Depositional history and palaeoecology of the Oxfordian/Kimmeridgian boundary beds at South Ferriby, South Humberside

Abstract

SUMMARY The uppermost Ampthill Clay and lowermost Kimmeridge Clay are well exposed in a pit at South Ferriby. This provides an exceptional record of depositional conditions across the Oxfordian/Kimmeridgian boundary. These are investigated using a combined palaeoecological and taphonomic approach which reveals marked changes in the structural attributes of the benthic associations. The Ampthill Clay contains a low-diversity fauna dominated by deep infaunal bivalves and epifaunal serpulids. Sedimentation rates may have been high and benthic oxygen levels moderately low. The faunas of the Kimmeridge Clay are distinctly more diverse and they suggest a range of substrate types from soft to firm. The presence of phosphatic nodules and high fragmentation indices attests to low sedimentation rates. The phosphatic nodules are reworked only in the Oxytoma Cementstone, which coincides with the base of the baylei Zone and is here taken as the base of the formation. Benthic diversity declines to zero in the topmost beds at South Ferriby. This is attributed to the rapid onset of bottom water anoxia at an earlier time in the Kimmeridgian than anywhere else in Britain, possibly due to rapid local subsidence.