Palaeoecological analysis of maximum flooding zones from the Tithonian (Upper Jurassic) of the Kachchh Basin, western India

The siliciclastic Jhuran Formation of the Kachchh Basin, a rift basin bordering the Malagasy Seaway, documents the filling of the basin during the late syn-rift stage. The marine, more than 700-m-thick Tithonian part of the succession in the western part of the basin is composed of highly asymmetric transgressive–regressive cycles and is nearly unfossiliferous except for two intervals, the Lower Tithonian Hildoglochiceras Bed (HB) and the upper Lower Tithonian to lowermost Cretaceous Green Ammonite Beds (GAB). Both horizons represent maximum flooding zones (MFZ) and contain a rich fauna composed of ammonites and benthic macroinvertebrates. Within the HB the benthic assemblages change, concomitant with an increase in the carbonate content, from the predominantly infaunal “Lucina” rotundata to the epifaunal Actinostreon marshii and finally to the partly epifaunal, partly infaunal Eoseebachia sowerbyana assemblage. The Green Ammonite Beds are composed of three highly ferruginous beds, which are the MFZ of transgressive–regressive cycles forming the MFZ of a 3rd-order depositional sequence. The GAB are highly ferruginous, containing berthieroid ooids and grains. GAB I is characterized by the reworked Gryphaea moondanensis assemblage, GAB II by an autochthonous high-diversity assemblage dominated by the brachiopods Acanthorhynchia multistriata and Somalithyris lakhaparensis, whereas GAB III is devoid of fossils except for scarce ammonites. The GAB are interpreted to occupy different positions along an onshore–offshore transect with increasing condensation offshore. Integrated analyses of sedimentological, taphonomic, and palaeoecological data allow to reconstruct, in detail, the sequence stratigraphic architecture of sedimentary successions and to evaluate their degree of faunal condensation.