Equence Stratigraphic Analysis of a Shallow Marine, Mixed Carbonate-Siliciclastic System, Early Miocene, Kutch

Abstract

The Early Miocene succession in Kutch (western India) is represented by a mixed carbonate-siliciclastic system. A combined sedimentological, trace fossil assemblage and micropaleontological analysis of the succession was carried out to refine sequence stratigraphic architecture. In this mixed carbonate-siliciclastic system, the trace fossil complement the micropaleontological data in the carbonate portion and provide crucial information in the siliciclastic part, particularly in identifying key stratal surfaces. The sedimentary succession records single cycle of third order fluctuation in relative sea-level (~7 Ma.) with higher order fluctuations of million year episodicity. The preserved sequence is asymmetric, consisting of a thick transgressive systems tract (TST) and a relatively thin highstand systems tract (HST). The transgressive systems tract exhibits a gradual change in foraminiferal assemblage indicating deepening upward succession in overall middle shelf environment. The trace fossils change from Psilonichnus ichnogenera to Skolithos assemblage to Thalassinoides assemblage, from the base of the section to the top. The reworked burrow tubes of Psilonichnus ichnogenera concentrated in the lower part of the TST indicates lag concentration, accumulated during onset of the transgression. The recurrence of larger foraminifera and trace fossil attributable to the Thalassinoides assemblage at various stratigraphic levels (i.e. upper part of TST and lower part of HST) marks the higherorder flooding surfaces and base of parasequence. A high abundance of planktonic foraminifera, including Cassigerinella, Globigerina and Globorotalia , characterizes the maximum flooding surface (MFS) corresponding to a maximum bathymetry of ~50m. The progressive shallowing in highstand systems tract exhibit siliciclastic dominated succession with increased coarse clastic sediments towards top and it is marked by a gradual decrease in the abundance of ichnogenera and shallow water foraminifera and their absence in the upper part of the HST.