DEEP WATER CANYONS IN THE SNUG HARBOR SILTSTONE AND POMEROY ARKOSE MEMBERS, NAKNEK FORMATION, ALASKA—NEW INSIGHTS INTO THE SEQUENCE STRATIGRAPHY OF THE LATE JURASSIC COOK INLET FOREARC BASIN

Recent field studies of Upper Jurassic strata in lower Cook Inlet yield insights into the depositional environments, stacking architecture, and sequence stratigraphy of the forearc basin margin. In the Iniskin Peninsula area, the Naknek Formation is >1300 m thick, comprising four members (in ascending order): Chisik Conglomerate (fan delta), lower sandstone (shelf), Snug Harbor Siltstone (outer shelf and slope), and Pomeroy Arkose (base of slope to basin floor). Outcrop-based observations, including geologic mapping of an ~50-km-long outcrop belt, led to the discovery of two deep water canyons in the Snug Harbor–Pomeroy interval. These km-scale canyons were incised into slope strata of Snug Harbor and host channelized to tabular fills of sandstone and mudstone to amalgamated sandstone successions that are hundreds of meters thick. The canyons—floored by unconformities overlain locally by boulder-bearing conglomerate—served as conduits through which sediment bypassed to the deep water depositional system of the Pomeroy and were ultimately backfilled by onlapping elements of that system. We interpret the canyon floors as a sequence boundary that we tie to a correlative interval in inter-canyon areas; furthermore, candidate transgressive and maximum flooding surfaces are proposed at the base of and within Snug Harbor, respectively. These three surfaces envelope a transgressive systems tract (lower Snug Harbor) and a highstand systems tract (mid-Snug Harbor) and define the base of a deep water lowstand systems tract (upper Snug Harbor and lower Pomeroy). The Snug Harbor transgressive surface caps the basal Naknek members—Chisik and lower sandstone (in part lateral equivalents)—that likely comprise a lowstand systems tract overlying the Middle Jurassic Chinitna Formation along a sequence boundary. Thus, a complete, probable third-order (m.y.-scale duration) stratigraphic sequence occurs in the lower three members of the Naknek, with record of a renewed accommodation succession cycle lying above the canyon sequence boundary. This new sequence stratigraphic framework sheds predictive light on facies distribution in this part of the basin’s underexplored petroleum system and renders a new play concept for Cook Inlet—coarse-grained canyon fill encased, at least in part, by fine-grained slope strata.