A Study of Bioerosion of Belemnites and Taphonomy of Cephalopods from Kuldhar Member of Jurassic Jaisalmer Formation from Jaisalmer, Rajasthan

Abstract

The Callovian-Oxfordian Kuldhar Member of Middle-Upper Jurassic Jaisalmer Formation, Rajasthan is characterized by a rich cephalopod (ammonites and belemnites) fauna along with other invertebrates. They are embedded in oolitic limestone, at places in nodular limestone, grey shale and occasional red mud. The belemnite rostra bear bioerosions in the form of borings of circular or elliptical type. The present study identified those as Trypanites isp. and Rogerella isp. and characterized the hitherto unnoticed bioerosions of these belemnite rostra to be post-mortal. Different dispositions of major diameter of belemnite rostra has been interpreted to represent churning effect of ichnofossil producing organisms which put evidences of several generations of omission surfaces. Vertical and horizontal orientations of stephanoceratid ammonites side by side have been critically analysed in the light of lithology and prevailing environmental condition and inferred to be an artifact of gravity and thixotropic property of clay. Ammonites like Stephanoceratids and Macrocephalitids inhabit within 110m depth which is also permissible depth of belemnites as well as probable depth of formation of oolites, carbonate mud and aluminosilicate mud. Considering the lithology of the rocks like oolites, carbonate mud, aluminosilicate, the habitat of stephanoceratid and habitat of acrothoracid barnacles (organisms responsible for Rogerella isp.), the environment has been delineated to be sublittoral which was previously designated as shoreface zone to offshore transition zone.