stratigraphy, age, and provenance of the eocene chumstick

For permission to copy, contact [email protected] © 2021 Geological Society of America

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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Stratigraphy, age, and provenance of the Eocene Chumstick basin, Washington Cascades; implications for paleogeography,

regional tectonics, and development of strike-slip basins

Erin E. Donaghy1,†, Paul J. Umhoefer2, Michael P. Eddy1, Robert B. Miller3, and Taylor LaCasse4

1 Department of Earth, Planetary, and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907, USA2 School of Earth Sciences and Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA3 Department of Geology, San Jose State University, San Jose, California 95192, USA4 Department of Geology, Carleton College, Northfield, Minnesota 55057 USA

ABSTRACT

Strike-slip faults form in a wide variety of tectonic settings and are a first-order control on the geometry and sediment accu-mulation patterns in adjacent sedimentary basins. Although the structural and depo-sitional architecture of strike-slip basins is well documented, few studies of strike-slip basins have integrated depositional age, lithofacies, and provenance control within this context. The Chumstick basin formed in central Washington during a regional phase of dextral, strike-slip faulting and episodic magmatism associated with Paleogene ridge-trench interaction along the North America margin. The basin is bounded and subdi-vided by major strike-slip faults that were active during deposition of the intra-basinal, non-marine Chumstick Formation. We build on the existing stratigraphy and present new, detailed lithofacies mapping, conglom-erate clast counts (N = 16; n = 1429), and sandstone detrital zircon analyses (N = 16; n = 1360) from the Chumstick Formation to document changes in sediment provenance, routing, and deposition. These data allow us to reconstruct regional Eocene paleo-drainage systems of Washington and Or-egon and suggest that drainage within the Chumstick basin fed a regional river system that flowed to a forearc or marginal basin on the newly accreted Siletzia terrane. More generally, excellent age control from five interbedded tuffs and high sediment accu-mulation rates allow us to track the evolving sedimentary system over the Formation’s ca. 4–5 m.y. depositional history. This is the first time lithofacies and provenance varia-

tions can be constrained at high temporal resolution (0.5–1.5 m.y. scale) for an ancient strike-slip basin and permits a detailed re-construction of sediment routing pathways and depositional environments. As a result, we can assess how varying sediment supply and accommodation space affects the depo-sitional architecture during strike-slip basin evolution.

INTRODUCTION

Classic strike-slip basins are typically char-acterized by (1) high sediment accumulation rates, (2) scarce igneous activity, (3) abrupt lat-eral lithofacies changes, (4) thickening of sedi-mentary sequences over short distances, (5) nu-merous unconformities that reflect syn-tectonic sedimentation and fault reorganization, and (6) locally derived fault-margin alluvial fans (Crow-ell, 1974a, 1974b; Sylvester, 1988). Many strike-slip basins have a well-defined stratigraphic and structural architecture (e.g., Crowell, 1974a, 1974b; Allen and Allen, 2013), but they rarely have integrated precise depositional ages and a robust provenance data set within this architec-ture. The resulting poor age control results in limited knowledge of the timing of how basin accommodation space and sediment accumula-tion patterns vary as basin-bounding fault pat-terns evolve in a strike-slip setting. For example, a rapidly migrating basin depocenter is a key component in strike-slip basin models (Chris-tie-Blick and Biddle, 1985; Sylvester, 1988; Crowell, 2003b) but is difficult to reconstruct in ancient strike-slip basins without basin-wide stratigraphic markers. Holistic data sets that combine excellent geochronologic and strati-graphic data permit detailed reconstructions of changing sediment routing pathways and depo-sitional environments relative to a strike-slip ba-sin’s faulting history.

Here we present a large provenance data set coupled with new lithofacies mapping from the Chumstick basin within the framework of a recently developed precise depositional chronol-ogy (Eddy et al., 2016b). This basin formed in a strike-slip setting in central Washington and provides a unique opportunity to track changes in sediment routing systems that are related to rapidly changing paleogeography in basin-bounding basement blocks. This is the first time that detailed lithofacies mapping and provenance variations can be constrained to 0.5–1.5 m.y. timescales within an ancient strike-slip basin, and our data demonstrate the importance of changes in localized topography, sediment sup-ply, and basin accommodation space in creating the complex depositional architecture of strike-slip basins.

TECTONIC SETTING OF THE CHUMSTICK BASIN

A tectonic belt from Oregon and Washington to British Columbia to southern Alaska is as-sociated with complex ridge-trench interaction during the Paleogene (e.g., Bradley et al., 2003; Madsen et al., 2006). The effects of this process include regional dextral strike-slip faulting as a result of oblique convergence of the Kula (or Resurrection) and North American plates (Friz-zell, 1979; Ewing, 1980; Vance and Miller, 1981; Johnson, 1982; Engebretson et al., 1983; Wells et al., 1984; Haeussler et al., 2003; Madsen et al., 2006), near-trench magmatism associated with migration of the Kula (or Resurrection)-Farallon spreading ridge along the continental margin (Madsen et al., 2006; Cowan, 2003; Bradley et al., 1993; 2003), and widespread exhumation of mid-crustal rocks (e.g., Miller et al., 2016). Strike-slip faulting was active within Washing-ton and southern British Columbia from at least 50 Ma until the start of the modern Cascades arc †[email protected].

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at 45–40 Ma (e.g., Umhoefer and Miller, 1996) and may have continued until 34 Ma, when the last major fault system was intruded by a

granitoid batholith (Tabor et al., 1984, 2003). During this period, the non-marine Chumstick basin formed in central Washington between

the Leavenworth and Entiat faults and was later subdivided by the Eagle Creek fault zone (Fig. 1; Tabor et al., 1984; Evans, 1991, 1994).

Figure 1. (A) Reference map shows the study area within the state of Washington. (B) Simplified geologic map em-phasizes the metamorphic and igneous terranes in the North Cascades core adjacent to the Chumstick and Swauk basin. Plutons are orange, red, and pink, and crystallization ages are defined by colors in key. (C) Map of the Chumstick ba-sin in respect to the adjacent Wenatchee and Chelan blocks. The eastern (ES) and western subbasins of the Chumstick basin are defined. Note that the western subbasin is split into the northern western sub-basin (NWS) and the southern western subbasin (SWS) by the Wenatchee River. Abbre-viations: CH—Chaval pluton; CPP—Cloudy Pass pluton; CRB—Columbia River Ba-salts; EFZ—Entiat Fault zone; ECFZ—Eagle Creek Fault zone; LFZ–Leavenworth fault zone; NQ—Napeequa Com-plex; SM—Sulfur Mountain pluton; WD—Wenatchee Dome; WRG—Wenatchee Ridge Gneiss. Figure modi-fied from Schuster (2005) and Miller et al. (2009).B

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Provenance and tectonics of the Chumstick basin

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There is controversy regarding the struc-tural setting and stratigraphic architecture of the Chumstick basin and its relationship to the adjacent Swauk basin to the west (Cheney and Hayman, 2009; Johnson, 1984; Tabor et al., 1984; Evans, 1996). Some workers argue that deposition occurred regionally, and that basin-bounding reverse faults cut and deformed the ba-sin following deposition (Cheney and Hayman, 2009). In contrast, others consider the Chum-stick basin to have formed as a strike-slip basin (Johnson, 1984, 1996) or during an early period of extension followed by strike-slip partition-ing as an extensional half-graben (Evans, 1994, 1996). Recent improvements in our understand-ing of regional tectonics and in constraining the depositional ages of Eocene sedimentary units throughout western Washington have started to resolve some of these controversies by showing that sedimentation within the adjacent Eocene sedimentary units was temporally distinct from deposition in the Chumstick basin (Eddy et al., 2016b).

Within this new tectonic framework, the Chumstick basin is interpreted to have formed during a period of regional strike-slip faulting immediately following the ca. 51–49 Ma ac-cretion of the Siletzia oceanic plateau to North America (Massey, 1986; Wells et al., 2014; Eddy et al., 2017). Both the basin-bounding Entiat and Leavenworth faults have been in-terpreted as dextral strike-slip faults that were active during basin formation and likely con-trolled basin development (Fig. 2). Estimates of displacement on both structures are between 20 km and 30 km (Tabor et al., 1987). How-ever, given the absence of clear piercing points and that the Columbia River basalts cover the basin to the southeast, these estimates are minimums.

STRATIGRAPHIC AND STRUCTURAL ARCHITECTURE OF THE CHUMSTICK BASIN

The Chumstick basin is ∼75 km long and 20 km wide and consists of ∼10.5 km of middle to late Eocene sedimentary strata and interbed-ded tuffs that can be divided into four distinct members and three structural zones (Fig. 2). Major unconformities separate the four members intro three packages, and from oldest to young-est, they are: the age-equivalent Clark Canyon and Tumwater Mountain, Nahahum Canyon, and Deadhorse Canyon (Fig. 3; Evans, 1988). The Clark Canyon and Tumwater Mountain Members occupy a structurally distinct, west-ern subbasin that is bounded by the Eagle Creek and Leavenworth faults (Fig. 2). This subbasin can be further divided into distinct northern and

Figure 2. Simplified geologic map shows the Chumstick basin. The locations of all data samples collected for geochronology, conglomerate clast counts, and measured stratigraphic sections are displayed. Solid lines represent where tuffs were mapped in the field in this study. Dotted lines represent the locations of tuffs from previous mapping (McClincy, 1986; Tabor et al., 1984, 1987) and projections based on structural map patterns. Adapted from Tabor et al. (1982). CC—Clark Canyon; CL—Camas Land; CR—Camprec Road; DG—Devil’s Gulch; DP—Dirtyface pluton; EC—Eagle Creek; EFZ—Entiat Fault zone; ECFZ—Eagle Creek fault zone; FV—Fairview Canyon; L—Leavenworth; LFZ—Leavenworth fault zone; MA—Malaga; MCR—Merry Canyon Road; MI—Mission Creek; MO—Monitor; MR—Mission Ridge Ski Hill; NP—North Plain; PP—Plain Pass; RT—Ranger Tower; SH—Ski Hill; SHI—South Highway; TW—Tumwater Mountain; VC—Van Creek; W—Wenatchee; WD—Wenatchee Dome; #2—Number 2 Canyon Road.


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southern zones (Fig. 1). The Nahahum Canyon Member occupies an eastern subbasin bounded by the Eagle Creek and Entiat faults (Fig. 1). The Deadhorse Canyon Member overtops the Eagle Creek fault and is unconformably depos-ited on top of the other members. Eddy et al. (2016b) presented six high-precision U-Pb zir-

con eruption/deposition dates from tuffs and two maximum depositional ages for sandstones within the Chumstick basin, and we utilize their geochronology to calculate sediment accumula-tion rates within the basin. These methods are outlined in the Supplementary Material, and our preferred sediment accumulation rates are pre-

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, mud-stone, and interbedded tuffs, and is exposed ex-clusively 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 ad-jacent to northwest-trending segments of the Leavenworth fault zone. During deposition of

1Supplemental Material. (1) Descriptions of spatial and temporal stratigraphic thickness variations in the Chumstick basin and methods for sediment accumulation rate calculations, (2) Detailed descriptions and photographs of each lithofacies association of the Chumstick Formation defined in the text of the manuscript, (3) Tables of raw and summary conglomerate clast count data for each member of the Chumstick Formation, (4) 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 collected within the Chumstick Formation, (6) Conglomerate clast raw data from LaCasse (2013) and (7) Tables of detrital zircon raw data from each individual sandstone sample within the Chumstick Formation (Donaghy, 2015). Please visit https://doi.org/10.1130/GSAB.S.13624076 to access the supplemental material, and contact [email protected] with any questions.

Figure 3. Generalized measured stratigraphic section shows poorly sorted boulder con-glomerate, poorly moderately sorted cobble conglomerate with interbedded tuffs and cross-stratified sandstones interbedded with massive mudstones, thinly interbedded sandstone and mudstone, and massive mudstones with rare sandstone. Ten of the interbedded tuffs are not shown. Stratigraphic thickness of the Chumstick Formation varies spatially at differ-ent locations within the basin. The Chumstick Formation is divided into the Clark Canyon and age-equivalent Tumwater Mountain Member, Nahahum Member, and the Deadhorse Member. Abbreviations: ecte—Eagle Creek tuff; tctc4—Clark Canyon 4 tuff; tctc2—Clark Canyon 2 tuff.


https://doi.org/10.1130/GSAB.S.13624076





the upper Clark Canyon Member, sediment ac-cumulation rates decrease to ∼2.6 mm/yr, which is possibly related to the rate being calculated from strata deposited in a different depositional system from the lower Clark Canyon Member strata (see discussion). Assuming that rates re-mained constant during deposition of the upper-most Clark Canyon Member, the age of the un-conformity that tops the Clark Canyon Member is estimated to be 46.5 Ma (Table A2).

The Tumwater Mountain Member consists of boulder-cobble conglomerate exposed only along the Leavenworth fault zone (Fig. 3) and is age-equivalent to the middle and upper Clark Canyon Member along most of the basin mar-gin. It interfingers with strata that are along strike with sandstones of the upper Clark Can-yon Member in the Ski Hill section (Fig. 2), providing a maximum age for the Tumwater Mountain Member in the northern part of the basin. A minimum age is provided by our ca. 46.5 Ma estimate for the top of the Clark Can-yon Member South of the Wenatchee River, the Clark Canyon 4 tuff strikes into the Tumwa-ter Mountain Member stratigraphically above the outcrops at Mission Ridge Ski Hill, sug-gesting that this part of the Member is at least 48.186 ± 0.026 Ma (Fig. 2; all tuff dates from Eddy et al. (2016b) are presented with 2σ ana-lytical uncertainties only). Age relationships to the south of, and consequently older than, Clark Canyon 4 tuff cannot be demonstrated. Howev-er, we interpret that >48.186 ± 0.026 Ma strata of the Clark Canyon and Tumwater Mountain Members interfinger in the southernmost ba-sin and possibly extend beneath the Columbia River basalts.

The Nahahum Canyon Member consists of ∼1.5–2 km of mudstone and sandstone and is exposed exclusively in the eastern sub-basin (Fig. 3; Evans, 1994). The base of the member is not exposed, and its relation to the Clark Canyon Member is uncertain. A maximum depositional age (MDA) is provided by the youngest zircon identified during laser ablation–inductively cou-pled plasma–mass spectrometry (LA-ICP-MS) analysis from a sandstone in the middle Nahahum Canyon Member, which was subsequently dated to 46.902 ± 0.076 Ma (Eddy et al., 2016b). The Wenatchee Dome intrudes the Nahahum Canyon Member along the Eagle Creek fault zone and yields a chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) zircon age of 44.447 ± 0.027 Ma (Gilm-our, 2012), constraining deposition between ca. 46.9 and 44.4 Ma (Eddy et al., 2016b). Strata are deformed into a major syncline that runs along the subbasin axis and further deformed in a series of en echelon synclines and anticlines trending ∼300° adjacent to the Eagle Creek fault zone

(Fig. 2). Some of this deformation is interpreted to be syn-depositional based on the orientation of folds relative to the Entiat fault and presence of soft-sediment deformation related to syn-dep-ositional earthquakes (Evans, 1994; Donaghy, 2015). However, some of these structures can be traced to broad folds within the Columbia River basalts, which suggests that they were reactivated during or after the Miocene (Cheney and Hay-man, 2007). Determining a precise age of the Nahahum Canyon Member is difficult due to the lack of interbedded tuffs. However, provenance data presented below strongly suggest that the Nahahum Canyon Member is largely younger than the western subbasin.

The Deadhorse Canyon Member is ∼2.2 km thick, consists of mudstone and minor sandstone, and is found in the northern and southeastern part of the Chumstick basin (Fig. 2; Evans, 1994). The Deadhorse Canyon Member sits uncon-formably on sediments of both the eastern and western subbasin and overtops the Entiat fault zone, Eagle Creek fault zone, and Leavenworth fault zone. A maximum depositional age for this unit is given by a CA-ID-TIMS detrital zircon age of 45.910 ± 0.021 Ma (Eddy et al., 2016b). Strata within the Deadhorse Canyon Member may have been part of a regional depositional system correlative with the Roslyn Formation to the west (Evans, 1994; Eddy et al., 2016b). The Deadhorse Canyon Member was excluded from the new total stratigraphic thickness of the Chumstick Formation due to the uncertainty of its relation to the underlying units and because of its potential correlation to the Roslyn Formation.

The presence of precisely dated tuff marker beds permits us to laterally trace coeval sedimen-tary units across the basin despite locally discon-tinuous outcrop, rapid lithofacies changes, and structural complexity. This technique is critical for recognizing thickness variations and refining previous thickness estimates of the Clark Can-yon Member. For example, The Clark Canyon 4 tuff in the southern and northern part of the western subbasin was not previously correlated but is now known to be offset by a fault along the Wenatchee River with left separation of ∼2.3 km (Fig. 2). Offset of tuffs and juxtaposition of older sediments in the north on younger sediments in the south suggest a steep fault with left-lateral and reverse slip along the Wenatchee River. Our ∼10.5 km estimate for the stratigraphic thick-ness of the Chumstick Formation differs from the previous interpretation of ∼13 km (Mc-Clincy, 1986; Evans, 1994) because our new stratigraphic correlations reveal repeated section in the McClincy (1986) and Evans (1994) esti-mates. Furthermore, the oldest part of the Clark Canyon Member is exposed exclusively in the southern part of the basin, and the upper Clark

Canyon Member is exposed only in the northern part of the basin. This creates a shingling effect of northwestward-younging strata that is further supported by vitrinite reflectance data from Ev-ans (1988), which indicate a significance differ-ence between stratigraphic thickness (>12 km) and basin thickness (>3.5 km). Based on our age control and correlation of tuffs from north to south in the western subbasin (Fig. 3), the strati-graphic thickness (∼8.5 km) is not thickened by faults but was deposited by rapidly shifting lateral depocenters. This refined stratigraphic ar-chitecture is consistent with an origin as a strike-slip basin and demonstrates the utility of having numerous well-defined, basin-wide marker beds throughout basin depositional history to resolve structural and stratigraphic complexities.

INFERRED DEPOSITIONAL ENVIRONMENTS

We use six new lithofacies associations (FA) that are slightly modified from Evans (1988, 1991) to characterize the Chumstick Formation: poorly sorted, boulder-cobble conglomerate (FA1); cobble-pebble conglomerate interbedded with coarse-medium-grained sandstone (FA2); coarse-medium-grained sandstone interbedded with massive sandy conglomerate (FA3); lentic-ular, cross-stratified sandstone interbedded with organic-rich mudstone (FA4); thinly interbedded sandstones and mudstones (FA5); and massive, organic-rich mudstone interbedded with minor intervals of lenticular sandstone (FA6). De-tailed descriptions, maximum particle size, and photographs of each lithofacies association are in the Supplementary Material. The map rela-tions of the lithofacies associations are shown in Figure 4. Based on the abundance of organic material in sandstone and mudstone beds and previous paleoclimatic studies, all strata were clearly deposited in a humid environment (New-man, 1981; Evans, 1988, 1991).

Strata of FA1 are interpreted to represent gravel deposition by alluvial-fluvial processes on steep-gradient alluvial fans that bordered basin-bounding faults. Boulder-cobble con-glomerates were deposited by braided stream systems, although uncommon debris-flow de-posits are also present (Evans, 1994). Interbed-ded conglomerates and sandstones of FA2 and FA3 suggest deposition by stream flow on the medial to distal part of a low-gradient, braided stream-dominated alluvial slope. Slope calcula-tions by Evans (1988) are between 5 m/km and 1 m/km depending on whether the calculation was from a coarse-grained (FA1–FA2) or fine-grained lithofacies (FA2, FA3, FA4). In contrast, strata of FA4, FA5, and FA6 are finer-grained and are interpreted to represent deposition by


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meandering stream, deltaic, and lacustrine processes. Thick mudstones and lenticular sandstones of FA4 were deposited by channel cut-and-fill and overbank flow processes in a

meandering stream system. Laminated organic-rich mudstone and minor sandstones of FA5 were deposited mainly in lacustrine settings. Lastly, massive mudstones of FA6 are interpret-

ed to represent sediment deposition in a lake that formed along the eastern subbasin axis (Evans, 1988; Donaghy, 2015).

Overall, the Chumstick Formation is domi-nated by coarser-grained lithofacies in the western subbasin (Fig. 4), suggesting that de-position 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 la-custrine 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 un-conformities during periods that were more tec-tonically active. It is important to reiterate that tuff ages facilitated the spatial correlation of dif-ferent facies based on age rather than lithology and sedimentologic features. For example, one would not expect to correlate the FA2 conglom-erates in the northern western subbasin to fine-grained sediments in the southern western sub-basin (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 litholo-gies. In areas where rock exposure is poor or discontinuous, and lithostratigraphic correla-tions are difficult, this technique is crucial for defining stratigraphy and understanding the spa-tial and temporal relationships between lithofa-cies in strike-slip basins.

SEDIMENT SOURCES IN THE WASHINGTON CASCADES

A diverse assortment of metamorphic and plutonic rocks surrounds the Chumstick basin (Fig. 1; Table 1) and reflects a complex series of tectonic events including final mid-Creta-ceous accretion of the Insular belt to the Inter-montane belt (the margin of North America at that time) and related deformation, metamor-phism, and magmatism (McGroder, 1991); long-lived arc magmatism and underthrusting of forearc sediments in the Late Cretaceous to Paleocene (Matzel et al., 2004; Sauer et al., 2018); and Eocene accretion of Siletzia (Eddy

Figure 4. Simplified geologic map of the Chumstick basin is based on new lithofacies map-ping and stratigraphic sections (blue circles) from Donaghy (2015). Lithofacies Associations (FA) show poorly sorted boulder conglomerate (FA1—red), cobble-boulder conglomerate and sandstone (FA2—pink), sandy conglomerate (FA3—dark yellow), lenticular sandstone and mudstone (FA4—light yellow), mudstone and minor sandstone (FA5—blue), and mud-stone (FA6—green). See Supplementary Material (see footnote 1) for full descriptions of lithofacies associations and photographs. Light colors represent inferred continuation of lithofacies based on structure and previous mapping (Evans, 1988). ECT—Eagle Creek tuff; ECFZ—Eagle Creek fault zone; EFZ—Entiat fault zone; FVT—Fairview tuff; L—Leavenworth; LFZ—Leavenworth Fault zone; SS—Sunnyslope; ST—Sunitsch tuffs; TCTC2—Clark Canyon tuff 2; TCTC4—Clark Canyon tuff 4; YT—Yaksum tuffs; tctc4—Clark Canyon 4 tuff; tctc2—Clark Canyon 2 tuff.




et al., 2017; McCrory and Wilson, 2013; Wells et al., 2014) regional strike-slip faulting (e.g., Umhoefer and Miller, 1996) and ridge-trench interaction (Eddy et al., 2016b; Miller et al., 2016). Magmatic episodes within the region are associated with largely tonalitic and grano-dioritic melts that provide abundant zircons in the potential sediment source areas surround-ing the Chumstick basin. Furthermore, the complex tectonic history has produced distinct regional differences in magmatic crystalliza-tion ages, metamorphic grade, and rock com-position that can be linked to sediment prov-enance (Table 1).

Distinguishing unique source terranes on both sides of the Chumstick basin is critical for deter-mining where sediments were derived and how sources change throughout basin evolution. The simplest distinction among the ages of crystalline rocks in this region is that rocks to the east of the Chumstick basin (Chelan block) have a younger magmatic history than the rocks to the west of the basin (Wenatchee block). In the Wenatchee block to the west, the Wenatchee Ridge Gneiss is the only location with a muscovite-rich gneiss and distinctive fuchsite (Table 1). In the Chelan block to the east, Eocene plutons (49–45 Ma) are dominantly granodiorite in contrast to the older tonalitic plutons (Misch, 1966; Miller et al., 2009). Felsic and mafic dike swarms of this age, which formed coeval with these plutons, are also restricted to the Chelan block with the notable exception of the basaltic Teanaway dike swarm (Tabor et al., 1982, 1987), which is unlikely to have crystallized zircon. Furthermore, the domi-

nant detrital zircon ages of 78–68 Ma in felsic injections into paragneisses of the Swakane Gneiss, and the 79–65 Ma zircon ages of the Entiat, Oval Peak, and Cardinal Peak plutons are distinctive of the Chelan block. The ability to identify these unique source signals in Chum-stick strata, combined with a robust paleocurrent data set from Evans (1988) and compositional data from this study, allows us to document how rapidly changing paleogeography impacted sedi-ment routing systems.

METHODS

Distinct sedimentary sources combined with our revised stratigraphic architecture of the Chumstick basin provide an opportunity to assess provenance through time and space. Con-sequently, we present new U-Pb detrital zircon and clast count data sets. The combination of these data sets allows for a more complete under-standing of evolving sediment source regions. For example, mafic igneous rocks tend to yield little zircon and could be missed as a potential source terrane if detrital zircon were exclusively used. Also, coarse conglomerates represent prox-imal alluvial fan deposits, and it is assumed that they derived sediments from proximal sources. Therefore, large clasts with distinctive litholo-gies and/or ages can also be used as piercing points to constrain the minimal amount of offset along basin margin strike-slip faults. Detrital zir-con can record a more regional signal given the mineral’s refractory nature. Consequently, our combined clast and detrital zircon data sets offer

an integrated view of sediment sources during deposition of the Chumstick basin.

Conglomerate clast counts were obtained from individual pebble-cobble conglomerate beds within the Chumstick Formation (N = 16 samples; n = 1255 total clasts; Supplementary Material). The lithologies of 50–150 clasts were identified during each count, and individual clasts from each lithology were collected for thin-sec-tion petrographic analyses. Clasts were chosen randomly on a ∼2 × 2 m surface within a single conglomerate bed. During conglomerate clast composition counts, the long axes of the 10 larg-est clasts per conglomerate bed were measured for the maximum particle size (MPS) to aid in understanding depositional environments and transport. These data are reported in the Supple-mentary Material.

Detrital zircons from 16 sandstone samples and from three boulder-sized tonalitic clasts were analyzed using LA-ICP-MS, and isotopic data are reported in the Supplementary Mate-rial. All U-Pb detrital zircon geochronology was completed at the University of Arizona LaserChron Center using a Nu Plasma multi-collector ICP-MS. Zircon grains were ablated using a Photon Machine Analyte G2 excimer laser equipped with a HelEX low-volume cell and a laser spot diameter of 30 μm. Between 100 and 110 zircon grains from each sample were randomly selected for isotopic analyses, and the natural zircon reference materials SL2 (535 ± 2.3 Ma; Gehrels et al., 2008) and R33 (419.3 ± 0.4 Ma; Black et al., 2004) were mea-sured to constrain fractionation and provide a

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 (Ma)

Lithology

Wenatchee block (east)*Wenatchee Ridge Gneiss ca. 91 Light-colored, banded granitic gneiss; muscovite-richDirtyface pluton ca. 91 TonaliteTenpeak pluton ca. 92–89 TonaliteMount Stuart batholith 91 (south part); 96–92 (north part) Tonalite and lesser granodiorite, diorite, and gabbroChiwaukum Schist/Nason Ridge Gneiss 125–120; 280–130; ca. 2.7 Ga Well-foliated pelitic schistIngalls Complex ca. 192–145 Ultramafic rock, gabbro, basalt, chert, minor sandstone

Chelan block (west)Railroad Creek pluton 46–45 GranodioriteDuncan Hill pluton 46–45 GranodioriteGolden Horn batholith 48.5–47.7 GranodioriteCooper Mountain batholith 49–47.9 GranodioriteEntiat pluton ca. 73–71 Tonalite and quartz-dioriteCardinal Peak pluton ca. 78–72 Tonalite, granodiorite, and dioriteOval Peak pluton ca. 65 TonaliteSkagit Gneiss Complex 52–45; 75–60; 80–70 Orthogneiss, banded bitotie gneiss, paragneiss; metasedimentary rocksBlack Peak pluton ca. 92–87 TonaliteEldorado pluton ca. 88 TonaliteSeven Fingered Jack pluton ca. 92–90 TonaliteSwakane Gneiss 70–65; 78–68; 87–81; 200–145; 1.8–1.4. Ga Biotite-paragneiss; volumetrically significant felsic sillsNapeequa Complex 70–66; 80–70 Zircon poor; quartzite and amphibolite; schist; volumetrically significant

felsic sillsChelan Complex ca. 120–100 Migmatitic tonalite gneiss, tonaliteDumbell and Marblemount plutons ca. 220 Tonalitic orthogneissCascade 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).


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measure of analytical reproducibility. Dates for zircon that are younger than 900 Ma use the 206Pb/238U dates, while dates that are older than 900 Ma use the 207Pb/206Pb date as they offer the highest precision for dates within these respec-tive age ranges.

RESULTS

Conglomerate Clast Compositions

Sixteen conglomerate clast counts were ob-tained from individual pebble-cobble conglom-

erate beds within the Chumstick Formation (Figs. 2 and 3). Overall, clasts are primarily felsic-intermediate plutonic (39% of all clasts) and metamorphic (38%) lithologies. Felsic to intermediate plutonic clasts remain a consistent source of sediment throughout deposition of

Figure 5. Pie diagrams show the temporal variation in the dis-tribution of felsic-intermediate plutonic and metamorphic clast lithologies. Summary pie diagrams also show the overall varia-tions 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.




the Clark Canyon Member (38%–43%; mainly tonalite and diorite). There is a significant de-crease of metamorphic clasts from the lower Clark Canyon to the lowermost part of the up-per 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 signifi-cantly from Tumwater Mountain conglomer-ates 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, quartz-ite, amphibolite) and minor felsic to intermedi-ate plutonic (18%; granodiorite, tonalite, and diorite). From the Nahahum Canyon to the Deadhorse Canyon Member, there is an in-crease in intermediate (38%; rhyodacite) and mafic plutonic clasts (27%; gabbro) (Fig. 5). One of the most diagnostic changes in prov-enance is the significant increase in reworked sedimentary lithologies from the Clark Canyon Member into the Nahahum Canyon Member (3%–10%) and the influx of granodiorite and rhyodacite conglomerate clasts in the Naha-hum Canyon and Deadhorse Canyon Members (Fig. 5). Significant variations in conglomer-ate clast compositions between samples from the Clark Canyon, Nahahum Canyon, and Deadhorse Canyon Members can be linked to different sediment source regions. Further-more, conglomerate clast counts in the basin margin Tumwater Mountain Member record significant changes in provenance from age-equivalent, basin-axis Clark Canyon Member strata as well as within the Tumwater Moun-tain Member from south to north in the western subbasin (Fig. 5).

U-Pb Geochronology of Detrital Zircons

Similar to the clast composition data, there are significant variations in 16 detrital zircon samples collected from the different members of the Chumstick Formation (Figs. 2, 3, and 6). Probability curves indicate that the lower Clark Canyon Member is dominated by latest Cretaceous–early Paleocene ages (81–60 Ma; 56%), whereas the lower part of the upper Clark Canyon Member has an even distribu-tion of latest Cretaceous (81–70 Ma; 21%) and Early Cretaceous ages (145–100 Ma; 21%). There is an increase in early Late Cretaceous ages (100–87 Ma) from the lower Clark Can-yon Member (24%) to the upper Clark Canyon Member (37%; Fig. 6). Specifically, we see an

increase in the 93–90 Ma peak age population in the Upper Clark Canyon and Nahahum Canyon Members (Fig. 6). From the lower Clark Can-yon Member to the Nahahum Canyon Member, there is also a significant increase in middle–early Eocene ages (53–40 Ma; 0% to 10%). The Nahahum Canyon Member has latest Creta-ceous (81–70 Ma; 23%), Late Cretaceous (100–87 Ma; 17%), Early Cretaceous (145–100 Ma; 15%), and Jurassic (200–145 Ma; 11%) zircon ages (Fig. 6). These variations in detrital peak ages and how they relate to changing sediment sources will be discussed below in detail. Ad-ditionally, boulder-sized tonalitic conglomerate clasts were dated from three sampling locations in the Tumwater Mountain Member along the Leavenworth fault zone (Table E4, see foot-note 1; LaCasse, 2013). U-Pb zircon dates of the three tonalite clasts, from south to north in sampling locations (Fig. 2), yielded weighted mean ages of 90.58 ± 0.53 Ma (n = 24, mean square of weighted deviates [MSWD] = 1.35), 91.72 ± 0.75 Ma (n = 24, MSWD = 0.96), and 90.95 ± 1.06 Ma (n = 24, MSWD = 0.53).

DISCUSSION

Evolution and Paleogeography of the Chumstick Basin

Paleoflow measurements to the southwest, south, and east (all data from Evans, 1988; Fig. 7) suggest that both eastern and western source terranes supplied sediment to the Chum-stick basin. Temporal and spatial changes in provenance and paleoflow between the different members of the Chumstick Formation constrain (1) when specific source terranes in the adjacent bounding basement blocks were uplifted and exhumed, (2) timing of local faulting, and (3) changes in depositional environments and sedi-ment routing systems. We track these changes through six time periods below. They were se-lected because they are bracketed by dated tuffs or correspond to major changes in basin archi-tecture. Within each time period, we discuss the depositional architecture in terms of an eastern low-gradient, stream-dominated fan formed off the Entiat fault zone—a series of steep alluvial

Figure 6. Age probability plots show distribution of U-Pb age determinations for detrital zir-con grains from the Clark Canyon and Nahahum Canyon Members. Ages represent individ-ual spot analyses from separate detrital zircon grains. U-Pb ages are plotted as a normalized relative-probability distribution (Ludwig, 2003). Gray bars were chosen to highlight the peak age populations and how they change throughout the Chumstick basin. The age range of each bar is labeled at the top of the diagram. Tc—Clark Canyon Member; N—number of samples; n—total number of zircon grains; Pz—number of Paleozoic grains not shown; Pc—number of Precambrian grains not shown.


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fans that bordered the western Leavenworth fault zone—and a mixing zone along the basin axis off the Leavenworth fault zone fan fringes. A distinctive detrital zircon signature and con-glomerate clast compositions, paleocurrent indi-cators, and lithofacies associations define each of these components.

Lower–Middle Clark Canyon Member and Inferred Age Equivalent Tumwater Mountain Member (49.2–48.5 Ma)

Our new provenance data are consistent with paleoflow measurements (Fig. 7; Evans, 1988, 1991) and demonstrate that most of the sediments

from the lower to middle Clark Canyon Member were derived from units to the east. The lower to middle Clark Canyon Member is exposed in the southern part of the basin and is characterized by interbedded conglomerates, sandstones, and mudstones (Fig. 3). It is believed that a majority of sediments were deposited on a low-gradient, stream-dominated alluvial fan system in a humid, tropical climate (Evans, 1988, 1991). Addition-ally, boulder conglomerates of the southernmost Tumwater Mountain Member indicate that a component of sediment was also derived from the west and deposited in steep, alluvial fans that bordered the Leavenworth fault zone at this time.

The latest Cretaceous detrital zircon age pop-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 interpre-tation that sediment in the lower Clark Canyon Member was sourced mainly or wholly from the 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 conglom-erate clasts (94% of felsic-intermediate plutonic clasts; Fig. 5) may be derived from multiple intru-sions 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 conglomerate data to further study whether the sediments that fed the lower and middle Clark Canyon Member are from sources that were proximal or distal to the basin-bounding Entiat fault. The 80–60 Ma zircon ages and gneissic conglomerate clasts overlap in age and compo-sition of both the Swakane Gneiss and Skagit Gneiss (Table 1). However, we consider the Swakane Gneiss to be the dominant source based on the relatively large sizes of gneissic clasts, suggesting that they have not traveled far from the source (Table B1; see footnote 1). This interpretation is further supported by Ju-rassic (200–145 Ma; 10.6%) and latest Creta-ceous (87–81 Ma; 8.7%) detrital zircons, which form major zircon populations in the Swakane Gneiss (Sauer et al., 2018) but only minor and/or

A B

C D

Figure 7. Paleoflow data from Evans (1988) are shown. Shaded gray areas represent the interpreted area of sediment deposition. Figure modified from Evans (1994).




variable zircon populations in metasedimentary rocks within the Skagit Gneiss (Sauer et al., 2017b). Finally, tonalite clasts (32% of felsic plutonic clasts) are dominant within the unit and match the composition of the Entiat pluton, and schist and quartzite clasts (14% of metamorphic clasts) match the composition of the Napeequa

Complex (Fig. 5). Thus, we consider most sedi-ment in the lower to middle Clark Canyon Mem-ber to have been sourced from within 30–40 km of the Entiat fault. Our stratigraphically lowest clast count, where nearly all of the clasts have compositions consistent with sources in the Swakane Gneiss, Napeequa Complex, and En-

tiat pluton, suggests that proximal sources were particularly dominant during initial opening of the basin (Table C1; see footnote 1).

The southernmost Tumwater Mountain Member is likely age equivalent to the lower to middle Clark Canyon Member based on its po-sition below the Clark Canyon #4 tuff (Fig. 2).

Figure 8. Schematic block dia-grams and fault restorations illustrate the structural and stratigraphic evolution of the Chumstick basin. Colors of the basin sediments indicate the following: yellow—proximal al-luvial fan facies, orange—me-dial to distal alluvial fan facies, green—basin axis meander-ing stream facies. (A) Dur-ing deposition of the lower to middle Chumstick Formation, sediments were derived primar-ily from distal to proximal east-ern 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 batho-lith adjacent to boulder con-glomerates 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-lat-eral 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, conglomer-ates 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.


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Conglomerate detrital modes of the Tumwater Mountain Member in this area are dominated by tonalite, matching the composition of the Mt. Stuart batholith to the west of the Leav-enworth fault zone. U-Pb zircon dates of two tonalite clasts yielded weighted mean ages of 90.58 ± 0.53 Ma and 91.72 ± 0.75 Ma, consis-tent with the crystallization ages (Matzel et al., 2004) of the southeastern part of the Mount Stu-art batholith where it is cut by the Leavenworth

fault zone. Both conglomerate clast composi-tions and ages suggest derivation of sediments from the west on a steep alluvial fan that bor-dered the Leavenworth fault zone (Fig. 8). Fur-thermore, minor Late Cretaceous detrital zircon ages (100–87 Ma; 8.7%) and Jurassic ages (200–145 Ma; 10.6%) in age-equivalent middle Clark Canyon Member strata along the inferred basin axis also match the 96–91 Ma age of the Mt. Stu-art batholith and Jurassic sedimentary rocks of

the Ingalls Complex (Table 1), which suggests that sediments from east- and west-derived fans mixed along the basin axis.

Initial opening and subsidence of the Chum-stick basin at ca. 49.2 Ma overlapped with the emplacement of the Teanaway Formation and associated dike swarm in a transtensional set-ting between the Leavenworth fault zone and the Straight Creek fault (Figs. 1 and 9; Eddy et al., 2016b). More broadly, the region to the

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 modi-fied half-graben in a transten-sional 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 domi-nantly 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.




east of the basin was undergoing rapid crustal extension and exhumation at this time (Miller et al., 2016; Kruckenberg et al., 2008, and refer-ences therein). Within the Chumstick basin this period of time is marked by rapid accumulation (6–7 mm/yr) of sediment that is dominantly sourced from proximal igneous and metamor-phic rocks to the east of the basin. Based on

paleocurrent data (Evans, 1988) and lithofacies mapping, these sediments were transported on a low-gradient, braided stream-dominated allu-vial fan system that transitioned into meandering stream systems in the southwestern part of the Chumstick basin (Donaghy, 2015). West-derived detritus and boulder conglomerates in a narrow belt along the Leavenworth fault zone are strong

evidence for alluvial-fluvial fans along this ac-tive 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 sug-gest 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 sedi-ment accumulation rate, and the inferred topog-raphy along the basin’s margins, we interpret that dextral strike-slip movement along the En-tiat fault zone and Leavenworth fault zone cre-ated the accommodation space necessary to form the Chumstick basin during this period of time.

Lowermost Part of the Upper Clark Canyon Member and Age Equivalent Tumwater Mountain Member (48.5–48.0 Ma)

The upper Clark Canyon Member marks a significant change in provenance and an influx of coarse-grained sediments into the Chumstick

Figure 9. (A) Paleogeographic reconstruc-tion 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 pa-leo-rivers that supplied detritus to forearc sediments that sit above Siletzia. The ba-salt 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 ad-jacent 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 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.

A

B


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basin (Fig. 3). The lowermost part of the upper Clark Canyon strata in the west-central Chum-stick basin is primarily composed of cobble-peb-ble conglomerates and coarse-grained sandstone (Fig. 4), which is consistent with deposition on the medial part of the low-gradient braided stream-dominated alluvial system derived from the east (Donaghy, 2015). Although sediments are still dominantly derived from the east at this time, the belt of Tumwater Mountain Member along the Leavenworth fault zone near the South Highway sampling location (Fig. 2) suggests that sediment deposition was also active in west-ern alluvial fans. These systems mixed along the basin axis and formed a southward-flowing axial drainage system.

Sediment accumulation rates calculated from this section are 2.6 mm/yr, which is slower than the sediment accumulation rates from the underlying strata but still high and consistent with strike-slip basins (Allen and Allen, 2013). Along with the significant change in provenance, this depositional package is also equivalent to the tuff-rich, Clark Canyon section that is ∼1350 m thick (Fig. 3; Evans, 1988). Abundant meter-scale tuffs (five undated tuffs between Clark Canyon tuff 2 and Eagle Creek tuff are not shown in Fig. 3) in the upper Clark Canyon Member suggest that sediments were deposited during a period of active volcanism in the region. Tuffs are typically overlain by debris-flow de-posits that are similar to alluvial-fluvial deposits adjacent to volcanic centers in humid climates (Kesel, 1985; Evans, 1988).

Latest Cretaceous (81–70 Ma; 21.3%) zircon ages remained a dominant source for the lower-most part of the upper Clark Canyon Member (Fig. 6). However, there is a significant increase up section in Early Cretaceous (145–100 Ma; 20.6%) and Eocene (53–40 Ma; 3.2%) zircon ages and a minor increase in Jurassic (200–145 Ma; 14.8%) and Triassic (251–200 Ma; 2.6%) zircon ages (Fig. 6; Table E2; see foot-note 1). Latest Cretaceous detrital zircons and tonalite clasts match the age and composition of the Entiat and Cardinal Peak plutons (Fig. 1; Table 1). Jurassic ages match the age of the Chel-an Complex and Dumbell orthogneiss–Marble-mount plutons (Miller et al., 2009). The abundant quartzite and biotite-rich gneissic conglomerate clasts match the composition of the Napeequa Complex and Swakane Gneiss, respectively. The increase in Early Cretaceous ages also points to the Swakane Gneiss continuing to be a promi-nent source at this time and the addition of the Chelan Complex as a source (Table 1). These data indicate that an east-derived, braided stream system was still the dominant depositional sys-tem, and there was an expansion of the sediment source region to include rocks to the east of the

Entiat pluton, including the Chelan Complex and the ca. 49–47.9 Ma Cooper Mountain batholith (Shea, 2008).

The most significant change in provenance is the increase of mafic-intermediate volcanic conglomerate clasts (1% to 33%; Fig. 5) and an increase in Eocene detrital zircon ages (53–47 Ma; Fig. 6). The clast compositions match many of the dikes in the Chelan block and are distinguished from the Teanaway dike swarm in the Wenatchee block based on the presence of intermediate compositions. Intermediate com-position dikes are found to be associated with the ca. 49–47.9 Ma Cooper Mountain pluton (Fig. 1; Shea, 2008), and they are particularly dense in our proposed sediment source region (Fig. 8; Raviola, 1988; Tabor et al., 1987). An increase in granodiorite clasts and Eocene zir-con ages further supports exhumation of the area surrounding the Cooper Mountain pluton. The proximal Swakane Gneiss, Napeequa Complex, and Entiat pluton remained the main sources of sediment, but the increase in Early Cretaceous ages and clasts that are inferred to have origi-nated from the region near the Cooper Mountain pluton further support our interpretation that the drainage area was likely cutting into more distal regions of the Chelan block at this time (Fig. 8).

The South Highway location of Tumwater Mountain Member conglomerates sits strati-graphically above the Clark Canyon section (Fig. 2; McClincy, 1986; this study). Conglom-erate clasts are dominated by tonalite that match-es the composition of the Mt. Stuart batholith, and a single tonalite clast from this location gave a U-Pb zircon date of 90.95 ± 1.06 Ma, consistent with this source (Fig. 8). Schist clasts in Clark Canyon Member conglomerates along the basin axis north of Leavenworth re-semble the Chiwaukum Schist (Fig. 2). Clark Canyon sandstones along the basin axis near Camas Land (Fig. 2) document an increase in Late Cretaceous (100–87 Ma; 17.1%) zircon. Additionally, conglomerate clasts in the Clark Canyon Member adjacent to the Ranger Tower section (Fig. 2) are interpreted to represent the Wenatchee Ridge Gneiss due to their light color, presence of fuchsite, and muscovite-rich com-position. This gneiss is located northwest of the Chumstick basin (Fig. 1), and based on the above data, there is strong evidence for increased sediment derivation from the west and mixing of these sediments along the basin axis at this time.

Our provenance data, combined with south-westerly paleoflow measurements (Fig. 7; Ev-ans, 1988), suggest that the dominant source of sediments to the lowermost part of the upper Clark Canyon Member remained the crystalline rocks immediately adjacent to the Entiat fault. However, the shift from low to moderate energy

stream systems to dominantly high-energy braid-ed stream systems increased Early Cretaceous, Triassic, and Eocene detrital zircon populations, and an influx of fine-grained igneous clasts, all suggest that the eastern catchment expanded dur-ing this time and that fluvial systems were effi-ciently transporting coarse sediment from across the Chelan block. We speculate that this expan-sion is related to regional tectonic processes in-cluding increased volcanism and exhumation of the Skagit Gneiss (Miller et al., 2016), as well as the natural evolution of river systems along young basins. Throughout this period, local to-pography was maintained on the Leavenworth fault as evidenced by the coarse conglomerates of the Tumwater Mountain Member A series of steep alluvial fans bordered the Leavenworth fault zone and derived sediments from only sources proximal to the fault and mixed with sediments from the east-derived fan along the basin axis (Fig. 8).

Upper Clark Canyon Member and Age Equivalent Upper Tumwater Mountain Member (48–46.5 Ma)

During this time, there was a well-established series of coalescing alluvial fans bordering the Leavenworth fault zone and mixing with the east-derived, low-gradient, stream-dominated alluvial fan along the basin axis (Fig. 8). The Upper Clark Canyon and Tumwater Mountain Members are exposed north of the city of Leavenworth within the Peshastin syncline and along the western side of the basin (Fig. 2). They interfinger near Leav-enworth, demonstrating their contemporaneous deposition and location of the mixing zone along the inferred basin axis. Age equivalent strata are absent from the southern Chumstick basin, sug-gesting a northward migration of the main dep-ocenter. Thick, amalgamated packages of poorly sorted, boulder-cobble conglomerate of the Tumwater Mountain Member represent deposi-tion on the western fans along the Leavenworth fault zone (Fig. 2; Evans, 1988; Donaghy, 2015). Interbedded sandstones and pebble-cobble con-glomerates and sandy conglomerates of the up-per Clark Canyon Member represent sediment deposition on the same east-derived medial to distal parts of a stream-dominated alluvial slope. Fine-grained deposits that represent deposition by meandering streams and possibly small la-custrine systems characterize the uppermost part of the Clark Canyon Member in the stratigraphic interval between the Tumwater Mountain Mem-ber conglomerates of the Tumwater Mountain sampling location (Fig. 2) and the upper uncon-formity (Fig. 4).

In contrast to the volcanic-rich detritus of the lowermost part of the upper Clark Can-yon Member, conglomerates of the upper




Tumwater Mountain Member and upper Clark Canyon Member along the basin axis, near the Ski Hill and Tumwater Mountain loca-tions (Fig. 2), are dominated by metamorphic and igneous clasts that match western sources (Fig. 5). Early Late Cretaceous (100–87 Ma) zircon ages from the Clark Canyon Member and tonalite conglomerate clasts from the inter-fingering Tumwater Mountain Member corre-late with the composition of the Mount Stuart batholith. (Table 1). Foliated tonalite clasts in the Clark Canyon Member are consistent with derivation from the ca. 92–89 Ma Tenpeak and ca. 91 Ma Dirtyface plutons, which also corre-late with Late Cretaceous zircon ages and lay to the northwest of the basin in the Wenatchee block. Schist clasts in both members can be matched to the Chiwaukum Schist and gneiss clasts to the Nason Ridge Gneisses (Tabor et al., 1987) of the Wenatchee block. The pres-ence of muscovite-rich gneiss conglomerate clasts consistent with the Wenatchee Ridge Gneiss is noted at several locations along the Leavenworth fault zone. Overall, the increase in easily eroded schist clasts indicates that gravel sediments were transported minimal distances and deposited proximal to the source. Paleocurrent measurements from Tumwater Mountain Member conglomerates indicate an eastward direction of paleoflow, in contrast to measurements along the basin axis in Clark Canyon Member strata that indicate southward to southwestward paleoflow (Fig. 7; Evans, 1988). The rapid lateral change in lithofacies and interfingering relationship between the Clark Canyon and Tumwater Mountain Mem-bers support the mixing of these systems along the basin axis (Fig. 4).

Upper Clark Canyon Member strata east of and along the basin axis are still dominated by latest Cretaceous–early Paleocene (81–60 Ma) detrital zircon ages and conglomerate clasts of quartzite, banded gneiss, and biotite-quartz gneiss (Figs. 5 and 6). Latest Cretaceous–Paleo-cene (70–60 Ma) zircon ages and biotite-quartz gneiss and quartzite clasts are consistent with derivation from the Swakane and Napeequa units, respectively, to the east (Table 1). The increase in Eocene zircon ages and presence of granodiorite conglomerate clasts can be at-tributed to continued exhumation and erosion of the felsic plutons to the east of the Entiat fault zone. These data suggest that although there was a greater influx of sediments from the west, the eastern derived alluvial-fluvial depositional system still remained a dominant route for sedi-ment transportation (Fig. 8). The fining trend of lithofacies toward the basin axis and to the south supports the idea that the two fan systems merged along the western basin axis and then

flowed southward along the basin axis in a me-andering stream system (Fig. 8).

The asymmetric nature of the western sub-basin with the axis near the Leavenworth fault zone was the result of two factors. First, strike-slip motion on the Leavenworth fault zone and its right bend (transition to north-trending near Leavenworth) may have resulted in greater sub-sidence near this fault. Clast compositions of the upper Tumwater Mountain Member vary significantly from the tonalite-dominated clast compositions of the middle and lower Tum-water Mountain Member (Fig. 5), supporting a northward shift in the main basin depocenter to be adjacent to more northerly sources. This is consistent with right-lateral slip and northward growth of the Leavenworth fault zone during deposition of the Clark Canyon and Tumwater Mountain Members (Fig. 8; Donaghy, 2015). The second factor that might have influenced the asymmetry of the western subbasin is the differ-ent uplift histories of the Wenatchee and Chelan blocks. The Wenatchee block was uplifted and exhumed by the time of deposition of the Swauk Formation (<59–51 Ma), and previous research indicates relatively low topography to the west of the basin in the Wenatchee block (Methner et al., 2016). In contrast, the Chelan block was largely exhumed during deposition of the Chum-stick basin in the middle Eocene (Miller et al., 2016), and we conclude from the basin history presented here that it was rising substantially in elevation during basin formation.

Basin Partitioning (46.5–44 Ma)A basin-wide to regional unconformity de-

veloped following deposition of the upper Clark Canyon Member. We estimate the age of this unconformity to be 46.5 Ma by assuming that sediment accumulation rates remained constant from the tuffaceous-rich Clark Canyon section upward to the top of the Clark Canyon Member. However, this is a maximum age because some of the Clark Canyon Member has been eroded. Continued rapid exhumation of the crystalline core in the Chelan block during this time is evi-denced by K-Ar cooling ages from biotites and hornblendes between 47 Ma and 44 Ma (Miller et al., 2016) and is compatible with faulting on the Entiat fault zone. Magmatism in the Chelan block also continued during this period with emplacement of the Duncan Hill and Railroad Creek plutons (ca. 46–45 Ma) east of the Chum-stick basin (Fig. 1).

Development of the unconformity in the west-ern subbasin represents a profound change in the local paleogeography. We infer that it was ac-companied by initial motion on the Eagle Creek fault zone and formation of the eastern subba-sin, which is described below. The NW-trending

segments of the Leavenworth fault zone likely became transpressive faults at this time and are related to a train of folds that formed parallel to these fault segments as the western subbasin inverted (Figs. 3 and 8). This uplift and deforma-tion disrupted the fluvial system that ran down the axis of the Chumstick basin and provided an uplifted region that provided sedimentary clasts to the eastern subbasin as it began to develop.

Nahahum Canyon Member (46–44? Ma)The Nahahum Canyon Member was deposited

exclusively in the eastern subbasin and derived from primarily eastern sources with important input from the inverted western Chumstick subbasin. The Nahahum Canyon Member is lithologically different than the older Chumstick Formation strata due to its dominantly finer-grained deposits (Fig. 3). Overall, paleoflow was toward the central to southern parts of the eastern subbasin (Fig. 7; Evans, 1988). During deposi-tion of the Nahahum Canyon Member, sediments were deposited by steep, small alluvial fans that bordered the Entiat fault zone (Fig. 8). Rapid fa-cies changes, from the Eagle Creek fault zone toward the eastern subbasin axis, suggest that the depositional systems changed laterally from al-luvial to fluvial and lacustrine environments over hundreds of meters to a few kilometers (Fig. 4).

Compositional and geochronologic data in-dicate that sediments in the Nahahum Canyon Member were derived primarily from eastern source terranes. The dominance of gneiss con-glomerate clasts and 60–80 Ma detrital zircon ages suggest that the dominant source rocks were the Swakane Gneiss, Skagit Gneiss Complex, Entiat pluton, and felsic sheets intruding the Nap-eequa Complex and Swakane Gneiss (Fig. 5). Furthermore, conglomerate clasts of hornblende-quartz-diorite, diorite, and tonalite are consistent with the Entiat pluton, and the 92–87 Ma peak detrital zircon age population matches the age of the Seven Fingered Jack and Black Peak plutons (Fig. 6; Miller et al., 2009; Shea et al., 2016). The increase in conglomerate clasts of quartzite and amphibolite indicates that the Napeequa Com-plex became more important as a source during deposition of the Nahahum Canyon Member likely due to its location proximal to the east-ern subbasin (Fig. 1; Table 1). Nevertheless, the lower part of the Nahahum Canyon Member con-tains reworked sedimentary lithics from the older Clark Canyon Member in the western subbasin, suggesting that sediments were also derived from west of the Eagle Creek fault zone during initial opening of the eastern subbasin at ca. 46–45 Ma (Supplementary Material) and that the western subbasin was inverted during deposition of the Nahahum Canyon Member. In addition, the Swakane gneiss is uplifted in the Eagle Creek


Donaghy et al.


fault zone horst, so biotite gneiss conglomerate clasts could also represent sediments shed from west of the eastern subbasin.

Two of the most important changes in prove-nance are the presence of granodiorite conglom-erate clasts (Fig. 5) and increase of 46–48 Ma detrital zircons (Fig. 6). Granodiorite clasts match the age and composition of plutons that intruded in the Chelan block to the east during basin partitioning and unconformity develop-ment (Table 1), indicating they were rapidly exhumed after their formation. There is also an increase in conglomerate clasts that match the composition of rhyodacitic dike swarms adjacent to the Cooper Mountain pluton, Duncan Hill plu-ton, and Golden Horn batholith. The increase of Eocene zircon ages also suggests reworking of older Chumstick Formation strata and tuffs into the eastern subbasin as a result of uplift and ex-humation along the Eagle Creek fault zone. The lack of Eocene igneous rocks in the Wenatchee block further supports local sources only from the inverting western Chumstick basin, locally exposed Swakane Gneiss along the Eagle Creek fault zone, and the exhuming Chelan block (Fig. 8).

Based on lithofacies mapping and previous research by Evans (1988), a large lake regularly filled the axis of the eastern subbasin. Deltaic deposits suggest that short streams flowed into the lake from both sides. This change in depo-sitional environments also suggests that any large rivers were south of the presently exposed Chumstick basin by this time. Subsidence and deposition in the Eastern subbasin ended as the Wenatchee Dome intrusive complex intruded the southern part of the Eagle Creek fault zone at ca. 44.447 ± 0.027 Ma (Gilmour, 2012). We infer that slip decreased on the NW-trending Entiat fault zone and Leavenworth fault zone as strike-slip motion localized on the N-trending Straight Creek fault to the west. This interpretation is consistent with apparent offset of northwest-trending lithologic units and structures along this fault. However, some limited strike-slip motion on the Entiat fault zone likely occurred follow-ing deposition of the Nahahum Canyon Member based on the presence of en enchelon folds in the eastern subbasin.

Deadhorse Canyon Member (44–42? Ma)Thick packages of mudstone and sandstone

in the Deadhorse Canyon Member are inter-preted to represent high-sinuosity meander-ing stream channels and floodplain deposits (Evans, 1994). Paleoflow indicators and the distribution of lithofacies suggest flow across the Leavenworth fault zone, and dominantly, a westerly paleoflow (Evans, 1994). Conglomer-ate clast lithologies are dominated by grano-

diorites and rhyodacites, which is consistent with derivation from 46–49 Ma felsic intrusive bodies in the Chelan block. There is also an in-crease of gabbroic clasts, which is consistent with rocks exposed in the Chelan block (e.g., Riddle Peaks gabbro, subordinate bodies within the Entiat and Seven Fingered Jack plutons, and the Chelan Complex). Other clast lithologies from the lower Deadhorse Canyon Member are muscovite-rich gneiss, gneiss, gabbro, and foliated tonalite, all of which were derived from source units adjacent to the north to northeast part of the Chumstick basin (Table C3; see footnote 1).

Deposition and stratigraphic relationships of the Deadhorse Canyon Member with basin-bounding faults indicate that strata were depos-ited during a time of local tectonic quiescence and final filling of the northern basin (Evans, 1994). Initially, sandy braided stream sediments were derived from the east and west before transitioning into an east-derived meandering stream system for the remainder of sediment deposition of the Deadhorse Canyon Member (Fig. 8; Evans, 1994; Donaghy, 2015). This unit is likely correlative with the Roslyn Forma-tion to the west of the Leavenworth fault zone, which also shows no proximal to distal facies changes relative to the Leavenworth fault zone and is broadly coeval (Evans, 1994; Eddy et al., 2016b). We speculate that both formed a broader depositional system between the Chelan block on the east and the Straight Creek fault on the west (Eddy et al., 2016b).

Paleogeography of Western Washington and the Pacific Northwest

During deposition of the early Chumstick Formation, significant basin-margin strike-slip faulting along the Leavenworth fault zone was responsible for rapid vertical and lateral lithofa-cies variations and changes in provenance. Al-luvial fans bordered the Leavenworth fault zone and Entiat fault zone, deriving sediments from both the low hills of the Wenatchee block to the west of the basin and highlands in the Chelan block to the east. Sediments from both systems mixed along the axis of the western subbasin and formed the south-flowing river, which we designate here as the Chumstick River (Fig. 8). During deposition of the Nahahum Canyon Member in the eastern subbasin between 46 and 44 Ma, lakes and small streams were in-ternally drained, and any large stream system, such as the Chumstick River, would have been south of the Chumstick basin. By the time of deposition of the Deadhorse Canyon Member at ca. 44–42 Ma, a continuous, regional west (and southwest?) -flowing fluvial system was estab-

lished between the Chelan block and Straight Creek fault.

We do not know the course of the Chumstick River as it flowed south from the Chumstick basin between 49 Ma and 45 Ma. It either turned west to the coast or continued south to join the proposed Idaho River (Dumitru et al., 2016). The Idaho River is postulated to have been sourced in western Idaho and flowed to the marginal marine to marine Tyee Formation in southwest Oregon, which was deposited over the recently accreted Siletzia terrane (Dumitru et al., 2016; cf. Dorsey et al., 2019). Eocene marine strata overlying the volcanic rocks of Siletzia on the Olympic Peninsula remain poorly studied but also record deltaic and ma-rine sedimentation. The relationship between these two areas is uncertain, but both record the formation of large forearc depositional systems immediately following the accretion of Siletzia. A large provenance data set exists for the Tyee Formation (Dumitru et al., 2016), but only limited provenance data exist for the rocks in western Washington. Detrital zircon geochronology from the Blue Mountain Unit in Washington demonstrates that it is slightly younger than the Chumstick basin (Eddy et al., 2017). However, these are the only data from this area that can be directly compared to those presented in this study.

To assess if the Chumstick River flowed south to join the Idaho River or turned west toward the coast to feed the forearc basin in western Washington, we have compared composite detrital zircon data from the Blue Mountain Group and Tyee Formation that are shown in Figure 9 to the detrital zircon signature from the Chumstick Formation. We would expect to see similar age populations if age-equivalent strata were sharing and/or mixing sediment sources. The two main age populations of 60–70 Ma and 93–96 Ma that characterize the Chumstick Formation are notably absent in the Tyee Formation (Fig. 9). We consider this absence to preclude sedi-ment transport from the Chumstick River to the Tyee Formation, because the large num-ber of 60–79 Ma and 93–96 Ma zircon in the Chumstick basin should be present down-stream even as the sediments were diluted with sediments in the Idaho River.

In contrast to the Tyee Formation, both the Blue Mountain Unit and the Chumstick Forma-tion are characterized by similar age populations of 45–50 Ma, 60–70 Ma, 90–96 Ma, and 150–160 Ma (Fig. 9). The correlation of peak zircon age populations suggests that the Chumstick River turned west after flowing south from the Chumstick basin and mixed with fluvial systems that supplied sediment to coastal sedimentary




basins, such as forearc basin sediments that unconformably overlie Siletzia basalts. Despite overlapping zircon age spectra, there are dif-ferences in the percentages of peak zircon age populations in each formation. Furthermore, there are also peak age populations of 112 Ma and 246 Ma that are not present in Chumstick Formation strata. These differences in the Blue Mountain Group can be accounted for by deriva-tion of sediments from local sources in the Coast Mountains batholith and Western Mélange belt (Sauer et al., 2017a; Gehrels et al., 2009). This suggests that the Western Mélange belt formed a topographic high in the forearc region that was possibly related to motion on the Straight Creek fault and fault splays of the Darrington fault system or shortening related to the accretion of Siletzia (Fig. 9).

The origin of Precambrian zircon ages in Eocene sedimentary basins in Oregon, Wash-ington, and Alaska has been a topic of debate (Garver and Davidson, 2015; Dumitru et al., 2013, 2015, 2016). Precambrian grains in the Chumstick Formation were likely derived from the adjacent Swakane Gneiss and yield ages of 1.38 Ga and a range of 1.6–1.8 Ga ages (Sauer et al., 2018). However, these zircons do not form a significant population. Similar age populations are seen in the Tyee Formation and Blue Mountain Group, but the Blue Mountain Group has significantly more grains. We interpret the influx of Precambrian grains in the Blue Mountain Group as the re-sult of derivation of sediments from the Mé-lange belt (Sauer et al., 2017a), because our regional drainage reconstruction (Fig. 9) sug-gests that the other potential source for these zircon, the southern belt basin (Dumitru et al., 2016), was feeding a depocenter in Idaho at this time.

GENERALIZED CONCLUSIONS FOR STRIKE-SLIP BASINS

The balance between subsidence and sedi-ment supply determines the architecture of sedi-mentary basins. Understanding the relationship between these variables in strike-slip basins is particularly challenging because of the difficulty in producing a basin-wide chronostratigraphy across rapidly changing facies due to incomplete exposure or a lack of radiometric age constraints (Crowell, 2003a, 2003b; Link, 2003; Hempton et al., 1983; Reading, 1980; Crowell, 1974a, 1974b). This study is the first that we know of that documents variation in sediment accumula-tion rates based on numerous precise tuff ages, lithofacies, and provenance to 0.5–1.5 m.y. in-tervals within an ancient strike-slip basin. As a result, we can assess how varying sediment

supply and accommodation space affects the depositional architecture during strike-slip ba-sin evolution.

Previous studies have shown that strike-slip basin geometry, depth, and stratigraphic archi-tecture are determined by the ratio between basin-bounding fault overlap (o) and separa-tion (s) (Fig. 10; Reading, 1980; Rodgers, 1980; Crowell, 1974a, 1974b). When this ratio (o/s) is low, subsidence is highest, and a continuous depositional system is expected to form between the tips of the en echelon strike-slip faults. On the other hand, when the o/s ratio is high, indi-vidual depocenters are expected to form adjacent to each fault tip, and subsidence is more muted. During initial formation of the Chumstick basin (ca. 49.3–48.5 Ma), fault overlap is estimated to be approximately equal to the separation between the basin-bounding faults (Fig. 10), although the southern termination of the Entiat fault is not ex-posed, and this estimate for the o/s ratio should be considered a minimum. In this geometry we expect localized topography around the fault tips and a zone of maximum subsidence con-necting these areas (Fig. 10). The stratigraphic architecture of the lower Clark Canyon Member shows that maximum basin thickness (∼4.5 km) overlaps with this axis of maximum subsidence for a basin forming between the north tip of the Leavenworth fault zone and a potential southern tip of the Entiat fault zone near the southern limit of its present-day exposure (Fig. 10). Sediment accumulation rates varied between 6 mm/yr and 7 mm/yr for the lower Clark Canyon Member, which is fairly high relative to rates calculated for classic strike-slip settings (2–3 mm/yr; Allen and Allen, 2013; Reading, 1980). We interpret this to reflect an initial phase of rapid subsidence coupled with high sediment supply as the basin formed with an initially low o/s ratio between active strike-slip faults. Numerical models by Petrunin and Sobolev (2006, 2008) support this interpretation and show that subsidence rates are initially rapid in pull-apart basins and reduce over the lifespan of the basin as the fault tips propagate and the o/s ratio increases. Distinctive northward shifts in the basin depocenter associ-ated with lower sediment accumulation rates in the upper Clark Canyon Member are consistent with a northward-propagating Leavenworth fault zone and likely track this process in the Chum-stick basin.

The region of greatest subsidence can be roughly estimated in strike-slip basins using the above o/s ratio (Rodgers, 1980) and plays a ma-jor role in predicting how accommodation space evolved through time in the Chumstick basin. In any active basin, the amount of available accom-modation space is a major control on the depo-sitional architecture of a basin (Allen and Allen,

2013). Huerta et al. (2011) documented how the relationship between accommodation space and sediment supply impacts the fluvial depositional architecture of nonmarine basins, which can then be used to understand similar relationships in the Chumstick basin. The majority of the lower Clark Canyon Member is finer-grained (Fig. 4) and characterized by thick sections of mudstone interbedded with lenticular sandstones and mi-nor sandy to pebble conglomerates that vary from the younger basin strata (Supplementary Material). We interpret these lithofacies to be consistent with deposition off the fringe of the east-derived fan system and within the fluvial-dominated basin-axis while initial relief was in-creasing along the eastern basin margin (Fig. 8). Based on the thick stratigraphic section of the lower Clark Canyon Member (Fig. 10) and high sediment accumulation rates, we assume that sediment supply was great enough to aggrade sediments in the basin. High accommodation space coupled with high sediment supply in nonmarine basins results in a fluvial depositional architecture with low interconnectivity and rib-bon-shaped channels (Huerta et al., 2011) and is similar to our interpretation of the lower Clark Canyon Member.

We see a significant change in the deposi-tional architecture of the upper Clark Canyon Member (48.5–46.5 Ma) following northward migration of the main basin depocenter as the Leavenworth fault zone propagated northward (Fig. 10). This interval of time is marked by an influx of coarse-grained material from both sides into the basin synchronous with a period of increased regional volcanism and increased relief across basin-bounding faults (Figs. 4, 5, and 8). Sheet-like sandstone and pebble-cobble conglomerate geometries characterize the upper Clark Canyon Member (Supplementary Mate-rial) and are representative of deposition on the medial part of the east-derived fan system, sug-gesting progradation of this fan toward the basin axis since deposition of the lower Clark Canyon Member (Fig. 8). Continuous northward propa-gation of the Leavenworth fault zone resulted in a high o/s ratio, and modeling by the Rodg-ers (1980) and Wu et al. (2009) models would predict a period of reduced accommodation space and two main basin depocenters form-ing near the tips of the Leavenworth fault zone and Entiat fault zone. Reduced accommodation space, while maintaining high sediment supply, is characterized by sheet-like channel fills and high interconnectivity nonmarine basins (Huer-ta et al., 2011), consistent with the depositional architecture of the upper Clark Canyon Mem-ber. While our study also clearly documents two to three episodes of a northward-shifting basin depocenter along the Leavenworth fault zone


Donaghy et al.


(Figs. 8 and 10), a separate basin depocenter near the end of the Entiat fault zone is difficult to document. First, we do not know where the southern Entiat fault zone tip is, and the south-ern Chumstick basin is dominated by the older strata of the basin. Alternatively, one depocenter may be favored by the exhumation of the entire Chelan block along the Entiat fault zone at this time, overpowering the signature of localized uplift. Sediments were increasingly supplied by more distal sources to the east during deposi-tion of the upper Clark Canyon Member (see discussion), suggesting that the catchment area in the Chelan block was expanding and that topography began to be controlled by regional tectonic processes in addition to localized uplift along the Entiat fault zone and Leavenworth

fault zone. This transition differs from the pre-diction for classic strike-slip systems, in which sediments are sourced from areas proximal to the faults, and it is likely a consequence of rapid exhumation and increased volcanism associated with the accretion of Siletzia and ridge-trench interactions.

Our study also gives us the unique opportu-nity to examine the variability of sediment accu-mulation rates for the same time interval in dif-ferent depositional systems and positions in the basin. When we look at the section between the Yaksum and Eagle Creek tuffs, in the southern part of the basin within the fine-grained “basin-axis” facies (see discussion), sediment accumu-lation rates are ∼7.12 mm/yr (Table A2). For the same time interval in the northern part of the ba-

sin on the “east-derived fan facies,” sediment ac-cumulation rates are ∼3.04 mm/yr (Table A2). A similar contrast is seen for the Eagle Creek to Clark Canyon 4 tuff, where sediment accumu-lation rates in the north are ∼2.57 mm/yr and ∼6.29 mm/yr in the south (Table A2). These systematic differences in sediment accumula-tion rates may reflect changes in where sedi-ments bypassed an area or are accumulated due to changes in depositional systems from the basin axis to nearer the eastern basin margin. Overall, there is still a decrease in stratigraphic thickness upsection in the Clark Canyon Mem-ber once the main depocenter shifts primarily to the north (Fig. 10), which is consistent with reduced accommodation space during matura-tion of a pull-apart basin (Petrunin and Sobo-

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.




lev, 2008, 2006). Previous studies have shown that sediment accumulation rates are greatest in mudstone-rich sections versus sandstone se-quences (Huerta et al., 2011; Crowell, 2003b, 1974a, 1974b). This is consistent with the idea that during high sediment supply, alluvial fan systems will be largely a zone of sediment by-pass. Additionally, as accommodation space is reduced, sediments will also start to bypass de-position within the basin if sediment supply re-mains high. This point highlights the importance of standardizing the facies in which sediment accumulation rates are calculated in strike-slip basins as well as integrating this information with an understanding of sediment supply and the development of accommodation space. The unique basin-wide deposition of numerous tuffs within the Chumstick basin has allowed us to begin to address the variability of depositional environments and sediment accumulation rates from initiation through maturation of pull-apart basins. These data confirm previous hypotheses about strike-slip basin development from mod-eling and provide a framework for future studies of similar basins.

CONCLUSIONS

Our study integrates new lithofacies map-ping and a robust provenance data set from the Chumstick Formation with previously published depositional ages to create a holis-tic view of basin evolution in a strike-slip set-ting. Deposition of the Chumstick Formation occurred primarily on a low-gradient, braided stream-dominated alluvial fan system that de-rived sediments from eastern source terranes in the Chelan block of the North Cascades. West-derived sediments from the Wenatchee block across the active Leavenworth fault zone on steep alluvial fans mixed with east-derived sediments in a meandering stream setting along the basin axis. Overall, felsic-intermediate plu-tonic (tonalite and diorite) and metamorphic (schist and biotite-gneiss) conglomerate clasts and Late Cretaceous zircon ages (60–80 Ma) are dominant throughout deposition of the Clark Canyon Member, which supports the interpretation that eastern terranes were an important source of sediment supply. There was a significant change in provenance and northward shift of the main basin depocenter during deposition of the Upper Clark Canyon Member. Numerous interbedded tuffs and an influx of coarse-grained volcanic material and Eocene detrital ages (53–44 Ma) support syn-chronous deposition of the upper Clark Canyon Member with a phase of local volcanism. The increase in Late Cretaceous ages (90–96 Ma) and metamorphic clasts supports northward

propagation of the Leavenworth fault zone and exhumation of adjacent source terranes, which caused northward migration of the main ba-sin depocenter during deposition of the Upper Clark Canyon Member. Fault reorganizations late in the basin’s history led to basin partition-ing, deposition of the fine-grained Nahahum Canyon Member in the eastern subbasin, and ultimately the cessation of strike-slip faulting on the basin-bounding faults. This transition was associated with the re-establishment of a regional depositional system in the Deadhorse Canyon Member and the gradual removal of localized topography. Our data set represents a well-constrained example of how sediment routing within an ancient strike-slip basin evolves through the basin’s lifetime and pro-vides insight into the competing local and re-gional processes that led to changes in sediment source areas through time during deposition of the Chumstick basin. These results are not only important for understanding the regional tectonic setting and paleogeography of Wash-ington at this time but also shed light on the fundamentals of strike-slip basin evolution.

ACKNOWLEDGMENTS

We thank Christopher Donaghy and Jason Muhl-bauer for their assistance in the field. Financial sup-port for this project was provided by National Science Foundation grants EAR-1119063 to P.J. Umhoefer and EAR-1119358 to R.B. Miller. The research was further supported by graduate research grants to E.E. Donaghy from the Geological Society of America, the American Association of Petroleum Geologists Nancy Setzer Murray Memorial fund, and the Pioneer Research Scholarship Fund. The manuscript has ben-efited from the thoughtful comments of two anony-mous reviewers and editorial feedback from Science Editor Wenjiao Xiao.

REFERENCES CITED

Allen, P.A., and Allen, J.R., 2013, Basin Analysis: Principles and Application to Petroleum Play Assessment (Third Edition): Hoboken, New Jersey, Wiley Blackwell, 632 p.

Black, L.P., Kamo, S.L., Allen, C.M., Davis, D.W., Aleini-koff, J.N., Valley, J.W., Mundil, R., Campbell, I.H., Korsch, R.J., Williams, I.S., and Foudoulis, C., 2004, Improved 206Pb/238U microprobe geochronology by the monitoring of a trace-element-related matrix ef-fect; SHRIMP, ID-TIMS, ELA-ICP-MS and oxygen isotope documentation for a series of zircon standards: Chemical Geology, v. 205, p. 115–140, https://doi .org/10.1016/j.chemgeo.2004.01.003.

Bradley, D.C., Haeussler, P., and Kusky, T.M., 1993, Tim-ing of Early Tertiary ridge subduction in southern Alaska: U.S. Geological Survey Bulletin 2068, p. 163–177.

Bradley, D.C., Kusky, T.M., Haeussler, P.J., Goldfarb, R., Miller, M., Dumoulin, J., Nelson, S.W., and Karl, S., 2003, Geologic signature of early Tertiary ridge subduction in Alaska, in Sisson, V.B., Roeske, S.M., and Pavlis, T.L., eds., Geology of a Transpressional Orogeny Developed during Ridge-Trench Interaction along the North Pacific Margin: Geological Society of America Special Paper 371, p. 19–49, https://doi .org/10.1130/0-8137-2371-X.19.

Brown, E.H., Cary, J.A., Dougan, B.E., Dragovich, J.D., Fluke, S.M., and McShane, D.P., 1994, Tectonic evolu-

tion of the Cascades crystalline core in the Cascade Riv-er area, Washington: Washington Division of Geology and Earth Resources Bulletin, v. 80, p. 93–113.

Cheney, E.S., and Hayman, N.W., 2007, Regional Tertiary sequence stratigraphy and structure on the eastern flank of the central Cascade Range, Washington, in Stelling, P., and Tucker, D.S., eds., Floods, Faults, and Fire: Geo-logical Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 179–208, https://doi.org/10.1130/2007 .fld009(09).

Cheney, E.S., and Hayman, N.W., 2009, The Chiwaukum Structural Low: Cenozoic shortening of the central Cas-cade Range, Washington State, USA: Geological Soci-ety of America Bulletin, v. 121, no. 7–8, p. 1135–1153, https://doi.org/10.1130/B26446.1.

Christie-Blick, N., and Biddle, K.T., 1985, Deformation and basin formation along strike-slip faults, in Biddle, K.T., and Christie-Blick, N., eds., Strike-Slip Defor-mation, Basin Formation, and Sedimentation: Society of Economic Paleontologists and Mineralogists Spe-cial Publication 37, p. 1–34, https://doi.org/10.2110/pec.85.37.0001.

Cowan, D.S., 2003, Revisiting the Baranof-Leech River hypothesis for early Tertiary coastwise transport of the Chugach-Prince William terrane: Earth and Plan-etary Science Letters, v. 213, p. 463–475, https://doi .org/10.1016/S0012-821X(03)00300-5.

Crowell, J.C., 1974a, Sedimentation along the San Andreas fault, California, in Dott, R.H., Jr., and Shaver, R.H., eds., Modern and Ancient Geosynclinal Sedimenta-tion: Society of Economic Paleontologists and Miner-alogists Special Publication 19, p. 292–303, https://doi .org/10.2110/pec.74.19.0292.

Crowell, J.C., 1974b, Origin of late Cenozoic basins in south-ern California, in Dickinson, W.R., ed., Tectonics and Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 22, p. 190–204, https://doi.org/10.2110/pec.74.22.0190.

Crowell, J.C., 2003a, Introduction to geology of Ridge Ba-sin, southern California, in Crowell, J.C., Evolution of Ridge Basin, Southern California: An Interplay of Sedimentation and Tectonics: Geological Society of America Special Paper 367, p. 1–15, https://doi .org/10.1130/0-8137-2367-1.1.

Crowell, J.C., 2003b, Tectonics of Ridge Basin region, south-ern California, in Crowell, J.C., Evolution of Ridge Ba-sin, Southern California: An Interplay of Sedimentation and Tectonics: Geological Society of America Special Paper 367, p. 157–203, https://doi.org/10.1130/0-8137-2367-1.157.

Donaghy, E.E., 2015, Structure, stratigraphy, and provenance of Eocene sedimentary rocks in the Chumstick basin, central Washington [M.S. thesis]: Flagstaff, Arizona, Northern Arizona University, 236 p.

Dorsey, R.J., Brutzkus, P., and Mortimer-Lamb, M., 2019, Basinal and paleoriver response to Eocene accretion of Siletzia in SW Oregon: Conflicting data and unresolved questions. Geological Society of America Abstracts with Programs, v. 51, no. 4.

Dumitru, T.A., Ernst, W.G., Wright, J.E., Wooden, J.L., Wells, R.E., Farmer, L.P., Kent, A.J.R., and Graham, S.A., 2013, Eocene extension in Idaho generated mas-sive sediment floods into the Franciscan trench and into the Tyee, Great Valley, and Green River basins: Geology, v. 41, p. 187–190, https://doi.org/10.1130/G33746.1.

Dumitru, T.A., Ernst, W.G., Hourigan, J.K., and McLaughlin, R.J., 2015, Detrital zircon U-Pb reconnaissance of the Franciscan subduction complex in northwestern Cali-fornia: International Geology Review, v. 57, p. 767–800, https://doi.org/10.1080/00206814.2015.1008060.

Dumitru, T.A., Elder, W.P., Hourigan, J.K., Chapman, A.D., Graham, S.A., and Wakabayashi, J., 2016, Four Cordil-leran paleorivers that connected Sevier thrust zones in Idaho to depocenters in California, Washington, Wyo-ming, and indirectly, Alaska: Geology, v. 44, p. 75–78, https://doi.org/10.1130/G37286.1.

Eddy, M.P., Bowring, S.A., Miller, R.B., and Tepper, J.H., 2016a, Rapid assembly and crystallization of a fossil large-volume silicic magma chamber: Geology, v. 44, p. 331–334, https://doi.org/10.1130/G37631.1.


https://doi.org/10.1016/j.chemgeo.2004.01.003

https://doi.org/10.1016/j.chemgeo.2004.01.003

https://doi.org/10.1130/0-8137-2371-X.19

https://doi.org/10.1130/0-8137-2371-X.19

https://doi.org/10.1130/2007.fld009(09)

https://doi.org/10.1130/2007.fld009(09)

https://doi.org/10.1130/B26446.1

https://doi.org/10.2110/pec.85.37.0001

https://doi.org/10.2110/pec.85.37.0001

https://doi.org/10.1016/S0012-821X(03)00300-5

https://doi.org/10.1016/S0012-821X(03)00300-5

https://doi.org/10.2110/pec.74.19.0292

https://doi.org/10.2110/pec.74.19.0292

https://doi.org/10.2110/pec.74.22.0190

https://doi.org/10.1130/0-8137-2367-1.1

https://doi.org/10.1130/0-8137-2367-1.1

https://doi.org/10.1130/0-8137-2367-1.157

https://doi.org/10.1130/0-8137-2367-1.157

https://doi.org/10.1130/G33746.1

https://doi.org/10.1130/G33746.1

https://doi.org/10.1080/00206814.2015.1008060

https://doi.org/10.1130/G37286.1

https://doi.org/10.1130/G37631.1

Donaghy et al.


Eddy, M.P., Bowring, S.A., Umhoefer, P.J., Miller, R.B., McLean, N.M., and Donaghy, E.E., 2016b, High-res-olution temporal and stratigraphic record of Siletzia’s accretion and triple junction migration from nonmarine sedimentary basins in central and western Washing-ton: Geological Society of America Bulletin, v. 128, p. 425–441, https://doi.org/10.1130/B31335.1.

Eddy, M.P., Clark, K.P., and Polenz, M., 2017, Age and volca-nic stratigraphy of the Eocene Siletzia oceanic plateau in Washington and on Vancouver Island: Lithosphere, v. 9, p. 652–664, https://doi.org/10.1130/L650.1.

Engebretson, D.C., Cox, A., and Gordon, R.G., 1983, Rela-tive Motions Between Oceanic and Continental Plates in the Pacific Basin: Stanford, California, Stanford University Publications. Geological Sciences, v. 18, p. 80–82.

Evans, J.E., 1988, Depositional environments, basin evolu-tion and tectonic significance of the Eocene Chumstick Formation, Cascade Range, Washington [Ph.D. thesis]: Seattle, University of Washington, 325 p.

Evans, J.E., 1991, Facies relationships, alluvial architecture, and paleohydrology of a Paleogene, humid-tropical alluvial-fan system: Chumstick Formation, Washington State, U.S.A: Journal of Sedimentary Research, v. 61, no. 5, p. 732–755.

Evans, J.E., 1994, Depositional history of the Eocene Chum-stick Formation; implications of tectonic partitioning for the history of the Leavenworth and Entiat-Eagle Creek fault systems, Washington: Tectonics, v. 13, no. 6, p. 1425–1444, https://doi.org/10.1029/94TC01321.

Evans, J.E., 1996, Reply to Comment on “Depositional history of the Eocene Chumstick Formation: Impli-cations of tectonic partitioning for the history of the Leavenworth and Entiat- Eagle Creek fault systems, Washington”: Tectonics, v. 15, p. 510–514, https://doi .org/10.1029/95TC03695.

Ewing, T.E., 1980, Paleogene tectonic evolution of the Pa-cific Northwest: The Journal of Geology, v. 88, no. 6, p. 619–638, https://doi.org/10.1086/628551.

Frizzell, V.A., 1979, Petrology and stratigraphy of Paleogene nonmarine sandstones, Cascade Range, Washington [Ph.D. thesis]: Stanford, California, Stanford Univer-sity, 169 p.

Garver, J.I., and Davidson, C.M., 2015, Southwestern Laurentian zircons in Upper Cretaceous flysch of the Chugach-Prince William terrane in Alaska: American Journal of Science, v. 315, no. 6, p. 537–556, https://doi.org/10.2475/06.2015.02.

Gatewood, M.P., and Stowell, H.H., 2012, Linking zircon U-Pb and garnet Sm-Nd ages to date loading and metamorphism in the lower crust of a Cretaceous magmatic arc, Swakane Gneiss, WA, USA: Lithos, v. 146–147, p. 128–142, https://doi.org/10.1016/ j.lithos.2012.04.030.

Gehrels, G.E., Valencia, V.A., and Ruiz, J., 2008, Enhanced precision, accuracy, efficiency, and spatial resolution of U-Pb ages by laser ablation-multicollector-inductively coupled plasma-mass spectrometry: Geochemistry, Geophysics, Geosystems, v. 9, no. 3, p. 1–13, https://doi.org/10.1029/2007GC001805.

Gehrels, G.E., Rusmore, M., Woodsworth, G.J., Crawford, M., Andronicos, C., Hollister, L.S., Patchett, P.J., Du-cea, M., Butler, R., Klepeis, K., Davidson, C., Fried-man, R., Haggart, J.W., Mahoney, B., Crawford, W., Pearson, D., and Girardi, J., 2009, U-Th-Pb geochronol-ogy of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evo-lution: Geological Society of America Bulletin, v. 121, p. 1341–1361, https://doi.org/10.1130/B26404.1.

Gilmour, L.A., 2012, U/Pb ages of Eocene and younger rocks on the eastern flank of the central Cascade Range, Washington, USA [M.S. thesis]: Seattle, Washington, University of Washington, 50 p.

Gordon, S.M., Bowring, S.A., Whitney, D.L., Miller, R.B., and Mclean, N., 2010, Time scales of metamorphism, deformation, and crustal melting in a continental arc, North Cascades USA: Geological Society of America Bulletin, v. 122, p. 1308–1330, https://doi.org/10.1130/B30060.1.

Gordon, S.M., Miller, R.B., and Sauer, K.B., 2017, Incor-poration of sedimentary rocks into the deep levels of continental magmatic arcs: Links between the North

Cascades arc and surrounding sedimentary terranes, in Haugerud, R.A., and Kelsey, H.M, eds., From the Puget Lowland to East of the Cascade Range: Geologic Excursions in the Pacific Northwest: Geological Soci-ety of America Field Guide 49, p. 101–142, https://doi .org/10.1130/2017.0049(06).

Haeussler, P.J., Bradley, D.C., Wells, R.E., and Miller, M.L., 2003, Life and death of the Resurrection plate: Evidence for its existence and subduction in the northeastern Pa-cific in Paleocene—Eocene time: Geological Society of America Bulletin, v. 115, no. 7, p. 867–880, https://doi.org/10.1130/0016-7606(2003)115<0867:LADOTR>2.0.CO;2.

Haugerud, R.A., and Tabor, R.W., 2009, Geologic Map of the North Cascade Range, Washington: Technical pamphlet to accompany Scientific Investigations Map 2940: U.S. Geological Survey, 32 p.

Haugerud, R.A., Van Der Heyden, P., Tabor, R.W., Stacey, J.S., and Zartman, R.E., 1991, Late Cretaceous and early Tertiary plutonism and deformation in the Skagit Gneiss Complex, North Cascade Range, Washington and British Columbia: Geological Society of Ameri-ca Bulletin, v. 103, no. 10, p. 1297–1307, https://doi .org/10.1130/0016-7606(1991)103<1297:LCAETP>2.3.CO;2.

Hempton, M.R., Dunne, L.A., and Dewey, J.F., 1983, Sedi-mentation in an active strike-slip basin, southeastern Turkey: The Journal of Geology, v. 91, p. 401–412, https://doi.org/10.1086/628786.

Hopson, C.A., and Mattinson, J.M., 1994, Chelan migma-tite complex, Washington—Field evidence for mafic magmatism, crustal anatexis, mixing, and protodiapiric emplacement, in Swanson, D.A., and Haugerud, R.A., eds., Geologic Field Trips in the Pacific Northwest, Se-attle, Washington, October 24–27: Geological Society of America Annual Meeting, v. 2, p. 2K2–2K21.

Huerta, P., Armenteros, I., and Silva, P.G., 2011, Large-scale architecture in non-marine basins: The response to the interplay between accommodation space and sediment supply: Sedimentology, v. 58, no. 7, p. 1716–1736, https://doi.org/10.1111/j.1365-3091.2011.01231.x.

Johnson, S.Y., 1982, Stratigraphy, sedimentology, and tec-tonic setting of the Eocene Chuckanut Formation, Northwest Washington [Ph.D. thesis]: Seattle, Univer-sity of Washington, 296 p.

Johnson, S.Y., 1984, Stratigraphy, age, and paleogeography of the Eocene Chuckanut Formation, northwest Wash-ington: Canadian Journal of Earth Sciences, v. 21, p. 92–106, https://doi.org/10.1139/e84-010.

Johnson, S.Y., 1996, Comment on “Depositional history of the Eocene Chumstick Formation: Implications of tectonic partitioning for the history of the Leaven-worth and Entiat-Eagle Creek fault systems, Washing-ton”: Tectonics, v. 15, no. 2, p. 506–509, https://doi .org/10.1029/95TC03694.

Kesel, R.H., 1985, Alluvial fan systems in a wet-tropical en-vironment, Costa Rica: National Geographic Research, v. 1, p. 450–469.

Kruckenberg, S.C., Whitney, D.L., Teyssier, C., Fanning, C.M., and Dunlap, W.J., 2008, Paleocene-Eocene mig-matite crystallization, extension, and exhumation in the hinterland of the northern Cordillera: Okanogan Dome, Washington, USA: Geological Society of America Bulletin, v. 120, p. 912–929, https://doi.org/10.1130/B26153.1.

LaCasse, T.J., 2013, Using a coarse-grained conglomerate to investigate lateral offset along the Leavenworth Fault Zone, North Cascades, Washington: Carleton College, unpublished undergraduate research, 40 p.

Link, M.H., 2003, Depositional systems and sedimentary facies of the Miocene-Pliocene Ridge Basin Group, Ridge Basin, southern California, in Crowell, J.C., ed., Evolution of Ridge Basin, Southern California: An Interplay of Sedimentation and Tectonics: Geological Society of America Special Paper 367, p. 17–87, https://doi.org/10.1130/0-8137-2367-1.17.

Ludwig, K.R., 2003, Isoplot 3.00: A Geochronological Tool-kit for Microsoft Excel: Berkeley, California, Berkeley Geochronology Center Special Publication 4a, 74 p.

MacDonald, J.H., Jr., Harper, G.D., Miller, R.B., Miller, J.S., Mlinarevic, A.N., and Schultz, C.E., 2008, The Ingalls ophiolite complex, central Cascades, Washington: Geo-

chemistry, tectonic setting, and regional correlations in Wright, J.E., and Shervais, J.W., eds., Ophiolites, Arcs, and Batholiths: A tribute to Cliff Hopson: Geological Society of America Special Paper 438, p. 133–159, https://doi.org/10.1130/2008.2438(04).

Madsen, J.K., Thorkelson, D.J., Friedman, R.M., and Mar-shall, D.D., 2006, Cenozoic to Recent plate configura-tions in the Pacific Basin: Ridge subduction and slab window magmatism in western North America: Geo-sphere, v. 2, no. 1, p. 11–34, https://doi.org/10.1130/GES00020.1.

Massey, N.W.D., 1986, Metchosin Igneous Complex, southern Vancouver Island: Ophiolite stratigraphy developed in an emergent island setting: Geol-ogy, v. 14, p. 602–605, https://doi.org/10.1130/0091-7613(1986)14<602:MICSVI>2.0.CO;2.

Matzel, J., 2004, Rates of tectonic and magmatic processes in the North Cascades Continental Magmatic Arc [Ph.D. thesis]: Cambridge, Massachusetts, Massachusetts In-stitute of Technology.

Matzel, J.E.P., Bowring, S.A., and Miller, R.B., 2004, Proto-lith age of the Swakane Gneiss, North Cascades, Wash-ington: Evidence of rapid underthrusting of sediments beneath an arc: Tectonics, v. 23, 18 p.

Matzel, J.E.P., Bowring, S.A., and Miller, R.B., 2006, Time scales of pluton construction at differing crustal levels: Examples from the Mount Stuart and Tenpeak intru-sions, North Cascades, Washington: Geological Society of America Bulletin, v. 118, p. 1412–1430, https://doi .org/10.1130/B25923.1.

McClincy, M.J., 1986, Tephrostratigraphy of the middle Eo-cene Chumstick Formation, Cascade Range, Douglas County, Washington [M.S. thesis]: Portland, Oregon, Portland State University, 125 p.

McCrory, P.A., and Wilson, D.S., 2013, A kinematic model for the formation of the Siletz- Crescent forearc ter-rane by capture of coherent fragments of the Farallon and Resurrection plates: Tectonics, v. 32, p. 718–736, https://doi.org/10.1002/tect.20045.

McGroder, M.F., 1991, Reconciliation of two-sided thrusting, burial metamorphism, and diachronous uplift in the Cas-cades of Washington and British Columbia: Geological Society of America Bulletin, v. 103, p. 189–209, https://doi.org/10.1130/0016-7606(1991)103<0189:ROTSTB>2.3.CO;2.

Methner, K., Fiebig, J., Wacker, U., Umhoefer, P., Cham-berlain, P.C., and Mulch, A., 2016, Eocene-Oligocene proto-Cascades topography revealed by clumped (Δ47) and oxygen isotope (δ18O) geochemistry (Chumstick basin, WA, USA): Tectonics, v. 35, p. 546–564, https://doi.org/10.1002/2015TC003984.

Miller, R.B., and Bowring, S.A., 1990, Structure and chronol-ogy of the Oval Peak batholith and adjacent rocks: Im-plications for the Ross Lake fault zone, North Cascades, Washington: Geological Society of America Bulletin, v. 102, p. 1361–1377, https://doi.org/10.1130/0016-7606(1990)102<1361:SACOTO>2.3.CO;2.

Miller, R.B., Paterson, S.R., and Matzel, J.P., 2009, Plu-tonism at different crustal levels: Insights from the ∼5–40 km (paleodepth) North Cascades crustal sec-tion, Washington, in Miller, R.B., and Snoke, A.W., eds., Crustal Cross Sections from the Western North American Cordillera and Elsewhere: Implications for Tectonic and Petrologic Processes: Geological Society of America Special Paper 456, p. 125–149, https://doi .org/10.1130/2009.2456(05).

Miller, R.B., Gordon, S.M., Bowring, S.A., Doran, B., McLean, N.M., Michels, Z., Shea, E., and Whitney, D.L., 2016, Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA): Geosphere, v. 12, p. 900–924, https://doi.org/10.1130/GES01262.1.

Misch, P., 1966, Tectonic evolution of the Northern Cascades of Washington State: A west- Cordilleran case history, in Gunning, H.C., ed., Symposium on the Tectonic History and Mineral Deposits of the Western Cordillera in Brit-ish Columbia and Neighboring Parts of United States: Quebec City, Canada, Canadian Institute of Mining and Metallurgy, Special Volume, No. 8, p. 101–148.

Misch, P., 1968, Plagioclase compositions and non-anatectic origin of migmatitic gneisses in Northern Cascade


https://doi.org/10.1130/B31335.1

https://doi.org/10.1130/L650.1

https://doi.org/10.1029/94TC01321

https://doi.org/10.1029/95TC03695

https://doi.org/10.1029/95TC03695

https://doi.org/10.1086/628551

https://doi.org/10.2475/06.2015.02

https://doi.org/10.2475/06.2015.02

https://doi.org/10.1016/j.lithos.2012.04.030

https://doi.org/10.1016/j.lithos.2012.04.030

https://doi.org/10.1029/2007GC001805

https://doi.org/10.1029/2007GC001805

https://doi.org/10.1130/B26404.1

https://doi.org/10.1130/B30060.1

https://doi.org/10.1130/B30060.1

https://doi.org/10.1130/2017.0049(06)

https://doi.org/10.1130/2017.0049(06)

https://doi.org/10.1130/0016-7606(2003)115<0867:LADOTR>2.0.CO;2



https://doi.org/10.1130/0016-7606(1991)103<1297:LCAETP>2.3.CO;2



https://doi.org/10.1086/628786

https://doi.org/10.1111/j.1365-3091.2011.01231.x

https://doi.org/10.1139/e84-010

https://doi.org/10.1029/95TC03694

https://doi.org/10.1029/95TC03694

https://doi.org/10.1130/B26153.1

https://doi.org/10.1130/B26153.1

https://doi.org/10.1130/0-8137-2367-1.17

https://doi.org/10.1130/0-8137-2367-1.17

https://doi.org/10.1130/2008.2438(04)

https://doi.org/10.1130/GES00020.1


https://doi.org/10.1130/0091-7613(1986)14<602:MICSVI>2.0.CO;2

https://doi.org/10.1130/0091-7613(1986)14<602:MICSVI>2.0.CO;2

https://doi.org/10.1130/B25923.1

https://doi.org/10.1130/B25923.1

https://doi.org/10.1002/tect.20045

https://doi.org/10.1130/0016-7606(1991)103<0189:ROTSTB>2.3.CO;2



https://doi.org/10.1002/2015TC003984

https://doi.org/10.1002/2015TC003984

https://doi.org/10.1130/0016-7606(1990)102<1361:SACOTO>2.3.CO;2

https://doi.org/10.1130/0016-7606(1990)102<1361:SACOTO>2.3.CO;2

https://doi.org/10.1130/2009.2456(05)

https://doi.org/10.1130/2009.2456(05)





Mountains of Washington State: Contributions to Mineralogy and Petrology, v. 17, p. 1–70, https://doi .org/10.1007/BF00371809.

Newman, K.R., 1981, Palynologic biostratigraphy of some early Tertiary nonmarine formations in central and western Washington, in Armentrout, J.M., ed., Pacific Northwest Cenozoic Biostratigraphy: Geological Soci-ety of America Special Paper 184, p. 49–65, https://doi .org/10.1130/SPE184-p49.

Petrunin, A., and Sobolev, S.V., 2006, What controls thick-ness of sediments and lithospheric deformation at pull-apart basins?: Geology, v. 34, no. 5, p. 389–392, https://doi.org/10.1130/G22158.1.

Petrunin, A., and Sobolev, S.V., 2008, Three-dimensional numerical models of the evolution of pull-apart basins: Physics of the Earth and Planetary Interi-ors, v. 171, p. 387–399, https://doi.org/10.1016/j.pepi.2008.08.017.

Raviola, F.P., 1988, Metamorphism, plutonism, and defor-mation in the Pateros-Alta Lake region, North-Central Washington [M.S. thesis]: San Jose, California, San Jose State University, 181 p.

Reading, H.G., 1980, Characteristics and recognition of strike-slip fault systems, in Ballance, P.F., and Reading, H.G., eds., Sedimentation in Oblique-Slip Mobile Zones: International Association of Sedimen-tologists Special Publication 4, p. 105–125, https://doi .org/10.1002/9781444303735.ch2.

Rodgers, D.A., 1980, Analysis of basin development pro-duced by en echelon strike-slip faults, in Balance, P.F., and Reading, H.G., eds., Sedimentation at Oblique-Slip Margins: International Association of Sedimen-tologists Special Publication 4, p. 27–41, https://doi .org/10.1002/9781444303735.ch3.

Sauer, K.B., Gordon, S.M., Miller, R.B., Vervoort, J.D., and Fisher, C.M., 2017a, Evolution of the Jura-Cretaceous North American Cordilleran margin: Insights from detrital-zircon U-Pb and Hf isotopes of sedimentary units of the North Cascades Range, Washington: Geo-sphere, v. 13, p. 2094–2118, https://doi.org/10.1130/GES01501.1.

Sauer, K.B., Gordon, S.M., Miller, R.B., Vervoort, J.D., and Fisher, C.M., 2017b, Transfer of metasupracrust-al rocks to midcrustal depths in the North Cascades continental magmatic arc, Skagit Gneiss Complex,

Washington: Tectonics, v. 36, p. 3254–3276, https://doi .org/10.1002/2017TC004728.

Sauer, K.B., Gordon, S.M., Miller, R.B., Vervoort, J.D., and Fisher, C.M., 2018, Provenance and metamorphism of the Swakane Gneiss: Implications for incorporation of sediment into the deep levels of the North Cascades continental magmatic arc, Washington: Lithosphere, v. 10, p. 460–477, https://doi.org/10.1130/L712.1.

Schuster, E.J., 2005, Geologic map of Washington State: Washington State Department of Natural Resources, scale 1:500,000, 1 sheet.

Shea, E.K., Miller, J.S., Miller, R.B., Bowring, S.A., and Sullivan, K.M., 2016, Growth and maturation of a mid- to shallow-crustal intrusive complex, North Cascades, Washington: Geosphere, v. 12, p. 1489–1516, https://doi.org/10.1130/GES01290.1.

Shea, E.K., Miller, J.S., Miller, R.B., Chan, C.F., Kent, A.J.R., Hanchar, J.M., Dustin, K., and Elkins, S., 2018, Cretaceous North Cascades magmatic arc, Washing-ton, and relationship to Cretaceous flare-up magma-tism: Lithosphere, v. 10, no. 6, p. 708–722, https://doi .org/10.1130/L1001.1.

Shea, E.K.M., 2008, Structure of the southern Skagit Geniss Complex, North Cascades, Washington [M.S. thesis]: San Jose, California, San Jose State University, 90 p.

Sylvester, A.G., 1988, Strike-slip faults: Geological Society of America Bulletin, v. 100, p. 1666–1703, https://doi .org/10.1130/0016-7606(1988)100<1666:SSF>2.3.CO;2.

Tabor, R.W., Frizzel, V.A., Jr., Whetten, J.R., Waitt, R.B., Swanson, D.A., Byerly, G.R., Booth, D.B., Hether-ington, M.J., and Zartman, R.E., 1987, Geologic Map of the Chelan 30-Minute by 60-Minute Quadrangle, Washington: U.S. Geological Survey Miscellaneous Investigations Series I-1661, scale 1:100,000, 1 sheet, 29 p. text.

Tabor, W., Waitt, R.B., Frizzel, V.A., Swanson, D.A., Byerly, G.R., and Bentley, R.D., 1982, Geologic map of the Wenatchee 1:100,000 Quadrangle, central Washington: U.S. Geological Survey Miscellaneous Investigations Series, 1–1311, 26 p.

Tabor, R.W., Frizzell, V.A., Vance, J.A., and Naeser, C.W., 1984, Ages and stratigraphy of lower and middle Ter-tiary sedimentary and volcanic rocks of the central Cascades, Washington: Application to the tectonic

history of the Straight Creek fault: Geological Society of America Bulletin, v. 95, no. 1, p. 26–44, https://doi .org/10.1130/0016-7606(1984)95<26:AASOLA>2.0.CO;2.

Tabor, R.W., Haugerud, R.A., Hildreth, W., and Brown, E.H., 2003, Geologic map of the Mount Baker 30- by 60-Minute Quadrangle, Washington: U.S. Geological Survey Geologic Investigations Series 1-2660, scale 1:000,000, 2 sheets, 73 p. text.

Umhoefer, P.J., and Miller, R.B., 1996, Mid-Cretaceous thrusting in the southern Coast Belt, British Colum-bia and Washington, after strike-slip fault reconstruc-tion: Tectonics, v. 15, no. 3, p. 545–565, https://doi .org/10.1029/95TC03498.

Vance, J.A., and Miller, R.B., 1981, The movement history of the Straight Creek fault in Washington state, in Monger, J.W.H., ed., The Last 100 Million Years (Mid-Creta-ceous to Holocene) of Geology and Mineral Deposits in the Canadian Cordillera: Geological Association of Canada Program with Abstracts, v. 6, p. 39–41.

Wells, R.E., Engebretson, D.C., Snavely, P.D., Coe, R.S., and Anonymous, 1984, Cenozoic plate motions and the volcano-tectonic evolution of western Oregon and Washington: Tectonics, v. 3, no. 2, p. 275–294, https://doi.org/10.1029/TC003i002p00275.

Wells, R., Bukry, D., Friedman, R., Pyle, D., Duncan, R., Haeussler, P., and Wooden, J., 2014, Geologic his-tory of Siletzia, a large igneous province in the Or-egon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot: Geosphere, v. 10, p. 692–719, https://doi.org/10.1130/GES01018.1.

Wu, J.E., McClay, K., Whitehouse, P., and Dooley, T., 2009, 4D analogue modeling of transtensional pull-apart basins: Marine and Petroleum Geology, v. 26, p. 1608–1623, https://doi.org/10.1016/j.marpetgeo.2008.06.007.

Science Editor: Wenjiao XiaoAssociate Editor: Troy Rasbury

Manuscript Received 1 May 2020Revised Manuscript Received 9 November 2020Manuscript Accepted 4 January 2021

Printed in the USA


https://doi.org/10.1007/BF00371809

https://doi.org/10.1007/BF00371809

https://doi.org/10.1130/SPE184-p49

https://doi.org/10.1130/SPE184-p49

https://doi.org/10.1130/G22158.1

https://doi.org/10.1130/G22158.1

https://doi.org/10.1016/j.pepi.2008.08.017

https://doi.org/10.1016/j.pepi.2008.08.017

https://doi.org/10.1002/9781444303735.ch2

https://doi.org/10.1002/9781444303735.ch2

https://doi.org/10.1002/9781444303735.ch3

https://doi.org/10.1002/9781444303735.ch3



https://doi.org/10.1002/2017TC004728

https://doi.org/10.1002/2017TC004728

https://doi.org/10.1130/L712.1



https://doi.org/10.1130/L1001.1

https://doi.org/10.1130/L1001.1

https://doi.org/10.1130/0016-7606(1988)100<1666:SSF>2.3.CO;2



https://doi.org/10.1130/0016-7606(1984)95<26:AASOLA>2.0.CO;2



https://doi.org/10.1029/95TC03498

https://doi.org/10.1029/95TC03498

https://doi.org/10.1029/TC003i002p00275

https://doi.org/10.1029/TC003i002p00275


https://doi.org/10.1016/j.marpetgeo.2008.06.007.

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