The cliff and foreshore exposures in the Devon part of the Dorset and East Devon Coast World Heritage Site expose an unbroken
late Triassic to early Jurassic succession. The change from the terrestrial, red-bed facies of the Triassic Mercia Mudstone Group to
the fully marine conditions of the Jurassic Lias Group takes place via the Penarth Group, a succession of mudstones, siltstones and
limestones deposited in lagoonal and sheltered shallow-marine environments of varying salinities. The Penarth Group as
currently defined is divided into the Westbury Formation overlain by the Lilstock Formation, based on type sections in the Severn
Estuary area. The lithology and sedimentology of the Westbury Formation strata exposed on the east Devon coast are closely
comparable with those of the type area, but those of the Lilstock Formation are not. It is therefore proposed on lithological and
historical grounds that this formation should be replaced by a Cotham Formation overlain by a White Lias Formation. This would
reinstate, without any change in their original definitions, two of the oldest formally defined stratigraphical names in the British
Phanerozoic. All three formations are lithologically distinctive, and are separated from their neighbours by erosion surfaces that
represent non-sequences. Those at the bases of the Westbury and Cotham formations are overlain by pebble beds rich in
vertebrate remains (‘bone beds’). The Cotham Formation is a highly condensed succession comprised of thinly interbedded
mudstones and limestones with ripple trains, stromatolites, desiccated surfaces and slumped beds, the last of which have been
attributed to earthquakes or a bolide impact. The limestones of the White Lias Formation exposed on the east Devon coast are
sedimentologically complex with channels, slumps and desiccated surfaces. The position of the Triassic-Jurassic boundary is
currently under review. Possible positions include the base of the Cotham Formation, a horizon within the formation, the base of
the White Lias, the base of the overlying Blue Lias Formation or a horizon within the Blue Lias Formation.
The Upper Jurassic Kimmeridge Clay Formation (KCF) underlies much of the Vale of Pickering where it is almost wholly concealed by the Cretaceous Speeton Clay Formation and Quaternary deposits. There are few KCF inland or coastal exposures in Yorkshire with the result that the succession was stratigraphically poorly known until the 1970s oil crisis when the British Geological Survey drilled continuously cored boreholes at Marton and Reighton to examine the formation as a possible source of hydrocarbons. These were supplemented in 1987 by continuously cored boreholes drilled at Marton, Reighton, Ebberston and Flixborough by the Institut Français du Pétrole for hydrocarbons research. Taken together, the boreholes have enabled the lithological, palaeontological, geochemical and geophysical characters of the full thickness of the formation to be examined. Comparison of the KCF successions proved in Yorkshire with those in the adjacent North Sea, the East Midlands and the Dorset coast type area, shows marked variations in thickness related to penecontemporaneous faulting. However, there are only minor variations in the lithologies and faunas at any particular stratigraphical level. This appears to be due to a combination of Milankovitch-driven climatic fluctuations and pulsed variations in sea level which combined to produce similar depositional conditions throughout the English KCF at any one time. The chronostratigraphical classification of the KCF developed in southern England has therefore been shown to be applicable to the Yorkshire outcrop and the southern North Sea. The changes in sea level may be eustatic rather than regional events, but there is insufficient palaeontological evidence to enable them to be correlated with confidence with those of the standard Jurassic sea-level curve.
Numerous site investigations have described ‘reconstituted’, ‘putty’, ‘soft’ and ‘rubbly’ chalks which have generally been grouped
together for geotechnical description purposes as ‘structureless’, to distinguish them from ‘structured’ (bedded and jointed) in situ
chalks. Structureless chalks have mostly been assumed to be redeposited materials or materials that have been intensely
mechanically reworked in place. They include sludge (Head), partially water-sorted (Coombe), fluvial (Dry Valley and Nailborne)
and cryoturbation (Head) deposits.
Recent surveys of the east Devon coast have shown that an extensive, thick chalk deposit, which in small exposures or in cored
boreholes could be mistaken for redeposited material, has formed in situ by the partial dissolution of a particular type of chalk.
The best exposures, in the cliffs at and adjacent to Beer, Devon [SY 230 890], show a 15 m- to 30 m-thick layer of partially
decalcified, nodular chalk that is underlain and overlain by intact clay-rich chalks. The boundaries of the decalcified unit are
stratigraphically controlled and sharply defined. It contains angular to rounded, granule to boulder sized litho-relics in a
degraded, chalk-fines matrix. Blocks of intact, largely unweathered chalk up to tens of metres thick occur as detached masses
‘floating’ in the partially decalcified unit, and even larger masses have settled down into it. Some of these contain the youngest
Chalk preserved in Devon.
The Shales-with-Beef Member of the Charmouth Mudstone Formation (Lower Jurassic) crops out in almost continuous cliff and
foreshore sections over a distance of c. 5 km between Pinhay Bay, east Devon and Charmouth, west Dorset. A fault-bounded
outlier, 3.5 km west of Pinhay Bay, exposes the lower part of the member. At its type section on the foreshore and in cliffs below
Black Ven, Charmouth, the member consists of c. 30 m of thinly interbedded organic-rich mudstones and calcareous mudstones
with numerous thin beds of fibrous calcite (‘beef’) and several beds of tabular and nodular limestone. Many of the individual beds
of mudstone are richly fossiliferous and this has previously been used, in combination with the lithological variations, to divide
the succession into over 100 numbered and lettered beds. However, as noted in the original study, many of the thinner beds are
laterally impersistent and few can be recognised with confidence away from the type section. In addition, the type section is
separated from the main Shales-with-Beef Member outcrop in the Lyme Regis area by a penecontemporaneously active fault belt
that had an effect on sedimentation. Other fault belts at Lyme Regis and westwards from there divide the outcrop into areas with
successions that differ in detail from the type section and from one another. As a result, few of the numbered beds can be traced
from one of these areas into the adjacent area. A simplified system of bed numbering is described here based on sections that crop
out on the west and east sides of Lyme Regis, supplemented by the successions proved in cored site-investigation boreholes drilled
at Lyme Regis. The proposed lithostratigraphy provides a framework that takes account of the lateral variations in the member over
its full outcrop distance, and enables material collected from any part of the exposure to be placed in its correct stratigraphical
context.
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