--> Origin and Stratigraphic Significance of Phosphatic Beds in Fine-Grained Thin-Bedded Turbidites of Neoproterozoic Upper Kaza Group, Windermere Supergroup: Implications for Source and Sink

AAPG ACE 2018

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Origin and Stratigraphic Significance of Phosphatic Beds in Fine-Grained Thin-Bedded Turbidites of Neoproterozoic Upper Kaza Group, Windermere Supergroup: Implications for Source and Sink

Abstract

Single or multiple, thin (1-2 cm) layers of hard, medium-grey, massive, phosphatic claystone form interbeds in up to decameter-thick, laterally-continuous successions of thin-bedded, fine-grained turbidites in basin floor to transitional zone deposits of the Upper Kaza Group (UKG). In contrast to most mudrocks in the UKG, which contain low phosphate content (<0.15% P2O5), phosphate content in phosphatic claystone ranges from 3.5 to 15.4% P2O5, and occurs in beds that commonly can be traced for up to several hundreds of meters laterally before being either eroded or covered by surface debris.

Electron-scanning microscopic images, x-ray fluorescence and microprobe results show that the phosphate is in fact francolite (calcium-rich fluoroapatite), and occurs as dispersed detrital grains that were transported into deep basinal areas and as interstitial patches that would have formed near the sediment-water interface during early marine phosphogenesis. Additionally, rare, corroded authigenic monazite (i.e. rare earth element-rich phosphate) might represent the activity of shallow seabed sulphate-reducing bacteria that microbially degraded organic matter and controlled the uptake and release of phosphorus during deeper burial diagenesis.

The occurrence of phosphate-rich beds in both shallow- and deep-marine strata is generally attributed to episodes of sediment starvation and the development of condensed horizons coinciding with maximum transgression. However, in the UKG, thin-bedded turbidites above and below the phosphate beds lack evidence of any lithological or geochemical change, suggesting that the phosphatic horizons have no stratigraphic significance. Instead, these chemically anomalous beds are interpreted to record intermittent deposition from low-concentration, phosphatic-rich turbidity currents whose high phosphate content was a consequence of increased continental weathering and/or strong upwelling events that possibly coincided with intensified oceanic paleoproductivity and nutrient input on the outer continental shelf and upper slope of late Neoproterozoic North America and worldwide.

The formation and preservation of phosphate deposits, especially those in deep-water turbidite systems, remains poorly understood. Thus, the microtextural and compositional analyses from phosphatic beds in the UKG might help constrain the requisite chemical and physical conditions for the deposition and diagenesis of similar deep-marine phosphatic rocks.