The Penarth Group-Lias Group succession (Late Triassic-Early Jurassic) in the East Irish Sea Basin and neighbouring areas: a stratigraphical review

Abstract

Abstract Small, widely separated outliers of latest Triassic and Early Jurassic rocks in and around the East Irish Sea are preserved in the hangingwall blocks of faults that were active during post-Pliensbachian-pre-Cenomanian times. The successions preserved in the outliers comprise the Penarth and Lias groups, in typical facies and with characteristic microfloras and macrofaunas, and are vestiges of a formerly extensive cover of those rocks to the northwest of their main outcrops in England. The Penarth Group, of Rhaetian (Late Triassic) age, comprises the Westbury Formation and the succeeding Lilstock Formation; the latter is represented largely, or entirely, by the Cotham Member. Lias Group successions preserved in the outliers include, above the basal (‘pre-planorbis’) beds, of latest Triassic age, Early Jurassic beds ranging in age from Hettangian ( planorbis Zone) to Pliensbachian ( spinatum Zone) and are considerably thicker than their correlatives in eastern England, indicating that subsidence was slower there than in western Britain in Hettangian, Sinemurian and Pliensbachian times.