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.

Recurrent hardgrounds and their significance for intra-basinal correlations: a case study of upper Bathonian rocks from the western margin of the Indian craton

“…1a; e.g., Biswas 1971Biswas , 1977Biswas , 1991Das Gupta 1975;Krishna 1987Krishna , 2017Fürsich et al 2001Fürsich et al , 2013Pandey et al 2014), about 400 km apart. These basins are well known for numerous fossiliferous horizons, shell beds, well-preserved trace fossils, condensed horizons, reworked concretions, hardgrounds, cyclic sedimentation and lateral thickening and thinning of stratigraphic units within the Jurassic succession (e.g., Singh 1989;Fürsich et al 1991Fürsich et al , 1992Fürsich et al , 2013Fürsich et al , 2017Fürsich et al , 2018Fürsich and Oschmann 1993;Paul et al 2018). The peak of the first transgression, which inundated the basins simultaneously, has been dated as late Bajocian (Singh et al 1982;Pandey and Fürsich 1994;Pandey et al 2006aPandey et al , 2006bPandey and Choudhary 2007).…”