Bombarded Britain: A Search for British Impact Structures

BOMBARDED BRITAIN A Search for British Impact Structures

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Richard S t r a t f o r d INSPEC, UK

BOMBARDED BRITAIN A Search for British I m p a c t

-iffi

Structures

Imperial College Press

Published by Imperial College Press 57 Shelton Street Covent Garden London WC2H 9HE Distributed by World Scientific Publishing Co. Pte. Ltd. 5 Toh Tuck Link, Singapore 596224 USA office: Suite 202, 1060 Main Street, River Edge, NJ 07661 UK office: 57 Shelton Street, Covent Garden, London WC2H 9HE

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

Maps reproduced from Ordnance Survey mapping on behalf of The Controller of Her Majesty's Stationery Office © Crown Copyright. Licence Number MC 100038893.

BOMBARDED BRITAIN: A SEARCH FOR BRITISH IMPACT STRUCTURES Copyright © 2004 by Imperial College Press All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the Publisher.

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ISBN 1-86094-356-X

Typeset by Stallion Press

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This book is dedicated to the memory of my beloved wife Sylvia, whose enthusiasm and encouragement were the main-spring of the work

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Contents Foreword Acknowledgements

vlii x

Part I. Impacts and Geology 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

A Curious Omission Of Calculations and Craters The Search for Impact Structures The Shetland Craters Midlands Geology The Ashby Inlier Charnwood Forest The Midlands Basin — A Cometary Impact Structure? The Herefordshire Domes The Rochford Basin — A Digression into Essex Fuller's Earth and Bagshot Sands — A Surrey Crater? Gabbro, Granite, and Grampians Other Circular Structures

3 21 31 39 45 53 60 65 82 96 106 112 124

Part II. Impacts in History 14. 15. 16. 17.

Small Craters, Airbursts, and Tsunami Dozmary Pool and Other Craterlets Levin-Bolt and Blast British Atlantis?

139 145 153 162

Epilogue: The Silverpit Crater

175

Appendix 1 Appendix 2 Bibliography Index

178 193 195 203

Foreword The last 40 years have seen a revolution in planetary science. Unmanned and manned missions to the Moon, studies of impact craters on Mars, Venus and Mercury and on the satellites of the outer planets, and the discovery of a large population of nearEarth asteroids have shown that impact cratering is an important process, and for many bodies the dominant surface process, throughout the solar system. The recognition of large terrestrial impact structures and the realisation that the Cretaceous-Tertiary mass extinction was almost certainly caused by the impact of an asteroid have shown that impact processes are important for the geological and biological history of the Earth. About 200 terrestrial impact structures are now known, and these structures have been discovered on every continent except Antarctica. However, no impact structures have yet been identified in Great Britain or Ireland. I have set out to remedy this omission for Great Britain by searching for circular landforms and reexamining their geology with explicit consideration of the impact hypothesis. This research has sometimes required a re-assessment of British geological history and of the actual formation of impact structures. In particular, atmospheric break-up of asteroids and comets before they hit the ground may radically alter the morphology of the resulting crater. Observations by satellites and from the Earth's surface have shown that large meteoric fireballs often explode in the atmosphere, and that these explosions can cause damage on the Earth's surface. I have analysed the frequency of such events, and suggest that damaging explosions may occur over the British Isles on a time-scale of decades. There are a number of historical records that may describe fireball explosions, some of which have killed people, but many of these have been previously identified as thunderstorms, tornadoes, and even earthquakes.

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Finally, I draw attention to the largely overlooked danger of tsunami created by impacts in the oceans. In particular, if the Carolina Bays of the south-eastern United States were produced by a cometary impact in late glacial times, this impact would have caused a tsunami tens of metres high on the western coasts of Europe and the British Isles. The date of this event corresponds to the date of the destruction of the legendary island of Atlantis.

Acknowledgements My thanks and acknowledgements are due, first to Mr. Nic Howes, who sent me the maps, diagrams and photographs of the Woolhope and Hope Mansell domes that appear as Figure 6 and Figures 8 to 12. Thanks also to Bedfordshire Libraries for providing me with information about the Stevington meteorite fall and for drawing my attention to the Chilterns fireball of 1887. Also to Darlington Library for their assistance in obtaining information about the Hell's Kettles craters, to Gloucester Library for information about the Coleford meteorite fall of 1946, to the Archive Service of the Natural History Museum for information about the Tetbury meteorite fall of 1929, to Sidmouth Library and the Sid Vale Heritage Centre for information about the fireball of 1970, and to Wells Library and the Somerset Studies Library (Taunton) for details of the Wells fireball of 1596. I should like to thank the anonymous referees for comments that improved the organisation and layout of the book. Finally, I cannot sufficiently express my gratitude to my wife Sylvia for her encouragement and for her handling of correspondence and of the business side of getting the book into print.

Impacts and Geology

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CHAPTER

A Curious Omission 'I have not got housemaid's knee. Why I have not got housemaid's knee, I cannot tell you, but the fact remains that I have not got it. Jerome K. Jerome, Three Men in a Boat The curious incident of the dog in the night-time.' The dog did nothing in the night-time' Sir Arthur Conan Doyle, Silver Blaze In t h e nearly 6 0 y e a r s since t h e end of t h e Second World War, there h a s b e e n a revolution in b o t h a s t r o n o m y a n d t h e E a r t h sciences, a s t h e importance of i m p a c t cratering a s a p l a n e t a r y process h a s come to be recognised. In 1945 m a n y scientists believed t h a t the craters of the Moon were of volcanic origin, a n d the few terrestrial meteorite craters t h a t h a d been identified were regarded a s local curiosities rather t h a n a s being geologically significant. There w a s some justification for this attitude: Hey's (1966) catalogue of meteorite craters included 18 craters t h a t were regarded a s authentic (the largest being Deep Bay, Saskatchewan, with a diameter of 12 km), b u t only 12 of these 18 craters h a d been recognised before 1945. These 12 craters were Aouelloul (Mauritania), Boxhole, Dalgaranga a n d H e n b u r y (Australia), C a m p o del Cielo (Argentina), Haviland

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(Kansas), Kaalijarv (Estonia), Meteor Crater (Arizona), Mount Darwin (Tasmania), Odessa (Texas), Tunguska (Siberia), and Wabar (Saudi Arabia). The largest of these craters was Meteor Crater (now re-named Barringer Crater), with a diameter of 1.2km. However, even as early as the end of the 19th century and the beginning of the 20th, a few scientists were willing to accept a role for impact cratering in both astronomy and geology. In particular the American geologist Grove Karl Gilbert (1893), the Estonian astronomer Ernst Opik (1916), and the German meteorologist Alfred Wegener (1920) all advocated an impact hypothesis for the origin of the craters of the Moon, even though their arguments attracted little attention at the time. The first terrestrial impact structure to be identified as such was, of course, Meteor Crater (35°02'N, 111°01'W), where A.E. Foote (1891) discovered large numbers of iron meteorites that were clearly associated with a deep, circular and non-volcanic depression. The impact interpretation for this crater was confirmed beyond dispute by the undaunted efforts of Daniel Moreau Barringer and E.M. Shoemaker (1928-97). However, the first application of the impact hypothesis to a really large terrestrial structure was the suggestion by Werner (1904) that the Ries Kessel 1 (48°53'N, 10°37'E), a circular depression in southern Germany with a diameter of 24 km, was a meteorite crater. In 1910 A. Hogbom compared Lake Mien and Lake Dellen, 2 in Sweden, to Meteor Crater, and suggested that they were also impact craters (von Engelhardt, 1972). In 1921, P. Eskola described supposedly volcanic rocks from Lake Janisjarvi, 3 and pointed out that these rocks were very similar to those of Lake Mien and Lake Dellen, and to those of Lake Lappajarvi (63°10'N, 23°40'E), in Finland. Later, M. MacLaren (1931) suggested that Lake Bosumtwi (6°32'N, 1°24'W), in 1

This structure is now called the Nordlinger Ries, or simply the Ries. The name Ries Kessel means Giant Kettle. ^ h e impact melt rock 'dellenite' was regarded by Tyrrell (1950) as the type for the volcanic rock rhyodacite. This shows how difficult it can be to distinguish impact melts from volcanic rocks. 3 Janisjarvi (61°58'N, 30C55'E) is now in Russian Karelia; however, during the 1920s it was in eastern Finland. The word jarvi is the Finnish for 'lake'.

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Ghana, was an impact crater; and in 1936 F.E. Suess and O. Stutzer separately suggested that Kofels Hollow (47°13'N, 10°58'E), a 4-km wide circular basin in the Otztal of the Austrian Tirol, was a meteorite crater of late Pleistocene or even Holocene age. I should also mention a mysterious person named J. Kalkun [alias J. Kaljuvee), apparently an Estonian, who in a 1933 work entitled 'Die Grossprobleme der Geologic' ['The Main Problems of Geology') suggested that the great Hungarian Plain was a meteorite crater. According to Baldwin (1963), Kalkun compared the Kaalijarv craters in Estonia to Meteor Crater as early as 1922. During the same period the Englishman William Comyns Beaumont (1873-1956) argued that most if not all natural disasters were due to meteorite impacts, and pointed to the Sedgemoor basin in Somerset and the glacial lochs of the Hebrides, the Orkneys and the Shetlands as examples of meteorite craters. During the same period, a number of anomalous terrestrial structures had been found that showed brecciation and faulting without any obvious geological cause. The prototype of these structures was the Steinheim Basin (48°42'N, 10°04'E), near Heidenheim in south Germany; this basin was described by Branca and Fraas (1905) and was called a 'crypto-volcanic structure.' W.H. Bucher (1933) described six similar structures in the United States, namely Serpent Mound (Ohio), Jeptha Knob (Kentucky), Upheaval Dome (Utah), Decaturville (Missouri), Wells Creek (Tennessee), and Kentland (Indiana). Bucher (1936) later added Hicks Dome (Illinois) and Crooked Creek (Missouri) to this list. These crypto-volcanic structures were characterised by a circular central uplift, a few kilometres in diameter, where concealed sedimentary rocks had been tilted and uplifted by a few hundred metres to be exposed at the surface, and had suffered faulting and brecciation. The oldest rocks exposed in the uplift were in the centre, and they were surrounded by successively younger rocks, which dipped steeply outwards. (In some of the cryptovolcanic structures the sedimentary rocks had actually been overturned and therefore dipped inwards; however, these structures could be distinguished from ordinary synclines by the fact

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that older rocks were exposed nearer to the centre.) Radial and concentric faults were also present. In well-exposed structures, the central uplift could be seen to be surrounded by a ring syncline (called s j , which was in turn surrounded by a ring anticline (called aY). In the Wells Creek Basin (36°23'N, 87°40'W), in Tennessee, the ring anticline was encircled by an outer ring syncline (s2), which was itself encircled by an outer ring anticline (02), with a diameter of about 8.5 miles (13.6km). On geological maps these crypto-volcanic structures appeared as circular inliers of older rocks surrounded by concentric circular outcrops of successively younger rocks; this concentric pattern was, however, disturbed, and often disguised, by intense faulting. When they were first discovered, these crypto-volcanic structures were thought to have been produced by explosive outbursts of hot, high-pressure volcanic gases. However, in 1933 Rohleder suggested that the Steinheim Basin, the prototype of these structures, was actually an impact crater. A few years later Boon and Albritton (1938, 1942), in America, suggested that the crypto-volcanic structures of Bucher (1933, 1936) were actually the roots of eroded meteorite craters. Boon and Albritton argued that the rocks under the crater would respond as a fluid to the shock wave produced by the impact and explosion of a giant meteorite, and that the geological structure created by the shock would consist of a central uplift formed by the rebound of the rock, encircled by concentric ring synclines and anticlines. These features were exactly those observed in the 'crypto-volcanic structures.' Boon and Albritton had, in fact, identified the 'crypto-volcanic structures' as complex impact structures, with flat floors and central peaks, rather than simple bowl-shaped craters like Meteor Crater. However, the distinction between the two types of crater was not to be recognised for many more years. European study of lunar and terrestrial impact craters was checked by the man-made catastrophe of the Second World War, and interest in the subject lapsed for nearly 20 years. Very few papers were published about the Nordlinger Ries and the Steinheim Basin during the 1940s and 1950s. The Swedish and Finnish impact structures appear to have been almost forgotten, and they were rediscovered only during the 1960s. Kofels Hollow was likewise forgotten, in spite of its location in a popular tourist area, and has received little serious attention since the war.

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In America, however, there was an increased interest in meteorites and impact craters. The impact hypothesis for the craters of the Moon was at last put on a firm footing by R.B. Baldwin (1949) in his book The Face of the Moon. Meanwhile, on Earth, R.S. Dietz showed that the striated conical rock fractures called shatter cones were indicators of high-pressure shocks that could be produced only by meteorite impacts. These shatter cones had been first discovered in the Steinheim Basin by Branca and Fraas (1905), and Dietz and other geologists now proceeded to find them in several of Bucher's crypto-volcanic structures. Dietz himself found large shatter cones in the Kentland (Indiana) structure in 1945; and shatter cones were later found in Wells Creek, Flynn Creek, Decaturville, Crooked Creek, Serpent Mound, and Sierra Madera (Texas). Outside the United States, shatter cones were also found in 1961 in the huge Vredefort crypto-volcanic structure (D= 140 km) in South Africa, confirming a suggestion by Daly (1947) that it was an impact structure. In the light of these discoveries, and to avoid the suggestion that they were volcanic, the 'crypto-volcanic structures' were renamed 'crypto-explosion structures.' Another demonstration of the reality and the importance of impact cratering came on February 12, 1947, when a large iron meteorite broke up in the atmosphere over the Sikhote Alin Mountains, north of Vladivostok, and fell as a hail of iron masses, some of which weighed several tons. The largest intact piece of this meteorite had a mass of 1.75 tons; larger pieces broke up when they hit the ground. This meteorite produced a field of 106 craters, the largest of which was 26.5 metres in diameter. The systematic search by C.S. Beals and his co-workers for impact structures in Canada must also be mentioned. This search yielded a large number of candidates, notably the Brent and Holleford craters (Ontario), Deep Bay (Saskatchewan), Clearwater Lakes and Lac Couture (Quebec), and West Hawk Lake (Manitoba). It is partly as a result of this work that Canada can boast 26 authenticated impact structures (Grieve 1991, 1996). In 1953 a new mineral called coesite, a dense, high-pressure form of silica (SiOa), was created in the laboratory, and was quickly recognised as a criterion for the identification of meteorite

Part I: Impacts and Geology craters. It was first found in nature in Meteor Crater (Chao et ah, 1960) and in the Ries (Shoemaker and Chao, 1961). Also in 1961, an even denser high-pressure form of Si0 2 , called stishovite was produced; and this mineral was also found in both Meteor Crater and the Ries. Later in the 1960s a new indicator of impact shock was recognised in the rocks of crypto-explosion structures. These were the so-called planar deformation lamellae, microscopic fractures in crystals of quartz and feldspar. According to Grieve (1987), these fractures correspond to glide planes filled by solid-state glass. These lamellae occur along specific crystallographic orientations, and they form at shock pressures between 5 and 35GPa (50 to 350kbar). At higher shock pressures (30-45 GPa) the crystal structure of minerals is destroyed, although the crystal habit is retained, and the mineral is converted to diaplectic or thetomorphic glass. At still higher pressures (>45GPa) the rock is actually melted and forms sheets of impact melt, which has often been mistaken for volcanic rock. The identification of these mineralogical stigmata of impact made it possible to identify impact structures with certainty, and to measure the shock pressures reached in them. Recognition of impact cratering as an important geological process was also advanced by space missions to other planets. The discovery by Mariner 4 in 1965 of craters on Mars took most scientists by surprise; and, for my own part, I was astonished by the close resemblance of the surface of Mercury (as observed by Mariner 10) to the lunar surface. Missions farther afield, to the satellites of the giant planets, showed that cratered planetary surfaces were the rule. It became clear that in this respect the Moon was typical of the solid bodies of the solar system and that it was the almost uncratered surface of the Earth that was exceptional. As if photographs of cratered planetary surfaces were not enough, the close approaches of the asteroid 1566 Icarus during 1968 and of many other asteroids since then provided immediate reminders that impact cratering of planetary surfaces remains an active process. The advent of piloted space missions and of Earth surveillance satellites led to the discovery of many new terrestrial impact structures, which could be identified by their circular shape.

A Curious Omission

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Some of these structures also showed a characteristic 'bull's-eye' appearance due to the concentric outcrops of rock in and around the central uplift. Examples of such structures were the Araguainha Dome and Serra da Cangalha in Brazil, Aorounga in Chad, and Gosses Bluff in Australia. These new developments, the identification of circular structures on satellite images and the application of the mineralogical criteria of impact shock, led to a rapid increase in the number of known impact structures, which have now been identified in every continent except Antarctica. Grieve (1987) listed 116 authenticated impact structures; the number had increased to 131 in Grieve (1991); and Grieve (1996) stated that 149 such structures were known in 1995. The most recent list, compiled by Fortes (2000), lists no fewer than 224 impact structures. When one includes possible but not yet authenticated impact structures the number is increased to more than 250. Classen (1977) already listed 230 terrestrial impact structures, although many of these have not yet been authenticated by detailed study. Three particularly significant recent discoveries are Chicxulub, in the Yucatan Peninsula of Mexico (D~ 180 km, age = 65Myr), which was probably responsible for the Cretaceous-Tertiary mass extinction; Chesapeake Bay (D~85km, age ~34Myr), which is probably the source of the North American tektite strewn field4 (Poag et ah, 1994); and Lake Tonle Sap, in Cambodia (about 100km x 35km, age ~0.78Myr), which may be the source crater of the Australasian tektites (Hartung & Koeberl, 1994). That three such large craters, all 2 5 k m , the central peak is replaced by a ring of peaks with a diameter 0.50 times the diameter of the crater. The Ries is regarded as an example of a terrestrial peak-ring crater of this sort (Melosh, 1989). On the Moon the transition from central-peak craters to peakring craters takes place at a crater diameter of D~140km, rather than D~25km as on Earth. This difference is a result of the weaker surface gravity of the Moon. In consequence lunar peakring craters are rather rare; and there are no good examples on the Earth-turned side of the Moon. In regions of crystalline (igneous and metamorphic) rocks, complex impact structures form either circular lakes, with or without central islands (e.g. Lake Mistastin, Lappajarvi, Deep Bay,

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Lake Mien), or arcuate chains of lakes surrounding a central uplift (e.g. Siljan and Lake Manicouagan). The most beautiful of these structures is the Clearwater Lakes, in Quebec; the western member of this pair of lakes contains a concentric ring of islands that forms the inner peak ring of the crater. In regions of sedimentary rocks, complex impact structures are characterised by a circular central uplift, consisting of steeply tilted or even overturned rocks brought up from beneath the floor of the crater. This central uplift is severely faulted, and its rocks are often brecciated and mixed together. In uneroded complex impact structures, such as Haughton Dome in Canada, the brecciated central uplift may be covered by a layer of allochthonous breccia that originally covered the floor of the crater. The central uplift is often covered by an annular peripheral depression, which is itself surrounded by a faulted outer rim; these components are particularly well developed in Wells Creek (Tennessee), Gosses Bluff (Australia), and Richat (Mauritania). Geologically, complex impact structures generally appear as circular faulted and brecciated inliers corresponding to the central uplift of the structure, surrounded by concentric circular outcrops of successively younger rocks towards the rim. In three dimensions these impact structures appear as circular domes, with the outward dips of the rocks increasing towards the centre; strictly, in view of this centripetal increase in dip, they would be better described as cusps. Some impact structures have, at first, been mistaken for tectonic domes or for alkaline igneous complexes, which often uplift the country rocks. Such misinterpretations are more likely in deeply eroded impact structures, since the signs of shock metamorphism inevitably die out as the shock pressure decreases with increasing depth beneath the original floor of the crater. The deformation of the rock may thus become almost indistinguishable from ordinary tectonic folding and faulting. The only remaining evidence for impact is then the circular shape of the uplift, and, in geologically stable regions, the isolated nature of the disturbance. One can turn this argument round to predict the existence of a class of eroded impact structures that are exposed at a level below the reach of megabrecciation and shock metamorphism, but that still show the characteristic circular central uplift and radial and concentric faulting. Such structures might be called

A Curious Omission

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'hypo-astroblemes' or 'infra-impact structures.' Studies of complex impact structures suggest that the effects of shock metamorphism are observable to a depth (dgh) below the crater floor of about a tenth of the final diameter of the crater (i.e. d^ ~ 0.1 x Df), whereas appreciable structural deformation occurs to depths ddej -0.2-0.3 xD f . Thus there should be 1-2 times as many 'hypoastroblemes', without shock metamorphism, as there are classical impact structures with shock metamorphism, shatter cones, and dense polymorphs of silica. The Richat and Semsiyat domes in Mauritania, which have almost the ideal morphology of impact structures but which lack evidence of shock, are probably examples of such 'hypo-astroblemes'. Although very large iron meteorites, with masses of thousands of tons, reach the Earth's surface intact and form single large craters with D~ 1km, smaller irons, with masses of a few hundred tons, generally break up in the atmosphere and fall as an 'iron hailstorm' which produces a group of craters. The largest craters of such groups have diameters between about 25 and 300 metres. Several such groups of craters are known, for example the Henbury, Odessa, Campo del Cielo, and Sikhote-Alin groups. This morphology is so characteristic that it can be used by itself as evidence for impact. For example, near Quillagua (21°30'S, 69°20'W), in northern Chile, there is a chain of five groups of craters and isolated craters, with diameters ranging up to 300 metres, strung out over a distance of about 97km from north-east to south-west. 7 These crater fields were probably formed by a disintegrating iron meteorite. Eight large hexahedrite meteorites have been found in the same area of northern Chile, and these may be fragments from the same fall. In the diameter range between these crater fields and single bowl-shaped craters like Barringer Crater is a transitional impact regime associated with iron meteorites with diameters of about 10-100 metres, where the effects of aerodynamic pressure are extremely important. This transitional regime is poorly understood, although it appears that the outcome of such impacts depends on both the 7

Or from south-west to north-east; it is not clear which direction the meteorite came from.

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initial size of the meteorite and the speed at which it enters the atmosphere. 8 However, there are differences of opinion over the details. Hills and Goda (1993) calculate that iron meteoroids with initial velocities v0 > 2 0 k m / s and radii between 1 and 20 metres will break up explosively in the atmosphere and yield meteorites with maximum masses of only a few kilograms. Of course such meteorites will not form craters, and the explosion will be essentially identical to a Tunguska-type airburst. On the other hand, Melosh (1989) argues that large iron meteoroids ( r > 2 0 metres) with u 0 > 2 5 k m / s will be crushed to fragments by aerodynamic pressure, and that the resulting 'meteorite swarm' will be compressed and flattened into the shape of a discus or a fat pancake. The impact of such a flattened swarm of meteorites may produce a 'shotgun blast' crater, very shallow and with a nearly flat floor and a very low narrow outer rim; such a crater may be regarded as a crater field in which all the individual craters overlap. There is indirect evidence for the reality of this transitional impact regime in the fact that there are few known meteorite craters with diameters (D) between 300 and 1000 metres, and none of them are of Holocene age, whereas there are many single craters and groups of craters with D< 300 metres, and a large proportion of these craters are Holocene, or at least post-glacial [t< 12kyr). The rarity of craters with D between 300 and 1000 metres might be explained on the supposition that the shallow, flat-floored craters proposed by Melosh have simply not been recognised as meteoritic owing to their unusual morphology; moreover, such craters will be destroyed by erosion more quickly than deep bowl-shaped craters (formed by impacts with u 0 < 2 0 k m / s ) like Barringer Crater and New Quebec Crater. This reasoning implies that understanding atmospheric interactions and surface impact processes in this transitional regime may be important for estimates of impact fluxes and cratering rates. There may be a similar transition from groups and clusters of craters through shallow, low-rimmed structures to the classical large complex impact structures among the impact structures

8

Hills and Goda (1993) suggest that even the height of the crater above sea level, which governs the surface atmospheric pressure, may be important. They point out that Barringer Crater, which is ~2 km above sea level, may be anomalous in this respect.

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formed by stony asteroids and even perhaps by cometary nuclei. Empirical evidence suggests that craters in groups formed by the impact of stony meteorites or asteroids have maximum diameters of about 2-4 km (e.g. Rio Cuarto, in Argentina) (Schultz and Lianza, 1992). The transitional regime for stones corresponds to asteroids with diameters between about 200 and 600 metres and impact structures with diameters of about 4-15 km. Melosh (1989) identifies Flynn Creek and Decaturville as shallow impact structures belonging to this transitional regime. It appears that disintegrating cometary nuclei with D < 500 m cause damage at the Earth's surface through their associated air blast, rather than producing crater fields by direct surface impact. The transitional impact regime corresponds to a cometary diameter of about 1 to 3 km, and to craters with diameters of roughly 20 to 80 km. Thus similar morphological transitions occur with increasing size in the craters and impact structures formed by all three of the main types of impactors. Examples of such transitional impact morphologies produced by all these types of impactors will be cited in this book. The importance of these aerodynamic effects has also been demonstrated by the discovery that complex impact structures, like simple meteorite craters, sometimes occur in chains. These chains are thought to have been formed by asteroids or comets that broke up shortly before colliding with the Earth. For example, the Aorounga structure in Chad has been found to be a member of a chain of three or four craters, the largest of which is 17 km in diameter (Ocampo and Pope, 1996). Aorounga itself is 12.6km in diameter. Again, a study by Rampino and Volk (1996) has identified a chain of no fewer than eight impact structures strung out along a distance of at least 600 km in the Midwestern United States, from Kansas to Illinois. These eight structures are the notorious members of the '38th parallel lineament' identified by Snyder and Gerdemann (1965). They are, from west to east, Rose Dome (Kansas), the Weaubleau area, Decaturville, Hazel Green, Crooked Creek, Furnace Creek and the Avon diatremes in Missouri, and Hicks Dome (Illinois). Crooked Creek and Decaturville were identified as impact structures in the 1950s and 1960s, but the other six structures were thought to be of igneous origin.

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The Steinheim and Ries craters in Germany appear to belong to a complex chain, which has been strangely neglected by scientists. Near the spa town of Bad Urach (48°29'N, 9°25'E), in the Swabian Alps of south-west Germany, is a field of breccia pipes or tuffisite pipes of supposedly volcanic origin. It may be significant that this volcanic area is directly in line with the Ries crater and Steinheim Basin, about 58 km west-south-west of Steinheim. East of the Ries, in central Bavaria, seven craters or groups of craters have been identified, ranging in diameter from 0.85 km to 2.5 km. These craters lie exactly between the Ries and a field of tektites in Bohemia (the Czech Republic); moreover, the craters diminish in size from west to east, that is, the largest craters are nearest to the Ries. North-east of the Ries is another possible impact structure, called the Stopfenheim Kuppel (or Stopfenheim Dome); this structure is 8 km in diameter (Classen, 1977). This structure lies between the Ries and another field of tektites in the region of Lausitz (or Lusatia), near Dresden, about 300 km from the Ries (Storr and Lange, 1992). The full length of this chain of possible impact structures and tektite strewn fields, from Bad Urach to yet another tektite field in Moravia, is about 500 km. However, the 38th parallel lineament and the Ries chain pale before the groups and chains of impact structures in southern Africa. The vast Bushveld Igneous Complex (about 25°S, 29°E) of Transvaal appears to consist of at least three Precambrian impact structures (Rhodes, 1975), which were probably formed at the same time as the Vredefort impact structure south of Johannesburg. The largest of the Bushveld impact structures, centred near 25°S, 291/2°E, is probably 200-250 km in diameter, and the whole complex is about 500 km from east to west. This enormous impact complex is similar in size to Mare Crisium on the Moon. Even the Bushveld Complex does not exhaust the possibilities of southern Africa. Recently a huge Upper Jurassic impact structure, at least 70 km in diameter, has been identified at Morokweng (26°08'S, 23°45'E) in northern Cape Province. Other geologists have suggested that the Bangweulu Basin (11°1'S, 29°45'E) and the Lukanga Swamps (14°26'S, 27°45'E) in Zambia

A Curious Omission

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are also impact structures, with diameters of about 150 km. Examination of a map shows that the Okavango Delta (19°27'S, 22°54'E) in Botswana is aligned with the Bangweulu Basin and the Lukanga Swamps. Moreover, the large complex depression of the Makgadikgadi Saltpan (20°41'S, 25°26'E), south-east of Okavango, is located essentially midway between Lukanga and Morokweng. These two basins are about 100-130 km in diameter. If these depressions form a chain of impact structures, the length of this chain is about 1800 km, approximately the distance from Liverpool to Naples. It may be noticed that both this chain and the Ries chain split in two along their length. The large number of known impact structures implies that they are quite thickly distributed over the Earth's surface. The total land surface area of the Earth is about 150 x 10 6 km 2 . If there are about 200 known terrestrial impact structures, the mean surface density is 1 impact structure per 750,000 km 2 . This figure is slightly more than three times the area of the United Kingdom (244,030km 2 ), and 2.4 times the total area of the British Isles (315,173 km 2 ). However, it would be an error to suppose that this density of impact structures is typical of any area of the Earth's land surface. Most impact structures have been identified on the stable continental cratons, particularly the Midwestern United States, the Canadian Shield, Europe north of the Alps, and the desert regions of Australia. The Earth's mobile belts (for example, the Alpine Himalayan belt and the circum-Pacific belt) have hardly any impact structures, owing to their rapid destruction by tectonic deformation and by erosion. 9 Likewise, there are no known impact structures in active sedimentary basins, such as Bangladesh, the Netherlands, or the Gulf States of America, where they are quickly covered by younger rocks. In addition, the continents of Asia, Africa, and South America, which have been less thoroughly explored than North America, Europe, and Australia, have so far yielded fewer impact structures. Grieve (1991) lists only twelve craters and impact structures in Africa (30.335 x 10 6 km2) and five in South America ^ h e r e is one exception to this rule: Lake Kara-Kul (D = 45 km), in the Pamir Mountains of Tajikistan, is identified as an impact structure by Grieve (1991).

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Part I: Impacts and Geology

(17.611 x l 0 6 k m 2 ) , against eight in Quebec (1.541 x 10 6 km 2 } and five in Ontario (1.069 x 10 6 km 2 ). The implied density of impact structures in Quebec and Ontario is thus about one per 2 x 10 5 km 2 . The stable European craton tells a similar story to the United States and the Canadian Shield; most of the countries of northern Europe have impact structures. The Ukraine has seven such structures 1 0 ; Belarus has one (Logoisk); European Russia has eight; and the Baltic States (Estonia, Latvia, and Lithuania) have four impact structures between them, as well as two groups of small Holocene craters (Ilumetsy and Kaalijarv). Poland has a group of seven small Holocene craters at Morasko (52°29'N, 16°54'E), north-west of Poznan, and a single crater, 100 metres in diameter, at Frombork (54°20'N, 19°41'E). No large impact structures have been discovered in Poland, probably because more than half the area of the country is covered by Quaternary sediments. Germany has the Ries and Steinheim Basin, and the smaller craters that may have been formed by the same impact. In addition, Gallant (1964) points to the 1-km Randecker Maar as a possible meteorite crater. However, the main search for European impact structures has focused on the countries of Fennoscandia, with spectacular results. A list compiled by A.D. Fortes (2000) includes four impact structures in Norway, where the terrain and recent glacial erosion must hamper searches, 12 in Finland, and no fewer than 42 in Sweden. Four of these structures (all of them of Proterozoic age) have D> 100 km, the largest being the vast Uppland structure, with its centre near Uppsala, with a diameter of about 320 km. It should be remembered that Norway, Sweden and Finland combined have an area less than four times that of the British Isles. Denmark and the Netherlands also have no large impact structures, again because they consist predominantly of Quaternary sediments. Belgium and Luxembourg also lack impact structures, probably because both are small countries and because Belgium consists largely of Tertiary rocks.

'By an odd quirk of nomenclature, the Odessa craters are not in the Ukraine.

A Curious Omission

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Elsewhere in Western Europe, France has one large impact structure, with a diameter of 23 km, at Rochechouart (45°50'N, 00°50'E), west of Limoges. It is of historical interest that the town of Chalus, where King Richard the Lionheart was killed, is near to the Rochechouart structure. It may be of more geological significance that the village of Montmorillon (46°26'N, 00°50'E), the original source of the clay mineral montmorillonite, is about 60 km from Rochechouart. In addition to this large structure, France has a group of six or seven small craters, which are described by Baldwin (1963), at 43°32'N, 03°08'E, near Cabrerolles and Faugeres in the departement of Herault. These craters have diameters between 45 and 220 metres, and are thought to be about 10,000 years old. Graham et al. (1985) regard these craters as discredited. However, as I have explained previously, meteorite craters with diameters of about 200 metres generally occur in groups, and the conformity of the Herault craters to this morphological pattern is evidence that they are indeed meteorite craters. A number of small lakes at Sucy-en-Brie and Alentours, near Paris, have also been mentioned as possible meteorite craters. Farther west still, Spain has the 30-km Azuara structure (41°10'N, 00°55'W), south of Zaragoza. Even the Alpine nations of Switzerland and Austria have possible meteorite craters. A 400-metre crater at St.-Imier (47°10'N, 07°00'E), in the Swiss J u r a Mountains, is said to have shatter cones and small iron particles (Graham et al, 1985); shatter cones have also been found at Lago di Tremorgio (d= 1.36 km), in Ticino canton. An impact origin was suggested for Kofels Hollow (47°13'N, 10°58'E; D=4km) in the Austrian Tirol in 1936; and this suggestion has been a matter of debate ever since. According to von Engelhardt (1972), Storzer et al. (1971) identified diaplectic glass and planar features in quartz in a glassy dyke from Kofels; however, Officer and Carter (1991) argue that the hollow is the trace of a giant landslide. The formation of the hollow has been dated at about 8150 B.C. (von Engelhardt, 1972), or 8700 BP (Officer and Carter, 1991); if it is an impact structure it is a very young one and probably of Holocene age. However, the largest suggested impact structure in Europe outside Sweden is in the heart of the continent, in the Bohemian

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Part I: Impacts and Geology

Massif of the Czech Republic. According to Rajlich (1992), the entire massif constitutes an impact structure 260 km in diameter, associated with breccias and shock-metamorphosed glass. The structure is probably of Late Proterozoic age (about 600-1000 Myr). Since impact structures are so abundant in other parts of Europe, it seems strange that there are none known in the United Kingdom or Ireland. The British Isles are, after all, part of the stable European craton; they have outcrops of rock belonging to every geological period since the Late Proterozoic; and they have been very thoroughly explored. It is difficult to believe that any large impact structure, or even a reasonably well-preserved meteorite crater larger than a few hundred metres in diameter, could have escaped the notice of the Ordnance Survey or the British Geological Survey. It must also be remembered that the British Isles are surrounded by the shallow seas of the northwest European continental shelf, and that this shelf covers a larger area than the islands themselves. It is therefore strange that exploration for oil and gas on the continental shelf has not yet led to the discovery of impact structures there. This anomaly is strengthened by the recent discoveries of the Montagnais, Toms Canyon, Mj0lnir and Neugrund structures. Thus the curious omission that forms the title of this chapter is the fact that there is not a single established impact structure or cryptoexplosion structure anywhere in the United Kingdom or Ireland, or on the surrounding continental shelf; there are not even any small groups of craters comparable to Morasko or Kaalijarv. The absence of British meteorite craters and impact structures is a geological anomaly; and this book will discuss the reasons for the absence of such structures, as well as identifying and describing some circular landforms that may in fact be the products of impacts. The first step in the search for British impact structures is to study the statistics of impacts, that is, to measure the surface densities of impact structures in other countries, and to obtain estimates of cratering rates. Such statistical analyses can give indications of the probable number and sizes of British impact structures, and even of their age distribution. These calculations will be the subject of the next chapter.

CHAPTER

Of Calculations and Craters Who hath laid the measures thereof, if though knowest? Or who hath stretched the line upon it? Job, 38, 5. The first chapter of this book argued that impact structures could no longer be regarded as merely local curiosities, and that instead they were common features of the Earth's surface a potentially of geological importance. However, it might have been felt that the suggestion that there are impact structures in Britain might have been based purely on nationalistic sentiment, a feeling that if Sweden, Finland, Germany and France have impact structures it is only fair that the United Kingdom should be similarly favoured. It is therefore necessary to provide quantitative arguments in favour of the thesis. These arguments, besides their bearing on the impact question, have interesting geological implications in their own right. It was stated in Chapter 1 that there are at least 250 authentic and possible terrestrial impact structures, implying a surface density on land of 1 impact structure per 6 x 10 5 km 2 . However, the fact that impact structures in the Earth's mobile belts are quickly destroyed by tectonic processes and by erosion means

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Part I: Impacts and Geology

that we must turn to the stable cratons to obtain an accurate estimate of the true surface density. Estimates of one impact structure per 2 x 10 5 km 2 have already been obtained for Quebec and Ontario. However, of the nine craters in Quebec, the two Clearwater lakes form a pair, and should be counted as a single impact; and the New Quebec Crater, with a diameter of only 3.44km, is likely to be short-lived, even as a crypto-explosion structure. Of the five craters in Ontario, the Sudbury structure is Precambrian (Lower Proterozoic), and should not be used in calculations of the areal density of Phanerozoic craters. These adjustments reduce the densities of impact structures in Quebec and Ontario to 1 per 2 . 2 x l 0 5 k m 2 and 1 per 2 . 7 x l 0 5 k m 2 respectively. Larger areal densities of impact structures are found on the continental platform of the eastern United States, west of the Appalachian Mountains. In analysing these data, it should be remembered that there are several probable impact structures in the eastern USA that are not on the lists of Grieve (1987 and 1991), or even of Fortes (2000), and that inclusion of these structures increases their areal density. For example, Grieve (1991) lists two impact structures (Wells Creek and Flynn Creek (36°16'N, 85°37'W)) in Tennessee (109,411 km 2 ). In addition to these two, Officer and Carter (1991) regard Howell (also mentioned by Baldwin (1963)) as an authentic impact structure; and Hey (1966) mentions the Dycus structure (36°22'N, 85°45'W), in Jackson County, as having upwardly directed shatter cones. The Howell structure is admittedly very small, only 1.6 or 2.4km in diameter; and Dycus is so close to Flynn Creek (only 16 km to the north-west) that they may be the result of a double impact. Thus there may be either two, three or four independent impact structures in Tennessee, yielding an areal density of between 1 per 55,000 km 2 and 1 per 27,000 km 2 . Likewise, Grieve (1991) mentions only one impact structure, Middlesboro (D = 6km), in Kentucky (104,623km 2 ). Officer and Carter, however, regard both Jeptha Knob (38°06'N, 85°06'W) and the Versailles structure (38°02'N, 84°42'W) (D = 1.5km) as authentic impact structures. If these are accepted as genuine, they yield an areal density of impact structures of 1 per 35,000 km 2 .

Of Calculations and Craters

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Baldwin (1963) describes Jeptha Knob as 'a small cryptovolcanic structure' with a total diameter of about 2^ miles (3.8 km). Other authorities have given larger diameters, up to 24 km (Seeger, 1985), and the morphology, with a central uplift and surrounding ring anticline, implies a diameter of at least 8-10 km. The proximity of Versailles and Jeptha Knob (40 km apart) suggests at first sight that they were formed by a double impact. However, the ages are different; Jeptha Knob is believed to be of Silurian or Ordovician age (about 410-490Ma), but the age of Versailles is given as 0.13% of the area of Mauritania. In Africa as a whole, the great chain of possible and established impact structures from Bangweulu to Morokweng alone covers 0.2% of the area of the continent.

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Part I: Impacts and Geology

Among the European nations, Sweden has 0.68% of its area covered by its eleven Phanerozoic impact structures. The 34 structures of uncertain age add another 0.29% of the area of the country to this figure. The other European nations mostly fall short of this figure, perhaps because they have been less thoroughly explored or because their impact structures have been concealed by later sediments or destroyed by erosion and tectonic activity. The four Finnish craters that are certainly Phanerozoic cover 0.08% of the area of Finland; Logoisk covers 0.11% of Belarus; the Ries and Steinheim together cover 0.13% of Germany; Rochechouart covers 0.08% of France; and the Azuara structure covers 0.14% of Spain. The Baltic States have 0.06% of their area covered by impact structures, Ukraine has 0.12%, and European Russia has 0.16%; this last figure, however, is dominated by the vast Puchezh-Katunki structure (D =80km). On a larger scale, the Bohemian Massif structure, with D = 260 km, covers 0.5% of the total area of Europe by itself. There are eleven large impact structures in Asiatic Russia, which together cover 0.11% of the country. Although no definite impact structures have yet been identified in China, the large Duolun structure (D~ 170 km) in Inner Mongolia and Taihu Lake (D~70km) in Kiangsu province have been named as possible impact structures. If so, about 45,000 km 2 of Asia are covered by known impact structures, or 0.1% of the area of the continent. The state of affairs in Australia is unusual. The Acraman and Woodleigh structures, with D = 160 km and D = 120 km respectively, cover 0.41% of the area of the continent by themselves. The remaining Australian impact structures (excluding the Teague structure 3 , which is Lower Proterozoic) add only another 0.02% to bring the total up to 0.43%. It appears from this analysis that in well explored stable continental shields or platforms, on average 0.1-0.2% of the territory is covered by medium-sized Phanerozoic impact structures. In most of the territories studied the largest of these impact structures occupies more than half (often >80%) of the total area •^he Teague structure has now been renamed the Shoemaker structure, after the famous astronomer and geologist E.M. Shoemaker (1928-97).

Of Calculations and Craters occupied by impact structures, and this largest structure is often 100 km in diameter (e.g. Bohemia, Acraman, Morokweng, Chicxulub and Popigai). It appears then that over a large enough area of the Earth's surface a 'saturation percentage areal coverage' of 0.1-0.2% is reached in 100 metres the bay is nearly a completely enclosed depression. The Firth forms a deep marine basin, with a maximum depth of about 145 metres (Mykura 1976), between the islands of Fetlar and Yell to the north and north-west, the peninsula of Lunna Ness (Mainland) to the west, and the islands of Whalsay and Out Skerries to the south. The centre of the basin is at 60°29'N, 00°54'W (approximately HU610785), and its diameter is about 14 km. The two basins are about 38 km apart. According to Hughes (1996) the craters may be of Late Tertiary age (i.e. Charnwood {

Oaks Fm.