The lithostratigraphy of the latest Triassic to earliest Cretaceous of the English Channel and its adjacent areas

Abstract

The stratigraphy of the latest Triassic through to the earliest Cretaceous of the Portland-Wight Basin and its adjacent areas may be subdivided using petrophysical (gamma-ray and interval transit time) criteria, in association with gross lithology to allow a total of 50 lithological units to be recognized. Three units occur within the latest Triassic, 46 in the Jurassic and one in the earliest Cretaceous. The lithostratigraphy can be integrated into a biostratigraphic template using published data and subsequent observations based on micropalaeontology (ostracods, foraminiferids) and palynology (dinocysts, miospores). Throughout the study area recognition of major lithostratigraphic and biostratigraphic events enables the chronostratigraphic framework of the basin to be determined, which has aided identification of regional stratigraphic breaks throughout much of the Portland-Wight Basin, including haitii in proximity to the Pliensbachian-Toarcian and the Aalenian-Bajocian boundaries, and also within the Late Oxfordian. Although the majority of the lithostratigraphic events have been deduced to be isochronous, a number of lithological units exhibit significant diachroneity.

Triassic to Middle Jurassic stratigraphy of the Kerr McGee 97/12-1 exploration well, offshore southern England

“…The high gamma spike at the top of the formation (1248 0 , log) is envisaged to represent a glauconite rich level, associated with a hardground development. Similar gamma spikes have been noted throughout these successions in the Portland-Wight Basin (Ainsworth et al, 1998b). The Upper Inferior Oolite is characterised by a blocky wireline log motif with low gamma-ray responses and high sonic velocities.…”

“…The lower and more thicker sequence (2503 0 , log-2653 0 , log) comprises medium grey to medium dark grey, calcareous claystones and marls, inter-bedded with light grey to medium light grey, cryptocrystalline, mud limestones and argillaceous limestones. A comparable two-fold subdivision has been recognised by Ainsworth et al (1998b) from wells located elsewhere in the PortlandWight Basin.…”

“…Those papers which have concentrated on the Dorset coast and the offshore PortlandWight Basin and are the more relevant to this study, include Ainsworth et al (1998b), Bristow et al (1991), Callomon and Cope (1995), Cope et al (1980a,b), Hamblin et al (1992), Hesselbo and Jenkyns (1995), Hollaway et al (1989), Lott et al (1982), Parkinson (1996), Ruffell (1990), Warrington et al (1980) and Whittaker et al (1985). More recently, the lithostratigraphic nomenclatures for both the Lower Jurassic Lias and the Triassic Mercia Mudstone Groups in England and Wales have been revised and rationalised; the former by Cox et al (1999), and the latter by both Gallois (2002) and Howard et al (2008).…”

“…Stratigraphic comments: The ''Paper Shales'' appear to have a very restricted geographical distribution in southern England. At present they have only been described from three wells (98/22-1, 98/22-2 and 98/23-1) located on the southern edge of the Portland-Wight Basin, attaining a maximum thickness of no more than 10 0 (Ainsworth et al, 1998b Lithology: Comprising light olive grey, very fine to fine grained, well sorted, subangular-subrounded, micromicaceous-micaceous, slightly glauconitic, calcareous, silty sandstones and sandy limestones. Locally they grade through to sandy mudstones and siltstones.…”

Pushing the boundaries: Integration of multi-source digital elevation model data for seamless geological mapping of the UK's coastal zone

“…The resistant limestones of the Portland Limestone Group (Coe 1996) also form strong featuring across Mupe Bay, in parallel to that formed by the overlying Purbeck Group limestone. These limestones are known to form a significant escarpment on the sea floor, extending up to 10 km offshore (Ainsworth et al 1998).…”