THE SCHRADER BLUFF–PRINCE CREEK FORMATION TRANSITION AT SHIVUGAK BLUFFS: A PALEOENVIRONMENTAL RECONSTRUCTION OF A MUDDY, POLAR DELTAIC SYSTEM FROM THE LATE CRETACEOUS OF ARCTIC ALASKA

Upper Cretaceous (Santonian to Campanian) shallow-marine, deltaic, and continental strata from the upper 200 m at Shivugak Bluffs along the Colville River of northern Alaska preserve a high-paleolatitude, fluvial-deltaic succession deposited along a prograding paleo-Arctic Ocean coastline. Strata are interpreted and subdivided lithostratigraphically into proximal shelf, shallow-marine, and deltaic deposits of the Schrader Bluff Formation and lower delta plain, coastal plain, and fluvial deposits of the continental Prince Creek Formation. Ichnology, sedimentary structures, sandbody geometries, and vertical stacking patterns were critical to identify eleven unique paleoenvironments within a dominantly muddy but overall coarsening upward succession. The Schrader Bluff–Prince Creek transition includes distal to proximal deposits of a river-dominated delta overlain by a meandering to braided fluvial system, recording the preservation of both autogenic deltaic processes and allogenic erosional surfaces. The delta front and interdistributary bays of a river-dominated delta transitioned, during lobe reorganization and abandonment, into lakes and floodbasins containing immature soils on the lower delta plain, alongside distributary channels. An ensuing marine transgression flooded over the floodbasin deposits, based on Phycosiphon and Helminthopsis found within proximal shelf and prodelta muds. The overlying delta front, interdistributary bay, distributary mouth bar, and subaqueous to subaerial distributary channel deposits signaled the reactivation of the delta lobe. These deltaic environments were ultimately incised into and overlain by braided channels during a marked drop in relative sea level and progradation of the fluvial system. Workers examining shallow, viscous-to-heavy oil reservoirs on the North Slope should consider Shivugak Bluffs as one of the best outcrop analogues for the Schrader Bluff–Prince Creek transition. Uniquely, this is one of the few documented cases of an ancient muddy deltaic shoreline. This study highlights the interplay of autogenic and allogenic processes recorded in facies trends, fluctuating sandbody and shale geometries, variable trace-fossil assemblages, and preserved surfaces within an ancient, high-latitude, mud-rich deltaic system.