Mudstone Sedimentation at High Latitudes: A Petrographic Study of the Lower Cretaceous Succession from the Mikkelsen Bay State #1 We l l , Alaska

By

J. Macquaker (University of Manchester, U.K.) and M. Keller (U.S. Geological Survey)

Studies of mudstones have concentrated on sediments deposited at relatively low latitudes. Here sediment transport and water column mixing are dominated by tide and storm activity and lithofacies variability is controlled by variations in primary production and clastic dilution. This study seeks to describe mudstone deposition at higher latitudes where additional sediment transport processes (e.g. ice rafting) operate, water column mixing are modified by seasonal ice cover, and lithofacies variations are influenced by productivity affected by variations in seasonal day length.

Lower Cretaceous mudstone (pebble shale unit and Hue Shale) deposited at high latitudes cored in Mikkelsen Bay #1 were investigated here. These analyses reveal distinctive and systematic lithofacies variations within the studied succession. Individual units are predominantly thin-bedded, upward-fining and partially bioturbated rather than being laminated. They exhibit a wide variety of lithofacies (e.g. silt-bearing clay-rich mudstones, silt and sand-bearing clayrich mudstones, carbonate cement-dominated mudstones), have varying source potentials (2–6% TOC), and commonly contain large quantities of very small pyrite framboids and coarse sand grains. Depositional textures indicate that the finest fractions of these mudstones were derived from fluvial input and transported to site of deposition by a combination of storms, tides and seasonal ice rafting. Depositional textures were then modified by burrowing meioinfauna. The presence of small framboids and bioturbation together raises an interesting paradox; a possible solution to which is that bottom waters were episodically anoxic because of short-term variations in bottom water O2 concentrations, perhaps caused by seasonal variations in day length and mixing. Depostional processes operating to preserve organic matter at high latitudes are therefore different from low latitudes. This means that depositional models of source rock formation at high latitudes may need modifying.

AAPG Search and Discovery Article #90008©2002 AAPG Pacific Section/SPE Western Region Joint Conference of Geoscientists and Petroleum Engineers, Anchorage, Alaska, May 18–23, 2002.