The Callovian-Oxfordian boundary in Oxfordshire and Wiltshire based on two new temporary sections

Abstract

New temporary exposures of the Lower, Middle and Upper Oxford Clay (Callovian-Oxfordian) provide palaeoecological data that record the changing marine depositional conditions during the Callovian to Oxfordian in southern England. The faunally impoverished Lower Oxford Clay indicates oxygen restricted depositional conditions. The succeeding late Callovian Spinosum Clay in Oxforshire contains a moderate diversity benthic macrofauna in pale mudstones. These grade upwards into the Lamberti Limestone which yields a distinctive gastropod and echinoid fauna. The Limestone thins to the south-west of Oxford and is represented in Wiltshire by a Gryphaea-Oxytoma shell bed. Benthic faunas in the dark mudstones of the Upper Oxford Clay (Mariae Clay) are of low diversity and dominated by deposit feeders although moderately diverse epibionts occur on the occasional Gryphaea . Very soft substrates are inferred. Ammonites from the Upper Oxford Clay are commonly preserved as small pyritic nuclei but in Wiltshire they retain their original aragonitic shells.