Early Diagenetic Glauconitization of Feldspar Grains in a Middle Proterozoic Tidal-Flat Complex: ABSTRACT

Abstract

Extensive glauconitization of feldspar grains exists in arkosic sedimentary rocks of a middle Proterozoic carbonate tidal-flat sequence in the Godavari valley, south India. Glauconitized sedimentary rocks occur as (1) well-defined thick layers, (2) small lenses at different stratigraphic levels, and (3) infilling in the intercolumnar areas of SH-stromatolite structures, in colonies of SS-type structures, and shrinkage cracks in algal mats. Glauconite replaces potash feldspar and plagioclase along cleavage and fractures. Varying degrees of replacement from incipient to complete are observed. Petrographic studies reveal two phases of glauconitization. The major phase, affecting detrital grains, is early diagenetic and is almost syndepositional. A second phase, affecting authigenic feldspar overgrowth, follows silicification. Primary structures and sediment characters of glauconitized sandstones indicate that glauconite formed in varying conditions of turbulence and bathymetry. A glauconitic layer in Ramgundam (18°45^primeN, 79°26^primeE) appears to have formed in a quiet environment, at moderate depth below wave base, during brief periods of transgression. Large bed forms in a thick layer near Manthani (18°33^primeN, 79°40^primeE), indicate a turbulent environment. Small sand lenses within carbonate rocks reflect rapid deposition by high-velocity currents. Close association of glauconite with SH and SS-type stromatolites indicates origin in agitated environments at very shallow depth, and supratidal origin is implied by its presence in shrinkage cracks within algal mats. Evidence of organic activity to aid glauconitization is absent. Even if blue-green algae might have effected glauconitization of detritus associated with stromatolites by providing small areas of reducing environment within a strongly oxidizing macroenvironment, glauconitization in the thick, sandy zones appears to be independent of biochemical processes. End_of_Article - Last_Page 691------------