PROVENANCE AND DEXTRAL STRIKE-SLIP FAULTING ON THE BORDERS OF THE CHUMSTICK FORMATION BASED ON CONGLOMERATE CLASTS TIED TO SOURCE AREAS, CENTRAL WASHINGTON

Early to Late Eocene nonmarine sedimentary and volcanic rocks (Chumstick Fm.) were deposited in a complex forearc setting adjacent to the precursor arc to the Cascades. The Chumstick basin formed in an extensional half-graben or transtensional setting, bounded by high-angle oblique faults. These major right-lateral strike-slip faults are the Leavenworth fault zone (LFZ) on the west and the Entiat fault zone (EFZ) on the east. The central Eagle Creek fault zone (ECFZ) divides the Chumstick basin into the western and eastern subbasins. As a result, sediment deposition was coeval with multiple tectonic partitioning events along the LFZ, ECFZ, and EFZ.

Narrow belts of a cobble-boulder conglomerate within the Chumstick Fm. are exposed adjacent to the LFZ in the western subbasin, and the ECFZ in the eastern subbasin. Clast counts were obtained from eight cobble-boulder conglomerate beds distributed throughout the conglomerate belt to document variations in provenance. In the western subbasin, conglomerate detrital modes are dominated by metamorphics (schist, banded gneiss – 50-60% of clasts) and felsic plutonics (foliated tonalite, tonalite, quartz diorite – 42-52%). There are no apparent variations in provenance from north to south along the conglomerate ridge. In the eastern subbasin, conglomerate detrital modes vary spatially from north to south. To the northeast, conglomerate detrital modes are dominated by metamorphics (biotite gneiss, schist, amphibolites – 42-62%) and other rock types (quartzite, various sedimentary rocks – 26-30%). In contrast, the conglomerate detrital modes in the central eastern subbasin are dominated by felsic plutonics (tonalite, diorite – 50-27%) and metamorphics (biotite gneiss, schist – 31-37%). All conglomerates within the eastern subbasin have minor clast populations from igneous dikes (5-21%), increasing towards the ECFZ. Thirty thin sections of conglomerate clasts were made to aid in identifying the lithology for correct correlation to adjacent source terranes.

This study will help us to quantify offset on the LFZ and possibly the EFZ by identifying clast lithologies derived from localized rock assemblages exposed across the faults. Mineral identification and geochemical analyses to match lithologies across the fault will allow us to determine minimum fault offsets.