A Mid-Cretaceous Association of Shell Beds and Organic-rich Shale: Bivalve Exploitation of a Nutrient-Rich, Anoxic Sea-floor Environment

Abstract

The Toolebuc Formation (Late Albian) is a thin (<40m), very widely distributed unit marking the maximum deepening of the Cretaceous epicontinental sea recorded by the infill of the Great Artesian Basin, eastern Australia. It consists of organic-rich shale, with TOC ranging to 35%, and limestone as laminae and thin beds comprised of Inoceramus sutherlandi McCoy and, less commonly, Aucellina hughendenensis Etheridge. Finely interlayered organic-rich shale and coquina are typical of the formation, resulting in a distinctively black-and-white, thinly bedded to laminated rock. Inoceramus valves are commonly disturbed by breakage, reorientation, and imbrication, and, in many cases, have disintegrated into prism horizons. Sedimentary laminae show that the sea floor was subject to some current activity, but benthic scavengers are considered to have been active agents of shell disturbance. Although shelly substrate suitable for encrusting epibenthos or colonization by endoliths was available in abundance, other benthic elements, inclusive of trace fossils, are very poorly represented. The formation contains diverse planktonic, pelagic, and nektonic fossil remains attesting to deposition beneath a water column of normal salinity, supporting a complex food chain. It displays a negative δ13Corg excursion, considered to relate to sea floor bacterial reworking of detrital organic material derived from plankton. In the context of the Toolebuc Formation, Inoceramus sutherlandi and Aucellina hughendenensis represent ecological specialists, with shell-growth strategies designed to cope with soft substrates and physiologies that were tolerant of oxygen-poor bottom conditions. Their intimate association with organic-rich shale suggests a trophic link with sea-floor bacterial productivity, supported from the organic-rich substrate. The lateral extent of Toolebuc coquinas suggests that Inoceramus sutherlandi and Aucellina hughendenensis were filter feeders rather than dependent on chemotrophic symbionts. Fine-scale interlayering of coquina and organic-rich shale attests to frequent alternation of sea-floor conditions conducive to colonization by bivalve communities and those that were not. Alternation is attributed in part to an autocyclic mechanism, with sea-floor accumulation of shelly debris having progressively isolated bivalve communities from trophic support, causing their episodic demise.