Origin of dolomite−ankerite cement in the Bravaisberget Formation (Middle Triassic) in Spitsbergen, Svalbard

Abstract

The organic carbon (OC)−rich, black shale succession of the Middle Triassic Bravaisberget Formation in Spitsbergen contains scattered dolomite−ankerite cement in coarser−grained beds and intervals. This cement shows growth−related compositional trend from non−ferroan dolomite (0-5 mol % FeCO3) through ferroan dolomite (5-10 mol % FeCO3) to ankerite (10-20 mol % FeCO3, up to 1.7 mol % MnCO3) that is manifested by zoned nature of composite carbonate crystals. The 13 C (−7.3‰ to −1.8‰ VPDB) and 18 O (−9.4‰ to −6.0‰ VPDB) values are typical for burial cements originated from mixed inor− ganic and organic carbonate sources. The dolomite−ankerite cement formed over a range of diagenetic and burial environments, from early post−sulphidic to early catagenic. It reflects evolution of intraformational, compaction−derived marine fluids that was affected by disso− lution of biogenic carbonate, clay mineral and iron oxide transformations, and thermal de− composition of organic carbon (decarboxylation of organic acids, kerogen breakdown). These processes operated during Late Triassic and post−Triassic burial history over a tem− perature range from approx. 40C to more than 100C, and contributed to the final stage of cementation of the primary pore space of siltstone and sandstone beds and intervals in the OC−rich succession. Key wor ds: Arctic, Svalbard, Middle Triassic, cementation, petrography, geochemistry, carbon and oxygen isotopes.