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The Middle Jurassic sedimentary succession of the Kachchh Basin shows significant changes in lithology, faunal content and diversity across the Bathonian-Callovian boundary. The change in lithology from predominantly carbonates in the Middle and Late Bathonian to siliciclastics in the Callovian is accompanied by a drop in species diversity from high to moderate. Characteristic faunal elements of the Bathonian (corals, sponges, the bivalves Eligmus and members of the Opinae) are absent or rare in the Callovian. In contrast, Callovian sediments contain comparatively high proportions of nuculid bivalves. The clay mineral assemblage reveals prominent variations in the smectite vs. kaolinite abundance, especially during the Bathonian to Callovian transition. From the Bajocian to the Middle Bathonian, smectite becomes the dominant clay mineral and is nearly exclusively present in the Late Bathoman. Towards the Middle Callovian, its content gradually decreases and kaolinite becomes the dominant clay mineral. Illite generally is only a minor component throughout the Middle Jurassic. Based on these results, the Bajocian to Middle Bathonian time interval is interpreted to represent subtropical climatic conditions with seasonal droughts and a moderate supply of terrigenous clastics to the basin. The Late Bathonian was a period with a semi-arid climate, hot seasonal droughts and a minor input of terrigenous clastics. The increase in kaolinite contents in the Callovian suggests a subtropical humid climate with less prominent seasonal droughts and a higher input of siliclastics into the basin accompanied by a higher nutrient influx. Oxygen isotope ratios measured on Bathonian to early Callovian brachiopod shells give palaeotemperatures of 19-24 degreesC. Callovian to Oxfordian paleotemperatures calculated from oxygen isotopes measured on belemnite rostra range from 11 to 21 degreesC. The lower palaeotemperatures estimated from the oxygen isotopic composition of belemnite calcite are explained by assuming that the belemnites spent most of their life span in colder regions of the Malagassy Gulf and migrated into the warmer subtropical waters of the Kachchh Basin. Combining these various lines of evidence, the changes in lithology and faunal composition taking place in the Kachchh Basin during the Middle Jurassic appear to reflect largely a change towards increasing humidity rather than a distinct decrease in water temperature. (C) 2004 Elsevier B.V. All rights reserved.