A PALAEOGEOLOGICAL MAP of the LOWER PALAEOZOIC FLOOR below the cover of UPPER DEVONIAN, CARBONIFEROUS AND LATER FORMATI...
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A PALAEOGEOLOGICAL MAP of the LOWER PALAEOZOIC FLOOR below the cover of UPPER DEVONIAN, CARBONIFEROUS AND LATER FORMATIONS with inferred and speculative reconstructions of Lower Palaeozoic and Precambrian outcrops in adjacent areas LEONARD JOHNSTON WILLS, M.A., Sc.D., Ph.D.
Not to seek in any subject greater accuracy than its nature admits. Aristotle.
Edited by PROFESSOR B. E. LEAKE
University of Glasgow
MEMOIR NUMBER 8
A JOINT PUBLICATION W I T H THE P E T R O L E U M E X P L O R A T I O N SOCIETY OF GRE A T BRITAIN
GEOLOGICAL SOCIETY LONDON 1978
Published for The Geological Society London
by
Scottish Academic Press Ltd. 33 Montgomery Street, Edinburgh EH7 5JX
First published 1978
All rights reserved. No part of this publication stored in a retrieval system, or transmitted, in means, electronic, mechanical, photocopying, wise, without the prior permission of the Burlington House, London, WIN 0JU.
may be reproduced, any form, or by any recording or otherGeological Society,
9 The Geological Society, London, 1978
SBN
7073
0226
9
Printed in Great Britain by Lindsay & Co. Ltd., Edinburgh
PREFACE It is four years since Professor Wills produced his outstanding pre-Permian Palaeogeological Map of England and Wales, published by this Society with an explanatory memoir and now to be found on the walls of many working offices across the country. He was then 89 and it was assumed that this was his final production, a remarkable achievement in the face of great physical frailty. Nevertheless he immediately moved on to a map of a stratigraphically deeper level m a palaeogeological map of the rocks following the Caledonian movements and the subsequent erosion, the surface of which would be exposed by the removal of Upper Devonian and later strata. This was completed and printed in 1975, and Wills began the equally complex task of writing an accompanying memoir to explain the way he had interpreted and applied the varied evidence of the deep structure. This process however led him to undertake still another map, that of the preDevonian rocks of a belt extending from East Anglia to South Wales, which has been completed this year. These two maps form the coloured plates accompanying this memoir. The memoir is also illustrated by a map of the Basal Elements, and by three Phanerozoic Time Scale sections, which illustrate the point which Wills has made before m that in the dominantly shelf environment of Britain the stratigraphical record is largely incomplete, with the lacunae at least comparable in time terms to the periods represented by sediments. This publication, then, represents the further remarkable achievement of a man of 93 ( n "and a half", as he emphasises - - he feels that months count now), working in a country study without technical support, his activity sharply limited by eye trouble and by a heart condition. It would have been a notable production for a fit man half his age. Both economic and academic geologists are already deeply indebted to Wills for his map of the pre-Permian surface; these further compilations add more organised data and more food for thought about the past history and deep structure of Britain. P. E. KENT, F.R.S. Fellow of the Geological Society Past Chairman, Petroleum Exploration Society September 17, 1977 Natural Environment Research Council.
FOREWORD During the preparation of the map in Memoir No. 7 I made sections across the map with the object of seeing whether the limits of the stratigraphical units shown in the sections agreed with the boundaries shown or implied by the reconstructions given in my Palaeogeographical Atlas, 1948. Nothing, however, came from this except the invention of the P.T.-S. Section, now known as a hiatograph, two of which were incorporated in Memoir No. 7 as Figs. 1 and 2. The present Fig. 2 shows the top of the 'Floor' and the bottom of the 'Cover' by ornamentation which shows how the stratigraphical age of the 'basal elements' varies from place to place; and how the 'Floor' on which a particular Basal Element rests may also vary. In the text of Mem. No. 7 no significance was attached to these features. Thus I failed to recognise the significance of 'Basal Elements' as a pile o f tattered carpets, each carpet giving a stratigraphical date to the particular part or parts of the Floor on which it lies. The sections are often very conjectural below the Carboniferous, but Fig. 2 shows clearly the well-known unconformity between the Upper and Lower Old Red Sandstone-Devonian in South Wales, which in my ponderings, I became convinced recorded the surface of the 'Old Red Sandstone Continent'. The same unconformity had long ago been discovered by James Hutton in Scotland. It is strikingly displayed by the break between Lower Palaeozoic rocks and the Carboniferous Limestone of the Pennines and North Wales. Here I thought was the surface of the very first continental land - - surely a suitable subject for a second palaeogeological map of England and Wales. I found however that there were far fewer boreholes than were available for the map in Memoir No. 7. There was also a dearth of papers (Bott's 1967 paper on the structure of the North of England is a brilliant exception) and a want of agreement on, for example, radiometric ages. As the map stands, it looks simple enough, but one should expect this since all the Upper Palaeozoic, Mesozoic and Tertiary complications of sedimentation, tectonics, magmatic intrusions and erosion are assumed to have been omitted. The Memoir however has to take account of these latter factors, which necessitate the inclusion of two supplementary maps and a complicated diagram (Fig. 1) illustrating the various manifestations of cyclicity throughout Phanerozoic Time, and five Hiatographs which demonstrate how much of Phanerozoic Time is unrecorded by sediment from place to place. However the map that I produced was welcomed by the few geologists and geophysicists who saw it as an attempt to depict the geology of England and Wales at the end of the Caledonian orogeny. It was redrawn beautifully by Mr A. A. Miles, Senior draughtsman of British Petroleum Ltd., at their expense, and colour printed at the expense of the Petroleum Exploration Society of Great Britain. Even before this, I discovered that the surface that I thought was Middle Devonian in age, was in reality the Palaeozoic Platform fashioned by erosion at seven different geological dates ranging from Upper Devonian to Gault! This involved the drawing of two more figures (Figs. 1 and 2). Later another map, Plate 2, became necessary to demonstrate the putative interpretation of the geology concealed by the outcrops of Lower and, in places, Middle O.R.S.-Devonian shown on Plate 1 in South Wales and S.E. England. These figures have been redrawn from my originals by Mr Colin Knipe. At long last the text of Memoir No. 8 has been completed about a map whose very title is wrong! It should read - - 'A palaeogeological map of the Palaeozoic Platform below the Cover of Upper Devonian and younger Formations'.
Partly as an outcome of this work I am now the proud recipient of the Honorary Fellowship of the Geological Society of London and of the Honorary Membership of the Petroleum Exploration Society of Great Britain, and I take the opportunity again to express my gratitude to the two Societies and to many individual members who have helped me, in particular Sir Peter Kent and Dr L. V. Illing, and others named in the acknowledgements. Work on the maps and memoir has taken an old man (I went to Cambridge in 1903) into the complexities of a good many modern 'ologies' such as sedimentology, ecology, radiometric dating and plate tectonics. The reader will notice that the memoir makes no reference to many modern theories although some must have inevitably invalidated many of the conceptions on which I was brought up. Ave atque Vale L. J. WILLS 1977
ACKNOWLEDGEMENTS
and T H A N K S
The author is deeply indebted for information on deep borings from the following Companies: British Gas Corporation, per K. W. Barr and V. S. Colter. British Petroleum Company Ltd., per Sir Peter Kent and F. Howitt. Burmah Oil Company. Cambrian Exploration Ltd., per J. A. C. Gage. Esso Exploration and Production U.K. Ltd. Shell U.K. Exploration and Production Ltd., per J. M. Bowen. Superior Oil Co. Ltd. Trend Cheshire U.S. Ltd., per J. Q. Stokstad. Tricentrol International Ltd., per C. A. Fothergill. Ultramar Exploration Ltd., per L. V. Illing. And for general assistance and advice from: Sir Peter Kent, N.E.R.C. L. V. Illing (of V. C. Illing and Partners). P. Lovelock (of Shell U.K. Exploration and Production Ltd.). Isles Strachan, Geology Dept., University of Birmingham. G. Bennison, Geology Dept., University of Birmingham. A. A. Miles (of British Petroleum Company Ltd.). C. Knipe (of Johnson, Poole and Bloomer, Civil and Mining Engineers, Dudley). Mrs D. Rae (Geology Dept., University of Glasgow). Mrs M. Darley (late of Geology Dept., University of Birmingham). Mrs P. A. Pyatt, East Worcestershire Waterworks Company. Miss S. Hodge (Geology Dept., University of Birmingham). Financial assistance towards the cost of publication has generously come from: British Gas, British National Oil Corporation, and the Petroleum Exploration Society of Great Britain.
CONTENTS PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
FOREWORD ..................................................................
4
ACKNOWLEDGEMENTSand
6
CONTENTS a n d
THANKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LIST OF ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
1.
Explanatory Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
2.
The general geography of Caledonian England and Wales . . . . . . . . . . . . . . . . . . . . . . . . . .
21
A.
The geosynclines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
B.
The English Microcraton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.
The Precambrian Basement, ridges and troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
4.
The topography of the pre-Phanerozoic Basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
5.
Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
6.
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
7.
Appendix: Explanation of Plate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
PLATES Plate 1. England and Wales. A palaeogeological map of the Lower Palaeozoic floor below the cover of Upper Devonian, Carboniferous and later formations with inferred and speculative reconstructions of the Lower Palaeozoic and Precambrian outcrops in adjacent areas. Scale: 1/625,000 or about ten miles to one inch. Completed October 1975. N.B. The crimson colour for the Silurian has been inadvertently omitted from the map between the putative interpretation and the coastline in Yorkshire. Bobbing should be 9 km S.W. of Sheerness B.H. Fobbing, add Cambrian? below M.Devonian. Plate 2. An inferred palaeogeological map of Wales and of England south of National grid Latitude 40 North, assuming the removal of all O.R.S.-Devonian and later formations, to reveal the outcrops at the close of Silurian times. The map records the sites of some 111 boreholes that reached the L. Palaeozoic or Precambrian formations, plus the sites of certain magnetic anomalies and other features. For a list of these, see Appendix. Scale: 1/625,000 or about ten miles to one inch. Completed August 1977.
TEXT-FIGURES Fig. 1. Fig. Fig. Fig. Fig. Fig. Fig.
2. 3. 4. 5. 6. 7.
Key to the Hiatographs and to the cyclic control of sedimentation throughout Phanerozoic time. Map of the basal elements or 'carpets' of the post/Mid-Devonian cover. Hiatograph A1, St. Davids to Norfolk coast. Hiatograph A2, Anglesey to Tunbridge, Kent. Hiatograph B1, Northumberland to Portsmouth, Hants. Hiatograph B2, Chesterfield to Brighton, Sussex. Hiatograph across the Shelve-Longmynd-Church Stretton-Wrekin country.
10 12-13 14-15 16 17 18 19
CONTENTS PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
FOREWORD ..................................................................
4
ACKNOWLEDGEMENTSand
6
CONTENTS a n d
THANKS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
LIST OF ILLUSTRATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7
1.
Explanatory Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
2.
The general geography of Caledonian England and Wales . . . . . . . . . . . . . . . . . . . . . . . . . .
21
A.
The geosynclines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
B.
The English Microcraton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
23
3.
The Precambrian Basement, ridges and troughs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
25
4.
The topography of the pre-Phanerozoic Basement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
5.
Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
30
6.
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
32
7.
Appendix: Explanation of Plate 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
PLATES Plate 1. England and Wales. A palaeogeological map of the Lower Palaeozoic floor below the cover of Upper Devonian, Carboniferous and later formations with inferred and speculative reconstructions of the Lower Palaeozoic and Precambrian outcrops in adjacent areas. Scale: 1/625,000 or about ten miles to one inch. Completed October 1975. N.B. The crimson colour for the Silurian has been inadvertently omitted from the map between the putative interpretation and the coastline in Yorkshire. Bobbing should be 9 km S.W. of Sheerness B.H. Fobbing, add Cambrian? below M.Devonian. Plate 2. An inferred palaeogeological map of Wales and of England south of National grid Latitude 40 North, assuming the removal of all O.R.S.-Devonian and later formations, to reveal the outcrops at the close of Silurian times. The map records the sites of some 111 boreholes that reached the L. Palaeozoic or Precambrian formations, plus the sites of certain magnetic anomalies and other features. For a list of these, see Appendix. Scale: 1/625,000 or about ten miles to one inch. Completed August 1977.
TEXT-FIGURES Fig. 1. Fig. Fig. Fig. Fig. Fig. Fig.
2. 3. 4. 5. 6. 7.
Key to the Hiatographs and to the cyclic control of sedimentation throughout Phanerozoic time. Map of the basal elements or 'carpets' of the post/Mid-Devonian cover. Hiatograph A1, St. Davids to Norfolk coast. Hiatograph A2, Anglesey to Tunbridge, Kent. Hiatograph B1, Northumberland to Portsmouth, Hants. Hiatograph B2, Chesterfield to Brighton, Sussex. Hiatograph across the Shelve-Longmynd-Church Stretton-Wrekin country.
10 12-13 14-15 16 17 18 19
1.
EXPLANATORY
DESCRIPTION
THE FLOOR The first truly-continental land surface in Phanerozoic Britain is that of the Old Red Sandstone continent which forms the subject of the present essay and map (P1. 1). As there is in west England and South Wales a great unconformity between Upper Devonian and Lower Old Red SandstoneDevonian formations, with the Lower O.R.S.-Devonian following the Silurian with little or no break; and the Upper Devonian merging upwards into the Lower Dinantian (Tournaisian), MidDevonian is taken as the date of the land surface, here termed the 'Ground Floor' (often referred to in the sequel as 'The Floor'), on which the overlying rocks of "The Cover' rest. There are four more newer Continental Floors indicated on Fig. 1. The geophysical properties of these Floors are very different; and this fact makes it imperative to distinguish them in present day exploration for coal, oil and gas. The term 'Basement Floor' should be restricted to the pre-Phanerozoic Floor, and the term Floor should be qualified as follows: 'Ground Floor' is pre-U. Devonian-Lower Carboniferous (here under discussion). This was the subject of the Yorkshire Geological Society's 1967 Symposium on the Sub-Carboniferous Basement. The '1st Floor' refers to the Sub-U. Permian (Zechstein) Floor described by Wills (1973) and which has also been referred to as 'the Basement' without any agequalification e.g. in Woodland (1975). The '2nd Floor' refers to the sub-U. Cretaceous Floor, the subject of Sir Aubrey Strahan's Geological Society Presidential Address 1913 in which it is rightly and consistently referred to as the 'Palaeozoic Platform '. Since completion of the Map, it has been shown that the supposed Mid-Devonian surface is composite, being made up of a patchwork of erosion surfaces ranging in age from Upper Devonian to Cretaceous, see Fig. 2. In fact the supposed Mid-Devonian Floor has turned out to be part of Strahan's Palaeozoic Platform. Likewise, the map of Wills (1973) ought to be a Palaeogeographical map of the Palaeozoic Platform below the Zechstein, etc.
Fig 1
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Fig. 1 Key to the hiatographs (Figs. 3--7), to the hypothesis of cyclic sedimentation and to deductions made from P1. 1. Cycles i--iv pre-Middle Devonian are deduced after removing the effects of the later cycles 1--9. Shaded strips show sedimentation due to prolonged rising ocean-level, v. volcanic rocks. Columns 1 - - 4 explained on Fig. Column 4 is constructed mainly from the following transitional areas: St. Davids-W. Dyfed; Llandeilo-Llandovery-Builth; Long Mynd-Church Stretton-Caradoc; Wrekin-Lilleshall-Edgmond; Furness-Cautley-Ingleborough-Horton. Column 5 shows the sequence on the English Microcraton. Column 6 Eo-North Sea Caledonian geosyncline, southern extension after Stormer (1967) and Bogdunoff et al. (1964). Evidence on P1. 1 and 2 and in Appendix. Columns 3 - - 6 The Ground Floor or the surface of the Palaeozoic Platform with three expanses of O.R.S.-Devonian as shown P1. 1. One underlies the North Irish Sea, the second stretches from the Severn Estuary to Herefordshire being an Anglo-Welsh delta and the third is the S.E. England Gulf which may have joined Kent with the Ardennes and the marine Devonian of the North Sea (Kent 1965b, Pennington 1975). The second Continental Floor was explained by Wills (1973). Column 7 Complete plantation to within 30m of ocean-level by terrestrial erosion. Column 8 Evidence available that present ocean-level is close to ocean level at these times. Column 9 Long suites of (a) plankton-bearing mudstones (graptolitic shales); coccolith limestones (chalk); (b) benthonic shelf-deposits-sandstones, mudstones, limestones, delta conglomerates, and Carboniferous cyclothems. Both (a) and (b) indicate long-continued rise of ocean-level with maintenance of the same environment. Column lOa Lower Palaeozoic cycles in the Floor (Roman numerals). Column lOb Cycles in the Cover, Nos. 1 to 7 being Basal Elements. Cycles of various lengths of time are shown by the curves on Fig. 1, ranging from c. 60 m.y. to a very short cycle e.g. Carboniferous cyclothems (Ramsbotton (1976), Calver (1969)). The base of each cycle represents sediment deposited on a virtually horizontal surface produced by erosion and planation of the preceding one which was graded to the contemporary ocean-level (often quite near to present ocean level). Column 11 Isostatic movements are due to movements in the asthenosphere (Bott 1967).
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THE MAP, PLATE 1 The index to Plate 1 explains the colours and symbols, and attempts to assign a time-interval to each of the groups of formations distinguished by colour, following Harland et al. (1964, 1971); and to show on the same time scale the approximate age of some of the tectonic events. South of the line from Pembrokeshire to Kent which is labelled 'Putative Armorican Front' (with emphasis on 'putative') the map has been left uncoloured, except for two provings of Silurian, because the Floor was so disturbed by the Variscan movements here that Mid-Devonian outcrops on it are not decipherable. It should be noted that throughout this text 'outcrop' is used for outcrops on a supposed Mid-Devonian land-surface. A possible Mid-Devonian coastline is indicated, with a speculative large promontory continuing the Mercian Mountains (site uncoloured) southwards which had the effect of dividing the Devonian Armorican Sea into a Devonshire-Somerset-S. Wales-AngloWelsh Bay from a SE England Gulf. A few important stratigraphical sites have been added. A second uncoloured area is the Lower Severn Permo-Triassic Graben, now proved to overlie the Lower Palaeozoic Mercian Mountains. The faults shown on the map are all believed to be basement faults that originated in Precambrian times and have moved throughout the Lower Palaeozoic era and~ or, later. Those faults that are known to reach the present-day surface are distinguished from inferred faults. The same horizontal scale was used in constructing the Phanerozoic Time-Scale Sections, here renamed 'Hiatographs' as in Wills (1973) where it is stated that such sections are designed to eliminate the effects of varying original formational thickness and of subsequent folding and faulting (with the exception of a few major dislocations), thereby elucidating the time-space distribution of the gaps in the Geological Column, and helping to distinguish between the original and the present limits of the formations. In the present hiatographs those parts that are conjectural have the shading lines of the 'ornament' broken to distinguish them from the parts based upon borehole and, or, surface exposures. As set out in Figs. 3--7, they show all the significant gaps in the entire Phanerozoic succession when traced from place to place, but they have not been revised to conform with the new discoveries in the North Sea (Woodland, 1975). If the data in Fig. 2 are compared with the hiatographs it becomes apparent that, at any particular place, the Mid-Devonian Ground Floor may be covered by its original cover of U. Devonian and L. Carboniferous, or by Coal Measures, or by the next younger covers (Permo-Trias plus Jurassic-Aptian or U. Cretaceous). Each geological formation that lies directly upon the MidDevonian Floor is termed a Basal Element or Carpet. Whereas the Map (P1. 1) should show outcrops as in Mid-Devonian times, they can only claim to have a semblance of accuracy where they have been proved to be directly covered by U. Devonian or basal L. Carboniferous formations, namely where they are seen to disappear under these rocks, as in N. England, S. Wales and parts of the Midlands. Elsewhere, in general to the east, the L. Carboniferous cover has been destroyed by the erosion and planation implicit in the growth of the major unconformities mentioned above: as has been proved only by the scattered boreholes shown on the map. Few of these have penetrated
A PALAEOGEOLOGICAL MAP OF THE ANGLO-WELSH MID-DEVONIAN FLOOR
21
the L. Palaeozoic rocks for more than a few feet, and so do not throw any light on the sequence of formations below. It must be recognised therefore that the boundaries suggested are at best inferences, and more usually conjectures. Where the Lower Palaeozoic rocks form the present-day surface, the boundaries of the map are for the most part those shown on the I.G.S. 1/650,000 Geological Map, 1957, but again the lines may be erroneous as boundaries on the Mid-Devonian Floor, especially in Wales and the Welsh Borderland where pre-Devonian formations may have been concealed below a more extensive cover of L. O.R.S.-Devonian than now exists. The internal basin filled by the L. O.R.S.-Devonian deposits of the Anglesey-N. Irish Sea, is almost entirely conjectural. Equally, the reader should note that the map assumes that effectively no significant internal movements affected the Lower Palaeozoic rocks after Middle Devonian times and in some instances after Silurian times. The formations that constituted the Cover are not shown on the map (P1. 1) which assumes their removal to display the Floor, and Fig. 2 has been constructed to show the distribution of the seven Basal Elements. The symbol of the U. Devonian (c3) has however been inserted on patches of stipple at various points to help in the assessment of the original extent of the U. Devonian phase of the L. Carboniferous transgression before Dinantian times. A putative limit of the (c3) transgression is shown by a line broken by crosses on Fig. 2. It may be commented that Mortimer & Chaloner (1972) doubt the existence of Fammenian in the concealed rocks of S. England, spores suggesting Frasnian-Givetian only.
2. T H E
GENERAL GEOGRAPHY OF CALEDONIAN ENGLAND AND WALES
This account would be incomplete without some reference to the author's admittedly hypothetical conceptions of how to interpret the available evidence in terms of palaeogeography and of the stratal record and the gaps within it. The following statements read like dogmas, yet they are not meant as such, but as brief summaries of the conceptions. Throughout Lower Palaeozoic times (c. 415-595 m.y.) geosynclinal or basinal seas (using the term geosyncline in its original sense: see the review of the conception of Umbgrove 1950, pp. 19 m 23) surrounded the English part of Britain which tended to behave as a rigid positive entitymthe English Microcraton, or Microcraton as it is referred to both here and on P1. 1. It covers too diminutive an area to be classed as a continental plate, like the East European Craton and the North American Craton, though it may have originated as a splinter from the former. In a recent paper (Scrutton 1976) states that some two dozen such 'Continental fragments' have been identified in oceanic areas between the six present-day continental 'Plates'. The English Microcraton here postulated, with its northwest and northeast limits shown on P1. 1, surrounded by geosynclinal developments, clearly resembles them. It could have broken away from the Baltic Shield area of the East European Craton. At times of high ocean-level the whole region may have been submerged, but sediment reaching the geosynclinal seas tended to be deposited in unbroken sequences except in places tectonically uplifted and converted into shoals; whereas coeval sediments laid down on the microcraton were typical shelf-deposits of littoral and benthonic origin, varying in thickness inversely related to the drowned topography. This, together with tight folding and strike faulting is partly responsible for the great expanses of Cambrian and Silurian in Wales and Cumbria; for both of these systems might have formed during protracted periods of world-wide rising ocean level (See Sloss (1963), Valentine & Moores (1970) and Fig. 1) though some would challenge this in view of the 10 Km of sediment involved.
A PALAEOGEOLOGICAL MAP OF THE ANGLO-WELSH MID-DEVONIAN FLOOR
21
the L. Palaeozoic rocks for more than a few feet, and so do not throw any light on the sequence of formations below. It must be recognised therefore that the boundaries suggested are at best inferences, and more usually conjectures. Where the Lower Palaeozoic rocks form the present-day surface, the boundaries of the map are for the most part those shown on the I.G.S. 1/650,000 Geological Map, 1957, but again the lines may be erroneous as boundaries on the Mid-Devonian Floor, especially in Wales and the Welsh Borderland where pre-Devonian formations may have been concealed below a more extensive cover of L. O.R.S.-Devonian than now exists. The internal basin filled by the L. O.R.S.-Devonian deposits of the Anglesey-N. Irish Sea, is almost entirely conjectural. Equally, the reader should note that the map assumes that effectively no significant internal movements affected the Lower Palaeozoic rocks after Middle Devonian times and in some instances after Silurian times. The formations that constituted the Cover are not shown on the map (P1. 1) which assumes their removal to display the Floor, and Fig. 2 has been constructed to show the distribution of the seven Basal Elements. The symbol of the U. Devonian (c3) has however been inserted on patches of stipple at various points to help in the assessment of the original extent of the U. Devonian phase of the L. Carboniferous transgression before Dinantian times. A putative limit of the (c3) transgression is shown by a line broken by crosses on Fig. 2. It may be commented that Mortimer & Chaloner (1972) doubt the existence of Fammenian in the concealed rocks of S. England, spores suggesting Frasnian-Givetian only.
2. T H E
GENERAL GEOGRAPHY OF CALEDONIAN ENGLAND AND WALES
This account would be incomplete without some reference to the author's admittedly hypothetical conceptions of how to interpret the available evidence in terms of palaeogeography and of the stratal record and the gaps within it. The following statements read like dogmas, yet they are not meant as such, but as brief summaries of the conceptions. Throughout Lower Palaeozoic times (c. 415-595 m.y.) geosynclinal or basinal seas (using the term geosyncline in its original sense: see the review of the conception of Umbgrove 1950, pp. 19 m 23) surrounded the English part of Britain which tended to behave as a rigid positive entitymthe English Microcraton, or Microcraton as it is referred to both here and on P1. 1. It covers too diminutive an area to be classed as a continental plate, like the East European Craton and the North American Craton, though it may have originated as a splinter from the former. In a recent paper (Scrutton 1976) states that some two dozen such 'Continental fragments' have been identified in oceanic areas between the six present-day continental 'Plates'. The English Microcraton here postulated, with its northwest and northeast limits shown on P1. 1, surrounded by geosynclinal developments, clearly resembles them. It could have broken away from the Baltic Shield area of the East European Craton. At times of high ocean-level the whole region may have been submerged, but sediment reaching the geosynclinal seas tended to be deposited in unbroken sequences except in places tectonically uplifted and converted into shoals; whereas coeval sediments laid down on the microcraton were typical shelf-deposits of littoral and benthonic origin, varying in thickness inversely related to the drowned topography. This, together with tight folding and strike faulting is partly responsible for the great expanses of Cambrian and Silurian in Wales and Cumbria; for both of these systems might have formed during protracted periods of world-wide rising ocean level (See Sloss (1963), Valentine & Moores (1970) and Fig. 1) though some would challenge this in view of the 10 Km of sediment involved.
22
LEONARDJOHNSTONWILLS
On the other hand, with falling ocean level the surface of the Microcraton would become dry land and subject to aerial erosion, as it did, for example, in mid-Ordovician times, leaving only one occurrence (so far discovered) of Arenig (at Eyam in Derbyshire) and creating the great subLlandovery-Wenlock unconformity stretching from the Llandeilo-Llandovery-Builth-Church Stretton-Wrekin outcrops and subcrops described by George (1963), Ziegler et al. (1968) and by Ziegler (1970) eastwards at least as far as Ware (Hefts.) and Cliffe No. 1 (Kent). See also Figs. 4-6.
A . THE GEOSYNCLINES
The geosynclinal regions were mobile trough-like belts or basins, and the thick marine deposits laid down in them were eventually tectonized by Caledonian orogenic forces into mountain chains whose trends were influenced by the shape of the microcraton. (i) On the west were the Welsh, the Lake District and the Southern Uplands geosynclines of the British Caledonian system and here where the Lower Palaeozoic and Lower Devonian rocks are exposed today the present-day outcrops have to be adopted because of the impossibility of restoring the Mid-Devonian ones after all the subsequent tectonic and erosional vicissitudes (Fig. 1). Accordingly, on the map the boundaries of the present-day outcrops on land have been taken from the I.G.S. 1/650,000 Geological Map, 1957 Edition, b u t those s h o w n w h e r e the sea and, or, u n e x p l o r e d cover n o w hide the M i d - D e v o n i a n Floor are inferences. (ii) On the east was the East Yorkshire-East Anglian, or to give it a distinctive name, 'the EoNorth Sea geosyncline' which perhaps was part of the Mid-European Caledonian System. (See Turner 1949, Stormer 1967, Kent 1975a). There was a syntaxis between (i) and (ii) probably under Stainmoor on the north side of the Askrigg Block. In the area covered by (ii), Lower Palaeozoic rocks never reach the surface, being hidden under varying, but often thick, Upper Palaeozoic and later formations which have been penetrated by boreholes in East Anglia and round the Thames Estuary. These holes have often entered Devonian, less often, geosynclinal U. Silurian and M. Ordovician at Burmah 47/29A in the North Sea off Skegness, U. Ordovician (Caradocian) at Bobbing in north Kent and reputed Cambrian at Fobbing, S. Essex (Baker 1935). For this reason the boundaries suggested on Pls. 1 and 2 must be regarded as conjectures. Kent (1975b) in his review of the discoveries in the North Sea that had been disclosed at the November 1975 Symposium (Woodland 1975) concluded that, apart from off-shore provings in the Danish Sector, there was "a solitary identification of Silurian below the Permian of the Dogger Bank area", and an important discovery of Upper and Middle marine Devonian in the Argyll Field in the latitude of North Berwick (Pennington, 1975). This discovery lies outside the region covered by P1. 1. The discovery of Ordovician (Llanvirn) in The Wash off Skegness was made too late for inclusion on P1. 1, but is shown on P1. 2. (iii) On the south lay the Lower Palaeozoic fore-runner of the Devonian-L. Carboniferous part of the Variscan geosyncline, now buried below great thicknesses of Permo-Triassic and later deposits except in the SW Cornubian Peninsula. Exceptions are the rocks of the crush-zone between Gerrans Head and Gorrans Haven from which Hendriks (1937) recorded Llandeilian, Caradocian and less certainly Silurian faunas. More recently Sadler (1974) has discovered Llandeilian trilobites in the Gorran Quartzite. As the main evidence of the geosyncline we have to rely on deep boreholes; for the Palaeozoic
22
LEONARDJOHNSTONWILLS
On the other hand, with falling ocean level the surface of the Microcraton would become dry land and subject to aerial erosion, as it did, for example, in mid-Ordovician times, leaving only one occurrence (so far discovered) of Arenig (at Eyam in Derbyshire) and creating the great subLlandovery-Wenlock unconformity stretching from the Llandeilo-Llandovery-Builth-Church Stretton-Wrekin outcrops and subcrops described by George (1963), Ziegler et al. (1968) and by Ziegler (1970) eastwards at least as far as Ware (Hefts.) and Cliffe No. 1 (Kent). See also Figs. 4-6.
A . THE GEOSYNCLINES
The geosynclinal regions were mobile trough-like belts or basins, and the thick marine deposits laid down in them were eventually tectonized by Caledonian orogenic forces into mountain chains whose trends were influenced by the shape of the microcraton. (i) On the west were the Welsh, the Lake District and the Southern Uplands geosynclines of the British Caledonian system and here where the Lower Palaeozoic and Lower Devonian rocks are exposed today the present-day outcrops have to be adopted because of the impossibility of restoring the Mid-Devonian ones after all the subsequent tectonic and erosional vicissitudes (Fig. 1). Accordingly, on the map the boundaries of the present-day outcrops on land have been taken from the I.G.S. 1/650,000 Geological Map, 1957 Edition, b u t those s h o w n w h e r e the sea and, or, u n e x p l o r e d cover n o w hide the M i d - D e v o n i a n Floor are inferences. (ii) On the east was the East Yorkshire-East Anglian, or to give it a distinctive name, 'the EoNorth Sea geosyncline' which perhaps was part of the Mid-European Caledonian System. (See Turner 1949, Stormer 1967, Kent 1975a). There was a syntaxis between (i) and (ii) probably under Stainmoor on the north side of the Askrigg Block. In the area covered by (ii), Lower Palaeozoic rocks never reach the surface, being hidden under varying, but often thick, Upper Palaeozoic and later formations which have been penetrated by boreholes in East Anglia and round the Thames Estuary. These holes have often entered Devonian, less often, geosynclinal U. Silurian and M. Ordovician at Burmah 47/29A in the North Sea off Skegness, U. Ordovician (Caradocian) at Bobbing in north Kent and reputed Cambrian at Fobbing, S. Essex (Baker 1935). For this reason the boundaries suggested on Pls. 1 and 2 must be regarded as conjectures. Kent (1975b) in his review of the discoveries in the North Sea that had been disclosed at the November 1975 Symposium (Woodland 1975) concluded that, apart from off-shore provings in the Danish Sector, there was "a solitary identification of Silurian below the Permian of the Dogger Bank area", and an important discovery of Upper and Middle marine Devonian in the Argyll Field in the latitude of North Berwick (Pennington, 1975). This discovery lies outside the region covered by P1. 1. The discovery of Ordovician (Llanvirn) in The Wash off Skegness was made too late for inclusion on P1. 1, but is shown on P1. 2. (iii) On the south lay the Lower Palaeozoic fore-runner of the Devonian-L. Carboniferous part of the Variscan geosyncline, now buried below great thicknesses of Permo-Triassic and later deposits except in the SW Cornubian Peninsula. Exceptions are the rocks of the crush-zone between Gerrans Head and Gorrans Haven from which Hendriks (1937) recorded Llandeilian, Caradocian and less certainly Silurian faunas. More recently Sadler (1974) has discovered Llandeilian trilobites in the Gorran Quartzite. As the main evidence of the geosyncline we have to rely on deep boreholes; for the Palaeozoic
A PALAEOGEOLOGICALMAP OF THE ANGLO-WELSHMID-DEVONIANFLOOR
23
platform lies under the Mesozoic and Tertiary of Wessex and The Weald, which bury it below great thicknesses of Permo-Triassic and later deposits, thus: 1. The Minety B.H. 17 km NW of Swindon (Wilts.), starting in Oxford Clay proved 1448 m of Jurassic and Trias above 2744 m of Cambrian with Tremadoc. The Minety hole, over 4 km deep (about 289 miles) is probably the deepest in Britain. 2. Cranborne (Dorset B.H.) proved 378+ m of supposed Middle Devonian below 1663 m of Trias and later formations. 3. Sonning Eye B.H. (5 km NE of Reading) proved 497 m of Tertiary and Mesozoic rocks on Upper Coal Measures with associated basalt sills, 265 m; on 231+ m of Upper Devonian to a total depth of 1048 m. 4. Shalford B.H. (2 km S. of Guildford) proved 1933 m of Mesozoic rocks on Carboniferous Limestone, 89 m on Silurian. The Carboniferous Limestone here and at some other provings in Surrey, Sussex and Kent map out as shelf deposits on the southern edge of the London Platform. The Warlingham B.H. proved Tournaisian, and Penshurst B.H. showed Visean. See Fig. 4. 5. Bolney B.H. proved M. Devonian, Brightling L. Devonian and Tatsfield M. Devonian (all by spores, Mortimer & Challoner 1972). These are all in East Sussex. In view of the dearth of information and the tectonic dislocations in this Armorican Zone, no outcrop boundaries have been inserted on the map, but the borehole sites and provings both in England and in north France, together with notes on a possible Mid-Devonian coastline, have been added to P1. 1. Since the completion of P1. 1, a conjectural interpretation of it has been worked out as a palaeogeological map (P1. 2) which assumes the removal of the Middle and Lower Old Red Sandstone-Devonian of the 'Anglo-Welsh Cuvette' and 'SE England Gulf' to reveal the inferred Lower Palaeozoic and Precambrian subcrops. B.
T H E ENGLISH MICROCRATON
The English Microcraton was essentially a triangular area with its northwest side bounded by a line from Pembrokeshire to The Long Mynd and Skipton, and on its northeast side by a line from Ripon to The Wash-Kent along an arcuate line shown on P1. 1. The southern side cannot be precisely defined owing to tectonic disturbance and deep burial; but for present purposes may be taken along the conjectural line of the Armorican Front as shown on Pls. 1 and 2. The line of the northwest side was first recognised by Jones (1927) and part of it was more fully defined by George (1963), and R. H. Hoare has pointed out to the author that this line is followed by recent earthquakes. The Microcraton tended to behave as a rigid positive element throughout L. Palaeozoic times, appearing to rise or fall in relation to contemporary ocean level, as a whole, rather than yielding by folding and thrusting as did the geosynclinal areas. That this rigidity was acquired in prePhanerozoic times is suggested by the form of the Basement Floor which appears to have been divided into blocks by faults, resulting in horst-like ridges and intervening graben-like troughs that influenced the pattern of sedimentation and planation throughout the L. Palaeozoic Era. The ridges have been located for the most part by reference to high magnetic anomalies but this thesis has only been demonstrated conclusively by Bott, (1969) for the Furness-Ingleborough-Norfolk Ridge. It is easier to consider the rising and falling relative to ocean level as due to eustatic changes in that level brought about by the opening or closing of an ocean between continental plates as demonstrated by McKerrow (1972) and Zieglier (1970) for the Llandovery transgression, than to attribute it to isostatic vertical movements of continental crust, though these may have taken place
A PALAEOGEOLOGICALMAP OF THE ANGLO-WELSHMID-DEVONIANFLOOR
23
platform lies under the Mesozoic and Tertiary of Wessex and The Weald, which bury it below great thicknesses of Permo-Triassic and later deposits, thus: 1. The Minety B.H. 17 km NW of Swindon (Wilts.), starting in Oxford Clay proved 1448 m of Jurassic and Trias above 2744 m of Cambrian with Tremadoc. The Minety hole, over 4 km deep (about 289 miles) is probably the deepest in Britain. 2. Cranborne (Dorset B.H.) proved 378+ m of supposed Middle Devonian below 1663 m of Trias and later formations. 3. Sonning Eye B.H. (5 km NE of Reading) proved 497 m of Tertiary and Mesozoic rocks on Upper Coal Measures with associated basalt sills, 265 m; on 231+ m of Upper Devonian to a total depth of 1048 m. 4. Shalford B.H. (2 km S. of Guildford) proved 1933 m of Mesozoic rocks on Carboniferous Limestone, 89 m on Silurian. The Carboniferous Limestone here and at some other provings in Surrey, Sussex and Kent map out as shelf deposits on the southern edge of the London Platform. The Warlingham B.H. proved Tournaisian, and Penshurst B.H. showed Visean. See Fig. 4. 5. Bolney B.H. proved M. Devonian, Brightling L. Devonian and Tatsfield M. Devonian (all by spores, Mortimer & Challoner 1972). These are all in East Sussex. In view of the dearth of information and the tectonic dislocations in this Armorican Zone, no outcrop boundaries have been inserted on the map, but the borehole sites and provings both in England and in north France, together with notes on a possible Mid-Devonian coastline, have been added to P1. 1. Since the completion of P1. 1, a conjectural interpretation of it has been worked out as a palaeogeological map (P1. 2) which assumes the removal of the Middle and Lower Old Red Sandstone-Devonian of the 'Anglo-Welsh Cuvette' and 'SE England Gulf' to reveal the inferred Lower Palaeozoic and Precambrian subcrops. B.
T H E ENGLISH MICROCRATON
The English Microcraton was essentially a triangular area with its northwest side bounded by a line from Pembrokeshire to The Long Mynd and Skipton, and on its northeast side by a line from Ripon to The Wash-Kent along an arcuate line shown on P1. 1. The southern side cannot be precisely defined owing to tectonic disturbance and deep burial; but for present purposes may be taken along the conjectural line of the Armorican Front as shown on Pls. 1 and 2. The line of the northwest side was first recognised by Jones (1927) and part of it was more fully defined by George (1963), and R. H. Hoare has pointed out to the author that this line is followed by recent earthquakes. The Microcraton tended to behave as a rigid positive element throughout L. Palaeozoic times, appearing to rise or fall in relation to contemporary ocean level, as a whole, rather than yielding by folding and thrusting as did the geosynclinal areas. That this rigidity was acquired in prePhanerozoic times is suggested by the form of the Basement Floor which appears to have been divided into blocks by faults, resulting in horst-like ridges and intervening graben-like troughs that influenced the pattern of sedimentation and planation throughout the L. Palaeozoic Era. The ridges have been located for the most part by reference to high magnetic anomalies but this thesis has only been demonstrated conclusively by Bott, (1969) for the Furness-Ingleborough-Norfolk Ridge. It is easier to consider the rising and falling relative to ocean level as due to eustatic changes in that level brought about by the opening or closing of an ocean between continental plates as demonstrated by McKerrow (1972) and Zieglier (1970) for the Llandovery transgression, than to attribute it to isostatic vertical movements of continental crust, though these may have taken place
24
LEONARD JOHNSTON WILLS
as well. This being granted, it follows that with each major rise of ocean-level, the whole rigid Microcraton with its diversified surface of ridges and troughs would be flooded; and transgressive sedimentation (of platform or shelf facies) tended to take place simultaneously over the whole region, but it was thinner over upstanding parts, and more complete and thicker over the troughs; but as sea-level began to fall, shoals and islands of an archipelago appeared, from which the newlyformed sediments were quickly removed by aerial erosion and by planation by wave action from those parts where they were thinnest, while tending to survive where thickest. With the next rise of ocean-level, the next set of strata would be conformable to the old set in the troughs, but transgressive and unconformable on the flanks and tops of the ridges. In this way came about the merging of the small gaps in the sequence into large composite unconformities so clearly shown on the hiatographs (Figs. 3-6, particularly Fig. 4). Since much, if not all, of the erosion would have been wave-cut, the base of each sedimentary cycle would also be horizontal (Fig. 1). The rise and fall of ocean-level would also be recorded by the stratigraphical sequences found in the geosynclinal areas, a feature helpful in correlating the records in the two contrasting environments; but this is made more difficult by the occurrence of orogenic phases that have been discovered in the exposed geosynclinal rocks of Wales and the Lake District, which cannot be recognised in the concealed and inadequately explored rocks of the Microcraton. Complete compliance with the general hypothesis outlined above cannot be expected. For example, modifications would be needed if the Microcraton were to be tilted as a whole, or one or more blocks within it depressed or elevated in relation to the whole, or the whole subjected to gentle open folding. Convincing stratigraphical evidence in favour of the above hypothetical interpretation is limited to those few places where the passage from geosynclinal to microcratonic conditions can be examined. (Fig. 1 Column 4 and Fig. 7). In addition, the reader when looking critically at the map, must recognise that the interpretation adopted has involved the really insuperable task of eliminating the effects of all later sedimentations, movements and erosions affecting the Cover, that have been superimposed upon the pre-Upper Devonian Palaeozoic and Precambrian rocks of the Microcraton, themselves already disturbed by the Caledonian stresses. Some of the complexity of that problem has been shown by a study of the geological age of the final planation of the Palaeozoic Platform from place to place which is revealed by the 'Basal Elements' or Carpets of the Cover. Each Basal Element is a geological formation that has been proved to lie, like a torn and moth-eaten carpet, directly on the Mid-Devonian Floor over areas that have now been found to be mappable with a fair degree of accuracy (Fig. 2). This problem has to some degree been dealt with by Wills (1973) and by Kent (1975a); but both papers date from before the realisation of both authors that the geological age of different parts of the surface of the Floor could be thus discovered, a realisation that in fact occurred after the completion of the Map, P1. 1. The Mid-Devonian Floor thought to be mapped has turned out to be a patchwork of floors falling into groups, three of which can be subdivided on a stratigraphical basis into two, thus giving the seven mappable Basal Elements whose real distribution is shown on Fig. 2. In general, each of the Basal Elements follows a major planation (Fig. 1), but there is no important break between the U. Devonian and the Dinantian, but from SE England and most of Wales Namurian is missing. In some places (e.g. Oxfordshire) Upper Coal Measures overstep the Lower Measures on to Silurian. The Jurassic oversteps the Trias on the same erosion surface - - The so-called 'Palaeozoic Platform' of the London-Brabant Massic (Strahan 1913). The U. Cretaceous Basal Element differs from all the others in that it only qualifies as a Basal Element where the others are missing through the destruction inherent in the production of the earlier floors; in other words, missing in the great composite gap so clearly shown on Figs. 3 & 4.
A PALAEOGEOLOGICAL MAP OF THE ANGLO-WELSH MID-DEVONIAN FLOOR
3. T H E P R E C A M B R I A N
BASEMENT,
RIDGES
25
AND TROUGHS
Precambrian rocks are generally separated from Cambrian rocks by a marked unconformity. In the Anglo-Welsh area Precambrian rocks are generally volcanic (Pebidian of Pembrokeshire, Uriconian of the Wrekin-Stretton area, Malvern, Barnt Green, and a hidden area centred on Droitwich and identified by the predominance of Uriconian-type rhyolitic clasts in the Clent Breccia near Stourbridge) although the Longmyndian is a thick development of mainly clastic rocks. The present-day outcrops are usually small, and the rocks difficult to correlate from one area to another; the only large one is a subcrop in the NE Midlands shown on P1. 1 as extending from Stamford to Buxton, and from Leicester almost to The Wash. Charnwood Forest gives the name 'Charnian' to this thick group of volcanic and intrusive rocks. For recent radiometric datings, see Cribb (1975). Similar rocks have been reached in borings at North Creake and Hunstanton, both in Norfolk. These holes lie on the magnetic ridge that joins Norfolk to the Askrigg Block (Bott 1967). Bott's paper is so lucidly written and as a result of two deep boreholes is now proved to be so accurate that it is quoted to explain the nature of the Precambrian Basement of the microcratonic parts of England and Wales. "The Lower Carboniferous rocks of the southern half of the Askrigg Block are underlain at shallow depth by a belt of magnetic basement rocks which are probably metamorphic or extrusive igneous rocks. This belt of basement rocks extends beyond the confines of the block at greater depth both to the west and to the south-east. The magnetic rocks extend downwards to a depth of several kilometres, and they probably underlie the Ingletonian rocks" (Bott 1967). The depositional age of the Ingletonian is Cambrian or possibly early Ordovician (O'Nions et al. 1973, 1974) and therefore the magnetic rocks are probably older than that age. In P1. 1 the Ingletonian is shown as Cambrian. West of the Dent Line, the magnetic rocks underlie the Silurian and have a different strike direction, suggesting they underwent folding before the Caledonian movements. Bott (1967) also wrote "A negative gravity anomaly covering the central part of the Askrigg Block is interpreted as caused by a granite underlying Wensleydale and penetrating the northern flank of the magnetic basement rocks. The contact between the granite and the magnetic rocks forms the steep northern boundary of the magnetic belt." The Raydale borehole into the Wensleydale Granite (Dunham 1974) has since proved Bott to have diagnosed the structure exactly. These magnetic basement rocks that probably underlie the Ingletonian and certainly underlie the Askrigg Block Carboniferous rocks "could be (1) plutonic intrusions, (2) lavas or minor intrusions or (3) metamorphic rocks: Absence of a major gravity anomaly suggests that (2) and (3) are the most likely hypotheses. Pebbles of metamorphic rocks and lavas were found in the Ingletonian by Rastall 1906." (Bott 1967). The author's idea is that the belt of magnetic basement rocks may be a great sheet of magnetic iron ore comparable with one of the Canadian greenstones in size, and underlying the whole microcraton (appropriately if the microcraton was part of the East European Craton). This I suggested to Bott (letter 17/6/76 and his reply of 18/6/76) was "I suspect the belts of magnetic high displayed by the Aeromagnetic map are mainly caused by deep-seated metamorphic rocks, although the possibility that some of them are magnetic iron ore of the bedded type seems to me to be quite possible. I think we should really need a drill hole through to them to be sure of their origin, but I suspect their amplitude is rather smaller than one would suspect with bedded iron oreswat least in most cases." This view tends to be confirmed by the Beckermonds oblique boring [SD 8636 8016] drilled by I.G.S. into the magnetic high crossing the Askrigg Block which proved magnetite-bearing metamorphic rocks similar to the magnetite-bearing Dolwen Grits of the Harlech Dome L. Cambrian. (pers. comm. from Mr B. J. Taylor, I.G.S.).
26
LEONARD JOHNSTON WILLS
Bott (1967) also pointed out that a ridge of high magnetic anomalies runs southeastwards from Furness to The Wash: it can now be extended to north Norfolk where it has been proved by two borings (North Creake and Hunstanton) to be Precambrian (Charnian ?). This is referred to subsequently and on P1. 2 as Magnetic Ridge No. 1. Guided by other 'magnetic highs' and 'magnetic lows' one can postulate from the I.G.S. Aeromagnetic map a number of similar Precambrian 'ridges' and 'troughs', some of which are defined by 'basement faults', shown up by subparallel, rather straight, magnetic contours (one good example runs from near Buxton to near Derby). Others clearly define the area of the Mercian Mountains of Lower Palaeozoic age (see P1. 1 and P1. 2). A negative gravity anomaly covering the central part of the Askrigg Block is interpreted as caused by the Wensleydale Caledonian granite which penetrates the northern flank of the magnetic basement rocks. The granite is similar in age to that of the Rookhope Granite in the Alston Block (Dunham et al. 1965) and both granites were proved by boring to have been uncovered before Carboniferous times (P1.1). Bott (1974) has shown that there is a large granite batholith under the northern part of the Lake District with shallow narrow ridges connecting the small present-day outcrops of granite indicated on P1. 1, and by a deep ridge joining it to the Weardale Granite of the Alston Block. This great composite batholith, together with the granites of the Isle of Man, are seen on P1. 1 to occur within the same major Caledonian-striking anticlinorium of Cambrian and Ordovician. The importance of these recent investigations in the north of England lies in the fact that they are the only systematic attempt to discover the ages, composition and structural relationships of the pre-Carboniferous rocks and of the Caledonian granites where these are hidden in England and Wales, and for this reason must be taken as our guide wherever major gravity and, or, magnetic anomalies occur. It is relevant that Bott has also (1976) suggested that blocks tend to stand high as a result of granitic intrusions at later dates being the ultimate cause of blocks remaining high. Kent (1974b) has estimated that the depth to the magnetic basement may be around 7000 m near Leeds and around 9000 m near the southeast part of the Yorkshire coast*, "implying a thickness of perhaps 3-5000 metres of rocks between the Carboniferous and the basement in the east"; the basement surface being possibly the surface of the Precambrian.
4. T H E T O P O G R A P H Y OF PRE-PHANEROZOIC BASEMENT In the microcraton two great Lower Palaeozoic transgressions can be recognised. The earlier one during Cambrian times was across the pre-Phanerozoic Basement Floor which everywhere consisted of Precambrian rocks. The second one began in U. Ordovician times and, after slight oscillations continued throughout the Silurian into the Lower Devonian. This is referred to as the "Silurian" or the "Llandovery" or the "Upper Llandovery-Wenlock" transgression which took place across a floor cut by pre-U. Silurian and earlier planations of rocks ranging from Precambrian to L. Silurian. The pre-Phanerozoic Basement to the southwest of Magnetic Ridge No. 1 seems to have been cut by basement faults of which a few can be recognised by a close spacing of parallel magnetic contours on the I.G.S. Aeromagnetic map, even where more recent movement has occurred along the same line. It appears probable that the pre-Phanerozoic basement may have developed a considerable topographic relief at the beginning of the Cambrian transgression by the production of * Owing to an oversight the crimson lake colour (for Silurian) has been omitted from the area between the 'putative interpretation' and the Yorkshire coast.
26
LEONARD JOHNSTON WILLS
Bott (1967) also pointed out that a ridge of high magnetic anomalies runs southeastwards from Furness to The Wash: it can now be extended to north Norfolk where it has been proved by two borings (North Creake and Hunstanton) to be Precambrian (Charnian ?). This is referred to subsequently and on P1. 2 as Magnetic Ridge No. 1. Guided by other 'magnetic highs' and 'magnetic lows' one can postulate from the I.G.S. Aeromagnetic map a number of similar Precambrian 'ridges' and 'troughs', some of which are defined by 'basement faults', shown up by subparallel, rather straight, magnetic contours (one good example runs from near Buxton to near Derby). Others clearly define the area of the Mercian Mountains of Lower Palaeozoic age (see P1. 1 and P1. 2). A negative gravity anomaly covering the central part of the Askrigg Block is interpreted as caused by the Wensleydale Caledonian granite which penetrates the northern flank of the magnetic basement rocks. The granite is similar in age to that of the Rookhope Granite in the Alston Block (Dunham et al. 1965) and both granites were proved by boring to have been uncovered before Carboniferous times (P1.1). Bott (1974) has shown that there is a large granite batholith under the northern part of the Lake District with shallow narrow ridges connecting the small present-day outcrops of granite indicated on P1. 1, and by a deep ridge joining it to the Weardale Granite of the Alston Block. This great composite batholith, together with the granites of the Isle of Man, are seen on P1. 1 to occur within the same major Caledonian-striking anticlinorium of Cambrian and Ordovician. The importance of these recent investigations in the north of England lies in the fact that they are the only systematic attempt to discover the ages, composition and structural relationships of the pre-Carboniferous rocks and of the Caledonian granites where these are hidden in England and Wales, and for this reason must be taken as our guide wherever major gravity and, or, magnetic anomalies occur. It is relevant that Bott has also (1976) suggested that blocks tend to stand high as a result of granitic intrusions at later dates being the ultimate cause of blocks remaining high. Kent (1974b) has estimated that the depth to the magnetic basement may be around 7000 m near Leeds and around 9000 m near the southeast part of the Yorkshire coast*, "implying a thickness of perhaps 3-5000 metres of rocks between the Carboniferous and the basement in the east"; the basement surface being possibly the surface of the Precambrian.
4. T H E T O P O G R A P H Y OF PRE-PHANEROZOIC BASEMENT In the microcraton two great Lower Palaeozoic transgressions can be recognised. The earlier one during Cambrian times was across the pre-Phanerozoic Basement Floor which everywhere consisted of Precambrian rocks. The second one began in U. Ordovician times and, after slight oscillations continued throughout the Silurian into the Lower Devonian. This is referred to as the "Silurian" or the "Llandovery" or the "Upper Llandovery-Wenlock" transgression which took place across a floor cut by pre-U. Silurian and earlier planations of rocks ranging from Precambrian to L. Silurian. The pre-Phanerozoic Basement to the southwest of Magnetic Ridge No. 1 seems to have been cut by basement faults of which a few can be recognised by a close spacing of parallel magnetic contours on the I.G.S. Aeromagnetic map, even where more recent movement has occurred along the same line. It appears probable that the pre-Phanerozoic basement may have developed a considerable topographic relief at the beginning of the Cambrian transgression by the production of * Owing to an oversight the crimson lake colour (for Silurian) has been omitted from the area between the 'putative interpretation' and the Yorkshire coast.
A P A L A E O G E O L O G I C A L MAP OF THE ANGLO-WELSH M I D - D E V O N I A N F L O O R
27
horsts and graben, here termed 'ridges' and 'troughs'. The Caledonian geology of the ridges has been more frequently proved by borings because of the shallow depth to the basement, whereas that of the troughs is rarely known. Ridge 1, the Furness-Ingleborough-Norfolk ridge has already been described. On P1. 1, only a few small outcrops are shown as Precambrian, the Ingletonian is coloured as Cambrian, while in the southeast (in east Lincs.) putative Cambrian quartzites with steep dips have been proved in the Nocton No. 1, Stixwould and Bardney borings, and these rocks are, according to Kent, (pers. comm.) only mildly tectonised (as also at Eakring). Trough 1, the Leeds-Pontefract-Worksop trough was probably separated from Ridge 1 by a basement fault, perhaps the fore-runner of the present-day South Craven Fault, and its continuation to Pontefract. The trough is shown on P1. 1 as a large embayment of transgressive Silurian overlapping on to a ring of Cambrian which was proved to be Lower Cambrian with a 90~ dip at Nocton No. 1 (connecting eastwards with Bardney and Stixwould (Ridge 1), and westwards with probable Cambrian at Eakring and with Arenig? at Eyam). Two small magnetic highs on the I.G.S. Aeromagnetic map are shown on P1. 1 as Precambrian inliers with respect to the Silurian, but as outliers of the large NE Midlands expanse of Precambrian Charnian? of Ridge 2. See Hiatograph B2, Fig. 6 and Cribb (1975) for radiometric dating. Ridge 2, the Rochdale-Buxton-Nottingham-Leicester-Charnwood-Oxenham-Orton-Bletchley ridge. Like Ridge 1, the Ridge 2 strikes NW-SE, and in several places has been proved to consist of Charnian Precambrian, with Caledonian major plutonic intrusions--Mount Sorrel and Enderby granodiorites in Leicestershire and probably the Warboys diorite in Cambridgeshire. Cope's (1949) view that the sub-Carboniferous rocks at Woo Dale may be Precambrian is disputed by Le Bas (1972) in favour of a L. Carboniferous age. Also dubious are the putative Precambrian subcrops at Bletchley (see section on P1. 2) after Jukes-Brown (1889). Trough 2, the Burton-on-Trent-Leicestershire-East Warwickshire-Buckinghamshire trough is shown on P1. 1 as floored by Cambrian which together with the Ordovician Llanvirn of Huntingdonshire, swings round the SW and SE flanks of the NE Midlands expanse of Precambrian. (P1. 2). The L. and M. Devonian outcrop of the SE Midlands shown on P1. 1 effectively conceals the L. Palaeozoic subcrops, and P1. 2 attempts to draw in putative boundaries that are little more than guesses. Ridge 3, the Walsall-Birmingham-Stratford-Witney-Newbury ridge has a general N-S trend. It is not known how many of the numerous faults defining the coalfields in the northern part of the ridge originated in the Caledonian orogeny, but several basement faults are evidenced by the magnetic and gravity maps. The Batsford-Marlborough Fault and faults of the Malvern and Severn areas are good examples, while the East Malvern Fault can be proved by the behaviour of the unconformable U. Llandovery both to the south and north of the main outcrops of the Malvern Precambrian (Hardie 1969, Bullard 1974). The country to the east of the Batsford-Marlborough Fault was an easterly extension of the Lower Palaeozoic Mercian Mountains that has escaped the down-faulting of the 'Lower Severn Graben' in Triassic times. South of Oxford is the small magnetic high of Abingdon-Stadhampton (43)*: further south is the larger Newbury Complex (44), both defined by very high magnetic anomalies, despite being deeply buried by at least 1000 m of non-magnetic cover as proved in the Sonning Eye B.H., Reading (34). The magnetic contours defining the Newbury complex, below 1000 m of non-magnetic cover compare with those in the Harlech Dome-Arenig region which have no cover at all. The magnetic Figures in brackets refer to ringed sites on P1.2 and listed in Appendix.
28
LEONARD JOHNSTON WILLS
pattern does not match that of the mountain of magnetite (Magnitnaya) in the Devonian-elevated Urals and the author is driven to postulate that the deeply buried source of the high magnetisation must be in a basement consisting of an exceptionally massive local concentration of magnetite possibly in the basaltic greenstone sheet postulated previously. The Newbury Complex, shown by Dunning (1975) and interpreted by him as an igneous complex, extends eastwards in a narrow tongue to Richmond B.H. (15) and Streatham B.H. (14), both proving Dittonian. Its south side appears to mark the edge of the L. Palaeozoic London Platform from Guildford, Shalford B.H. (32), roughly to Cliffe (13) (P1. 2 and Hiatograph B3, Fig. 8), but one must be aware of the many erosions and sedimentations that have affected that region since Mid-Devonian times. Trough 3 is perhaps represented by the western sector of the Mercian Mountains, uncoloured on P1. 1, save for a tiny patch of green denoting L. Ordovician round Ultramar's Netherton B.H. (75), 4~ km, SW of Evesham. This is the region known to petroleum geologists as the 'Lower Severn Permo-Triassic Graben' where the stumps of the old mountains have been down-faulted and covered by 1568 m of Trias at Netherton in the north, and in the south by 1124 m of Jurassic and Triassic at the Minety B.H. (78), 9 km NW of Swindon. The only other deep hole in the graben is at Stowell Park, Northleach (96). It ended in Trias at 1501 m (Green & Melville 1956). As regards the Netherton B.H. (76), surface 175 feet or 53 m O.D., Dr L. V. Illing has kindly supplied the following: "The Trias at Netherton is 1568 m thick. The andesitic tufts below were bored through for 561 m without bottoming them. Their petrography and radiometric age of at least 424 m.y. (determined by the I.G.S.) suggest that they are Ordovician, perhaps contemporary with the Borrowdale Volcanics of the Lake District." The author considers that at Netherton we have evidence of an andesitic ash volcano at least half as high as the present-day Vesuvius (1353 m) as one of the features of the Lower Palaeozoic Mercian Mountains. Dr Illing informed the author that a series of seismic sections from the Inkberrow Fault towards Malvern and the Worcester gravity low suggests the occurrence of Coal Measures between the reflector and the Triassic rocks; but the author considers that transgressive Silurian may equally well be the cause of this. The Minety B.H. (78), Cooles Farm 1: Mr J. M. Bowen and Dr P. Lovelock of Shell U.K. Exploration and Production Ltd. generously supplied the following information: All measurements from derrick floor at 310 ft. OD (94.5m). Mesozoic rocks (Oxford Clay to base of Keuper Sandstone) with underlying unconformity 1448 m (3592 ft) Cambrian including some Tremadoc 2744 m (7485 ft) Total depth
4193 m (289 miles)
Only two samples were recovered. One is a soft black uncleaved and nearly horizontally-bedded shale with tiny microzoans? in a one inch thick and 5 inch dia. core. This may have contained the Tremadoc microfauna. The other is a fragment of dark quartzite said to be from near the base. Dr Lovelock gave them both to the author; and they are now in the Geology Museum, Birmingham University, numbered 1928 and 1929 undergoing further investigation. Neither have been metamorphosed. The nearest borehole to the south (about SW) of Minety is the Lucknam Boring (Water Supply Wiltshire, Mem. G.S. Engl. & Wales, 1925, p. 55) which Professor Dineley in a letter states is probably a boring in the cutting of the Box Tunnel near Bath (O.D. 542 ft (165 m) N.G. 829689). It reached the base of the Rhaetic (the Hercynian unconformity) at 72 ft O.D. (22 m), on limestones with purple marl and ironstone bands which the Memoir ascribed to the Carboniferous, but which B. Hawkins, of Bristol University thinks is a mistaken identification of Silurian (Wenlock?).
A PALAEOGEOLOGICAL MAP OF THE ANGLO-WELSH MID-DEVONIAN FLOOR
29
The author accepts this view. The 'Hercynian unconformity' at 72 ft (22 m) O.D. at Luckham falls to 3282 ft O.D. (1000 m) at Minety some 31 km away to the northeast, giving a gradient of about 1 in 160 for the base of the Triassic rocks. The only other known occurrence of Cambrian with A c r o t r e t a in the south of England is at Fobbing near Canvey Island, in the Thames Estuary, briefly recorded by Baker (1935), who states that Cambrian fossils occur in the bottom 25 ft (7.6 m). It underlies supposed U. Devonian or Carboniferous. A notable feature of both the Netherton and Minety borings is the absence of any Silurian or O. R. S.-Devonian. Transgressive U. Llandovery is known at several places that must have been marginal parts of the L. Palaeozoic Mercian Mountains, namely Tortworth; the Malverns; Rubery (north end of Lickey Hills), in neptunean dykes in a Lickey Quartzite beach cliff; Batsford (48); and Withycombe Farm (47) Banbury, see later. All those occurrences are valuable evidence of the great sub-Llandovery-Wenlock unconformity, well known along the western edge of the microcraton (George 1963, 1970; Ziegler et al. 1968) to extend over much of the rest of it (section on P1. 2), as witnessed by the gently folded Wenlock-Ludlow of Shropshire, Herefordshire, south Staffordshire and Worcestershire (sharp folding as at Dudley and in the Abberley Hills is due to post-Caledonian compression). Shelf-facies of Wenlock at Ware and Cliffe No. 1 appear to fall within the microcraton. On the other hand the Llandovery proved at Shalford near Guildford (32) carried a mixture of shelf and geosynclinal faunas. The O.R.S.-Devonian appears widely both to the east and west of the Mercian Mountain tract where that tract has not been down-faulted in Triassic times. It may be that any O.R.S.-Devonian deposited on the mountain tract has been removed by pre-U. Devonian and, or, pre-Carboniferous and, or, pre-Triassic erosion. See Fig. 1. On the eastern side of the Batsford-Marlborough Fault and 44 km east of Netherton, the Withycombe Farm boring (47) at Banbury revealed that here the Triassic and younger formations are 495 m thick, resting on U. Coal Measures 489 m; which rests on Silurian (?L. Llandovery) 194 m; which lies unconformably upon deeply weathered andesitic rock (almost certainly Ordovician, though too weathered to be dated radiometrically). In Poole's (1974) account, this rock was said to be basaltic, but this has been corrected to andesitic in Dunham & Poole (1974). (Figs. 3m5). The discovery of Ordovician? volcanics and of the inversion of an ancient Lower Palaeozoic mountain tract which became a deep rift valley in Permo-Triassic times almost exactly matches the conditions revealed by a boring by Petrotrend Ltd. (now Trend Cheshire (U.S.) Ltd.) at Prees in the southeast part of the Cheshire Basin and released for publication by that Company. Here, Liassic, Triassic and Permian were 4301 m thick, resting unconformably upon Upper Coal Measures (Westphalian C?), 27 m, and these on 80+ m of L. Palaeozoic rocks, nearly vertical in the cored interval near the bottom of the hole at a total depth of 4572 m. The latter were dated as Tremadocian by acritarchs; while Chitinozoa from 4546 m were Ordovician, possibly Llandeilo or early Caradocian. (Information from the Assistant Director, I.G.S. Leeds by permission of Petrotrend Ltd.) The site lies outside the limits of the microcraton close to the Prees Fault which can be traced into the fault on the northern side of the Breidden Hills, where Upper Llandovery rests unconformably on Caradocian with volcanics some of which are andesitic. Andesitic rocks were found at Milton Green, near Chester (101) that are probably Lower Ordovician. Clearly, the Banbury district lay outside the Mercian Mountains at a point where transgressive Silurian covered a deeply weathered surface of Ordovician that had been exposed for a very long time during the pre-Llandovery planation of the microcraton.
30
LEONARD JOHNSTON WILLS
To the west of the Malvern line and the Malvern-Stroud magnetic high which should be referred to as Precambrian Ridge 4, the Old Red Sandstone of the Anglo-Welsh Basin and the overlying South Wales Coal Field conceal the Lower Palaeozoic outcrops. The Maesteg Borehole at Maesteg proved below Coal Measures, successively Upper and Lower Old Red Sandstone and then typical red marls of the Dittonian lying on Ludlow, Wenlock and Llandovery which was found to overlie Ordovician volcanics. Other possible areas of Ordovician, Cambrian and possibly Precambrian have been suggested between Builth and Malvern by George. The author would agree with this and locate such areas where magnetic highs, such as those at Kidwelly, Llanelli, Pontrilas, are shown on the I.G.S. Aeromagnetic Map from which the magnetic contours on Plate 1 and P1. 2 have been taken. The Ordovician Volcanics of Pembrokeshire do not coincide with conspicuous magnetic anomalies: and it can be argued that the anomalies there are also due to a deep-seated magnetic zone in the Precambrian. See P1. 2. 5. S T R A T I G R A P H Y
Cambrian The large outcrops of Cambrian between the Precambrian ridges and at times even on them (as depicted on P1. 1) and the known great thicknesses of Cambrian and Tremadoc in North Wales and at Minety are consistent with a hypothesis of formation during a period of world-wide rise of ocean level on a stable basement of Precambrian having a subdued topography of wide low ridges and intervening troughs, the two often being delimited by major faults (horst and graben arrangement). This is borne out by the widespread Cambrian outcrop (including much of the Ingletonian) shown on PI. 1, and by the findings of the Minety B.H. The hypothesis of long continued rise of oceanlevel giving persistent shaUow-water sedimentation has recently received support from Rickards (1975) who points out that the sessile Dendroid graptolite Dictyonema which was fixed to objects on the sea-bed at shallow depths, persists throughout great thicknesses of mudstones in the Upper Cambrian and Tremadoc.
Lower Ordovician This system was characterised by frequent earth movements and volcanic eruptions combined with changes in sea-level in the geosynclinal areas and in the Lower Palaeozoic Mercian Mountains and possibly other areas not yet explored. In the rest of the microcraton Lower Ordovician rocks are scarce; Arenig deposits are known only at Eyam and Llanvirn rocks in borings in Huntingdonshire (Great Paxton (18) and near Huntingdon (19)) and offshore in The Wash (27). Uplift and resultant profound erosion of pre-Caradoc and pre-Ashgill age may have been responsible for this absence. This too might account for the great differences of thickness, and even for the absence, of any volcanic deposit in south central England comparable with the Netherton pile of ashes.
Upper Ordovician Profound earth movement, erosion and volcanic activity ushered in the Upper Ordovician Caradocian and Ashgillian of Furness and Ingleborough, the corresponding Bala formation in North Wales, near Shelve and Church Stretton (Fig. 7) along the western edge of the microcraton and along its SE edge at Bobbing (12), as the first phase of the Silurian major transgression which
30
LEONARD JOHNSTON WILLS
To the west of the Malvern line and the Malvern-Stroud magnetic high which should be referred to as Precambrian Ridge 4, the Old Red Sandstone of the Anglo-Welsh Basin and the overlying South Wales Coal Field conceal the Lower Palaeozoic outcrops. The Maesteg Borehole at Maesteg proved below Coal Measures, successively Upper and Lower Old Red Sandstone and then typical red marls of the Dittonian lying on Ludlow, Wenlock and Llandovery which was found to overlie Ordovician volcanics. Other possible areas of Ordovician, Cambrian and possibly Precambrian have been suggested between Builth and Malvern by George. The author would agree with this and locate such areas where magnetic highs, such as those at Kidwelly, Llanelli, Pontrilas, are shown on the I.G.S. Aeromagnetic Map from which the magnetic contours on Plate 1 and P1. 2 have been taken. The Ordovician Volcanics of Pembrokeshire do not coincide with conspicuous magnetic anomalies: and it can be argued that the anomalies there are also due to a deep-seated magnetic zone in the Precambrian. See P1. 2. 5. S T R A T I G R A P H Y
Cambrian The large outcrops of Cambrian between the Precambrian ridges and at times even on them (as depicted on P1. 1) and the known great thicknesses of Cambrian and Tremadoc in North Wales and at Minety are consistent with a hypothesis of formation during a period of world-wide rise of ocean level on a stable basement of Precambrian having a subdued topography of wide low ridges and intervening troughs, the two often being delimited by major faults (horst and graben arrangement). This is borne out by the widespread Cambrian outcrop (including much of the Ingletonian) shown on PI. 1, and by the findings of the Minety B.H. The hypothesis of long continued rise of oceanlevel giving persistent shaUow-water sedimentation has recently received support from Rickards (1975) who points out that the sessile Dendroid graptolite Dictyonema which was fixed to objects on the sea-bed at shallow depths, persists throughout great thicknesses of mudstones in the Upper Cambrian and Tremadoc.
Lower Ordovician This system was characterised by frequent earth movements and volcanic eruptions combined with changes in sea-level in the geosynclinal areas and in the Lower Palaeozoic Mercian Mountains and possibly other areas not yet explored. In the rest of the microcraton Lower Ordovician rocks are scarce; Arenig deposits are known only at Eyam and Llanvirn rocks in borings in Huntingdonshire (Great Paxton (18) and near Huntingdon (19)) and offshore in The Wash (27). Uplift and resultant profound erosion of pre-Caradoc and pre-Ashgill age may have been responsible for this absence. This too might account for the great differences of thickness, and even for the absence, of any volcanic deposit in south central England comparable with the Netherton pile of ashes.
Upper Ordovician Profound earth movement, erosion and volcanic activity ushered in the Upper Ordovician Caradocian and Ashgillian of Furness and Ingleborough, the corresponding Bala formation in North Wales, near Shelve and Church Stretton (Fig. 7) along the western edge of the microcraton and along its SE edge at Bobbing (12), as the first phase of the Silurian major transgression which
APALAEOGEOLOGICALMAPOFTHEANGLO-WELSHMID-DEVONIANFLOOR
31
was brought about by the rise of ocean-level that drowned first the edges and hollows of the microcraton and ultimately the whole region (Fig. 1). Silurian As in the case of the Cambrian, so a rising ocean-level during the Silurian accounts for the very extensive transgressive outcrops shown on Pls. 1 and 2. This Silurian Transgression, more commonly referred to as the Upper Llandovery Transgression (George 1963, 1970, Ziegler et al. 1968, 1969; Zeigler 1970) is recorded on the microcraton by gently flexured shelf-sandstones, mudstones and limestones which pass very abruptly westwards into strongly tectonised basin sediments of the Welsh and Furness geosynclines. On the eastern side of the microcraton Lower Palaeozoic rocks have never been reached in east Yorkshire and Lincolnshire, but from Norfolk to the Thames Estuary there are scattered borings, some new exploratory ones (Chroston 1975, see also P1. 1) and others old searchings for coal or water. Probably the putative Silurian is in the Denbigh Flags facies and the passage from geosynclinal to shelf-development is everywhere hidden by the Devonian and newer formations. Boring has revealed the shelf-facies of the Wenlockian only at Ware (16), many miles from the coastal strip running from near Cromer, through Norfolk,-Suff0ik-afid Essex Where high dipping basin-facies of Silurian, or probable Silurian rocks appear to be part of the Eo-North Sea Geosynclinal rocks of the Caledonian System. Further data are proved by borings in Surrey, Kent and the Boulonnais (Wallace 1967). Within the latter large area only Cliffe No. 1 (13) appears to be Wenlockian in shelffacies; the rest are open sea graptolitic mudstones. A putative edge of the shelf is indicated on P1. 2 which also records the sites of some of the boreholes in the eastern Weald area (apart from the Kent Coalfield). These scattered few holes have been sited on anticlines in the Palaeozoic floor, and ended in Devonian or more commonly L. Carboniferous. Had borings been sited in the intervening synclines, coals like those in the Kent Coalfield might have been found. Since the O.R.S.-Devonian effectively conceals the subcrops of the L. Palaeozoic formations in South Wales and in SE England, P1. 2 has been constructed to show them as it is thought they would appear, were the O.R.S.-Devonian stripped off. This map which is largely conjectural, reveals the hitherto unrecognised importance of the Precambrian, Cambrian and Ordovician, and the transgressive relationship of the Silurian to those older formations in the Southern Midlands. Tectonisation and concertina folding (dating from about 415 m.y.) crumpled and uplifted the two geosynclinal regions into subaerial mountain tracts (additions to the Caledonian ranges of the Dalradian Highlands and the Southern Uplands). The erosional debris from these tracts was carried by rivers towards the Silurian Armorican sea to build the Downtonian-Dittonian red-bed development of the L. O.R.S.-Devonian of three areas. (a) the Anglesey/North Irish Sea intermont basin inferred from a few marginal fragments in NE Anglesey; (b) the Anglo-Welsh coastal plain and delta area that merged into the Devonian Armorican Sea in Devon and Cornwall, and (c) a large SE England gulf that possibly joined a North Sea Middle Devonian marine basin discovered in the Auk Oilfield, east of Aberdeen (described in Woodland 1975 and Kent 1975b) to an eastern extension of the Cornubian Armorican Sea of Devonian age, the existence of which was proved by the discovery of the marine L. Devonian of Little Missenden (35) and of marine Middle Devonian in the Brightling boring in Sussex (P1. 1) and by Middle and Upper Devonian in borings at Tatsfield near Warlingham, Surrey (Mortimer & Chaloner 1972), in the Ferques Inlier in North France (Wallace 1968), and by the presence of Mid-Devonian coral-reef limestones at Torquay.
REFERENCES BAKER, H. A. 1935. An undiscovered coalfield; probability of its existence beneath North Kent, the Thames estuary and SE Essex. Iron & Coal Trades Review, 12 pp. BATH, A. H. 1974. New isotopic age data on rocks from the Long Mynd, Shropshire. J. Geol. Soc. Lond. 130, 567-574. BOGDANOFF, A. A. MOURATOV, M. V. & SCHATSKY, N. S. (Eds.) 1964. Tectonics of Europe, Explanatory note to the International Technic Map of Europe. Intern. Geol. Congress. 1-260. Bo'rr, M. H. P. 1967. Geophysical Investigations of the northern Pennine Basement Rocks. Proc. Yorks. geol. Soc. 36, 139-168. 1974. The geological interpretation of the gravity survey of the English Lake District and the Vale of Eden. J. geol. Soc. Lond. 130, 309-331. BRENNAND, T. P. & VAN VEEN, F. R. 1975. The Auk Oil-field. In Woodland, A.W. (Ed.) Petroleum and the Continental Shelf of North- West Europe. Volume 1 - - Geology. Appl. Sci. Publ., London. BUCHAN, S. 1938. The water supply of the County of London. Mere. Geol. Surv. U.K. BULLARD, D. W. 1974. Western boundary of the Malvernian, North Malvern Hills, Worcestershire. Mercian Geologist 5, 65-70. BULMAN, O. M. B. and RUSTON, A. W. A. 1973. Tremadoc faunas from Boreholes in Central England. Bull. Geol. Surv. Gt. Britain No. 43, 1-40. British Regional Geology: Northern England 4th Ed. 1971; The Pennines and adjacent areas, 3rd Ed. 1954; North Wales 3rd Ed. 1961; Welsh Borderland 3rd Ed. 1971; Central England 3rd Ed. 1969; South Wales 3rd Ed. 1970; Bristol & Gloucester 1st Ed. 1948; Thames Valley 3rd Ed. 1960; The Weald 4th Ed. 1965. H.M.S.O. CALVER, M. A. 1969. Westphalian o f Britain. C. R. 6e Congres intern. Strat. Geol. Carbonif. Sheffield 1967. 1, 233-254. CHROSTON, N. 1975. The Sub-Mesozoic Floor in Norfolk. Proc. geol. Soc. Norfolk 27, 3-19. COCKS, L. R. M. HOLLAND, C. H., RICKARDS, R. B., STRACHAN, I. 1971. Geol. Soc. London Special Report No. 1. J. geol. Soc. Lond. 127, 103-136. COLTER, V. S. & BARR, K. W. 1975. Recent developments in the Geology of the Irish Sea and Cheshire Basins. In Woodland A.W. (Ed.) Petroleum and the Continental Shelf of North-West Europe Volume 1 - - Geology. App. Sci. Publ., London. COPE, F. W. 1949. Report on a boring at Woo Dale, Buxton. Abs Proc. Geol. Soc. Lond. No. 1446, 24. CRIBB, S. J. 1975. Rubidium-Strontium ages and Strontium ratios from the igneous rocks of Leicestershire. J. geol. Soc. Lond. 131, 202-212. DAVIES, A. M. & PRINGLE, J. 1913. On two deep borings at Calvert Station (N. Bucks.); and on the Palaeozoic Floor north of the Thames. Q. J. geol. Soc. Lond. 69, 308-342. DUNHAM, K. C. et al. 1965. Granite beneath Visean sediments with mineralization at Rookhope, northern Penines. Q. J. geol. Soc. Lond. 121,383-417. - - - - . 1974. Granite beneath the Pennines in North Yorkshire. Proc. Yorks. Geol. Soc. 40, 191-194. - - - - . & Poole, E. G. 1974. The Oxfordshire Coalfield. J. geol. Soc. Lond. 130, 387.394. DUNNING, F. 1975. In Harris, A.L. et. al. A correlation of Precambrian rocks in the British Isles. Geol. Soc. Lond. Special Report No. 6, 11-14. FALCON, N. L. & TARRANT, L. H. 1951. Gravitational and magnetic exploration of parts of the Mesozoic-covered areas of Central England. Q. J. geol. Soc. Lond. 106, 141-170. GEORGE, T. N. 1961. British Regional Geology, North Wales 3rd Edition, I . G . S . H . M . S . O . - - - - . 1962. Tectonics and palaeogeography in Southern England, Sci. Progress 50, 192-217. - - - - . 1963. Tectonics and palaeogeography in Northern England, Sci. Progress 51 32-50. 1963. Palaeozoic growth of the British Caledonides, In. JOHNSON: M. R. W. and STEWART, F. H. (Ed.) The Britbh Caledonides Oliver and Boyd, Edinburgh. - - - - , 1970. British Regional Geology, South Wales 3rd Ed. I . G . S . H . M . S . O . GREEN, G. W. & R. V. MELVILLE, 1956. Stratigraphy of the Stobwell Park Borehole, 1949-51. Bull. Geol. Surv. Great Britain, No. 11, pages 1-66. HANCOCK, J. M. 1976. The petrology of the Chalk. Proc. Geol. Ass. 86, 499-535. HARDIE, W. G. 1969. A guide to the geology of the Malvern Hills and adjacent areas. Worcestershire Education Committee, 54pp. HARLAND, W. B., SMITH, A. G. & WILCOX, B. 1964. The Phanerozoic Time-scale. Q. J. geol. Soc. Lond. 120S, 458 pp. - - - - . & FRANCIS. E. H. 1971. The Phanerozoic Time-scale: a supplement. Spec. Pub. No. 5, Geol. Soc. Lond. 43, 356 pp. HENDRIKS. E. M. L. 1937. The rock succession and structure in South Cornwall. Quart. J. geol. Soc. Lond. 93, 332-367. INESON, P. R. & MITCHELL, J. G. 1974. K-Ar isotopic age determinations from some Lake District mineral localities. Geol. Mag. 111, 521-37. JONES, O. T. 1927. The foundations of the Pennines, J. Manchester Geol. Ass. 1, 4-14.
A PALAEOGEOLOGICAL MAP OF
THE ANGLO-WELSHMID-DEVONIANFLOOR
33
JUKES-BROWNE, A. J. 1889. The occurrence of granite in a boring at Bletchley. Geol. Mag. 6, 356-361. KAY, M. 1951. North American geosynclines. Mem. geol. Soc. Am. 48, 1-143. KENT, P. E. 1967a. Outline geology of the southern North Sea basin. Proc. Yorks. Geol. Soc. 36, 1-22. 1967b. Contour map of the sub-Carboniferous Surface in the north east Midlands. Symposium on Carboniferous basement rocks of N.E. England. Proc. Yorks. Geol. Soc. 36, 127-133. 1968. The buried floor of Eastern England. In Sylvester-Bradley, P. and Ford, T. D. (Ed.) The geology of the East Midlands. University Press, Leicester. 138-148. 1975a. The Tectonic development of Great Britain and the surrounding Seas. In Woodland, A. W. (Ed.)Petroleum and the Continental Shelf of North-West Europe. Volume 1 - - Geology. Appl. Sci. Publ., London. 1975b. Review of North Sea Basin development. J. geol. Soc. Lond. 131, 435-468. 1974. Structural history. In Rayner, D. H. and Hemingway, J. E. (Ed.) The geology and mineral resources of Yorkshire. Yorkshire Geol. Soc. Leeds. KING, R. E. 1973. Petroleum Extraction and Production in Europe in 1972. Bull. Am. Ass. Petr. Geol. 57, 1961. 1974. Petroleum Extraction and Production in Europe in 1973. Bull. Am. Ass. Petr. Geol. 58, 2024. LAMONT, A. 1940. Derived Upper Llandovery Fossils in Bunter Pebbles from Cheadle, North Staffordshire. Cement, Lime and Gravel 15, 26-30. LE BAS, M. J. 1972. Caledonian igneous rocks beneath central and eastern England. Proc. Yorks. Geol. Soc. 39, 71-84. LISTER, T. R. 1970. (Palynological data on E. Anglia etc., London Platform and Kent) LG.S. Annual Report for 1969, H.M.S.O.p. 93. MCKERROW, W. S. 1972. Silurian palaeogeographic development of the Proto-Atlantic Ocean. 24th Int. Geol. Cong. Section 6, 4-10. MITCHELL, J. G. & INESON, P. R. 1974. Discussion of the Age of the Ingletonian J. geol. Soc. Lurid. 130, 285-287. MORTIMER, M. G. & CHALONER, W. G. 1972. The palynology of the concealed Devonian rocks of southern England. Bull. Geol. Surv. Gt. Br., No. 39, 1-56. O'NIONS, R. K. et al. 1973. New isotopic and stratigraphic evidence on the age of the Ingletonian: probable Cambrian. J. geol. Soc. Lurid. 129, 445-452. 1974. Discussion of the age of the Ingletonian J. geol. Soc. Lond. 130, 285-289. PENNINGTON, J. J. 1975. The geology of the Argyll Field. In Woodland, A.W. (Ed.) Petroleum and the Continental Shelf of North-West Europe, Volume 1 -- Geology. Appl. Sci. Publ., London, 285. POOLE, E. G. 1974. The Withycombe Farm Borehole, Banbury 201 sheet in I.G.S. Report No. 74/7, H.M.S.O.p. 6. RAMSBOTTOM, W. H. C. 1976. Major cycles of transgression and regression in the Namurian. Proc. Yorks. Geol. Soc. 41,261-292 1970. Carboniferous faunas and palaeogeography of the South West England Region. Proc. Ussher Soc. 2, 144-157. RASTALL, R. H. 1906. The Ingletonian Series: West Yorkshire. Proc. Yorks. Geol. Soc. 16, 87-100. RICKARDS, R. B. 1975. 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LEONARDJOHNSTON WILLS
34
WHITAKER, W. & JUKES-BROWN, A. 1894. On deep borings north of London. Q. J. geol. Soc. Lond. 50, 488. WILLIAMSON, I. A. 1968. 'The Landscape' - - Geology, In Natural History of the Barnsley Area. BarnsIey Corpn. WILLS, L. J. 1973. A palaeogeological map of the Palaeozoic Floor beneath the Permian and Mesozoic Formations in England and Wales. Mere. geol. Soc. Lond. No. 7 . 2 3 pp. WOODLAND, A. W. (Ed.) 1971. The Llanbedr (Mochras Farm) Borehold Rep. Int. Geol. Sci. 71/18, H.M.S.O. 115 pp. ZIEGLER, A. M. 1970. Geosynclinal development of the British Isles during the Silurian period. J. Geol. 78, 445-479. - - - - . COCKS, L. R. M., & MCKERROW, W. S. 1968. The Llandovery transgression of the Welsh Borderland. Palaeontology l l , 736-782. - - - - . et al. 1969. Correlation and environmental setting of the Skomer Volcanic Group, Pembrokeshire. Proc. Geol. Assoc. 80, 409-439. ZIEGLER, W. H. 1975. Outline of the geological history or the North Sea In Woodland, A.W. (Ed.) Petroleum and the Continental Shelf of North-West Europe. Volume 1 - - Geology. App1. Sci. Publ., London. 163-187. ZIEGLER, P. A. 1975. North Sea Basin History in the framework of North-Western Europe. In Woodland, A. W. (Ed.) Petroleum and the Continental Shelf of North-West Europe. Volume 1 --Geology. Appl. Sc. Publ., London. 131-148.
7.
AP PEND IX
EXPLANATION OF PLATE2
Numerical list of borings and localities with stratigraphy, established or putative. The numbers are ringed on the map. No. Locality and Boreholes No. Locality and Boreholes 1
Le Waast graptolites
b~
2
Ferques graptolites
b'
3
Brabourne Acritarchs
bs
4
Chilham graptolites
b5
5
Herne and Reculver
Sil?
6
Weeley
b 5?
7
Stutton
b6?
8
Lowestoft
U. Sil.
9
Somerton
Sil?
10
East Ruston
SiI?
11
Saxthorpe
Sil?
12
Bobbing graptolites and brachiopods
b3
1S
Richmond pi'oved Dittonian on
16
Ware Shelf-facies
b6
16A Culford
12A Crossness, Lithology
c'
12B Battersea Lithology
c1
13
b6
Cliffe No. 1 Shelf-facies
13A Fobbing See H. A. Baker 1935 a (a?) 14
Streatham proved Dittonian on
14A Canvey I. Lithology
Sil. by inference
Sil. by inference
Ord?
17
Wyboston
a3
18
Great Paxton
b2
19
Huntingdon
b2
20
Upwood
P-C
21
Gas Council
P-C
22
Warboys
23
West Wittering
P-C
24
Glinton
P-C
25
Hunstanton
P-C
26
North Creake
P-C
27
Burmah 47, 29, 2/1 off Skegness
b2
28
Bardney
a1
29
Stixwould 8000 ft est. non-magnetic
a1
30
Nocton No. 1
a~
31
Cranbourrle to 6735'
Caledonian? Diorite
(Dorset)
non-magnetic U / M Dev
A PALAEOGEOLOGICAL
MAP OF THE ANGLO-WELSH
MID-DEVONIAN
35
FLOOR
No. Locality and Boreholes
No. Locality and Boreholes b $
52
Calvert non-mag, cover 443
a a
Kingsclere non-magnetic to 5060'
Mesoz.
53
Westcote non-mag, cover 537
a 3
Sonning Eye Reading non-magnetic to 2862'
54
Apley Barn, Witney non-mag, to 4941'
C3
Mesoz.
32
Shalford
33 34
UCM C3
35
Little Missenden marine
36
Beacon Hill, East Mendips, (Moons Q.) Andes.-Basalt
bS?
Hamswell
cl?
C1
55
Nuneaton, Hartshill, Moon's
56
Nuneaton, east of, b s postulated by
b5
Shotton 1927 on a ~
a 1
The Lickey; Rubery and Barnt Green
b $
C1
57
Q.
P-C
a 1
P-C 37
58
Merevale, near Atherstone
C3 a 2 3
37A, Lucknam
?Silurian or Carb L.
59
Walsall BH and surface
60
Sandwell Park Coll., Warley
a~
61
Barr Common (N.C.B.)
Sil.
62
Middleton (N.C.B.)
bs 7
a~
38
Tortworth
b 5
39
Usk Inlier
b6-7
40
Rumney (near Cardiff)
bS-7
41
Senghenydd (Pontypridd)
bS-7
42
Maesteg
b 5 7
e3
a
43
44
Stadhampton magnetic H.300 near Abingdon and Oxford Newbury magnetic H.300 (Newbury Complex)
45
Deanshanger
46
Bletchley Rly Sta. (Pebs. of Charnian in Kellaways) H. 1950 ~
47
63
:Source of area of P-C, a 1 and b s pebs. in Clent Breccia
bs P-C?
Ord. Andes.
64
Coombe Abbey (Coventry)
a3
P-C?
65
Desford
a3
66
Spinney (Leicester)
a3
67
Countesthorpe
68
Willesden non -magnetic Tert. and Meso. 1010" c 3 proved to 2400'; Dev. to T.D. 2680'
All c3?
69
Eyam
bl ?
70
Woo Dale
71
Eakring (Dukes W.)
72
Nocton 1
73
Foston Charnian
74
Mount Sorrel
75
Charnwood, Charnian
P-C? aa
a3
Withycombe Farm, Banbury. Mesozoics non-magn & U. Coal Measures 4741' bS? Ord. Andes. b3
48
Batsford non-magnetic
49
Shellingford (Faringdon) Silurian inferred below c ~
Sil?
50
Bracknell magnetic High 190
?P-C
51
Marlow magnetic High 290
?P-C
Diorite
P-C? a~? 60 ~ a 190 ~ P-C Caledonian granite P-C
36
LEONARD
No. Locality and Boreholes 76
77
WILLS
No. Locality and Boreholes
Netherton Trias
5800 ft.
L. Ord. Andesitic ash
1835+ft.
Steeple Aston, non-mag. Bunter+ca including ? mag. dol.
JOHNSTON
3199 ft. less dolerite 569+ ft.
78
Minety (Swindon)
79
Tattenhoe (Bletchley)
a 3
80
Meriden
a 3
81
Ryton
a3
82
Caerphilly mag. High, a small outcrop of?
95
The Wrekin and Lilleshall
b $
a
P-C 96
Stowell Park, North Leach
Trias 1651+m
97
Arenig Fawr
b I volc.
98
Snowdon N.B. No Llandeilo
Caradoc
See text
Llanvirn 99
83
84
85
LlanelliMagn. H.
Wiston Magn. H. over unrelated outcrops
b 5
b~A Ord? Ord? Camb? P-C
Kidwelly-TenbyMagn. H.
Conway
Both Ord. Camb.
100
Prees
bS? a3
101
Milton Green
bl?
102
Malvern, North and South
b 5-7 a3 a t
P-C
b 5
b3
103
Church Stretton
b 5
b3 a~ &
86
Brecon Beacon
87
Black Mountains
!
Camb? continues into and
P-C 104
Cyrn-y-Brain and Llanellidan inliers
surrounds
North of Pontrilas small outcrops of
Putative Cambrian?
105
Meifod
P-C 89
Mynydd Eppynt
90
Beacon Hill (Radnor)
91
Shelve
92
Hanwood Colliery
93 94
Syncline in ,:1
b 5
b3A 106
Batsford
107
St Davids& Ramsey Island
?P-C bl-3
O.b 7
U.b s-~ L.b s b3A
P-C? 88
a3
b5 a
P-C 108
Edgmond
P-C
a~
109
Orton
P-C
N.C.B Alveley
b7
110
Oxenham
P-C
Smestow
Sil
111
Sproxton
P-C
