Recurrent hardgrounds and their significance for intra-basinal correlations: a case study of upper Bathonian rocks from the western margin of the Indian craton

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.

Kilometre-scale coral carpets on mixed carbonate-siliciclastic platforms; a sedimentological study from the Lower Cretaceous of northwestern Africa

“…The sub-circular scours and pits observed may also have formed from the erosion of soft sediment by increased wave-action (e.g. Bromley., 1992;Pandey et al, 2018). These surfaces show no evidence of sub-aerial exposure in thin section for example vadose zone diagenetic features or pedogenetic features like root traces at the outcrops.…”

The earliest-published recognition of a trace fossil and its producer

“…B: The first figure of identified traces: Aldrovandi's specimen identified as having being inhabited by dactyli, attributed by Baucon (2009) to Gastrochaenolites and another, compared with the shape of serpents, and attributed by Baucon (2009) to Cosmorhaphe; image from (Aldrovandi, 1648) piddocks, which have specialised drilling ridges on their shells and rotate down into the rock. These then live in their chambers for their entire lives (Pinn et al, 2005). Other bivalve groups, such as hiatelloids, are associated with Gastrochaenolites, but also occupy empty boreholes created by others (Kelly, 1980).…”