Abstract
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.
Highlights
In sequence stratigraphic terminology, a maximum flooding surface/zone is marking the end of shoreline transgression (Catuneanu 2006), i.e., it marks the position of the seafloor furthest away from the coastline during a cycle of fluctuating sea level
At HB I, the sandy bioclastic wackestone immediately below the ferruginous crust that forms the base of the Hildoglochiceras Bed is very similar in lithology but contains abundant cm-sized irregular cavities filled with several generations of sparite (Fig. 6b, c)
The Green Ammonite Beds (GAB), called Umia Ammonite Beds, constitute a well-known marker horizon that can be traced from Katesar in the west to beyond NNE of Lakhapar (Fig. 1), for a distance of more than 10 km, before it is lost at a fault
Summary
A maximum flooding surface/zone is marking the end of shoreline transgression (Catuneanu 2006), i.e., it marks the position of the seafloor furthest away from the coastline during a cycle of fluctuating sea level. The two fossil-rich units at the focus of this study occur in the western part of the basin (Fig. 1), in the Tithonian part of the Jhuran Formation as is evidenced by ammonites. The lower of the two condensed units, the so-called Hildoglochiceras Bed, is Early Tithonian in age (Pandey et al, submitted) and occurs in the informal Shivparas Siltstone member. The upper unit, the Green Ammonite Beds, forms most of the Umia Ammonite member and is late Early Tithonian to early Berriasian in age. The Hildoglochiceras Bed, in contrast, has only recently been discovered It is characterized by a high carbonate content, which is unique in the Jhuran Formation, whereas the Green Ammonite Beds are rich in iron minerals such as berthieroid ooids and grains.
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