FAUNAL RESPONSE TO TRANSGRESSIVE REGRESSIVE CYCLES - EXAMPLE FROM THE JURASSIC OF WESTERN INDIA

Upper Callovian to Oxfordian shelf sediments at Ler in the Kachchh Basin, western India, are interpreted in terms of transgressive-regressive cycles. The transgressive phases are represented by thin layers of reworked and bored concretions, sometimes in association with skeletal concentrations, the regressive phases are documented by much thicker units of largely fine-grained sediments. The benthic fauna of transgressive and regressive phases differs markedly and thus mirrors the sedimentary cycles: During transgressive phases non-sedimentation produced hard substrates colonized mainly by byssate, cemented, or pedicle-attached suspension-feeding epifaunal species, whilst sediment input during regressive phases lead to soft substrate conditions characterized by infaunal deposit- and suspension-feeders as well as by epifaunal opportunists. Whereas the composition of the "regressive fauna" remains fairly constant through time, that of successive "transgressive faunas" often differs drastically. This is possibly due to a greater environmental sensitivity of epifaunal taxa as compared to infaunal taxa.