PETROGRAPHIC OBSERVATIONS SUGGESTIVE OF MICROBIAL MATS AND METAZOAN ACTIVITY IN MIDDLE PROTEROZOIC BLACK SHALES FROM INDIA

In order to understand the significance of various microfabrics in black shales of the Kaimur and Rhotas Formation (Vindhyan Supergroup; Middle Proterozoic), samples were examined by petrographic microscope and SEM. Mineral identifications were verified with an EDS unit attached to the SEM.

Examination of thin sections and hand specimens revealed the following features: 1) generally wavy lamination; 2) wavy pyritic laminae; 3) thin shreds of organic matter (up to 20mm in diameter) that may be contorted and folded; 4) rounded organic particles (0.2-0.3mm in size); 5) clay/silt lenses (50-300 mm in size); 6) clusters of pyrite in organic matter; 7) disseminated diagenetic quartz. SEM observations show that pyrite is primarily octahedral in habit (4- 20ƒÝm in size), and forms irregular clusters or discontinuous laminae. Differential compaction around pyrite grains indicates formation very early in diagenesis. Deposition of diagenetic quartz between pyrite grains is ubiquitous and indicates early diagenetic formation of both quartz and pyrite in these shales.

Whereas many Phanerozoic black shales are characterized by parallel lamination and framboidal pyrite, their Vindhyan counterparts differ significantly by way of their wavy laminae and pyrite textures. While the former are commonly interpreted to reflect deposition in stagnant, oxygen-deficient, deeper water settings, what we observe in Vindhyan black shales is suggestive of surface morphology and cohesive behavior, characteristics typically associated with microbial mats. The Vindhyan examples may therefore add to the growing list of Precambrian black shales that resulted from growth of subtidal microbial mats.

The clay/silt lenses show rounded outlines in plan view and resemble in size and morphology fecal pellets described from Phanerozoic shales. Although they may eventually be explained as current sorted clay rip-ups, the close resemblance to fecal pellets produced by sediment feeding organisms is intriguing. Could they be an indication of pre-Ediacaran metazoans?