Abstract

The ubiquitous occurrence of glauconites in specific stratigraphic levels is observed within Sylhet Formation that belongs to an overall fining-up 2nd order transgressive systems tract (TST). The mixed siliciclastic-carbonate Sylhet Formation implies deposition in a tide affected marginal marine (deltaic) to inner-shelf carbonate ramp with episodic clastic supply from western hinterland by isolated proto-rivers during regressive phases. The glauconites indicate an open shallow marine environment with a low sedimentation rate, normal marine salinity and weakly reducing environments associated with transgression of relative sea-level. The glauconites occur mainly in three forms, viz. glauconite pellets/pelloids, glauconite infillings within bioclasts and glauconite cements. The Sylhet glauconites represent a complete spectrum of maturity with presence of all four types, nascent, slightly evolved, evolved and highly evolved varieties and can be readily identified under petrographic microscope, XRD and SEM analysis. High resolution sequence stratigraphic analysis based on core and electrolog data on Sylhet Formation allows to subdivide the 2nd order TST into three systems tracts of 3rd order, (i) basal TST, represented by shale and limestone followed by (ii) a highstand systems tract (HST) incorporating sandstone, limestone and shale, and (iii) a TST at the top. The intervening 3rd order HST can be further subdivided into 4th order parasequence-sets separated by marine flooding surfaces. Electrolog correlation along dip and strike profiles reveal that the ubiquitous presence of glauconite-rich horizons corresponds to intervals of marine flooding events (MFE) of 3rd and 4th order sealevel change. The glauconites associated with MFE result in a condensed section with minimum sediment supply. Thus glauconitic horizons provide a key parameter to build the sequence stratigraphic architecture of Eocene sequence of Assam and AssamArakan Basin. The sources of K and Fe required for glauconitization are sea-water and detrital minerals respectively and are achieved by diffusion-nucleationcrystal growth process.