SEDIMENTOLOGY OF THE "COBBLESTONE SANDSTONE" OF THE FORTRESS MOUNTAIN FORMATION, CENTRAL NORTH SLOPE, ALASKA

The “Cobblestone sandstone” (informal) of the Fortress Mountain Formation was deposited during Aptian(?) time in the proximal foredeep of the Colville Basin. At Cobblestone Creek, several structurally dismembered sections display a variety of turbidite facies. One 130-m thick partial section comprises at least six depositional units composed of turbidite sandstones and massive to graded sandstones with little mudstone. Depositional units are dominated by solitary to amalgamated sandstones 0.5 to 4 m thick exhibiting massive to graded, coarse- to fine-grained Bouma A facies with basal flute casts and other sole marks with an east to northeast paleocurrent direction. Some beds contain sequences with Bouma A, B, and C divisions (massive to parallel laminated to ripple laminated). One section includes up to 15 m of channelized, cobble- to boulder conglomerate. Collectively, these sandstones and conglomerates on Cobblestone Creek might represent a succession of proximal-fan turbidites. Most of the turbidite section at Cobblestone Creek is oil bearing, and the oil was typed as having a possible source from the Shublik Formation. Near Toolik Lake, a 100-m thick partial section of the Cobblestone sandstone exhibits a proximal succession of amalgamated cobble- and boulder-bearing channel deposits with clasts up to 1m that have cut into thinner, massive to graded Bouma A turbidites and thin-bedded turbidites. This section might represent slope or proximal fan-channel conglomerates. Given the grain size of the Cobblestone sandstone and its possible distribution in the subsurface, the Cobblestone sandstone might be a potential petroleum reservoir in the Foothills area of the central North Slope.