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

Abstract

The Late Triassic to Early Jurassic aged succession of SW Britain (the Penarth and lower Lias Groups) comprises mudstone, sandstone and limestone strata deposited in a variety of marine to non-marine environments. Faunal and floral characteristics of these successions have led to the proposal that one location in SW England, St Audrie's Bay, should serve as the Global Stratotype Section and Point (GSSP) for the base of the Hettangian Stage and, thus, for the Triassic-Jurassic (Tr-J) boundary. The sections of SW Britain have also been used previously to infer sea-level change history and relate this to potential kill mechanisms associated with the Tr-J boundary mass extinction. Chemostratigraphic, biofacies and lithofacies data are used here to suggest alternative models of sea-level change in relation to possible Tr-J boundary horizons in the sections of SW Britain. A sea-level lowstand surface of erosion is inferred to occur within the Cotham Member of the Lilstock Formation, a unit deposited in an environment that was often subaerially exposed. In contrast to previous interpretations, the top surface of the overlying Langport Member (here inferred to be deposited on a carbonate ramp of depositional or tectonic origin) represents a drowning event of at least regional extent. All horizons regarded as plausible levels at which to place the Tr-J boundary based on fossil distributions lie within strata deposited during relative sea-level rise. However, it is doubtful whether the higher horizons proposed to mark the boundary faithfully record times of true biotic change on a global scale and, additionally, there is no positive evidence that sea-level fall had any relation to the genesis of proposed Tr-J marker horizons. It is unlikely that sea-level fall played a significant role in the Tr-J boundary extinctions in either a local or a global context.

A marine vertebrate fauna from the Late Triassic of Somerset, and a review of British placodonts

“…The Westbury Formation is described as lying uncomformably on Carboniferous limestone, followed by the Cotham beds, Langport beds and the Jurassic Inferior Oolite (Moore, 1867;Richardson, 1911). The Cotham and Langport beds consist of micritic limestones, believed to have formed during a temporary regression in the late Rhaetian, before a second transgression turned the area fully marine in the Hettangian (Swift, 1995;Hallam, 1997;Hesselbo et al, 2004). Curtis measured the Rhaetian sediments to be 0.5 m thick, consisting of yellow limestone shales, sandstone beds and black clays (Fig.…”

The Evolution of Marine Reptiles

“…3). It has been argued that some English plesiosaurian fossils may have come from the highest Triassic strata but this view has been questioned (Hesselbo et al 2004). Second, their limbs had been modified into rigid flippers.…”

Episodicity of Mesozoic terrane accretion along the Pacific margin of Gondwana: implications for superplume-plate interactions

“…The early Toarcian was a period of sea-level rise and marine transgression (Schouten et al 2000) and the Triassic-Jurassic boundary has also recently been recognized as a time of sea-level rise (Hesselbo et al 2004). …”