Wyke Regis Fossil Hunting

1875 – Blake discussed the boundary beds of the Wyke Regis and Fleet coast
J. F. Blake treated the uppermost Corallian and basal Kimmeridge Clay beds of the Wyke Regis coast as distinctive boundary beds, showing that this stretch of shore was already important in nineteenth-century work on the Oxfordian–Kimmeridgian transition.

1877 – Blake and Hudleston further documented the Kimmeridge passage beds of the Wyke Regis coast
Later nineteenth-century work by Blake and Hudleston refined understanding of the beds around Wyke Regis and the Fleet, helping establish the scientific importance of this coast for the uppermost Corallian and lowermost Kimmeridge Clay succession.

1947 – Arkell described the Weymouth district geology and recorded the fossil-rich beds of the Wyke Regis coast
By the mid twentieth century, Arkell’s Geological Survey memoir had helped fix the Wyke Regis and Fleet shore as classic Upper Jurassic coastal sections, with fossil-rich beds yielding oysters, other bivalves, gastropods and ammonites.

1978 – Birkelund and co-workers used the Fleet section in important work on Rasenia faunas
The East Fleet succession was used in important work on the ammonite genus Rasenia and the Lower Kimmeridgian Cymodoce Zone. This showed that the Wyke Regis–Fleet coast was a key section for understanding Upper Jurassic ammonite succession.

1981 – Cox and Gallois refined the basal Kimmeridge Clay succession of the Wyke Regis area
Detailed later twentieth-century work clarified the lithostratigraphy of the beds above the Corallian in the Wyke Regis and Fleet area, helping place the local fossil horizons into a more precise modern framework.

1999 – East Fleet and Small Mouth were formally treated as Geological Conservation Review sites
Modern review work recognised the East Fleet and Small Mouth sections as key localities because they expose the Baylei and Cymodoce zones of the basal Kimmeridgian together with the underlying Oxfordian. This confirmed the Wyke Regis west-shore coast as one of the most important Dorset sections for the basal Kimmeridgian boundary beds.

Modern collecting – the west side of Wyke Regis remains known for Corallian and Kimmeridge Clay fossils
Today the Chesil Beach and East Fleet shore west of Wyke Regis is still known for large oysters, plentiful bivalves, gastropods and ammonites, with occasional marine reptile bones and teeth also reported from the Kimmeridge Clay. Its importance lies more in the richness of the fossil horizons and the boundary succession than in a long list of famous single named finds.

Section Architecture

This is not a single continuous cliff log. The west Wyke Regis locality is best treated as two linked Fleet-side sections: first, the East Fleet Corallian reference section about 1 km west of Wyke Regis, where a gently dipping 14 m succession is exposed in foreshore and low cliff; and second, the low, slumped and weathered cliffs north-west of Ferry Bridge at the eastern end of The Fleet, where the uppermost Oxfordian and basal Kimmeridgian boundary beds are exposed patchily in low cliffs and on the foreshore. Modern protection from marine erosion by Chesil Beach and, farther east, Portland Harbour works has degraded many of the old exposures, so the locality must be handled honestly as a composite one.

Numbering Note

The WR numbers below are site-use numbers. Formal published bed numbers are retained exactly where they exist, especially Wright’s East Fleet Beds 1–13 and the classic named boundary beds of the basal Kimmeridge Clay. No attempt is made to invent a false single continuous numbered log across concealed or slipped ground between East Fleet and Ferry Bridge.

CORALLIAN GROUP

Osmington Oolite Formation (Upper Oxfordian)

Upton Member

Bed WR1 — Wright Beds 1–3, Burrowed Sandy Siltstones Of The Upton Member

The lowest exposed beds at East Fleet are highly bioturbated sandy siltstones and sandy mudstones forming the basal part of the 14 m Corallian section. Their most distinctive feature is the unusually well-preserved trace fossil Teichichnus, which is especially well shown here and makes East Fleet the key district locality for this facies. Macrofossils are comparatively subdued when compared with the shell-rich beds higher up, but the sedimentological message is clear: these beds represent shallow offshore shelf deposition with substantial clastic input, a more marginal and shallower-water development than the more open-marine equivalent seen farther east near Osmington.

Shortlake Member

Bed WR2 — Wright Bed 4, Lower Oolitic Shoal Bed

The lower Shortlake Member is dominated by oolite, here partly cross-bedded, and forms the first clearly higher-energy carbonate unit above the clastic Upton beds. The cross-bedding and ooidal texture indicate tidal or storm-reworked ooid shoals and small deltas, while the bed’s place within a strongly facies-variable westward Corallian tract suggests repeated migration of mobile carbonate sand bodies across a shallow marine shelf.

Bed WR3 — Wright Beds 5–7, Quartz Sands And Clays Of The Middle Shortlake Member

Quartz sand and clay succeed the lower oolite and form a mixed clastic interval in the middle Shortlake Member, with Bed 7 only about 0.9 m thick. These beds show that the ooid shoal phase was interrupted by renewed influx of fine quartz sand and mud. The alternation of sandier and muddier lithologies suggests repeated switching between current-swept shallow-water conditions and quieter lagoonal or shelf-floor accumulation, rather than a uniform carbonate platform setting.

Bed WR4 — Wright Beds 8–10, Fossiliferous Oomicrites Of The Upper Shortlake Member

Three beds of very fossiliferous oomicrite occur at the base of the low cliff and are among the most useful collector and palaeoecological horizons in the East Fleet Corallian section. Recorded fossils include the bivalves Myophorella, Plicatula, Lucina, Chlamys, Opis, Nanogyra nana and Ostrea, the gastropods Nerinea, Bourguetia, Pseudomelania, Ampullina, Dicroloma, Littorina and Procerithium, and the echinoid Nucleolites scutatus. These shelly micritic beds indicate an increasingly lagoonal and carbonate-mud-rich upper Shortlake setting, still marine but quieter than the ooid shoals below and rich in benthic life.

Nodular Rubble Member

Bed WR5 — Wright Bed 11, Nodular Rubble

The Nodular Rubble is exposed about 100 m to the south-east as three prominent concretionary limestone beds with nodules larger than those seen in equivalent strata at Osmington. These irregular nodular limestones form hummocky, resistant reefs and record a phase of early cementation and diagenetic concentration within otherwise shallow-marine carbonate sediment. The top of the unit is erosional and heavily burrowed, and it forms the substrate on which the lower Clavellata sands were later laid down.

Clavellata Formation (Upper Oxfordian)

Bed WR6 — Sandy Block Subdivision And Wright Bed 12

At East Fleet only the lower part of the Clavellata Formation is exposed, and the lower subdivision is the Sandy Block. A thin bed of fine quartz sand, about 0.07–0.14 m thick, wraps around the eroded hummocks of the Nodular Rubble, and sand-filled borings descend from above into the underlying limestones. This is followed by soft argillaceous sand and then by about 1.2 m of flaggy fine-grained sandstone, best seen near the centre of the bay. The basal quartz sand resting directly on the eroded carbonate surface is clear evidence of a non-sequential break and renewed clastic influx. These beds are markedly more marginal than the equivalent horizons at Osmington and represent shallow-water sandy marine conditions encroaching across an eroded carbonate bench.

Bed WR7 — Chief Shells Beds Subdivision And Wright Bed 13 (0.55 m)

The overlying Chief Shells Beds are represented at East Fleet by about 0.55 m of flaggy, immature, sandy oolite containing Myophorella clavellata and Nanogyra nana. Though thin, this bed is very important because it captures the classic clavellata fauna in a distinctly sandy, marginal westward facies. The reduction in quartz sand upward from the Sandy Block is noticeable, but the bed still belongs to a clastic-influenced shallow shelf or marginal-lagoon setting rather than to a fully open carbonate shelf.

Covered Interval Note

On the Fleet side, the passage from the lower Clavellata Formation at East Fleet into the highest Oxfordian boundary beds near Ferry Bridge is not continuously exposed. Much of the upper Clavellata Formation and much or all of the intervening Sandsfoot Grit Member are concealed, slipped or inaccessible on this side of Chesil, so the page deliberately uses a composite Fleet-side section rather than inventing missing beds.

Sandsfoot Formation (Upper Oxfordian; Partial Fleet-Side Exposure Only)

Ringstead Clay Member

Bed WR8 — Upper Ringstead Clay Member

At the eastern end of The Fleet, the low cliffs north-west of Ferry Bridge expose the highest part of the Ringstead Clay Member beneath the basal Kimmeridge section. Lithologically these beds are pale- to dark-grey variably calcareous mudstones, locally with fine calcareous laminae, passing upward into reddish silty mudstone with siltstone lenses and the characteristic red-weathering lens-shaped sideritic concretions of the member. Fossils are comparatively sparse, with bivalves, echinoids and rare ammonites recorded regionally. The beds dip gently and steadily south, are badly slumped, and are often best understood from their relative position on the foreshore rather than from any one clean cliff face.

Osmington Mills Ironstone Member

Bed WR9 — Osmington Mills Ironstone Member / Highest Oxfordian Ironshot Oolite

The youngest Oxfordian stratum in the Fleet-side Wyke Regis section is the pale ironshot oolite traditionally miscalled the “Ringstead Coral Bed”. At Wyke Regis it is a thin ferruginous ooidal bed full of fossils, notably serpulids and the coarsely corrugated bivalve Ctenostreon, but without true corals. This is a condensed hiatal surface bed formed at the top of the Sandsfoot Formation and represents the last Oxfordian marine condensation before the abrupt incoming of the basal Kimmeridge muds above.

Kimmeridge Clay Formation (Lower Kimmeridgian: Baylei And Cymodoce Zones)

Bed WR10 — Inconstans Bed (0.7 m)

The Inconstans Bed marks the base of the Kimmeridge Clay Formation and the Oxfordian–Kimmeridgian stage boundary. It is a very dark grey shelly and bioturbated mudstone with ferruginous ooids and phosphatic pebbles, resting sharply and non-sequentially on the Oxfordian ironstone below. The eponymous brachiopod Torquirhynchia inconstans characterizes the bed. This is the critical datum for the whole Wyke Regis west boundary succession and records abrupt deepening from the condensed Oxfordian ironstone into darker basal Kimmeridge mud deposition.

Bed WR11 — Nana Bed (0.25 m)

A thin but very distinctive bed rich in the small oyster Nanogyra nana. This is one of the classic named beds of the basal Kimmeridge in the Weymouth district and lies low in the Pictonia baylei Zone. The concentration of small oysters indicates a short-lived shelly event within otherwise muddy sedimentation, and the bed remains a practical field marker wherever the Fleet-side section is clean enough to be traced.

Bed WR12 — Lower Baylei-Zone Dark Grey Mudstones With Deltoideum delta (c. 3.0 m)

Dark grey mudstones above the Nana Bed are rich in the flat oyster Deltoideum delta. These beds represent the lower part of the Pictonia baylei Zone and are more argillaceous and darker than the pale mudstones above. They indicate a low-energy offshore muddy shelf with a reduced but characteristic benthic fauna dominated by oysters and scattered molluscs.

Bed WR13 — Upper Baylei-Zone Pale Grey Mudstones With Red-Weathering Tabular Clay Ironstones (c. 6 m Or More)

This is the thickest single visible package in the Fleet-side basal Kimmeridge section. Pale grey mudstones with thin red-weathering tabular clay ironstones overlie the darker Deltoideum-rich beds and still belong to the Pictonia baylei Zone. Pictonia is reported from beds below the Wyke Siltstone, and these pale muds represent a relatively expanded Baylei-Zone interval when compared with several other Dorset coastal sections. The thin tabular clay ironstones provide a useful practical marker where shell faunas are sparse.

Bed WR14 — Wyke Siltstone (Up To c. 1.0 m; KC5; Base Of The Cymodoce-Zone Succession In Dorset Practice)

The Wyke Siltstone is the most prominent single bed in the Fleet-side basal Kimmeridge succession. It is a thin siltstone crowded with myid and other bivalves and has yielded at least 17 bivalve species as well as gastropods and occasional ammonites. Ammonites from this horizon include the oldest Fleet-side Rasenia fauna recognized by Birkelund and co-workers, including microconchs referred to Rasenia cf. triplicata and macroconchs close to R. cymodoce. The base of the bed correlates with KC5 of the Dorset Kimmeridge scheme and has long been treated in practical Dorset correlation as the base of the Rasenia cymodoce Zone.

Bed WR15 — Higher Cymodoce-Zone Shales With Deltoideum delta (c. 2.0 m Or More)

Above the Wyke Siltstone, the section returns to mudstone and shale, again with Deltoideum delta present. These higher shales yielded the next younger Fleet-side Rasenia horizon recognized by Birkelund and colleagues, characterized by microconchs referred to Rasenia cf. and aff. involuta and macroconchs assigned to Eurasenia. The beds record continuation of offshore muddy deposition after the shelly siltstone event of the Wyke Siltstone, but with ammonite faunas now clearly within the Cymodoce-Zone succession.

Bed WR16 — Black Head Siltstone (0.5 m; KC8)

A second thin siltstone bed of similar lithology occurs 1–2 m above the Wyke Siltstone and is known as the Black Head Siltstone. Though thinner and less visually dominant than the Wyke Siltstone, it is an important correlation bed and represents a higher Cymodoce-Zone shelly event, correlating with KC8. Younger Fleet-side Cymodoce-zone clays near the southern end of the exposure have yielded the highest Rasenia horizon recognized here, including Rasenioides lepidula and Semirasenia askepta, but those beds are patchy and slumped and should not be forced into a spurious separate clean bed in this Fleet-side page.

Scope Limit Note

Higher lower Kimmeridge mudstones of the mutabilis Zone are historically better known from the harbour-side cliffs between Sandsfoot and Small Mouth, not from the Fleet-side Wyke Regis west locality defined here. They are therefore not forced into this page, which is restricted to the East Fleet and Ferry Bridge / eastern Fleet shoreline exposures along Chesil Beach and The Fleet.

Depositional Environment

The Wyke Regis west succession records a marked westward transition from shallow Oxfordian carbonate-shelf facies into the deeper muddy regime of the basal Kimmeridge. The Upton Member represents clastic-rich offshore shelf sedimentation; the Shortlake Member records ooid shoals, sandy episodes and increasingly lagoonal shelly micrites; the Nodular Rubble represents early cemented carbonate benches later eroded at the top; and the lower Clavellata Formation records renewed quartz-sand influx and marginal sandy oolite deposition on an eroded shelf surface. The highest Oxfordian boundary beds near Ferry Bridge pass from calcareous mudstone of the Ringstead Clay into the condensed hiatal Osmington Mills Ironstone. Above the Oxfordian–Kimmeridgian boundary, the Inconstans Bed and overlying Baylei- and Cymodoce-zone mudstones show establishment of a quieter offshore muddy shelf in which oysters, ammonites and thin shelly siltstones alternate with thicker mudstone packages. The Fleet-side basal Kimmeridge is notably expanded, which is why it became one of the key Dorset sections for lower Kimmeridgian ammonite correlation.

Total Thickness Note

The East Fleet reference section exposes about 14 m of upper Corallian strata. At the eastern end of The Fleet, the Fleet-side boundary section exposes the uppermost Ringstead Clay and thin Osmington Mills Ironstone, overlain by about 13.5 m or more of basal Kimmeridge Clay up to and including the Black Head Siltstone. These are separated on the Fleet side by concealed, slipped or poorly exposed ground, so no single honest total thickness can be given for a continuous Wyke Regis west log.

References

British Geological Survey Lexicon of Named Rock Units: Corallian Group, Osmington Oolite Formation, Clavellata Formation, Sandsfoot Formation, Ringstead Clay Member and Kimmeridge Clay Formation.
Wright, J.K. (1986). A new look at the stratigraphy, sedimentology and ammonite fauna of the Corallian Group (Oxfordian) of south Dorset.
Cox, B.M. & Gallois, R.W. (1981). The stratigraphy of the Kimmeridge Clay of the Dorset type area and its correlation with some other Kimmeridgian sequences.
Cox, B.M. & Sumbler, M.G. (1994). The Oxfordian–Kimmeridgian boundary beds in southern England.
Wright, J.K. & Cox, B.M. (2001). Geological Conservation Review site accounts for East Fleet, East Fleet–Small Mouth and Sandsfoot in British Upper Jurassic Stratigraphy (Oxfordian to Kimmeridgian).
Arkell, W.J. (1947). Geology of the Country around Weymouth, Swanage, Corfe and Lulworth.
Birkelund, T., Thusu, B. & Vigran, J. (1978). Jurassic–Cretaceous biostratigraphy of Norway, with comments on the British Rasenia cymodoce Zone.
Williams, M.E. (2003). The development of hiatal surfaces in the Osmington Mills Ironstone Member of south Dorset.