Phosphogenic facies and processes in the Triassic of Svalbard

• Autorzy: Krajewski K. P.
• Języki publikacji: EN

Abstrakty

EN

The Middle Triassic sedimentary sequence in Svalbard provides an insight into ancient organic carbon-rich, phosphogenic shelf depositional system showing broad facies associations, from coastal to deep shelf. This paper presents results of the sedimentologic, petrographic and micro- structural investigation of the phosphogenic system, with special emphasis being placed on the role of deep-water benthic microbial communities in phosphorite formation. The investigation was carried out in an attempt to explore earlier hypotheses suggesting a prominent role of gradient-type microbial mats in focusing phosphogenesis on some ancient organic-rich seafloors. The microbial mat-generated structures were recognized at recurrent levels in the Triassic phosphogenic sequence, which mark local intraformational discontinuities associated with suppressed or halted sedimentation. The horizons with well-defined microbial mat structures show high concentrations of carbonate fluor-apatite (up to 30% P2O5), though their contribution to the phosphorus pool of the phosphogenic facies is subordinate. Other types of phosphate, including pristine and allochthonous peloidal and nodular accumulations, dominate the phosphorite fraction of the Middle Triassic sequence, but these phosphates usually give lower burial phosphorus concentrations in the sediment. The microbial-mat generated phosphorite in the Triassic phosphogenic system of Svalbard is interpreted to have been produced by white filamentous sulphur bacteria that proliferated on sulphidic gradients during periods of substantially lowered depositional rates in the organic-rich shelf environment. These mats provided local depositional systems capable of depositing and significantly concentrating apatite in the sediment, though their development was a supplementary factor promoting seafloor phosphogenesis in the shelf environment. There is no evidence to support earlier presumptions that gradient-type microbial mats acted as an immediate and direct phosphorus source for apatite deposition at the sediment/water interface. It seems more plausible that these mats allocated the apatite concentration close to the interface due to sealing of the sediment surface to phosphate diffusion, and narrowing chemical gradients essential for the phosphorus pumps into the sediment, reactive phosphate buildup and mineral precipitation. Thus, these mats supported development of interface-linked sedimentary collectors of the formational phosphate, which, without the mats, would be partly lost from the sedimentary system or remained dispersed in the fine-grained facies.

• Wydawca: Instytut Nauk Geologicznych PAN
• Czasopismo: Studia Geologica Polonica
• Rocznik: 2000
• Tom: Vol. 116
• Strony: 7--84

Twórcy

autor

Krajewski K. P.

• Polish Academy of Sciences, Institute of Geological Sciences, ul. Twarda 51/55, 00-818 Warszawa, Poland,