The Confusion Range, west-central Utah: Fold-thrust deformation and a western Utah thrust belt in the Sevier hinterland David C. Greene* Department of Geosciences, Denison University, Granville, Ohio 43023, USA ABSTRACT INTRODUCTION tions together while delineating the lateral and oblique thrust ramps that form a signifi cant The Confusion Range in west-central Utah The Confusion Range is a collection of ridges complicating factor in the structure of the fold- has been considered a broad structural trough and small ranges that together form a low moun- thrust system. Together, these fi ve cross sections or synclinorium with little overall shorten- tain range in western Utah, between the more total almost 300 km in map length. Enlarged ing. However, new structural studies indicate imposing Snake Range on the west and House versions of the cross sections at a scale of that the Confusion Range is more accurately Range on the east (Figs. 1 and 2). The range is 1:50,000, along with a discussion of the petro- characterized as an east-vergent, fold-thrust named for its “rugged isolation and confusing leum potential of the region, may be found in system with ~10 km of horizontal shortening topography” (Van Cott, 1990). The Confusion Greene and Herring (2013). during Late Jurassic to Eocene Cordilleran Range exposes ~5000 m of Ordovician through Similar structural style and fold-thrust struc- contractional deformation. For this study, Triassic strata in what has been considered a tures are continuous southward throughout the four balanced and retrodeformable cross broad structural trough or synclinorium (e.g., length of the originally proposed synclinorium, sections across the Confusion Range and Hose, 1977; Anderson, 1983; Hintze and Davis, forming a fold-thrust belt more than 130 km in adjacent Tule Valley were constructed using 2003; Rowley et al., 2009). Hintze (in Hintze length that is herein named the western Utah existing mapping and new fi eld data, and and Davis, 2003) described the envisioned syn- thrust belt. Comparison with the central Nevada these were tied with a fi fth strike-parallel sec- clinorium as a structural feature 130 km long, thrust belt and similar zones of fold-thrust tion. Ramp anticlines and anticlinal duplexes up to 24 km wide, and extending the entire deformation in eastern Nevada and western characteristic of strong Lower Paleozoic length of western Millard County, with the Con- Utah suggests that this region is not a little- carbonate units are balanced by faulted and fusion Range comprising approximately the deformed “hinterland” to the Sevier thrust belt, rotated detachment folds in more ductile northern half. as has been previously envisioned, but is instead Upper Paleozoic strata. The apparently syn- Complex near-surface structures have been rec- a zone of signifi cant distributed Mesozoic con- clinal aspect results from two different sets ognized in the Confusion Range since the map- tractional deformation. of thrust structures that uplift and expose ping of Richard Hose, Lehi Hintze, and others In this paper, the term “thrust system” is used Lower Paleozoic rocks on the fl anks of the in the 1960s and 1970s, (e.g., Hose and Ziony, for a zone of closely related thrusts and associ- range. Fold-thrust structures are continuous 1963; Hose and Repenning, 1963; Hose, 1965a, ated folds that are geometrically and mechani- southward for more than 130 km, forming a 1974a; Hintze, 1974a). These previous structural cally linked (McClay, 1992), and “thrust belt” thrust belt of regional extent herein named interpretations have generally featured compli- is used more generally for a relatively narrow the western Utah thrust belt. This thrust belt cated and variable internal deformation, but with zone of fold-thrust deformation of regional is comparable in length and magnitude of little overall shortening (e.g., Hose, 1977). extent. The continental-scale Cordilleran thrust shortening to the central Nevada thrust belt, In contrast to early interpretations of synclino- belt is considered to consist of multiple smaller and both merge southward with the Sevier ria, more recent work (Dubé and Greene, 1999; thrust belts of regional extent that may show dif- frontal thrust belt. Together, these and related Nichols et al., 2002; Yezerski and Greene, 2009; ferences in location, timing, structural style, and fold-thrust zones in eastern Nevada indicate Matteri and Greene, 2010; Greene and Herring, magnitude of shortening. signifi cant, broadly distributed Mesozoic 2013) indicates that the Confusion Range is more shortening in the Sevier hinterland. The Cor- accurately characterized as an east-vergent, fold- Regional Setting dilleran thrust belt in eastern Nevada and thrust system with signifi cant (~10 km) horizon- western Utah thus consists of a frontal zone— tal shortening during Late Jurassic to Eocene In what is now western Utah, more than the Sevier frontal thrust belt—where major Cordilleran contractional deformation. 13 km of Neoproterozoic to Triassic strata were thrust faults with 50–100 km of displacement I present here a series of four balanced and deposited on the rifted western edge of cratonic breach the surface, and a hinterland zone, restorable cross sections across the Confusion North America (Fig. 3). Four kilometers or characterized by a number of distributed Range and adjacent Tule Valley (location map, more of Neoproterozoic and Lower Cambrian fold-thrust belts, each accommodating on the Plate 1; sections, Plates 2–5) that characterize predominantly clastic strata, exposed in the order of 10 km of shortening. the structural architecture and style of defor- Snake and Deep Creek Ranges to the west of mation. A fi fth strike-parallel cross section the Confusion Range, are overlain by 8–9 km *E-mail: [email protected]. (Plate 6) ties the strike-perpendicular cross sec- of Middle Cambrian to Devonian strata, largely Geosphere; February 2014; v. 10; no. 1; p. 148–169; doi:10.1130/GES00972.1; 9 fi gures; 6 plates. Received 30 July 2013 ♦ Revision received 2 December 2013 ♦ Accepted 20 December 2013 ♦ Published online 14 January 2014 148 For permission to copy, contact [email protected] © 2014 Geological Society of America Western Utah thrust belt 113° 30′W Nevada Utah Deep Creek 2006; Yonkee and Weil, 2011). The Sevier frontal Range thrust belt in central Utah consists of four main thrust systems, the Canyon Range, Pavant, Pax- ton, and Gunnison thrusts, with total shortening of at least 220 km (DeCelles and Coogan, 2006). The Canyon Range and Pavant thrust sheets, in particular, include a strong Precambrian to Lower Middle Range Cambrian quartzite sequence that supported long- distance eastward transport. These thrust sheets apparently root at midcrustal levels beneath the Fig. 2 Juab County Confusion Range (Allmendinger et al., 1983; Millard County DeCelles and Coogan, 2006) and are continuous eastward under the Sevier Desert basin as long 39° Confusion Tu 39° 30′ 30′ l N Range e N hanging wall-on-footwall fl ats, initially ramping Va to the surface in the Canyon Range. Snake lley Late Cretaceous to middle Cenozoic paleo- Range geography in the region likely consisted of a Snake Va high-elevation, low-relief plateau referred to as the “Nevadaplano,” with a steep topographic front and foreland basin to the east (Coney lley 25p7.061 House and Harms, 1984; DeCelles, 2004; Best et al., Range 2009; Henry et al., 2012). Paleogene conglom- erates, lacustrine limestones, and interbedded volcanics were deposited in local basins and paleochannels, possibly related to early exten- sional collapse within the plateau (Vandervoort and Schmitt, 1990; Constenius, 1996; Greene U Sevier Lake S Highway 50 and Herring, 1998; Hintze and Davis, 2003; Snake 39° Range Druschke et al., 2011; Lechler et al., 2013). 00′ N Widespread pyroclastic volcanism, the “ignim- Confusion brite fl areup” (Coney, 1978; Best et al., 2009, Range 2013), began in the late Eocene and contin- ued through the early Miocene. Beginning in the early Miocene, predominantly high-angle Burbank extensional faulting formed the typical Basin Hills and Range topography observed today (e.g., Dickinson, 2006). North-striking, fault-bounded valleys such as Snake Valley and Tule Valley, which formed adjacent to uplifted ranges, con- White Pine County Lincoln County tain up to 3000 m of fl uvial, alluvial, and lacus- Utah trine sediments, with interbedded volcanics. 0 10 20 30 km Nevada Utah N CROSS-SECTION CONSTRUCTION Figure 1. Map showing location and topography of the Confusion Range and surround- ing region, modifi ed from U.S. Geological Survey orthophoto state map series. Central box Four cross sections transverse to structural indicates the location of Figure 2. strike and one tie section parallel to structural strike were constructed for this study. Existing surface mapping, detailed new surface mapping carbonates deposited in a stable passive-margin ler et al., 2004; Dickinson, 2006). By late Meso- over the lines of section, and the very limited setting (Link et al., 1993; Cook and Corboy, zoic time, an organized subduction system was subsurface data available were incorporated into 2004; Hintze and Davis, 2003; Hintze and established along the Cordilleran continental the cross sections. Kowallis , 2009) and now well exposed in the margin, and a major retroarc fold-thrust belt The Confusion Range is well covered by geo- House Range and Confusion Range. developed inboard of the magmatic arc. The seg- logic mapping completed mostly in the 1960s The carbonate-dominated passive margin was ment of this fold-thrust belt in southern Nevada and 1970s (Hose, 1963a, 1963b, 1965a, 1965b, disrupted in early Mississippian time by clastic and Utah has been termed the Sevier belt (Arm- 1974a, 1974b; Hose and Repenning, 1963, foredeep sediments associated with the Antler strong, 1968; DeCelles and Coogan, 2006). 1964; Hose and Ziony, 1963, 1964; Hintze, orogeny to the west, and by subsequent devel- In Utah, fold-thrust deformation began in the 1974a; Sack, 1994a; Hintze and Davis, 2002a).