Tidally influenced coal-bearing sediments in the Tertiary Bering River Coalfield, south-central Alaska

Abstract

Tertiary coal-bearing sediments of the Tokun Formation in the structurally complex Bering River Coalfield, south-central Alaska, comprise stacked, glauconitic, fining- and rare coarsening-upward sequences up to 7 m thick. Fining-upward sequences consist of a scoured base, overlain by shale pellet conglomerate, coarse- to medium-grained, well-sorted, planar and trough cross-bedded sandstone, containing skeletal remnants of crabs and bivalves, and carbonaceous shale and coal beds. Coarsening-upward sequences are the inverse of the fining-upward sequences; stacking of these sequences produces megasequences up to 50 m thick. The fining- and coarsening-upward sequences are interpreted to have been deposited in tidal inlet and back barrier lagoon, and marsh environments that formed part of a mesotidal barrier island-estuarine complex. Seaward migration of inlet channels and flood tidal deltas produced fining-upward sequences capped by back barrier lagoon and marsh deposits. Vertical stacking of sequences is attributed to a series of transgressive-regressive events controlled by differential rates of sediment supply and subsidence. Consideration of the plate tectonic setting of the Bering River Coalfield and the established tectonic control on Tertiary sedimentation in Alaska, suggests that megasequence deposition was most probably related to longer term regional tectonic events, such as fault-controlled basin subsidence.