Speeton Sands

[TA 142 764]

B.M. Cox

Introduction

The Yorkshire coast has long been known for its magnificent exposures of Jurassic rocks. Northwards from Filey Brigg, at the northern end of Filey Bay, Jurassic beds from the Upper Jurassic Corallian Group down to the Lower Jurassic Lias Group are displayed in superb cliff and foreshore exposures giving an unprecedented insight into the Hettangian to Oxfordian geological history of the Cleveland Basin. South of Filey Brigg, younger Jurassic beds are much less well exposed, and Kimmeridgian strata are seen only intermittently in generally poor exposures at the base of the cliffs and, occasionally, on the foreshore at the southern end of Filey Bay (Leckenby, 1859; Judd, 1868; Fox-Strangways, 1892; Pavlow and Lamplugh, 1892; Lamplugh, 1896, 1924; Arkell, 1933; Callomon in Callomon and Cope, 1971; Cope, 1974) (see (Figure 4.26)). The GCR site comprises over a kilometre of foreshore south-eastwards from near Reighton Gap. It is usually covered by beach sand but occasional exposures of the underlying strata have provided sections of the Lower Kimmeridgian Autissiodorensis Zone and Upper Kimmeridgian Elegans Zone; these zones are not exposed elsewhere in Yorkshire.

Description

The following description is based on the section recorded by Prof. J.H. Callomon in the spring of 1964 near Reighton Gap when the foreshore between tide-marks was free of sands and exposure exceptionally good; the outcrop occurs in a small planed-off, south-pitching anticline near an old concrete gun-emplacement bunker [TA 143 764] (Callomon in Callomon and Cope, 1971). No new stratigraphical data have since been added to this original record because of the rare and largely unpredictable availability of these exposures.

The cliffs adjacent to the GCR site are mainly of slumped Quaternary till. At their base, exposures of the underlying Speeton Clay (Lower Cretaceous) and Kimmeridgian strata vary with the state of the beach and cliff, but the Wheatleyensis, Hudlestoni and Pectinatus zones of the Upper Kimmeridgian have been recorded from time to time (Cope, 1974, emend. 1980; Wignall, 1990a, 1993; Rawson and Wright, 2000) and, in recent years, have been regularly exposed. Younger Kimmeridgian and Portlandian strata are cut out by the unconformity, marked by the Coprolite Bed, at the base of the Speeton Clay. Cored boreholes through the Kimmeridge Clay have been drilled for the British Geological Survey (Gallois, 1979a) and the Institut Francais du Petrole (Herbin et al., 1991; Oschmann, 1994) on or near the cliff top here.

Interpretation

The two units into which Callomon (in Callomon and Cope, 1971) divided his section were separated on the basis of a sharp faunal break rather than a marked lithological change. The ammonite fauna of the upper unit, comprising species of Pectinatites, indicates the Upper Kimmeridgian Elegans Zone, and the more varied ammonite assemblage, including Aulacostephanus, of the lower unit indicates the Lower Kimmeridgian Autissiodorensis Zone. The section thus shows the Lower–Upper Kimmeridgian substage boundary. Callomon (in Callomon and Cope, 1971) referred the 'pectinatitids' of the lower unit to Pectinatites (Arkellites) but Cope (1974), on the basis of material seen by him on a subsequent visit to the site, believed they belonged to the genus Propectinatites, including forms apparently intermediate between Propectinatites websteri Cope and early P (Arkellites) ; according to Cope (1974), true Pectinatites had yet to be recorded from the Lower Kimmeridgian strata.

The concretionary horizons with uncrushed ammonites recorded in the lower unit are almost certainly the source of many museum specimens, marked 'Filey Bay' or 'Speeton', which probably came from material washed up on the shore in the past (Callomon in Callomon and Cope, 1971). These include species figured and described by Pavlow (in Pavlow and Lamplugh, 1892), Spath (1925) and Ziegler (1961, 1962), amongst which are the type specimens of Aulacostephanus fallax Ziegler, A. rigidus Ziegler and Subdichotomoceras lamplughi Spath (Figure 4.7). According to Callomon (in Callomon and Cope, 1971), this latter genus, together with Sphinctoceras, is definitely more abundant here than in southern England and, because these genera are also known from East Greenland, he suggested that they represented a genuinely Boreal group.

Conclusions

The elusive exposures on the foreshore near Reighton Gap in Filey Bay provide an occasional opportunity, depending on the erosional state of the beach, to observe the Lower–Upper Kimmeridgian substage boundary succession, including diagnostic ammonite faunas. This interval is not exposed elsewhere in the Cleveland Basin where it is otherwise known only from cored boreholes. Although ammonites are the only fauna to have been reported from here, these show a more Boreal aspect than those of southern England (see also the 'Description' section for Golden Hill Pit, this volume), including relatively abundant Sphinctoceras and Subdichotomoceras. Material from these rarely seen exposures near low water mark are almost certainly the source of ammonite material, including type and figured specimens, found in many museum collections. Younger Kimmeridgian strata can usually be seen at the base of the slumped cliffs adjacent to the site.

References