NEW OUTCROPS OF AMPTHILL CLAY NORTH OF MARKET WEIGHTON, NORTH YORKSHIRE, AND THEIR STRUCTURAL IMPLICATIONS

Abstract

SUMMARY Significant outcrops of Ampthill Clay occur beneath the Upper Calcareous Grit Formation south-east of Malton and some 24 km north-north-west of Market Weighton. A marginal facies of the Ampthill Clay is here named the Limekilns Member. Marked overstep of the Ampthill Clay Formation by the Upper Calcareous Grit Formation extends the major discontinuity in the upper part of the Gregoryceras transversarium Zone elsewhere in Europe to the Yorkshire Corallian basin. The unconformity previously mapped beneath ‘Kimmeridge Clay’ lies at the base of the Ampthill Clay. Minor cyclic sedimentation within the Limekilns Member and the Upper Calcareous Grit Formation is described and related to local movements along a prominent fault line which marks the northern edge of the East Midland basement shelf; the fault line extends from Gilling through the Howardian Hills and the Wolds to Flamborough Head on the coast. Sediments indicate that the Market Weighton structure had little (if any) influence on Upper Oxfordian facies in the area and that the northern margin of the Ampthill Clay Basin lay along the Gilling-Flamborough fault-line and not at the axis of the Market Weighton hinge.