New Insights from Uniformitarianism Applied to Hauterivian-Barremian Organic Carbon-Rich Mudstones of Arctic Alaska
Over the past decade, our studies of three sections of the Lower Cretaceous pebble shale unit and lower part of the Hue Shale on Alaska’s North Slope have produced informative new petrographic images with linked geochemical data. The studied interval consists of a stacked succession of organic carbon-rich (2-6 wt % TOC), thin-bedded (2-5 mm) mudstone. Individual beds are sharp-based, commonly crudely upward fining, and rarely show evidence of erosion. Common to rare mud-supported, “outsized” sand- to pebble-size grains are present as either isolated clasts, or clusters concentrated at the base of beds. The succession is predominantly silt-bearing clay-rich mudstones and clay-dominated mudstones with clay and silt organized into organo-mineralic aggregates, pelleted, or homogenous. These mudstones are variously bioturbated by a diminutive meiofauna and contain calcispheres, foraminifera, and radiolaria. Rare macro-burrows cut across bedding. Subordinate tuff, tuffaceous mudstone, silt-dominated mudstone, and sandstone are present. Authigenic pyrite and carbonate are present throughout the succession, forming rare to common carbonate cement-dominated mudstone or pyrite laminae.
From its high latitude origin (≈70˚N), organic richness, distinctive mud-supported “outsized” clasts, poorly sorted discontinuous silt- and sand-rich basal laminae with commonly wavy sharp bases, and crude upward fining, we interpret much of this succession to result from intermittent deposition by suspension settling from melting sediment-laden seasonal sea ice and associated primary productivity similar to modern processes in the Beaufort Sea. Carbonate cement-dominated intervals indicate important breaks in sedimentation. Pelleted organic-carbon rich units associated with tuffs exhibit some of the best petroleum source rock potential in the pebble shale unit, probably indicating that ash falls episodically contributed otherwise limiting nutrients. These results document considerable newly recognized variation in sedimentary facies and fabrics in this mudstone succession, and provide critical evidence to interpret these mudstones directly from modern marine processes. This approach shows great promise for interpreting mudstones.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009