Stratigraphy, Structure, and Geochronology of Eocene Sedimentary Rocks in the Chumstick Basin, Central Washington

Erin Donaghy
Northern Arizona University, School of Earth Sciences and Environmental Sustainability
Flagstaff, Arizona, United States of America
[email protected]

The purpose of this study is to reconstruct the evolution of the Chumstick basin through stratigraphic correlations, structural analyses, and geochronologic analyses of middle to late Eocene sedimentary and volcanic rocks. The Chumstick Formation consists of 8-12 km of nonmarine strata and 15+ interbedded tuffs that formed in the forearc region in central Washington as an extensional half-graben or transtensional basin at ~50 Ma. Basin development occurred coeval with regional deformation and magmatism from a well-documented episode of spreading ridge subduction ~60-40 Ma, and the collision of the Siletzia terrane with the Pacific Northwest ~50 Ma. Consequently, two major Eocene dextral strike-slip fault zones bound and subdivide the Chumstick basin and caused multiple events of tectonic partitioning that created a complex lithofacies distribution. Correlation of lithofacies across the basin to establish depositional architecture and structural relationships is poorly defined because past work was based on poor chronological datasets. Fieldwork consisted of geologic mapping to document vertical and lateral architecture of tuff and lithofacies packages in order to establish their relationships with faults throughout the study area. Sedimentary and volcanic samples were collected throughout the Chumstick Formation for geochronologic and thermochronologic analyses to determine the age of sediment source terranes and the timing of uplifted sources. These analyses, integrated with conglomerate clast counts, will yield spatial and temporal changes in provenance. As a result, this study will significantly progress our understanding of the long-term sedimentary and structural response to spreading ridge subduction and oceanic ridge collision in the Pacific Northwest.

AAPG Search and Discovery Article #90183©2013 AAPG Foundation 2013 Grants-in-Aid Projects