0016-7622/2008-71-5-731/$ 1.00 © GEOL. SOC. INDIA JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.71, May 2008, pp.731-738 Ichnofossils from the Fort Member (Middle Jurassic), Jaisalmer Formation, Rajasthan KANTIMATI G. KULKARNI, V. D. BORKAR and TEJASHREE J. PETARE Geology and Palaeontology Group, Agharkar Research Institute, G. G. Agarkar Road, Pune - 411 004 Email: kantimatik@yahoo.co.in Abstract: An ichnoassemblage from the Fort Member, Jaisalmer Formation, occurring in a hill north of the city of Jaisalmer is described. It comprises Arenicolites tenuis isp. nov., (?)Bichordites isp., Planolites isp., Rhizocorallium irregulare Mayer, Rhizocorallium jenense Zenker, Taenidium serpentinum Heer and Thalassinoides isp. Overall paucity of repichnia in an ichnoassemblage vis-à-vis noticeable dominance of domichnia and fodinichnia implies abundance of detrital nutrients. Host rocks yielding this ichnoassemblage are current bedded calcarenite showing fine shell hash along current beds and clayey limestones with presence of small, lenticular storm bed intercalations. They suggest near-shore, moderate- to strong-energy marine conditions rich in organic nutrients, supporting inferences drawn on the basis of trace fossils. The beds containing this ichnoassemblage are assigned ages ranging from Bathonian to Callovian, and hence considered here Middle Jurassic. Keywords: Trace fossils, Fort Member, Middle Jurassic, Jaisalmer, Rajasthan. was given by Kumar (1979). A fossil polychaete described by Gupta et al. (1966) was justifiably identified as a specimen of the ichnogenus Ichnyspica by Chiplonkar et al. (1981). On the basis of ichnoassemblages, a low energy, shallow marine environment was interpreted for the Bhadasar Formation (Sudan et al. 2000). Borkar and Kulkarni (2001) recorded presence of Rhizocorallium karaiensis from the Habur Formation, while occurrence of Planolites isp. cf. P. montanus and Thalassinoides isp. in the Fatehgarh Formation (Borkar and Kulkarni, 2002) questioned the anaerobic condition of its deposition advocated by Muktinath (1967). The present paper describes trace fossils from the Fort Member of the Jaisalmer Formation exposed in the hill, north of Jaisalmer City (Fig.1). All the trace fossils were collected from the bivalve assemblage Zone 4 viz. Modiolus patchamensis - M. sahai zone of Kachhara and Jodhawat (1999), who ascribed a Lower Callovian age to it. Type specimens described in this communication, are housed in the Fossil Repository of the Agharkar Research Institute, Pune. SYSTEMATIC PALAEONTOLOGY Ichnogenus Arenicolites Salter 1857 Arenicolites tenuis isp. nov. (Pl. 1, figs. a-d) INTRODUCTION Presence of fossiliferous rocks in vicinity of Jaisalmer came to be known sometime in the late forties of the nineteenth century, when Impey found an ammonoid near village Kuchhri in the Jaisalmer District (Carter, 1861). A preliminary yet authentic report about the geology of the area around Jaisalmer became available through the work of Blanford (1877). Revising this, Oldham (1886) proposed a lithostratigraphic classification. Revision of stratigraphy of entire Marwar region was undertaken with renewed concern after the inception of the Oil and Natural Gas Corporation. Data collected was integrated by Das Gupta (1975, 1976), who recognized this region as a tectonic province and termed it as ‘Western Rajasthan Shelf’. Palaeontological contributions from the marine Mesozoics of Rajasthan include that by Sahni and Bhatnagar (1958) assigning Callovian and Argo-Portlandian ages respectively to Jaisalmer and Bhadasar Formations. Micropalaeontological investigations mainly include studies on foraminifera (Subbotina et al. 1960) and ostracodes (Lubimova et al. 1960). Biozonations were proposed by Jai Krishna (1987), Dave and Chatterjee (1996) and Kachhara and Jodhawat (1999), latest being that by Khosla et al. (2006) on the basis of ostracodes. Though occurrence of trace fossils was noticed by earlier workers, the first systematic account on trace fossils