The stratigraphy of the Inferior Oolite at South Main Road Quarry, Dundry, Avon

Abstract

A new section opened on the site of the former South Main Road Quarry, Dundry, Avon, is described. The biostratigraphy of the ammonite succession has been revised through careful bed-by-bed collecting and is presented in a range chart. It is recast in the form of a sequence of discrete ammonite fauna! horizons as has become customary in the classical areas of Somerset and Dorset first described in this way by Buckman. One new ammonite faunal horizon has been identified in the Lower Bajocian: Bj-10b, Sonninia micracanthica (Buckman). This carries several of what have been regarded in the past as leading guide-fossils of the Sauzei Zone. But it is now seen to be also the type horizon of the index of the Laeviuscula Zone, Witchellia laeviuscula. It is therefore retained in the Laeviuscula Zone as its youngest horizon on grounds of nomenclatural stability. The basal boundary of the overlying Sauzei Zone is drawn above it and its lowest ammonite horizon renamed, Bj-11a, Stephanoceras kalum (Buckman). The systematics of some important species based on types from Dundry are reviewed briefly and the origins and phylogeny of some important ammonite genera of the Ovale to Sauzei Zones are discussed. The succession of forms of Witchellia is now separated as a phyletic subfamily Witchelliinae subf. nov. within the polyphyletic clade of the Sonniniidae. A new species of Emileia, E. dundriensis, is described from the Laeviuscula Zone. The nautiloids are also reviewed briefly.