DECIPHERING LOWER CRETACEOUS MUDSTONE OF ALASKA'S NORTH SLOPE: HIGH-RESOLUTION STUDIES OF LITHOFACIES AND SEQUENCE, DEPOSITIONAL PROCESSES, AND PETROLEUM SOURCE-ROCK POTENTIAL

Inspired by Gil Mull and the complex variation in petroleum source rock potential suggested by few existing data, we are doing high-resolution studies of the pebble shale unit (PSU) and lower part of the Hue Shale (HS) of the North Slope. Our aim is to answer fundamental questions about lithofacies variability, distribution, and depositional processes for this predominantly mudstone Lower Cretaceous succession (LCS) that is said to represent the transition from northern sourced sediment to post-rift, southern or Brooks Range sourced sediment.

Petrographic and SEM studies of 3 sections (Canning River, Mikkelsen Bay State-1 and Orion-1 wells) indicate that except for a much coarser basal unit of pebbly to sandy mudstone and sandstone of the PSU, the LCS is primarily clay dominated mudstone and silt-bearing clay-rich mudstone with less, but variable, amounts of interbedded carbonate-cemented mudstone, tuff and tuffaceous mudstone, and sandstone. These lithofacies are present with different abundance and sequence in all sections. Sand lamina are commonly crudely layered and poorly sorted in the PSU in contrast to well layered and well sorted in the HS. Mud-supported sand-sized clasts with rare granules and pebbles occur throughout much of the studied succession; variable bioturbation is present throughout; and a distinctive pelleted fabric occurs in much of the clay-dominated intervals. Rock-Eval pyrolysis for the Mikkelsen well indicates good petroleum source rock potential (TOC's 2.1-5.9 wt %; HI's 110-449) and different kinetics for the PSU and HS.

In as much as the LCS was deposited at similar latitude to its position today, although during greenhouse conditions, it is not surprising that some modern Arctic processes are important to its interpretation. We argue that melting seasonal sea-ice was a major source both of coarse- and fine-grained sediment to the LCS. In addition, nutrients released from melting ice probably fueled high primary organic productivity in the marginal ice zone, and organisms associated with the ice pelleted much of the sedimentary detritus. This decreased its residence time in the water column and along with high productivity led to the deposition of organic-rich sediments. In addition to melting seasonal sea-ice, eolian, fluvial and volcanic sources also contributed sediment.