AGE AND EVOLUTION OF MID-CENOZOIC STEPOVER BASINS IN SOUTHEASTERN ALASKA

The Paleocene-Oligocene Kootznahoo Formation in SE Alaska provides a glimpse of sedimentation and transform tectonics during and after the transition from subduction to a transform margin. The Kootznahoo Formation consists of distinctive fluvial deposits of cross-bedded sandstone, gravel and conglomerate up to several kilometers thick, and lies 10-80 km west of the Coast Mountains. Paleocurrent indicators consistently show paleoflow to the SW, indicating the sediment was mainly derived from the high topography of the Coast Mountains, consistent with provenance work. There are about a dozen small depocenters and two larger depocenters, which we refer to as the Angoon and Kake basins. These two basins (1) have strata that dip moderately to the SE, (2) are equidimensional and about 10-15 km across, and (3) have late Paleocene to late Oligocene strata, although the Kake basin is dominantly Paleocene and the Angoon basin is dominantly Oligocene. These two basins are asymmetric, with a wedge-shaped geometry that thickens to the SE, consistent with being ‘trap door’ or stepover basins within a nascent right-lateral fault system. We infer NW-striking dextral faults defining the basin margins, with one or more NE-striking normal faults connecting them in the stepover region and creating accommodation space for deposition. The NW-striking dextral faults are truncated by the Chatham Strait fault, and thus these faults were active prior to Neogene activity on the Chatham Strait – eastern Denali faults. Subsequent late Oligocene-Miocene volcanism and magmatism of the Tkope-Portland Peninsula belt were focused along the inferred basin-bounding dextral faults.

Kootznahoo deposition began before the end of Coast Mountains batholith magmatism at 55 Ma, but was primarily after ~50 Ma when SE Alaska became a transform margin. Some Kootznahoo deposition and associated faulting was coeval with deposition in basins along the Denali fault in the Yukon and central interior Alaska. Cessation of Kootznahoo deposition is coincident with the rise of the Alaska Range and the collision of the Yakutat terrane into southern Alaska. We infer a large-scale reorientation of crustal stresses related to Yakutat collision, and dextral faulting focused on the modern Queen Charlotte-Fairweather fault resulted in cessation of Kootznahoo deposition.