Pinhay Bay Fossil Hunting

1906 – Cotham Member fossils and White Lias structures recorded
Richardson described the Pinhay Bay Rhaetian beds, including the Cotham Marble, the “Estheria Bed” with Estheria minuta, ripple marks and the distinctive Sun Bed at the top of the White Lias. These observations helped establish Pinhay Bay as one of the key Devon sections for the latest Triassic coastal succession.

1924 – Pre-planorbis Beds measured above the White Lias
Lang recorded the basal Blue Lias at Pinhay Bay, including the Pre-planorbis Beds, also known as the Ostrea Beds because of their abundant oysters. This was important because the first appearance of Psiloceras planorbis lies above these beds, making the bay useful for discussing the Triassic–Jurassic boundary interval.

1960 – classic study of the Langport Member fauna
Hallam produced the main detailed account of the Pinhay Bay White Lias, listing a diverse Langport Member assemblage of bivalves, gastropods, rarer solitary corals and trace fossils. The work showed that many fossils occur in small dense patches within the limestones and marls, recording shallow, repeatedly disturbed marine conditions.

1961 – Cotham Marble interpreted as algal growth
Hamilton’s work on Cotham Marble helped explain the “landscape” structures seen in the Cotham Member at Pinhay Bay as algal or microbial growths rather than ordinary inorganic patterns. This gave the bay’s Cotham Marble a clearer biological significance within the late Triassic lagoonal setting.

1983 – deformed Cotham Member bed linked to seismic disturbance
Mayall recorded a deformed bed in the middle of the Cotham Member at Pinhay Bay and correlated it with nearby East Devon sections. The bed includes disturbed sediment, microfaulting and water-escape features, providing evidence that some fossil-bearing late Triassic lagoonal sediments were affected by earthquake-related movement soon after deposition.

2001 – Triassic–Jurassic boundary beds re-examined after new exposure
Newly exposed sections at Pinhay Bay allowed Wignall to document the White Lias–Blue Lias contact in detail, including bored hardgrounds, intraformational conglomerates, truncated burrows such as Diplocraterion, and rapid change into organic-rich basal Blue Lias shale. This made the bay especially important for interpreting sea-level change and environmental stress across the end-Triassic interval.

2020 – dewatering structures described in the oldest Blue Lias
Gallois and Thompson described unusual dewatering and diapiric structures in the oldest Rhaetian part of the Blue Lias at Pinhay Bay. Although mainly sedimentological, the work matters for fossils because it refines the unstable sea-floor conditions in which the earliest Blue Lias oyster-rich beds and associated marine fossils were preserved.

Section Architecture

Pinhay Bay is not a single simple cliff log. The key exposures are the low cliffs and foreshore in the middle and eastern part of the bay, where the upper Penarth Group and basal Blue Lias are seen, together with the faulted western end of the bay where Blue Lias is downthrown against the older Langport Member. Landslides, beach build-up and foreshore cover repeatedly obscure parts of the section, so the succession has to be treated as a composite of cliff, foreshore and temporary erosion windows rather than as one uninterrupted face.

Scope Note

This page is restricted to the classic Pinhay Bay boundary section itself. It therefore emphasizes the Westbury, Cotham and Langport members of the Penarth Group and the basal ammonite-bearing Blue Lias beds above them. Higher Blue Lias and younger lower Jurassic strata continue eastwards toward Seven Rock Point, Devonshire Head and Lyme Regis, but those are better treated as the broader Pinhay Bay–Lyme Regis continuation and are not forced into this more tightly defined locality page.

PENARTH GROUP

Westbury Formation (Rhaetian)

Bed PB1 – Basal Westbury Burrow-Fill Bone-Bed And Pebbly Transgressive Lag

The base of the Westbury Formation is exposed in the intertidal area at Pinhay Bay as dark mudstone and pebble-bearing infillings penetrating the top of the older Blue Anchor surface. Like the equivalent east Devon boundary beds elsewhere, this is a transgressive lag developed in burrows and shallow irregularities rather than a perfectly even sheet. The sediment includes dark grey mudstone, shell debris, fish and vertebrate remains, angular green mudstone clasts and quartz grains, and it records the first major marine flooding of the latest Triassic surface in this area. The bed is an important marker, but it is best treated honestly as a burrowed erosional base rather than as a tidy planar seam.

Bed PB2 – Main Westbury “Contorta” Shales (c. 5.0 m)

Above the basal transgressive lag, the Westbury Formation consists of dark grey to black fossiliferous mudstones with shelly intervals and local shell-detrital limestones. In older descriptions of the Pinhay Bay section this unit is simply the “Contorta Shales”, but the fauna and lithology are those of the standard Rhaetian Westbury Formation. Thin-shelled bivalves dominate, and the environment was a quiet-water brackish to shallow-marine embayment or restricted shelf. Compared with the more complex Cotham and Langport carbonates above, the Westbury succession is more monotonously muddy and distinctly darker in colour.

Cotham Member

Bed PB3 – Cotham Member, Including The “Estheria Bed” And Cotham Marble (c. 1.5 m Visible At Pinhay Bay)

At Pinhay Bay the Cotham Member is less fully visible than the overlying Langport Member, but older descriptions record the “Estheria Bed”, black shales and the Cotham Marble in the section. In regional east Devon correlation, this interval represents thinly interbedded greenish-grey mudstones, laminated and nodular limestones and subordinate sandy or silty beds deposited in shallow lagoonal to brackish-water conditions. The Cotham Marble or “Landscape Marble” marks the top of the member and represents stromatolitic or algal-mat growth in very shallow water. Where exposed in east Devon the member may also show slumps, dewatering structures and ooid ripple trains, but at Pinhay Bay those details are commonly obscured by beach cover and intermittent exposure.

Langport Member

Langport Member Note

The Langport Member at Pinhay Bay is the classical thick east Devon “White Lias” development. The currently accessible cliffs expose about the upper 7.0 m including the junction with the Blue Lias, but older measurements suggest that the full local succession was about 7.84–8.0 m thick. The member is composed mainly of very pure off-white fine-grained limestone with subordinate grey porcellanous limestone, marl partings and a series of hardgrounds, slumps, channels and clast-rich remobilized beds. Published local Pinhay bed numbers 1a–12 are retained here where they can be used confidently.

Bed PB4 – Pinhay Beds 1a–1b, Lower Slumped Limestones And Porcellanous Hardgrounds

The lower exposed Langport beds at Pinhay Bay consist of slumped fine-grained limestones separated by distinctive porcellanous hardground surfaces. These are among the first strong indications that the “White Lias” here was not deposited as a simple undisturbed limestone sheet. The limestones were repeatedly lithified very early, fractured or destabilized, and then draped or overlain by renewed micritic sediment. This lower part of the member forms the start of the exceptionally thick east Devon White Lias and probably corresponds to the basal several metres of the accessible cliff section.

Bed PB5 – Pinhay Beds 2–3, Nodular And Channelled Limestones With Marl Partings

Above the lower slumped units, the succession passes into nodular limestone with marl partings and then into the distinctive channelled base and channel infill of Pinhay Bed 3. The base is erosive and locally cut into the bed below; infill includes slumps and dewatering structures. This is one of the most revealing units in the whole bay because it shows that the middle White Lias surface was locally incised and re-filled, rather than merely accumulating by quiet parallel bedding. The member was clearly deposited in a very shallow, laterally variable marine setting where local relief could develop on the sea floor.

Bed PB6 – Pinhay Beds 4–6, Middle Slumped And Wavy-Laminated White Lias

Pinhay Bed 4 is a slumped limestone with abundant angular clasts, followed by Bed 5, a wavy-laminated limestone with load structures, and Bed 6, a more simply wavy-laminated limestone. Together these beds record repeated soft-sediment instability and short-distance remobilization of semi-lithified carbonate muds. The load structures and slump fabrics show that deformation took place early, while the sediment was still only partly consolidated. At the same time, the fine-grained micrite and sheet-like geometry show that the original depositional setting remained one of very shallow, low-energy carbonate sedimentation.

Bed PB7 – Pinhay Beds 7–8, Remobilised Clast-Rich Limestones And Hardground Surface

Pinhay Bed 7 comprises parallel-bedded remobilised limestones containing common clasts, shells and bored pebbles, and it is capped by the hardground surface of Bed 8. This interval is one of the clearest field signs that previously lithified or semi-lithified carbonate beds were being broken up, reworked and re-deposited in place or after only very short transport. The bored pebbles and shell debris indicate repeated pauses in sedimentation, colonization of firm seafloor and subsequent erosion. It is therefore a marker of intermittent condensation within the larger White Lias succession.

Bed PB8 – Pinhay Bed 9, Main Slump Bed (c. 1.37 m)

This is the most conspicuous single deformation unit in the Pinhay Bay White Lias. It consists of slumped limestone with abundant clasts, some up to boulder size, concentrated especially toward the base. The unit has long been recognised as the “Main Slump Bed” and is one of only two obvious marker beds that can be correlated readily between Pinhay Bay and Charton Bay. The large clasts and chaotic internal fabrics show failure and remobilization of already lithified or partly lithified carbonate sheets on a very shallow shelf or tidal-flat surface.

Bed PB9 – Pinhay Beds 10–11, Upper Finely Wavy Laminated Limestones

Above the Main Slump Bed, the succession becomes more regularly bedded again and is dominated by finely wavy laminated limestones, in places with marl partings and minor local clasts or hardground fragments. These upper beds are more uniform than the slump-rich lower part of the member, but they still preserve evidence of early lithification and interrupted sedimentation. Their fine lamination and purity show renewed quiet-water micrite deposition after the instability of the Main Slump Bed.

Bed PB10 – Pinhay Bed 12, Sun Bed And Locally Replacing Scour Hollows

The top of the Langport Member is the Sun Bed, a thin wavy laminated limestone forming the final White Lias surface beneath the Blue Lias. At most localities this surface is marked by the classic Diplocraterion burrows of the Sun Bed, but at Pinhay Bay it may be locally absent and replaced by scour hollows infilled with brecciated limestone. Detailed sedimentological work has shown that the uppermost White Lias at Pinhay first developed a burrowed firmground and marine hardground, was then eroded and reworked into an intraformational conglomerate, underwent brief subaerial emergence with fissuring and pitting, and was finally drowned beneath the Paper Shale of the Blue Lias. It is one of the most important boundary surfaces on the British Triassic–Jurassic coast.

Total Thickness Of The Langport Member At Pinhay Bay: About 7.84 Metres Recorded, With The Complete Local Succession Probably Close To 8.0 Metres

LIAS GROUP

Blue Lias Formation (Latest Rhaetian? To Hettangian)

Blue Lias Note

At Pinhay Bay the base of the Blue Lias is one of the most critical boundary intervals in southern Britain. The formation begins sharply above the Sun Bed with the Paper Shale of Bed H1. The lowest 2.7 m of the Blue Lias at Pinhay Bay are the traditional Pre-planorbis or Ostrea Beds, but the first unequivocal psiloceratid ammonites do not appear until Bed H25, so the lowest Blue Lias beds here may in part still represent the very latest Triassic or the earliest pre-planorbis phase immediately below the conventional Jurassic ammonite boundary.

Bed PB11 – Bed H1, Paper Shale

The Paper Shale is a laminated, organic-rich, silty shale resting sharply on the irregular top of the White Lias and draping fissures, hollows and small erosional relief on the Sun Bed surface. It is dark, finely laminated and rich in pyrite, including abundant small framboids that indicate sulphidic bottom-water conditions. Fossils are almost absent except for a single bedding plane crowded with disarticulated valves of Modiolus minimus and echinoid spines. The bed has yielded articulated and associated Diademopsis material in the Pinhay Bay succession and is of exceptional sedimentological significance because it records the rapid marine transgression and abrupt establishment of an anoxic lower water column after brief emergence of the White Lias surface.

Bed PB12 – Beds H2–H5, Basal Limestone–Shale Couplets Above The Paper Shale

Bed H2 is a limestone with fine lamination at its base and top but strong bioturbation and a shelly fauna dominated by Liostrea through its centre. Bed H3 is an overlying laminated organic-rich shale, locally cut out where a dome of limestone rises from the top of H2. Bed H4 is another limestone, again laminated at base and top but bioturbated in the middle, and Bed H5 is a further finely laminated organic-rich shale with a thin bioclastic limestone parting. These first couplets show the beginning of the classic Blue Lias rhythm, alternating between more oxygen-poor laminated shale deposition and more oxygenated, bioturbated limestone phases. At Pinhay Bay they are also affected by syn-sedimentary folding and small westward-downthrowing growth faults.

Bed PB13 – Beds H6–H24, Higher Pre-planorbis / Ostrea Beds (Total Pre-planorbis Succession c. 2.7 m)

The rest of the Pre-planorbis Beds continue the limestone–mudstone rhythm established in H1–H5. Oysters, especially Liostrea hisingeri, are abundant at several levels, which is why this package was long known as the Ostrea Beds. Articulated or associated Diademopsis tests and spines are known from H6 as well as H1 and H3, and these lowest Blue Lias echinoid-bearing levels lie below the first ammonites in the Pinhay Bay succession. The lithology and faunal evidence together suggest repeated alternation between anoxic laminated deposition and more oxygenated seabed colonization on a shallow but fully marine shelf. Because the first psiloceratids are not yet present, the higher part of this interval remains crucial to debate over the precise practical position of the Triassic–Jurassic boundary in east Devon.

Bed PB14 – Beds H25–H29, Earliest Ammonite-Bearing Boundary Interval

The first known psiloceratids at Pinhay Bay occur in Bed H25. They are small, crushed, poorly preserved forms associated with a burrowed horizon filled by greyish white marl just below Bed H26; they are thought most likely to be Neophyllites rather than the traditionally expected Psiloceras erugatum fauna. Small evolute psiloceratids are also present in Bed H26. For this reason the base of the Jurassic System in the Pinhay Bay succession is still most cautiously placed somewhere within Bed H25, and the burrowed horizon below the first ammonites may represent a small non-sequence. Higher in the same lower ammonite-bearing package, about 0.18 m above the base of Bed H29, a typical plicatulum-level fauna including Psiloceras cf. plicatulum and P. ex grp. sampsoni marks a more secure early Hettangian ammonite horizon.

Higher Blue Lias Note

Higher Hettangian Blue Lias continues eastwards from Pinhay Bay around Seven Rock Point toward Lyme Regis, and cliffs on the west side of the bay also enter higher Blue Lias levels. Those beds yield further planorbis-, johnstoni- and liasicus-chronozone faunas in the wider district, but they are not forced into this page because the core geological importance of Pinhay Bay itself is the Penarth–basal Blue Lias boundary section rather than the full later Hettangian cliff log of the Lyme Regis coast.

Depositional Environment

The Pinhay Bay succession records a major environmental shift from latest Triassic marginal marine muds and lagoonal limestones into earliest Jurassic offshore mudstone–limestone rhythms. The Westbury Formation represents quiet-water brackish to shallow-marine mud deposition following transgressive reworking at its base. The Cotham Member records shallow brackish lagoonal conditions with stromatolitic and shell-bearing beds. The Langport Member, traditionally the White Lias, formed in a broad very shallow marine seaway of slightly abnormal salinity, with repeated early lithification of micritic carbonate mud, hardground development, burrowing, erosion, slumping, brecciation and brief emergence of the sea floor. The contact with the Blue Lias is highly dynamic, recording marine hardground formation, local reworking into intraformational conglomerate, short-lived subaerial exposure and then rapid transgression. The Paper Shale and lowest Blue Lias were deposited under an anoxic to euxinic lower water column, later passing into the repeated alternation of laminated, more oxygen-poor shales and more bioturbated limestones that typifies the basal Blue Lias rhythms.

Total Thickness Covered Here: About 5.0 Metres Of Westbury Formation, About 1.5 Metres Of Visible Cotham Member, About 7.8–8.0 Metres Of Langport Member, And At Least The Lowest 2.7 Metres Of The Blue Lias Pre-planorbis Beds Plus The Basal Ammonite-Bearing Hettangian Interval

References

Hallam, A. (1960). The White Lias of the Devon Coast.
Wignall, P.B. (2001). Sedimentology of the Triassic–Jurassic boundary beds in Pinhay Bay (Devon, SW England).
Gallois, R.W. (2007). The stratigraphy of the Penarth Group (late Triassic) of the east Devon coast.
Page, K.N. (2002). A review of the ammonite faunas and standard zonation of the Hettangian and Lower Sinemurian succession of the east Devon coast.
Page, K.N. & Paul, C.R.C. (2017). Articulated echinoids from the basal Blue Lias Formation near Watchet, with discussion of equivalent low Blue Lias horizons at Pinhay Bay.
British Geological Survey Lexicon of Named Rock Units: Westbury Formation, Lilstock Formation, Cotham Member, Langport Member and Blue Lias Formation.
Geological Conservation Review account: Pinhay Bay, Devon, in British Upper Triassic Stratigraphy.
Woodward, H.B. & Ussher, W.A.E. (1906, 1911). The Geology of the Country near Sidmouth and Lyme Regis.