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

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