JOUR.GEOL.SOC.INDIA, VOL.72, DEC. 2008 774 KANTIMATI G. KULKARNI AND OTHERS Gastrochaenolites Bioerosion in the Kalyanpur Limestone (Pliocene) of Dwarka Area, Kathiawar, Gujarat KANTIMATI G. KULKARNI, V.D. BORKAR and TEJASHREE PETARE Palaeobiology Group, Agharkar Research Institute, G.G. Agarkar Road, Pune – 411 004 Email: kantimatik@yahoo.co.in Abstract: Well preserved flask shaped borings belonging to ichnogenus Gastrochaenolites Leymerie, (1842) are being reported from the Kalyanpur Limestone Member (Pliocene) of Dwarka Formation, Jamnagar district, Kathiawar Region, Gujarat. The ichnofauna consists of Gastrochaenolites ampullatus Kelly and Bromley, G. cluniformis Kelly and Bromley, G. lapidicus Kelly and Bromley, G. orbicularis Kelly and Bromley, G. ornatus Kelly and Bromley, G. torpedo Kelly and Bromley, G. turbinatus Kelly and Bromley, and Gastrochaenolites isp. Though individuals of groups other than bivalves are known to create Gastrochaenolites-like borings, these borings were evidently emplaced by bivalves alone, confirmed by the presence of bivalve casts in some of the specimens. This hardground suggests an intraformational break in deposition Keywords: Bioerosion, Gastrochaenolites, Neogene, Pliocene, Kathiawar, Gujarat. Mude (2004) from the Gaj Formation, are actually from the Kalyanpur limestone. In area around Bhavpara, the Kalyanpur limestone directly overlies the Deccan Trap and there is no development of Gaj Formation. All the studied specimens occur as natural casts. Type specimens of the borings described herein bear registration numbers MACS G 5147 to 5152 and are housed in the Palaeontological Repository of the Agharkar Research Institute, Pune. SYSTEMATIC TAXONOMY Ichnogenus Gastrochaenolites Leymerie, 1842 Diagnosis: “Clavate borings in lithic substrates. The apertural region of the boring is narrower than the main chamber and may be circular, oval, or dumb-bell shaped. The aperture may be separated from the main chamber by a neck region, which in some cases may be widely flared. The main chamber may vary from sub-spherical to elongate, having a parabolic to rounded truncated base and a circular to oval cross section, modified in some forms by a longitudinal ridge or grooves to produce an almond- or heart- shaped section” (Kelly and Bromley, 1984). Remarks: Specimens under consideration show diagnostic characters of Gastrochaenolites including clavate-shape, aperture narrower than main chamber, INTRODUCTION Post Deccan Trap marine Tertiary sediments in the Dwarka-Okha tract of the Kathiawar peninsula are of Neogene age. Rocks of the Ashapura clay [equivalent to the Gaj Formation of Sind (Pakistan) and the Chhasra Formation of Kachchh], Burdigalian in age, overlie the Deccan Trap with an unconformity. The succeeding formation, termed the Dwarka Formation, consists of three members viz. Positra limestone member, Shankhodhar sand-clay member and Kalyanpur limestone member in ascending order (Bhatt, 2000). These three members are assigned Middle Miocene, Upper Miocene and Lower Pliocene ages respectively. This entire sequence is followed by the Chaya Formation and Holocene beach deposits (Bhatt, 2000) (Table 1). The present communication deals with certain borings referred to ichnogenus Gastrochaenolites, occurring within the Kalyanpur limestone member, Dwarka Formation, exposed in the tract between villages Gadhechi [22°21'18" N : 69°17'41" E] and Batisa (Fig. 1), indicating presence of a hardground represented by bivalve bioerosion. An allusion to some coralline algae and trace fossils from the Neogene sediments has been made by Kundal and Dharashivkar (2003a, b). A preliminary report on trace fossils by Kundal and Dharashivkar (2006) includes five ichnoforms from the Kalyanpur limestone. It may be mentioned that coralline algae reported by Kundal and JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.72, December 2008, pp.774-780 0016-7622/2008-72-6-774/$ 1.00 © GEOL. SOC. INDIA