Stratigraphy, age, and provenance of the Eocene Chumstick basin, Washington Cascades; implications for paleogeography, regional tectonics, and development of strike-slip basins: Reply

Abstract

We welcome the comment by Evans (2022) and the opportunity to further discuss our study of the Chumstick Formation. The correlation of fault-bound nonmarine sedimentary units in central and western Washington has been a topic of interest, and debate, for many years (Frizzell, 1979; Taylor et al., 1988; Gresens et al., 1981; Gresens, 1983; Evans and Johnson, 1989; Evans, 1994; Cheney and Hayman, 2009). However, many questions about the regional correlation of these units were resolved with the publication of a suite of internally consistent high-precision 206Pb/238U zircon dates from volcanic interbeds throughout the early to middle Eocene stratigraphy (Eddy et al., 2016). This data set confirmed the timing of sediment deposition of the different members within the Chumstick Formation. Donaghy et al. (2021) provides a detailed study of the Chumstick Formation, which builds on earlier research by Gresens et al. (1981, 1983), McClincy (1986), and Evans (1994) by incorporating new geochronologic information and additional clast counts, detrital zircon geochronology, and facies mapping. We interpret large parts of the Chumstick Formation to represent a spatially and temporally distinct sedimentary system between the Leavenworth and Entiat fault zones that likely formed as a pull-apart basin. Evans (2022) objects to several of the interpretations presented in Donaghy et al. (2021) regarding the relationship between different members of the Chumstick Formation and surrounding sedimentary units, the timing of strike-slip faulting, and the regional tectonic setting of these rocks. We discuss each of these points in the following sections.