Abstract

Growth faults and synorogenic sedimentary strata preserved in Upper Cretaceous units on the margin of the Kaiparowits Basin in southern Utah pinpoint the timing of onset of the Laramide orogeny in this region between 80 and 76 Ma. The newly identified listric normal faults, exposed in the steep limb of the East Kaibab monocline, sole into shales and evaporites of the Jurassic Carmel Formation. Faults lose displacement up-section through the Cretaceous Wahweap Formation and are associated with numerous coseismic sedimentary features. Fault orientations and slip vectors yield strain directions consistent with fold-related extension parallel to the axis of the growing East Kaibab monocline, or with development of a pull-apart basin at a bend in the trend of the fold. The association of the faults with the steep limb of a major basement-cored structure links them to initial Laramide movement along the Kaibab Uplift. When combined with recent radiometric ages of rock units bracketing the fault-induced growth strata, these sedimentary and structural features narrowly define the onset of Laramide deformation in the western Colorado Plateau.

INTRODUCTION

The timing and kinematic progression of Laramide deformation in the Colorado Plateau and Rocky Mountain regions of the western United States have been controversial topics since early exploration of the American West (Powell, 1875; Dutton, 1882; Walcott, 1890). To explain variations in the orientations and structural styles of Colorado Plateau and Rocky Mountain basement-cored uplifts, geologists have suggested processes including changing stress fields (Chapin and Cather, 1981; Gries, 1983; Varga, 1993; Bird, 1998; Bump, 2004), rotation or northward translation of the Colorado Plateau crustal block (Chapin and Cather, 1981; Bryan, 1989; Wawrzyniec and Geissman, 1995; Cather, 1997, 1999), lithospheric buckling (Tikoff and Maxson, 2001), and oblique reactivation of preexisting crustal weaknesses (Kelley, 1955; Stone, 1969; Marshak et al., 2000; Timmons et al., 2001; Tetreault and Jones, 2007). A definitive understanding of the growth of foreland basement-cored uplifts, both as individual structures and as tectonic systems, hinges on unraveling the timing and sequence of deformation. Unfortunately, determination of timing is complicated by limitations of erosion and exposure. On the Colorado Plateau in particular, the key Late Cretaceous and early Tertiary syntectonic strata have been eroded from the crests of major basement-cored uplifts or are deeply buried in intervening basins. Across the spatial transition from Sevier fold-and-thrust deformation to Laramide basement-cored uplifts in southwestern Utah, the relevant synorogenic sedimentary rocks are exposed only in narrow, discontinuous belts across the westernmost Colorado Plateau (Goldstrand, 1994; Lawton et al., 2003) (Fig. 1). Examination of the limited exposures of sedimentary rocks spanning the time of the Laramide orogeny reveals a regional unconformity separating Late Cretaceous strata from Paleocene and Eocene rocks, and most researchers have assumed that Laramide deformation across the southwestern Colorado Plateau generally coincided with development of this unconformity (Gregory and Moore, 1931; Bowers, 1972; Chapin and Cather, 1981; Goldstrand, 1994). Regional studies of sedimentation patterns and basin styles provide a broad framework for the timing and progression of Laramide deformation (Chapin and Cather, 1981; Dickinson et al., 1986; Goldstrand, 1994), but the different orientations and structural styles of individual uplifts, and the underlying tectonic causes of basement-involved foreland deformation, are still actively debated (e.g., Bump, 2004, 2007; Jones et al., 2007; Tetreault and Jones, 2007; Wawrzyniec et al., 2007).

Synorogenic sedimentary rocks are instrumental in unraveling timing, progression, and kinematics of mountain-building and in understanding the effects of orogenesis on Earth surface processes and environments. Several studies have investigated syntectonic sediments and growth strata associated with Sevier fold-and-thrust deformation in the western United States (DeCelles, 1994; DeCelles et al., 1995; Latta, 1999; Anastasio et al., 2002). Within the Rocky Mountain foreland uplifts bordering the Colorado Plateau, the magnitude of uplift and subsidence favored preservation of syntectonic strata, and, in many cases, subsequent Basin and Range normal faulting resulted in their dissection and exposure (e.g., Hoy and Ridgeway, 1997; Seager et al., 1997; Johnson and Andersen, 2009). The low amplitude of deformation and the presence of a regional unconformity between Late Cretaceous and Tertiary deposits of the western Colorado Plateau have precluded widespread investigation of synorogenic sedimentary rocks associated with the start of the Laramide orogeny in this enigmatic region.

Close examination of Upper Cretaceous strata exposed at the boundary between the Kaibab Uplift and the Kaiparowits Basin in southern Utah (Fig. 2) has revealed faulting and associated growth strata linked with early movement of a major Laramide uplift. Fault kinematics indicate extension in the upper crust at the onset of Laramide deformation, and associated syntectonic strata reveal that the basement-cored Kaibab Uplift influenced surface structure and sedimentation during Campanian time—specifically, between 80 and 76 Ma. The study area lies near the boundary between the Sevier, “thin-skinned” fold-and-thrust belt style of deformation and the Laramide, “thick-skinned” basement-cored uplifts of the Cordilleran foreland (Fig. 2). The transition in structural style across this region makes it a key area for understanding kinematic and tectonic development of orogens, while the temporal progression of deformation across the boundary is a necessary element in modeling the Colorado Plateau system of basement-cored uplifts. This paper analyzes the timing and kinematics of surface faulting marking the onset of Laramide deformation along the Kaibab Uplift and discusses tectonic and sedimentologic implications.

GEOLOGIC SETTING

The Kaibab Uplift is a Laramide-age, basement-cored uplift near the western edge of the Colorado Plateau, occupying the foreland for ∼180 km parallel to the edge of the Sevier thin-skinned fold-and-thrust belt (Fig. 1). In southern Utah, the eastern edge of the Kaibab Uplift is marked by steeply east-dipping Jurassic and Cretaceous strata of the East Kaibab monocline, and to the east, a thick sequence of Upper Cretaceous strata preserved in the neighboring Kaiparowits Basin (Figs. 1 and 2). Previous work suggests that the East Kaibab monocline formed above steeply dipping Precambrian basement faults that were reactivated as reverse or dextral-reverse faults during the Late Cretaceous to early Tertiary Laramide orogeny, causing west-side-up monoclinal folding of Paleozoic and Mesozoic sedimentary strata (e.g., Stern, 1992; Tindall and Davis, 1999; Marshak et al., 2000; Timmons et al., 2001; Bump and Davis, 2003) (Fig. 2). The maximum vertical offset across the monocline is ∼1600 m near the northern end of the Kaibab Uplift in southern Utah.

This study focuses on faults and sedimentary strata preserved in the Late Cretaceous Wahweap Formation at the boundary between the Kaibab Uplift and Kaiparowits Basin (Fig. 2). Recent radiometric dating of a bentonite layer in the middle Wahweap provided an age of 80.1 ± 0.3 Ma (Jinnah et al., 2009), and a tuff near the base of the overlying Kaiparowits Formation yielded an age of 75.96 ± 0.14 Ma (Roberts et al., 2005). These radiometric ages complement studies of microvertebrate biostratigraphy (Eaton, 1991, 2002) in supporting a Campanian age for the Wahweap Formation. West and south of the study area, the Wahweap Formation has been uplifted and removed by erosion, while to the north and east, it is buried in the subsurface of the Kaiparowits Basin (Fig. 1). However, along the East Kaibab monocline, a narrow strip of tilted Cretaceous rocks provides perspective on structure and sedimentation at the onset of Laramide deformation (Fig. 2).

GROWTH FAULTS AND SYNTECTONIC STRATA

Modern Fault Geometry

Two northeast-striking, northwest-dipping faults exposed in the steep, east-dipping limb of the East Kaibab monocline each display ∼0.5–1 km of right-handed separation of Jurassic Entrada through Cretaceous Wahweap strata (Figs. 2 and 3). Slickenlines on exposed fault surfaces rake moderately (∼45°–55°) southwest, indicating oblique, northwest-side-up relative displacement (Fig. 4). The faults lose displacement up-section within the Cretaceous Wahweap Formation; separation at the base of the Wahweap is up to 0.5 km, but the Wahweap-Kaiparowits contact is undisturbed by the southern fault and is offset only a few meters by the northern fault (Fig. 3). Toward the southwest, each fault trace curves into parallelism with shales and evaporites of the Jurassic Carmel Formation, a valley-forming unit through which stratigraphic markers and fault surfaces are difficult to trace. However, the faults clearly do not displace the underlying Jurassic Navajo Sandstone in the field area (Figs. 2 and 3). Figure 4 summarizes orientations of major northeast-striking fault surfaces and southwest-raking slickenlines in their modern-day orientations.

Stern (1992) and Tindall and Davis (1999) interpreted these faults as oblique reverse faults that developed during a final phase of Laramide, ENE-directed horizontal shortening and monocline growth. Stern (1992) noted their association with a bend in the trend of the monocline, and inferred them to represent displacement transfer between right-stepping basement fault segments. A reverse right-lateral kinematic interpretation is logical given the modern fault and slickenline orientations and the apparent right-handed separation across the faults. However, closer examination of the informally defined lower, middle, upper, and capping sandstone members of the Cretaceous Wahweap Formation across the faults reveals apparent growth strata in the upper member immediately south of each fault trace. Today, the Wahweap strata dip 20°–60° east as a result of folding in the steep limb of the monocline. The east-dipping upper member growth strata complicate the interpretation of fault timing and kinematics because bedding must have been approximately horizontal, not tilted in the steep monoclinal limb, at the time faulting occurred. The presence of Wahweap growth strata indicates that the faults predate significant monoclinal folding. Furthermore, if faulting can be linked to early rather than late Laramide movement across the East Kaibab monocline, then the Campanian age of the Wahweap growth strata also coincides with the initiation of Laramide deformation.

Evidence for Growth Faults

The upper member of the Wahweap Formation appears to thicken from south to north as it approaches the faults (Fig. 3). The apparent thickening is particularly evident when comparing the separation of the base of the Wahweap (Ksc-Kwl contact, Fig. 3) with the separation of the top of the Wahweap (Kwu-Kk contact, Fig. 3).

Because the steep limb of the monocline is tilted eastward up to 70° in the field area, the map pattern exposed in the tilted limb presents an oblique cross section of strata as they would have appeared before monoclinal folding. In other words, the eroded map view “slice” across tilted strata is analogous to a nearly vertical cross section through the same structural and stratigraphic features when bedding was horizontal, prior to monoclinal folding. Using this analogy, the faults and sedimentary layers exposed in map view resemble a cross section through dip fans in growth strata on the hanging wall of listric normal faults (Figs. 3 and 5) (Hamblin, 1965; Shelton, 1984; Twiss and Moores, 2007). A localized westward turn in the strike of Jurassic and Cretaceous strata south of the southern fault resembles a rollover anticline associated with listric normal faulting, and short, northwest-striking antithetic faults accommodate extension and rollover in the hanging wall of the listric normal fault system (Figs. 2 and 5). The growth fault interpretation can be confirmed by comparing true stratigraphic thickness of the upper member of the Wahweap Formation at proximal and distal locations.

Evaluation of Stratigraphic Thickness

The upper member of the Wahweap Formation consists primarily of thick lenses of fluvial channel sands and minor conglomerates with overbank silt and shale deposits (Pollock, 1999; Lawton et al., 2003; Wizevich et al., 2008). Although exposure in the field area is excellent, treacherous topography, steep bedding dip, and the discontinuous nature of the channel and overbank deposits make accurate measurement of thickness of stratigraphic sections problematic. Instead, evaluation of stratigraphic thickness was accomplished by careful measurement and Global Positioning System (GPS) location of contacts and dip inflections along transects crossing the suspected syntectonic strata at varying distances from the faults. Bedding orientations were measured along four transects (lines A–D on Fig. 3) across the upper and capping sandstone members of the Wahweap, with particular emphasis on recording lower and upper contacts of these units and notable changes in bedding dip. Field data were used to construct cross sections using the kink method (Faill, 1969; Suppe, 1985). Cross sections display true thickness of the upper member for easy visual comparison (Fig. 6). Accurate thicknesses were calculated geometrically using horizontal distance and elevation change between data points (provided by GPS coordinates) and bedding dip across changing dip domains (Fig. 6).

Transect A lies north of the two-fault system, across a section of the Wahweap that appears relatively thin in map view. The transect yields an upper member true thickness of 92 m at this location. In a listric growth fault system (Fig. 5), this transect location is analogous to the footwall bordering a pair of linked, listric normal faults; the uplifted footwall is expected to accumulate the thinnest section of syntectonic strata.

Transects B and C lie immediately south of the northern and southern faults, respectively, where the upper member appears thicker. These locations are hypothesized to represent the down-dropped hanging-wall blocks of the northern and southern faults, where thickened syntectonic strata fill fault-bounded half-graben. The true thickness along transect B is 132 m, and along transect C, the upper member is 262 m thick.

Transect D crosses the upper member of the Wahweap ∼2 km south of the southern fault, where the thickened wedge of growth strata begins to taper. The upper member true thickness at this location is reduced to 160 m.

The true thickness calculations take into account variations in elevation and bedding orientation across the field area, eliminating the possibility that the apparent variations in thickness are illusions created by orientation and exposure. Our calculations indicate that the upper member of the Cretaceous Wahweap Formation is almost three times thicker on the southernmost fault block (originally the hanging wall) than on the northern block (the original footwall) of the fault system, and that thickness tapers again toward the south as distance from the faults increases. These thickness variations could not have developed if faulting postdated monoclinal folding, because the Wahweap is incorporated in the steep monoclinal limb. Instead, structural data and thickness calculations support the interpretation that the Wahweap Formation contains growth strata on the southeastern side of each fault, meaning that the faults were active during deposition of the Wahweap Formation, before significant monoclinal folding occurred.

Cretaceous Fault Kinematics

As exposed today, the faults dip northwest and display reverse right-handed separation in the steep monoclinal limb; however, the growth strata in the Wahweap Formation indicate that faulting occurred when the Wahweap was more or less horizontal, prior to significant monoclinal tilting. Therefore, the faults must be rotated stereographically to their Cretaceous orientations before correct kinematic interpretation can occur.

Original (Cretaceous) fault orientations were determined by rotating faults and associated slickenlines around the strike of adjacent footwall bedding and thus returning bedding dips to horizontal (Fig. 7). Fault traces are curved, so their orientations vary slightly according to stratigraphic position, but, in general, the faults rotate back to easterly and northeasterly strikes with moderate to steep southward dips when bedding is returned to horizontal. Slickenlines on major fault surfaces, although southwest-raking today, restore to rakes of 72W–78E, indicating southeast-side-down, nearly dip-slip normal offset. An additional 29 measurements were gathered from minor fault surfaces and cataclastic deformation band shear zones containing both slickenlines and offset markers in Straight Cliffs Formation sandstones throughout the field area. Unfolding and inversion of these fault measurements yield incremental strain orientations (following Unruh and Twiss, 1998) of S1 = 162°, 9°; S2 = 68°, 24°; and S3 = 271°, 64° (Fig. 8). The fault plane solution based on the 29 measurements, rotated back to their Cretaceous orientations, reveals slightly oblique normal faulting (Fig. 8). The strain directions and the focal mechanism are consistent with an interpretation of near-surface normal faulting in the regional context of Laramide, ENE-directed horizontal shortening (Coney, 1976; Reches, 1978; Anderson and Barnhard, 1986). The faults exposed in the steep limb of the East Kaibab monocline therefore represent northeast- to east-striking, south-dipping, listric normal faults, detached within the Jurassic Carmel Formation, that were active during deposition of the Cretaceous Wahweap Formation and that mark the onset of Laramide monoclinal folding.

DISCUSSION

Laramide Timing

The fragmented distribution of Late Cretaceous and early Tertiary rock exposures has limited the investigation of synorogenic strata associated with Laramide deformation in the western Colorado Plateau, complicating the determination of the timing of individual uplifts. Apatite fission-track data from the Grand Canyon suggest that regional Laramide uplift was under way by ca. 75 Ma (Dumitru et al., 1994). Working near our field area in southwestern Utah, Goldstrand (1994) associated the initiation of Laramide-style deformation in this region with deposition of the Lower Paleocene Canaan Peak and Grand Castle Formations. However, the presence of growth strata in the Wahweap Formation suggests an earlier start to Laramide movement along the Kaibab Uplift. The upper member of the Wahweap Formation has not been dated directly, but it is bracketed by a bentonite layer in the middle member dated to be 80.1 ± 0.3 Ma (Jinnah et al., 2009) and a dateable tuff in the lower unit of the Kaiparowits at 75.96 ± 0.14 Ma (Roberts et al., 2005). This marks the time of deposition, and therefore of fold-related faulting, as Campanian (80–76 Ma) rather than Maastrichtian–early Paleocene.

The Campanian age indicated by growth faulting in the Wahweap Formation instead agrees closely with estimates of initial Laramide deformation in central Utah and in the surrounding Rocky Mountains. Chapin and Cather (1981) examined basin types and facies distributions in Rocky Mountain basins around the margins of the Colorado Plateau and broadly distinguished two pulses of Laramide tectonism, a slow beginning from 80 to 55 Ma and a rapid pulse 55–40 Ma. Cather (2004) identified an initial stage of deformation between 80 and 75 Ma in Laramide basins of northern New Mexico and southern Colorado. Dickinson et al. (1986) linked changes in petrofacies of sandstones deposited in central Utah during the latest Campanian and earliest Paleocene to initial Laramide fragmentation of the Sevier foreland basin into smaller subbasins. However, recent work by Horton et al. (2004) determined that the Charleston-Nebo salient of the Sevier thrust belt in central Utah was still highly active during the Campanian, implying temporal overlap of Sevier and Laramide deformation in that area. These studies imply concurrent movement on Laramide structures around the margins of the Colorado Plateau and within the frontal thrusts of the Sevier thin-skinned thrust belt.

Close identification of the onset of Laramide deformation along the Kaibab Uplift and other major uplifts of the Colorado Plateau and Rocky Mountains can help to distinguish among proposed causes of foreland basement-involved deformation. For example, if Colorado Plateau and Rocky Mountain uplifts arose as a result of indentation or rotation of a rigid Colorado Plateau crustal block (e.g., Hamilton, 1981, 1988; Karlstrom and Daniel, 1993; Cather, 1999; Wawrzyniec et al., 2002; Cather et al., 2006), the timing of deformation around the margins of the Colorado Plateau should coincide. However, if foreland basement-cored uplifts developed above a gradually flattening lithospheric slab (Coney, 1976; Dumitru, 1991) or from eastward injection of ductile material from the overthickened Sevier wedge at the midcrustal level (McQuarrie and Chase, 2000), then deformation should progress temporally from west to east across the Colorado Plateau. Yet a third possibility is that a changing stress field favored movement across different uplifts at different times, depending on favorable orientations of preexisting crustal weaknesses (Chapin and Cather, 1981; Gries, 1983; Bird, 1998; Bump, 2004). In this model, the initiation of movement should correlate with uplift orientation. Evaluation of these hypotheses requires a detailed knowledge of the timing of movement along major foreland basement-cored uplifts. A closer examination of minor faults and preserved syntectonic sedimentary rocks in Cretaceous and Tertiary strata along the margins of uplifts might yield the necessary level of detail to make such distinctions.

Crustal Kinematics

The timing, rate, and sequence of deformation relate not only to underlying tectonic cause but also to the kinematic history of individual basement-cored uplifts. By defining the changing rate of growth through time of specific uplifts, and comparing them across the Colorado Plateau, we can determine whether uplifts developed by a uniform or a widely variable set of processes. Basement-cored uplifts have been described or modeled as fault-propagation folds (Erslev, 1991; Erslev and Rogers, 1993; Stone, 1993; Mitra and Mount, 1998) or as drape folds (Stearns, 1971; Reches and Johnson, 1978; Reches, 1978); with or without folding of the basement-cover interface (Erslev and Rogers, 1993; Schmidt et al., 1993; Stone, 1993; Narr and Suppe, 1994; Mitra and Mount, 1998; Bump, 2003); overlying steep, reactivated faults (Huntoon, 1993; Marshak et al., 2000); or newly formed Laramide thrusts (Hamilton, 1988; Huntoon, 1993; Yin, 1994); or originating as broad lithospheric buckles (Tikoff and Maxson, 2001). Detailed analyses of secondary structures in sedimentary cover and in the basement, where exposed, coupled with a detailed knowledge of timing and deformation rate, provide clues to understanding the ways in which each uplift formed, and therefore to explaining the observed variability in orientation and structural style among foreland uplifts.

Our study indicates that the Kaibab Uplift experienced an early phase of extension in the upper crust along the rising eastern limb, with faults soling into a shale or evaporite detachment horizon. The normal faulting could be a response to localized, fold-parallel extension; the field area lies near an inflection where the northward plunge of the monocline begins to steepen from 5° to nearly 15° (Sargent and Hansen, 1982). Unruh and Twiss (1998) observed similar fold-parallel lengthening in geodetic data and focal mechanisms following the 1994 Northridge earthquake, and in inversion of fault-slip data from several Laramide basement-cored anticlines. Varga (1993) interpreted fold-parallel lengthening along Laramide basement-cored uplifts to be coeval with basement-involved shortening, rather than indicative of an earlier episode of extension. The transverse orientation of the faults in our field area relative to the fold axis is consistent with fold-parallel extension as observed by Varga (1993) and Unruh and Twiss (1998).

Alternatively, the normal faults may be caused by the right-stepping bend in the trend of the East Kaibab monocline (Figs. 2 and 3). Other authors have related this bend, and similar bends southward along the monocline, to basement fault segmentation (Stern, 1992; Rosnovsky, 1998), and Tindall and Davis (1999) documented evidence for a right-lateral component of basement-rooted fault offset across the monocline. A right bend in a right-lateral strike-slip system typically produces a pull-apart basin. If a strike-slip component of offset was significant during the earliest phases of basement fault reactivation beneath the growing East Kaibab monocline, small pull-apart basins could have developed at the surface between right-stepping basement fault segments, providing accommodation space for the observed syntectonic sediments. Three similar right-stepping bends are spaced ∼15 km apart along the East Kaibab monocline to the south, but they do not display normal faults or growth strata. At each bend, however, the level of structural and stratigraphic exposure is deeper than in our field area, so pull-apart basins may have been present before uplift and erosion.

Effects on Sedimentation and Paleoenvironment

Although they are minor structures, the faults in the field area ruptured the ground surface and affected sedimentation and environment in the immediate vicinity. During deposition of the Wahweap Formation, meandering and braided fluvial systems flowing across the Sevier foreland were sensitive to minor changes in climatic and tectonic factors (Lawton et al., 2003; Simpson et al., 2008; Wizevich et al., 2008; Jinnah et al., 2009). Pervasive sedimentary structures preserved in the Wahweap Formation near the northern end of the East Kaibab monocline are consistent with active surface faulting during deposition. For example, Hilbert-Wolf et al. (2009) described extensive seismites in the capping sandstone along the northern East Kaibab monocline, and attributed fluidization features and large-amplitude soft-sediment folds to proximal faulting before lithification. The normal faults in our field area were active during the deposition of the upper member and through to the beginning of capping sandstone deposition, and they are the most likely source of some of the local seismicity (Wolf et al., 2006, 2008; Hilbert-Wolf et al., 2009). Orsulak et al. (2006) described an increase in locally derived sandstone and mudstone clasts in conglomerates of the capping sandstone from north to south across the traces of the two faults. Farther north along the East Kaibab monocline, a Cretaceous sag pond deposit has been reported in the Wahweap Formation adjacent to a northeast-striking, northwest-dipping normal fault at a left bend in the trend of the monocline (Wolf et al., 2007; Hilbert-Wolf et al., 2009; Simpson et al., 2009). These features are consistent with the interpretation that the faults in the field area were seismically active and produced surface rupture during deposition of the Wahweap Formation.

CONCLUSIONS

Two northeast-striking, northwest-dipping faults within the steep limb of the East Kaibab monocline preserve thickened wedges of Cretaceous Wahweap Formation immediately southeast of each fault trace. True thickness of the Wahweap upper member is 92 m immediately north of the fault pair, but it thickens to 262 m immediately south of the southern fault trace before gradually thinning again southward. These thickness changes represent wedges of growth strata deposited during faulting. Faults and syntectonic sediment wedges subsequently have been rotated 20°–60° eastward in the steep limb of the East Kaibab monocline. Restoration of faults and slickenlines to their Cretaceous, pre-monoclinal folding orientations reveals that the faults formed as northeast-striking, southeast-dipping, listric normal faults detached within the Jurassic Carmel Formation. The association of faulting with the steep limb of the Kaibab Uplift, and the compatibility between the faults and Laramide strain directions, imply that faults formed during initial Laramide deformation. Radiometric ages of units bracketing the Wahweap upper member growth strata delimit the onset of Laramide deformation between 80 and 76 Ma (Roberts et al., 2005; Jinnah et al., 2009).