Carbonate platforms and coral reefs: the Coralline Oolite Formation of the Yorkshire Upper Jurassic – a prime source of palaeontological information

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.

Figure 1 (1)
Fig. 1. Locality map of the Betton Farm and Spikers Hill quarries. Geological outcrops from BGS Sheet 54 (Scarborough) (1998), (Wright, 2001, p.157, fig.4.20). Total image © Joint National Conservation Committee; geological outcrop map – British Geological Survey © NERC. Redrawn and reproduced with permission.

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.

Figure 2
Fig. 2. Formations and members of the Corallian Group in NE Yorkshire (Wright and Rawson, 2014, p.126, fig. 3). © Yorkshire Geological Society. Reproduced with permission.

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.

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