Abstract
A set of two to three prominent hardgrounds can be traced for more than 40 km from east to west within the Jurassic succession of the Jaisalmer Basin at the western margin of the Indian Craton. The hardgrounds started to form under subtidal conditions in a mixed carbonate–siliciclastic setting during the last phase of a transgressive systems tract, i.e. the maximum flooding zone. The age difference between the hardgrounds is very small, but they differ lithologically. Typically, the stratigraphically oldest hardground occurs at the top of a 1-m-thick calcareous sandstone. It is characterized by a spectacular megaripple surface encrusted with oysters and subsequently occasionally bored by bivalves. The hardground is overlain by 10–25 cm of biowackestone to biopackstone, at the top of which another hardground is developed. This second hardground is characterized by abundant bivalve (Gastrochaenolites isp.) and “worm” borings (Trypanites and Meandropolydora isp.) and occasional oyster encrustations. The third hardground can be found within the overlying 60-cm-thick, bioturbated, fossiliferous silty marly packstone. It shows common to abundant oyster encrustations and occasional borings together with reworked concretions. The individual hardground can be well recognized throughout the basin based on lithology and biotic components. The second hardground (biowackestone to biopackstone) with abundant bivalve and worm borings is most prominent and widespread. Lithostratigraphically, these three hardground surfaces belong to the uppermost part of the Bada Bag Member of the Jaisalmer Formation. Based on ammonites, such as Perisphinctes congener (Waagen), brachiopods, and corals, this interval of the Bada Bag Member has been assigned a late Bathonian age. The entire succession above the first hardground is bioturbated up to the overlying marly silt of the Kuldhar Member of the Jaisalmer Formation, which is already Callovian in age. The characteristic hardground lithologies, together with the ammonite record, allow long-distance correlations within the basin emphasizing their importance as valuable marker horizons. The biotic components associated with the hardgrounds and alternating sediments represent high diversity community relicts developed in shallow-water, open-marine environments.
Highlights
Kachchh Basin and Jaisalmer Basin are two neighbouring sedimentary basins at the western margin of the Indian Craton (Fig. 1a; e.g., Biswas 1971, 1977, 1991; Das Gupta 1975; Krishna 1987, 2017; Fürsich et al 2001, 2013; Pandey et al 2014), about 400 km apart
5 Results A set of two to three recurrent hardgrounds has been traced within the upper Bathonian Bada Bag Member of the Jaisalmer Formation from the southwestern to eastern part of the Jaisalmer Basin (Figs. 2, 3)
The hardground surfaces have been investigated at the following localities: hardgrounds are more common than siliciclastic ones (Taylor and Wilson 2003)
Summary
Kachchh Basin and Jaisalmer Basin are two neighbouring sedimentary basins at the western margin of the Indian Craton (Fig. 1a; e.g., Biswas 1971, 1977, 1991; Das Gupta 1975; Krishna 1987, 2017; Fürsich et al 2001, 2013; Pandey et al 2014), about 400 km apart These basins are well known for numerous fossiliferous horizons, shell beds, well-preserved trace fossils, condensed horizons, reworked concretions, hardgrounds, cyclic sedimentation and lateral thickening and thinning of stratigraphic units within the Jurassic succession (e.g., Singh 1989; Fürsich et al 1991, 1992, 2013, 2017, 2018; Fürsich and Oschmann 1993; Paul et al 2018). Further field investigations in other parts of the basin have revealed that these hardgrounds are widespread throughout the basin from east to west and are excellent marker beds of the upper Bathonian rocks This is very significant for stratigraphic correlation in the Jaisalmer Basin, where stratigraphic units commonly disappear or change in lithological character laterally. The present study deals with a detailed description of hardgrounds in the Jaisalmer Basin and discusses the implication of their wide extension
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