CLAY MINERAL STRAT IGRAPHY IN THE PERMO-TRIASS IC RED BED SEQUENCES OF BNOC 72/10-1A, WESTERN APPROACHES, AND THE

ABSTRACT: The Permo-Triassic red bed sequence penetrated in BNOC Well 72/10-1A, located in the Western Approaches, i divided into eight clay mineral zones. The zones are defined by variations in the clay assemblage in which iren- and magnesium-rich chlorite, corrensite and mica occur. This zonal sequence is matched mineralogicaUy and stratigraphically with the pattern of clay mineral variation recorded from the Permo-Triassic ediments ofthe south Devon coast, some 500 km distant. Dating and correlating Triassic red bed sequences have always presented problems. The use of vertebrate remains or lithofacies has met with limited success but, for the oil industry, the greatest advances have been made with the application of palynology. One major disadvantage of palynology is that preservation of palynomorphs i restricted largely to the non-red lithologies in dominantly red bed successions; thick, monotonous red mudstone sequences are invariably barren. Unfortunately, the red mudstone lithofacies comprises much of the Upper Triassic in the North Sea and Western Approaches Basins. Electric-log correlation is effective in the Southern North Sea Basin where lateral facies continuity is most pronounced and where Triassic sedimentary successions are relativel