An environmental study of the Upper Domerian and Lower Toarcian in Great Britain

Abstract

An integrated petrological and palaeoecological study has been undertaken of the Spinatum, Tenuicosta-tum and Falciferum zones of the Lias in all the major British sections. After a brief stratigraphical review the deposits are described syste-matically in terms of four regions, Yorkshire, the Midlands, south-west England and the Inner Hebrides. The Spinatum Zone consists of a series of ironstones, bioclastic limestones and fine sandstones with a rich and diverse fauna dominated by brachiopods, bivalves and belemnites. The Tenuicostatum Zone, thin or absent in most areas, tends to be finer grained and more argillaceous, with a fauna related to that of the underlying beds. The Falciferum Zone is characterized especially by the extremely widespread development of laminated bituminous shales with an impoverished invertebrate fauna. A brief world strati-graphical review leads to the conclusion that there was a notable eustatic rise of sea level in the early part of the Toarcian. The British deposits are considered to have been laid down in a very shallow shelf sea in an area of great tectonic stability and very slight relief. Within this context, the facies was controlled primarily by rates of deposition and subsidence, local topography, liability to wave action and proximity of rivers. The early Toarcian transgression, following a late Domerian regression, had the effect of inducing widespread stagnation below wave base, until the sea had deepened sufficiently in mid Toarcian times to allow freer circulation. Four different facies associations in the fauna can be distinguished and related to environmental conditions. The development of faunal provinces among the later Domerian am-monites and brachiopods and some extinction is attributed to the existence of extremely shallow seas extensively broken up by newly emergent land. The widespread phase of bottom stagnation in the Falciferum Zone led to extinction of most of the benthos, so that the overlying beds contain a substantially new fauna, with Middle Jurassic affinities.