The lower part of the Lias Group in south Gloucestershire: zonal stratigraphy and structure

Abstract

Temporary exposures in the Blue Lias Formation and the Charmouth Mudstone Formation, Lower Jurassic, between Hawkesbury and Gloucester, Gloucestershire, yielded many ammonites of biostratigraphical significance. In the southern part of the area the Denotatus Subzone and the Oxynotum Subzone are believed to be missing, but in the northernmost part of the area the zonal and subzonal succession in the Hettangian, Sinemurian and Lower Pliensbachian stages is probably complete, apart from possibly the Figulinum Subzone. The narrow Lower Jurassic outcrop in the southern area is due to cut-out by strike faults, and the overlying Inferior Oolite by steeper than normal dips. This structural zone may be the southernmost expression of the Malvern Fault belt and may have determined the position of the Middle Jurassic escarpment of the south Cotswolds. The broad outcrop north of Dursley is due to a dome structure.