St Audries Bay Fossil Hunting

1824 – Psiloceras planorbis named from the west Somerset coast
The classic early Jurassic ammonite Psiloceras planorbis was named by Sowerby in 1824, and St Audries Bay is now recognised as the type locality for this important species. Because Psiloceras planorbis became the index fossil of the Planorbis Zone, the locality gained major importance in Jurassic stratigraphy.

1911 – Richardson published the classic description of the St Audries Bay succession
Richardson’s work provided one of the key early detailed accounts of the St Audries Bay section, including the uppermost Triassic beds, the transition into the Blue Lias, and the local marker beds used by later workers. This became one of the foundation studies for the geology of the bay.

1969 – the St Audries Bay area was discussed as a possible global standard for the base of the Jurassic
By the late 1960s, the importance of the St Audries Bay succession had grown to the point that it was being discussed as a possible global reference section for the base of the Jurassic System. This reflected the value of the locality for tracing the appearance of the earliest Jurassic ammonites.

1994 – St Audries Bay formally proposed as a candidate GSSP for the base of the Jurassic
In 1994, the St Audries Bay section was formally proposed as a candidate Global Stratotype Section and Point for the base of the Jurassic System. This was one of the most important scientific milestones in the history of the locality and confirmed its international value.

1995 – the bay’s importance for the base of the Hettangian and the planorbis Zone was reinforced
Further work in 1995 reinforced the importance of St Audries Bay as the type locality for Psiloceras planorbis and as one of the key British sections for the base of the Hettangian Stage. This cemented the bay’s place as one of the most important Lower Jurassic reference sites in Britain.

Section Architecture

St Audries Bay is a composite cliff-and-foreshore section rather than a single simple vertical cliff log. The beds dip south-west, so progressively younger strata are exposed from east to west; the upper Mercia Mudstone Group dominates the eastern cliffs, the Penarth Group is best seen on the western side of the bay and on the foreshore, and the basal Blue Lias is most easily followed west of St Audries Slip and into the immediate westward continuation of the coast. Small faults, active landslip and foreshore stripping continually refresh the exposure, but they can also cause local repetition, omission and concealment.

Boundary And Nomenclature Note

The former White Lias Formation and Watchet Mudstone Formation used in some Somerset papers are treated here within the Langport Member of the Lilstock Formation, following current BGS usage. St Audries Bay was long discussed as a candidate reference section for the base of the Jurassic, but it is best treated as a key British boundary section rather than forced into a single globally definitive boundary horizon.

MERCIA MUDSTONE GROUP

Branscombe Mudstone Formation (Upper Triassic: Norian–Rhaetian)

Bed SA1 — Upper Branscombe Mudstone Formation (c. 67–68 m Seen)

The highest exposed Branscombe Mudstone Formation at St Audries Bay consists of red-brown to reddish and green mudstones with a few green siltstone beds. The mudstones weather blockily, and gypsum or anhydrite was originally abundant in nodules and veins, much of it now weathered out or represented by cavities and softer disrupted zones in the cliff; older cliff descriptions simply treated these as the red and green mudstones beneath the Tea Green Marl. Small faults affect the section, so minor repetition and omission can occur. Interpretation: low-relief continental mudflat, playa-lake and sabkha deposition close to sea level under strongly evaporitic conditions; body fossils are scarce and the unit is chiefly important as the substrate to the boundary succession above.

Blue Anchor Formation (Upper Triassic: Latest Norian–Rhaetian)

Rydon Member

Bed SA2 — Rydon Member (c. 30–34 m; Published Logs Vary Slightly)

The Rydon Member forms the classic Tea Green Marl and Grey Marl interval of older Somerset usage. At St Audries Bay it consists of grey, black, green and occasional red-brown dolomitic mudstones with more resistant dolomite bands, especially in the higher part of the member, and with common gypsum crystals that have often dissolved to leave cavities and local collapse breccias. The overall colour change from the red Branscombe mudstones below to cooler green-grey dolomitic beds above is very distinctive in the cliff. Fossils are sparse through most of the member. Interpretation: saline to hypersaline lakes and mudflats on an evaporitic plain, with repeated early dolomitization and intermittent emergence.

Williton Member

Bed SA3 — Williton Member (c. 2–3 m)

The thin Williton Member rests disconformably on the Rydon Member across an erosional surface penetrated by Diplocraterion burrows. It consists predominantly of grey and dark grey shales and marls with flaser- and lenticular-bedded fine sandstone and silt, together with harder siltstones at the top; older literature commonly referred to these beds as the Sully Beds. This is the most overtly marine part of the Blue Anchor Formation at St Audries Bay and has yielded trace fossils, fragmentary gastropods, vertebrate remains including Gyrolepis and Hybodus, and one of the most diverse palynomorph assemblages of its age known from Britain in the uppermost metre. Interpretation: shallow marine to marginal-marine transgression across the evaporitic plain, with tidal or storm-reworked silt and sand laminae.

Total Thickness Of The Upper Mercia Mudstone Group Here: Approximately 104 Metres Seen, Comprising c. 67 m Of Branscombe Mudstone Formation And About 37 m Of Blue Anchor Formation, Though Minor Faulting Produces Local Repetition Or Omission

PENARTH GROUP

Westbury Formation (Rhaetian)

Bed SA4 — Basal Westbury Transgressive Shale And Bone-Bed Interval (Basal c. 0.36 m)

The basal Westbury Formation marks the main Rhaetian marine transgression at St Audries Bay. The lowest c. 0.36 m is black shale with minor bone-bed material and vertebrate debris, locally with reworked coarse particles and phosphatic grains, forming the classic transgressive lag at the base of the Penarth Group. Pyrite is common, and the fauna is already fully marine in aspect. Typical fossils from this basal transgressive interval and closely adjacent beds include scales and teeth of fishes such as Acrodus, Birgeria, Dalatias, Gyrolepis and Hybodus, together with rarer reptile debris. Interpretation: rapid marine flooding of the west Somerset shelf, with storm reworking and concentration of vertebrate remains into thin laggy horizons.

Bed SA5 — Main Westbury Shale–Limestone–Beef Cycles (Remaining c. 1.8–2.3 m; Full Composite c. 2.2–2.6 m)

Above the basal lag, the Westbury Formation consists of repeated black shale–dark limestone–beef cycles. Thinly laminated black and very dark grey shales with green marl wisps and partings alternate with hard dark grey earthy limestones, dark silty calcareous marl and several fibrous calcite “beef” seams with cone-in-cone structure. Crushed bivalves are common at many levels, and the fauna includes Chlamys, Protocardia and Rhaetavicula, together with ophiuroids, fish remains and marine reptile debris. Interpretation: shallow marine shelf sedimentation under fluctuating energy conditions, with low-energy organic-rich mud deposition punctuated by storm reworking, shell concentration and early diagenetic cementation.

Lilstock Formation (Rhaetian)

Cotham Member

Bed C1 — Basal Slumped Lag Bed (0.12–0.35 m)

This formally numbered basal Cotham bed is one of the most distinctive soft-sediment horizons in the whole St Audries Bay boundary section. It is highly variable laterally, made up mainly of greenish-grey calcareous mudstones and silty mudstones with irregular slumped masses of more strongly cemented mudstone, streaks and clasts of dark grey Westbury mudstone, and an irregular erosive base. A basal lag with fish debris, sand grains and small black phosphatic pebbles is developed locally. Interpretation: abrupt change from open-marine Westbury shelf muds to very shallow Cotham lagoonal sedimentation, accompanied by erosion, reworking and local slumping of still-unconsolidated substrate.

Bed C2–C4 — Lower Cotham Slumped Mudstone And Muddy-Limestone Package (c. 1.0 m)

Beds C2 to C4 comprise a lower Cotham alternation of muddy limestones and greenish-grey weakly calcareous mudstone. Bed C3 is laminated and thinly bedded with coarse-silt lenses and isolated ripple patches, and both limestone beds may be locally slumped or internally disturbed. The interval records repeated early cementation of parts of the substrate within otherwise soft lagoonal mud. Interpretation: shallow-water lagoon or broad mudflat with intermittent traction currents, local dewatering and slump deformation.

Bed C5 — Rippled Sandstone / Calcarenite With Desiccation Cracks (0.33 m)

Bed C5 is a key marker horizon in the Somerset Cotham succession. It is a fine- to medium-grained sandstone or calcarenite with shell debris and siltstone, showing stacked wave-ripple sets, wavy bedding and mudstone partings. Small desiccation polygons occur within the bed, and larger sand-filled cracks descend from its base into the beds below. Interpretation: very shallow lagoonal or tidal-flat sand sheet laid down under wave or current action and then exposed, cracked and locally reworked.

Bed C6–C9 — Upper Cotham Mudstones And Porcellanous Limestones (c. 1.33–1.47 m)

The upper Cotham succession is thicker on the Somerset coast than in the Penarth area and at St Audries Bay forms a greenish-grey mudstone and muddy-limestone package with two especially important marker beds. Bed C7 is a thinly bedded laminated porcellanous micritic limestone that splits into thin slabs and has long been compared with a Cotham Marble equivalent; Bed C8 is laterally very variable, passing between mudstone, muddy limestone and porcellanous limestone in alternating lenticular or nodular beds; Bed C9 returns to greenish-grey mudstone. Bed C6 below remains thinly bedded, with laminated clay-rich beds alternating with more calcareous beds. Interpretation: fluctuating shallow lagoonal deposition with periodic carbonate precipitation, local emergence and strong lateral facies variability.

Bed C10 — Burrowed Hardground Limestone (0.07 m)

Bed C10 is a thin but highly significant limestone at the top of the Cotham Member. It is argillaceous and strongly bioturbated, with abundant Thalassinoides and other burrow systems, and is capped by an irregular mineralized hardground with encrusting oysters. This surface marks a marked omission and forms the immediate foundation for the White Lias facies above. Interpretation: firmground to hardground development at the top of a very shallow lagoon or restricted shelf, following reduced sedimentation and marine reworking.

Langport Member

Bed SA6 — Basal Langport White Lias / Sun Bed Limestone Pair (0.20–0.21 m In The Platform Section)

At St Audries Bay the basal Langport Member begins with a dense micritic laminated porcellanous limestone pair, two beds about 0.10 m thick separated by a very thin calcareous mudstone that fills hollows on the irregular top of the lower limestone. This is the classic White Lias facies of the Somerset coast and corresponds to the pale Sun Bed interval below the basal Blue Lias mudstone. The rock is hard, splintery and locally conchoidal, and forms one of the sharpest and most laterally persistent marker beds in the entire boundary succession. Interpretation: warm, very shallow carbonate lagoon or emergent carbonate-flat deposition with rapid precipitation of fine micrite.

Bed SA7 — Upper Langport Calcareous Mudstones And Shelly Limestones (0.28 m In The Platform Section; Thicker In The Composite Cliff Log)

Above the White Lias lies the upper Langport muddy interval, historically separated by some authors as the Watchet Mudstone Formation but treated here within the Langport Member. On the St Audries wave-cut platform it comprises pale grey to pale grey-green calcareous mudstone infilling hollows on the White Lias, a tightly cemented argillaceous limestone with moulds of Liostrea, Modiolus and other bivalves, and a further pale calcareous mudstone at the top; in the classical composite cliff log the same interval expands into several green-tinged marls and hard splintery limestones with pyrite and calcite stringers. It forms the hard grey calcareous mudstone package immediately beneath the fissile basal Blue Lias shale. Interpretation: slightly deeper but still very shallow restricted marine shelf or lagoonal mud deposition above the White Lias carbonate-flat phase.

Total Thickness Of The Penarth Group At St Audries Bay: About 5.5–6 Metres In The Classical Composite Section, Although Individual Platform Measurements May Expose Only Part Of The Westbury Succession Beneath The Better-Seen Cotham–Langport Boundary Beds

LIAS GROUP

Blue Lias Formation (Hettangian; Basal Sinemurian In The Immediate Westward Continuation)

Blue Lias Bed Numbering Note

The numbered beds below follow the formal Whittaker & Green bed numbering used throughout the west Somerset Lower Jurassic literature. At St Audries Bay the lowest Blue Lias is best seen in the cliff and foreshore west of St Audries Slip, and because the beds dip south-west and are locally faulted, the succession is followed laterally rather than as one single simple vertical face.

Beds 1–7 — Traditional Pre-planorbis Beds (Basal c. 5 m)

Beds 1–7 are the traditional Pre-planorbis Beds of the St Audries Bay boundary section. At the very base, the Blue Lias begins at the sharp planar base of a fissile organic-rich mudstone resting on the hard grey calcareous mudstone at the top of the Langport Member; in the platform log this basal fissile mudstone is about 1.40 m thick and is capped by a 0.11 m tightly cemented bluish-grey argillaceous limestone. Higher beds in this lower package continue as mudstone–nodular limestone couplets. The macrofauna is dominated by bivalves, especially Liostrea hisingeri, with Camptonectes, Protocardia and Pteromya also recorded, but unequivocal ammonites were not recognized in the original scheme. Interpretation: normal-marine offshore shelf mud deposition with repeated early cementation into nodular limestones, representing the lowermost Blue Lias boundary interval.

Beds 8–12 — Lower Psiloceratid Boundary Interval

Beds 8–12 yield the lowest psiloceratids known from the St Audries Bay section and are critical to boundary work. Bed 8 has produced weakly ribbed Psiloceras, including forms referred to P. cf. erugatum, the lower part of Bed 9 has yielded cf. Neophyllites, and the upper part of Bed 9 has yielded ?Psiloceras cf. P. planorbis. Lithologically the interval still belongs to the basal Blue Lias rhythm of mudstones and rather nodular limestones, and preservation of ammonites is generally poorer than in the higher laminated mudstones. Stratigraphic significance: this is the lowest ammonite-bearing part of the St Audries Bay boundary succession and the level on which much of the British base-Jurassic discussion at the site has turned.

Beds 13–19 — Main Psiloceras planorbis Interval

Beds 13–19 contain unequivocal Psiloceras planorbis and form the classic lower Hettangian ammonite interval in the St Audries Bay section. The beds are alternating mudstones and limestones typical of the lower Blue Lias offshore facies, and are among the most important horizons for correlation within the wider Watchet district. Typical associated fossils in the lower Blue Lias include abundant bivalves such as Liostrea, Camptonectes, Protocardia and Pteromya, together with echinoids including Diademopsis and Eodiadema bechei. Ammonites: abundant Psiloceras planorbis. Interpretation: open offshore shelf sedimentation with periodic limestone formation and the first well-developed psiloceratid fauna of the area.

Beds 20–24 — Upper Planorbis Subzone, Including Bed 24

Beds 20–24 make up the upper part of the Planorbis Subzone and culminate in the famous laminated mudstone of Bed 24. This upper interval is more mudstone-rich, and Bed 24 itself comprises 2 m or more of hard indurated laminated mudstone packed with crushed iridescent ammonites referred to Psiloceras ex gr. planorbis; the upper part has also yielded bluntly ribbed forms referred to Psiloceras ex gr. plicatulum. Small pyritized colonies of Heterastraea, fish remains and occasional ichthyosaur material have been recorded from the Planorbis-zone mudstones. Stratigraphic significance: Bed 24 is one of the most distinctive and laterally useful lower Hettangian marker beds on the Somerset coast.

Beds 25–42 — Johnstoni Subzone (c. 3.4 m)

Bed 25 is a limestone band marking the first appearance of Caloceras and therefore the base of the Johnstoni Subzone. Beds 25–42, about 3.4 m thick, comprise limestones and mudstones, including hard laminated mudstone horizons that have yielded Caloceras johnstoni in Bed 36 and C. intermedium in Bed 37. The interval is less famous with collectors than the Psiloceras planorbis beds below, but it is crucial for detailed Hettangian subdivision in west Somerset. Interpretation: continued offshore Blue Lias deposition, with laminated mudstone episodes and recurring limestone development in a fully marine shelf setting.

Beds 43–79 — Liasicus Zone (c. 28 m)

The appearance of Waehneroceras sensu lato in Bed 43 marks the base of the Liasicus Zone. Beds 43–79 total about 28 m and are dominated, especially below about Bed 69, by mudstones with only subordinate limestones. Laqueoceras appears around Bed 67 and marks the boundary between the Portlocki and Laqueus subzones. The macrofauna becomes more diverse than in the Planorbis-zone beds below and includes Camptonectes, Gervillia, Lucina, Liostrea, Modiolus, Plagiostoma, Pseudolimea and the lowest local Gryphaea cf. obliquata. Interpretation: relatively expanded offshore mudstone deposition in the Bristol Channel Basin, with fewer limestones and a more mature early Jurassic marine benthos.

Beds 80–145 — Angulata Zone (c. 40 m To The Hettangian–Sinemurian Boundary Interval)

The base of the Angulata Zone is taken at the first appearance of Schlotheimia cf. amblygonia in Bed 80. From Bed 80 to Bed 145, a further c. 40 m of mudstones and nodular limestones are exposed in the wider St Audries Bay continuation, with the Complanata Subzone beginning at Bed 95 and the Depressa Subzone at Bed 134. The fauna is relatively sparse compared with lower Hettangian levels, but Gryphaea arcuata becomes increasingly common and the brachiopod Calcirhynchia calcaria is characteristic. These beds lead up to the Hettangian–Sinemurian boundary interval in upper Bed 145 west of the bay proper. Interpretation: continued open-marine Blue Lias deposition in a thick, basinward Hettangian succession.

Total Thickness Of The Blue Lias Formation Covered Here: About 80 Metres From Bed 1 To Bed 145 In The St Audries Bay–Immediate Westward Coastal Section, With Younger Lower Sinemurian Beds Continuing Farther West Toward East Quantoxhead And Lilstock

Depositional Environment

The St Audries Bay succession records a major environmental shift across the late Triassic–early Jurassic transition. The upper Branscombe Mudstone Formation represents continental playa-lake and sabkha deposition on a low-relief evaporitic plain; the Rydon Member continues that evaporitic regime in dolomitic lake and mudflat facies; the Williton Member records the first clear shallow-marine incursion. The Westbury Formation marks a fully marine transgression across the shelf, with black shales, storm-reworked shell debris, beef seams and vertebrate-rich lag deposits. The Cotham Member records a return to very shallow lagoonal, tidal-flat and intermittently emergent conditions, including dewatering, slumping and desiccation cracking, while the Langport Member formed in warm very shallow carbonate lagoons and restricted shelf settings. The Blue Lias Formation then records establishment of an offshore early Jurassic sea in the Bristol Channel Basin, with alternating mudstones and limestones, progressive development of ammonite zonation from the lowest psiloceratids upward, and one of the fullest British Hettangian successions known onshore.

Total Thickness Covered Here: Approximately 185–190 Metres Of Upper Triassic To Lower Jurassic Stratigraphy Across The St Audries Bay Cliff, Foreshore And Immediate Westward Continuation

References

Whittaker, A. & Green, G.W. (1983). Geology of the Country around Weston-super-Mare.
Mayall, M.J. (1979, 1981, 1983) on the sedimentology and depositional environments of the Blue Anchor Formation and Penarth Group at St Audries Bay.
MacQuaker, J.H.S. (1984, 1994) on Westbury Formation sedimentology, cyclicity and transgressive bone-bed reworking.
Warrington, G. & Whittaker, A. (1984) on the Blue Anchor Formation and Triassic–Jurassic boundary beds of west Somerset.
Warrington, G., Cope, J.C.W. & Ivimey-Cook, H.C. (1994) on St Audries Bay as a candidate reference section for the base of the Jurassic System.
Warrington, G. & Ivimey-Cook, H.C. (1995) on the Triassic and Jurassic boundary succession of the Blue Anchor–Lilstock coast.
Page, K.N. & Bloos, G. (1998), and Bloos, G. & Page, K.N. (2000a, 2000b, 2002), on the basal Hettangian ammonite succession and the Hettangian–Sinemurian boundary in west Somerset.
Hounslow, M.W., Posen, P.E. & Warrington, G. (2004) on magnetostratigraphy and biostratigraphy at St Audries Bay.
Gallois, R.W. (2009). Lithostratigraphy of the Penarth Group of the Severn Estuary region and Somerset coast.
Benton, M.J., Cook, E. & Turner, P. (2002). Permian and Triassic Red Beds and the Penarth Group of Great Britain — St Audries Bay GCR account.
Simms, M.J., Chidlaw, N., Morton, N. & Page, K.N. (2004). British Lower Jurassic Stratigraphy — Blue Anchor–Lilstock Coast GCR account.
British Geological Survey Lexicon of Named Rock Units: Branscombe Mudstone Formation, Blue Anchor Formation, Westbury Formation, Lilstock Formation, Cotham Member, Langport Member and Blue Lias Formation.