Carbonate platforms and coral reefs: The Coralline Oolite of the Yorkshire Upper Jurassic – a prime source of palaeontological information
Keith Eastwood (UK)
The Malton Oolite Member of the Coralline Oolite Formation (Corallian Group), as exposed in the Betton Farm South Quarry (TA00158555) at East Ayton, near Scarborough (Fig. 1), provides a wealth of fascinating palaeontological and sedimentological information. Examination of outcrops within this small quarry enables the geologist to reconstruct the palaeoenvironment of deposition of the Betton Farm Coral Bed, a localised system of patch, ribbon and framework reefs that developed during the Upper Jurassic.
The lithology and textural characteristics of the Malton Oolite Member provide a sedimentological basis for the interpretation, but the fossil content adds definitive ecological and climatic insights. The Malton Oolite is the upper of two oolite members in the Coralline Oolite Formation (Fig. 2). The lower one, the Hambleton Oolite Member, is not seen in the Betton Farm Quarries (which consist of two quarries: Betton Farm North Quarry and Betton Farm South Quarry, north and south of the A170, respectively) but is fully exposed in the Spikers Hill Quarry (SE 980863) just 3km to the WNW (Fig. 1). This location is important in providing a regional depositional context for the Betton Farm deposits, even though the upper surface of the intervening Middle Calcareous Grit Member is a minor unconformity.
The Hambleton Oolite Member is primarily an ooidal limestone succession, 16m thick, with variable texture (Wright, 2001). The lowest layer (termed the Cordatum Subzone) dominates the formation at a thickness of 13m. It is a white, thick-bedded oolitic grainstone. Fossils include frequent bivalves (Givillella aviculoides), corals (Thamnasteria sp) and ammonites (Cardioceras sp). A 2.45m blue, unoxidised, non-oolitic limestone (floatstone/rudstone) follows with oncoids (pisolites), shell fragments and the occasional ammonite. The uppermost beds are a 0.9m thick white oolitic packstone, containing whole small colonies of Thamnasteria concinna (Goldfuss), followed by a 0.15m bed of oolitic packstone with an abundant fauna of ammonites, bivalves and gastropods.
The blue non-oolitic, oncoid-bearing limestone is not only laterally persistent within the quarry (Fig. 3), is an excellent marker band over a distance of 40km across the Corallian outcrop from Spikers Hill in the east to Helmsley in the west – it is also a remarkable sight to behold in the sheer faces of the quarry.
Powell (2010, p.65) likens the overall Hambleton Oolite depositional environment to that of the present-day Great Bahama Bank, with the periodic migration of oolitic shoals on a shallow-water carbonate platform influenced by waves and oscillating tidal currents. Just try to imagine that – a much more equable climate than the present-day Yorkshire, by far.
In the current exposures of the carbonate sand platform with patch reefs at Betton Farm South Quarry (Fig. 4), five facies that are lateral equivalents are identified from west to east (modified after Wright and Rawson, 2014):
- Coral-shell sand.
- Massive ribbon coral, Thamnasteria concinna (Goldfuss).
- Coral-shell sand, rich in bivalves and echinoids.
- Fine carbonate mud [lime mud on Figure 4] with gastropods.
- Patches of the coral, Thamnasteria concinna (Goldfuss)
A massive Thamnasteria concinna coral (Fig. 5) forms a north-south elongate ribbon reef framework that is no more than 3 to 4m wide. It is bordered to the west by an apron of coral-shell sand facies, comprised of a bioclastic micrite with fragments of bivalves (for example, Nanogyra nana (J. Sowerby)), small gastropods, spines from the echinoid Cidaris and fragments of the corals Rhabdophyllia phillipsi, Thecosmilia and Thamnasteria concinna. Growth of the Thamnasteria concinna framework was occasionally overtaken by that of the coral-shell sand apron and vice versa (Fig. 6).
To the east of the ribbon reef, a coral-shell sand with bivalves and echinoids facies reflects material washed in from the reef and the organisms that lived and thrived near the coral. In addition to debris from the coral ribbon reef, the bivalves, Chlamys nattheimensis (de Loriol), Actinostreon gregarium (J. Sowerby) and Nanogyra nana (J. Sowerby), are present, as are spines of the echinoid, Nenoticidaris smithi (Wright). These shelly carbonates then pass laterally into a fine carbonate mud (Fig. 7) with a population of the large gastropod, Bourgetia saemanni (Oppel) (Fig. 8), scattered thin-shelled bivalves (Modiolus sp) and echinoids, such as Pseudodiadema sp and Hemicidaris cf. intermedia Fleming.
Few corals or coral fragments are present in the fine carbonate mud facies, which is interpreted as having been deposited in a lagoon sheltered from strong currents by the ribbon reef (Fig. 9). In the easternmost part of the quarry, the fine carbonate mud facies comes into contact with patches of the coral, Thamnasteria concinna (Goldfuss).
From a palaeoenvironmental perspective and going beyond the lithological and sedimentological aspects, the coral Thamnasteria gives additional ecological information. Thamnasteria is a scleractinian hermatypic coral, which depended on symbiotic photosynthetic algae to produce organic carbon that the coral polyps took up and used as food. The algae needed light and, in turn, this meant that the coral could only flourish at depths normally less than 50m in the photic zone, with a good supply of oxygen (Clarkson, 1998, p.132).
Thamnasteria’s preference was for a firm anchorage and it grew most abundantly within the zone of wave action at depths less than 20m in normal salinity water, at temperatures of 25 to 29°C. It is considered likely that these conditions prevailed during the deposition of the Betton Farm Coral Bed within the Malton Oolite Member of the Coralline Oolite Formation.
After reading this article, I hope that you will agree that both the Spikers Hill and Betton Farm Quarries provide us with a remarkable insight into the varied carbonate environments of the Yorkshire Upper Jurassic. The Betton Farm quarries, in particular, offer an extraordinary amount of fascinating information, in a compact and easily accessible area, relating to the development of a localised Upper Jurassic coral reef system.
Acknowledgements
I thank Messrs. Wright and Rawson for agreeing to my submission of this article which draws freely on data in their Betton Farm Coral Bed (2014) paper. I also thank Dr Annette McGrath for on-site discussions and guidance on writing style. I am indebted to the Joint Nature Conservation Committee (JNCC) and the Natural Environment Research Council (NERC) as represented by the British Geological Survey for permission to reproduce figure 4.20 from Wright (2001) as my Fig. 1 – Permit Number CP17/083 British Geological Survey © NERC 2017. All rights reserved. I am similarly indebted to the Principal Editor of the Proceedings of the Yorkshire Geological Society (©Yorkshire Geological Society) for permission to reproduce figures 3, 7, 12 and 13 from Wright and Rawson (2014) as my Figs. 2, 4, 6 and 9 respectively.
About the author
Keith Eastwood is a current second year student on the Postgraduate Diploma in ‘The Geology of Yorkshire and northern England’ at the University of York – a programme that is run entirely online by distance learning.
References
Clarkson, E.N.K. (1998). Invertebrate palaeontology and evolution, 4th ed. Oxford: Blackwell Science.
Eastwood, K.M. (2017a). Spikers Hill Quarry. [Photograph]. (Keith Eastwood’s own private collection).
Eastwood, K.M. (2017b). Thamnasteria concinna (Goldfuss), Betton Farm South Quarry. [Photograph]. (Keith Eastwood’s own private collection).
Eastwood, K.M. (2017c). Coral-shell with bivalves and echinoids facies on the left passing into the lime-mud facies with Bourguetia gastropods to the right, Betton Farm South Quarry. [Photograph]. (Keith Eastwood’s own private collection).
Eastwood, K.M. (2017d). Gastropod Bourguetia saemanni (Oppel). Betton Farm South Quarry. [Photograph]. (Keith Eastwood’s own private collection).
Powell, J.H. (2010). Jurassic sedimentation in the Cleveland Basin: a review. Proceedings of the Yorkshire Geological Society, 58(1), 21-72.
Wright, J.K. (2001). Spikers Hill. In: Wright, J.K. & Cox, B.M. (Eds.). British Upper Jurassic Stratigraphy (Oxfordian to Kimmeridgian). Geological Conservation Review Series No. 21. Peterborough: Joint Nature Conservation Committee, pp.157–161.
Wright, J.K. and Rawson, P.F. (2014). The development of the Betton Farm Coral Bed within the Malton Oolite Member (Upper Jurassic, Middle Oxfordian) of the Scarborough District, North Yorkshire, UK. Proceedings of the Yorkshire Geological Society, 60(2), 123-134.
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