Ringstead Bay Fossil Hunting

1865 – the Oxfordian–Kimmeridgian boundary at Ringstead was already being recorded in the literature
The junction between the Ringstead Coral Bed and the overlying Kimmeridge Clay at Ringstead was recorded in nineteenth-century work, making the bay one of the classic Dorset localities for the base of the Kimmeridgian succession.

1877 – Blake and Hudleston further documented the Ringstead boundary beds
Later nineteenth-century work refined knowledge of the Ringstead succession and helped establish the importance of the bay for the uppermost Corallian and lowermost Kimmeridge Clay beds.

1914 – Salfeld recorded the boundary beds and ammonite succession at Ringstead
Early twentieth-century work continued to build the case for Ringstead as one of the most important localities for understanding the Oxfordian–Kimmeridgian transition in southern England.

1933–1936 – W. J. Arkell published his classic work on the Ringstead Corallian succession
Arkell’s studies of the Corallian beds between Osmington and Ringstead became the classic reference for the area. His bed-by-bed work helped fix the importance of Ringstead for corals, oysters, ammonites and other Upper Jurassic fossils.

1947 – Arkell’s later synthesis cemented Ringstead as a classic Upper Jurassic Dorset locality
Arkell’s later publications kept Ringstead central to discussions of Dorset Upper Jurassic stratigraphy and fossil faunas, especially the richly fossiliferous upper Corallian beds below the Kimmeridge Clay.

1969 – Ringstead was recommended as the basal boundary stratotype for the Kimmeridgian Stage
For many years, the basal beds at Ringstead Bay were the recommended boundary stratotype for the base of the Kimmeridgian Stage. This made the site internationally important, not just as a fossil locality, but as a reference point in Jurassic stratigraphy.

1978–1981 – the Ringstead boundary beds were revised in detail
Later twentieth-century work by Brookfield and by Cox and Gallois refined the lithostratigraphy and faunas of the upper Oxfordian and lower Kimmeridgian beds at Ringstead. This improved understanding of the Inconstans Bed and the faunal changes across the stage boundary.

1986 – the Ringstead Coral Bed was highlighted as a distinctive coral-rich Upper Jurassic bed
Modern work showed that the Ringstead Coral Bed, with masses of Thamnasteria arachnoides, remained one of the most distinctive fossil horizons in the bay. It is unusual in south Dorset because it is the only Corallian bed there to contain abundant corals.

1994 – Ringstead was still being treated as the recommended boundary stratotype for the base of the Kimmeridgian
Detailed studies of the Oxfordian–Kimmeridgian boundary beds continued to use Ringstead as the recommended basal boundary stratotype for the Kimmeridgian Stage, confirming the long-standing importance of the bay in international Jurassic correlation.

1996 – a rare giant chalk ammonite was found at Ringstead Bay
A fine specimen of the rare chalk ammonite Parapuzosia leptophylla, about 3 feet in diameter, was found at Ringstead Bay by two members of the Bournemouth Natural Science Society. It is one of the more notable individual fossil finds reported from the locality.

Late 1990s–2000s – Ringstead remained central to debate over the Kimmeridgian boundary
Although later work favoured other sections for the eventual formal global boundary, Ringstead continued to be recognised as one of the classic Dorset reference localities for the base of the Kimmeridgian and the transition from the Corallian into the Kimmeridge Clay.

2021 – the formal global boundary for the base of the Kimmeridgian was fixed elsewhere, but Ringstead retained its historic importance
When the formal GSSP for the base of the Kimmeridgian was ratified in Scotland in 2021, Ringstead lost its long-standing role as the recommended boundary stratotype. Even so, it remains one of the historic classic sections for the stage boundary and one of Dorset’s best-known Upper Jurassic fossil localities.

CORALLIAN GROUP

Sandsfoot Formation (Upper Oxfordian)

Oxfordian Section Character At Ringstead Bay

At Ringstead Bay the uppermost Sandsfoot Formation is exposed in low, landslipped clifflets and foreshore embayments rather than in a single clean wall. The type Ringstead Clay Member and Bed 25 are the key Oxfordian units here; the underlying Sandsfoot Grit is attenuated and is much better exposed westwards toward Black Head and Osmington Mills, so it is not forced into a false bed-by-bed Ringstead log below.

Ringstead Clay Member

Bed RB1 — Lower Ringstead Clay (Arkell Beds 24a–24b; up to c. 1.0 m visible at the type locality)

At Ringstead Bay, favourable foreshore conditions expose pale grey, very fine-grained, variably calcareous mudstone with numerous fine carbonate laminae that are little affected by bioturbation. This lower part of the old Ringstead Waxy Clay includes Arkell’s Beds 24a–24b and records quiet, restricted mud accumulation in a shallow sheltered lagoon or embayment. Fossils are sparse, but thin-shelled bivalves occur locally and Deltoideum delta may appear toward the base. Westward the member rests sharply and non-sequentially on ferruginous sandy beds of the Sandsfoot Grit. Only part of the member is normally seen at Ringstead, but the full unit is about 3.5 m thick at the type section and 3.5–5 m thick on the Dorset coast.

Bed RB2 — Upper Ringstead Clay (Arkell Beds 24c–24d; c. 0.5 m visible at the type locality)

Above the pale laminated mudstone lies reddish laminated silty mudstone with siltstone lenses and numerous red-weathering, lens-shaped sideritic concretions and nodules, representing the upper part of Arkell’s Beds 24c–24d. Ringsteadia anglica was recorded from this level, and the ferruginous concretions are one of the most useful field characters of the member across south Dorset. The fine unbioturbated lamination, the very limited fauna and the presence of siderite all indicate strongly restricted bottom-water circulation, probably lagoonal and possibly locally hypersaline, with conditions stressful or toxic for much of the benthos.

Osmington Mills Ironstone Member

Bed 25 — Ringstead Coral Bed (Ringstead Coral Facies Of The Osmington Mills Ironstone Member; c. 0.16 m at Ringstead Bay)

This very thin but famous limestone is the youngest Oxfordian horizon at Ringstead and the type locality of the Ringstead Coral Bed. It represents the Ringstead coral facies of Bed 25 within the Osmington Mills Ironstone Member; westward, the same condensed member is only about 0.48 m thick and consists of dark silty clay overlain by limonite-oolite calcareous mudstone, and the facies change can be followed within the bay. At Ringstead the bed is a hard marly limestone in clay, crowded with serpulids, bivalves and corals. Tabular to foliaceous colonies of Thamnasteria concinna, occasional solitary Thecosmilia annularis, and bivalves including Nanogyra nana, Lopha genuflecta, Astarte and Chlamys nattheimensis are typical; the coral colonies are partly in life position, partly broken, bored or abraded in a fine micritic matrix. The locality also gives its name to the late Oxfordian ammonite genus Ringsteadia. The bed records a short-lived return to fully marine, very slow sedimentation in a low-clastic setting below fairweather wave base, with coral and bivalve colonization followed by only gentle reworking.

Kimmeridge Clay Formation (Lower To Upper Kimmeridgian)

Section Character

Ringstead Bay should not be treated as a single simple measured cliff log. The basal Kimmeridge beds crop out in low grassy landslip sections near the village slipway; much of the lower formation is obscured or destroyed by landslip; the next good exposures lie farther east in small overlapping Eudoxus–Autissiodorensis sections; and the highest beds reappear beneath the Portland Group near Burning Cliff on the downthrown side of the Holworth House Fault. In the breakdown below, published Arkell bed numbers are retained where they exist; the RB beds above them are site-use interval labels only, based directly on named marker horizons and composite east-bay sections.

Bed 26 — Inconstans Bed (c. 0.61 m)

Purplish-grey clay, locally hardened, with large serpulid tubes and rolled or bored phosphatic pebbles. Characteristic fossils include Torquirhynchia inconstans, Pictonia densicostata, Nanogyra nana and Exogyra praevirgula. This condensed, slightly erosive marine bed marks the base of the Kimmeridge Clay Formation and the Oxfordian–Kimmeridgian stage boundary in the Dorset type area. It is the classic basal Baylei Zone marker at Ringstead and records renewed open-marine mud deposition after the restricted late Oxfordian Ringstead Clay and Coral Bed interval.

Bed 27 — Nana Bed (c. 0.2–0.3 m)

A dark shell-packed bed dominated by the small oyster Nanogyra nana, locally cemented into a hard limestone-like band. Rare Pictonia occurs in situ. Although thin, this is one of the most immediately recognizable beds in the basal Kimmeridge at Ringstead. It represents a condensed shelly marine horizon formed under very low net sedimentation on the mud-dominated shelf immediately above the stage boundary.

Bed 28 — Deltoideum Delta Mudstones (historically seen to c. 4.6 m)

Blue shaly clay with repeated shell layers rich in Deltoideum delta. This interval continues the basal lower Kimmeridgian mudstone succession above the Nana Bed and records quieter offshore shelf sedimentation than the more condensed boundary beds below. Thin shelly partings, soft weathering and slumping make it less conspicuous than Beds 26–27, but it is important because it shows the boundary interval passing quickly into typical Kimmeridge clay deposition.

Bed RB3 — Wyke Siltstone And Black Head Siltstone Interval

Higher in the lower Kimmeridge, the Wyke Siltstone and Black Head Siltstone are the next securely recognized named markers at Ringstead. They occur within otherwise mud-dominated lower Kimmeridge strata and record short-lived increases in silt input and slightly more energetic bottom conditions on the marine shelf. Because exposure is discontinuous, they are best treated here as correlation horizons rather than as a single clean measured run of beds, but they are crucial for tying Ringstead into the wider Dorset type-area framework.

Bed RB4 — Landslipped Lower–Middle Kimmeridge Composite Interval (Lower Formation Of Order 95 m In Total)

Between the basal boundary beds and the next well-measured east-bay sections, a large part of the lower formation is either unexposed or too damaged by landslip for accurate bed-by-bed logging. Cox and Gallois estimated the Lower Kimmeridge at Ringstead to be of the order of 95 m thick overall. This obscured tract must include much of the intervening lower and middle Kimmeridgian zonal succession represented more clearly elsewhere on the Dorset coast, but Ringstead does not provide a trustworthy single continuous log through it. Any usable website stratigraphy therefore has to preserve the composite nature of the section rather than invent continuity that the cliff does not possess.

Bed RB5 — ?Hobarrow Bay Stone Band And Lower Nannocardioceras Oil Shales

The lowest of the better exposed upper Kimmeridge sections begins with a densely cemented coccolith-rich oil-shale or limestone horizon correlated as the ?Hobarrow Bay Stone Band. It passes into dark oil shales and bituminous mudstones with Nannocardioceras, abundant large Nanogyra virgula and associated ammonites. This is an organic-rich open-marine shelf facies deposited under low-oxygen bottom conditions with episodic carbonate concentration. At Ringstead this interval belongs to the upper part of the Eudoxus-zone succession recognized in the small overlapping east-bay exposures.

Bed RB6 — Nannocardioceras Cementstone

Above the lower oil shales lies the Nannocardioceras Cementstone, an important marker horizon composed of oil shale with septarian nodules, in which Nannocardioceras may occur preserved in translucent calcite. The bed provides one of the clearest tie-points between Ringstead and the better known Kimmeridge Bay and Hounstout sections. Its cementstone character reflects early diagenetic carbonate concentration within otherwise bituminous clay-rich sediment on the offshore shelf.

Bed RB7 — Upper Eudoxus To Mid-Autissiodorensis Rhythmic Mudstones

Overlying the cementstone are rhythmic alternations of oil shale, bituminous mudstone and calcareous mudstone with scattered ammonites including Aulacostephanus and Nannocardioceras; higher in the interval crushed pectinatitid ammonites become more important. Some beds are shelly, others nearly barren, and several carry septarian concretions. This composite interval spans the upper part of the Eudoxus Zone to about the middle of the Autissiodorensis Zone and records repeated small-scale shifts in organic productivity, carbonate supply and bottom oxygenation on the offshore marine shelf.

Bed RB8 — ?Cattle Ledge Stone Band, ?Grey Ledge Stone Band And ?Blackstone Interval

Farther up, an almost continuous tabular cementstone with characteristic dicey weathering is correlated with the ?Cattle Ledge Stone Band. Nearby sections also show widely spaced septarian concretion levels, a prominent ?Grey Ledge equivalent and a thick oil shale with calcareous concretions interpreted as the ?Blackstone. At Ringstead this Blackstone-equivalent interval is commonly deeply weathered, and the tiny pyritized plates of the pelagic crinoid Saccocoma, so conspicuous in fresher type-area examples, have not been recorded. These beds belong to the higher upper Kimmeridge marker-band succession above the obscured Lower–Upper Kimmeridgian boundary.

Bed RB9 — White Stone Band

The White Stone Band is a prominent finely laminated coccolith-rich limestone, locally with an oil-shale parting near its base. At Ringstead it forms one of the most easily recognized upper Kimmeridge marker beds and can be followed into the eastern faulted cliffs. Its hard pale weathering contrasts strongly with the surrounding dark mudstones and oil shales, reflecting a brief interval of concentrated carbonate accumulation within the otherwise clay-rich, organic-bearing offshore succession.

Bed RB10 — ?Middle White Stone Band Interval

Above the White Stone Band, about 4.5 m of poorly exposed rhythmic calcareous mudstone and oil shale include a laminated coccolith-rich horizon correlated with the ?Middle White Stone Band. Pectinatitid ammonites occur higher in the interval. Though commonly obscured by vegetation and slipped material, this package is stratigraphically important because it bridges the White Stone Band upward into the highest preserved marker-band part of the Ringstead upper Kimmeridge section.

Bed RB11 — Freshwater Steps Stone Band

The Freshwater Steps Stone Band is another finely laminated coccolith-rich limestone high in the upper Kimmeridge succession and was used as the top marker of the classic measured Ringstead upper section. Its presence shows that Ringstead Bay reaches well up into the pectinatitid-dominated upper Kimmeridgian marker-bed interval even though the succession is not exposed as a single continuous cliff log.

Bed RB12 — Burning Cliff And The Highest Kimmeridge Beds Beneath The Portland Group

At the east end of the bay, the high cliff west of Holworth House is the famous Burning Cliff, where a bituminous shale caught fire spontaneously in 1826 and burned for several years. The combusted shale is almost certainly the local equivalent of the Blackstone. The Holworth House Fault downthrows the succession eastward by about 45 m, and on its downthrown side the highest Kimmeridge beds, at least from the level of the White Stone Band upward, are intermittently exposed beneath the Portland Group. This is therefore a faulted composite top-of-formation section rather than a single undisturbed terminal Kimmeridge log; the very top Fittoni-zone part of the succession was regarded as probably present, though not firmly proved in the classic exposed sections.

PORTLAND GROUP

Portland Sand Formation (Upper Jurassic)

Bed RB13 — Upper Portland Sand Interval (Upper 8.5 m Exposed At Ringstead Bay)

On the downthrown Holworth House block the Portland Group overlies the highest Kimmeridge. At Ringstead Bay the reference section shows only the upper 8.5 m of the Portland Sand Formation, from within the Upper Black Nore Beds up to the Portland Clay. Lithologies are mixed siliciclastic–carbonate mudstones, siltstones and fine sandstones that become more calcareous upward. These beds record the shift away from the organic-rich Kimmeridge mud shelf into sandier, shallower latest Jurassic shelf conditions.

Portland Stone Formation (Upper Jurassic)

Portland Chert Member

Bed RB14 — Lower Cherty Beds

The lower part of the Portland Stone at Ringstead Bay belongs to the Portland Chert Member. Pale brown to grey bioturbated calcilutites with peloids, sponge spicules, shell debris and common black nodular chert make up this interval. In the Ringstead Bay reference section these beds rest on Portland Clay and mark the strong lithological change from dominantly siliciclastic Portland Sand to more carbonate-rich Portland Stone.

Portland Freestone Member

Bed RB15 — Portland Freestone Limestones

Above the cherty beds, the Portland Stone passes into thick-bedded ooidal, shell-rich calcilutites of the Portland Freestone Member. Bioclastic material increases upward, and small patch-reef development by the red alga Solenopora is characteristic of the upper part of the formation in Dorset. At Ringstead Bay the full Portland Stone reference section is about 18 m thick in total from the base of the cherty beds to the top of the Freestone Member. These limestones represent much shallower, higher-energy carbonate-shelf conditions than the mudstones below.

PURBECK GROUP

Lulworth Formation (Latest Jurassic To Earliest Cretaceous)

Mupe Member

Bed RB16 — Basal Purbeck Ostracod-Rich Limestones And Marls

The Portland Stone is sharply overlain by the Mupe Member of the Lulworth Formation, marking the start of Purbeck sedimentation. The boundary is defined by the first finely laminated, ostracod-rich limestones above the more massive Portland limestone. In Dorset the basal Lulworth succession includes pale-weathering rippled calcarenites, stromatolitic or algal micrites, evaporitic material and interbedded mudstones, recording the familiar switch from open-marine Portland carbonate sedimentation to highly variable restricted lagoonal, sabkha, brackish and freshwater Purbeck environments. At Ringstead these beds belong to the faulted eastern part of the bay rather than to the classic Kimmeridge boundary section.

Depositional Environment

Ringstead Bay captures a major environmental swing through the latest Oxfordian and Kimmeridgian. The Ringstead Clay records quiet restricted lagoonal mud deposition with poor circulation and possible hypersalinity; the Ringstead Coral Bed marks a brief fully marine condensed interval with corals, serpulids and bivalves on a low-sedimentation sea floor; the Kimmeridge Clay records return to open-marine bituminous shelf mud sedimentation with repeated coccolith limestones, cementstones, oil shales and shell-bearing marker bands, interrupted by condensation at the stage boundary and later modified by landslip and faulting; and the Portland–Purbeck succession at the east end shows the later shift into shallower sandy and carbonate shelf conditions followed by restricted lagoonal to evaporitic and freshwater facies.

Total Thickness And Stratigraphic Range Represented Here

Ringstead Bay does not provide a single continuously measurable cliff section, but the composite exposures represent nearly the whole local Upper Jurassic passage from the uppermost Oxfordian Ringstead Clay and Ringstead Coral Bed through an almost complete Kimmeridgian succession and into the Portland Group, with basal Purbeck above on the faulted eastern block. The lower Kimmeridge alone is of the order of 95 m thick, though much of it is obscured or landslipped; the Ringstead Clay Member is about 3.5–5 m thick regionally; the Portland Sand shows 8.5 m in the Ringstead Bay reference section; and the Portland Stone is about 18 m thick there. An almost complete Kimmeridgian zonal sequence is represented, although the very top Fittoni interval is inferred rather than securely proved in the classic exposed sections.

References

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. Report of the Institute of Geological Sciences, 80/4, 44 pp.
Arkell, W.J. (1947). The Geology of the Country around Weymouth, Swanage, Corfe and Lulworth. Memoir of the Geological Survey of Great Britain (England and Wales).
Arkell, W.J. (1933). The Jurassic System in Great Britain.
Arkell, W.J. (1949). “The Kimeridge Clay succession at Burning Cliff, Ringstead.” Proceedings of the Dorset Natural History and Archaeological Society, 70, 124.
Brookfield, M.E. (1978). “The lithostratigraphy of the Upper Oxfordian and Lower Kimmeridgian beds of south Dorset.” Proceedings of the Geologists’ Association, 89, 1–32.
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.
Cox, B.M. & Sumbler, M.G. (1994). “The Oxfordian–Kimmeridgian boundary beds in southern England (Dorset to Fenland).” Geobios, Mémoire Spécial, 17, 133–140.
JNCC Geological Conservation Review site accounts: Ringstead (B.M. Cox) and Osmington (J.K. Wright).
British Geological Survey Lexicon entries: Sandsfoot Formation, Ringstead Clay Member, Kimmeridge Clay Formation, Portland Sand Formation, Portland Stone Formation and Lulworth Formation.
House, M.R. (1989). Geology of the Dorset Coast. Geologists’ Association Guide.