Petrography of the Lower Oligocene Limestones of Anklesvar Oil Field, Cambay Basin, Gujarat

The limestones of Dadhar Formation (Lower Oligocene) in the Anklesvar oil field of Cambay basin, provides a typical example of erosional transgression. Interbedded with grey, fossiliferous shales and fine grained sandstones, these carbonate layers have been found to be biomicrudites comprising foraminiferal shells, glauconite, terrigenous sand, silt and clay, microspar patches and development of pseudospar in fossil shells. The above assemblage, especially the glauconite and recrystallized microspar and pseudospar, typically indicates a transgressive marine environment, characterised by calm, weak and shortlived currents. Most earlier workers have invoked a regressive phase for the deposition of this limestone but recent microfaunal studies and the salinity data point to an inner neritic environment transgressive phase.

The petrography provides evidences to reveal a marine environment of shallow tidal sea, wherein the marine incursion must have been characterised by tidal effects and reworking of the older sediments.