Abstract:
An integrated study of the sedimentology, micropalaeontology, mineralogy and geochemistry of glauconites in the Oligocene Maniyara Fort
Formation (western Kutch, India), has been undertaken. Authigenic glauconites, mostly of evolved type, formed within a back-barrier lagoonal
environment. Foraminifera help constrain the biostratigraphy and along with sedimentological evidence, provide information on the
depositional conditions. Glauconite in the Maniyara Fort Formation occurs either as infillings within intra-particle pores of larger foraminifers,
or as an altered form of faecal pellets. X-ray diffraction studies reveal the less mature nature of glauconite infillings compared to the glauconite
pellets. Electron microprobe investigation confirms a relative enrichment of K2O and total Fe2O3 in the latter. Both varieties of glauconite
formed by initial authigenic precipitation of K-poor glauconite and subsequently matured by addition of potassium in the interlayer sites
and fixation of total iron in the octahedral sites; calcium, magnesium and aluminum were released from the glauconite structure concomitantly.
Alkaline conditions during the entire process of glauconite formation did not allow dissolution of foraminiferal tests. Mineralogical
and chemical characteristics of the Maniyara Fort Formation glauconites are more similar to deep marine glauconites than those reported from
other shallow or marginal marine settings. A low negative cerium anomaly, as well as abundant pyrite, suggests formation of glauconite in
sub-oxic micro-environments, created by decay of organic matter associated with foraminiferal chambers and faecal pellets. Sub-oxic condition
apparently prevailed relatively longer within the Maniyara Fort Formation lagoons.
