Biofilm microfacies in phosphoritic units of the Neoproterozoic Halkal Shale, Bhima basin, South India

Abstract

The phosphatic bands of the Halkal Shale of the Neoproterozoic Bhima Basin, record an anoxic paleoenvironment, where only a few episodic depositional events introduced oxygen. In thin-sections, five distinct microfacies, caused by biofilms colonizing the ancient sea floor, can be distinguished. The microfacies document that low sedimentation rate allowed growth of biomass-rich laminae sets, whereas episodic influx of sand led to stacks of organic laminae alternating with fine sandy interlayers. Such depositional events may have also caused local rupture of biofilms and the release of small biofilm roll-ups and clasts. The ancient biofilms led to precipitation of phosphorous and iron-rich minerals and likely contributed to phosphorite formation. In modern equivalent environments, sea floor-colonizing biofilms include a high abundance of sulfide oxidizing and sulphate reducing bacteria.