Organic C-Rich Mudstone Sedimentation in Arctic Alaska During a Seasonally Cold Early Cretaceous (Hauterivian-Barremian)

Keller, Margaret A.¹, Joe H.S. Macquaker² (1) USGS Menlo Park, Menlo Park, CA (2) University of Manchester, Manchester, United Kingdom

New models for mudstone deposition and diagenesis have come from recent studies utilizing high-resolution petrography and SEM. These and geochemical techniques applied to the Lower Cretaceous of Arctic Alaska address basic questions about lithofacies, organic facies, depositional settings and processes, and petroleum source potential.

Results from the pebble shale unit (psu) and Gamma Ray Zone (GRZ) of the Hue Shale at 3 localities (Canning River, Mikkelsen Bay State-1 and Orion-1 wells) indicate that the succession consists of a basal pebbly to sandy mudstone and sandstone, overlain primarily by burrowed and pelleted clay-rich mudstones that are either silt or fine-sand-bearing or clay dominated. Subordinate, bedding-parallel carbonate-cemented mudstones, tuff and tuffaceous/bentonitic mudstones, and sandstones are also present. Importantly, mud-supported sand-sized clasts, some rounded and pitted, with rare granules and pebbles occur scattered throughout much of this succession; and psu genetic beds (2-5 mm) are more crudely bedded and poorly sorted than GRZ. Rock-Eval pyrolysis on Mikkelsen cores indicates that both formations contain primarily Type II, oil prone kerogen.

During deposition of the psu, in addition to distal fluvial, volcanic, and eolian sources, melting of sediment-laden, seasonal sea-ice -- recently recognized in the modern Arctic – probably episodically contributed silt, mud, primary organics, and coarser sediment to this basin. While regionally, fluvial input to the studied psu successions is considered less important than sea-ice, locally its influence is likely to be greater. For the GRZ, distal turbidites and volcanic ash contribute more sediment than sea-ice, and hiatuses with carbonate cementation are more common.