FACIES STACKING PATTERNS IN STORM-INFLUENCED DELTA FRONT AND SHOREFACE SUCCESSIONS, NANUSHUK FORMATION, CENTRAL NORTH SLOPE, ALASKA

Fluvial-deltaic strata of the Albian-Cenomanian Nanushuk Formation form the top of a thick wedge of basinal to alluvial depositional systems extending from the Chukchi Sea eastward to the central North Slope. These systems record the west-to-east filling of a large foreland basin formed in response to north-vergent thrust loading of continental crust starting in Late Jurassic time. The marine part of the Nanushuk along the south side of its outcrop belt includes thick parasequences that consist, in ascending order, of burrowed mudstone and bioturbated HCS sandstone, recording deposition in storm wave-influenced lower shoreface and delta-front settings. Parasequences are characterized by subtle facies differentiation and sequence boundaries are cryptic. This facies motif continues upward to the base of the marine-nonmarine transition, where parasequences are capped by a variety of nearshore and marginal-marine facies and display significant facies differentiation, recording deposition in upper shoreface/delta-front to backshore settings. In the northern part of the outcrop belt, the marine Nanushuk includes thinner parasequences that consist, in ascending order, of burrowed mudstone, interbedded mudstone and HCS sandstone, bioturbated HCS sandstone, and amalgamated HCS and SCS sandstone, recording deposition in storm-influenced offshore transition through shoreface/delta-front settings. Parasequences are characterized by pronounced facies differentiation. Abrupt facies dislocations from offshore to upper shoreface facies and offshore transition to fluvial channel sandstone facies are associated with prominent sequence boundaries, and are characteristic of the uppermost Nanushuk in this part of the outcrop belt. These relations suggest the possibility of significant shelf bypassing to supply deepwater systems in latest Albian and Cenomanian time.