Progressive Palaeontology
Progressive Palaeontology 2013
22
FIELD TRIP - CAYTON BAY
SCARBOROUGH
Overview:
Cayton Bay is on the North Yorkshire coast, between the headlands of Osgodby or Knipe Point and Yons Nab, between
Scarborough and Filey (Fig. 1). The bay contains an excellent and varied Middle and Upper Jurassic sedimentary sequence –
in places abundantly fossiliferous, as well as large deposits of glacial boulder clay containing exotic clasts and fossils
transported from as far away as Scotland.
Location:
Fig. 1
Fig. 2
Fig. 2. Locality map of Cayton Bay, showing the position of the Gristhorpe Plant Bed. Car Parking (P) and access points are
indicated. Grid lines are 1km apart. HWM and LWM correspond to High Water Mark and Low Water Mark.
Geology: Cayton Bay exposes a varied succession of Middle and Upper Jurassic marine and fluvio-deltaic sedimentary
rocks deposited in the Cleveland Basin between 171 and 161 Ma. The following sections give a brief introduction to the
various stops we will be making through the morning. An outline of the stratigraphy is given below, based on nomenclature
used by Rawson and Wright (2000) and Powell (2010).
Fig. 3. Middle Jurassic stratigraphy exposed in the main portion of Cayton Bay (STOP 1). (Modified from Powell (2010))
Fig. 4. Middle Jurassic stratigraphy exposed east of Cayton Bay and at Yons Nab (STOP 2). (Modified from Powell (2010))
The northern section of Cayton Bay is characterized by two large landslide complexes known as Cayton Cliff and Tenants’
Cliff .The presence of these landslides is conditioned by the local geology, which features incompetent Jurassic clays of the
Oxford Clay Formation cropping out at sea level, overlain by competent Jurassic sandstones and limestones of the Lower
Calcareous Grit Formation. We will be concentrating on the geology exposed in south-eastern portion of the bay, beyond
the large outcrops of slumped glacial boulder clay near the path from the car park.
STOP 1: High Red Cliff, east side of Cayton Bay
Hazards: Falling shale, hard hats must be worn at all times.
Lower Calcareous Grit Fm
Oxford Clay Formation
Red Cliff Rock Member
Figure 5: Photo and cross-section of Red Cliff, south-east Cayton Bay. (Section from Rawson and
Wright (2000)).
The bulk of the accessible section on the east side of the bay is represented by the Red Cliff Rock Member of the Callovianaged Osgodby Formation. Many fallen blocks of chamosite oolite sandstone from the upper Red Cliff Rock Member occur
on the upper beach and yield abundant bivalves and occasional ammonites, especially Kepplerites.
The Hackness Rock member (Osgodby Formation) comprises 1-2m thick of chamosite oolite limestone; it has occasionally
yielded the ammonites Quenstedtoceras and Kosmoceras.
Fallen blocks of the overlying Oxford Clay Formation are occasionally fossiliferous with ammonites, and the blocks of Lower
Calcareous Grit which litter the shore contain abundant Thallasinoides burrows.
At the base of the cliff towards the eastern end of the Red Cliff section, the Cornbrash Formation is exposed along the foreshore. This 1m thick unit represents the early Callovian marine transgression and yields a benthic fauna dominated by
epifaunal bivalves such as Lopha, Entolium, oysters and infaunal genera such as Pleuromya.
The lower portion of the formation also contains well preserved networks of Rhizocorallium burrows extending down into
the underlying Scalby Formation. These burrows frequently preserve scratch marks made by the original inhabitants –
thought to be shrimp-like crustaceans.
Rhizocorallium burrow networks
in the base of the Cornbrash
Formation.
Figure 6: Collecting fossils from the Cornbrash Formation, including Rhizocorallium burrow networks.
East of the Cornbrash outcrops further large fallen blocks of Lower Calcareous Grit containing abundant Thallasinoides
burrows litter the fore-shore. In places the in-situ stratigraphy is obscured by debris and vegetation.
At the eastern end of Red Cliff the path of the Red Cliff fault is marked by a large gully. Beyond this on the up-thrown side
of the fault, the stratigraphy examined at Red Cliff is now exposed high in the cliffs. The nature of the accessible outcrop
changes as we move into a sequence of middle Jurassic (Bajocian - Bathonian) fluvio-deltaic and marine rocks exposed
between here and Yons Nab.
STOP 2: Yons Nab
Yons Nab is composed of a gently westerly dipping succession of Middle Jurassic strata lying beneath the Red Cliff Member.
The outermost reef is formed of the marine limestone/sandstone-dominated Millepore Bed, and it and the overlying Yons
Nab beds of the Cloughton Formation run straight across the rock platform to meet the Red Cliff fault as it runs out
northwards into the sea.
The following section is described in reverse; with the stratigraphically highest beds first, as encountered on our traverse.
The first beds you will come to in the low cliff section are Moor Grit member sandstones and siltstones of the fluvio-deltaic
Scalby Formation. There are irregular alternations of sandstones containing much carbonised wood, with laminated silty
shales. Soft-sediment deformation structures present in this unit have been interpreted as saurian footprints
(‘dinoturbation’). Below this the marine Scarborough Formation represents transgression and establishment of marine
conditions, with occasional bivalve fossils present (Fig 7). The underlying mudstones and sandstones of the Cloughton
Formation record a range of environments from shallow marine to deltaic, including the Grisethorpe Plant bed.
In the upper part of the 3.3m of the Scarborough Formation the
bivalves Lopha and Meleagrinella are abundant.
Below the Scarborough Formation the Cloughton Formation
represents a mix of shallow marine and deltaic environments.
In places marine fauna are evident.
In the Gristhorpe Member well preserved stems and leaves of
Bennetitales, Ginkgoales, conifers, fers, Pteridophytes and
Caytoniales can be found.
The carbonate-rich Millepore Bed represents shallow marine
conditions during marine transgression.
Figure 7: Stratigraphy exposed at Yons Nab. (Taken from Van Koninjenburg-Van Cittert and Morgans
(1999)).
Figure 8: Possible
‘dinoturbation’ in the
Scalby Formation (Powell,
2010).
Figure 9: Characteristic plant fossils of the Grisethorpe Plant Bed (Powell, 2010).
References:
Powell, J.H. 2010. Jurassic sedimentation in the Cleveland Basin: a review. Proceedings of the Yorkshire
Geological Society, 58 (1), pp. 21-72
Rawson, P.F., Wright, J.K. 2000.The Yorkshire Coast. Geologists’ Association Guide No. 34. DinkyPrint. 130 pp.
Van Koninjenburg-Van Cittert, J.H.A., Morgans, H.S. 1999. The Jurassic flora of Yorkshire. Palaeontological
Association Field Guide No. 8. 134 pp.
THINGS TO LOOK FOR:
STOP1:
Kepplerites
Thallasinoides
Kosmoceras
Quenstedtoceras
STOP 2:
Lopha:
Meleagrinella: