THE STRUCTURAL AND DEPOSITIONAL EVOLUTION OF PULL-APART BASINS: AN EXAMPLE FROM THE EOCENE CHUMSTICK BASIN, CENTRAL WASHINGTON

Strike-slip basins are a fundamental class of basin and their structural and depositional evolution has been widely studied for decades. Previous studies on pull-apart basins, formed at strike-slip releasing bends or fault step-overs, have shown that basin geometry, subsidence, and stratigraphic architecture is determined by the ratio between basin-bounding fault overlap and basin width. However, the timing of changes to the basin’s length to width ratio as basin-bounding faults propagate and link is debated. We interpret the Eocene Chumstick basin in the North Cascades, WA as a pull-apart basin with a fault bend at its northern end that formed during a period of regional, dextral strike-slip faulting. The Chumstick basin is between the Leavenworth and Entiat faults and contains clastic rocks deposited in alluvial-fluvial and lacustrine settings. Basin strata are interbedded with 18 tuffs. Some of these tuffs serve as distinctive marker beds throughout the basin. Excellent age control from five of these tuffs allows us to document variations in lithofacies, provenance, and sediment accumulation rates at the 0.5-1.5 m.y. scale over the 4-5 m.y. depositional history of the basin. By putting the basin architecture into context of detailed chronostratigraphy, we are able to assess: 1) how sediment accumulation rates vary in different depositional environments of the basin for age-equivalent strata, and 2) better constrain the relationship between basin-margin fault propagation and migration of the main basin depocenter (area of greatest subsidence). Fault propagation and eventual linkage to create a fault bend plays an important role in the evolution of a pull-apart basin. This is the first time these variables have been temporally constrained with this precision in an ancient pull-apart basin, and provides valuable insight into how accommodation space and sediment supply varies during changing basin geometry in the lifespan of a pull-apart basin.