Tasmanites algae—contributors to the Middle Triassic hydrocarbon source rocks of Svalbard and the Barents Shelf

Abstract

When moving from west to east across Svalbard, organic geochemical, palynological and sedimentological data from the Middle Triassic dark shales of the Sassendalen Group show an improved quality of hydrocarbon source rock (kerogen type II/III), and an increased abundance of structured algal material, including Tasmanites. Triassic specimens of Tasmanites in palynological residues from Svalbard are generally of small size (<100 mm in diameter). Tasmanites of variable size, and the associated degradation products, occur frequently in Middle Triassic dark shales, and extraordinarily large-sized specimens (phycomata >500 mm in diameter) are recorded in silty shales and siltstones palynologically dated as being of Ladinian–Carnian age. Similar occurrences are also present in deposits of the Norwegian Barents Sea. Our organic geochemical analyses suggest that Tasmanites is a major source for the hydrocarbons encountered in these rocks. Tasmanites algae are also enriched in Ladinian–Norian deposits in Taimyr, Siberia. The development of a marine embayment, the direction of the ocean currents, the supply of clastic material and freshwater, and the palaeolatitude all show similarities to the conditions in the present Mediterranean, and adjacent Atlantic Ocean. Ecological conditions in the water column (light intensity, nutrient supply and temperature) are expected to have been similar, and favoured the growth and accumulation of Tasmanites. The accumulation of monotypic, large Tasmanites cells in silty shales is explained as a result of contemporaneous recirculation and sorting.