Helwell Bay (Doniford) Fossil Hunting

Early Jurassic fossils at Helwell Bay – among the oldest ammonites recorded in Britain
Helwell Bay is well known for yielding very early Jurassic ammonites, including smooth Psiloceras forms regarded as among the oldest ammonites recorded in Britain. This gives the locality real importance within the earliest Jurassic of Somerset.

Modern recognition of Helwell Bay as a key ammonite locality
Helwell Bay became especially well known to collectors for its abundant white ammonites, iridescent ammonites, bivalves and occasional reptile remains. These fossils, exposed on the foreshore and in fragile shales and limestones near the cliffs, helped establish the bay as one of the best known fossil beaches in the Watchet area.

2021 – unusual nautilid-dominated beds discovered at Helwell Bay
Recent work identified two adjacent limestone beds at Helwell Bay in which nautilids dominate the cephalopod assemblage and ammonites are almost absent. These unusual beds, described as “Cenoceras islands”, are important because they differ markedly from the normal ammonite-rich Lower Jurassic faunas of the area.

2023 – large arietitid ammonite recorded in situ at Helwell Bay
In August 2023, visitors discovered a large and unusually complete arietitid ammonite at Helwell Bay. Although too fragile to remove safely, it was recorded and photographed in situ, making it one of the more notable recent fossil finds from the beach.

Section Architecture

Helwell Bay and Doniford do not form one simple continuous cliff log. The Rhaetian boundary beds are best seen at very low tide on the Doniford foreshore east of Watchet, whereas Helwell Bay itself exposes much higher Blue Lias and basal Charmouth Mudstone in low cliffs and foreshore platforms north of the railway line and beside the revetment and groynes. Pleistocene gravels and head locally cap or obscure the western cliffline, and modern beach cover can hide large parts of the bedrock for long periods.

Structural Note

This is a faulted composite locality. Rhaetian strata on the Doniford foreshore are folded and heavily faulted; the Doniford Bay Fault brings lower Sinemurian strata down against Mercia Mudstone, and the Helwell Bay Fault throws Blue Lias against Mercia Mudstone farther west. The result is that upper Triassic and lower Jurassic rocks are juxtaposed in short coastal distances, and some intervals are repeated, truncated or concealed. The succession below therefore follows published bed numbers where they exist, but it is not presented as a falsely continuous single face.

MERCIA MUDSTONE GROUP

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

Rydon Member

Bed HB1 — Highest Exposed Rydon Member Mudstones And Siltstones

The highest exposed Blue Anchor strata at Doniford consist of green to dark grey dolomitic mudstones and siltstones with thin laminae, local dolomitized siltstone beds and the evaporitic overprint typical of the upper Mercia Mudstone Group. In regional terms these beds correspond to the Tea Green Marl–Grey Marl interval of older usage, but at Doniford they are seen only in fault slices beneath the Rhaetian marine beds. Their lithology and sparse marine influence indicate a subtidal to supratidal sabkha or ephemeral coastal-flat setting, more marine than the classic red Mercia mudstones below but still markedly restricted.

Williton Member

Bed HB2 — Williton Member / Sully Beds Equivalent

Immediately beneath the basal Westbury bone bed at Doniford is a thin but distinctive muddy, calcite-cemented limestone bed rich in Rhaetavicula contorta. This is the local expression of the Williton Member, the North Somerset and South Wales transgressive unit formerly called the Sully Beds. Its erosive top and shelly, firmground character show that it records the earliest marine incursion before the main Westbury transgression. At Doniford it is much thinner than at some other Somerset sections, but it remains one of the most important indicators that the marine flooding began before the classic basal bone-bed lag was laid down.

PENARTH GROUP

Westbury Formation (Rhaetian)

Bed HB3 — Lower Bone Bed (c. 5 cm, But Variable)

The lower Doniford bone bed is a black sandy marl with an eroded upper surface and local conglomeratic development where marly limestone intraclasts have been ripped up from below. It is dominated by well-rounded quartz grains, mostly 0.1–2 mm across, with wavy lamination and a thin calcitic matrix; small black and brown abraded bones and teeth are common. The bed rests on a bored and burrowed hardground at the top of the Blue Anchor Formation, and it is best interpreted as the principal lag of the initial Rhaetian transgression. Typical fossils are fish teeth and scales with rarer reptile material, all showing abrasion and reworking. This is one of the classic Somerset basal bone-bed facies.

Bed HB4 — Intervening Basal Westbury Shales And Limestones (c. 50–60 cm)

Above the lower bone bed lies an interval of black shales, laminated marly limestone and more massive limestone. The beds are pyritic, dark and clearly marine, but they still retain the variable, storm-influenced and restricted character typical of the lower Westbury Formation. Sedimentary structures and lithological contrasts suggest alternating quiet muddy deposition and brief higher-energy events. This short interval is important because it separates the two Doniford bone beds and shows that the upper bed is not simply a reworked residue from the lower one.

Bed HB5 — Upper Bone Bed (c. 6.5 cm)

The upper bone bed is a black siltstone to fine sandstone with wavy bedding and isolated vertebrate remains. Unlike the lower bone bed it lacks conspicuous intraclasts, contains much less calcite, and its fossils are more sparsely distributed within a finer, darker matrix. Some larger fossils are pyritized, including reported ichthyosaur material. This bed represents a second concentration horizon within the lower Westbury, distinct in both lithology and faunal taphonomy from the lower bone bed, and probably formed during a separate reworking event under restricted shallow-marine conditions.

Bed HB6 — Main Westbury Shale–Limestone–Beef Succession

Above the upper bone bed the Westbury Formation continues as grey to black laminated pyritic shales, mudstones and fine sandstones with occasional silty or muddy limestones, commonly associated with fibrous calcite “beef”. Ripple marks and trace fossils show that these were deposited in shallow water, but the strongly pyritic, dark character indicates restricted bottom conditions and at least episodic oxygen deficiency. Bivalves, fish debris and shell-rich horizons occur patchily. The overall environment was a shallow marine embayment or shelf-lagoon system with rapid fluctuations in salinity, water depth and oxygenation, probably intensified by storm events.

Lilstock Formation (Rhaetian)

Cotham Member

Bed HB7 — Basal Contorted Cotham Bed

The base of the Cotham Member at Doniford is marked by a disturbed bed with contorted bedding, long recognized as a laterally traceable deformed unit and commonly interpreted as a seismite. Greenish-grey calcareous mudstones and siltstones are internally disrupted and rest abruptly above the darker Westbury marine beds. This bed records an abrupt change from the open restricted-marine Westbury setting to very shallow brackish Cotham conditions, accompanied by synsedimentary disturbance, dewatering or slump-like deformation.

Bed HB8 — Lower Cotham Calcareous Mudstones And Siltstones

The lower and middle Cotham succession at Doniford consists of greenish-grey calcareous mudstones and siltstones with occasional argillaceous limestones, the limestone proportion increasing upward. The beds are cyclic and were deposited under very shallow brackish to lagoonal conditions during a prolonged phase of lower relative sea level. Fossils are much sparser than in the Westbury and the facies is more restricted. The rhythmic alternation of mud-rich and carbonate-rich beds shows repeated shifts between quieter water, firmer substrate and short-lived exposure-prone shallows.

Bed HB9 — Desiccation-Cracked Sandstone / Calcarenite Level

At one level in the Cotham succession a more arenaceous bed occurs, represented by a thin sandstone or calcarenitic horizon with evidence of subaerial exposure, including desiccation cracking. This is one of the clearest indicators that the Doniford Cotham beds were deposited on very shallow flats or lagoon margins that periodically dried out. It marks a brief higher-energy influx within an otherwise mud-dominated restricted succession and is an important environmental marker even where the exact bed geometry is obscured by weathering or faulting.

Bed HB10 — Upper Cotham Porcellanous Limestones And Burrowed Top Bed

The upper Cotham beds comprise greenish-grey mudstones with laterally variable porcellanous and argillaceous limestones, including pale dense micritic beds comparable to the more conspicuous marker limestones seen farther east at St Audries Bay. The top of the member is capped by a strongly bioturbated limestone with abundant burrow systems and hardground development. This uppermost Cotham interval shows repeated shallowing, firmground development and condensation before the distinctive White Lias facies of the Langport Member was established.

Langport Member

Bed HB11 — Langport White Lias Limestones And Sun Bed

The Langport Member at Doniford is better developed than at some other British boundary sections and consists of light grey laminated micritic limestones with rare calcareous shales. The limestones are dense, pale and splintery, and several are capped by desiccated erosional surfaces showing repeated brief emergence. Corals indicate clearer and more oxic water than in the underlying Cotham or Westbury beds, yet the fauna remains low-diversity and the environment was still highly stressed. The top bed has long been called the “Sun Bed” because of its dried and cracked appearance and is the most distinctive uppermost Penarth marker at the locality.

Bed HB12 — Upper Marly Langport Interval / Former Watchet Beds (c. 0.5 m Near Watchet)

Above the pale White Lias limestones lies a thin marly interval of grey fissile and locally bituminous calcareous mudstones with occasional silty limestones. Older literature often separated these as the Watchet Beds or Watchet Mudstone, but they are retained here as the upper muddy part of the Langport Member in keeping with current broad usage. Around Watchet and Doniford the interval is only about half a metre thick. It records renewed deepening, greyer marine mud deposition and the first signs of the darker organic-rich facies that become more fully established in the basal Blue Lias.

LIAS GROUP

Blue Lias Formation (Latest Rhaetian To Lower Sinemurian)

Blue Lias Numbering Note

The standard west Somerset bed numbers of Whittaker & Green are retained below where they are well established in the literature. Doniford provides the key basal Hettangian ammonite succession, whereas Helwell Bay exposes much higher Blue Lias and the transition into Charmouth Mudstone facies. The sequence is therefore composite: it uses formal published bed numbers where available, but it follows the beds laterally across foreshore and low cliff sections rather than pretending that Helwell Bay and Doniford form one uninterrupted wall section.

Bed HB13 — Beds 1–7, Pre-planorbis Beds (c. 5 m)

The basal Blue Lias at Doniford consists of calcareous silty mudstones, laminated “paper shales” and thinner argillaceous micritic limestones. These lowest beds are largely ammonite-barren in the classical sense, though rich in marine bivalves such as Liostrea hisingeri, and they record the first establishment of a more open offshore marine regime above the Penarth boundary facies. The lithology already shows the classic Blue Lias alternation of mudstone and limestone, but the basal interval still carries the latest Rhaetian to earliest Hettangian boundary signature and remains central to discussion of the practical base of the Jurassic in Britain.

Bed HB14 — Beds 8–9, Lowest Psiloceratid Interval

Bed 8 has yielded the earliest known psiloceratids in the west Somerset coastal succession, including weakly ribbed forms referred to Psiloceras cf. erugatum, while Bed 9 has yielded cf. Neophyllites and very early psiloceratid material. Lithologically these beds are still part of the lower Blue Lias alternation of laminated mudstones and rather nodular limestones, but biostratigraphically they are crucial because they demonstrate that ammonites enter before the main Psiloceras planorbis interval higher in the succession.

Bed HB15 — Beds 13–19, Main Psiloceras planorbis Interval

These beds contain abundant unequivocal Psiloceras planorbis and form the main lower Hettangian marker interval at Doniford. The rocks remain typical Blue Lias mudstone–limestone alternations, but the ammonite fauna is now well established and provides one of the best regional correlations in the basal Jurassic. Associated fossils include common bivalves and small echinoids; articulated Diademopsis tests and spines are known from beds within this lower Planorbis succession. The interval is fully marine, offshore and low-energy, with periodic limestone formation during pauses in mud accumulation.

Bed HB16 — Beds 20–24, Upper Planorbis Subzone And Laminated Bed 24

The upper part of the Planorbis succession becomes more mudstone-rich and culminates in the famous Bed 24, more than 2 m of indurated laminated mudstone packed with crushed iridescent ammonites of the Psiloceras planorbis group. The upper part of the bed has also yielded more bluntly ribbed forms referred to Psiloceras ex gr. plicatulum. Bed 24 is one of the most distinctive lower Jurassic fossil beds on the Somerset coast, and it is a leading candidate source for some of the historic nacreous Watchet-area ammonites, though exact old provenance is not always certain. The fine lamination indicates unusually quiet bottom-water conditions with rapid fine-sediment settling and limited disturbance.

Bed HB17 — Beds 25–37, Johnstoni Subzone And Lower Caloceras Interval

Bed 25 marks the first appearance of Caloceras and therefore the base of the Johnstoni Subzone. Through beds 25–37 the Blue Lias continues as alternating mudstones, laminated shales and limestones; Caloceras johnstoni occurs in Bed 36 and C. intermedium in Bed 37. This is a key interval for detailed west Somerset Hettangian subdivision and is one of the horizons that helped establish Doniford as the superior ammonite section to St Audries Bay for the lowermost Jurassic biostratigraphy. Fossils are commonly flattened in the darker mudstones and better preserved in the limestones.

Bed HB18 — Higher Blue Lias Continuation At Doniford And Helwell Bay

Above the basal Hettangian psiloceratid succession, the Blue Lias continues through many tens of metres of alternating medium- to dark-grey calcareous blocky mudstones, laminated bituminous shales and hard blue-grey limestones. In the wider west Somerset standard section these higher beds span the later Hettangian and pass upward into lower Sinemurian strata, but within the Helwell Bay–Doniford locality they are exposed as faulted slices rather than a single measured continuous run. The faunas diversify upward and include Arnioceras, Coroniceras, Paracoroniceras, oysters and rare vertebrate remains, while the lithology becomes increasingly mudstone-dominated toward the uppermost Blue Lias facies seen at Helwell Bay.

Bed HB19 — Helwell Bay Upper Blue Lias Beds Below The Main Cenoceras Bed

In Helwell Bay the upper Blue Lias is exposed in low cliff sections and east–west foreshore ledges as gently northward-dipping dark shales, paler grey calcareous mudstones and occasional persistent limestones. The beds immediately below the main nautilid horizon include grey-black shale, blocky grey mudstone and calcareous mudstone, with bioturbation and local uneven limestone tops. They lie in the lower Sinemurian Semicostatum Chronozone and lead up toward the local Lyra Subchronozone ammonite horizons that characterize the Blue Lias–Charmouth transition in west Somerset.

Bed HB20 — Main Cenoceras Bed (Whittaker & Green Bed 228 / Palmer Bed E(V))

This is the most distinctive Helwell Bay bed for lower Sinemurian collectors and palaeoecologists. It is a persistent limestone horizon crowded in places with nautilids referred to Cenoceras, many lying on bedding surfaces and commonly associated with encrusters that formed small shell-supported “islands” above the surrounding sediment. Ammonites are relatively sparse compared with the sheer abundance of nautilids. The bed records a phase of slow sedimentation or localized shell concentration on the sea floor rather than a simple storm lag, and it has become one of the best-known faunal marker beds in the upper Blue Lias of west Somerset.

Bed HB21 — Intervening Mudstones And Upper Cenoceras Bed (Bed 231; c. 0.7 m Above Bed 228)

About 0.7 m above the Main Cenoceras Bed lies a second nautilid-rich horizon, the Upper Cenoceras Bed. The intervening beds are grey mudstones and calcareous mudstones with minor limestones, and the upper bed again yields abundant Cenoceras with relatively few ammonites. Together the two beds define a short interval of unusual nautilid dominance within the lower Sinemurian Blue Lias. Their limited thickness range, persistence and strong faunal contrast make them exceptionally useful local markers in the Helwell Bay upper foreshore sequence.

Bed HB22 — Doniford Shales, Uppermost Blue Lias Transitional Interval

The highest local Blue Lias facies in the Helwell Bay–Doniford area are Palmer’s Doniford Shales, transitional beds between the limestone–mudstone alternations of the Blue Lias below and the more uniform Charmouth Mudstone facies above. They comprise dark shales, paler calcareous mudstones and thin limestones. The highest ammonite fauna in this interval represents Sn15b within the Lyra Subchronozone, and the beds are closely associated with forms such as Coroniceras lyra. They are important because they show that on the west Somerset coast the Blue Lias–Charmouth boundary is relatively complete rather than strongly condensed.

Charmouth Mudstone Formation (Lower Sinemurian)

Black Ven Marl Member (Local Helwell Marls At Base)

Bed HB23 — Basal Helwell Marls / Lowest Black Ven Marl Member

The basal Charmouth Mudstone at Helwell Bay is represented by Palmer’s Helwell Marls, now best treated as the lowest local expression of the Black Ven Marl Member. These beds are dark grey thinly interbedded mudstones and organic-rich mudstones with a few thin muddy limestones or doggers, markedly less rhythmic than the Blue Lias below. The lowest fauna represents Sn16, and low levels in the Helwell Marls have yielded Arnioceras bodleyi. This marks the formal incoming of true Charmouth Mudstone facies on the west Somerset coast.

Bed HB24 — Higher Helwell Marls

Higher in the Helwell Marls the mudstones remain dark, fissile and marl-rich, with only sparse limestone development. The next fauna upward represents Sn17a and includes early Paracoroniceras together with Arnioceras. These beds continue the lower Sinemurian marine mudstone regime and show that the Blue Lias–Charmouth transition at Helwell Bay is stratigraphically fuller than in many more condensed south-western sections. In practical field terms they form the highest Jurassic mudrocks normally examined at the site.

Depositional Environment

The Helwell Bay–Doniford succession records the late Triassic to early Jurassic marine flooding of the Bristol Channel Basin in exceptional detail. The upper Blue Anchor Formation represents restricted sabkha and coastal-flat sedimentation with increasing marine influence upward, culminating in the Williton Member transgressive shell bed. The Westbury Formation records shallow restricted marine conditions, commonly pyritic and probably at times anoxic, with storm-reworked and transgressive bone-bed lags at its base. The Cotham Member marks very shallow brackish lagoonal to tidal-flat conditions with contorted beds, cyclic lime-mud deposition and local emergence; the Langport Member records clear-water restricted carbonate sedimentation repeatedly interrupted by desiccation, followed by the marlier upper Langport or Watchet facies and renewed deepening. The Blue Lias then records establishment of an open offshore early Jurassic sea with alternating mudstone and limestone, increasingly rich ammonite faunas and locally exceptional nacreous preservation in laminated mudstones. At Helwell Bay the uppermost Blue Lias passes into the more persistent marine mudstones of the Charmouth Mudstone Formation, showing continuation of lower Sinemurian offshore deposition rather than abrupt local omission.

Total Thickness Note

The uppermost Blue Anchor and Penarth boundary succession at Doniford is only a few to several metres thick in any one fault slice, but it preserves a highly detailed Rhaetian transgressive record. Above it, the wider Watchet–Helwell coastal tract represents nearly 205 m of Blue Lias and lower Charmouth Mudstone strata in published composite logs, although at Helwell Bay and Doniford these lower Jurassic beds are exposed only in faulted and partly beach-masked slices rather than as one continuous measurable face.

References

Tayler, J., Duffin, C.J., Hildebrandt, C., Parker, A. & Benton, M.J. (2023). Geology and microvertebrate faunas of the Rhaetian Westbury Formation of Doniford Bay, Somerset.
Whittaker, A. & Green, G.W. (1983). Geology of the Country around Weston-super-Mare.
Mayall, M.J. (1981, 1983) on the Williton Member, Blue Anchor Formation and Cotham sedimentology of west Somerset.
Gallois, R.W. (2009). The lithostratigraphy of the Penarth Group (late Triassic) of the Severn Estuary area, UK.
Warrington, G., Ivimey-Cook, H.C. & Cope, J.C.W. (1994), and Warrington, G. and co-authors (2008), on the St Audrie’s Bay–Doniford Bay boundary succession and Jurassic boundary proposal.
Page, K.N. & Bloos, G. (1998); Bloos, G. & Page, K.N. (2000a, 2000b, 2002) on the basal Hettangian and lower Sinemurian ammonite succession of west Somerset.
Page, K.N. (1994). On the sequence of ammonite-correlated chronostratigraphical horizons in the British Sinemurian.
Page, K.N. (2009). High resolution ammonite stratigraphy of the Charmouth Mudstone Formation (Lower Jurassic: Sinemurian–Lower Pliensbachian) in south-west England, UK.
Page, K. & Paul, C.R.C. (2017). Articulated echinoids from the basal Blue Lias Formation (Lower Jurassic) near Watchet, Somerset, England.
Paul, C.R.C. (2018). An unexpected crinoid–cephalopod association from the Blue Lias Formation (Lower Sinemurian, Lower Jurassic) near Watchet, Somerset, England.
Evans, D.H. & King, A.H. (2019). ‘Cenoceras islands’ in the Blue Lias Formation (Lower Jurassic) of West Somerset, UK: nautilid dominance and influence on benthic faunas.
British Geological Survey Lexicon of Named Rock Units: Blue Anchor Formation, Williton Member, Westbury Formation, Cotham Member, Langport Member, Blue Lias Formation, Charmouth Mudstone Formation and Black Ven Marl Member.
Geological Conservation Review accounts: Blue Anchor–Lilstock Coast and Doniford.