The base of the Jurassic System: new data on the first appearance of <i>Psiloceras planorbis</i> in southwest Britain

Hodges, Peter

doi:10.1017/s0016756800012905

Cited by 28 publications

(10 citation statements)

References 9 publications

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“…A significant problem with this proposal is that it is now recognized that other species of Psiloceras appear below P. planorbis, and it is doubtful whether any first appearance of Psiloceras was globally synchronous (e.g. Hodges 1994;Bloos & Page 2000;Hesselbo et al 2002;Robinson 2002).…”

Section: Placement Of the Tr-j Boundary In The Sw British Successionmentioning

confidence: 99%

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

Hesselbo

Robinson

Surlyk

2004

JGS

174

167

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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.

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Section: Placement Of the Tr-j Boundary In The Sw British Successionmentioning

confidence: 99%

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

Hesselbo

Robinson

Surlyk

2004

JGS

174

167

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“…Ammonites had not previously been recorded any lower in the immediate area and, indeed, characteristically Triassic ammonoids are entirely lacking in Britain (although there is a problematic record of an indeterminate, and therefore completely undiagnostic psiloceratid, from presumed latest Triassic deposits elsewhere in the region; Donovan et al 1989). This definition, therefore, relies to a certain extent on negative evidence, as emphasised by the subsequent discovery of earlier ammonites at the same locality by Hodges (1994) in Beds A18 and A19 (Beds 8 and 9 of and the author, westwards along the coast in Doniford Bay nearer Watchet. Further work on the sections has revealed a succession of ammonite faunas, previously unrecognised, which correlates well with faunas recovered from the Wilkesley Borehole in Cheshire in north-west England (Bloos & Page 1997, 2000aPage & Bloos 1998).…”

Section: The Base Of the Hettangian Stage And The Jurassic Systemmentioning

confidence: 99%

The Lower Jurassic of Europe: its subdivision and correlation

2003

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The Lower Jurassic Sub-system comprises four stages, in chronological order, the Hettangian, Sinemurian, Pliensbachian and Toarcian. Each stage is subdivided into a sequence of ‘standard zones’ (= chronozones) and subzones – each correlated primarily on the basis of its ammonite fauna. A further increase in stratigraphical resolution is available by the use of intra-subzonal units known collectively as ‘horizons’. The close link between ammonites and chronostratigraphy means that faunal provincialism may determine which zonal framework, and therefore which subdivision of the Lower Jurassic, applies in different regions of Europe. Such provincialism is of minor importance in the early Jurassic (Hettangian – Lower Pliensbachian) but increases significantly in the Upper Pliensbachian and into the Toarcian where at least three ammonoid faunal provinces are distinguishable. The standard zonal schemes for each relevant faunal area are discussed here, with greatest emphasis being placed on the Northwest European Province, which is characteristic of much of northern Europe throughout most of the Early Jurassic. Intra-subzonal units have only been described in certain regions for parts of the Lower Jurassic but where recognisable these are introduced.

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“…The first Jurassic ammonite Psiloceras planorbis (J. de C. Sowerby) appears some distance above the base of the Blue Lias (Hodges, 1994), with the result that basal beds of this unit are known as the Pre-planorbis Beds, or sometimes the Ostrea Beds due to the abundance of the oyster Liostrea hisingeri (Nilsson) at this level (Lang, 1924). Lateral thickness variations of the Pre-planorbis Beds are probably partly due to the diachronous appearance of P. planorbis (Hodges, 1994). At Pinhay Bay they are 2.7 m thick (Lang, 1924).…”

Section: Stratigraphymentioning

confidence: 99%

“…This trend need not necessarily indicate a shallowing of the depositional event because benthic oxygenation is often observed to improve during the course of a transgression (Wignall, 1994). The predominance of oysters in the aerobic strata has led several workers to suggest that conditions were shallower in the Pre-planorbis Beds than at higher levels within the Blue Lias (Hodges, 1994;Hallam, 1997).…”

Section: Environmental Change At the T-j Transitionmentioning

confidence: 99%

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

Wignall

2001

Proceedings of the Geologists' Association

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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.

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The base of the Jurassic System: new data on the first appearance of Psiloceras planorbis in southwest Britain

Cited by 28 publications

References 9 publications

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

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

The Lower Jurassic of Europe: its subdivision and correlation

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

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