ANALYSIS OF MOLLUSCAN SHELL BEDS FROM LOWER EOCENE DEPOSITS OF CAMBAY BASIN, WESTERN INDIA REVEALS PROLIFERATION OF OPPORTUNIST TAXA IN THE COASTAL SHELTERED ENVIRONMENT IN RESPONSE TO TRANSGRESSIONS

The Cenozoic sedimentary succession of the Cambay basin, western India was developed over the undulating topography of Deccan Traps. The lower Eocene Cambay Shale is composed of various shales and lignite. The argillaceous sediments with its content of profuse plant materials like fossilized leaves, wood and amber, and pyrite were deposited in continental anoxic to dysoxic set up. This succession was punctuated by deposition of marine fossil bearing lithounits. These units are rich in well preserved benthic molluscs, mainly bivalves and gastropods.

Surface exposure of the Cambay Shale is rare. Recent exploration for lignite in the district of Surat, Gujarat made this succession accessible for investigation. This study is carried out in the Mangrol mine. Three molluscan shell beds characterize the succession here. The middle of these yielded maximum number of well preserved mollusc shells and is studied in detail. Taphonomic features indicate autochthony of the fossils. This study documents molluscan diversity of the shell beds and quantitatively analyzes their relative abundance to reconstruct the relationship between faunal diversity and abundance.

The analysis reveals that species diversity is relatively low and distribution of relative abundance is strongly skewed. Two species belonging to the bivalve genus Caryocorbula share 60% or more of the micro-mollusc (0.15 – 3 mm. in length) community in all the studied levels. Two other large Caestocorbula spp. dominate the co-occurring macro-mollusc assemblage (Roy and Mukherjee 2017). It appears from this diversity-abundance pattern that opportunist taxa proliferated in this sheltered basin when ocean water inundated this coastal lagoon in response to transgressions.

Corbulids have a wide range of tolerance and are known from marine to brackish water environments (Etim et al. 1998). The brackish environment of the lower Eocene Cambay basin could be tolerated by corbulids. It may be pertinent to mention that the sedimentary rocks of these shell beds also yielded terrestrial plant debris indicating mixture of fresh and marine waters.

References:

Etim, L., Sankare, Y., Brey, T. and Arntz, W. (1998). Archive of fishery and marine research, 46, 253-262.

Roy, P. and Mukherjee, P. (2017). Journal Geological Society of India, 89, 315-320.