AGES AND PROVENANCE OF THE SWAUK AND CHUMSTICK FORMATION UNITS NEAR THE EOCENE LEAVENWORTH FAULT ZONE, NORTH CASCADES, WASHINGTON

The North Cascades is an excellent area to study basins and strike-slip faults that formed during regional Eocene transtension. This study focuses on the southern flank of the Cascades crystalline core where the Eocene Leavenworth fault zone (LFZ) separates Eocene non-marine clastic rocks of the Chumstick Formation to the east, from the older Swauk Formation to the west. U-Pb dating of tuffs reveals that depositional ages are between 49-46 Ma and >59-51 Ma, respectively. The age and provenance of these formations provide insights to the controversial movement history of the LFZ. Samples of the Swauk Formation are from two stratigraphic levels ~1.5-2 km west of the LFZ. The Chumstick sample was collected ~2 km east of the main fault in the Beehive Reservoir area of the Devil’s Gulch unit, which consists mostly of unsorted and unstratified diamictite.

Most of the detrital zircons (n=109) from the lower Swauk sandstone cluster in three age peaks. The youngest and most prominent peak is ~60 Ma, a second peak is ~68 Ma, and the third is ~90 Ma. The sample only yielded one Precambrian (~1741 Ma) grain. Likely source terranes include the Cascades crystalline core, which is subdivided into the Wenatchee and Chelan blocks, separated by the dextral-normal Entiat fault. The Wenatchee block, located west of the Entiat fault and LFZ, is a potential source for the ~90 Ma zircons, and some may be derived from the 96-91 Ma Mt. Stuart batholith, which is directly north of the Swauk Formation and NW of the LFZ. Conversely, the Chelan block, which contains younger magmatic rocks than the Wenatchee block, is most likely responsible for the ~60 and 68 Ma peaks. Detrital zircons (n=45) from the upper sandstone of the Tronsen Ridge member cluster in 3 peaks. The youngest and most prominent peak is ~95 Ma, the second peak is ~146 Ma, and the third peak is ~560 Ma. The likely source for the ~95 Ma zircons is the Mt. Stuart batholith. Collectively, the age data for the Swauk samples are compatible with local sources in the Cascades core. The Chumstick sample is a sandstone interbedded with monolithologic (tonalite) breccia. Detrital zircons (n=110) yield a single peak at ~91 Ma and have a clear Mt. Stuart batholith source. This sample is ~20 km SE of the batholith along the strike of the LFZ, supporting other evidence for 20-30 km of dextral slip on the fault.