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Phosphorus concentration and organic carbon preservation in the Blanknuten Member (Botneheia Formation, Middle Triassic), Sassenfjorden, Spitsbergen

Krajewski Krzysztof P.

Studia Geologica Polonica

2000

Vol. 116

139--173

The paper attempts to reconstruct depositional to diagenetic processes that led to the increased contents of mineral phosphorus and organic carbon in the Middle Triassic organic-rich phosphatic Blanknuten Member sequence in Sassenfjorden, Spitsbergen. The results of petrographic and geochemical analyses suggest that enhanced preservation of both organic carbon (1-8%) and mineral phosphorus (3-26% P2O5) in the sequence is a result of excess production, deposition, and diagenesis of indigenous marine organic matter in the shelf environment under prevailing oxygen- deficient bottom conditions. The nature of organic sources and the character of depositional environment aided the preferential preservation of oil-prone kerogen of the Type II. Diagenesis of organic phosphorus in surficial sediments led to the formation of phosphate peloids and nodules that occur dispersed in fine-grained facies (3-10% P2O5), or are concentrated at recurrent granular horizons due to dynamic environmental processes (12-18% P2O5). Maximum phosphorus concentra- tion (up to 26% P2O5) is noted in granular phosphorite horizons that were covered and stabilized by deep-water microbial mats during periods of diminished bottom dynamics and non-deposition. An interplay between the deposition and diagenesis of organic matter in oxygen-deficient environment, the dynamic processes leading to reworking and redeposition of sediments, and the growth and phosphatization of microbial mats accounts for the observed complexity of the phosphorus and organic carbon distributions in the Blanknuten Member sequence.