Marine-continental transitions in a greenhouse world : reconstructing late Cretaceous deltas of paleopolar Arctic Alaska and Utah
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Near horizontal (6° dipping) outcrop exposures of Upper Cretaceous (Santonian-Campanian) strata at Shivugak Bluffs in northern Alaska preserve an extensive record of a clinoform-topset system. These strata are generally subdivided lithostratigraphically into proximal shelf, deltaic, and shallow marine deposits of the Schrader Bluff Formation and lower delta plain, coastal plain, and fluvial deposits of the continental Prince Creek Formation. Shivugak Bluffs includes 400 m of continuous marine deposits overlain by 140 m of strata containing the marine–continental transition between the lower Schrader Bluff and Prince Creek formations. The marine-continental transition is one of the few outcrop expressions of an ancient, muddy, prograding river-dominated deltaic system that contains interdistributary bays that shoal upward into floodbasins with pedogenic modification. The lowermost 400 m of the lower Schrader Bluff Formation is divided into the Rogers Creek, Barrow Trail, and Sentinel Hill members interpreted as recurring deposits of river-dominated deltas comprising distributary mouth bars (DMBs), subaqueous terminal distributary channels (TDCs), interdistributary bays, medial delta front deposits, distal delta front deposits, and prodelta deposits interbedded with proximal shelf deposits. One interval within the Rogers Creek Member comprising the most hummocky cross-stratified (HCS) interval at Shivugak Bluffs is interpreted as wave-reworked DMB-TDC complexes or storm sheets. The Schrader Bluff (West Sak and Tabasco equivalent in the subsurface) and Prince Creek (Ugnu equivalent in the subsurface) formations are relevant to industry as outcrop analogs for numerous shallow, viscous- to heavy-oil reservoirs on the central North Slope, Alaska. From a reservoir perspective, a 36-m-thick subset of the Alaska succession within the Rogers Creek Member is compared to 36- and 34-m-thick wave-dominated successions of the Kenilworth and Grassy Members of the Blackhawk Formation in the Book Cliffs in eastern Utah. The Rogers Creek Member includes amalgamated DMB-TDC complexes (54%) with minor HCS wave-reworked deposits (46%). This succession is compared with the Kenilworth and Grassy members that exhibit predominantly swaley and HCS intervals (75–81%) with minor channel complexes (14–25%). The Blackhawk Formation, based on this analysis, is a poor reservoir analog for the lower Schrader Bluff Formation of Arctic Alaska.