STRATIGRAPHY, STRUCTURE, AND PROVENANCE OF EOCENE SEDIMENTARY ROCKS IN THE CHUMSTICK BASIN, CENTRAL WASHINGTON; IMPLICATIONS FOR BASIN EVOLUTION AS A RESULT OF SPREADING RIDGE SUBDUCTION

The Chumstick basin formed during the early to middle Eocene in a complex transtensional zone adjacent to the magmatic belt that preceded the Cascades. Major Eocene right-lateral strike-slip fault zones bound and subdivide the Chumstick basin and caused at least two episodes of tectonic partitioning during sediment deposition. The faults are the western Leavenworth fault zone (LFZ), the eastern Entiat fault zone (EFZ), and the central Eagle Creek fault zone (ECFZ). Nonmarine sedimentary and volcanic rocks (Chumstick Fm.) were deposited coeval with regional deformation and magmatism from an episode of spreading ridge subduction west of the study area.

Five lithofacies associations consisting of conglomerate, arkosic sandstone, mudstone, and interbedded tuffs characterize the ~6-8 km thick Chumstick Fm. Sixteen sandstone samples reveal four main U-Pb detrital zircon age populations: Late Cretaceous (100-65 Ma; 60% of 1369 grains), Early Cretaceous (145-100 Ma; 13%), Jurassic (176-145 Ma; 9%) and Middle Eocene to Paleocene (40-65 Ma; 8%). A total of 1202 clasts from fifteen conglomerate clast counts were identified within the Chumstick. Felsic plutonic (foliated tonalite, tonalite, quartz diorite – 64% of 445 clasts) and metamorphic (schist, banded gneiss - 33%) clasts dominate western conglomerates. Metamorphic (biotite gneiss, quartzite, schist – 50% of 423 clasts), felsic plutonic (27%), and mafic-intermediate volcanic (dikes - 12%) clasts characterize eastern conglomerates. Proximal to the ECFZ, conglomerate clasts consist of metamorphics (gneiss, quartzite, amphibolites - 32% of 334 clasts), felsic plutonics (28%), and mafic-intermediate volcanics (dikes - 22.5%).

These new datasets confirm past research that Chumstick strata were derived from dominantly eastern source terranes by alluvial-fluvial processes within a pull-apart basin between the EFZ and LFZ. Upper Chumstick strata were deposited within the eastern transtensional subbasin following fault reorganization and initiation of the ECFZ, while lower basin strata were inverted and folded. More broadly, this study will significantly progress our understanding of how strike-slip faulting affects lithofacies distribution and provenance changes as a result of the long-term sedimentary and structural response to ridge subduction.