OLIGOCENE TRANSPRESSIONAL STRIKE-SLIP BASIN DEVELOPMENT AND SEDIMENTATION ALONG THE DENALI FAULT SYSTEM, COLORADO CREEK BASIN, ALASKA RANGE

New sedimentological, petrofacies, and palynological data from the Colorado Creek basin (CCB) help evaluate the displacement history of the 2000-km-long Denali fault system (DFS). The CCB consists of ~455 m of Cenozoic strata that are bound by thrust faults oriented subparallel to the McKinley segment of the DFS. AGE – The age of the CCB was previously interpreted as Paleocene on the basis of sparse plant megafossils. Palynological analysis of mudstones (n=9) from a continuous stratigraphic section indicates early Oligocene deposition. DEPOSYSTEMS – Measured stratigraphic sections document two lithofacies associations. The lowermost 350 m of strata consist of imbricated, clast-supported conglomerate, cross-stratified sandstone, and carbonaceous mudstone interpreted as stream-dominated alluvial fan deposits. The uppermost 55 m of strata consist of lava flows, tuff, and pumice interpreted as subaerial volcanic deposits. PROVENANCE – Conglomerate clast counts (n=1012) document mainly sedimentary (71%) and igneous (15%) clast types. Source-diagnostic clasts in combination with southeastward-directed paleocurrent indicators (n=47) suggest that detritus was derived from local source terranes exposed along the DFS. BASIN DEVELOPMENT – New data from this study indicate that the CCB formed as a transpressional basin during the early Oligocene, concurrent with strike-slip faulting along the DFS. From a regional perspective, development of the CCB was synchronous with dextral displacement and strike-slip basin development along the DFS in southwestern Alaska, British Columbia, and Yukon Territory. Collectively, the presence of these basins indicates a major phase of late Eocene-Oligocene displacement along the DFS.