Early silica diagenetic fabrics in Late Quaternary sediments, south India

Abstract

Late Pleistocene aeolian and marine sediments on the coastal plain of south India show extensive evidence of very early diagenetic change that has involved the leaching of original carbonates and subsequent lithification of the sediments by opal-A. The diagenesis is closely associated with both intense pedological weathering of the sands in the vadose environment and the former presence of roots. Silica rhizoliths (petrifactions and micro-petrifactions) are major components of the diagenetic fabric together with pore-filling opaline silica precipitates. Three distinct fabric types can be identified within the diagenetic profile: an upper deeply weathered fabric with a matrix of clays and iron oxides and limited preservation of silica petrifactions and clay root cutans (tubules); a mid-profile, mottled fabric having a distinctive high porosity and in which well-preserved silica petrifactions and micro-petrifactions are best developed, together with opaline silica cements and clay root cutans; and a lower profile fabric in which less common silica petrifactions occur within carbonate-cemented sediments. The early diagenetic fabrics, the first described of their kind, provide evidence of widespread and rapid silicification of organic matter in a vadose, seasonally moist, tropical weathering environment. As such, they have potentially important implications for the interpretation of silica diagenesis in the terrestrial geological record.