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

A P A L A E O G E O L O G I C A L MAP of the P A L A E O Z O I C F L O O R below the PERMIAN and M E S O Z O I C F O R M A T I O N S in E N G L A N D AND WALES with inferred and speculative reconstructions of the Palaeozoic outcrops in adjacent areas as in Permo-Triassic times L. J. WILLS

" I n the foreground you see the smooth sheets of the Old Boy's bed, here and there

rumpled where he has stirred in his sleep; and there (pointing to the Malverns), that's where he's put his toe through the bedclothes". Charles Lapworth (looking south and west from the Lickey Beacon Hill).

MEMOIR NUMBER

PETROLEUM

7

A JOINT PUBLICATION WITH EXPLORATION SOCIETY OF

GEOLOGICAL LONDON I973

THE GREAT

SOCIETY

BRITAIN

Published for The Geological Society London

by

Scottish Academic Press Ltd. 25 Perth Street, Edinburgh EH3 5DW

First published 1973

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form, or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the Geological Society, Burlington House, LondonW1N 0JU

O The Geological Society, London, 1973

SBN

7073 0048 7

Printed in Northern Ireland at The Universities Press, Belfast

CONTENTS PREFACE

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I.

INTRODUCTION

II.

THE

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III. REGIONAL PALAEOGEOLOGY AND ITS I N T E R P R E T A T I O N (A) The Northern Province . (B) The Median Province . . . . . (c) The Southern or Armorican Province

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10

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11 16 22

IV.

R E P R E S E N T A T I O N OF THE L O W E R P E R M I A N

REFERENCES

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23

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FIGURE 1. PHANEROZOIC TIME SCALE (P.T.S.) SECTION N W - S E . .

12 and 13

FIGURE 2. PHANEROZOIC TIME SCALE (P.T.S:) SECTION S W - N E . . .

14 and

A

15

SEPARATE~PALAEOGEOLOGICAL M A p OF T H E PALAEOZOIC F L O O R BELOW THE P E R M I A N AND MESOZOIC F O R M A T I O N S I1# E N G L A N D AND W A L E S .

CORRIGENDA

TO

THE

MAP

The Map bears the date of completion, 1971 below the Title and the date of printing, 1973 on the southeast margin. Owing to an oversight, the whole outcrop of the Lizard Complex has been coloured in error as Precambrian. National Grid 3827, outcrop eoloured L. P A L should be ORS/D and the lettering UCM/WCM/ORS refers to Baggeridge Colliery (not shown). National Grid 3929, outcrop coloured ORS should be L. PAL. National Grid 409308, for Witlington read Whittington. National Grid 411339, for Chertley read Chartley. SE Corner, 15 lines from bottom, for Oxen read Oxfordshire.

PREFACE The map with the accompanying short memoir represents a notable addition to British geological literature. Professor Wills has compiled from many sources and with many consultants a map showing the surface of England and Wales as it existed in early Permian times, itself a remarkable achievement. In doing this he has developed to a novel degree concepts of the history of erosion surfaces and of the sub-Permian/Mesozoic massifs which are of major importance in considering the structural history of the region. He has suggested that in the English and Welsh highland areas Permian and later erosion modified only slightly much older surfaces, quoting relics of pre-existing peneplains extending back into the Palaeozoic times which were only a short distance below the postulated sub-Permian surface. Thus he lists pre-Upper Coal Measures in North Wales, pre-Lower Carboniferous in North Wales and the Lake District, pre-Late Devonian from Shropshire across into southeastern England and pre-Llandovery in the Welsh borderland. This is a major extension of previous concepts (for example the view that Wales represented either a dissected Jurassic or late Tertiary surface), but a close analogy is available in east-central England. There (as Wills has shown) the Mesozoic sequence becomes progressively more defective southeastwards on to the London platform, whilst the unconformities at the bases of the individual transgressive units converge at very low angles towards the Thames Valley. Finally there is no distinction between surfaces developed mainly during the Jurassic and the sub-Albian surface. This Mesozoic history had its earlier precursor, for the sub-Carboniferous surface, where preserved, lies so close to the main sub-Mesozoic surface as to be not significantly different on a regional scale, and on present evidence the sub-Old Red Sandstone surface, where known on the block, also lies within very few hundreds of feet below the subMesozoic plain. Feather edges of other formations--in particular the Coal Measures--transgressed on to the platform further west and north. Thus by the accident of escaping regional uplift and denudation, the London-Brabant massif in southeastern England provides a close homologue to the concept of the uplifted Welsh area, with its buried multiple nearly-coincident peneplained surfaces extending over a very long time span. One may extend Professor Wills' concept by making a secondary deduction relevant to the history of the Welsh mountain mass. The Mochras boring, on the north coast of Cardigan Bay, proved 1974 feet of Tertiary upon some 2000 feet of Lias in a fine grained facies, faulted down against the Palaeozoic rocks of Harlech. This has led to deductions that the adjoining highland area was submerged by the Lias sea and was the former site of thick Mesozoic rocks, contrary to early concepts such as those of Arkell. By analogy with the London platform it is however possible to envisage a broad massif, maintained close to sea level throughout the Mesozoic, with the various formations thickening off its margins, in some cases probably increasing sharply across submerged intra-formational faults. According to this concept there need never have been more than feather edges of any marine Mesozoic formations across the Welsh Highland area, and these would have been quickly obliterated by erosion following post-Cretaceous uplift. The compilation of this map and memoir by a Professor Emeritus in his 89th year is a unique achievement. In 1966 the Petroleum Exploration Society recognised Professor Wills' prowess as a palaeogeographer by making him an Honorary Member and in addition that Society has carried most of the cost of this map. With them the Geological Society is happy to have the opportunity of organising its publication. P. E. K ~ T Fellow of the Geological Society Past Chairman, Petroleum Exploration Society

PART

I

INTRODUCTION

The Map 1 sets out to show by colours the outcrops of pre-Permian formations on the Palaeozoic Floor (hereafter called 'the Floor') as they were in Permo-Triassic times, with the lower Cover consisting of L. Permian breccias, sandstones and volcanics, shown by means of symbols overprinted in black. The formations are grouped as follows :-Precambrian, undifferentiated (P-C); the Lower Palaeozoic (L. PAL); the Devonian/Old Red Sandstone (ORS/D and DEV); the Lower Carboniferous (Dinantian) (LC or CL); the Namurian (~A and NA CULM); the Lower Westphalian (LWCM and WCM); the Upper Westphalian, the Stephanian plus the lowest Permian, grouped together as UCM. The upper Cover consisting of U. Permian (Zechstein) and later formations is not shown at all, being assumed removed to display the Floor. The scale of the map is the same as that of the Geological Survey's 10 miles to 1 inch Geological Map. The area coincides with that of a map entitled A Structure contour map of the surface of the buried prePermian rocks of England and Wales by Kent (1949) and an unpublished amended version, which was circulated privately in 1964. In this text, as on the map, the letters shown above in brackets are used instead of the full geological terms. The reader is asked to study the Index for a fuller explanation, and to note that there an attempt has been made to assign a time-interval to each of the groups of formations distinguished by a colour, following The Phanerozoic time-scale (Harland et al. 1964, 260-62). The faults indicated on the map are mostly known post-Triassic ones which are thought to have been active in Permo-Triassic times, though the direction of down-throw may have been in an opposite direction to that shown on the map, which is the downthrow side today (see Wills 1948, p. 59). The Map is accompanied by two Phanerozoic Time-Scale or P.T.S. sections Figs. 1 and 2, which have been 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 (Tables I, II), and helping to distinguish between the original and the present limits of the formations. With the exception of the two most recent ones, the complicated series of post-L. Permian erosional phases listed in Table I and shown in the London Platform on Fig. 1, each modified to some extent the surface of the Floor that had been produced during the Palaeozoic/Mesozoic interval. This involves full recognition of the principle that the sediments, whether transgressive or localized, that were laid down on one erosion-surface were liable to be quickly removed by the next phase of planation, particularly from areas of positive tendency. In this way parts of erosion-surface No. 1 became incorporated in erosionsurface No. 2--and so on, making it difficult to distinguish an area that perhaps never received certain formations because it remained a land-area during a marine submergence, from another area where the same formations were deposited and later removed. The Midland Barrier in Carboniferous times is an I The Map was completed in December 1971 and demonstrated to the Geological Society by Dr. P. E. Kent on 6th January 1972.

8

L.J.

W-ILLS

example of the first. Yet the Map can show only the final result; for it is this that is revealed by the borings and by surface exposures along the edges of the Cover. The numerous gaps known to occur in the pre-Permian Palaeozoic sequences where they outcrop in different parts of the country are set out in Table II. They have to be taken into account when considering composition of the Floor below the Cover. They are most numerous and conspicuous in sequences that originated in areas that over long periods tended to behave as positive axes or regions. When traced into negative areas the gaps, as it were, fade out in thick unbroken sequences. T A B L E I : - - I M P O R T A N T GAPS I N THE SUCCESSION CONSTITUTING THE COVER

post -Tertiary post-U. Cretaceous post-L. Cretaceous post-U. Jurassic post-Lias post-Bunter post-L. Permian post-UCM

pre- Quaternary pre-Tertiary pre-U. Cretaceous pre-L. Cretaceous pre-M. Jurassic pre-L. Keuper Sandstone pre-Trias (Bunter Pebble~ Beds and Zechstein)~ pre-L. Permian )

TABLE II:--IMI'ORTANT GAPS IN THE SUCCESSIONOF

PALAEOZOIC OUTCROPPING ON THE F L O O R

post-Ammanian WCM post-l:)inantian inter-Dinantian post-L. ORS]D post-U. Silurian post-U. Ordovician post-Llanvirnian post-U. Cambrian (Tremadoc) inter-Cambrian post-Precambrian

pre-UCM pre-NA

Maj or Major Major Local Local ?Major 1 Universal 2

FORMATIONS Major s Local Local Major/4

pre-U. ORS/D and} pre-Dinantian pre-L. ORS/D pre-Silurian pre-Caradoc pre-Arenig

Major) Major Major Major

pro-L. Cambrian

Local Universal

1 ? = Hardegsen Unconformity of Germany 2 These two gaps merge locally and together constitute the only gap within the Phanerozoic succession that is universally present in England and Wales and Western Europe = the 'Post-Variscan' or 'Post-Hercynian' or Tost-Armorican Unconformity'. a Post-Malvcrnian Unconformity 4 Post-Caledonian Unconformity

The nature of the gap between Cover and Floor is strikingly displayed by Figs. 1 and 2 which cross the Midland Barrier almost at right angles. In Fig. 2 the base of the Cover is seen to be consistently Permo-Triassic, whereas in Fig. 1 it is shown to vary from Permo-Triassie to U. Cretaceous where the Mesozoic sequence is modified by the unconformities listed in Table I. Both figures demonstrate how greatly the age of the Floor varies from place to place and how the gap separating Floor from Cover can be a simple unconformity where the base of the Cover is Permian lying on the Floor of U. Coal Measures (as in Lincolnshire, Fig. 2), but a highly composite one (as in the region of the London Platform and Kent, where U. Cretaceous may rest on Silurian, and Lias on Cambrian or Caradocian).

Reliability of boundaries It must be concluded that at any one spot, each gap represents a stage or stages in the production of the Floor, but only the last stage can be dated (by the basal member of the Cover at that spot). However, the Map is intended to show only those outcrops on the Floor that were exposed in Permo-Triassic times. It follows that :--

A P A L A E O G E O L O G I C A L MAP OF T H E PALAEOZOIC FLOOR IN E N G L A N D AND

WALES

(a) only where the basal element of the Cover is known to be Permian or Triassic can the outcrops on the map approach correctness. (b) elsewhere they are at best approximations, as in East Anglia and the London Platform where the basal member of the Cover varies in age from Triassic to U. Cretaceous, and where some parts of the Floor have been exposed to denudation on at least three occasions since Triassic times. Other outcrops shown a r e (c) inferences from indirect evidence, as in the 20 mile strip east of the Malvern Axis, and in areas adjacent to the present limits of the Cover, as in North England, the Welsh Borderland and Cornubia: (d) conjectures where there a r e no relics of the Cover as in North Wales (except at the Mochras borehole) and as in Herefordshire and South Wales, north of the South Wales Coalfield: (e) speculations where the Cover is so thick t h a t no borehole data are available from the Floor, as in south England between Devon and Sussex.

PART

II

THE

REPRESENTATION PERMIAN

OF

THE

LOWER

~

The difficulties in representing the L. Permian arise from the vast interval of time covered by the Period (L. Permian 40 m.y., U. Permian 15 m.y.), during which earth-movements, continental conditions, and desert denudation and sedimentation predominated. There is a general absence of fossils from all but the oldest L. Permian Stages as developed in the Central Midlands. Elsewhere the rocks ascribed to the L. Permian cannot be more precisely dated t h a n as intermediate between the age of the rocks forming the underlying Floor at t h a t spot and t h a t of the overlying Cover of U. Permian (Zechstein) or Trias or later formation. No reliable correlation from place to place is possible. The continental deposits are mainly breccias (desert scree and wadi-distributed scree), sandstones (often dune-bedded aeolian, but also water-laid ones), and water-laid conglomerates, siltstones and mudstones (alluvial fans and temporary lake deposits). I n synclinal (negative) areas of the Floor, the earliest members of the Permian (the Enville Beds) follow conformably upon Upper Coal Measures. For this reason they are classified by the Geological Survey as Carboniferous or Permo-Carboniferous. B u t vertebrate footprints from the lowest member and one well preserved skull (Dasyceps bucklandi Lloyd) from the upper part, at Kenilworth, are certainly L. Permian in age ;3 yet they are with equal certainty part of the Floor in the sense of this Map. As such they are coloured and labelled UCM on the Map with the symbol for Permian breccia over-printed in black on those outcrops where appropriate. I n anticlinal (positive) areas of the Floor, breccias generally accepted as approximately the same age as the above, b u t possibly somewhat newer, rest with marked unconformity on UCM, ORS/D and L. PAL The Map incorporates certain data from a generalized map of the L. Permian of the British Isles by D. B. Smith (1972}. a yon Huene 1910 redeseribed the Dasyceps skull and assigned it to the Permian family Zatracheidae. A few years ago D. M. S. Watson said in a letter that it is "almost indistinguishable from Zatrachys from the Wichita and Clear Fork of Texas", which is proved Artinskian (L. Permian) by ammonites. One other old find is a maxilla of Oxyodon, a Pelycosaurid which is also a Permian Family.

10

L.J.

WILLS

e.g. the Clent Breccia near the Lickey Axis (not obviously unconformable) and the Haffield-Abberley Breccia near the Malvern-Abberley Axis (violently unconformable). In this category also are the Dawlish Breccias of South Devon and their submarine extension to Start Point and The Lizard, if, as is accepted here, the Ugbrook Formation is of UCM age (Scott Simpson 1959). Other breccias (including the so-called brockrams in Cumberland) lying unconformably on the Carboniferous predate the Zechstein, but cannot be more precisely dated. There are also breccias (e.g. the Moira and Hopwas Breccias that probably are of L. Permian age) that lie unconformably on the Floor, but can only be dated as pre-Triassic. There are also in each of the major Mesozoic Basins great expanses of sandstones, likewise not more precisely datable than post-the Second Malvernian movements and pre-Zeehstein or pre-Bunter Pebble Beds, which latter are taken by the author to be the earliest Triassic Formation in Britain (Wills 1948). On this view, which is in line with decisions by the Permian and Triassic working groups of the Geological Society Stratigraphic Committee, the Lower Mottled Sandstone, Bunter (fl) of the Geological Survey, is U. Permian. The following sandstones are now generally accepted as of that age (dune sands being asterisked) :-(i) Those proved to be pre-Zeehstein are the Penrith Sandstone*, the Yellow Sands of Durham*, the Rotliegend Gas Sands of the North Sea Basin and the Collyhurst Sandstone; (ii) Those proved or inferred to be pre-Triassic include part, if not all, of the so-called Lower Mottled Sandstone of Lancashire, Cheshire, Vale of Clwyd* and of the West Midlands (the Bridgnorth Dunesandstone*) and the Clyst St. Mary Dune-sandstone* of the Exeter area.

PART

III

REGIONAL PALAEOGEOLOGY INTERPRETATION

AND

ITS

Three provinces can be recognized in England and Wales: A. T H E N 0 R T H E R N P R 0 VI NC E, with continuations beneath the North Sea and Irish Sea, extends from the Southern Uplands of Scotland to an irregular east-west line running roughly from The Wash through Derby, Stoke-on-Trent, Ruabon, Wrexham to Liverpool. This line approximates to the southern limits of a region with a virtually complete Carboniferous sequence. The Northern Province was rigidified before U. ORS/D and LC times. B. T H E M E D I A N P R O V I N C E extends from the southern limit of the Northern Province to a line running roughly from Calais through Folkestone, Guildford, Reading, Bristol, Swansea, Tenby and the south side of St. Brides Bay. This mainly hypothetical line (shown on the Map by long dashes and small crosses) defines approximately the southern edges of the positive stable belt of St. George's Land, the Midland Barrier, and the Brabant Massif (in Britain the Wales-Brabant Island). This had been rigidified before U. ORS/D times, to form the Armorican foreland. Against this and locally on to it the sediments of the Armorican shelf, foredeep and possibly even those of the geosyncline were pushed in the Variscan orogeny. C. T H E S O U T H E R N OR A R M O R I C A N P R O V I N C E extends from the line just described to beyond the south coast, and continues southwards under the English Channel to the edge of the map. The Southern Province was not rigidified until UCM times. Since the date of rigidification, each province has undergone only gentle open folding with extensive vertical dislocations by faulting, the latter sometimes associated with narrow areas of intense folding and even thrusting. Details follow.

A P A L A E O G E O L O G I C A L MAP O F T H E

PATJAEOZOIC F L O O R I N

A. T H E N O R T H E R N

ENGLAND AND

WALES

11

PROVINCE

Owing to the limited areas of Permo-Triassic outliers to the west of the present Zechstein outcrop, great difficulty arises in attempting to distinguish the amount of movement and denudation t h a t had taken place in pre-Permo-Triassic times from that which has since occurred. However the Author accepts the view of Kent (1949) that the major tectonic features in this westerly area are due to the pre-Permian movements and erosion along persistent positive and negative belts, whereas broad open flexuring and large scale faulting characterizes the post-Permian tectonics further east. The outcrops depicted on the map in the Pennine region can only be said to be conjectural. This is particularly so in the case of the WCM outcrops over the Alston Block (adapted from Shotton 1956) over the Askrigg Block and the present Millstone Grit country between the Yorkshire and Lancashire Coalfields. On the other hand the composition of the Floor where it lies beneath the Cover has been extensively explored by mining, boreholes and geophysical techniques, and the outcrops shown (adapted from Kent 1966 and from Howitt and Brunstrom 1966) can be regarded as broadly correct. THE CARBONIFEROUS SYSTEM. The Carboniferous Stages have been so intensively studied and shown to vary so much lithologically and tectonically that on the Map only a simplified representation of outcrops, as in Permo-Triassic times, has proved feasible. The Map shows that, with the exception of the L. PAL of the Lake District and Howgill Fells, of the U. ORS of the Scottish Border area, and of supposed U. ORS/LC conglomerates (Mell Fell, Roman Fell), the whole of the Floor was composed of Carboniferous Formations. The U. ORS referred to above passes upwards without any break into L. Carboniferous of Tournaisian age--an arrangement comparable w i t h that found in the Bristol Coalfield and at Titterstone Clee, and possibly with the relationship discovered in the Galley B.H. (Northampton) and in the Cambridge B.H. It is now established that LC, NA and WCM in the Northern Province are each developed as virtually unbroken sequences of varying rhythmic facies and thicknesses, reflecting differential rates of subsidence of a region that for the most part received its sediments from a landmass to the north and north-west. There are however minor breaks, especially in thin developments in positive areas; e.g.S.E, of Lincoln (Lees and Taitt 1946). The fourth group, the UCM, is" apparently conformable and unbroken in areas of maximum subsidence, but towards the Midland Barrier it lies in many places unconformably upon WCM (First Malvernian or Symon Fault unconformity) or even on pre-Carboniferous rocks. This relationship is well known in the Median Province, but has also been discovered below the Cover in the area south-east of Lincoln in the Nocton, Dunston and Stixwold B.H's (Lees and Taitt 1946). On the Map the Lincolnshire outcrops of the UCM are shown ending at the coast line. This device is employed to denote that in the North Sea no subdivision has so far been made of the Coal Measures. THE DEVONIAN SYSTEM. Everywhere in the Northern Province where the base of the U. ORS (in the Border Country) or that of the LC has been seen at the surface or in boreholes, it is unconformable on P-C or L. PAL. Since the youngest L. P A L involved in the Caledonian movements is here uppermost Silurian (Ludlovian), both the mountain building and subsequent peneplanation in the Northern Province can be precisely dated as post-Silurian and pre-U. ORS]D (in the Border Country) and pre-LC elsewhere. It may be concluded therefore that it was the destruction of this particular range of the Caledonides with its extension into North Wales that provided most of the detritus of the L. ORS of the southern half of the Median Province as a sort of delta-fan thrown down by rivers draining into a narrow foredeep of the Armorican geosynclinal sea. The final planation was completed by U. ORS]D times in some places and by L. Carboniferous times in others. The general absence of M. ORS from the whole of England and Wales north of a line running from the

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