Origin and significance of glauconites in a sequence stratigraphic perspective: Sylhet Formation, Assam and Assam–Arakan Basin, India

The paper documents occurrence, origin and significance of glauconite-rich intervals at certain specific stratigraphic levels within Early to Middle Eocene Sylhet Formation of Assam and Assam–Arakan Basin. The microfacies with which glauconite is associated are fossiliferous silty shale, glauconitic sandstone, lime mudstone–wackestone and wackestone–packstone. The glauconites occur in three forms, viz., glauconite pellets, infillings within bioclasts and cements (i.e., non-pellet stringers and blebs), representing a complete spectrum of maturity with presence of four types, nascent, slightly evolved, evolved and highly evolved varieties. The elliptical, immatured pellets occur in glauconitic sandstone and exhibit yellowish-green to grass green colour, while relatively matured pellets are dark brownish green to brown, irregular, broken and mostly associated with shale. The Sylhet glauconites indicate deposition in a tide-affected marginal marine to inner-shelf carbonate ramp with low sedimentation rate, and normal marine salinity and represent an overall fining-up 2nd order TST. High-resolution sequence stratigraphic analysis allows to subdivide the 2nd order TST into 3rd order systems tracts: (i) a basal TST, represented by shale/limestone, (ii) an HST incorporating sandstone, limestone and shale, and (iii) a TST at the top. The intervening 3rd order HST is further subdivided into 4th order parasequence sets separated by marine flooding surfaces. Electrolog correlation reveals that the ubiquitous presence of glauconite-rich horizons corresponds to marine flooding events (MFE) of 3rd and 4th order sea-level changes. The glauconites associated with MFE result in a condensed section with minimum sediment supply and provide a key parameter to build the sequence stratigraphic architecture. The occurrences of green clays may possibly owe their origin due to migration of K+/Fe2+ ions from seawater or surrounding sediments to a given reducing microsystem centered on organic debris. The sources of K and Fe are seawater and detrital minerals, respectively, and are achieved by the diffusion-nucleation-crystal growth process.