Rivista Italiana di Paleontologia e Stratigrafia volume 110 pp. 181-190 April 2004 ENVIRONMENTS AND FAUNAL PATTERNS IN THE KACHCHH RIFT BASIN, WESTERN INDIA, DURING THE JURASSIC FRANZ THEODOR FÙRSICH 1 , J O H N H. CALLOMON 2 , DHIRENDRA K. PANDEY' & ANAND K. JAITLY 4 Recived September 19, 2002; accepted July 13, 2003 Key-words: Jurassic, taphonomy, palaeoecology, cycles, Kach- chh, India. Abstract. Marine Jurassic sediments (Bajocian-Tithonian) of the Kachchh Basin were deposited in a ramp setting. Except during the Middle and Late Bathonian, when a carbonate regime became estab- lished, the fill of the basin consists predominantly of siliciclastics. The sediments represent environments that range from coastal plains (riv- ers and associated flood plains with caliche nodules), deltas, brackish water lagoons, nearshorc sand and iron-oolite bars of the inner ramp, generally situated above fair-weather wave-base, to the middle ramp in- fluenced by storm-waves and by storm-generated currents, and finally to the outer ramp which is characterised by low energy, fine-grained sediments. Changes in relative sea level produced a cyclic sedimenta- tion pattern. The rich benthic fauna of macroinvertebrates is dominated by bivalves, followed by brachiopods, gastropods, corals, serpulids, and sponges. The analysis of 370 statistical samples and more than 27,000 specimens produced more than 40 benthic associations and assemblages. They show a relationship to several environmental parameters, two of which, salinity and climate, are briefly discussed. The spatial distribu- tion of the facies and biota is outlined for two time slices, the Batho- nian and the Callovian-Oxfordian, respectively. Riassunto. I sedimenti marini giurassici (Bajociano-Tithoniano) del Bacino di Kachchh furono deposti in un ambiente di rampa. Eccetto che durante il Bathoniano medio e superiore, quando si stabilì un regime carbonatico, il riempimento del bacino consiste predominantemente di materiale silicoclastico. I sedimenti rappresentano ambienti che vanno dalle pianure costiere (fiumi e relative piane d'inondazione con noduli a caliche), delta, lagune d'acqua salmastra, barre di sabbia litorale e oo- liti ferrosi della rampa interna, generalmente situati sopra la base d'on- da di bel tempo, alla rampa media (influenzata dalle onde di tempesta e dalle correnti generate da tempesta), ed infine alla rampa esterna, che è caratterizzata da sedimenti a grana fine, di bassa energia. I cambia- menti nel livello marino relativo hanno prodotto una sedimentazione ad andamento ciclico. La ricca fauna bentonica dei macroinvertebrati è dominata dai bivalvi, seguiti da brachiopodi, gasteropodi, coralli, scr- pulidi e spugne. L'analisi di 370 campioni statistici ed oltre 27000 esem- plari ha prodotto più di 40 associazioni e raggruppamenti bentonici. Essi mostrano una relazione con molti parametri ambientali, due dei quali, salinità e clima, vengono brevemente discussi. Viene delineata la distribuzione areale delle facies e dei biota per due porzioni temporali, rispettivamente il Bathoniano ed il Calloviano-Oxfordiano. Introduction The rift basin of Kachchh originated at the west- ern margin of the Indian Plate in the Triassic (e.g. Biswas 1982). After a phase that was characterised by non-ma- rine sediments (alluvial fans, fluvial systems), the sea in- undated the basin sometime in the early Middle Jurassic, when it became an appendix of the so-called Malgassy Gulf, a southern extension of the Tethyan Ocean. The precise timing of the transgression is still not clear; the oldest ammonite, a Leptosphinctes, is from the Late Ba- jocian (Singh et al. 1982). However, the bed containing this ammonite is underlain by about 250 m of sediments of partly marine, partly fluvial origin so that the earliest marine influence may date even from the Aalenian (Fiir- sich et al. 2001). The Jurassic sedimentary fill consists mainly of sil- iciclastics (except for a carbonate interlude in the central and western areas of the basin during the Middle and Late Bathonian) and records a general deepening culminat- ing in the Middle Oxfordian Schilli Subzone (Krishna et al. 2000). This was followed by regression which result- / Institut fiir Palàoniologie der Università! Wùrzburg, Pleicherwall 1, D 97070 Wurzburg, Germany. E-mail: franz.fuersich@mail.uni-wuerzburg.de 2 129 Moffats Lane, Brookmans Park, Hatfield, Herts. AL9 7RP, United Kingdom. E-mail: johncallomon(a lineone.net.uk 3 Department of Geology, University of Rajasthan, Jaipur 302 004, India. E-mail: dhirendrap@satyam.net .in 4 Department of Geoloirv. Banaras Hindu University. Varanasi 221 005. India. E-mail: akiaitlvff'banaras.ernet.in