Sedimentological evidence for bottom-water oxygenation during deposition of the Natih-B Member intrashelf-basinal sediments: Upper Cretaceous carbonate source rock, Natih Formation, North Sultanate of Oman

Abstract

ABSTRACT Geologists have commonly argued that the deposition of the excellent carbonate source rock (up to 13.7% total organic carbon) in the Upper Cretaceous Natih-B Member intrashelf basin (water depth circa 50 m) was mainly controlled by the presence of bottom-water “anoxia” in the basin centre. Some authors have even linked the formation of the Natih-B organic-carbon-rich sediments to the global development of “oceanic anoxia” that occurred a number of times during the Late Cretaceous. Recent research, however, suggests that the mechanisms that underpin organic-carbon enrichment in intrashelf-basinal settings are complicated and, instead, controlled by the complex interplay of variations in primary production of organic carbon, clastic dilution, bottom-water anoxia, early diagenesis and optimising rates of sediment accumulation, and are not necessarily related to global-forcing mechanisms. In this study, the requirement for persistent bottom-water anoxic conditions for the preservation of organic matter in this setting is assessed, evidence for oxic/dysoxic bottom-water conditions during deposition of the Natih-B organic-carbon-rich sediments is presented, and alternative models to explain organic-matter enrichment are considered. Natih-B sediments (collected both spatially and temporally from both core and exposures in North Oman) have been investigated using a combination of optical and electron-optical (backscattered electron imagery) techniques, which provide additional data to those gathered by traditional field and geochemical methods. Natih-B lithofacies alternate between two main types: organic-carbon-rich carbonate mudstones and calcite-cement-rich wackestones. The organic-rich mudstones are typically fine grained, dark grey, exhibit remnant parallel lamina, and are partially burrowed. These units commonly contain planktonic foraminifera, coccoliths and organic matter (average about 5.4%, up to 13.7% total organic carbon). In addition, in-place bivalves (including thick-shelled oysters and flattened pectens) are present. The calcite-cement-rich wackestones are lighter in colour and extensively bioturbated (in most cases < 1.5% total organic carbon). This lithofacies comprises a mix of reworked skeletal fragments (including bivalves, gastropods, echinoderms, brachiopods and corals), ostracods, calcispheres, and both benthic and planktonic foraminifera that are pervasively cemented by calcite. Given the above observations, bottom waters during deposition of the Natih-B intrashelf-basinal sediments must have contained at least some oxygen, and it is unlikely that they were persistently “anoxic”. Instead, it is likely that short-term enhanced organic productivity, rapid delivery of organic components to the sediment/water interface, optimal rates of sediment accumulation and episodic burial were the fundamental parameters that controlled organic-carbon production and preservation. Organic-matter enrichment was, therefore, not restricted to anoxic depositional environments, and exploration strategies within intrashelf-basinal settings need to be expanded beyond times when basinal anoxia is thought to have existed.