Discussion on sea-level change and facies development across potential Triassic–Jurassic boundary horizons, SW Britain

Abstract

Tony Hallam & Paul Wignall write: we welcome the paper by Hesselbo et al . (2004) for its thorough description and careful evaluation of the environmental significance of an important group of system boundary strata, but beg to take issue on a number of key points. (1) Most importantly, while we agree with the evidence of a regressive phase within the Cotham Member, we maintain in marked contrast to their opinion that there was also a sharp regression followed by a rapid transgression at the junction of the Langport Member (of the Lilstock Formation) and Blue Lias. It is pertinent before discussing the evidence in SW England further to take into account the broader European context (Hallam & Wignall 1999, with many relevant references cited therein). The evidence is especially clear in Germany where there was extensive shallowing in the latest Rhaetian marked by progradation of sandstone over shales. In northern Frankonia (Bavaria) fluvial sandstones infill channels incised into late Rhaetian marine strata and are overlain by marine Hettangian. The sea-level rise in the earliest Hettangian ( planorbis Zone) was evidently rapid, with the limit of marginal marine sandstones in the eastern part of southern Germany being pushed back at the expense of fully marine shales to the maximum extent achieved during the whole Hettangian Stage. Both in the north and south of Europe a similar pattern of successive sea-level fall and rise can be inferred. A clear end-Triassic regressive pulse can be recognized in the Danish Basin, while both in southern Sweden and NW Poland the upper Rhaetian is missing and there is an unconformity at the base of the Jurassic. In the Northern Calcaerous Alps of Austria, widespread emergence at the end of the Triassic is recognized, with the creation of karst surfaces on emergent reef complexes, while …