TEMPORAL AND GEOGRAPHIC DISTRIBUTION OF ANOXIA IN THE HEMI-PELAGIC CARBONATES OF THE TURONIAN-CAMPANIAN NIOBRARA FORMATION, U.S. WESTERN INTERIOR

Sometimes called OAE3, organic-rich sediment accumulation and anoxia characterized some restricted basins and continental margins during the Coniacian-Campanian. This was also a time of massive pelagic carbonate deposition across large parts of the world ocean, including epicontinental seas. In the Western Interior Sea (WIS) of North America, organic-rich, hemi-pelagic carbonates were deposited across a wide geographic interval over >3 myr. In the central part of the basin, detailed geochemical and paleontological data have documented the onset and continued strengthening of anoxia in the Smoky Hill Chalk member of the Niobrara Formation. We use planktic and benthic foraminifera to extend this record from the central basin into eastern and western CO, KS, WY and NM to study the timing of the onset of anoxia and its duration in order to understand its impact on the biological community of the WIS.

Foraminiferal data reveal several important trends across the sea: 1) during the transgression of the Niobrara Sea, prior to the onset of anoxia, benthic foraminiferal abundance generally increases, and keeled planktic foraminifera invade the central and eastern portions of the study area where carbonate content is highest. 2) As the transgression slows in the early Coniacian and reverses into regression, markers for oxygenation also slow and reverse, slowly declining until a point in the middle Coniacian S. ventricosus ammonite zone, equivalent to the base of the lower shale unit of the Smoky Hill, where they rapidly drop, suggesting benthic and deep water column anoxia. 3) On the western margin of the sea, these changes occur later than those observed to the east. For example, in the S. ventricosus zone, characterized by strong anoxia and a near complete lack of benthics in the central WIS, benthic foraminifera are abundant and diverse to the west. This suggests an east-west gradient in anoxia, where carbonate-rich sediments to the south and east are more prone to anoxia.