ANALYSIS OF ORGANIC CARBON ACCUMULATION ACROSS A BULK SEDIMENTATION GRADIENT: LATE CENOMANIAN, HARTLAND SHALE MEMBER, WESTERN INTERIOR SEAWAY

The accumulation of organic carbon in black shales may be controlled by preservational conditions, primary production, or dilution mechanisms, however it is commonly difficult to discern the relative influence of these controls in ancient strata. These depositional factors may be resolved and quantified if an accurate time-scale is developed that permits calculation of accumulation rates for organic carbon, as well as for proxies of redox state (preservation), siliciclastic input (dilution), and paleo-productivity. The Hartland Shale Member (late Cenomanian) of the Greenhorn Formation, Western Interior Basin, provides a rare opportunity to analyze organic carbon accumulation within such a temporal framework. This framework consists of high-resolution litho- and biostratigraphy integrated with a published Ar-Ar time scale based on bentonite ages from the section. To assess the spatial (proximal-distal) variability of organic matter accumulation, this time scale was applied to data from four sites in an east/west transect across the Western Interior Seaway, representing a major gradient in detrital sediment supply. These temporal and spatial records permit quantification of concentration and accumulation rate for selected proxies in time-equivalent strata of the western clastic margin, the central basin, and the eastern clastic-starved platform. As a complement to this accumulation rate record, elemental C:N:P ratios were analyzed to assess changes in degradation of organic matter both temporally and spatially. Preliminary results indicate: 1) There is an inverse relationship between %TOC and bulk sedimentation rate within the Hartland Shale Member. 2) Accumulation rate of organic carbon is highest in the center of the basin and diminishes landward. 3) The decrease in accumulation in the west can be attributed to dilution by terrigenous flux. 4) The decrease in accumulation in the east can be attributed to very low bulk sedimentation rate, increased exposure time of organic matter, and extensive microbial degradation.