 Chickahominy Formation, 57, 259277,380,389-402 community, 389 Exmore breccia, 193 generic equitab ility, 402-406 generic predominance, 402-406 Kiptopeke,389-402 nominate species, 395 nutrient supply, 4 15 outer neritic, 412-414

Index paleoenvironmental indicators, list, 412-414 postimpactcommunity of, 390 reworked, 391 species, 394, 401, 412 species richness, 407, 408 subzones, 395, 396 tests, 260, 457 Berkeley Alloformation, 54 Big Bethel scarp, 441, 443 shoreline, 443 Bigach crater, 155 biochron NPI9-20,279 P15,279 biochronology, 279, 283, 291, 388 biogenic methane, 431 biosphere global, 287 late Eocene, 419 biospheric effects, Chesapeake Bay impact, 387-432 biostratigraphic framework, 279 record, 283 biostratigraphy boreholes, 155 Chickahominy Formation, 279 biotopes, 58, 410, 411 Biozone Cibicidoides pipp eni, 391 , 394 PI5,283 ,291 ,297 PI6,283,29 1,297 bivalves, Chickahominy Formation, 411 BlackMember, OnapingFormation, 356,360,361 blanket. See ejecta blanket Bolboforma latdorfensis,398 sp inosa, 398

bolboformids, 259, 279, 280, 281, 282, 411 bolide bombardment, 1 Chesapeake Bay, 48, 57, 64, 380, 421, 447 Chicxulub, 332 Icrater diameter, 421 diameter, 377, 421 evidence, 64

491

explosion, 438 -generated, 255 impact, 1, 4, 48, 208, 292, 298, 318, 343,357,369,380,386,407,4 19, 422,424,447,449,450,45 1. late Eocene, 4 properties, 370 vaporized, 373 Bolivina by ramensis,410

co-predornonance, 411 , 415, 416, 419 gracilis , 405 jacksonensis Subassemblage, 401

opportunist, 404 "praevirginiana", 405 predominance, 402, 403 tectiformis, 395, 399, 400 tectiformis Subassemblage, 401 tectiformis Subzone, 407 tectiformis Taxon-rangeSubbiozone, 396 virginiana, 405

bombs, 345, 351 Bonheur crater, 329 borehole(s) basement rocks, 43, 44, 47, 75, 92 biostratigraphy 155 *CBC, 4-6, 56, 78, 92, 110, 142,CDROM .I cored,69 list, 17-39 lithostratigraphy, 155 Manson crater, 319 map, *CBC, 5, CD-ROM.1 Maryland and New Jersey, 389 Mjelnir crater, 315 Montagnais crater, 354, 355 near seismictracklines, 85 noncored, 69-72 borrow pits, 55 bottom water, 425, 432 anoxic, 411, 415 chemistry, 410, 411 disoxic, 411 microxic, 411 Bouguer gravity anomaly data, 86 map, 88, 89, 147, 313 boulder CIB ratio, 383 clay, 417

492

Index

Cretaceous, 212, 213 Exmore breccia, 202, 212-214 limestone, 213 Lockne crater, 154 M/B ratio, 382 scalyclay,210, 212, 214 sedimentary, 215 breccia apron, 155, 189, 190, 193,360 authigenic, 326, 370 basin-fill, 336 bodies, 356, 361 clasts,286, 288, 376 clast-supported, 193,357,384 compaction, 170 comparisons, 343-363 crater-fill, 139 crystalline-clast, 348, 351, 354, 357, 383 dikes,348 Exmore, 185-253 fill,52,69 -filledgullies, 322 low-density, 338 matrix, 202, 217 originand deposition, 301 stratigraphies, 343-363 unit, 171,318 upward-fining, 193 Brentcrater, 10, 307 Brightseat Formation, 50 brine aquifers, 433 Exmore breccia, 436-439 hypothesis, 435, 438 impact-generated,4 interstitial, 433 map, 435 model, 436-438 reservoir, 438, 439 British Institutions Reflection Profiling Syndicate (BlRPS), 333 Bulimina

co-predominance, 405, 411, 416 jacksonens~,395,399,400,410

jacksonensis Interval Subbiozone, 395 jacksonensis Subassemblage, 398 jacksonens~

Subzone,405,407 opportunistic, 411 ovata, 419

Buliminella, 411

Buliminellita curta, 400

Bunte Breccia, 345, 348, 350, 360-362 burning, impact-generated, 422 burrow casts, 259, 391, 410, 411, 415, 417 Chickahominy Formation, 52, 258263 -fills,202 ButlarsBluff Member, Nassawadox Formation, 441 calcareous nannofossils, 51, 57, 193, 259,279-282,291,411 CalvertFormation, 54, 266 Cambrian blackshale, 319 shale,350 Campanian fossils, 50 Canyon Grand,4 Rappahannock,94, 104, 120,268, 444 Cape Charles Delmarva Peninsula, 289 harbor, 140 Cape Hatteras, 48 Cape Henry, 86, 441, 443 Caravaca, Spain,419 carbonaceous chondrites, 240 carbonate ramp, 58 reflections, Chicxulub basin, 336-341 rocks, Chicxulub basin, 333 carbondioxide(C0 2), 422 Caribbean Sea, 64, 65, 288, 291, 394 CarteretAlloformation, 51 Cassidulina tenuicarinata, 410 Caucasina

co-predominance, 405, 415, 416, 419 marylandica, 399, 400, 405 COP profiles, 454. See also seismic reflection profile Cenomanian fossils, 50 Cenozoic age, 345 deposition rate, 58 deposition sequence, 57 deposits, Virginia, 50 lowstands, 52 marine regressions, 389 marine strata, 402

Index

sea-level falls, 387 sedimentary layer, 336 sediments, 363 strata, 383 central peak *CBC, 8, 9,140-146,304,321 ,325328,345,385,386 Chicxulub basin, 341 collapse, 330 craters, 330 crystalline, 301, 307, 318, 322 Euler crater, 154 formation, 366, 373, 376 Lockne crater, 324, 325, 332, 350, 370,371 ,385 low, 370 Manson crater, 318-321, 348, 349 Mjelnir crater, 316, 369, 370, 385 Montagnais crater, 312, 318, 354, 355 Popigai crater, 303, 305, 306, 351 Ries crater, 304, 345-347 cerium (Ce) anomalies, 242 Cfu (crater-fill unit), *CBC - 1, 357- 359, 361 -2,357-359 -3,357-360 -4,359-361 -5,359-361 -6,359-361 channel-fill, 69 channels radial, 370, 382 river, 444 steep-walled, 294 Charles City Formation, 55

Char/tonina madrugaensis, 405 Charpentier crater, 329 chemostratigraphic data, 292 Chesapeake Block, 41,163 Virginia, 55, 433 Chesapeake Bay basement beneath, 292 Bridge tunnel, 446 eastern margin, 292 impact, 61, 62, 69, 249, 280-298, 362,373 ,381 -387,417,421 ,432, 449 map, 3, 5,44, 60, 70, 78, 79, 87, 94, 434,441 ,442 mouth, 110

493

region, 242, 444-450 seawater from, 435 seismic profiles, 73, 77, 287, 453 southern (lower) part, 77,433-450 strata beneath, 69 west side, 85 Chickahominy assemblages, 410 benthic foraminifera, 402 deposition, 411 foraminiferal association, 401 Formation, 6, 51, 52, 57, 69, 99, 102, 103, 106-112, 116-119, 122, 123, 126-137,141-145,174,175,193, 212-215,255,259-284,387-410, 425,438,440,454-457 paleoenvironments, 407 River, 44 second depositional episode, 387 section, 399 third depositional episode, 387 Chicxulub impact basin, I, 10, 139, 154,336, 338,339,363,387,421 impact site, 417 chlorinity, 433, 435 Choptank Formation, 54 Chowan River Formation, 55 chromium contents, Exmore breccia, 249 Chron 13,429 13r, 425, 431 15n,429 15r, 283, 286, 425 16n.ln, 283, 286, 425, 429,431 16n.2n, 283, 291, 425, 431 16r.lr, 425, 429 chronodepositional framework , Chickahominy Formation, 388 chronostratigraphic chart, Chickahominy Formation, 431 Chuckatuck Formation, 55, 440

Cibicidoides pippeni, 395 pippeni Assemblage, 391, 394, 400, 407,411 ,419

pippeni f. speciosus, 457 pippeni Taxon-range Biozone, 394 pippeni Zone , 391 "rugoumbonatus",399

494

Index

CIPW normative compositions, Exmorebreccia,233, 242 proportions, Exmorebreccia, 246 citizens, southeastern Virginia, 438 clams, Chickahominy Formation, 4I0 c1ast(s) angular, 184,197,204,208,298 Bayside, 202 beddingplanes, 204 Bunte Breccia, 345 carbonate, 225, 226 chaoticallymixed, 176,345 clay, 184, 200, 208, 214, 382 clay-injected, 176 contact, 198, 204 crystalline basement, 171 , 177, 178, 193,208,2 1I , 216, 217, 224, 225, 230,242,345-363,373,38 1-383, 447--451 ejecta blanket, 153 ejecta curtain,38I entrained, 193 Exmore breccia, 67, 193, 200-204, 212,224-226,233,242,253 ,293, 360-363,373,376 fallback breccia, 377 flowin facies, 205 frequency, 202 impact melt, 233 Jurassic, 345 K-feldspar,224-226 laminations, 204 limestone,204, 298, 354 Locknebreccia, 350, 35I Locknecrater, 348, 350, 360 Manson crater, 348, 350, 360 matrix-supported, 2I4 melt rock, 348, 350 metamorphosed, 233, 363, 376 metasediment, 356 Montagnais crater, 354 Paleogene strata, 293 parent, 208 petrographic analyses, 218-233 plagioclase, 225 Popigai crater, 35I population, 233 Potomac Formation, 292, 294 quartz, 225 rind, 177

rounded, 197 sand, 226 sandstone, 171 sedimentary, 193, 215, 223, 233, 242, 246,345 ,350-360,377 ,384,450 shocked, 348 soft-sediment, 199 Sudburycrater, 356, 357 surgeback breccia, 383 Tandsbyn Breccia, 350 Triassic, 345 with mud rims, 198, 208 clay altered glass, 204 band, 202 block, 215 boulder, 212 Cenozoic, 345 ChickahominyFormation, 52, 57, 255,259,262,263,266,410,4 11, 438,449 clast, 178, 184, 200, 208, 382 dead zone, 255, 361, 390 Exmore breccia, 196, 2I8-226 , 234, 247,253,258,261,357,380,382, 437 fillings, 208 glass-derivative, 448 -injected clasts, 176, 179 K-T boundary, 417, 419 late Eocene, 255 Mansoncrater, 348 marine, 6,47, 50-52, 193,449 Marlboro, 50 massive, 204 megablocks, 176 Nanjemoy Formation, 51 nonmarine, 47 Paleocene, 208 paleosol, 181, 184, 215 plastic, 50 rim, 208 scaly, 179, 184,208-214 shelly, 54 silty, 214 SP curve, 212 St Marys Formation, 54 varicolored, 176 climate cooling, 28I, 423 impacteffect, 42I

Index modem, 58 postimpact, 449 shifts, 415 wann, 416, 424 clinopyroxene crystals, 295 coastalplain, 42, 47,59,445 cobalt, 249 Cobb Islandfault system, 291 cobble-to-boulder ratio (CIB), 384 coesite, 64, 295 coherency loss, 91 coherent sheets, impactmelt, 363 collapse factor, 370 features, 176 fireball, 356 gravitational, 377 oceanic water column, 350, 370, 377 structures, 155 transientcrater, 339 violent, 322 zone, 94, 286 Colonial Beach, Virginia, 158, 170, 362 comet shower, 423, 424, 432 community structure, benthic foraminifera, 402, 407 compaction breccia, 51 differential, 270, 273-275, 339 faults, 79, 270 comparison biotic changesat K-T boundary, 417 Chicxulub impact basin, 332 impactcraters, 301, 302 impactites, 302, 343 impactmodels, 385 compression impact-generated, 376 ridges, *CBC, 103, 120-123,291 computer modeling, 368 simulation, 372, 374, 381 conceptual model(s) brine formation, *CBC, 436, 437 Chesapeake Bay crater, 372, 386 comparison, 385 crater-fill deposition, 377 crater formation, 6, 30I, 365 excavation and deposition, 367, 369 holistic, 372 Lockne crater, 371

495

Mjolnircrater, 369 conceptual reconstruction, pyrite lattices, 207 concussive debris, 382 confiningunits, 433 Connecticut, 438 consequences Chesapeake Bay impact, 281, 287300 environmental, 301, 421 hypothetical, 281 natureof, 281 contact and compression, 365, 373 clast-to-matrix, 204 crateringstage I, 365, 373 inclined, 198, 384 vertical, 184 contamination hazard,438 Contessa Highway section, 286, 291 convoluted flow bands,204 cooling atmospheric dust, 422 event, 423, 429 global,429 gradient, 429 impact-generated, 287, 422, 423 long-term, 423 short-tenn,422 step-wise, 423 cool-water interval, 431 corals, ChickahominyFormation, 411 core(s) Antarctic, 429 basement, 216 Bayside, 177,205,257,357,438 boulders, 213-215 *CBC,324, 360, 380 Chickahominy Formation, 52, 259 deep, *CBC,243 deep-sea, 64, 67, 251, 387, 425, 429, 449 Exmore, 197-204 , 234 Exmorebreccia, 193,204, 208, 210, 211 ,253 ,381 ,382 extracrater, 263370 flow directions, 383 flowin lithofacies, 202 glauconite, 214 Hammond, 391 highlyfragmented, 213

496

Index

Kiptopeke, 401, 403, 408, 429, 454457 lithologies, 6, 259 Manson, 348 matrix-domonated, 213 Mjolnir crater, 314, 384 NASA Langley, 45, 178-183,202, 206,210,360,390,438 North, 205, 438 Oak Grove, 50 ODP 689B, 425, 429 photographs, 117-183, 192, 196-205, 209-211,258,260-262,271 recovery, 193,213,214,216 scaly clay, 214 sediment, 171, 193,204 Southern Ocean, 423 split, 6, 204, 211, 260, 262 stable isotopes, 425 thin sections, 228, 229, 231, 232 weathered, 262 Windmill Point, 189,210 coreholes, list, 17-39. Seealso by name correlation geochronological, 285 geophysical logs, 263 gravimetric data, 333 other craters and impactites, 283 problems, 291 counterpart species, 410, 415 crater(s) Acraman, I, 10 Aglaonica, 329 Ames, 10, 307 Barringer, 10, 224, 362 Bartan,329 Bigach, 10, 155 Bonheur, 329 Brent, 10, 307 Charpentier, 329 Chicxulub, 1, 10, 139, 154,336-339, 363,387,417,421 concentric, 319 Euler, 154 excavation, 361 floor rebound, 373 Granby, 11,307 Haughton, 12, 139 Kaluga, 12, 307 Kamensk,12,307,324,326 Klirdla, 12,307,326

King, 330-332 Lagerof, 329 list, 10-16 Lockne, 13, 154,318,319,351 Lonar, 13, 224 Manicouagan, I, 13 Manson, I, 13, 139, 154,307,318, 326,343,348,357,360 Meteor, 224, 362, 365 Mjelnir, 1,3,13,154,307,314,315 modification, 350 Montagnais, 3,13,154,307,314,326 Morokweng, I, 13 Popigai, 1,4,14,295-307,314,343, 351,357,360-363,420-424,432 Puchezh-Katunki, I, 14 Ragozinka, 14, 362 Ries, 1, 14, 153,224,303,307,314, 350,360-362 rim,4,43 ,85,94,189,433,440-444 subaerial, 301, 306, 365, 366 submarine, 3, 301,307,326,354, 366, 368 Sudbury, I, 15,343,354-360 suevite, 345 terrestrial, 30 I Toms Canyon, 3, 66, 294, 307, 326, 423--425,432 transient, 345, 348, 363-373 Ust Kara, 3,16 variability, 372 Vredefort, I, 16,433 Yablochinka, 329 crater-fill breccia, 139,324,333-341 ,354 debris, 171 deposits, 171, 330 lithofacies, 382, 384 model, 382 unit I (Cfu-I), 357-359, 361 unit 2 (Cfu-2), 357-359 unit 3 (Cfu-3), 357-360 unit 4 (Cfu-4), 359-361 unit 5 (Cfu-5), 359-361 unit 6 (Cfu-6), 359-361 crenulate folding, 208 crest central peak, 318, 354, 370, 384, 385 elevation, 146 peak ring, 120, 138, 139, 149, 189, 268,290,303,306,351,365

Index subpeak, 146 Cretaceous boulder, 212, 213 carbonate and evaporite section,336, 338,339 Early, 47, 158,383 ,391 foraminifera, 69 F unit, 292, 293 interiorseaway, 318 Late, 50, 318, 348, 391 Lower, 48,75, 158, 181,209,292294 shales, 350 -Tertiary boundary, 417 Upper, beds, 50 Cribrostomoides sp., 401 Crisfield borehole, 77 Maryland, 75 unit, 48 criticalthreshold, 381 cross bedding, 55,176 ,180,351,384 cross section(s) *CBC, 8, 9, 93, 172, 256, 304, 308, 321 , 325, 327, 341, 358, 439, CDROM.6 Chicxulub crater, 341 Locknecrater, 325 Mansoncrater, 321, 349 Mjelnir crater, 308 Montagnais crater, 309 Popigai crater, 305, 353 Ries crater, 304, 347 southeastern Virginia, 59, 71, 72 Virginia continental margin, 63 crust, continental, 64 cryosphere, 431 crystallographic orientation, PDFs, 217, 230 CTH shock physics hydrocode, 372 Cuba, 64 Cubitostrea sellaeformis, 51 Culpeper basin, 42 curtain, ballistic, 381, 382 Cyclammina cancellata, 40I Cycle 1,407 Cycle2,407 Cycle3, 407 cycles, low-to-high speciesrichness, 407 Czech Republic, 363

497

DalbyLimestone, 322 Darcy's law, 438 dead zone, *CBC, 6, 57,193,202,255, 257,361,380,390,391,407,417 debriite, 69, 350 debris flow deposit, 193,208, 350, 385 storm-generated, 380, 381, 448 decollement Mansoncrater, 318 Mjolnircrater, 314-317, 370 zone, 193, CD-ROM.7 DeepSea Drilling Project(DSDP) cores, 251 Site 612, 66-68,279-289,425 DefenseMapping Agency, 458 deformation brittle, 204, 345 plastic, 176, 184, 199,345,348,383 soft-sediment, 176, 179, 182, 204, 215 squeeze, 209 Delaware, 421 Delmarva basin, 42 beds, 52, 266 Peninsula, 6, 48, 77, 80, 86, 97, 110, 112, 124, 142, 149, 156, 159, 162, 289,290,292,438,441,443 unit, 259 (i

13

C

excursion, 431 negative shift, 416 record, 415, 425-431 table, 457 (iISO

negative shift, 416 planktonic foraminifera, 423 profile, 429 record, 415, 425-431 Southern Ocean cores,423 table, 457 deposit age, 47 back-barrier, 55 basin-fill, 52 breccia, 343 carbonate ramp, 58 Cenozoic, Virginia, 50 channel-fill, 69

498

Index

clast-supported, 350 crater-fill, 171-233,326,330,357, 448 dead zone, 361 debrisflow, 193,350,384 ejecta, deep-sea, 295 ejecta, distal, 417 ejecta, Massignano, 297 Eoceneclay, 255 extracrater, 381, 384 fallout ejecta, 204, 361, 384 flowin, 361, 380-386 graded,381-384 hydrothermal, 178 hypercane, 372, 380 impact, 49, 57, 204, 224, 287, 303, 318,326,354,356,362,381 ,384, 425 Locknecrater,322, 350, 351, 372 lowerOligocene, Virginia, 52 lowstand, 48 megablock, 448 Mjelnircrater,370 Montagnais crater,354 Popigai crater,361 postimpact, 51-55, 75, 255-270, 307, 322,361,449 preimpact, 48-51 Quaternary, Virginia, 270 Ries crater, 361 rift, 75 sand,64 seafloor-surge, 383 sedimentary, 47, 336 shelf,314 - siliciclastic, 52 silt, 64 slumpback, 348, 350 subaqueous, 448 subsurface, 193 Sudbury crater, 356, 361 sulfide, 184 surgeback,350, 351 , 356,361,372, 448 synimpact, 171-233,447,448 Taylorsville basin, 163 uniform, 263 upper Eocene, 48, 51 Virginia, 47 deposition abiotic, 391

rates,58, 291 depositional episodes, 387 facies, 417 lithofacies, *CBC, 377-379, 382 processes, 362 regime, *CBC, 6, 358, 359, 377-379, 382 setting, 255 depth conversion, 85, 86 excavation, 365 sections, 86, CD-ROM.6 deuterium eH), 438 devolatilization, 251 dewatering structures, 351 diabase, 75 diagenesis, 263, 456 diameter bolide, 370 centralpeak, 366 outerrim, 366 transient crater,366 diamicton, 185 Diamond Springs scarp,443 diatoms, Chickahominy Formation, 411 dike, 350, 370 dike breccia, 343, 348 dinoflagellates, 193, 259, 411,422,429, 431 diorite, 41 Discriminant Function analysis, Exmore breccia, 242, 247, 248 Dismal Swampcorehole, 50 displaced megablocks *CBC, 77, 79, 85, 91, 95-99, 106109,111-113,117 ,122 ,137,171 , 176,180-184,193 ,214,215,385 Chicxulub basin, 336 Eulercrater, 154 Kingcrater,331 Lockne crater, 384 Mansoncrater, 349, 350 Mjelnircrater, 385 Montagnais crater,354 disruption, preimpactsedimentary column, 292 distal ejecta deposition, 287, 294 deposits, 285, 417 diversity, foraminiferal genera, 407, 419

Index dominance, matrixover clasts, 360 downhole geophysical logs, 184, 212, 263-265, CD-ROM.7. See also logs dust in atmosphere, 432 loading, 422 particles, 423 submicrometer,422 earthquake displacement, 440 epicenters, southeastern Virginia 440, 441

East Coast, 48 EasternSlate Belt, 41 EastoverFormation, 54 echinoid spines, Chickahominy Formation, 259, 260, 410 echinoids, Chickahominy Formation, 411 effects atmospheric, 287 biospheric, 387-432 climatic, 421 cooling, 422 globalpaleoenvironmental, 419, 421, 425 greenhouse, 424,425 hydraulic, 366 impact, 294, 420-423, 433-446 lag, 407 loading, 366 long-term, 423 paleoenvironmental, 6, 387 residual, *CBC,433 seismic, 287, 298 shock, 91, 217, 226, 233, 234 short-term,421 subduction, 154 surgeback, 91 washback, 368 ejecta ballistic, *CBC, 381-384 -bearingimpactite, 283 blanket, 64, 153, 189, 330, 331 , 345, 362 blocks, 153 bombardment, 163 Chesapeake Bay impact, 69 curtain, 368, 370, 373,376 deposits, 419

499

distal, 294 field, 64-66 generation, 30I high-velocity, 385 in atmosphere, 422 layer, 193,425, 431 Massignano, Italy, 431 Mjelnir impact, 314 New Jersey, 67 North American, 66 rays, 65 strewnfields, 363 ejection ballistic, 345, 361, 368, 382 process, 204, 365 velocity, 153 elevation data, 269 E1 Kef, Tunisia,417, 419 embayment, 45, 292 energyyielded, scaledto crater diameter, 421 environmental changes, 431 circumstances, 420 conditions, 404 consequences, 421 damage, 422 department, 69 effects, 421 limits, 410 perturbations, 301 properties, 415 Eocene early, 307, 362, 391 epoch, 280,424 foraminifera, 69 late. See late Eocene middle, 51, 58,158,391 -Oligocene boundary, 295, 400, 425 -Oligocene contact, 407 origin, 438 pelagic limestones, 294 sediments, 423 shelf break, 58 upper, 48, 52, 297 epicenters, 441 . See also earthquake epicontinental sea, 318 epifauna, 411 Epistomin ella

co-predominance, 405, 411 , 415, 416, 419

500

Index

minufa,405 equitability generic, 402-405, 415 species, 402 erosional escarpment, 319 features, 441, 443 scarp, 324, 354 surface, 318 Estonia, 326 Euler crater, 154 europium (Eu) anomaly, 240, 242 evaporites, Chicxulub basin, 333-341 event EPi-2, 429 Ewing seismic profiles, 283. See also seismic reflection profile excavation impact, 389 initial stage, 376 maximum depth, 365 stage, 345, 365 Exmore breccia, 51, 52, 57, 67-70, 77, 85, 92, 93,99-103,106-109,111,112, 116-119,122,123,126-137,141 146,158,185-255,259,260,266, 267,279,283,284,287,326-330, 339,357,360,372,373,382,383, 389,416,433,435,438,445 corehole,53 , 112, 171-174, 186, 188, 193-201 ,211,214,216,233,235, 241,242,250,253,260,263,382, 390,405 matrix, 193, 196-198,216 Virginia, 185 extinction K-T boundary, 417 event, 417 mass, 301, 419, 423, 429 extracrater Chickahominy assemblages, 410 cores, 263 deposits, 385 jetting, 381 lithofacies, 377, 382 regimes, 381 washback deposits, 381 extraterrestrial craters, 326 Exxon Exploration Company, 77, 155 model, 48, 57

fall line, 64 fallback breccia , 177,357,370,377,383,385 ejecta, 356 particles, 351 process, 204, 322 suevite, 345, 348, 357 fallout debris, 361 deposit, 361 ejecta, 204 layer, *CBC, 6, 51,193,202,255, 361,385,391,407 lithofacies, 381, 382, 385 particles, 351 process, 204 silt layer, 206 suevite, 345, 351 unit, 360, 361 far-field seismic effects, 298-300 fault(s) blocks, 270 Chickahominy Formation, 438 clusters, 270 compaction, 440 concentric, *CBC, 95, 96, 103, 106, 107, 111,290,306 cores, *CBC, 204 crystalline basement, 77 en echelon, 157 extensional, 176 growth, *CBC, 270, 273-277, 440 high-angle, 307 hinge zone, 64 listric, 314, 318 near-surface, 440 normal, 290, 333 planes, 440 radial, 94, 120,284,306 reverse, 163,291 scarp, 91, 324 systems, 270 faunal shift, 416 Fay, 80 feeding strategies, 415 Fentress corehole, 52 Fe7Sg,456 F~SIO, 456 Fick's law, 438 field

Index evidence, 385 studies, 368, 377 filamentous organic detritus, 411 fine-grained matrix, 343, 357, 381 fireball, Chesapeake Bayimpact, 373, 376 fires, impact-generated, 422 fish fossils, Chickahominy Formation, 411 skeletal debris, 259 flame structures, Exmore breccia, 201, 208 flashevaporation, Chesapeake Bay impact, 438 floor crystalline, 345 innerbasin, *CBC, 139, 140, 146 Locknecrater, 370 Mjolnircrater,369 morphological,326-330 secondary craters,Chesapeake Bay, 158 structural,326-330 flow bands,204 direction, 381 high-velocity, 322 multidirectional, 360, 384 structures, 204 turbulent, 322, 350, 360, 363 flowin depositional facies, 205 deposits, 384, 385, 387 layer, 361 lithofacies, 202, 385 multidirectional,372 regime, 381 turbulence, 204 unit, 193 fluids, interstitial, 438 fluidized megablocks, *CBC, 376 sands, *CBC, 176 flux labileorganic carbon, 415 organic carbon, 417 rates, 415 fold axis, 209 chevron, 169 food supply, benthic foraminifera, 411

501

foraminifera assemblages, 417, 431, 457 bathyal, 412-417 benthic. See benthic foraminifera Chickahominy Formation, 387-419 Exmore breccia, 51 Massignano, Italy, 422 planktonic, 280, 282 reworked, 202 scanning electron micrographs, 395, 397,404 trends, 431 Formation Aquia, 50,158,249 Brightseat, 50 Calvert, 54, 266 Charles City,55 Chickahominy. See Chickahominy Formation Choptank, 54 Chowan River, 55 Chuckatuck,55,440 Eastover, 54 Joynes Neck, 55 KentIsland, 55 Marlboro Clay, 50 Mattoponi, 69, 70, 170, 190 Nanjemoy, 51, 249 Nassawadox, 55, 441 Norfolk,55 Old Church, 52, 53, 259 Omar,55, 441 PineyPoint, 51,158,249 ,391 Potomac, 48, 158 Shirley, 55, 440 St. Marys, 54 Tabb,55,440,441 Wachapreague, 55 Wastegate, 75 Windsor, 55, 440 Yorktown, 55 fracture cores,204 pattern, 228 fracturing, 286, 292,360 freshwater aquifer, 433, 438 Ft. Monroe high,45 corehole, 109 gabbro, 41, 42

502

Index

gamma-ray (GR) logs, 263, 264 gas hydrates, 431 Gaudryina alazanensis, 402

Gaultmodel, 365 genera, opportunistic, 4I5 generic density, 458 equitability, 402, 403, 405 predominance,402, 403, 405, 415 geochemistry, Exmorebreccia, 233-253 geochronostratigraphic chart, 280 framework, 279 geohazard, 440 geologic expression, 440 maps, 303 map, Virginia, 440, 442 modeling, 146 record, li 13C, 431 geological circumstances, 420 consequences, Chesapeake Bay impact, 281, 287 data, 6 differences, 330 framework, 4, 4 I history, Mars, 330 studies, 73 synthesis, *CBC, 447 time scale, 204 geomagnetic, 318 field, 457 surveys, 318 geophysical data, 6 framework,*CBC,73 logs, *CBC, 263-265, 438, CDROM.7 Georgia, 64, 294 georgiaites, 64, 249 geothermal study, 85 Germany, 363 GI gun, 453 glacial Lake Missoula Flood, 322 glass bodies, 345 bombs, 345 impact, 351, 354 microspherules, 206, 207, 216, 232, 233,360 ,385

-rich suevite, 356 splash, 232 glauconite Chickahominy Formation, 52, 259, 263,266 dead zone, 391 Exmorebreccia, 196,224-226, 235239 Exmorematrix, 382 increase, 214 /quartz matrix, 197 -quartz sand, 202, 213, 293, 360, 448 -rich fraction, 233 sand, 224, 226 glauconitic quartz sand, 197, 234, 360, 383 globalpositioning system(GPS) navigation, 453 station, 86 transceivers, 86 Globobulimina

co-predominance, 405, 416 opportunistic, 411 ova/a, 405, 419 Globocassidulina

co-predominance, 405, 415, 416 opportunistic, 411 Gloucester Point station, 445 Virginia, 446 Gloucester, Virginia, 443 Goochland terrane, 41 graben basement, 120, 158 Chicxulub basin, 333 concentric, 270 ring, 118-120,272 gradient basement, 45, 291 chlorinity, 433 structural, 289 temperature, 423, 429 Granbycrater, 307 GrandCanyon,4 granite basement, *CBC, 41, 171 , 184, 214, 373,382 Bayside corehole, 171 borehole, 43 Exmore breccia, 224-227, 382 Locknecrater, 319, 350

Index Montagnais core, 354 NASA Langley corehole, 184, 214 Petersburg, 41 Portsmouth, 41 Revsund,319,350 granodiorite, 41 granophyre, 356 gravimeter, 86 gravitational collapse, 377 gravity anomalies, 41, 86, 87, 139,289,307, 458 anomaly map, 314 data, 318, 454 -drivencollapse, 366 model, 150, 151 ,290 modeling, 292, 303, 458 profile, *CBC, 134 residual, 88, 89, 146-149 signature, 290 surveys, 4, 326 stations, 6, 87 GrayMember, Onaping Formation, 356, 357 greenhouse effects, 425 warming, 287,422,423 ,429,431 GreenMember, Onaping Formation, 356 greenschist-facies, 41, 42 greigite, 456 Grigelis annulospinosa, 405 cookei, 405

co-predominance, 405, 416, 419 ground shock, 91 surface, 85, 86, 381, 445 surge,345 ,360,361,381,382 zero,57,208,291 ,292,298,373 groundwater analysis, 4 chlorinity, 435 data, 433 extraction, 445 high-salinity, 450 hypersaline, 433 potable, 450 salinity, 416 sources, 438, 440 tests, 433

503

Gulf of Mexico, 64, 65, 288, 291 Gyre, 80 Gyroidinoides aequilateralis , 405 byramensis, 399, 400, 405

co-predominance, 405, 416 Hammond core, 391 corehole, 390, 394 Hampton, Virginia, 104,433 HamptonRoads, Virginia, 108, 109,446 Hanzawaia blanpiedi, 396, 398 Harpersville scarp, 441 Haughton crater, 12, 139 heat pulse late Eocene, 423 shock-induced, 422 heavymetals, 422 helium isotopes CHe),423, 429, 431 highstand, 48, 57 hingeline, 63 hingezone, 64 HMX mixingcalculation, 234, 243 Hoeglundina elegans, 410

Holocene deposits, 47 sand units, 255 sediments, 64, 440, 449 shoreline, 441 subsidence, 266 Hopewell, Virginia, 50 horsts, 120, 158,270 hospitals, around *CBC,433 hot plume, 373 HREE pattern, Exmorebreccia,242 HudsonCanyonAlloformation, 55 Hugoniot-e1astic-limit,372 hurricanes, runaway, 360, 381 hydraulic effects, 366 erosion, 170,322, 377 processes, 357 hydrocode CTH shock physics, 372 SOVA,372 hydrodynamic model, 330 hydrophone, 454 hydrothermal activity, 184 deposits, 178

504

Index

mineralization, 177, 179,384 hyperbolic reflections, 146 hypercane (flowin) concept, 380 deposit, 372, 380, 381, 384 deposition, 360, 376, 381, 385 succession, 380 unit, 448 hypersaline groundwater, 433 hypervelocity impacts, 163 ice-advance, 431 icehouse, 423 ice sheet buildup, 423, 429 Wisconsinan, 444 impact breccia, 4, 69,158, 171,322 -326,343 crater, 1-7, 10-16,42,66-69,73,7780,86,92, 100, 123, 124, 138, 140, 142, 146, 147, 150-158, 163, 170-175, 185-189,213,217,224227,249,255,257,269,277-280, 287-301,306,314,318-322,326, 357,365,377,386,390,419-423, 433-435,443,446,448,451,453, 458 debris, 4 deposits, 425 ejecta, 279, 384, 431 features, I fireball, 373 -generated deposits, 287, 303 glass, 351, 354 pressures, 208, 224, 376 process, 365 shock, 104 shock wave, 176 site, 57, 58, 73 structures, I subaerial, 351 trajectory, 373 tsunamiite, 343 velocity, 373 impactites comparison, 343-363 correlation, 283-286 Eocene, 431 terrestrial, 343 impactmelt breccia, 233, 348

*CBC, 224, 231 rock, 224, 333, 348, 350, 354, 361 363 -rock matrix, 351 sheet, 326, 351, 356 impactor asteroid, 373 Chesapeake Bay, 332, 432, 447 composition, 420 contact, 233 diameter, 365 incidence angle, 330 low-angle trajectory, 330 primarycrater, 153 properties, 373 rarefaction wave, 365 shock wave, 365 size, 301, 420, 421 speed, 420 trajectory, 420 impedance contrast, 139,266 implications s13C data, 431, 432 0180 data, 424-431 impactmodels, 365 INAA,249 incandescent meteors, 373 indexedPDFs, 230 IndianOcean, 289, 425 indigenous biota, 390 late Eocene specimens, 28 I microfossils, 255 indochinite, 25 I infauna, 411 inner basin *CBC, 88-91 , 124, 139-142, 146, 149,171 ,176,185,188,189,267, 270,272,289,292,294,308,321, 325-328,341,357,383,433 Chicxulub basin, 341 fill, 363 impactcraters, 301 Locknecrater, 322, 350 Manson crater, 3I8 Mjelnircrater, 308 Montagnais crater, 309, 312, 313 Ries crater, 306, 345 interiorfireball, 373 Interstate Highway 64, 104, 108 interval

Index transit-time, 85 velocity, 73, 75 intrabed multiples, 186, 187 intracrater, 377 breccias, 385 coreholes, *CBC, 215, 357, 360, 383 fallout, 380 lithofacies, *CBC, 377 marinesediment, *CBC,361 Paleogene strata, *CBC, 292 regimes, *CBC, 377 sites, *CBC,263 stratigraphic unit, 377 invertebrates, Chickahominy Formation 259 iridium (Ir) analysis, 243 anomaly, 295 -enriched layer, 286, 292 Exmore breccia, 249 Island Beach Alloformation, 50 isopach map Chickahominy Formation, 268 Exmore breccia, 190 postimpactsediments, *CBC, 257 isopleths, chlorinity, *CBC, 435 isotopes. See stable isotopes isotropization, quartz, 348, 354 James River, 50, 51, 77, 78,104 ,108,109 , 120,284,443 ,444 Store, 444 Japan, 421 Japanese Broadcasting Company (NHK),372 jetting, high-velocity, 382 Joynes Neck Sand, 55 Jurassic Early, 43 Late, 314 Period, I salt beds, 435 sandstones, 345 section, 77 subsurface unit, 48 Upper, 64 WastegateFormation, 75 Kaluga crater, 12,307 Kamensk crater, 12,307, 324,326

505

Kardla crater, 12,307,326 Kazakhstan, 155 Kent Island Formation, 55 Keweenawan Red Clastics, 350 shale-clast breccia, 348 kill curve, 419, 420 King crater, 330, 332 Kiptopeke borehole, 53, 171 , 188,260-266, 283, 382,425,431,435,457 core, 401, 403, 408, 410 corehole, 172-175 , 186,216, 224, 259,281 ,284,387,415,456 core site, 388-39 1, 396, 400, 402, 407,412 ,416,417 magnetochronology, 391 Virginia, 417, 446, 454 K-T study sections, 417 lag effect, 407 time, 456 -time hypothesis, 429 Lag enoglandulina virginiana

Interval Subbiozone, 395 Subassemblage, 398, 399 Subzone, 402, 407 Lagerof crater, 329 laminae azimuth, 383 Chickahominy Formation, 258 clay, 255 dead zone, 202, 255, 391 horizontal, 204, 255-258 inclined, 383, 384 parallel, 255, 257, 260 ripple, 202 sandy, 383 silt, 255 truncated, 204 laminated K-T boundary clay, 417 sediments, 381 silt and clay, 204 Lamont-Doherty Earth Observatory, 78, 79 landplants, 423 landslide, 350 last glacial maximum, 444

506

Index

late Eocene age, 155, 193,301,351 analogues, 410 atmospheric perturbations, 432 benthic foraminifera, 389--402 bolideimpact, 4 clay, 255 ejecta, 286 Epoch, 57, 69,279,407 greenhouse, 423 ocean, 163 paleoenvironments, 255, 410 seafloor, 322 time, 387 Late Paleocene Thermal Maximum, 431 lateralground surge,345 lattices, pyrite, 202-207, 255, 257, 385, 391 leaf-margin analysis, 423 lenses, in dead zone, 360, 384, 391 limestone bioclastic, 51, 158 fractured, 213 pebbles, 213 pelagic, 292 Lindenkohl Alloformation, 51, 58 lithofacies *CBC,378-386 depositional, 377-384 dominant, 383 Exmorebreccia,57, 193 extracrater, 381 intracrater, 377, 379 laterally extensive, 385 Potomac Formation, 48 submarine impacts, 384 updip,47 lithostratigraphy calibration, 155 interpretation, 190 loading effects, 366 pressures, 377 Lockne Breccia, 350, 360, 361 crater, I, 13, 154,307,318-326 ,343 , 351,357,370-372 Lake, 324 surgeback processes, 372 Loftarsten Breccia, 351, 360, 361 log(s), geophysical

all *CBCcoreholes, CD-ROM.7 Bayside corehole, 264 Exmorecorehole, 265 Kiptopeke corehole, 264 NASA Langley corehole, 264 NewportNews corehole, 265 North corehole, 264 Windmill Pointcorehole, 265 Lonar crater, 13,224 lowstand deposits, 48 systems tracts, 57 Loxostomina vicksburgensis f. reticulata, 400 LREE pattern, Exmorebreccia, 242 lunar analogue, 330 craters, 318 Lynnhaven Member, Tabb Formation, 440 Magellan radar images, Venusian craters, 329 magnetic inclination, 455 surveys, 326 magnetobiochronology, 284 magnetochron boundaries, 389 magnetochronology, 283, 387,388 magnetostratigraphic analyses, 280 record, 283, 292 studies, 291 magnetostratigraphy, 285, 429 MaidensGneiss, 41 MaIm limestone, 153 Manicouagan crater, I Manson crater, 1, 139, 154,307,318,326, 343,348,357,360 breccias, 348 Iowa,318 map basement structure, *CBC, 46, 47, 288, CD-ROM.5 boreholes, *CBC, 5, CD-ROM.2 boreholes to basement, *CBC,44 boreholes with Exmorebreccia, 70 boreholes with Mattaponi Formation, 70

Index breccia distribution, Popigai crater, 352 concentric ring grabens, *CBC, 272 earthquake epicenters, *CBC, 441 ejectadistribution, Ries crater, 346 geology, Locknecrater, 323 geology, southeastern Virginia, 442 gravityanomalies, *CBC,41,86-89, 147, 148 gravityanomalies, Montagnais crater, 313 gravitystations, southeastern Virginia, 87 groundwater chlorinity, *CBC, 435 isopach, Chickahominy Formation, 268 isopach, Exmore breccia, 190 isopach, postimpact sediments, *CBC, 257 late Eocenepaleogeography, 61 location, Chicxulub crater, 334 location, Manson crater, 319 location, Mjelnircrater, 315 location, Montagnais crater, 310 location, terrestrial impactcraters, 2 location, tide gauges, *CBC, 441 location, TomsCanyon crater,66 municipalities, *CBC, 434 North American tektite strewnfield, 65,296 outlines, *CBCand Popigai crater, 306 rivercourses, *CBC, 445 seismic tracklines, *CBC, 78, CDROM.2. See also trackline map structure, *CBC, 92, CD-ROM.3 structure, Lockne crater,324 structure, Montagnais crater, 313 structure, Potomac Formation, 293, 294 tectonostratigraphic terranes, Virginia basement rocks, 42 margin, continental, 3, 58, 63, 64, 87 marginulinids, 394 marine bolide impact, 381 microfossils, 259 sedimentation, 255, 322 seismic reflection profiles, 287 -targetimpacts, 318 watercolumn, 362, 368

507

Marlboro Clay,50 Mars, craters, 330 Maryland basement, 46, 47, 73, 75 boreholes, 43, 44, 389 coastalwells, 193 Marlboro Clay, 50 PineyPoint, 51, 391 seismic basement, 73 sequence stratigraphy, 56 state,433 Vibroseis profiles, 77 mass extinction, 301, 419, 423, 429 failure, 104 mortality, 419 spectrometry, 457 massif, 41, 146 Massignano, Italy correlation with othersites, 426 ejecta, 283-286, 291 , 431 helium isotopes, 423, 429 impactite, 283 sediments, 425 massive character, 259, 380, 381 clay, 204 collapse, 104 crossbedded units, 384, 385 disruption, 368, 382 failure, 91, fluidized? sand, 193 fluxes, organic carbon, 417 impact meltrock, 363 injection, CO2, 432 marine clay, 52, 449 sands, 176, 184,215 slumps, 384, 385 structureless units, 376 tuff,41 turbidites, 384 volumes, water, 380 matrix basalbreccia, NASA Langley, 184 clastic, 356 crystalline, 356 -dominated, 212-215, 348, 383 Exmore breccia, 193-202, 208, 216, 216-232,255,360,361,382,383 fallback suevite, 345 glassy, 345

508

Index

glauconite, 193, 197,213,214,293, 360,382 granite, 348 Manson centralpeak, 348, 350 M/B ratio, 382, 383 meltrock, 351 microcrystalline, 356 percentage, 202 phytodetritus, 415 -rich interval, 212, 213 sand,214 submarinecrater, 377 -supported, 171 , 176, 177 -supported blocks, 202 -supported breccia, 171, 193,214, 215,345,351 ,382,383,447,448 Tandsbyn Breccia, 350 versusboulders (M/B)ratio, 383 washback facies, 380 Mattaponi Formation, 69, 70, 170, 190 River, 54 Maud Rise, 285, 289 megablocks. See also displaced megablocks crystalline, 184 destabilized, 176 slumped, 318, 377 tilted,333 megabreccia, 171,357,385 megaclasts, 385 megafossils, 50 megaslide blocks, 91, 171, 176 megaslump, 91 megaturbidite, 193 Meguma Group, 354 melt inclusions, 356 features, 231 lapilli, 351 matrix, 213 products, 363 rock, 345, 356 -rockclasts, 348 -rock particles, 363 vein, 231 zones, 224, 363 melted basement clasts, 373 mineral grains,384 melting, 348

membrane filtration, 435 Mercury, craters, 330 Mesozoic sedimentary rocks, 318 metagranite, 184 metamorphic basement rocks, 354 bodies, 41 rocks, 41, 345 shockeffects, 217,221,234,235, shockfeatures, 216, 243, 345, 348, 350,354,356,380 metaquartzite, 354 metasubgraywacke, 354 Meteor(Barringer) crater, 10, 224, 362, 365 meteoritic component, 234, 249 Mey Alloformation, 54 microfaunas, 52, 57 microfossils, 50, 57, 193,202 ,279,383, 410 microhabitat, 410, 411 microkrystite, 295-297 micropaleontological analysis, 279 record,387 microspherules, 224, 233, 391 microtektite(s) Exmore breccia, 384 fallout deposits, 204 North American tektite strewn field, 64,283,294-296 strewn fields, 224 MiddleNeck Peninsula, 433, 440-443 migration, 287, 423, 435 military bases, around*CBC, 433 Minerals Management Service, 80 Miocene Calvert Formation, 266 faults, 440 late, 301 middle, 54, 158 Ries crater, 343, 362 sands, 363 upper, 54 mixed assemblages, 193, 195, 297, 390 bolboformids, 195 calcareous nannofossils, 194 planktonic foraminifera, 195 sedimentary-crystalline target, 30I suite, benthic foraminifera, 384

Index targetrocks, 387 tropical-temperate microfossils, 297 mixingcalculations, 234, 243 Mjelnircrater, 1,3 ,13,154,307,314318,326,368-370,385 model calculations, 290 conceptual, 301, 365-371 , 377-386 crater-fill, 382 cratering, 366, 380, 42I Exxon,48 Gault, 365 gravity, 150, 151 , 290 hydrodynamic, 330 numerical, 322 Oberbeck, 368, 385 Ormo, 370, 385 Ormo-Lindstrom,370 seven-step, crater formation, 368 Tsikalas, 368-372, 385 modern bay floor, 270 foraminiferal ecology, 410 foraminiferal populations, 410 rivers, 444 modification stage, crater formation, 322,365,366 moldavite, 251, 363 molecular diffusionrates, 438 molluscs, Chickahominy Formation, 423 Montagnais breccias, 354 crater, 3,13,154,307,309,314,326, 343,419 1-94 borehole, 354 Montpelier Metanorthosite, 41 monzogranite, 41 Moon craters, 330 farside, 330 Morokweng crater, 1, 13 morphology *CBC, 4-9 , 73, 77, 91-151 Chickahominy Formation, 266 Chicxulub basin, 332 Popigai crater, 303 morphometric data annulartrough, *CBC, 123 central peak, *CBC, 146 compression ridges, *CBC, 123 innerbasin, *CBC, 140

509

outerrim, *CBC, 114, 115 peak ring, *CBC, 138 secondarycraters,*CBC, 157 Mt. Zion Church, Virginia, 443 mud rims, clasts, 198 multichannel seismic reflection. See also seismicreflection profile data, 69 system, 332 multiring basin, I, 10, 15, 16, 154,334, 344 354, 363, 433 municipalities, around*CBC, 434 muscovite, 384 NanjemoyFormation, 51, 249 Nankai Trough, 208 nannofloras, 57, 423 NASA Langley core, 45, 178-183, 202, 203, 206, 210,360,390 corehole, 43, 53, 85, 108, 172-174, 184-188, 193,204, 213-216, 257266,279,383 ,384,425,431,438, 457 core site, 255, 410, 412, 417 Nassawadox Formation, 55, 441 NationalGeodetic Survey, 86 NationalGeographic Society, 79 nearshore region, 368 Neecho profiles, 78. See also seismic reflection profile nekton, Chickahominy Formation 411 neritic, 57, 58, 410, 411, 415, 417 nested centralbasin, 370 crater, 372 neutron activation analysis, instrumental, 249 New Jersey boreholes, 389 CoastalPlain., 193 Continental Slope, 298-300 ejecta, 67 Newark Supergroup, 43 NewportNews corehole, 55,106,171 ,172,186,189, 193,213 -216,225,233,239,242, 253,263 site, 390,410 unit, 54 Virginia, 433

510

Index

nickel (Ni), 249 nickel-rich spinels , 289 Ninetyeast Ridge, 289 NO" 422 NOAA Atlantic Marine Center, 454 nodosariids, 394 nodules, pyrite, 411 nominate species, benthic foraminifera , 395 Norfolk arch, 45, 291 basin, 42, 43 Formation, 55 naval base, 104 Virginia, 433, 441, 443 normalized magnetic intensities, 455 North America, 444 Atlantic, 64, 65, 288 Carolina, 193 corehole, 53,171-175,186,193,199, 201-205,215,216,259-266,383 , 384,391,438 North American Atlantic Continental Shelf, 372 Commission on Stratigraphic Nomenclature, 48 craton, 318 ejecta, 66 tektite debris, 66 tektites, 249, 251 tektite strewn field, 64, 67, 249, 283, 289,363 Northern Neck Peninsula , 43 Siberia, Russia, 30 I Northwest Geophysical Associates, 458 Norway, 314 Nova Scotian Shelf, 307 NP 19-20, 279 NP21,279 nuclear explosions, 365, 421 nutrient supply, 410, 415 Oak Grove corehole, 50 Oberbeck model, 368, 385 Occohannock Member, Nassawadox Formation, 441 Ocean Atlantic, 43, 425, 435, 444 Indian, 245, 295

Pacific , 425 Southern, 285,423, 424 Oceana Ridge, 443 Ocean Drilling Program (ODP) Site 216 Site 6898, 285, 295, 422-425 Site 703, 424 Site 903, 66, 67, 279 Site 904, 66, 67, 279 Site 10908, 295 oceanic crust, 64 Ohio Oil-Larry G Hammond #1 well, 389 Old Church Formation, 52, 53, 259 Oligocene, 52, 57, 259, 266, 280, 423, 429 Omar Formation, 55, 441 Onaping Formation, 356 Ontario , Canada , 354 ophiuroids, Chickahominy Formation, 411 opportunist genera, 415 life strategies, 404, 405, 411 Ordovician limestone, 319, 350 middle, 322 submarine crater, 326 organic carbon, 411, 415-417 Ormo model, 370, 385 Ormo-Lindstrorn model, 370 osmosis , reverse, 435 ostracodes, 193,259,411 outcrop, 4, 48, 51,155,343,372,417, 425 outer rim *C8C, 7, 78, 91-120, 148, 185, 187, 214,215,267-269,272,324,376, 417,441 escarpment, 188 formation, 376 Manson crater, 318, 350 Montagnais crater, 307, 354 Popigai crater, 303 scarp, 120, 176 oxides, Exmore breccia , 249 oxygen content, 411 deficiency, 415 -depleted, 411 depletion, 416

Index low, 419 18 0 , 438 oxygenated, 411 Ozone, 422 P15INP19-20 overlap interval, 289 P15-P14 boundary, 292 P15/P16 boundary, 398, 407 PI6-PI8,279 Pacific Ocean,425 Paleocene Aquia Formation, 50, 158 Brightseat Formation, 50 clay clasts,208 foraminifera, 69 lower, 50 Marlboro Clay,50 microbiota, 391 Thermal Maximum, 431 upper, 50 paleoclimate, 58, 424 paleodepth, 57, 59, 372, 381, 410, 415419 paleoecology, 402 paleoenvironmental aspects, *CBC, 451 effects, global, 419--432 effects, local, 387--419 interpretation, 407,429,431 implications, 402 record, 417 succession, 389 summary, 415 paleoenvironment(s) assessment, 402 Chickahominy Formation, 407 euxinic,356 foraminiferal indicators, 407--417 hostile, 417 postimpact, 6 Paleogene deposits, 410 formations, Virginia, 56 strata, 292, 293 paleogeography, 57 paleomagnetic boundaries, 387 data, 286 paleomagnetics, 454 paleoproductivity, 432 paleosol

511

displaced megablocks, 176, 184 intervals, 383 NASA Langley core, 181 Paleozoic carbonates, 350 Era, 41 sedimentary rocks, 318, 354 Pamunkey River, 51, 52, 190 parabolic reflections, 357 peak, central. See central peak peak ring Amescrater,302 *CBC, 4, 77, 79, 88-91,120-139, 148-150,175,176,185 ,188,215, 216,257 ,267-270,289-292,302308,321 ,325-328,332,341,357361,373,374 *CBC model, 385 Chicxulub basin, 333, 335-341 crystalline, 330 extraterrestrial craters, 326 formation, 366, 376 Kamensk crater, 302, 326 Kingcrater, 330-332 Lockne crater,302, 322-325, 350 low-relief, 307 Manson crater, 302, 320, 321 Mercurian craters, 333 Mjelnircrater,302, 308, 316 Montagnais crater,302, 307-313 Mooncraters, 333 non-proportional, 332 Oberbeck model, 385 Ormomodel, 384, 385 Popigai crater, 301-306, 351-353 Ries crater, 301-304, 346, 347 TomsCanyon crater,302 Tsikalas model, 384, 385 Venusian craters, 329 Peedee belemnite (PDB)standard, 457 periglacial bulge, 444 rebound, 445 permeability high, 212, 215 low,215, 259, 263 relative, 212, 260 Petersburg Granite, 41, 42 petrography, Exmore breccia, 216-233 Phanerozoic-clast breccia, 348

512

Index

megabreccia, 360, 361 Phoenix Canyon Alloformation, 54 photochemical fog, 422 photosynthesis, 422 phyllite, 354 phytodetritus, 415, 416 Piankatank River, 444 Piedmont Fall Line, 64 Province, 42, 58 PineyPointFormation, 51,158,249, 391 planardeformation features (PDFs) Exmore breccia, 216, 217, 229, 230, 233 multiple sets, 348 NorthAmerican tektite strewn field, 64 planarfractures, 348 planktonic foraminifera, 57,193 ,195 ,259 ,279282,297,398,423,457 foraminiferal assemblage, 389 foraminiferal Biozone P16, 297 foraminiferal ZoneP15, 396 organisms, 417 Pleistocene age, 362 sediments, 441 transgressions, 443 units,440 Pliocene age, 55 deposition, 54 late, 54, 55 sections, 440 plutons, 41, 42 pollen grains, 193,411 polygon plot, benthic foraminiferal abundance, 406, 407 polymict impactbreccia, 171 , 177, 193,356 megabreccias, 360 sedimentary clasts, 360 Popigai crater, 1,4, 14,289,301-307,314, 343,351,357,360-363,420-424, 432 breccias, 351 impact, 286, 291, 363, 425 Poquoson, 440

pore water, 438 Portsmouth Granite, 41 Virginia75, 86,433 postimpact benthic foraminifera, 390, 417 bioticchanges, 387 compaction, 354 deposition, 407 depositional changes, 387 deposits, 51, 53, 255 sedimentary section, 187 sediments, 77-79, 85, 170,336 storms, 381 tectonism, 155 units, 52 postriftdeposits, 75 postshock temperatures, 345 Potomac Formation, 48, 158,292-294 River,45, 50, 51, 55, 77,155, 158, 170 Riverprofile, 158-163,291, CDROM.14a--
Index pulse, warm, 425, 429, 431 P-wavevelocity, 333 pyrite Chickahominy Formation, 411 framboidal, 202, 206, 259, 384 lattices, 202-204, 206, 207, 257, 385, 391 nodular, 202 oxidation, 456 pyritization, 263 pyrotoxins, 422 pyrrhotite, 456 quartzite, 41 Quaternary formations, 55 sediments, 158 shorelines, 442 QueenAnne basin, 43 radioisotopic impactage, 283 radiolarians, Chickahominy Formation, 259,411 radiometric chronology, 283 Ragozinka crater, 14,362 rainforests, 58 raised lip absence, 314 seawater, 370 raisedrim, *CBC, 103, III , 112, 116, 119, 160-169 Rappahannock Canyon,94, 101-104, 120, 123,268, 444 River, 50, 54, 55, 77, 94, 97, 101 ,444 rare earth elements (REEs), Exmore breccia, 240-242, 250, 253 rarefaction impactshock, 204, 376 wave, 365 ratio, outer-rim diameterto peak-ring diameter, 330 ravinement, 57 rays, ejecta, 64, 65, 153 rebound crystalline basement, 369, 370 periglacial buldge, 444 targetrocks, 366 recrystallization, 348 red beds, 50 red sandstone, 171

513

refill,excavated craters, 361 reflection arcuate, 289 high-amplitude, 187,266,336-339, 357,361 hyperbolic, 187 inclined, 176 incoherent, 171 , 187 packages, 176 seismic, 185. See also seismic reflection profile signature, 266 regime ballistic ejection, 381 crater,217 depositional, *CBC,6, 358, 359, 377382,450 extracrater, 381 fallout, 380, 384 flowin, 380 intmcrater,377 jetting, 381 seafloor, 384 sedimentary, 266 slumpback, 382 stress,420 washback,381 relief, structural centralpeak, *CBC, 146, compression ridges, *CBC, 123 outer rim, *CBC, 100, 104, 114, 115, 187-189 peak ring, *CBC, 138 peak ring, Popigai crater, 303 Ries cmter, 306 researchinstallations, around *CBC, 433 research vessel Atlantis II, 80 Fay, 80 Gyre, 80 Neecho , 80 RIV Ewing , 80 Seaward Explorer, 79,453

return flow, tsunamis, 368 Revsund Granite, 319, 350 reworked microfossils, 255 specimens, 391 Rhode Island, 4, 433 Richmond, 42

514

Index

ridge. See also compression Ames, 443 Oceana, 443 low, 289 Ries breccias, 343, 360-362 crater, I, 153,224,301 ,307,314, 343- 351, 357, 363 rift basins, 77 deposits, 75 grabens, 42 rim clay, 208 escarpments, 157 fault, 104, 158 outer. See outerrim raised. See raised rim ring inner, 333 outer,333 peak. See peak ring ring crest, 139 ringinginterference, 176 ripple crossbeds, 180 laminae, 202 river alteredcourses,444 channels, 445 Chickahominy, 44 James, 50, 51, 77, 78,104 , 108, 109, 120,284,443 ,444 Mattaponi, 54 Piankatank, 444 Potomac, 45, 50, 51, 55, 77, 155, 158, 170 Rappahannock, 50,54,55, 77,94,97, 101,444 Susquehanna, 444 York, 51, 52, 77,104,106,120,138, 185,284,444 RMS velocities, 85, 86 rock density, 458 melting, 233 metasedimentary, 354 volcanic, 41 Roddy, OJ, dedication to, VI Roseway Unit, 307 rotarycuttings, 354

rotational motion, 176, 204 runawayhurricanes, 360, 381 runup, tsunami, 185, 190 RIV Maurice Ewing, 78, 79, 86,454 Sabot Amphibolite, 41 sag Chickahominy Formation, 267, 269 Exmorebreccia, 185-189 secondary craters, 158 sagging, 120 salinity, 416, 433 Salisbury embayment, 45 Maryland, 389, 390, 394 salts, 435 saprolite, 73 S. Atlantic, 289 scaling calculations, 421 relations, 365, 366 scalyclay, 179, 184, 208, 210 scanningelectron micrographs, 206 microscopy, 458 scaphopods, Chickahominy Formation, 259,411 scarp Big Bethel, 441-443 Diamond Springs, 443 erosional, 319, 324, 354 Harpersville, 441 rim, 120, 176, 188 steep, 301, 307, 314 Suffolk,442, 443 seafloor physiography, 410 surge, 382 -surgedeposits, 384 -surge lithofacies, 382 sea level, 52 eustatic, 387 fall, 416 lowstands,57 relative change, 281 , 444 rise, 444-446 seaports, around *CBC, 433 SeawardExplorer, 79, 453 seawater ancient,435 boil-off, 438

Index

vaporized, 432,438 secondary breccia, 362 craters, *CBC, 153-172, 382 craters, Euler crater, 154 craters, profile T-I-CB, 155-158 craters, profile T-II-PR, 158-169, CD-ROM .14a-d Sedgefield Member, Tabb Formation, 440,441 sediment(s) above basement, 80 accumulation rate, 62, 283, 284, 290, 387-391,394,399,407,416,449, accumulation rate curve, 388 accumulation rate drop, 399 alluvial, 57 anoxic, 415 Baltimore Canyon trough, 47 benthic foraminifera , 390, 391, 410, 417 brecciated, 382 Cenozoic, *CBC, 363 chaotically deposited, 314 Chickahominy Formation, 259, 268, 387,411 ,448 -clast breccia, 171, 177, 193, 345362,450 -clast megabreccia, 351, 354 collapse, 376 composition, 263, 416 continental shelf, 79,448 Cretaceous, 249, 357, 447 dead zone, 380, 39\ deep-sea, 297 deformation , 176--184, 204, 215 delivery rates, 415 displaced megablocks, 176 disruption, 157,370 failed, 91 -filled grabens, 42 flowin,380 fluvial, 57 gravity flows, 368 indurated, 184 loading, 64 Massignano, Italy, 423 Mattaponi Formation, 170 microtektite layer, 295 multidirectional flow, 448 normally stratified , 213

515

oldest, 48 Paleogene, 292 Pleistocene, *CBC, 441 poorly consolidated , 193,301,357, 362,376 postimpact, 51, 77, 78, 85, 104, 158, \70,176,318,322,332,336,450 postrift, 64 preimpact , 48, 78, 104,322,336--339 Quaternary , *CBC, 440 seafloor, 373, 380, 384, 401 shock-weakened,448 siliciclastic, 158,233,354,449 sources, 263 studies, 4 surface, Ries crater, 363 target layer, 373, 450 thickness, offshore, 64 thickness, postrift, 64 types, 234 Upper Jurassic to Holocene, 64 volume, *CBC, 433 -water interface, 411, 414, 415 water-saturated, 438 weathering, 242, 249 sedimentary clasts, 351 basin, 435 breccias, 182 megablocks, 186,314,348,376,377 megaslump blocks, 171 provenance, 242 regime, 266 rocks, 4, 318 structures , Exmore breccia, 204-211 target rocks, 287, 303, 314, 362, 368 seismic expression, 73 interpretation, 91 reflection, 92, 318 reflection features, 361 reflection signatures, 363 reflection survey, 4, 79, 85, 307, 314, 453 reflection tracklines, 78,124,315, 319, CD-ROM .2. See also trackline map refraction, 45 ringing, 186 signature, 73,171 ,185,186,266 stratigraphy, 78

516

Index

Systems Inc., 453 velocities, 85 seismicreflection profile *CBC, 6, 43-45, 52, 54, 73, 77-85 , 91,11O,148,149,440,CDROM.8-16 I-CB, 160, 161,CD-ROM.II Chicx-A, 336 Chicx-AI,336 Chicx-B, 338 Chicx-C, 339 crystalline basement, 73-77 E-2, 132-134, 143,CD-ROM.l5 E-3, 111 ,135, CD-ROM.16 high-resolution, 276, 277, 440 list, 80-84 Manson crater,320, CD-ROM.18 Montagnais crater, 311 , 312, CDROM.17 II-PR, 74, 95,164-169, CDROM.14a-d SEAX-I ,I13 SEAX-3,106 SEAX-4,99, 127 SEAX-6, 112, 144 SEAX-7, 128, 141 SEAX-8, 117, 128 SEAX8-7-6, CD-ROM.9 SEAX 9-10, CD-ROM. I0 SEAX-1O,98, 129, 131, 145 SEAX-12, 102, 121 SEAX-16, 109 SEAX 16-4a-4, CD-ROM.8 SEAX-22,119 SEAX-25, 118 SEAX-27, 116 SmithPoint, Virginia, 74 T-8-S-CB-E, 125, 130, 136, 137,CDROM.12 T-9-CB-F, 96, CD-ROM.13 T-lO-RR, 103 TomsCanyon crater, 66 trackline. See trackline map 13-YR,107 seismostratigraphic analysis, 77, 88, 155,292 framework, 73 signature, 266 sequence boundary, 57, 79 Exxonmodel, 48

stratigraphy, 56, 57 upward-fming, 184,381,384,385 seven-step model, craterformation, 368 shelf, continental, 45, 58, 64, 80, 110, 255,281,283,332,362,410 shift climate, 415 contour, 45 faunal,416 generic predominance, 405, 415, 416 negative, 013C, 416 negative, 0180 , 416 nutrient source, 146 permeabili~, downhole, 215 positive, 01 C,416 positive, 0180 , 416 positive SP, 213 sediment accumulation rate, 407, 416, 417 sediment sources, 263 species richness, 407 Shirley Formation, 55, 440 shock alteration, 171 ,287 compression, 204 -deformation features, 208 fluidization, 179 -fluidized collapse, 376 fractures, 216 fracturing, 233 melt,216 -melted minerals, 224 melting, 208, 232 metamorphic features, 243, 345, 350, 354,356 metamorphism, 216, 228-231, 343, 348,357,360,376 -metamorphosed rock, 343 pressures, 217, 345 wave, 163,365,373,422 shocked clasts, 348 grains, 64, 385 minerals, 294, 384 quartz, 228, 295 shoreline Big Bethel, 443 features, 443 late Eocene, 58 present, 64

Index side echoes,centralpeak, *CBC, 144, 145,289 siderophile elements, 234 silicification, 263 silt-rich facies, 202 tlowin layer, 384 interval, 204 layer, 205 unit, 360 simulation, *CBC impact, 373, 374 Sixtwelve Alloformation , 50 slope,continental, 292 slumpback deposit, 348, 350 lithofacies, 377, 383 megablocks, 351 process, 322 regimes, 383 slumping, 91, 170, 204, 314, 366 slurry rim, 368 spout, 368 water and rock debris, 373 Smith Point crysrtalline basement, 75 RMS velocities, 85 Virginia, 155 solitarycorals, Chickahominy Formation, 259 sonic logs, 86 soot, impact, 422 sorting, vertical, 345 source, North American tektitestrewn field, 294 South Africa, 433 Southern Ocean cores, 285,423 sites, 424, 429 SOVA computer hydrocode, 372 multimaterial code, 376 SP. See spontaneous potential species abundance, 410 agglutinated foraminifera, Chickahominy Formation, 391, 401,402,419 benthic foraminifera, Chickahominy Formation, 390-402, 410-419, 449 calcareous nannofossils, 297

517

composition, 410 diversity, 419 equitability, 402 event,turnover, 398 large, 394 Lazarus, 419 loss, 419 nominate, subzones, 395, 396 opportunistic, 404, 407 paleoenvironmental markers, 405, 412-414 planktonic foraminifera, 297, 389 predominance, 402, 403 richness, 402, 407, 408, 415, 449 spherules, 204 Spiroplectammina

K-Tboundary clay, 419 mississippiensis, 402

sponges, Chickahominy Formation, 411 spontaneous potential (SP) curve, 184,212,259,264,265,383, CD-ROM.7 high values, 213, 216 log, 174, 175,214,260 spores, Exmore breccia, 193 squeeze deformation, 209 -outs, Exmorebreccia, 200 stable isotopes Bath Cliff, Barbados, 426 *CBC,424-432 DSDP Site 612, 426 globalocean sites,428 Massignano, Italy, 426 ODP Site 689B, 427 late Eocene sediments, 423-432 table, Chickahominy Formation, 457 stage I - contactand compression, 365, 373 stage 2 - excavation, 365, 373 stage 3 - modification, 366, 376 State FarmGneiss, 41 Stilostomella cocoaensis ,405

co-prodominance, 405, 416, 419 "exilispinata",405 spp.,410 stilostomellids, 394 stishovite, 64, 295 St. Marys Formation, 54 strain rates, 208

518

Index

stratal geometry, 205 stratification , 183, 184, 193 stratified sedimentary beds, 383 sands, 176, 180, 184 stratigraphic attributes, 389 chart, 424 column, Chickahominy Formation, 388 correlation, 173 evidence, 333 framework, 47 range chart, Chickahominy Formation, 281, 391-393, CDROM range chart, DSDP Site 612, 282 ranges, microfossils, 279, 280 section using *CBC coreholes, CDROM.7 succession, 49,53,212,281,282, 362,382 variability, 407 stratigraphy, 4, 69,80,281 streamer, seismic, 453 stress regime, ambient, 420 strewn field. See also map Australasian, 363 Central European, 363 North American, 64, 283, 294-296, 363 Pacific (cpx), 296 striations, radial, 382 structural boundary, 366 differences, 303 floor, 326, 328 high, crystalline basement, 339 map, 92,186 peak ring, 339 relaxation, 444 rings, I sag, 333 sagging, 104 shelf, 292 structure *CBC, 4-9, 73, 77, 91-151 , CDROM.3,.5 Chickahominy Formation, 266 Chicxulub basin, 332 Exmore breccia, 188

map. See map Potomac Formation, 293, 294 terrestrial impact, 2 terrestrial impact list, 10-16 subassemblages, benthic foraminifera, 394-406 submarine bolide impact, 367-370, 387 submersibles, 292 subpeaks, 146 subpulse greenhouse warmth, 423--431 W-1, 425, 429, 430 W-2, 425, 426, 430 W-3, 425, 426, 430 subsidence basement, 87 breccia, 51 differential , 170, 266,443,445 rates, 58 Virginia continental margin, 64 substrate chemistry, 415 sedimentology, 410 subunit (gamma-ray) OR-A,266 OR-B,263 OR-C,263 OR-D,263 OR-E,263 subunit (spontaneous potential) SP-2, 260, 263, 266 SP-3,263 SP-4,263 SP-5,263 subzone (benthic foraminifera) Bathysiphon , 396, 409 Bolivina tectiformis, 396, 407, 409 Buliminajacksonensis, 392, 393, 395, 396,405,407.409 Lagenoglandulina virginiana, 395, 396,407,409 Uvigerina dumblei, 396, 407, 409 succession foraminiferal assemblages , 391 surface impacts, 432 Sudbury breccias, 354 Igneous Complex, 356 impact melt sheet, 356 multiring basin, 1,343,357,360 structure , 356

Index suevite breccia, 348, 361 Exmore breccia, 224 Suffolk scarp, 441 , 443 shoreline, 443 Virginia, 433 sulfate-reducing conditions, 411 surgeback breccia, 322, 324, 326, 350, 370, 383, 384 channels, *CBC, III deposit, 350, 351, 356, 361 ,386 erosion, 366 flow, 351, 370 gullies, 322, 324, 326, 351 lithofacies, 360, 377 process, 170, 204, 322, 385 velocity, 322 watercolumn, 185 Susquehanna River, 444 Sussex terrane, 163 synimpact crater-fill, 171 , 213 flowin, 361 lithofacies, 382 to postimpact, 407 systems tract, 48, 57 Tabb Formation, 55, 440, 441 tagamite, 351, 361 , 363 Tandsbyn Breccia, 322, 350, 357 targetrocks carbonate, 422, 432 composition, 243, 366, 420 compression, 365 consolidation, 420 crystalline, 281, 361 deepest, 377 disrupted, 158 ejected, 153,365 evaporites, 435 fragmented, 368 geochemistry, 249 hot plume, 373 melting, 224 mixed, 301, 386, 387 near-surface, 233 preimpact, 158 rebound, 366

519

sedimentary, 232, 303, 314, 361 , 362, 368,450 subaerial, 30I vaporized,363,365,373 Virginia, 287 volatiles, 366 taxonomic nomenclature, 458 Taylorsville basin,42, 43, 75, 163, 167169 tectonism Chicxulub basin, 333 syndepositional, 387 tectonostratigraphy, 41 teeth, fish, Chickahominy Formation, 259 tektite(s) Australasian, 363 glass, 289 NorthAmerican, 64, 294 strewn field, 251 strewn fieldmap, 65, 296 Teledyne Exploration Company, 77,454 seismic streamer, 453 temperature decline, 423 elevated, 416 impact, 438 temporal succession depositional regimes, 377 stages, 365 ternarydiagrams, 242 terrace, 96, 104,292 terranes, tectonostratigraphic, 41, 42, 163 test size, foraminifera, 415 Tethyan assemblages, 291 Texaco seismicdata, 69, 77, 454 seismic profiles, 78, 155, 186, 187, 283. See also seismicreflection profile Texas,64, 294 thickness Chickahominy Formation, 269 data, 269 Exmore breccia, 188-193 variations, 267, 268 thin bivalves, Chickahominy Formation, 259

520

Index

thin section, Exmore breccia, 228, 229, 231,232 3-D perspectives, *CBC, 3, 328 structure model, *CBC, CD-ROMA three-layered target, 301 threshold events, 429 thrust folds, 163 tide (sea-level) gauges, 441, 444 TNT equivalent, 420, 421 TomsCanyon Alloformation, 55 crater,3,66,294,295,307,326,423425,432 impact, 294, 295 structure, 67,295 tonalite, 41 topographic features, 440 high, 333, 339 peak ring, 336, 339 relief,443 touristattractions, around*CBC, 433 toxic substances, 411 trace element(s) analysis, 234 compositions, 253 Exmore breccia, 249 trackline map Chicxulub impactbasin, 334 entire*CBC, CD-ROM.2 Ewing profiles, 79 James River, 108 lowerChesapeake Bay, 124 middle Chesapeake Bay, 156 mouth, Chesapeake Bay, 110 northern rim, *CBC, 97 Potomac River, 156, 159, 162 Rappahannock River, 101 Virginia Continental Shelf, 110 York River, 105 trajectory, impactor, 301 ,370,420 transgression, marine, 57, 389 transgressive systems tract,48, 57 transient-crater collapse, 336 Triassic Late,43 sandstone, 171 , 345 subsurface unit,48 Wastegate Formation, 75 Triassic-Jurassic, 42

Tsikalasmodel, 368, 370, 372, 385 tsunami damage, 422 initial, 376 runup, 185, 190 secondwave, 368 washback,4, 170, 185, 189, 190,204, 322,366,372,376 washback breccia, 362, 381-385 washback deposits, 370, 372, 381 washbackprocess, 368, 376, 377, 380,383-385 washback unit, 360 waves, 4, 170, 185, 189,368,376, 377,384,385,448 tuff,41 tuned array, airguns, 454 turbidite distal,41 lacustrine, 361 Lockne crater,351 Oberbeck cratering model, 384 sequence, 361 turbidity currents, 381 turbulence gas cloud,345 marine processes, 361 turnover faunal, 287, 399, 407 floral,287 planktonic foraminifera, 398, 417 Turrilina robertsi, 410 2-D computer simulation, Chesapeake Bay impact, 373, 374 SALEcode, 376 two-waytraveltimes, 85 type I normal faults, *CBC, 284 type 2 normal faults, *CBC, 284 type A Venusian crater,329 type B Venusian crater, 329 type C Venusian crater, 329 type D Venusian crater, 329, 330 Ukraine-Russia border, 324 ultra-high-temperature mineral phases, 363 unconformity, 49,53,57,286,429 unconsolidated sediments, 362 undulating thin layers, 360 uplift, regional, 292

Index urban sprawl,around *CBC, 433 US Geological Survey(USGS), 4, 69, 77-80,85,243 US Gulf Coast, 394 US Gulf and Atlantic CoastalPlain, 458 Ust Kara crater, 3 Uvigerina cookei, 399, 414

co-predominance, 411, 416 dumblei, 395, 399,414 dumblei Interval Subbiozone, 396 dumblei Subassemblage, 400 dumblei Subzone, 407

equitability, 405 gardnerae,405 ,414 in Cibicidoides pippeni Assemblage,

411 vicksburgens~ ,400

vaporization, 292, 389 variability crater, 372 lithic, 212 natural,420 velocity bolide, 370 checkshot, 86 high, 381 impactor, 330 interval, 73, 75 profiles, 85 P-wave,333 RMS,85,86 seismic, 85 surgeback, 322 wind, 381 Venus, craters, 330 vertebrates, Chickahominy Formation, 259 Vibroseis, 73, 75, 77, 85 videoanimation, Chesapeake Bay impact, 372, 375 Virginia basement, 47 Beach,433 boreholes, 43, 44,69. See also boreholes CoastalPlain,4, 41, 48-57 ,73,281 cores, 387 cross section, 71, 72. See also cross section

521

Department of Environmental Quality, 69 deposition, 389 earthquakes, 440 formations, 49,53 . See also Formation geological studies,73 geophysical studies,73 outcrops, 48 population, 3, 433 seismicprofile, 75. See also seismic reflection profile subsidence rates, 58, 64 subsurface, 48, 52 State Highway 14,443 State WaterControl Board,69 Tech,86 volatiles, 366 volcanic degassing, 432 Vredefortcrater, I, 16,433 W-I warm subpulse, 425, 431 W-2 warm subpulse, 429 W-3 warm subpulse, 429 Wabarcrater, 16,224 Wachapreague Formation, 55 Virginia, 446 warm pulse, 425, 429, 431 washback breccia,383, 385 channel,*CBC, 190, 191 deposits, 386 facies, 361 lithofacies, 377-379, 381, 382, 384 process, 383, 384 regime, 377, 378 tsunami, 4, 170, 185, 189, 190,204, 322 unidirectional, 381 unit, 360, 361 Washington's Birthplace, 170 Washington State, 322 Wastegate Formation, 75 water depth,377 saturated, 440 vapor, 422 water column collapse, 104, 185,366 surgeback, 357

522

Index

wave oscillations, 368 weapons depots, around *CBC, 433 wetland loss, Chesapeake Bay, 445 wildfires, impact, 422 Williamsburg, Virginia, 190 Windmill Point core, 210 corehole, 171-175, 186, 189, 193, 211-216,226,239,242,253,260, 263,266,390,410 log, 212 Virginia, 444 Windsor Formation, 55, 440 Woods Hole Oceanographic Institution, 86,457 X-ray fluorescence (XRF) analysis, 234 Yablochinka crater, 329 Yaxcopoil corehole, 333-336 York-James Peninsula, 433, 440-443 York River, 51, 52, 77,104,106,120, 138, 185,284,444 Yorktown Formation, 55 Yucatan, Mexico, 334, 418 zone benthic foraminifera, 395 Bolboforma latdorfens is, 398 Bolboforma spinosa, 398 brecciated, 382 Cibicidoides pippeni, 391-394 collapse, 94, 292

crustal weakness, 163 crystallographic, 217 dead. See dead zone decollement, 91,176, 179, 193,376, 450 detachment, 303, 314 dike breccias, 345 dipping reflections, 339 disruption, 208, 333, 458 evaporation, 438 excavation, 292 fault, 292 foraminiferal, 395 fracture-weakened , 322 hinge, 64 indurated, 75 megablocks, 318 meh,224,354,363 microfossil, Chickahominy Formation, 279 NP16,298 NPI9/20,298 P15, 283, 396, 407 NPI5-PI4 boundary, 297 P16, 283, 407 seismic reflection, 149 shattered limestone, 350 shear, 333, 339 structural weakness, 440 target, 389 vaporization, 292, 438

*In order to conserve space, we have used CBC as an abbreviation for "Chesapeake Bay impact crater" throughout the index.

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