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Interpretation of authigenic vs. allogenic green peloids of ferric clay in the Proterozoic Penganga Group, southern India

Geological Survey of India, Salt Lake City, Kolkata-91, India

^* E-mail: hiyabando{at}yahoo.co.uk

(Received 15 March 2007; revised 6 August 2007)

Ferric illite in the Neoproterozoic Penganga Group, southern India, occurs in a lenticular unit of reworked arkoses conformably enclosed in carbonate mudstones, deposited in a shelf-margin carbonate slope setting influenced by deep marine sediment gravity flows. The structures and textures of the arkoses suggest emplacement by fine-grained debris flows and deposition in channels incised into the carbonate slope.

Ferric illite is sparse in the arkoses and has small concentrations and grainsizes within the same size fraction of the associated detritus, uniform chemical composition and an association with reworked facies. It consists predominantly of granular grains that are mostly sorted spherical, ovoid, capsule-shaped and vermicular peloids; many broken and micaceous. These attributes provide a comprehensive framework for suggesting an allogenic origin for the ferric illite previously interpreted as authigenic. The methodology adopted here is a useful approach in the interpretation of in situ vs. reworked origins of glauconitic minerals.

KEYWORDS: Proterozoic, ferric illite, marine sequence, Penganga Group, India