Pirates Cove Fossil Hunting

1875 – Blake discussed the boundary beds at Wyke Regis and the Fleet area
J. F. Blake treated the uppermost Corallian and basal Kimmeridge Clay beds of the Wyke Regis area as distinctive boundary beds, showing that this stretch of coast 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 to 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 clays containing Deltoideum delta, Nanogyra nana, Torquirhynchia inconstans and ammonites such as Rasenia.

1978 – Birkelund and co-workers used the Fleet section in their work on Rasenia faunas
The nearby 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 to place the local fossil horizons into a more precise modern framework.

Modern collecting – Pirates Cove remains known for Corallian fossils from the Osmington Oolite
Modern field guides and collecting notes describe Pirates Cove itself as a small but productive Corallian locality, with abundant bivalves and gastropods, large oysters, and echinoids including Nucleolites scutatus. The cove is best known for this general fossil richness rather than for a long list of named individual discoveries.

CORALLIAN GROUP

At Pirates Cove the usable measured log is the East Fleet foreshore and low-cliff section, exposed over roughly 600 m and dipping gently across the shore. The poorer, slipped Ringstead Clay and basal Kimmeridge exposures farther east toward Small Mouth and Ferry Bridge are separate sections and are not forced into this stratigraphic breakdown.

For East Fleet, the formal published numbering is Wright’s Beds 1–13. Arkell’s older Dorset coast bed numbers could not be continued here because of mis-correlation, so the bed numbers used below follow Wright’s revised section; site-use labels are added only where a distinct erosional surface or marker horizon needs to be separated for clarity.

Osmington Oolite Formation (Middle Oxfordian)

Upton Member

Bed PC1 — Beds 1–3, Lower Upton Sandy Siltstones

Beds 1–3 form the lower Upton Member and lie immediately above the erosive, burrowed contact with the Bencliff Grit Member of the Redcliff Formation, although the underlying sandstone is better exposed westward near Camp Road. They consist of highly bioturbated sandy siltstones with subordinate calcareous beds and remarkably well-preserved Teichichnus burrows, noted as the only clear occurrence of this burrowing in the district. Their strongly clastic character indicates shallow offshore shelf sedimentation with abundant sand and mud input rather than clean ooid-shoal deposition.

Shortlake Member

Bed PC2 — Bed 4, Lower Shortlake Oolite

Bed 4 is the lower oolitic bed of the Shortlake Member and is partly cross-bedded at East Fleet. It marks the abrupt shift from the clastic-rich Upton facies into ooid-rich carbonate sedimentation and is interpreted as a tidal ooid shoal or ooid-delta deposit laid down in shallow, energetic marine water.

Bed PC3 — Beds 5–7, Quartz-Sand And Clay Interval

Beds 5–7 are the quartz-sand and clay interval above the basal oolite. They show that at East Fleet the Shortlake Member becomes much sandier and more marginal than at Osmington, with quartz sand and clay succeeding the lower oolite and lagoonal influence increasing upward. In the revised log Bed 7 is only 0.9 m thick; the 3.5 m value printed in Wright’s 1986 paper was a misprint.

Bed PC4 — Beds 8–10, Fossiliferous Oomicrites

Beds 8–10 are three very fossiliferous oomicrites exposed at the base of the low cliff and form the key shelly interval at Pirates Cove. Arkell recorded the bivalves Myophorella sp., Plicatula sp., Lucina sp., Chlamys sp., Opis sp., Nanogyra nana and Ostrea sp., gastropods including Nerinea sp., Bourguetia sp., Pseudomelania sp., Ampullina sp., Dicroloma sp., Littorina sp. and Procerithium sp., and the echinoid Nucleolites scutatus. These beds represent shelly micritic to oomicritic carbonate accumulation in quieter shallow-marine lagoonal or sheltered shoal-margin conditions, and they are especially important because East Fleet is the only south Dorset locality where such richly fossiliferous beds are known in the Osmington Oolite Formation.

Nodular Rubble Member

Bed PC5 — Bed 11, Nodular Rubble

Bed 11 is the Nodular Rubble, exposed about 100 m to the south-east as three prominent concretionary limestone beds whose nodules are much larger than those at Osmington. Irregular limestone and marl, strong burrowing, and the general association with the sponge Rhaxella indicate a return to quieter, deeper offshore shelf conditions. The nodular fabric is generally attributed to intense bioturbation, probably enhanced by diagenetic cementation.

Total Thickness Of Osmington Oolite Formation In The Main East Fleet / Pirates Cove Section: About 12 Metres Exposed

Clavellata Formation (Upper Oxfordian)

Only the basal part of the Clavellata Formation is exposed at Pirates Cove. The contact is a clear transgressive non-sequence cut into the Nodular Rubble, and at East Fleet it is interpreted to omit strata of the Nunningtonense Subzone.

Sandy Block Subdivision (informal)

Bed PC6 — Basal Clavellata Transgressive Surface And Quartz-Sand Veneer (informal marker horizon; 0.07–0.14 m)

A thin bed of fine quartz sand wraps around the irregular bored hummocks at the top of the Nodular Rubble, and sand-filled borings descend from above into the carbonate surface. This is the clearest field expression of the transgressive surface at Pirates Cove and shows definite erosion of the underlying Nodular Rubble before renewed sedimentation.

Bed PC7 — Bed 12, Sandy Block Sandstone (1.2 m)

Above the basal veneer lies soft argillaceous sand followed by 1.2 m of flaggy, fine-grained sandstone, the Sandy Block facies best seen near the centre of the bay. The unit is much more quartz-rich and marginal than the equivalent upper Oxfordian facies at Osmington and records shallow marine transgression with continued clastic input over the eroded carbonate surface.

Chief Shell Beds Subdivision (informal)

Bed PC8 — Bed 13, Chief Shell Beds (0.55 m)

Bed 13 is 0.55 m of flaggy, immature, sandy oolite containing Myophorella clavellata and Nanogyra nana. It represents the incoming of the Chief Shell Beds fauna and the lower part of the classic Myophorella shell-bed facies. Depositional conditions were still shallow marine but less carbonate-pure than farther east, with shell accumulation probably enhanced by storm reworking on a fine-grained offshore to inner-shelf floor.

Only The Basal c. 2 Metres Of The Clavellata Formation Are Reliably Exposed At Pirates Cove, Represented By The Sandy Block And Chief Shell Beds Succession

Ammonite Age And Correlation

Regional ammonite evidence from the Dorset Corallian places the Upton Member in the lower Antecedens Subzone, the Shortlake Member in the uppermost Antecedens Subzone, and the Nodular Rubble Member in the Parandieri Subzone of the Middle Oxfordian. The overlying Clavellata Formation is Upper Oxfordian, and at East Fleet the basal transgressive contact is thought to span a significant break that omits the Nunningtonense Subzone.

Typical Fossils

The principal in-situ fossil horizons at Pirates Cove are Beds 8–10 and Bed 13. Characteristic finds include gastropods such as Bourguetia, Ampullina, Nerinea, Pseudomelania and Procerithium; bivalves such as Myophorella, Chlamys, Opis, Lucina, Nanogyra nana and Ostrea; and the small echinoid Nucleolites scutatus. Trace fossils are also important, notably Teichichnus in Beds 1–3 and the large borings and infilled burrow systems at the base of the Clavellata transgression.

Depositional Environment

The Pirates Cove succession records a westward, more marginal version of the south Dorset Corallian. The Upton Member represents clastic-rich offshore shelf sedimentation; the Shortlake Member records the development of ooid shoals, ooid-deltas and associated lagoonal intramicrites; the Nodular Rubble marks a return to quieter offshore carbonate-mud accumulation with sponge growth and strong bioturbation; and the basal Clavellata records renewed marine transgression, with quartz-rich Sandy Block sediment and shell-rich Chief Shell Beds laid down over an erosional surface. Overall, East Fleet shows the passing of the fuller, more open-marine Osmington succession into sandier western facies.

Total Thickness Covered Here: About 14 Metres Of Middle To Upper Oxfordian Corallian Stratigraphy In The Main Pirates Cove / East Fleet Section

References

Arkell, W.J. (1936). The Corallian Beds of Dorset, Part 1: The Coast.
Arkell, W.J. (1947). Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain (England and Wales).
Wright, J.K. (1986). “A new look at the stratigraphy, sedimentology and ammonite fauna of the Corallian Group (Oxfordian) of south Dorset.” Proceedings of the Geologists’ Association, 97, 1–21.
Wright, J.K. (2011). “The ammonite faunas of the Osmington Oolite Formation (Jurassic, Middle Oxfordian) of the Dorset coast.” Proceedings of the Geologists’ Association.
Wright, J.K. & Cox, B.M. (2001). British Upper Jurassic Stratigraphy (Oxfordian to Kimmeridgian), Geological Conservation Review Series No. 21, especially the East Fleet and Osmington site accounts.
British Geological Survey Lexicon of Named Rock Units: Osmington Oolite Formation and Clavellata Formation.
West, I.M. Geology of the Fleet Lagoon and related Wessex Coast field guides for Camp Road / East Fleet context.