Depositional Systems, Paleoclimate, And Provenance Of The Late Miocene To Pliocene Beluga And Sterling Formations, Cook Inlet Forearc Basin, Alaska

Abstract

The sedimentary record of forearc basins provides critical clues to the complex geologic history of subduction zones. This study focuses on Cook Inlet forearc basin, part of a larger forearc basin complex in southern Alaska. Specifically, I investigated the sedimentology, paleoclimate, and provenance of the Beluga and Sterling formations, comprising the late Miocene to Pliocene Cook Inlet basin record. These interpretations are used to reconstruct the Miocene-Pliocene basin history and better understand forearc development. Before ~11 Ma anabranching/single channel depositional systems of the Beluga Fm. deposited sediment on the western and eastern margins of Cook Inlet. Sandstone compositional data suggest sediment from the eastern margin was sourced from the accretionary prism. Between ~11 Ma and ~8 Ma deposition of the Beluga Fm. waned and sandstone compositional data indicate increases in volcanic lithic fragments derived from the volcanic arc to the northwest. Deposition by the southward-directed, sandy braided fluvial systems of the Sterling Fm. started on the western margin of the basin and migrated eastward, reaching the eastern margin ~8 Ma. By ~6 Ma, sandstone compositional data suggest that the volcanic arc was the dominant sediment contributor to the basin. Palynological results suggest that forests were predominately confined to coal swamps, and the surrounding floodplains were occupied by shrubs, herbs, and dispersed tree taxa. Thermophilic taxa persisted until at least ~6 Ma. Mean annual precipitation (MAP), estimated from delta13C values, ranged from 420 to 3900 mm a-1 with the greatest variability ~8 Ma. This ~8 Ma event correlates with a decline in sea surface temperatures of the Alaska Gyre and a North Pacific climate optimum. Climate likely played a minor role in fluvial style change. The dramatic change in depositional style between the Beluga and Sterling fms. is attributed to a change in sediment flux from the accretionary prism to the volcanic arc and western Alaska Range, most likely due to orogen-scale tectonic processes driven by far-afield flat-slab subduction of the Yakutat microplate. The change in fluvial style and sediment flux starting ~11 Ma suggests a previously unrecognized exhumation in the western Alaska Range at this time.