LePain, David L.¹, Paul J. McCarthy², Russell Kirkham³
(1) Wisconsin Geological and Natural History Survey, Madison, WI 
(2) University of Alaska, Fairbanks, AK
(3) Department of Geology & Geophysics, University of Alaska, Fairbanks, AK

ABSTRACT: Outcrop Expression of Sequence Boundaries in the Albian-Cenomanian Nanushuk Formation, Central North Slope, Alaska

The Nanushuk Formation represents the shallow marine to nonmarine part of a thick clastic wedge of basinal through alluvial depositional systems that extend from the Chukchi Sea eastward to the Sagavanirktok River in the central North Slope of Alaska. These systems record west to east filling of an asymmetric foreland basin (Colville basin) that formed in response to north-vergent thrust loading of continental crust beginning in latest Jurassic and culminating in Neocomian time. Flexural subsidence and resulting accommodation were greatest near the present-day Brooks Range mountain front and decreased northward. Changes in facies stacking patterns in marine deposits from south to north across the Nanushuk outcrop belt illustrate regional variations in accommodation. Along the south side of the outcrop belt, where accommodation was greatest, parasequences up to 90 m thick are arranged in thick progradational sets. In shelf and shoreface deposits from this setting, sequence boundaries are cryptic and identified by subtle seaward shifts in facies belts near the tops of sandier-upward successions. Along the north side of the outcrop belt, where accommodation was less, parasequences range from 10 to 25 m thick and are arranged in easily delineated sets that include sand-rich amalgamated parasequences near their tops. Abrupt seaward shifts in facies belts within these amalgamated successions allow easy identification of sequence-bounding unconformities. Discontinuous exposure between the north and south sides of the outcrop belt and poor to nonexistent biostratigraphic control make correlation between high- and low-accommodation areas difficult and pose significant challenges for future work in the region.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.