STRATIGRAPHIC AND STRUCTURAL CHARACTER OF THE KUSKOKWIM GROUP, NORTHERN KUSKOKWIM MOUNTAINS, SOUTHWESTERN ALASKA

The Late Cretaceous Kuskokwim Group, a six to twelve kilometer thick sedimentary sequence in southwestern Alaska, was deposited during a period of regional thrust belt and foreland basin development in the Cordillera. Despite its temporal significance, studies of the Kuskokwim Group have largely been in the context of regional mapping and biostratigraphy. Previous work has generally interpreted the Kuskokwim Group as mainly submarine fan deposits with minor deltaic, marginal marine, and nonmarine strata, but little is published on the sedimentological development of the basin.

New basin analysis data from the Kuskokwim Group in the central part of the outcrop belt document three distinct depositional environments in an overall upward-shallowing sequence. The oldest facies association (middle Turonian based on macrofossils) is characterized by sandstone beds with massive bases that grade upward into ripple laminated siltstone and shale, and interbedded thick shale. We interpret this facies association to represent delta-front, turbidite deposits. The second facies association (Campanian-Maastrichtian based on limited palynology) contains progradational packages of massive shale overlain by interlayered sandstone and shale, and finally by internally reworked, thick, tabular sandstone sheets. This facies association is interpreted to represent delta-front sediments deposited above storm wave base, but below normal wave base. The third facies association (Campanian based on limited palynology) contains interlayered organic-rich, coaly shale with abundant plant detritus interlayered with trough cross-bedded sandstone. We interpret this facies association to represent sedimentation in an abandoned delta/inner distributary bay environment that may have been influenced by tidal processes.

Consistant north and north-northwest verging structures in the Kuskokwim Group are well exposed along the Kuskokwim River. Detailed mapping in the vicinity of Crooked Creek and Sleetmute revealed dominantly asymmetric, closed to tight, upright and overturned, short wavelength (1 km) folds and south-southeast dipping faults. Folds are consistantly overturned to the north-northwest and have long upright limbs with relatively short overturned limbs. These structures represent overall north-northwestward tectonic transport.