Clay mineral stratigraphy of Mesozoic and Paleozoic red beds, Northern North Sea

Abstract

Differences in the clay mineralogy and stable isotope geochemistry of Triassic, Permian and Devonian red beds in UK Quadrant 9 aid in distinguishing samples of different age and provide a basis for delineating stratigraphic position during drilling of these unfossiliferous units. Triassic red beds are characterized by the presence of expandable clays (smectite and illite/smectite), kaolinite, illite and chlorite. Smectitic and kaolinitic samples are isotopically heavy (δ 18 O = 18.7–20.7‰) and probably represent clays formed near the earth’s surface from circulating meteoric water. The illite, mixed-layer illite/smectite (I/S), chlorite clay assemblage is isotopically lighter (δ 18 O = 16.7‰) and probably formed during burial diagenesis at elevated temperatures. The clay mineral assemblage of Permian Rotliegendes red beds is characterized by the presence of illite, ordered interstratified illite/smectite, kaolinite (shallow samples) and chlorite. Devonian red beds in Quadrant 9 are characterized by an isotopically light (δ 18 O = 14.5‰) illite-chlorite clay mineral assemblage, in conjunction with accessory albite and calcite. Clay geothermometry suggests that the mineralogy of Devonian red beds developed by metamorphic alteration and was either inherited from metamorphic provenance, or developed diagenetically during the Caledonian orogenesis. Clay crystal morphology provides additional criteria for distinguishing different-aged red beds: Triassic clays contain extremely small plates and laths of smectite or small plates of illite and chlorite; Permian red beds contain abundant lath-shaped and platy illite; Devonian red beds are dominated by coarse recrystallized mica and chlorite and lack lath-shaped or small dispersed crystallites.