Jurassic sequence stratigraphy in the Hebrides Basin, NW Scotland

Abstract

Stratigraphical analysis of the Triassic-Jurassic succession outcropping in the Hebrides in terms of genetic sequences has identified seven major sequences and seven minor sequences. The major sequence boundaries are defined at abrupt facies changes which usually represent deepening events, often associated with a hiatus. They are dated as (1) Late Triassic; (2) Mid Sinemurian ( Birchi subzone, Turneri zone diachronous to Oxynotum zone); (3) Early Toarcian ( Exaratum subzone, Falciferum zone); (4) Latest Toarcian ( Aalensis subzone, Levesquei zone); (5) Late Bajocian ( Dichotoma subzone, Garantiana zone); (6) Late Bathonian (? Discus zone); (7) Early Callovian ( Medea subzone, Jason zone). Integrating stratigraphical with subsidence history analyses enables identification of basin tectonics as the causal mechanism for sequence boundaries (1), (2), (4) and (5). Eustatic rises of sea level during Toarcian and Callovian phases of relative tectonic quiessence were the causes of sequence boundaries (3), (6) and (7). Late Jurassic tectonic events are not reflected in the sequence stratigraphy on the basis of available data.