SEDIMENTOLOGY,STRUCTURAL FRAMEWORK, AND BASIN ANALYSIS OF THE EASTERN ARKOSE RIDGE FORMATION, TALKEETNA MOUNTAINS, ALASKA

New sedimentological and structural data from Paleocene-Eocene strata exposed in the southern Talkeetna Mountains record active subsidence, fluvial-lacustrine deposition, and pyroclastic volcanism in a remnant forearc basin. This study focuses on the eastern part of the 60-km-long Arkose Ridge Formation (ARF) outcrop belt.

Measured sections document siliciclastic and volcanic strata with preserved thicknesses >900m. Exposures between the Chickaloon River and Boulder Creek consist of conglomerate with pumice/tuff clasts, cross-stratified sandstone, carbonaceous siltstone, and coal that represent fluvial-lacustrine environments routinely influenced by pyroclastic eruptions. Volcanic interbeds include welded tuff-breccia with bombs, welded lapilli tuff, crystalline tuff, pyroclastic flows, and sparse lavas. To the southwest, sections between the Kings and Chickaloon Rivers are dominated by carbonaceous mudstone and cross-stratified sandstone that represent fluvial-lacustrine environments. Volcanic interbeds include pumice-rich lapilli tuff, laminated crystalline tuff, pyroclastic flows, and rare lavas. Southwestward lithofacies transitions and paleocurrent data indicate that volcanogenic strata were partly derived from the Caribou Creek volcanic field (CV).

Geologic mapping demonstrates that eastern ARF strata unconformably overlie Jurassic igneous and sedimentary rocks. This relationship implies latest Cretaceous-Paleocene erosion of Cretaceous marine forearc strata. Four structure types deform eastern ARF strata. A northeast-trending shear zone with high-angle faults juxtaposes northern ARF strata and Jurassic rocks. Northeast-trending folds and thrust faults shorten ARF strata. North-trending normal faults with down to the east offset juxtapose ARF strata and Jurassic igneous rocks.

In summary, eastern ARF strata record fluvial-lacustrine deposition influenced by pyroclastic eruptions in a remnant forearc basin. Our new data suggest a southwestward proximal to distal trend away from the CV, which appears to have been a major sediment source. The depositional and deformational history is consistent with tectonic models that infer Paleocene-Eocene spreading ridge subduction followed by Oligocene-Recent flat-slab subduction of the Yakutat microplate.