West, Ian M. 2018. Dinosaur footprints of quarries of the Isle of Purbeck . Internet geological field guide. http://www.southampton.ac.uk/~imw/Purbeck-Dinoprints.htm. Version: 15th July 2018.

Dinosaur footprints of the quarries of the Isle of Purbeck
By Ian West,
Romsey, Hampshire
and Visiting Scientist at:
Faculty of Natural and Environmental Sciences
Southampton University,
Webpage hosted by iSolutions, Southampton University
Website archived at the British Library

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[CURRENTLY IN PREPARATION - NEW SECTIONS, IMAGES AND TEXT WILL BE ADDED]

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INTRODUCTION:

Sequence of Lithologies

Sequence of petrographic lithotypes in the Middle Purbeck Group of Durlston Bay, Dorset, by El-Shahat and West

Correlation of some quarry sections in the Intermarine Member of the Purbeck Group with the type-section at Durlston Bay, Swanage, Dorset

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INTERMARINE MEMBER:

Introduction

Graphic log of the Cinder Bed, Intermarine Member and Scallop Member of the Purbeck Group, Durlston Bay, Dorset, modified after Clements

Correlation of some quarry sections in the Intermarine Member of the Purbeck Group with the type-section at Durlston Bay, Swanage, Dorset

Trev Haysom's quarry at Acton, west of Swanage, showing an almost identical sequence of the lower part of the Intermarine Member as in Durlston Bay, Dorset

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INTERMARINE MEMBER:

Correlation

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A similar Durlston Formation, upper part of Middle Purbeck, at Lewis Quarry and at Haysom's Quarry near Acton, west of Swanage, Dorset, both showing an almost identical sequence of the lower part of the Intermarine Member

Correlation of the Intermarine Member, Purbeck Group, from Ridgeway Railway Cutting to Durlston Bay, Dorset

The Intermarine Member consists of thin-bedded, lagoonal and lacustrine limestones and shales above the conspicuous oyster bed - the Cinder Bed. Many of the limestones have been quarried for Purbeck Stone. These are the main limestones of the Upper Building Stones. The old traditional name, the "Intermarine Beds" presumably applies well to the upper part which is dominantly of brackish water origin. The unit lies beneath the near-marine Scallop Member with "Chlamys" and other more typically marine molluscs.

The general features of the succession can be seen from the diagrams above, which include a modified extract of part the classic log of Clements ,1969; 1993, and a modified correlation diagrams of El-Shahat, 1977. It consists of beds of hard shell-debris (biosparrudite) limestones with some pyritic shales in the central part (" mid-intermarine shales ").

Physa bristovii Ptychostylus

The lower part (Downs Vein etc, just above the Cinder Bed), though, is of almost freshwater origin and has similarities to the Cherty Freshwater Member in containing pond-snails like Physa bristovii, which is in beds DB 115 and 116, and Ptychostylus , abundant in DB 112, and charophyte algae. It, thus, starts as freshwater lake deposits but marine water gradually gained access to what then became a large lagoon. Austen (1852) and Fisher (1856) referred to the unit as the "Turtle Beds" which may, perhaps, be more appropriate than the " Intermarine term "used in Bristow and Fishers' (1857) vertical section and reproduced in Damon (1884). Nevertheless, the name " Intermarine " is so well-established and well-known that it is preferable to retain it.

The shelly limestones are mostly coarse enough to be termed "biosparrudites "(coarse shell debris with a sparry cement) in accordance with Folk's (1962) classification, but sometimes referred to for simplicity as " biosparites " (shell debris sands) as in Clements' (1969; 1973; 1992) log. They represent debris mostly from the brackish water bivalve Neomiodon . The shells were originally aragonite but have been replaced by calcite now. They have been accumulated by storms with easterly winds at the western margin of the Purbeck basinal area. This basinal region which extended eastward to the Isle of Wight and part of the Weald of Sussex was in basinal in relative terms with thicker sequences and less subaerial exposure - but actually still lagoonal and lacustrine and very shallow). The shell beds originated as extensive white shell sediments. Some of which were usually at or just below water-level and others formed shell beaches, above water-level for much of the time (El-Shahat and West, 1983).

Water-levels fluctuated, probably seasonally and over longer periods in the seasonal subhumid climate, and dinosaurs left their footprints in the damp shell deposits which were sometimes cemented by carbonate early, thus favouring preservation. Some footprints may have been preserved by shell debris being washed in by storms over dried muds with footprints and desiccation cracks, as happened in the case of the Cherty Freshwater Member. The apparent abundance of dinosaur footprints in the Intermarine Member in Isle of Purbeck is the result of several factors. The extensive shell beaches and marginal mud-flats were adjacent to a vegetated peninsula (the Wytch Farm or South Dorset High) and thus likely to be traversed by dinosaurs, especially during drought. The quarrying in the area has revealed footprints frequently, although they are also found commonly at Durlston and Worbarrow Bays. (For dinosaur footprints in the Lower Purbeck of Portland see the Portland Dinosaur Footprint webpage).The climatic and environmental factors are more important than an apparent abundance because of quarrying. The Intermarine Member originated in a climate less arid than that in which beds lower in the sequence were formed. This accounts for generally more sand, kaolinite clay, pyrite, plant debris and lower salinities than those indicated for the lower members, and almost no evaporites. The subhumid conditions seem to have been particularly favourable to turtles, crocodiles and dinosaurs.

Some of the limestones continue onto the shelf area of Lulworth but the main deposits are east of Worbarrow Bay. There were many quarries in the Swanage area, but later quarrying has moved westward to near Langton Matravers and Worth Matravers. The quarrying in the past has resulted in the naming of many of the individual beds.

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LEWIS QUARRY -

Details

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LEWIS QUARRY -

Footpring Details

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ADD DETAILS HERE

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ACKNOWLEDGEMENTS [prearation in progress]

I am much obliged to Francis Charig for further information on the Purbeck dinosaur footprints shown in relation to the reference to Dr Alan Charig. I am very grateful for various reprints and information on papers. I am particularly grateful to Alan Holiday for help when visiting fossil footprint sites in the Isle of Purbeck. I very much appreciate the permission for access and the help given by the quarry owners, Mr Lewis (Lewis Quarries) and .... [this is in preparation and will be continued]. Many thanks to ............................
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|Home and List of Webpages | Purbeck Bibliography - by Topics | Isle of Portland Field Guide |Durlston Bay - Peveril Point - Field Guide |Durlston Bay, Swanage, Middle Purbeck |Durlston Bay - Lower Purbeck |Durlston Bay - Central Zigzag Part & Coast Erosion |Durlston Head - Lower Purbeck Group & Portland Stone |Lulworth Cove Field Guide | |Portesham Rocket Quarry|




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Dr Ian West, author of these webpages

Webpage - written and produced by:


Ian West, M.Sc. Ph.D. F.G.S.

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at his private address, Romsey, Hampshire, kindly supported by Southampton University,and web-hosted by courtesy of iSolutions of Southampton University. The website does not necessarily represent the views of Southampton University. The website is written privately from home in Romsey, unfunded and with no staff other than the author, but generously and freely published by Southampton University. Field trips shown in photographs do not necessarily have any connection with Southampton University and may have been private or have been run by various organisations.


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