Stratigraphy, age, and provenance of the Eocene Chumstick basin, Washington Cascades; implications for paleogeography, regional tectonics, and development of strike-slip basins Erin E. Donaghy1,†, Paul J. Umhoefer2, Michael P. Eddy1, Robert B. Miller3, and Taylor LaCasse4 1 Department of Earth, Planetary, and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA 2 School of Earth Sciences and Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA 3 Department of Geology, San Jose State University, San Jose, California 95192, USA 4 Department of Geology, Carleton College, Northfield, Minnesota 55057 USA ABSTRACT tions can be constrained at high temporal Here we present a large provenance data set resolution (0.5–1.5 m.y. scale) for an ancient coupled with new lithofacies mapping from Strike-slip faults form in a wide variety strike-slip basin and permits a detailed re- the Chumstick basin within the framework of a of tectonic settings and are a first-order construction of sediment routing pathways recently developed precise depositional chronol- control on the geometry and sediment accu- and depositional environments. As a result, ogy (Eddy et al., 2016b). This basin formed in mulation patterns in adjacent sedimentary we can assess how varying sediment supply a strike-slip setting in central Washington and basins. Although the structural and depo- and accommodation space affects the depo- provides a unique opportunity to track changes sitional architecture of strike-slip basins is sitional architecture during strike-slip basin in sediment routing systems that are related well documented, few studies of strike-slip evolution. to rapidly changing paleogeography in basin- basins have integrated depositional age, bounding basement blocks. This is the first time lithofacies, and provenance control within INTRODUCTION that detailed lithofacies mapping and provenance this context. The Chumstick basin formed in variations can be constrained to 0.5–1.5 m.y. central Washington during a regional phase Classic strike-slip basins are typically char- timescales within an ancient strike-slip basin, of dextral, strike-slip faulting and episodic acterized by (1) high sediment accumulation and our data demonstrate the importance of magmatism associated with Paleogene ridge- rates, (2) scarce igneous activity, (3) abrupt lat- changes in localized topography, sediment sup- trench interaction along the North America eral lithofacies changes, (4) thickening of sedi- ply, and basin accommodation space in creating margin. The basin is bounded and subdi- mentary sequences over short distances, (5) nu- the complex depositional architecture of strike- vided by major strike-slip faults that were merous unconformities that reflect syn-tectonic slip basins. active during deposition of the intra-basinal, sedimentation and fault reorganization, and (6) non-marine Chumstick Formation. We build locally derived fault-margin alluvial fans (Crow- TECTONIC SETTING OF THE on the existing stratigraphy and present ell, 1974a, 1974b; Sylvester, 1988). Many strike- CHUMSTICK BASIN new, detailed lithofacies mapping, conglom- slip basins have a well-defined stratigraphic and erate clast counts (N = 16; n = 1429), and structural architecture (e.g., Crowell, 1974a, A tectonic belt from Oregon and Washington sandstone detrital zircon analyses (N = 16; 1974b; Allen and Allen, 2013), but they rarely to British Columbia to southern Alaska is as- n = 1360) from the Chumstick Formation to have integrated precise depositional ages and a sociated with complex ridge-trench interaction document changes in sediment provenance, robust provenance data set within this architec- during the Paleogene (e.g., Bradley et al., 2003; routing, and deposition. These data allow ture. The resulting poor age control results in Madsen et al., 2006). The effects of this process us to reconstruct regional Eocene paleo- limited knowledge of the timing of how basin include regional dextral strike-slip faulting as drainage systems of Washington and Or- accommodation space and sediment accumula- a result of oblique convergence of the Kula (or egon and suggest that drainage within the tion patterns vary as basin-bounding fault pat- Resurrection) and North American plates (Friz- Chumstick basin fed a regional river system terns evolve in a strike-slip setting. For example, zell, 1979; Ewing, 1980; Vance and Miller, 1981; that flowed to a forearc or marginal basin a rapidly migrating basin depocenter is a key Johnson, 1982; Engebretson et al., 1983; Wells on the newly accreted Siletzia terrane. More component in strike-slip basin models (Chris- et al., 1984; Haeussler et al., 2003; Madsen et al., generally, excellent age control from five tie-Blick and Biddle, 1985; Sylvester, 1988; 2006), near-trench magmatism associated with interbedded tuffs and high sediment accu- Crowell, 2003b) but is difficult to reconstruct migration of the Kula (or Resurrection)-Farallon mulation rates allow us to track the evolving in ancient strike-slip basins without basin-wide spreading ridge along the continental margin sedimentary system over the Formation’s stratigraphic markers. Holistic data sets that (Madsen et al., 2006; Cowan, 2003; Bradley ca. 4–5 m.y. depositional history. This is the combine excellent geochronologic and strati- et al., 1993; 2003), and widespread exhumation first time lithofacies and provenance varia- graphic data permit detailed reconstructions of of mid-crustal rocks (e.g., Miller et al., 2016). changing sediment routing pathways and depo- Strike-slip faulting was active within Washing- sitional environments relative to a strike-slip ba- ton and southern British Columbia from at least †[email protected]. sin’s faulting history. 50 Ma until the start of the modern Cascades arc GSA Bulletin; Month/Month 2021; 0; p. 1–21; https://doi.org/10.1130/B35738.1; 10 figures; 1 table; 1 supplemental file. For permission to copy, contact [email protected] 1 © 2021 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B35738.1/5246152/b35738.pdf by Central Washington University user on 09 March 2021 Donaghy et al. A C Figure 1. (A) Reference map shows the study area within the state of Washington. (B) Simplified geologic map em- phasizes the metamorphic and igneous terranes in the North Cascades core adjacent to the Chumstick and Swauk basin. Plutons are orange, red, and pink, and crystallization ages are defined by colors in key. (C) Map of the Chumstick ba- sin in respect to the adjacent Wenatchee and Chelan blocks. The eastern (ES) and western subbasins of the Chumstick basin are defined. Note that the western subbasin is split into the northern western sub- basin (NWS) and the southern western subbasin (SWS) by the Wenatchee River. Abbre- viations: CH—Chaval pluton; CPP—Cloudy Pass pluton; CRB—Columbia River Ba- salts; EFZ—Entiat Fault zone; ECFZ—Eagle Creek Fault zone; LFZ–Leavenworth fault zone; NQ—Napeequa Com- plex; SM—Sulfur Mountain pluton; WD—Wenatchee Dome; WRG—Wenatchee Ridge Gneiss. Figure modi- fied from Schuster (2005) and B Miller et al. (2009). at 45–40 Ma (e.g., Umhoefer and Miller, 1996) granitoid batholith (Tabor et al., 1984, 2003). the Leavenworth and Entiat faults and was later and may have continued until 34 Ma, when During this period, the non-marine Chumstick subdivided by the Eagle Creek fault zone (Fig. 1; the last major fault system was intruded by a basin formed in central Washington between Tabor et al., 1984; Evans, 1991, 1994). 2 Geological Society of America Bulletin, v. 130, no. XX/XX Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/doi/10.1130/B35738.1/5246152/b35738.pdf by Central Washington University user on 09 March 2021 Provenance and tectonics of the Chumstick basin There is controversy regarding the struc- tural setting and stratigraphic architecture of the Chumstick basin and its relationship to the adjacent Swauk basin to the west (Cheney and Hayman, 2009; Johnson, 1984; Tabor et al., 1984; Evans, 1996). Some workers argue that deposition occurred regionally, and that basin- bounding reverse faults cut and deformed the ba- sin following deposition (Cheney and Hayman, 2009). In contrast, others consider the Chum- stick basin to have formed as a strike-slip basin (Johnson, 1984, 1996) or during an early period of extension followed by strike-slip partition- ing as an extensional half-graben (Evans, 1994, 1996). Recent improvements in our understand- ing of regional tectonics and in constraining the depositional ages of Eocene sedimentary units throughout western Washington have started to resolve some of these controversies by showing that sedimentation within the adjacent Eocene sedimentary units was temporally distinct from deposition in the Chumstick basin (Eddy et al., 2016b). Within this new tectonic framework, the Chumstick basin is interpreted to have formed during a period of regional strike-slip faulting immediately following the ca. 51–49 Ma ac- cretion of the Siletzia oceanic plateau to North America (Massey, 1986; Wells et al., 2014; Eddy et al., 2017). Both the basin-bounding Entiat and Leavenworth faults have been in- terpreted as dextral strike-slip faults that were active during basin formation and likely con- trolled basin development (Fig. 2). Estimates of displacement on both structures are between 20 km and 30 km (Tabor et al., 1987). How- ever, given the absence of clear piercing points and that the Columbia