Sedimentology of the Triassic-Jurassic boundary beds in Pinhay Bay (Devon, SW England)

Abstract

in Pinhay Bay (SE Devon) of the White Lias (Langport Member of the Lilstock Formation) and basal Blue Lias reveal rapidly changing palaeoenvironments during the Triassic-Jurassic (T-J) boundary interval. During deposition of the topmost White Lias a soft seafloor of micritic mudstone was lithified and bored. The resultant hardground was locally eroded, probably in a shallow marine setting, to form a spectacular intraformational conglomerate that was itself lithified. Brief subaerial emergence then followed and produced a fissured and pitted top surface to the White Lias. The regression was short lived and rapid transgression at the base of the Blue Lias established organic-rich shale deposition with a small framboidal pyrite population and low Th/U ratios indicative of a stable, sulphidic lower water column (euxinic conditions). The White Lias/Blue Lias contact thus records a short duration, high amplitude relative sea-level change. This sea-level oscillation has also been postulated for other T-J boundary sections in Europe, although the failure to identify it in regional-scale sequence stratigraphic studies is probably due to its brief duration. Deposition of the basal beds of the Blue Lias was marked by a discrete phase of syn-sedimentary folding and small growth fault activity that may record a regional pulse of extensional tectonic activity.

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

“…Correlation between key SW British sections across the Tr-J boundary. The regressive surface of erosion of the present study is contrasted with the sea-level lowstand surface implied by Hallam (1988Hallam ( , 1990Hallam ( , 1995 and Wignall (2001). The position of Hallam's lowstand surface at St Audrie's Bay is based on Hallam (1995, fig.…”

“…4); south Wales and Mendips logs are based on new data, except for the lowest 1 m of the Mendips section,which is from Duffin (1980). Langport Member at Lyme Regis is from Hesselbo & Jenkyns (1995), modified on the basis of Wignall (2001), and the Cotham Member is from Mayall (1983). (See Fig.…”

“…Period boundaries commonly mark episodes of major global environmental changes and this is undoubtedly true of the transition from the Triassic to the Jurassic (Wignall 2001;Hesselbo et al 2002;Olsen et al 2002 and references therein). These environmental changes occurred coincidentally with a period of accelerated extinction that may have been particularly intense some time in the Late Triassic and at the Rhaetian-Hettangian (Triassic-Jurassic) boundary (Benton 1986;Hallam 2002;Sephton et al 2002).…”

Evolution and Geological Significance of Larger Benthic Foraminifera

“…The vast majority of the lava volume appears to have been extruded at 183 Ma in about 1 Ma. The anoxic event identified by Hallam (1961) and Wignall (2001) in NW European marine sections and in South America, may have been triggered by the eruption of volcanic CO 2 and ensuing global warming (Jenkyns, 1999;Wignall, 2005). Unlike the Tr-J boundary, where the CAMP volcanism seems to have triggered a major catastrophic extinction, the Pliensbachian event is more diffuse.…”

Switching off the carbonate factory: A-tidality, stratification and brackish wedges in epeiric seas

“…The overlying Lilstock Formation begins with the restricted, freshwater-influenced lagoonal to tidal flat calcareous mudstones and siltstones of the Cotham Member (Mayall, 1981(Mayall, , 1983. The Langport Member represents a low energy setting with abnormal salinities (Wignall, 2001), possibly a schizohaline lagoon (Arzani, 1997;Swift, 1999). Based on the broad extent and fine grained nature of this unit Wignall (2001, p.356) favoured a broad shallow seaway setting for the Langport Member, with abnormal salinities, damped tides and limited wave fetch.…”