Stratigraphy, Age, and Provenance of the Eocene Chumstick Basin

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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 1Department of Earth, Planetary, and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA 2School of Earth Sciences and Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA 3Department of Geology, San Jose State University, San Jose, California 95192, USA 4Department 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.

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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 basin 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 subbasin (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 modified 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).

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There is controversy regarding the structural 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 basin 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 partitioning as an extensional half-graben (Evans, 1994, 1996). Recent improvements in our understanding 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 accretion 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 interpreted as dextral strike-slip faults that were active during basin formation and likely controlled 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 River basalts cover the basin to the southeast, these estimates are minimums.

STRATIGRAPHIC AND STRUCTURAL ARCHITECTURE OF THE CHUMSTICK BASIN

Figure 2. Simplified geologic map shows the Chumstick basin. The locations of all The Chumstick basin is ∼75 km long and data samples collected for geochronology, conglomerate clast counts, and measured 20 km wide and consists of ∼10.5 km of middle stratigraphic sections are displayed. Solid lines represent where tuffs were mapped in to late Eocene sedimentary strata and interbed- the field in this study. Dotted lines represent the locations of tuffs from previous mapping ded tuffs that can be divided into four distinct (McClincy, 1986; Tabor et al., 1984, 1987) and projections based on structural map members and three structural zones (Fig. 2). patterns. Adapted from Tabor et al. (1982). CC—Clark Canyon; CL—Camas Land; Major unconformities separate the four members CR—Camprec Road; DG—Devil’s Gulch; DP—Dirtyface pluton; EC—Eagle Creek; intro three packages, and from oldest to young- EFZ—Entiat Fault zone; ECFZ—Eagle Creek fault zone; FV—Fairview Canyon; L— est, they are: the age-equivalent Clark Canyon Leavenworth; LFZ—Leavenworth fault zone; MA—Malaga; MCR—Merry Canyon and Tumwater Mountain, Nahahum Canyon, Road; MI—Mission Creek; MO—Monitor; MR—Mission Ridge Ski Hill; NP—North and Deadhorse Canyon (Fig. 3; Evans, 1988). Plain; PP—Plain Pass; RT—Ranger Tower; SH—Ski Hill; SHI—South Highway; TW— The Clark Canyon and Tumwater Mountain Tumwater Mountain; VC—Van Creek; W—Wenatchee; WD—Wenatchee Dome; #2— Members occupy a structurally distinct, west- Number 2 Canyon Road. ern subbasin that is bounded by the Eagle Creek and Leavenworth faults (Fig. 2). This subbasin can be further divided into distinct northern and

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sented in Table A21 and Figure 3. Importantly, the tuffs within the Chumstick Formation are laterally continuous over broad areas (Evans, 1994; McClincy, 1986) and provide important marker beds that can be used for stratigraphic correlation. The Clark Canyon Member is ∼8.5–8.8 km thick, consists of conglomerate, sandstone, mudstone, and interbedded tuffs, and is exposed exclusively in the western subbasin (Fig. 3). Tuff ages from Eddy et al. (2016b) and calculated sediment accumulation rates indicate that the Clark Canyon Member was deposited between 49.2 Ma and 46.5 Ma. In the southern part of the western subbasin (south of the Wenatchee River; Fig. 2), strata are ∼7 km thick and the basal to lowermost part of the upper Clark Can- yon Member is exposed in a homocline that dips ∼40° to the west between the Eagle Creek fault zone and Leavenworth fault zone. The oldest strata of the Clark Canyon Member are exposed adjacent to the west side of a rhyolitic intrusive complex that intrudes the Eagle Creek fault zone (Wenatchee Dome; Fig. 2) and are likely faulted. Sediment accumulation rates for the lower and middle Clark Canyon Members range between 6 mm/yr and 7 mm/yr. Extrapolating our age model to the lowermost exposed part of the unit gives a minimum age for basin initiation of 49.2 Ma (Supplementary Material). Strata in the northern part of the western subbasin are ∼4.5 km thick and span the middle and upper Clark Canyon Member (Fig. 3). These strata form the northwest-plunging Peshastin syncline north of the town of Leavenworth as well as the doubly plunging Eagle Creek anticline (Fig. 2; Cheney and Hayman, 2007). The uppermost Clark Canyon Member strata are deformed in a series of smaller anticline-syncline pairs adjacent to northwest-trending segments of the Leavenworth fault zone. During deposition of

Figure 3. Generalized measured stratigraphic section shows poorly sorted boulder con- 1 glomerate, poorly moderately sorted cobble conglomerate with interbedded tuffs and cross- Supplemental Material. (1) Descriptions of spatial and temporal stratigraphic thickness stratified sandstones interbedded with massive mudstones, thinly interbedded sandstone variations in the Chumstick basin and methods and mudstone, and massive mudstones with rare sandstone. Ten of the interbedded tuffs are for sediment accumulation rate calculations, (2) not shown. Stratigraphic thickness of the Chumstick Formation varies spatially at differ- Detailed descriptions and photographs of each ent locations within the basin. The Chumstick Formation is divided into the Clark Canyon lithofacies association of the Chumstick Formation defined in the text of the manuscript, (3) Tables of and age-equivalent Tumwater Mountain Member, Nahahum Member, and the Deadhorse raw and summary conglomerate clast count data Member. Abbreviations: ecte—Eagle Creek tuff; tctc4—Clark Canyon 4 tuff; tctc2—Clark for each member of the Chumstick Formation, (4) Canyon 2 tuff. Summary tables of conglomerate detrital modes for each member of the Chumstick Formation, (5) Summary tables and age probability plots of detrital zircon ages from each sandstone sample southern zones (Fig. 1). The Nahahum Canyon con eruption/deposition dates from tuffs and collected within the Chumstick Formation, (6) Member occupies an eastern subbasin bounded two maximum depositional ages for sandstones Conglomerate clast raw data from LaCasse (2013) by the Eagle Creek and Entiat faults (Fig. 1). within the Chumstick basin, and we utilize their and (7) Tables of detrital zircon raw data from each The Deadhorse Canyon Member overtops the geochronology to calculate sediment accumula- individual sandstone sample within the Chumstick Formation (Donaghy, 2015). Please visit https:// Eagle Creek fault and is unconformably depos- tion rates within the basin. These methods are doi.org/10.1130/GSAB.S.13624076 to access ited on top of the other members. Eddy et al. outlined in the Supplementary Material, and our the supplemental material, and contact editing@ (2016b) presented six high-precision U-Pb zir- preferred sediment accumulation rates are pre- geosociety.org with any questions.

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the upper Clark Canyon Member, sediment ac- (Fig. 2). Some of this deformation is interpreted Canyon Member is exposed only in the northern cumulation rates decrease to ∼2.6 mm/yr, which to be syn-depositional based on the orientation part of the basin. This creates a shingling effect is possibly related to the rate being calculated of folds relative to the Entiat fault and presence of northwestward-younging strata that is further from strata deposited in a different depositional of soft-sediment deformation related to syn-dep- supported by vitrinite reflectance data from Ev- system from the lower Clark Canyon Member ositional earthquakes (Evans, 1994; Donaghy, ans (1988), which indicate a significance differ- strata (see discussion). Assuming that rates re- 2015). However, some of these structures can be ence between stratigraphic thickness (>12 km) mained constant during deposition of the upper- traced to broad folds within the Columbia River and basin thickness (>3.5 km). Based on our most Clark Canyon Member, the age of the un- basalts, which suggests that they were reactivated age control and correlation of tuffs from north to conformity that tops the Clark Canyon Member during or after the Miocene (Cheney and Hay- south in the western subbasin (Fig. 3), the strati- is estimated to be 46.5 Ma (Table A2). man, 2007). Determining a precise age of the graphic thickness (∼8.5 km) is not thickened The Tumwater Mountain Member consists Nahahum Canyon Member is difficult due to the by faults but was deposited by rapidly shifting of boulder-cobble conglomerate exposed only lack of interbedded tuffs. However, provenance lateral depocenters. This refined stratigraphic ar- along the Leavenworth fault zone (Fig. 3) and data presented below strongly suggest that the chitecture is consistent with an origin as a strike- is age-equivalent to the middle and upper Clark Nahahum Canyon Member is largely younger slip basin and demonstrates the utility of having Canyon Member along most of the basin mar- than the western subbasin. numerous well-defined, basin-wide marker beds gin. It interfingers with strata that are along The Deadhorse Canyon Member is ∼2.2 km throughout basin depositional history to resolve strike with sandstones of the upper Clark Can- thick, consists of mudstone and minor sandstone, structural and stratigraphic complexities. yon Member in the Ski Hill section (Fig. 2), and is found in the northern and southeastern part providing a maximum age for the Tumwater of the Chumstick basin (Fig. 2; Evans, 1994). INFERRED DEPOSITIONAL Mountain Member in the northern part of the The Deadhorse Canyon Member sits uncon- ENVIRONMENTS basin. A minimum age is provided by our ca. formably on sediments of both the eastern and 46.5 Ma estimate for the top of the Clark Can- western subbasin and overtops the Entiat fault We use six new lithofacies associations (FA) yon Member South of the Wenatchee River, the zone, Eagle Creek fault zone, and Leavenworth that are slightly modified from Evans (1988, Clark Canyon 4 tuff strikes into the Tumwa- fault zone. A maximum depositional age for this 1991) to characterize the Chumstick Formation: ter Mountain Member stratigraphically above unit is given by a CA-ID-TIMS detrital zircon poorly sorted, boulder-cobble conglomerate the outcrops at Mission Ridge Ski Hill, sug- age of 45.910 ± 0.021 Ma (Eddy et al., 2016b). (FA1); cobble-pebble conglomerate interbedded gesting that this part of the Member is at least Strata within the Deadhorse Canyon Member with coarse-medium-grained sandstone (FA2); 48.186 ± 0.026 Ma (Fig. 2; all tuff dates from may have been part of a regional depositional coarse-medium-grained sandstone interbedded Eddy et al. (2016b) are presented with 2σ ana- system correlative with the Roslyn Formation with massive sandy conglomerate (FA3); lentic- lytical uncertainties only). Age relationships to to the west (Evans, 1994; Eddy et al., 2016b). ular, cross-stratified sandstone interbedded with the south of, and consequently older than, Clark The Deadhorse Canyon Member was excluded organic-rich mudstone (FA4); thinly interbedded Canyon 4 tuff cannot be demonstrated. Howev- from the new total stratigraphic thickness of the sandstones and mudstones (FA5); and massive, er, we interpret that >48.186 ± 0.026 Ma strata Chumstick Formation due to the uncertainty of organic-rich mudstone interbedded with minor of the Clark Canyon and Tumwater Mountain its relation to the underlying units and because of intervals of lenticular sandstone (FA6). De- Members interfinger in the southernmost ba- its potential correlation to the Roslyn Formation. tailed descriptions, maximum particle size, and sin and possibly extend beneath the Columbia The presence of precisely dated tuff marker photographs of each lithofacies association are River basalts. beds permits us to laterally trace coeval sedimen- in the Supplementary Material. The map rela- The Nahahum Canyon Member consists of tary units across the basin despite locally discon- tions of the lithofacies associations are shown ∼1.5–2 km of mudstone and sandstone and is tinuous outcrop, rapid lithofacies changes, and in Figure 4. Based on the abundance of organic exposed exclusively in the eastern sub-basin structural complexity. This technique is critical material in sandstone and mudstone beds and (Fig. 3; Evans, 1994). The base of the member is for recognizing thickness variations and refining previous paleoclimatic studies, all strata were not exposed, and its relation to the Clark Canyon previous thickness estimates of the Clark Can- clearly deposited in a humid environment (New- Member is uncertain. A maximum depositional yon Member. For example, The Clark Canyon man, 1981; Evans, 1988, 1991). age (MDA) is provided by the youngest zircon 4 tuff in the southern and northern part of the Strata of FA1 are interpreted to represent identified during laser ablation–inductively cou- western subbasin was not previously correlated gravel deposition by alluvial-fluvial processes pled plasma–mass spectrometry (LA-ICP-MS) but is now known to be offset by a fault along the on steep-gradient alluvial fans that bordered analysis from a sandstone in the middle Nahahum Wenatchee River with left separation of ∼2.3 km basin-bounding faults. Boulder-cobble con- Canyon Member, which was subsequently dated (Fig. 2). Offset of tuffs and juxtaposition of older glomerates were deposited by braided stream to 46.902 ± 0.076 Ma (Eddy et al., 2016b). The sediments in the north on younger sediments in systems, although uncommon debris-flow de- Wenatchee Dome intrudes the Nahahum Canyon the south suggest a steep fault with left-lateral posits are also present (Evans, 1994). Interbed- Member along the Eagle Creek fault zone and and reverse slip along the Wenatchee River. Our ded conglomerates and sandstones of FA2 and yields a chemical abrasion–isotope dilution– ∼10.5 km estimate for the stratigraphic thick- FA3 suggest deposition by stream flow on the thermal ionization mass spectrometry (CA-ID- ness of the Chumstick Formation differs from medial to distal part of a low-gradient, braided TIMS) zircon age of 44.447 ± 0.027 Ma (Gilm- the previous interpretation of ∼13 km (Mc- stream-dominated alluvial slope. Slope calcula- our, 2012), constraining deposition between ca. Clincy, 1986; Evans, 1994) because our new tions by Evans (1988) are between 5 m/km and 46.9 and 44.4 Ma (Eddy et al., 2016b). Strata are stratigraphic correlations reveal repeated section 1 m/km depending on whether the calculation deformed into a major syncline that runs along in the McClincy (1986) and Evans (1994) esti- was from a coarse-grained (FA1–FA2) or fine- the subbasin axis and further deformed in a series mates. Furthermore, the oldest part of the Clark grained lithofacies (FA2, FA3, FA4). In contrast, of en echelon synclines and anticlines trending Canyon Member is exposed exclusively in the strata of FA4, FA5, and FA6 are finer-grained ∼300° adjacent to the Eagle Creek fault zone southern part of the basin, and the upper Clark and are interpreted to represent deposition by

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ed to represent sediment deposition in a lake that formed along the eastern subbasin axis (Evans, 1988; Donaghy, 2015). Overall, the Chumstick Formation is dominated by coarser-grained lithofacies in the western subbasin (Fig. 4), suggesting that deposition of the older Chumstick sediments was primarily by proximal to distal alluvial-fluvial processes. In the eastern subbasin, the Naha- hum Canyon and Deadhorse Canyon Members were deposited by meandering stream and lacustrine processes. There are rapid lateral facies changes in age-equivalent strata from FA1 and FA2 facies along faults bounding FA3 and FA4 along the basin axis in both subbasins. Using the tuffs as marker beds, there is also a fining trend of lithofacies in age-equivalent strata from the north to the south within the western subbasin (Fig. 4). As a result of tectonic activity along basin- bounding faults in strike-slip settings, rapid facies changes and gaps in the “expected” transition between lithofacies associations are frequent. These gaps possibly represent intra- formational disconformities or basin-wide unconformities during periods that were more tec- tonically active. It is important to reiterate that tuff ages facilitated the spatial correlation of different facies based on age rather than lithology and sedimentologic features. For example, one would not expect to correlate the FA2 conglomerates in the northern western subbasin to fine- grained sediments in the southern western subbasin (Fig. 4). However, precise ages of Clark Canyon tuff 4 (tctc4) in both the northern and southern parts of the western subbasin allowed for correlation of strata across the Wenatchee River (Fig. 4) in distinctively different lithologies. In areas where rock exposure is poor or discontinuous, and lithostratigraphic correlations are difficult, this technique is crucial for defining stratigraphy and understanding the spa- Figure 4. Simplified geologic map of the Chumstick basin is based on new lithofacies map- tial and temporal relationships between lithofa- ping and stratigraphic sections (blue circles) from Donaghy (2015). Lithofacies Associations cies in strike-slip basins. (FA) show poorly sorted boulder conglomerate (FA1—red), cobble-boulder conglomerate and sandstone (FA2—pink), sandy conglomerate (FA3—dark yellow), lenticular sandstone SEDIMENT SOURCES IN THE and mudstone (FA4—light yellow), mudstone and minor sandstone (FA5—blue), and mud- WASHINGTON CASCADES stone (FA6—green). See Supplementary Material (see footnote 1) for full descriptions of lithofacies associations and photographs. Light colors represent inferred continuation of A diverse assortment of metamorphic and lithofacies based on structure and previous mapping (Evans, 1988). ECT—Eagle Creek plutonic rocks surrounds the Chumstick basin tuff; ECFZ—Eagle Creek fault zone; EFZ—Entiat fault zone; FVT—Fairview tuff; L— (Fig. 1; Table 1) and reflects a complex series Leavenworth; LFZ—Leavenworth Fault zone; SS—Sunnyslope; ST—Sunitsch tuffs; of tectonic events including final mid-Creta- TCTC2—Clark Canyon tuff 2; TCTC4—Clark Canyon tuff 4; YT—Yaksum tuffs; tctc4— ceous accretion of the Insular belt to the Inter- Clark Canyon 4 tuff; tctc2—Clark Canyon 2 tuff. montane belt (the margin of North America at that time) and related deformation, metamorphism, and magmatism (McGroder, 1991); meandering stream, deltaic, and lacustrine meandering stream system. Laminated organic- long-lived arc magmatism and underthrusting processes. Thick mudstones and lenticular rich mudstone and minor sandstones of FA5 of forearc sediments in the Late Cretaceous to sandstones of FA4 were deposited by channel were deposited mainly in lacustrine settings. Paleocene (Matzel et al., 2004; Sauer et al., cut-and-fill and overbank flow processes in a Lastly, massive mudstones of FA6 are interpret- 2018); and Eocene accretion of Siletzia (Eddy

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TABLE 1. COLOR-CODED TABLE SHOWING AGE AND COMPOSITION OF SOURCE TERRANES IN THE ADJACENT CHELAN AND WENATCHEE BLOCKS. COLORS CORRELATE TO AGE RANGES THAT ARE SHOWN IN FIGURE 1 Name Expected detrital zircon peak ages Lithology (Ma) Wenatchee block (east) *Wenatchee Ridge Gneiss ca. 91 Light-colored, banded granitic gneiss; muscovite-rich Dirtyface pluton ca. 91 Tonalite Tenpeak pluton ca. 92–89 Tonalite Mount Stuart batholith 91 (south part); 96–92 (north part) Tonalite and lesser granodiorite, diorite, and gabbro Chiwaukum Schist/Nason Ridge Gneiss 125–120; 280–130; ca. 2.7 Ga Well-foliated pelitic schist Ingalls Complex ca. 192–145 Ultramafic rock, gabbro, basalt, chert, minor sandstone Chelan block (west) Railroad Creek pluton 46–45 Granodiorite Duncan Hill pluton 46–45 Granodiorite Golden Horn batholith 48.5–47.7 Granodiorite Cooper Mountain batholith 49–47.9 Granodiorite Entiat pluton ca. 73–71 Tonalite and quartz-diorite Cardinal Peak pluton ca. 78–72 Tonalite, granodiorite, and diorite Oval Peak pluton ca. 65 Tonalite Skagit Gneiss Complex 52–45; 75–60; 80–70 Orthogneiss, banded bitotie gneiss, paragneiss; metasedimentary rocks Black Peak pluton ca. 92–87 Tonalite Eldorado pluton ca. 88 Tonalite Seven Fingered Jack pluton ca. 92–90 Tonalite Swakane Gneiss 70–65; 78–68; 87–81; 200–145; 1.8–1.4. Ga Biotite-paragneiss; volumetrically significant felsic sills Napeequa Complex 70–66; 80–70 Zircon poor; quartzite and amphibolite; schist; volumetrically significant felsic sills Chelan Complex ca. 120–100 Migmatitic tonalite gneiss, tonalite Dumbell and Marblemount plutons ca. 220 Tonalitic orthogneiss Cascade River-Holden unit ca. 220 Hornblende-biotite schist and gneiss, amphibolite, calc-silicate rock and minor metaconglomerate Notes: Source ages and compositions compiled from Eddy et al. (2016a), Gatewood and Stowell (2012), Gordon et al. (2017, 2010), Haugerud and Tabor (2009); Haugerud et al. (1991), Hopson and Mattinson (1994), MacDonald et al. (2008), Matzel et al. (2006, 2004), Matzel (2004), Miller and Bowring (1990), Miller et al. (2016, 2009), Misch (1968, 1966), Sauer et al. (2018, 2017a, 2017b), Shea et al. (2016, 2018), and Tabor et al. (1987, 1982).

et al., 2017; McCrory and Wilson, 2013; Wells nant detrital zircon ages of 78–68 Ma in felsic an integrated view of sediment sources during et al., 2014) regional strike-slip faulting (e.g., injections into paragneisses of the Swakane deposition of the Chumstick basin. Umhoefer and Miller, 1996) and ridge-trench Gneiss, and the 79–65 Ma zircon ages of the Conglomerate clast counts were obtained interaction (Eddy et al., 2016b; Miller et al., Entiat, Oval Peak, and Cardinal Peak plutons from individual pebble-cobble conglomerate 2016). Magmatic episodes within the region are distinctive of the Chelan block. The ability beds within the Chumstick Formation (N = 16 are associated with largely tonalitic and grano- to identify these unique source signals in Chum- samples; n = 1255 total clasts; Supplementary dioritic melts that provide abundant zircons in stick strata, combined with a robust paleocurrent Material). The lithologies of 50–150 clasts were the potential sediment source areas surround- data set from Evans (1988) and compositional identified during each count, and individual clasts ing the Chumstick basin. Furthermore, the data from this study, allows us to document how from each lithology were collected for thin-sec- complex tectonic history has produced distinct rapidly changing paleogeography impacted sedi- tion petrographic analyses. Clasts were chosen regional differences in magmatic crystalliza- ment routing systems. randomly on a ∼2 × 2 m surface within a single tion ages, metamorphic grade, and rock com- conglomerate bed. During conglomerate clast position that can be linked to sediment prov- METHODS composition counts, the long axes of the 10 larg- enance (Table 1). est clasts per conglomerate bed were measured Distinguishing unique source terranes on both Distinct sedimentary sources combined for the maximum particle size (MPS) to aid in sides of the Chumstick basin is critical for deter- with our revised stratigraphic architecture of understanding depositional environments and mining where sediments were derived and how the Chumstick basin provide an opportunity to transport. These data are reported in the Supple- sources change throughout basin evolution. The assess provenance through time and space. Con- mentary Material. simplest distinction among the ages of crystalline sequently, we present new U-Pb detrital zircon Detrital zircons from 16 sandstone samples rocks in this region is that rocks to the east of the and clast count data sets. The combination of and from three boulder-sized tonalitic clasts Chumstick basin (Chelan block) have a younger these data sets allows for a more complete under- were analyzed using LA-ICP-MS, and isotopic magmatic history than the rocks to the west of standing of evolving sediment source regions. data are reported in the Supplementary Mate- the basin (Wenatchee block). In the Wenatchee For example, mafic igneous rocks tend to yield rial. All U-Pb detrital zircon geochronology block to the west, the Wenatchee Ridge Gneiss little zircon and could be missed as a potential was completed at the University of Arizona is the only location with a muscovite-rich gneiss source terrane if detrital zircon were exclusively LaserChron Center using a Nu Plasma multi- and distinctive fuchsite (Table 1). In the Chelan used. Also, coarse conglomerates represent prox- collector ICP-MS. Zircon grains were ablated block to the east, Eocene plutons (49–45 Ma) are imal alluvial fan deposits, and it is assumed that using a Photon Machine Analyte G2 excimer dominantly granodiorite in contrast to the older they derived sediments from proximal sources. laser equipped with a HelEX low-volume cell tonalitic plutons (Misch, 1966; Miller et al., Therefore, large clasts with distinctive litholo- and a laser spot diameter of 30 μm. Between 2009). Felsic and mafic dike swarms of this age, gies and/or ages can also be used as piercing 100 and 110 zircon grains from each sample which formed coeval with these plutons, are also points to constrain the minimal amount of offset were randomly selected for isotopic analyses, restricted to the Chelan block with the notable along basin margin strike-slip faults. Detrital zir- and the natural zircon reference materials SL2 exception of the basaltic Teanaway dike swarm con can record a more regional signal given the (535 ± 2.3 Ma; Gehrels et al., 2008) and R33 (Tabor et al., 1982, 1987), which is unlikely to mineral’s refractory nature. Consequently, our (419.3 ± 0.4 Ma; Black et al., 2004) were mea- have crystallized zircon. Furthermore, the domi- combined clast and detrital zircon data sets offer sured to constrain fractionation and provide a

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Figure 5. Pie diagrams show the temporal variation in the distribution of felsic-intermediate plutonic and metamorphic clast lithologies. Summary pie diagrams also show the overall variations in conglomerate detrital modes in different parts of the Chumstick Formation. The age-equivalent Tumwater Mountain Member has been split out from the Clark Canyon Member to emphasize along-strike changes in provenance within the basin- margin facies.

measure of analytical reproducibility. Dates for RESULTS erate beds within the Chumstick Formation zircon that are younger than 900 Ma use the (Figs. 2 and 3). Overall, clasts are primarily 206Pb/238U dates, while dates that are older than Conglomerate Clast Compositions felsic-intermediate plutonic (39% of all clasts) 900 Ma use the 207Pb/206Pb date as they offer the and metamorphic (38%) lithologies. Felsic to highest precision for dates within these respec- Sixteen conglomerate clast counts were ob- intermediate plutonic clasts remain a consistent tive age ranges. tained from individual pebble-cobble conglom- source of sediment throughout deposition of

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the Clark Canyon Member (38%–43%; mainly tonalite and diorite). There is a significant decrease of metamorphic clasts from the lower Clark Canyon to the lowermost part of the upper Clark Canyon Member (43% to 16%) and an increase of mafic-intermediate volcanic or dike clasts (1% to 33%). In the southern Chum- stick basin, the Tumwater Mountain Member conglomerate clasts are dominated by felsic plutonics (97%; tonalite). These differ significantly from Tumwater Mountain conglomerates in the northern part of the basin, which contain less felsic to intermediate plutonic (43%; mainly tonalite) and more metamorphic (52%; schist and gneiss) content. Conglomer- ate clasts in the Nahahaum Canyon Member are mainly metamorphic (54%; gneiss, quartzite, amphibolite) and minor felsic to intermediate plutonic (18%; granodiorite, tonalite, and diorite). From the Nahahum Canyon to the Deadhorse Canyon Member, there is an increase in intermediate (38%; rhyodacite) and mafic plutonic clasts (27%; gabbro) (Fig. 5). One of the most diagnostic changes in provenance is the significant increase in reworked sedimentary lithologies from the Clark Canyon Member into the Nahahum Canyon Member Figure 6. Age probability plots show distribution of U-Pb age determinations for detrital zir- (3%–10%) and the influx of granodiorite and con grains from the Clark Canyon and Nahahum Canyon Members. Ages represent individ- rhyodacite conglomerate clasts in the Naha- ual spot analyses from separate detrital zircon grains. U-Pb ages are plotted as a normalized hum Canyon and Deadhorse Canyon Members relative-probability distribution (Ludwig, 2003). Gray bars were chosen to highlight the (Fig. 5). Significant variations in conglomer- peak age populations and how they change throughout the Chumstick basin. The age range ate clast compositions between samples from of each bar is labeled at the top of the diagram. Tc—Clark Canyon Member; N—number the Clark Canyon, Nahahum Canyon, and of samples; n—total number of zircon grains; Pz—number of Paleozoic grains not shown; Deadhorse Canyon Members can be linked Pc—number of Precambrian grains not shown. to different sediment source regions. Further- more, conglomerate clast counts in the basin margin Tumwater Mountain Member record increase in the 93–90 Ma peak age population in DISCUSSION significant changes in provenance from age- the Upper Clark Canyon and Nahahum Canyon equivalent, basin-axis Clark Canyon Member Members (Fig. 6). From the lower Clark Can- Evolution and Paleogeography of the strata as well as within the Tumwater Moun- yon Member to the Nahahum Canyon Member, Chumstick Basin tain Member from south to north in the western there is also a significant increase in middle– subbasin (Fig. 5). early Eocene ages (53–40 Ma; 0% to 10%). The Paleoflow measurements to the southwest, Nahahum Canyon Member has latest Creta- south, and east (all data from Evans, 1988; U-Pb Geochronology of Detrital Zircons ceous (81–70 Ma; 23%), Late Cretaceous (100– Fig. 7) suggest that both eastern and western 87 Ma; 17%), Early Cretaceous (145–100 Ma; source terranes supplied sediment to the Chum- Similar to the clast composition data, there 15%), and Jurassic (200–145 Ma; 11%) zircon stick basin. Temporal and spatial changes in are significant variations in 16 detrital zircon ages (Fig. 6). These variations in detrital peak provenance and paleoflow between the different samples collected from the different members ages and how they relate to changing sediment members of the Chumstick Formation constrain of the Chumstick Formation (Figs. 2, 3, and sources will be discussed below in detail. Ad- (1) when specific source terranes in the adjacent 6). Probability curves indicate that the lower ditionally, boulder-sized tonalitic conglomerate bounding basement blocks were uplifted and Clark Canyon Member is dominated by latest clasts were dated from three sampling locations exhumed, (2) timing of local faulting, and (3) Cretaceous–early Paleocene ages (81–60 Ma; in the Tumwater Mountain Member along the changes in depositional environments and sedi- 56%), whereas the lower part of the upper Leavenworth fault zone (Table E4, see foot- ment routing systems. We track these changes Clark Canyon Member has an even distribu- note 1; LaCasse, 2013). U-Pb zircon dates of through six time periods below. They were se- tion of latest Cretaceous (81–70 Ma; 21%) and the three tonalite clasts, from south to north in lected because they are bracketed by dated tuffs Early Cretaceous ages (145–100 Ma; 21%). sampling locations (Fig. 2), yielded weighted or correspond to major changes in basin archi- There is an increase in early Late Cretaceous mean ages of 90.58 ± 0.53 Ma (n = 24, mean tecture. Within each time period, we discuss the ages (100–87 Ma) from the lower Clark Can- square of weighted deviates [MSWD] = 1.35), depositional architecture in terms of an eastern yon Member (24%) to the upper Clark Canyon 91.72 ± 0.75 Ma (n = 24, MSWD = 0.96), and low-gradient, stream-dominated fan formed off Member (37%; Fig. 6). Specifically, we see an 90.95 ± 1.06 Ma (n = 24, MSWD = 0.53). the Entiat fault zone—a series of steep alluvial

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B The latest Cretaceous detrital zircon age pop- A ulation (66–70 Ma; 29.2%) of the Lower Clark Canyon Member correlates with igneous zircon crystallization ages in volumetrically significant felsic sills in the Swakane Gneiss (∼5%–10%) and the Napeequa Complex (30%) and the Skagit Gneiss to the northeast of the Chumstick basin (Miller et al., 2016). Older latest Creta- ceous zircon ages (81–70 Ma; 26.2%) match the emplacement ages of the Cardinal Peak, Entiat, Riddle Peaks, and Kelly Mountain plutons; some of the Skagit Gneiss Complex; the felsic sheets in the Swakane and Napeequa units (Fig. 1; Miller et al., 2016); and other smaller intrusions in the Chelan block (e.g., Marble Creek and Hid- den Lakes intrusions and the Chelan tonalite) (Misch, 1966; Brown et al., 1994; Tabor et al., 2003). Igneous units of this age are absent in the Wenatchee block (Table 1). Therefore, these detrital zircon populations support the interpretation that sediment in the lower Clark Canyon Member was sourced mainly or wholly from the C D east and that crystalline rocks of Late Cretaceous age were exposed by 50–49 Ma in the Chelan block (Fig. 8). Plagioclase-rich and biotite-rich gneiss conglomerate clasts (61% and 14% of all metamorphic clasts) make up the majority of metamorphic clasts in the Lower Clark Can- yon Member and match the composition of the Swakane and Skagit Gneisses. Compositional variations of tonalite and quartz-diorite conglomerate clasts (94% of felsic-intermediate plutonic clasts; Fig. 5) may be derived from multiple intrusions in the Chelan block, but they are thought to be from the Entiat pluton because of its proximal position (Fig. 1). Furthermore, minor Triassic age populations match the age of the Chelan Com- plex immediately east of the Entiat pluton, and the Cascade River-Holden unit, Marblemount plutons, and detrital zircons in the Methow basin farther to the north (Fig. 1; Table 1). We can use the combined detrital zircon and Figure 7. Paleoflow data from Evans (1988) are shown. Shaded gray areas represent the conglomerate data to further study whether the interpreted area of sediment deposition. Figure modified from Evans (1994). sediments that fed the lower and middle Clark Canyon Member are from sources that were fans that bordered the western Leavenworth from the lower to middle Clark Canyon Member proximal or distal to the basin-bounding Entiat fault zone—and a mixing zone along the basin were derived from units to the east. The lower fault. The 80–60 Ma zircon ages and gneissic axis off the Leavenworth fault zone fan fringes. to middle Clark Canyon Member is exposed in conglomerate clasts overlap in age and compo- A distinctive detrital zircon signature and con- the southern part of the basin and is characterized sition of both the Swakane Gneiss and Skagit glomerate clast compositions, paleocurrent indi- by interbedded conglomerates, sandstones, and Gneiss (Table 1). However, we consider the cators, and lithofacies associations define each mudstones (Fig. 3). It is believed that a majority Swakane Gneiss to be the dominant source of these components. of sediments were deposited on a low-gradient, based on the relatively large sizes of gneissic stream-dominated alluvial fan system in a humid, clasts, suggesting that they have not traveled Lower–Middle Clark Canyon Member and tropical climate (Evans, 1988, 1991). Addition- far from the source (Table B1; see footnote 1). Inferred Age Equivalent Tumwater Mountain ally, boulder conglomerates of the southernmost This interpretation is further supported by Ju- Member (49.2–48.5 Ma) Tumwater Mountain Member indicate that a rassic (200–145 Ma; 10.6%) and latest Creta- Our new provenance data are consistent with component of sediment was also derived from ceous (87–81 Ma; 8.7%) detrital zircons, which paleoflow measurements (Fig. 7; Evans, 1988, the west and deposited in steep, alluvial fans that form major zircon populations in the Swakane 1991) and demonstrate that most of the sediments bordered the Leavenworth fault zone at this time. Gneiss (Sauer et al., 2018) but only minor and/or

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Figure 8. Schematic block diagrams and fault restorations illustrate the structural and stratigraphic evolution of the Chumstick basin. Colors of the basin sediments indicate the following: yellow—proximal alluvial fan facies, orange—medial to distal alluvial fan facies, green—basin axis meandering stream facies. (A) Dur- ing deposition of the lower to middle Chumstick Formation, sediments were derived primarily from distal to proximal eastern sources in a low-gradient, braided stream-dominated fan system. Restoration of 30 km of right-lateral slip along the Leavenworth fault zone (LFZ) puts the Mount Stuart batholith adjacent to boulder conglomerates at Devil’s Gulch and the Mission Hill Ski Ridge. (B) Deposition of the lower part of the Chumstick Formation was coeval with increased volcanism in the Teanaway and right-lateral slip along the Leavenworth fault zone, resulting in a shift of the main basin depocenter to the north and influx of coarse- grained material to the basin. With this northward shift in the basin depocenter, conglomerates deposited adjacent to the Wenatchee Ridge Gneiss at the Leavenworth Ski Hill are en- riched in muscovite-rich gneiss clasts. (C) A series of coalescing, steep alluvial fans formed along the Leavenworth fault zone in the north part of the Chumstick basin following right-lateral slip along the Leavenworth fault zone. Although sediments are still derived from the east, there is now a major influx of sediments from western sources. The two systems merged along the basin axis and flowed south.

variable zircon populations in metasedimentary Complex (Fig. 5). Thus, we consider most sedi- tiat pluton, suggests that proximal sources were rocks within the Skagit Gneiss (Sauer et al., ment in the lower to middle Clark Canyon Mem- particularly dominant during initial opening of 2017b). Finally, tonalite clasts (32% of felsic ber to have been sourced from within 30–40 km the basin (Table C1; see footnote 1). plutonic clasts) are dominant within the unit and of the Entiat fault. Our stratigraphically lowest The southernmost Tumwater Mountain match the composition of the Entiat pluton, and clast count, where nearly all of the clasts have Member is likely age equivalent to the lower to schist and quartzite clasts (14% of metamorphic compositions consistent with sources in the middle Clark Canyon Member based on its po- clasts) match the composition of the Napeequa Swakane Gneiss, Napeequa Complex, and En- sition below the Clark Canyon #4 tuff (Fig. 2).

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Figure 8. (Continued) (D) Right-lateral motion along the Leavenworth fault zone, Eagle Creek fault zone (ECFZ), and Entiat fault zone (EFZ) created a basin-wide unconformity, inversion of the Clark Can- yon and Tumwater Mountain members, and formation of the eastern subbasin as a modified half-graben in a transtensional setting between the Eagle Creek fault zone and Entiat fault zone. Felsic intrustions of the Wenatchee Dome intruded along the Eagle Creek fault zone and the eastern Chelan block. (E) Deposition of the Nahahum Member occurred exclusively in the eastern subbasin. Sedi- ments were deposited dominantly from the east and were deposited by lacustrine and meandering stream processes along the basin axis. (F) Follow- ing an episode of latest Eocene deformation and folding of the Clark Canyon and Nahahum Members, the Deadhorse Mem- ber is deposited and overtops the Leavenworth fault zone and Entiat fault zone.

Conglomerate detrital modes of the Tumwater fault zone. Both conglomerate clast composi- the Ingalls Complex (Table 1), which suggests Mountain Member in this area are dominated tions and ages suggest derivation of sediments that sediments from east- and west-derived fans by tonalite, matching the composition of the from the west on a steep alluvial fan that bor- mixed along the basin axis. Mt. Stuart batholith to the west of the Leav- dered the Leavenworth fault zone (Fig. 8). Fur- Initial opening and subsidence of the Chum- enworth fault zone. U-Pb zircon dates of two thermore, minor Late Cretaceous detrital zircon stick basin at ca. 49.2 Ma overlapped with the tonalite clasts yielded weighted mean ages of ages (100–87 Ma; 8.7%) and Jurassic ages (200– emplacement of the Teanaway Formation and 90.58 ± 0.53 Ma and 91.72 ± 0.75 Ma, consis- 145 Ma; 10.6%) in age-equivalent middle Clark associated dike swarm in a transtensional set- tent with the crystallization ages (Matzel et al., Canyon Member strata along the inferred basin ting between the Leavenworth fault zone and 2004) of the southeastern part of the Mount Stu- axis also match the 96–91 Ma age of the Mt. Stu- the Straight Creek fault (Figs. 1 and 9; Eddy art batholith where it is cut by the Leavenworth art batholith and Jurassic sedimentary rocks of et al., 2016b). More broadly, the region to the

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A Figure 9. (A) Paleogeographic reconstruction of Washington and Oregon during the Eocene is shown. We interpret that the Chumstick River flowed south before turn- ing to the west and mixing with other paleo-rivers that supplied detritus to forearc sediments that sit above Siletzia. The basalt of Hembre Ridge and the Western Mé- lange belt likely formed paleo-topographic highs during this time, which is evidenced by reworking of these sediments into adjacent coastal basins of the Tyee and Blue Mountain Group. Shaded red area shows the present day extent of Siletzia beneath Washington and Oregon. Note that Vancou- ver Island and coastal terranes have been restored for rotation and right-lateral offset back to their Eocene positions. Abbrevia- tions: DDMF—Darrington-Devil Mountain fault; WMB—Western Mélange belt. (B) Age probability plots showing distribution of U-Pb age determinations for composite sections of the Blue Mountain Group, Tyee Formation, and Chumstick Formation. Note that the Chumstick detrital zircon spectra best overlaps with the age spectra of the B Blue Mountain Group, which suggests that sediments from the interior part of Wash- ington made it out to coastal basins at this time. The influx of Precambrian grains in the Blue Mountain Group and differences in peak age populations suggest that local sources, such as the Western Mélange belt, provided a significant amount of detritus at the time.

evidence for alluvial-fluvial fans along this active fault during early opening of the Chumstick basin. We infer that both east- and west-derived alluvial-fluvial systems merged along the basin axis (Fig. 8). Importantly, our observations suggest that significant topography and erosion was concentrated along the basin-bounding Entiat and Leavenworth fault zones during deposition of the lower to middle Clark Canyon Member. Based on the basin architecture, the high sediment accumulation rate, and the inferred topography along the basin’s margins, we interpret that dextral strike-slip movement along the En- tiat fault zone and Leavenworth fault zone created the accommodation space necessary to form east of the basin was undergoing rapid crustal paleocurrent data (Evans, 1988) and lithofacies the Chumstick basin during this period of time. extension and exhumation at this time (Miller mapping, these sediments were transported on et al., 2016; Kruckenberg et al., 2008, and refer- a low-gradient, braided stream-dominated allu- Lowermost Part of the Upper Clark Canyon ences therein). Within the Chumstick basin this vial fan system that transitioned into meandering Member and Age Equivalent Tumwater period of time is marked by rapid accumulation stream systems in the southwestern part of the Mountain Member (48.5–48.0 Ma) (6–7 mm/yr) of sediment that is dominantly Chumstick basin (Donaghy, 2015). West-derived The upper Clark Canyon Member marks a sourced from proximal igneous and metamor- detritus and boulder conglomerates in a narrow significant change in provenance and an influx phic rocks to the east of the basin. Based on belt along the Leavenworth fault zone are strong of coarse-grained sediments into the Chumstick

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basin (Fig. 3). The lowermost part of the upper Entiat pluton, including the Chelan Complex and stream systems to dominantly high-energy braid- Clark Canyon strata in the west-central Chum- the ca. 49–47.9 Ma Cooper Mountain batholith ed stream systems increased Early Cretaceous, stick basin is primarily composed of cobble-peb- (Shea, 2008). Triassic, and Eocene detrital zircon populations, ble conglomerates and coarse-grained sandstone The most significant change in provenance and an influx of fine-grained igneous clasts, all (Fig. 4), which is consistent with deposition is the increase of mafic-intermediate volcanic suggest that the eastern catchment expanded dur- on the medial part of the low-gradient braided conglomerate clasts (1% to 33%; Fig. 5) and ing this time and that fluvial systems were effi- stream-dominated alluvial system derived from an increase in Eocene detrital zircon ages (53– ciently transporting coarse sediment from across the east (Donaghy, 2015). Although sediments 47 Ma; Fig. 6). The clast compositions match the Chelan block. We speculate that this expan- are still dominantly derived from the east at this many of the dikes in the Chelan block and are sion is related to regional tectonic processes in- time, the belt of Tumwater Mountain Member distinguished from the Teanaway dike swarm in cluding increased volcanism and exhumation of along the Leavenworth fault zone near the South the Wenatchee block based on the presence of the Skagit Gneiss (Miller et al., 2016), as well Highway sampling location (Fig. 2) suggests intermediate compositions. Intermediate com- as the natural evolution of river systems along that sediment deposition was also active in west- position dikes are found to be associated with young basins. Throughout this period, local to- ern alluvial fans. These systems mixed along the the ca. 49–47.9 Ma Cooper Mountain pluton pography was maintained on the Leavenworth basin axis and formed a southward-flowing axial (Fig. 1; Shea, 2008), and they are particularly fault as evidenced by the coarse conglomerates drainage system. dense in our proposed sediment source region of the Tumwater Mountain Member A series of Sediment accumulation rates calculated from (Fig. 8; Raviola, 1988; Tabor et al., 1987). An steep alluvial fans bordered the Leavenworth this section are 2.6 mm/yr, which is slower increase in granodiorite clasts and Eocene zir- fault zone and derived sediments from only than the sediment accumulation rates from the con ages further supports exhumation of the area sources proximal to the fault and mixed with underlying strata but still high and consistent surrounding the Cooper Mountain pluton. The sediments from the east-derived fan along the with strike-slip basins (Allen and Allen, 2013). proximal Swakane Gneiss, Napeequa Complex, basin axis (Fig. 8). Along with the significant change in provenance, and Entiat pluton remained the main sources of this depositional package is also equivalent to sediment, but the increase in Early Cretaceous Upper Clark Canyon Member and Age the tuff-rich, Clark Canyon section that is ages and clasts that are inferred to have origi- Equivalent Upper Tumwater Mountain ∼1350 m thick (Fig. 3; Evans, 1988). Abundant nated from the region near the Cooper Mountain Member (48–46.5 Ma) meter-scale tuffs (five undated tuffs between pluton further support our interpretation that the During this time, there was a well-established Clark Canyon tuff 2 and Eagle Creek tuff are drainage area was likely cutting into more distal series of coalescing alluvial fans bordering the not shown in Fig. 3) in the upper Clark Canyon regions of the Chelan block at this time (Fig. 8). Leavenworth fault zone and mixing with the east- Member suggest that sediments were deposited The South Highway location of Tumwater derived, low-gradient, stream-dominated alluvial during a period of active volcanism in the region. Mountain Member conglomerates sits strati- fan along the basin axis (Fig. 8). The Upper Clark Tuffs are typically overlain by debris-flow de- graphically above the Clark Canyon section Canyon and Tumwater Mountain Members are posits that are similar to alluvial-fluvial deposits (Fig. 2; McClincy, 1986; this study). Conglom- exposed north of the city of Leavenworth within adjacent to volcanic centers in humid climates erate clasts are dominated by tonalite that match- the Peshastin syncline and along the western side (Kesel, 1985; Evans, 1988). es the composition of the Mt. Stuart batholith, of the basin (Fig. 2). They interfinger near Leav- Latest Cretaceous (81–70 Ma; 21.3%) zircon and a single tonalite clast from this location enworth, demonstrating their contemporaneous ages remained a dominant source for the lower- gave a U-Pb zircon date of 90.95 ± 1.06 Ma, deposition and location of the mixing zone along most part of the upper Clark Canyon Member consistent with this source (Fig. 8). Schist the inferred basin axis. Age equivalent strata are (Fig. 6). However, there is a significant increase clasts in Clark Canyon Member conglomerates absent from the southern Chumstick basin, sug- up section in Early Cretaceous (145–100 Ma; along the basin axis north of Leavenworth re- gesting a northward migration of the main dep- 20.6%) and Eocene (53–40 Ma; 3.2%) zircon semble the Chiwaukum Schist (Fig. 2). Clark ocenter. Thick, amalgamated packages of poorly ages and a minor increase in Jurassic (200– Canyon sandstones along the basin axis near sorted, boulder-cobble conglomerate of the 145 Ma; 14.8%) and Triassic (251–200 Ma; Camas Land (Fig. 2) document an increase in Tumwater Mountain Member represent deposi- 2.6%) zircon ages (Fig. 6; Table E2; see foot- Late Cretaceous (100–87 Ma; 17.1%) zircon. tion on the western fans along the Leavenworth note 1). Latest Cretaceous detrital zircons and Additionally, conglomerate clasts in the Clark fault zone (Fig. 2; Evans, 1988; Donaghy, 2015). tonalite clasts match the age and composition Canyon Member adjacent to the Ranger Tower Interbedded sandstones and pebble-cobble con- of the Entiat and Cardinal Peak plutons (Fig. 1; section (Fig. 2) are interpreted to represent the glomerates and sandy conglomerates of the up- Table 1). Jurassic ages match the age of the Chel- Wenatchee Ridge Gneiss due to their light color, per Clark Canyon Member represent sediment an Complex and Dumbell orthogneiss–Marble- presence of fuchsite, and muscovite-rich com- deposition on the same east-derived medial to mount plutons (Miller et al., 2009). The abundant position. This gneiss is located northwest of distal parts of a stream-dominated alluvial slope. quartzite and biotite-rich gneissic conglomerate the Chumstick basin (Fig. 1), and based on the Fine-grained deposits that represent deposition clasts match the composition of the Napeequa above data, there is strong evidence for increased by meandering streams and possibly small la- Complex and Swakane Gneiss, respectively. The sediment derivation from the west and mixing of custrine systems characterize the uppermost part increase in Early Cretaceous ages also points to these sediments along the basin axis at this time. of the Clark Canyon Member in the stratigraphic the Swakane Gneiss continuing to be a promi- Our provenance data, combined with south- interval between the Tumwater Mountain Mem- nent source at this time and the addition of the westerly paleoflow measurements (Fig. 7; Ev- ber conglomerates of the Tumwater Mountain Chelan Complex as a source (Table 1). These ans, 1988), suggest that the dominant source of sampling location (Fig. 2) and the upper uncon- data indicate that an east-derived, braided stream sediments to the lowermost part of the upper formity (Fig. 4). system was still the dominant depositional sys- Clark Canyon Member remained the crystalline In contrast to the volcanic-rich detritus of tem, and there was an expansion of the sediment rocks immediately adjacent to the Entiat fault. the lowermost part of the upper Clark Can- source region to include rocks to the east of the However, the shift from low to moderate energy yon Member, conglomerates of the upper

14 Geological Society of America Bulletin, v. 130, no. XX/XX

Tumwater Mountain Member and upper Clark flowed southward along the basin axis in a me- segments of the Leavenworth fault zone likely Canyon Member along the basin axis, near andering stream system (Fig. 8). became transpressive faults at this time and are the Ski Hill and Tumwater Mountain loca- The asymmetric nature of the western sub- related to a train of folds that formed parallel tions (Fig. 2), are dominated by metamorphic basin with the axis near the Leavenworth fault to these fault segments as the western subbasin and igneous clasts that match western sources zone was the result of two factors. First, strike- inverted (Figs. 3 and 8). This uplift and deforma- (Fig. 5). Early Late Cretaceous (100–87 Ma) slip motion on the Leavenworth fault zone and tion disrupted the fluvial system that ran down zircon ages from the Clark Canyon Member its right bend (transition to north-trending near the axis of the Chumstick basin and provided an and tonalite conglomerate clasts from the inter- Leavenworth) may have resulted in greater sub- uplifted region that provided sedimentary clasts fingering Tumwater Mountain Member corre- sidence near this fault. Clast compositions of to the eastern subbasin as it began to develop. late with the composition of the Mount Stuart the upper Tumwater Mountain Member vary batholith. (Table 1). Foliated tonalite clasts in significantly from the tonalite-dominated clast Nahahum Canyon Member (46–44? Ma) the Clark Canyon Member are consistent with compositions of the middle and lower Tum- The Nahahum Canyon Member was deposited derivation from the ca. 92–89 Ma Tenpeak and water Mountain Member (Fig. 5), supporting a exclusively in the eastern subbasin and derived ca. 91 Ma Dirtyface plutons, which also corre- northward shift in the main basin depocenter to from primarily eastern sources with important late with Late Cretaceous zircon ages and lay be adjacent to more northerly sources. This is input from the inverted western Chumstick to the northwest of the basin in the Wenatchee consistent with right-lateral slip and northward subbasin. The Nahahum Canyon Member is block. Schist clasts in both members can be growth of the Leavenworth fault zone during lithologically different than the older Chumstick matched to the Chiwaukum Schist and gneiss deposition of the Clark Canyon and Tumwater Formation strata due to its dominantly finer- clasts to the Nason Ridge Gneisses (Tabor Mountain Members (Fig. 8; Donaghy, 2015). grained deposits (Fig. 3). Overall, paleoflow was et al., 1987) of the Wenatchee block. The pres- The second factor that might have influenced the toward the central to southern parts of the eastern ence of muscovite-rich gneiss conglomerate asymmetry of the western subbasin is the differ- subbasin (Fig. 7; Evans, 1988). During deposi- clasts consistent with the Wenatchee Ridge ent uplift histories of the Wenatchee and Chelan tion of the Nahahum Canyon Member, sediments Gneiss is noted at several locations along the blocks. The Wenatchee block was uplifted and were deposited by steep, small alluvial fans that Leavenworth fault zone. Overall, the increase exhumed by the time of deposition of the Swauk bordered the Entiat fault zone (Fig. 8). Rapid fa- in easily eroded schist clasts indicates that Formation (<59–51 Ma), and previous research cies changes, from the Eagle Creek fault zone gravel sediments were transported minimal indicates relatively low topography to the west toward the eastern subbasin axis, suggest that the distances and deposited proximal to the source. of the basin in the Wenatchee block (Methner depositional systems changed laterally from al- Paleocurrent measurements from Tumwater et al., 2016). In contrast, the Chelan block was luvial to fluvial and lacustrine environments over Mountain Member conglomerates indicate an largely exhumed during deposition of the Chum- hundreds of meters to a few kilometers (Fig. 4). eastward direction of paleoflow, in contrast to stick basin in the middle Eocene (Miller et al., Compositional and geochronologic data in- measurements along the basin axis in Clark 2016), and we conclude from the basin history dicate that sediments in the Nahahum Canyon Canyon Member strata that indicate southward presented here that it was rising substantially in Member were derived primarily from eastern to southwestward paleoflow Fig. ( 7; Evans, elevation during basin formation. source terranes. The dominance of gneiss con- 1988). The rapid lateral change in lithofacies glomerate clasts and 60–80 Ma detrital zircon and interfingering relationship between the Basin Partitioning (46.5–44 Ma) ages suggest that the dominant source rocks were Clark Canyon and Tumwater Mountain Mem- A basin-wide to regional unconformity de- the Swakane Gneiss, Skagit Gneiss Complex, bers support the mixing of these systems along veloped following deposition of the upper Clark Entiat pluton, and felsic sheets intruding the Nap- the basin axis (Fig. 4). Canyon Member. We estimate the age of this eequa Complex and Swakane Gneiss (Fig. 5). Upper Clark Canyon Member strata east of unconformity to be 46.5 Ma by assuming that Furthermore, conglomerate clasts of hornblende- and along the basin axis are still dominated by sediment accumulation rates remained constant quartz-diorite, diorite, and tonalite are consistent latest Cretaceous–early Paleocene (81–60 Ma) from the tuffaceous-rich Clark Canyon section with the Entiat pluton, and the 92–87 Ma peak detrital zircon ages and conglomerate clasts upward to the top of the Clark Canyon Member. detrital zircon age population matches the age of of quartzite, banded gneiss, and biotite-quartz However, this is a maximum age because some the Seven Fingered Jack and Black Peak plutons gneiss (Figs. 5 and 6). Latest Cretaceous–Paleo- of the Clark Canyon Member has been eroded. (Fig. 6; Miller et al., 2009; Shea et al., 2016). The cene (70–60 Ma) zircon ages and biotite-quartz Continued rapid exhumation of the crystalline increase in conglomerate clasts of quartzite and gneiss and quartzite clasts are consistent with core in the Chelan block during this time is evi- amphibolite indicates that the Napeequa Com- derivation from the Swakane and Napeequa denced by K-Ar cooling ages from biotites and plex became more important as a source during units, respectively, to the east (Table 1). The hornblendes between 47 Ma and 44 Ma (Miller deposition of the Nahahum Canyon Member increase in Eocene zircon ages and presence et al., 2016) and is compatible with faulting on likely due to its location proximal to the east- of granodiorite conglomerate clasts can be at- the Entiat fault zone. Magmatism in the Chelan ern subbasin (Fig. 1; Table 1). Nevertheless, the tributed to continued exhumation and erosion of block also continued during this period with lower part of the Nahahum Canyon Member con- the felsic plutons to the east of the Entiat fault emplacement of the Duncan Hill and Railroad tains reworked sedimentary lithics from the older zone. These data suggest that although there Creek plutons (ca. 46–45 Ma) east of the Chum- Clark Canyon Member in the western subbasin, was a greater influx of sediments from the west, stick basin (Fig. 1). suggesting that sediments were also derived from the eastern derived alluvial-fluvial depositional Development of the unconformity in the west- west of the Eagle Creek fault zone during initial system still remained a dominant route for sedi- ern subbasin represents a profound change in the opening of the eastern subbasin at ca. 46–45 Ma ment transportation (Fig. 8). The fining trend local paleogeography. We infer that it was ac- (Supplementary Material) and that the western of lithofacies toward the basin axis and to the companied by initial motion on the Eagle Creek subbasin was inverted during deposition of the south supports the idea that the two fan systems fault zone and formation of the eastern subba- Nahahum Canyon Member. In addition, the merged along the western basin axis and then sin, which is described below. The NW-trending Swakane gneiss is uplifted in the Eagle Creek

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fault zone horst, so biotite gneiss conglomerate diorites and rhyodacites, which is consistent lished between the Chelan block and Straight clasts could also represent sediments shed from with derivation from 46–49 Ma felsic intrusive Creek fault. west of the eastern subbasin. bodies in the Chelan block. There is also an in- We do not know the course of the Chumstick Two of the most important changes in prove- crease of gabbroic clasts, which is consistent River as it flowed south from the Chumstick nance are the presence of granodiorite conglom- with rocks exposed in the Chelan block (e.g., basin between 49 Ma and 45 Ma. It either erate clasts (Fig. 5) and increase of 46–48 Ma Riddle Peaks gabbro, subordinate bodies within turned west to the coast or continued south to detrital zircons (Fig. 6). Granodiorite clasts the Entiat and Seven Fingered Jack plutons, and join the proposed Idaho River (Dumitru et al., match the age and composition of plutons that the Chelan Complex). Other clast lithologies 2016). The Idaho River is postulated to have intruded in the Chelan block to the east during from the lower Deadhorse Canyon Member been sourced in western Idaho and flowed to basin partitioning and unconformity develop- are muscovite-rich gneiss, gneiss, gabbro, and the marginal marine to marine Tyee Formation ment (Table 1), indicating they were rapidly foliated tonalite, all of which were derived from in southwest Oregon, which was deposited over exhumed after their formation. There is also an source units adjacent to the north to northeast the recently accreted Siletzia terrane (Dumitru increase in conglomerate clasts that match the part of the Chumstick basin (Table C3; see et al., 2016; cf. Dorsey et al., 2019). Eocene composition of rhyodacitic dike swarms adjacent footnote 1). marine strata overlying the volcanic rocks of to the Cooper Mountain pluton, Duncan Hill plu- Deposition and stratigraphic relationships Siletzia on the Olympic Peninsula remain ton, and Golden Horn batholith. The increase of of the Deadhorse Canyon Member with basin- poorly studied but also record deltaic and ma- Eocene zircon ages also suggests reworking of bounding faults indicate that strata were depos- rine sedimentation. The relationship between older Chumstick Formation strata and tuffs into ited during a time of local tectonic quiescence these two areas is uncertain, but both record the eastern subbasin as a result of uplift and ex- and final filling of the northern basin (Evans, the formation of large forearc depositional humation along the Eagle Creek fault zone. The 1994). Initially, sandy braided stream sediments systems immediately following the accretion lack of Eocene igneous rocks in the Wenatchee were derived from the east and west before of Siletzia. A large provenance data set exists block further supports local sources only from transitioning into an east-derived meandering for the Tyee Formation (Dumitru et al., 2016), the inverting western Chumstick basin, locally stream system for the remainder of sediment but only limited provenance data exist for the exposed Swakane Gneiss along the Eagle Creek deposition of the Deadhorse Canyon Member rocks in western Washington. Detrital zircon fault zone, and the exhuming Chelan block (Fig. 8; Evans, 1994; Donaghy, 2015). This unit geochronology from the Blue Mountain Unit (Fig. 8). is likely correlative with the Roslyn Forma- in Washington demonstrates that it is slightly Based on lithofacies mapping and previous tion to the west of the Leavenworth fault zone, younger than the Chumstick basin (Eddy et al., research by Evans (1988), a large lake regularly which also shows no proximal to distal facies 2017). However, these are the only data from filled the axis of the eastern subbasin. Deltaic changes relative to the Leavenworth fault zone this area that can be directly compared to those deposits suggest that short streams flowed into and is broadly coeval (Evans, 1994; Eddy et al., presented in this study. the lake from both sides. This change in depo- 2016b). We speculate that both formed a broader To assess if the Chumstick River flowed sitional environments also suggests that any depositional system between the Chelan block south to join the Idaho River or turned west large rivers were south of the presently exposed on the east and the Straight Creek fault on the toward the coast to feed the forearc basin Chumstick basin by this time. Subsidence and west (Eddy et al., 2016b). in western Washington, we have compared deposition in the Eastern subbasin ended as the composite detrital zircon data from the Blue Wenatchee Dome intrusive complex intruded the Paleogeography of Western Washington Mountain Group and Tyee Formation that southern part of the Eagle Creek fault zone at ca. and the Pacific Northwest are shown in Figure 9 to the detrital zircon 44.447 ± 0.027 Ma (Gilmour, 2012). We infer signature from the Chumstick Formation. We that slip decreased on the NW-trending Entiat During deposition of the early Chumstick would expect to see similar age populations fault zone and Leavenworth fault zone as strike- Formation, significant basin-margin strike-slip if age-equivalent strata were sharing and/or slip motion localized on the N-trending Straight faulting along the Leavenworth fault zone was mixing sediment sources. The two main age Creek fault to the west. This interpretation is responsible for rapid vertical and lateral lithofa- populations of 60–70 Ma and 93–96 Ma that consistent with apparent offset of northwest- cies variations and changes in provenance. Al- characterize the Chumstick Formation are trending lithologic units and structures along this luvial fans bordered the Leavenworth fault zone notably absent in the Tyee Formation (Fig. 9). fault. However, some limited strike-slip motion and Entiat fault zone, deriving sediments from We consider this absence to preclude sedi- on the Entiat fault zone likely occurred follow- both the low hills of the Wenatchee block to the ment transport from the Chumstick River to ing deposition of the Nahahum Canyon Member west of the basin and highlands in the Chelan the Tyee Formation, because the large num- based on the presence of en enchelon folds in the block to the east. Sediments from both systems ber of 60–79 Ma and 93–96 Ma zircon in the eastern subbasin. mixed along the axis of the western subbasin Chumstick basin should be present down- and formed the south-flowing river, which we stream even as the sediments were diluted Deadhorse Canyon Member (44–42? Ma) designate here as the Chumstick River (Fig. 8). with sediments in the Idaho River. Thick packages of mudstone and sandstone During deposition of the Nahahum Canyon In contrast to the Tyee Formation, both the in the Deadhorse Canyon Member are inter- Member in the eastern subbasin between 46 Blue Mountain Unit and the Chumstick Forma- preted to represent high-sinuosity meander- and 44 Ma, lakes and small streams were in- tion are characterized by similar age populations ing stream channels and floodplain deposits ternally drained, and any large stream system, of 45–50 Ma, 60–70 Ma, 90–96 Ma, and 150– (Evans, 1994). Paleoflow indicators and the such as the Chumstick River, would have been 160 Ma (Fig. 9). The correlation of peak zircon distribution of lithofacies suggest flow across south of the Chumstick basin. By the time of age populations suggests that the Chumstick the Leavenworth fault zone, and dominantly, a deposition of the Deadhorse Canyon Member at River turned west after flowing south from the westerly paleoflow (Evans, 1994). Conglomer- ca. 44–42 Ma, a continuous, regional west (and Chumstick basin and mixed with fluvial systems ate clast lithologies are dominated by grano- southwest?) -flowing fluvial system was estab- that supplied sediment to coastal sedimentary

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basins, such as forearc basin sediments that supply and accommodation space affects the 2013). Huerta et al. (2011) documented how the unconformably overlie Siletzia basalts. Despite depositional architecture during strike-slip ba- relationship between accommodation space and overlapping zircon age spectra, there are dif- sin evolution. sediment supply impacts the fluvial depositional ferences in the percentages of peak zircon age Previous studies have shown that strike-slip architecture of nonmarine basins, which can then populations in each formation. Furthermore, basin geometry, depth, and stratigraphic archi- be used to understand similar relationships in there are also peak age populations of 112 Ma tecture are determined by the ratio between the Chumstick basin. The majority of the lower and 246 Ma that are not present in Chumstick basin-bounding fault overlap (o) and separa- Clark Canyon Member is finer-grained Fig. 4( ) Formation strata. These differences in the Blue tion (s) (Fig. 10; Reading, 1980; Rodgers, 1980; and characterized by thick sections of mudstone Mountain Group can be accounted for by deriva- Crowell, 1974a, 1974b). When this ratio (o/s) interbedded with lenticular sandstones and mi- tion of sediments from local sources in the Coast is low, subsidence is highest, and a continuous nor sandy to pebble conglomerates that vary Mountains batholith and Western Mélange belt depositional system is expected to form between from the younger basin strata (Supplementary (Sauer et al., 2017a; Gehrels et al., 2009). This the tips of the en echelon strike-slip faults. On Material). We interpret these lithofacies to be suggests that the Western Mélange belt formed the other hand, when the o/s ratio is high, indi- consistent with deposition off the fringe of the a topographic high in the forearc region that was vidual depocenters are expected to form adjacent east-derived fan system and within the fluvial- possibly related to motion on the Straight Creek to each fault tip, and subsidence is more muted. dominated basin-axis while initial relief was in- fault and fault splays of the Darrington fault During initial formation of the Chumstick basin creasing along the eastern basin margin (Fig. 8). system or shortening related to the accretion of (ca. 49.3–48.5 Ma), fault overlap is estimated to Based on the thick stratigraphic section of the Siletzia (Fig. 9). be approximately equal to the separation between lower Clark Canyon Member (Fig. 10) and high The origin of Precambrian zircon ages in the basin-bounding faults (Fig. 10), although the sediment accumulation rates, we assume that Eocene sedimentary basins in Oregon, Wash- southern termination of the Entiat fault is not ex- sediment supply was great enough to aggrade ington, and Alaska has been a topic of debate posed, and this estimate for the o/s ratio should sediments in the basin. High accommodation (Garver and Davidson, 2015; Dumitru et al., be considered a minimum. In this geometry we space coupled with high sediment supply in 2013, 2015, 2016). Precambrian grains in the expect localized topography around the fault nonmarine basins results in a fluvial depositional Chumstick Formation were likely derived tips and a zone of maximum subsidence con- architecture with low interconnectivity and rib- from the adjacent Swakane Gneiss and yield necting these areas (Fig. 10). The stratigraphic bon-shaped channels (Huerta et al., 2011) and is ages of 1.38 Ga and a range of 1.6–1.8 Ga architecture of the lower Clark Canyon Member similar to our interpretation of the lower Clark ages (Sauer et al., 2018). However, these shows that maximum basin thickness (∼4.5 km) Canyon Member. zircons do not form a significant population. overlaps with this axis of maximum subsidence We see a significant change in the deposi- Similar age populations are seen in the Tyee for a basin forming between the north tip of the tional architecture of the upper Clark Canyon Formation and Blue Mountain Group, but the Leavenworth fault zone and a potential southern Member (48.5–46.5 Ma) following northward Blue Mountain Group has significantly more tip of the Entiat fault zone near the southern limit migration of the main basin depocenter as the grains. We interpret the influx of Precambrian of its present-day exposure (Fig. 10). Sediment Leavenworth fault zone propagated northward grains in the Blue Mountain Group as the re- accumulation rates varied between 6 mm/yr and (Fig. 10). This interval of time is marked by sult of derivation of sediments from the Mé- 7 mm/yr for the lower Clark Canyon Member, an influx of coarse-grained material from both lange belt (Sauer et al., 2017a), because our which is fairly high relative to rates calculated sides into the basin synchronous with a period regional drainage reconstruction (Fig. 9) sug- for classic strike-slip settings (2–3 mm/yr; Allen of increased regional volcanism and increased gests that the other potential source for these and Allen, 2013; Reading, 1980). We interpret relief across basin-bounding faults (Figs. 4, 5, zircon, the southern belt basin (Dumitru et al., this to reflect an initial phase of rapid subsidence and 8). Sheet-like sandstone and pebble-cobble 2016), was feeding a depocenter in Idaho at coupled with high sediment supply as the basin conglomerate geometries characterize the upper this time. formed with an initially low o/s ratio between Clark Canyon Member (Supplementary Mate- active strike-slip faults. Numerical models by rial) and are representative of deposition on the GENERALIZED CONCLUSIONS FOR Petrunin and Sobolev (2006, 2008) support this medial part of the east-derived fan system, sug- STRIKE-SLIP BASINS interpretation and show that subsidence rates gesting progradation of this fan toward the basin are initially rapid in pull-apart basins and reduce axis since deposition of the lower Clark Canyon The balance between subsidence and sedi- over the lifespan of the basin as the fault tips Member (Fig. 8). Continuous northward propa- ment supply determines the architecture of sedi- propagate and the o/s ratio increases. Distinctive gation of the Leavenworth fault zone resulted mentary basins. Understanding the relationship northward shifts in the basin depocenter associ- in a high o/s ratio, and modeling by the Rodg- between these variables in strike-slip basins is ated with lower sediment accumulation rates in ers (1980) and Wu et al. (2009) models would particularly challenging because of the difficulty the upper Clark Canyon Member are consistent predict a period of reduced accommodation in producing a basin-wide chronostratigraphy with a northward-propagating Leavenworth fault space and two main basin depocenters form- across rapidly changing facies due to incomplete zone and likely track this process in the Chum- ing near the tips of the Leavenworth fault zone exposure or a lack of radiometric age constraints stick basin. and Entiat fault zone. Reduced accommodation (Crowell, 2003a, 2003b; Link, 2003; Hempton The region of greatest subsidence can be space, while maintaining high sediment supply, et al., 1983; Reading, 1980; Crowell, 1974a, roughly estimated in strike-slip basins using the is characterized by sheet-like channel fills and 1974b). This study is the first that we know of above o/s ratio (Rodgers, 1980) and plays a ma- high interconnectivity nonmarine basins (Huer- that documents variation in sediment accumula- jor role in predicting how accommodation space ta et al., 2011), consistent with the depositional tion rates based on numerous precise tuff ages, evolved through time in the Chumstick basin. In architecture of the upper Clark Canyon Mem- lithofacies, and provenance to 0.5–1.5 m.y. in- any active basin, the amount of available accom- ber. While our study also clearly documents two tervals within an ancient strike-slip basin. As modation space is a major control on the depo- to three episodes of a northward-shifting basin a result, we can assess how varying sediment sitional architecture of a basin (Allen and Allen, depocenter along the Leavenworth fault zone

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Figure 10. Fence diagram shows the spatial variations in stratigraphic thickness of the Chumstick Formation within the western Chumstick subbasin. Each column represents a generalized measured section at that location within the basin (Donaghy, 2015; Evans, 1988, 1991). Solid lines represent tuff beds that were dated using isotope dilution-thermal ionization mass spectrometry by Eddy et al. (2016b). Tuff beds were correlated across the basin using new, precise ages as well as stratigraphic relationships observed in the field. Areas denoted by thin, dashed black lines and #1, #2, and #3 represent the northward-migrating basin depocenter through time. The heavy dashed line along the Leavenworth fault zone represents the section of the Leavenworth fault zone that is not active yet. Black arrows represent right-lateral motion along basin-bounding faults. Blue arrows represent average paleoflow based on paleocurrent measurements from Evans (1988) (Fig. 7). Highlighted gray areas are areas of maximum localized topographic relief along basin-bounding faults. Colors used in the fence diagram are correlative to the mapped lithofacies (Fig. 4). Please see Figure 4 for key. LFZ—Leavenworth fault zone; ECFZ—Eagle Creek fault zone; EFZ—Entiat fault zone; yak3—Yaksum tuff 3; ecte—Eagle Creek tuff; tctc4—Clark Canyon 4 tuff; tctc2—Clark Canyon 2 tuff; S2—Sunitsch 2 tuff; S1—Sunitisch 1 tuff; FVT—Fairview tuff.

(Figs. 8 and 10), a separate basin depocenter fault zone. This transition differs from the pre- sin on the “east-derived fan facies,” sediment ac- near the end of the Entiat fault zone is difficult diction for classic strike-slip systems, in which cumulation rates are ∼3.04 mm/yr (Table A2). to document. First, we do not know where the sediments are sourced from areas proximal to A similar contrast is seen for the Eagle Creek to southern Entiat fault zone tip is, and the south- the faults, and it is likely a consequence of rapid Clark Canyon 4 tuff, where sediment accumu- ern Chumstick basin is dominated by the older exhumation and increased volcanism associated lation rates in the north are ∼2.57 mm/yr and strata of the basin. Alternatively, one depocenter with the accretion of Siletzia and ridge-trench ∼6.29 mm/yr in the south (Table A2). These may be favored by the exhumation of the entire interactions. systematic differences in sediment accumula- Chelan block along the Entiat fault zone at this Our study also gives us the unique opportu- tion rates may reflect changes in where sedi- time, overpowering the signature of localized nity to examine the variability of sediment accu- ments bypassed an area or are accumulated due uplift. Sediments were increasingly supplied by mulation rates for the same time interval in dif- to changes in depositional systems from the more distal sources to the east during deposi- ferent depositional systems and positions in the basin axis to nearer the eastern basin margin. tion of the upper Clark Canyon Member (see basin. When we look at the section between the Overall, there is still a decrease in stratigraphic discussion), suggesting that the catchment area Yaksum and Eagle Creek tuffs, in the southern thickness upsection in the Clark Canyon Mem- in the Chelan block was expanding and that part of the basin within the fine-grained “basin- ber once the main depocenter shifts primarily topography began to be controlled by regional axis” facies (see discussion), sediment accumu- to the north (Fig. 10), which is consistent with tectonic processes in addition to localized uplift lation rates are ∼7.12 mm/yr (Table A2). For the reduced accommodation space during matura- along the Entiat fault zone and Leavenworth same time interval in the northern part of the ba- tion of a pull-apart basin (Petrunin and Sobo-

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lev, 2008, 2006). Previous studies have shown propagation of the Leavenworth fault zone and tion of the Cascades crystalline core in the Cascade Riv- er area, Washington: Washington Division of Geology that sediment accumulation rates are greatest exhumation of adjacent source terranes, which and Earth Resources Bulletin, v. 80, p. 93–113. in mudstone-rich sections versus sandstone se- caused northward migration of the main ba- Cheney, E.S., and Hayman, N.W., 2007, Regional Tertiary quences (Huerta et al., 2011; Crowell, 2003b, sin depocenter during deposition of the Upper sequence stratigraphy and structure on the eastern flank of the central Cascade Range, Washington, in Stelling, 1974a, 1974b). This is consistent with the idea Clark Canyon Member. Fault reorganizations P., and Tucker, D.S., eds., Floods, Faults, and Fire: Geo- that during high sediment supply, alluvial fan late in the basin’s history led to basin partition- logical Field Trips in Washington State and Southwest systems will be largely a zone of sediment by- ing, deposition of the fine-grained Nahahum British Columbia: Geological Society of America Field Guide 9, p. 179–208, https://doi.org/10.1130/2007 pass. Additionally, as accommodation space is Canyon Member in the eastern subbasin, and .fld009(09). reduced, sediments will also start to bypass de- ultimately the cessation of strike-slip faulting Cheney, E.S., and Hayman, N.W., 2009, The Chiwaukum Structural Low: Cenozoic shortening of the central Cas- position within the basin if sediment supply re- on the basin-bounding faults. This transition cade Range, Washington State, USA: Geological Soci- mains high. This point highlights the importance was associated with the re-establishment of a ety of America Bulletin, v. 121, no. 7–8, p. 1135–1153, of standardizing the facies in which sediment regional depositional system in the Deadhorse https://doi.org/10.1130/B26446.1. Christie-Blick, N., and Biddle, K.T., 1985, Deformation accumulation rates are calculated in strike-slip Canyon Member and the gradual removal of and basin formation along strike-slip faults, in Biddle, basins as well as integrating this information localized topography. Our data set represents K.T., and Christie-Blick, N., eds., Strike-Slip Defor- with an understanding of sediment supply and a well-constrained example of how sediment mation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Spe- the development of accommodation space. The routing within an ancient strike-slip basin cial Publication 37, p. 1–34, https://doi.org/10.2110/ unique basin-wide deposition of numerous tuffs evolves through the basin’s lifetime and pro- pec.85.37.0001. Cowan, D.S., 2003, Revisiting the Baranof-Leech River within the Chumstick basin has allowed us to vides insight into the competing local and re- hypothesis for early Tertiary coastwise transport of begin to address the variability of depositional gional processes that led to changes in sediment the Chugach-Prince William terrane: Earth and Plan- environments and sediment accumulation rates source areas through time during deposition etary Science Letters, v. 213, p. 463–475, https://doi .org/10.1016/S0012-821X(03)00300-5. from initiation through maturation of pull-apart of the Chumstick basin. These results are not Crowell, J.C., 1974a, Sedimentation along the San Andreas basins. These data confirm previous hypotheses only important for understanding the regional fault, California, in Dott, R.H., Jr., and Shaver, R.H., about strike-slip basin development from mod- tectonic setting and paleogeography of Wash- eds., Modern and Ancient Geosynclinal Sedimenta- tion: Society of Economic Paleontologists and Miner- eling and provide a framework for future studies ington at this time but also shed light on the alogists Special Publication 19, p. 292–303, https://doi of similar basins. fundamentals of strike-slip basin evolution. .org/10.2110/pec.74.19.0292. Crowell, J.C., 1974b, Origin of late Cenozoic basins in south- ACKNOWLEDGMENTS ern California, in Dickinson, W.R., ed., Tectonics and CONCLUSIONS Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 190–204, We thank Christopher Donaghy and Jason Muhl- https://doi.org/10.2110/pec.74.22.0190. Our study integrates new lithofacies map- bauer for their assistance in the field. Financial sup- Crowell, J.C., 2003a, Introduction to geology of Ridge Ba- ping and a robust provenance data set from port for this project was provided by National Science sin, southern California, in Crowell, J.C., Evolution the Chumstick Formation with previously Foundation grants EAR-1119063 to P.J. Umhoefer of Ridge Basin, Southern California: An Interplay and EAR-1119358 to R.B. Miller. The research was of Sedimentation and Tectonics: Geological Society published depositional ages to create a holis- further supported by graduate research grants to E.E. of America Special Paper 367, p. 1–15, https://doi tic view of basin evolution in a strike-slip set- Donaghy from the Geological Society of America, .org/10.1130/0-8137-2367-1.1. the American Association of Petroleum Geologists Crowell, J.C., 2003b, Tectonics of Ridge Basin region, south- ting. Deposition of the Chumstick Formation ern California, in Crowell, J.C., Evolution of Ridge Ba- occurred primarily on a low-gradient, braided Nancy Setzer Murray Memorial fund, and the Pioneer sin, Southern California: An Interplay of Sedimentation Research Scholarship Fund. The manuscript has ben- stream-dominated alluvial fan system that de- and Tectonics: Geological Society of America Special efited from the thoughtfulcomments of two anony- Paper 367, p. 157–203, https://doi.org/10.1130/0-8137- rived sediments from eastern source terranes mous reviewers and editorial feedback from Science 2367-1.157. in the Chelan block of the North Cascades. Editor Wenjiao Xiao. 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Manuscript Received 1 May 2020 Sauer, K.B., Gordon, S.M., Miller, R.B., Vervoort, J.D., Tabor, R.W., Frizzell, V.A., Vance, J.A., and Naeser, C.W., Revised Manuscript Received 9 November 2020 and Fisher, C.M., 2017b, Transfer of metasupracrust- 1984, Ages and stratigraphy of lower and middle Ter- Manuscript Accepted 4 January 2021 al rocks to midcrustal depths in the North Cascades tiary sedimentary and volcanic rocks of the central continental magmatic arc, Skagit Gneiss Complex, Cascades, Washington: Application to the tectonic Printed in the USA

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