Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology

The Colorado Plateau of the western United States in North America was situated in southwest Laurentia during the Mesozoic rifting of Pangea and the opening of the Gulf of Mexico (Kocurek and Dott, 1983; Blakey, 1994, 2008; Blakey and Ranney, 2008; Miall and Blakey, 2008). The basin was arid and centered around 30°N. The opening of the Gulf of Mexico provided abundant sediments that were dispersed via Triassic paleodrainages to the NW from the Grenville and Ouachita orogens in the central Texas uplift (Lawton and McMillan, 1999; Dickinson and Lawton, 2001; Mickus et al., 2009). In the Early to Middle Jurassic, eolian deposition dominated this basin; the depositional environment then shifted to hypersaline lacustrine conditions (Peterson, 1994; O’Sullivan, 2004). Today, these sediments constitute much of the exposures of central western North America on the Colorado Plateau. The Jurassic shift to a hypersaline depositional environment is evidenced by the so-called Wanakah Formation and the Tidwell Member of the Morrison Formation—two units that are widely exposed on the Colorado Plateau (Figs. 1 and 2).

The Middle Jurassic Wanakah Formation is exposed throughout western Colorado and extends south into New Mexico; stratigraphically, the unit is above the eolian Entrada Sandstone and below the hypersaline lacustrine Tidwell Member in western Colorado (Fig. 1; Peterson, 1994; O’Sullivan, 2004). A regional unconformity (known as the J-5) is typically placed between the Wanakah Formation and the overlying Tidwell Member and is delineated by a regional sandstone bed called “Marker Bed A” (Fig. 1; Peterson, 1988a, 1994; Turner and Peterson, 1992, 2004; O’Sullivan, 2004).

Controversies exist over Wanakah Formation depositional environmental interpretations (marine or lacustrine) and whether the unit is even present in western Colorado (Tanner, 1970; Adler, 1974; Ridgley and Goldhaber, 1983; Peterson, 1988a, 1994; Kirkland et al., 1995; Turner and Peterson, 1992; Anderson and Lucas, 1992, 1994; O’Sullivan, 2004; O’Sullivan et al., 2006; Lucas et al., 2006). The Wanakah Formation is lithologically variable and contains several dissimilar members. For example, in Ouray, Colorado, the type section includes an 18-m-thick basal limestone member (The Pony Express Limestone) that is not present north of the western Black Canyon of the Gunnison (O’Sullivan, 1992, 2004; O’Sullivan et al., 2006). In the Black Canyon of the Gunnison, the Wanakah Formation contains 3–7-m-thick gypsum lenses, also not present at any other exposure (O’Sullivan, 1992, 2004; O’Sullivan et al., 2006).

The paleogeography is currently reconstructed largely from depositional environment and provenance studies on the Colorado Plateau (Kocurek and Dott, 1983; Peterson, 1988b; Blakey, 1994; Dickinson and Gehrels, 2003, 2010; DeCelles, 2004; Demko et al., 2004). Recently, advances in U/Pb age dating of detrital zircons have allowed for more refinement in provenance studies and insight into the maximum ages of the Mesozoic units (Dickinson and Gehrels, 2003, 2010). This study presents new insight into the paleogeographic evolution of southwest Laurentia during the Middle to Late Jurassic. Specifically, this new study uses detailed analyses of petrography and stratigraphy coupled with the first detrital zircon geochronology data for the Middle Jurassic rocks on the Colorado Plateau to (1) provide new insights on the landscape and drainage evolution of the Ancestral Front Range and southwestern Laurentia during the opening of the Gulf of Mexico, and (2) refine the Middle Jurassic stratigraphy, specifically, the depositional environments and unconformities present on the eastern Colorado Plateau.

During the Mesozoic, the subduction of the Farallon plate along the western edge of North America created a nonmarine back-bulge basin between the forebulge in central Nevada to the east and the remnant Ancestral Rocky Mountains to the west that subsequently became the Colorado Plateau (Lawton, 1994). Thick packages of fluvial, eolian and fluvio-lacustrine sediments were deposited into this basin throughout the Mesozoic, eroding from the Cordilleran magmatic arc to the west, the Appalachian orogen to the east, and the Grenville and Ouachita orogens to the south (Dickinson and Gehrels, 2003, 2008, 2010).

A sea extended from the north onto the northern edge of the Colorado Plateau (Kocurek and Dott, 1983; Turner and Fishman, 1991; Blakey, 2008; Blakey and Ranney, 2008; Miall and Blakey, 2008). During the Middle to Late Jurassic, the depositional environments on the northwest part of the Colorado Plateau were tidal environments (e.g., Summerville and Curtis Formations; Gilluly, 1929; Peterson, 1994; Bernier and Chan, 2006). In the southeast part of the basin (in western Colorado and New Mexico), the Wanakah Formation records a hypersaline lake system (O’Sullivan and Pipiringos, 1983; Peterson, 1994; O’Sullivan, 2004). The Summerville Formation in southern Utah is interpreted as coeval to the Wanakah Formation (Gilluly, 1929; Peterson, 1994; Bernier and Chan, 2006). In the study area, the Late Jurassic Morrison Formation consists of three members: Tidwell Member, a hypersaline lacustrine system, Salt Wash Member, a fluvial unit, and the Brushy Basin Member, a fluvio-lacustrine environment. The paleodrainages of the Morrison Formation flow to the northwest (Turner and Fishman, 1991; Turner and Peterson, 1992; Peterson, 1994; Demko et al., 2004; Blakey, 2008; Blakey and Ranney, 2008; Miall and Blakey, 2008). Although the relative ages of these formations are clear, and the Morrison Formation has yielded some absolute age dates from volcanic ash abundant in the formation (interpreted to be deposited from 153 to 145 Ma; Kowallis et al., 1998), absolute ages of the Entrada Sandstone (a Middle Jurassic eolianite) and the overlying Wanakah Formation are not known.

Several major unconformities mark the Jurassic section on the Colorado Plateau, and this paper focuses on the J-5, which is present as an angular unconformity between the Summerville Formation and the Tidwell Member in southern Utah at Shadscale Mesa (Fig. 2; Gilluly, 1929; Bernier and Chan, 2006). The J-5 unconformity is typically extended into western Colorado and placed in between the Wanakah Formation and the Tidwell Member (O’Sullivan, 2004; O’Sullivan et al., 2006) because the Summerville Formation and the Wanakah Formation are interpreted as coeval (Peterson, 1994). However, this interpreted extension of the J-5 unconformity into Colorado is controversial (O’Sullivan and Pipiringos, 1983; O’Sullivan, 2004). Herein, we investigate the presence of the J-5 unconformity and provenance of the Entrada Sandstone, Wanakah Formation, and the Tidwell Member of the Morrison Formation.

The purpose of this study is to refine the regional paleogeography and landscape evolution of southwest Laurentia during Middle Jurassic rifting of Pangea by evaluating the provenance and depositional environment of the Middle Jurassic section in western Colorado. We utilized detrital zircon geochronology to determine the stratigraphic position of unconformities above and below the Wanakah Formation and to establish provenance. The specific hypotheses tested herein are: (1) A distinct and detectable change in provenance occurred that resulted in a change of depositional environment from eolian (Entrada Sandstone) to hypersaline lacustrine (Wanakah Formation and Tidwell Member of the Morrison Formation). (2) The Entrada Sandstone, the Wanakah Formation, and the overlying Tidwell Member are conformable rather than separated by the regional J-5 unconformity.

Field sites were chosen for their location along a west-east transect as well as for exposure quality (Fig. 2). Samples were collected for detrital zircon analysis from sandstones at Escalante Canyon (n_samples = 7; Figs. 1 and 2). Escalante Canyon is located south of Grand Junction, Colorado, at 38°42.634′N, 108°16.683′W. The section is accessed by heading south on Route 50 from Grand Junction and heading west on Road 650 past the Escalante Ranch. The section from the bottom exposure of the Entrada Sandstone to “Marker Bed A” of the Tidwell Member is 15 m (for a photo of the section and the units present, see Fig. 1).

Two Entrada Sandstone samples were collected: one from below a prominent, regional scour surface (n = 1) and one from above the scour surface (n = 1; note that this sample is reinterpreted herein as the basal Wanakah Formation; see results and discussion for details). Samples from sandstones within the Wanakah Formation (n = 2) were collected. Two samples from the Tidwell Member (one from the basal sandstone, Marker Bed A, and another from the second major sandstone) and one sample from the overlying Salt Wash Member (n = 1) were also collected. Representative shale and sandstone samples were acquired for quantitative elemental mineralogy using scanning electron microscopy (QEMSCAN) analyses.

Using methods defined by Gehrels et al. (2008) and Gehrels (2011), 105 grains were analyzed from each sample (n = 7) at Arizona LaserChron Center at the University of Arizona (see raw data in GSA Data Repository¹; and see Arizona LaserChron Center Web site for analytical protocol). However, from each sample, ∼5–10 measurements were discarded for discordance; results show the actual number of grains used for each sample. Data analyses (Kolgomorov-Smirnov [K-S] tests, age probability distributions, AgePick) were performed using the tools available from Arizona LaserChron Center.

QEMSCAN produces mineralogical “maps” of samples in two dimensions using the energy-dispersive X-ray spectroscopy (EDS) technology associated with a scanning electron microscope (SEM). A species identification protocol (SIP) in the software is used to convert elemental abundances into mineralogical interpretations. This instrument and the software were used at the Energy and Geoscience Institute at the University of Utah. This tool essentially replaces traditional point counting. For this method, six samples (from the same sandstones used for detrital zircon analysis, excluding the Salt Wash Member sample) were evaluated using 20 µm pixel spacing over an area of ∼15 × 20 mm.

In order to determine stratigraphic relations and depositional environment, we constructed a regional characterization of the Wanakah Formation. One of the defining characteristics of the formation is a lower red diagenetic facies and an upper green diagenetic facies (Figs. 3A and 3B). A black volcanic ash separates the red from the green diagenetic facies and is observed at Escalante Canyon, Ribbon Trail, Artists’ Point, and Ten Mile Graben (Figs. 3C and 3D). The unit at Ten Mile Graben is interpreted in the literature as the Tidwell Member; however, the red and green diagenetic facies separated by a black ash are present in this section too. In the lower part of the section at Escalante Canyon, a tabular sandstone with three distinct subunits is topped with microbial mats (Figs. 3E and 3F). Gypsum beds are present at Chukar Trail and Duncan Trail.

A sharp transition from sandstone to shale typically marks the boundary between the Entrada Sandstone and the Wanakah Formation; however a scour surface is present within the uppermost part of the Entrada Sandstone in Escalante Canyon, and at Ribbon Trail (Fig. 1). In western Colorado, the top contact between the Wanakah Formation and the Tidwell Member of the Morrison Formation is typically placed at the base of the first major sandstone, Marker Bed A (Fig. 1). This is a cross-bedded sandstone with distinctive coarse sand chert lags, and the J-5 unconformity is placed at this boundary (Fig. 1; Peterson, 1994; O’Sullivan, 2004).

Major lithofacies in the Wanakah Formation and the Tidwell Member of the Morrison Formation are described next. The major lithologies are siliciclastic mudstones, siltstones, and sandstones, but micrite is present in the Wanakah Formation and common in the Tidwell Member. A gypsum lithofacies is common in the Wanakah Formation, but only in the Black Canyon of the Gunnison. The description of the uppermost eolian Entrada Sandstone is limited to the uppermost 3 m, which consist of planar stratification. A stratigraphic section of Escalante Canyon illustrates typical lithofacies relationships (supplemental Fig. 1 [see ^{footnote 1}]).

Mudstone exhibits three distinct lithofacies: laminated, massive, and mottled. The laminated mudstone lithofacies exhibits horizontal planar lamination with some bioturbation. This lithofacies is common throughout all sections. The massive mudstone lithofacies is bioturbated, and most bedding is destroyed; however, minor horizontal planar lamination is present. This lithofacies is commonly interbedded with the siltstone lithofacies and is common in all sections. The mottled mudstone is bioturbated with some blocky textures (peds), root traces, and mottled color (red and green). Calcite concretions (<10 cm in diameter) are common in this lithofacies. A siltstone lithofacies is also common that is horizontally laminated to massive and exhibits bioturbation. This lithofacies is commonly interbedded with the massive mudstone and can exhibit a fining-upward into the massive mudstone lithofacies.

Three categories of sandstone lithofacies are observed: cross-bedded, wavy, and planar stratified. The cross-bedded sandstone lithofacies is fine- to medium-grained quartz arenite (determined via hand lens in the field and quantitatively with QEMSCAN, sample 2Jmt). Feldspar and matrix content is <10%. Coarse-grained chert lags are common.

The wavy sandstone lithofacies is a fine-grained feldspathic arenite with <10% matrix or limey sandstone with <10% feldspar but ∼35% calcite, as quantified by QEMSCAN analysis (samples 2Jw, 3Jw, 3Jmt). Stratification is horizontally laminated to wavy laminated to massive bedding. Bioturbation and salt casts are present. This lithofacies is present in <40-cm-thick beds and is typically topped by the micrite lithofacies. The planar stratified sandstone lithofacies is a fine- to medium-grained sandstone. It is present in the Entrada Sandstone and the basal Wanakah Formation. This lithofacies ranges from a feldspathic arenite to a feldspathic wacke based on QEMSCAN analysis (samples 1Jeeb, Jeef)

A micrite lithofacies and a gypsum lithofacies are also present. The micrite lithofacies exhibits wavy lamination with alternating light and dark lamina. Beds are <20 cm tall and can be mounded in <10-cm-tall ridges. This lithofacies is commonly directly above the wavy sandstone lithofacies. Gypsum beds are only exposed in the Black Canyon of the Gunnison. Beds are between 1 and 3 m thick and are fine-grained and horizontally planar laminated or globular and horizontally planar bedded. The beds are commonly interbedded with horizontally planar laminated mudstone. One bed in the Chukar Trail location exhibits large-scale cross-stratification interpreted as eolian reworking of gypsum sand into dune deposits.

Normalized age probability spectra for each sample (using macros available from the LaserChron Laboratory) are graphically represented with major age accumulations annotated in Figure 4. A histogram of percentages of age populations is presented in Figure 5. Raw data and cumulative distribution plots are available as supplemental data (see ^{footnote 1}).

The interesting feature of the petrographic analysis is the amount of K-feldspar in the samples from the lower section. Data are included in supplemental data (see ^{footnote 1}). The Entrada Sandstone is primarily a quartz arenite (from field observations); however, there is a distinct lithological change observable with a hand lens in the upper facies and above the scour surface (interpreted herein as the basal sandstone of the Wanakah Formation; see discussion for details). QEMSCAN analysis supports this field observation. Entrada Sandstone sample 1Jeeb is a subarkosic arenite with 14 area % K-feldspar. The unit directly above the scour surface (1Jeef; herein reinterpreted as basal Wanakah Formation; see following for discussion) is an arkosic arenite with 32% K-feldspar and 1% biotite. The Middle Wanakah Formation sample (2Jw) contains 13% K-feldspar and is a subarkosic arenite. The upper sample of the Wanakah Formation (3Jw) and the upper Tidwell Member sample (3Jmt) are quartz wackes that contain ∼30% calcite cement and less than 10% K-feldspar. The Lower Tidwell Member sample (2Jmt) is a quartz arenite.

The Wanakah Formation exhibits the following diagnostic sedimentary features: (1) laminated and massive (bioturbated) mudstones, (2) paleosols, (3) tabular sandstones with algal mats, (4) a lack of marine fossils, and (5) gypsum and salt casts. Laminated and bioturbated mudstones suggest quiet-water deposition. Paleosol features such as root traces, blocky peds, disrupted bedding, and carbonate concretions suggest subaerial exposure of the sediments and periods of nondeposition where soils could develop in a terrestrial setting. Tabular sandstones topped with microbialites (wavy laminated micrites) are interpreted as shoreline sandstones with transgressive water levels allowing for algal precipitation of carbonate.

Most depositional environment interpretations of the Wanakah Formation suggest a lacustrine origin, particularly those done on the Todilto Limestone to the south of the study area (Peterson, 1988a, 1994; Turner and Peterson, 1992; Anderson and Lucas, 1992, 1994; Kirkland et al., 1995; Benan et al., 2000; O’Sullivan, 2004; O’Sullivan et al., 2006; Lucas et al., 2006). In the study area, the lack of marine fossils lends support for a lacustrine interpretation, and the overall restricted biodiversity (limited to trace fossils and microbialites) suggests hypersaline water chemistry. Additional support for a hypersaline lacustrine interpretation is the presence of thick (<3 m) gypsum lenses in the Black Canyon of the Gunnison. A restricted marine environment interpretation is possible, but the development of paleosols and lack of marine fossils favor a terrestrial depositional setting. An arid climate setting is interpreted, due to the presence of gypsum and paleosols and reworking of gypsum into sand dune deposits (suggesting episodic desiccation).

Detrital zircon U/Pb age populations from the six samples fit within major age categories defined by Dickinson and Gehrels (2003, 2008, 2010), and the spectra for the Entrada Sandstone and Morrison Formation samples are fairly typical for Colorado Plateau Mesozoic sandstones (Fig. 5; Dickinson and Gehrels, 2003, 2008, 2010). The Entrada Sandstone and the Morrison Formation populations exhibit a majority of grains from Cordilleran back-arc activity (165–145 Ma), Paleozoic Appalachian orogeny (504–285 Ma), Neoproterozoic Appalachian orogeny (defined by Dickinson and Gehrels as 723–512 Ma, but separated for this study), the Grenville orogeny (1.3–0.9 Ga), Proterozoic anorogenic craton (1.53–1.3 Ga), and the Proterozoic Yavapai-Mazatzal orogeny (1.8–1.55 Ga; Fig. 5). The increase in 241–181 Ma grains in the Morrison Formation samples compared to older samples could reflect an increase in tephra air-fall deposition, which is abundant as tuffs in the Morrison Formation related to Cordilleran volcanism (Turner and Fishman, 1991; Kowallis et al., 1998).

The most interesting aspect of the detrital zircon data is the quantity of grains between 512 and 539 Ma in the Wanakah Formation samples along with the concomitant decrease in Grenville orogeny grains (1300–900 Ma). The Grenville orogeny is typically the most prevalent source in most Colorado Plateau Mesozoic deposits (Dickinson and Gehrels, 2003, 2008, 2010). A gradual increase in the 512–529 Ma age population begins directly above the scour surface in the Entrada Sandstone (Figs. 4 and 5), with 20% of the grains clustering at 519 Ma. This age population dominates the Wanakah Formation sandstones, with >50% of grains clustering at 523–527 Ma (Figs. 4 and 5). The cluster at 523 Ma is less but still present in the basal Tidwell Member (Marker Bed A) sandstone and is absent in the overlying Tidwell Member sample (Figs. 4 and 5).

The dramatic increase in 512–539 Ma sediments that begins above the scour surface in the Entrada Sandstone and ends after Marker Bed A of the Tidwell Member suggests the denudation of some unit with an abundance of ca. 512–539 Ma grains. The source of these 512–539 Ma grains is interpreted herein as the Cambrian intrusives in central Colorado, specifically the McClure Mountain syenite (523.98 ± 0.12 Ma; Bickford et al., 1989; McMillan and McLemore, 2004; Schoene and Bowring, 2006). The abrupt and abundant input of Cambrian grains that tapers off through the deposition of the Tidwell Member suggests several possibilities: (1) a late-stage uplift on the southwest side of the Ancestral Front Range that exposed the McClure Mountain syenite, which then eroded into a lacustrine basin to the northwest; (2) stream capture that diverted a major drainage into the basin from highlands where the McClure Mountain syenite was exposed; or (3) a combination of these two. The following section outlines our interpretation by discussing the source of the sediment and the depositional sequences and environments, and it ends with a summation of provenance.

The Cambrian McClure Mountain syenite is part of a complex of mafic-ultramafic cumulates, hornblende-biotite syenites, nepheline syenites, and mafic nepheline-clinopyroxene rocks intruded by carbonatite, several kinds of syenite, and lamprophyre dikes exposed in the Wet Mountains area of central Colorado (Fig. 6; Parker and Hildebrand, 1963; Shawe and Parker, 1967; Armbrustmacher, 1984). The U-Pb age of the hornblende-biotite syenite is 523 ± 0.12 Ma (Schoene and Bowring, 2006). This unit would have potentially been exposed in the remnants of the Ancestral Front Range during the Middle Jurassic, and the abundance of K-feldspar in the syenite (Armbrustmacher, 1984; Schoene and Bowring, 2006) could well explain the surge in K-feldspar and the trace amounts of biotite present in the Wanakah Formation sediments that are coincident with the input of 512–539 Ma zircons.

Several other possible sources for the 512–539 Ma grains were considered as well. One is a reworking of the Triassic Santa Rosa Formation (within the Chinle-Dockum system), which has a similar detrital zircon signature to that of the Wanakah Formation at Escalante Canyon with a marked peak at 516 Ma (Dickinson and Gehrels, 2008, 2010). The Santa Rosa Formation is interpreted to be derived from the Amarillo-Wichita Province in Texas and Oklahoma and deposited by paleorivers flowing northwest into the Triassic Eagle paleovalley (Dickinson and Gehrels, 2008, 2010). Additionally, reworking of the Santa Rosa Formation can also account for the grains in the unknown population range from 750 to 880 Ma present in the Wanakah Formation (Dickinson and Gehrels, 2008, 2010). However, the Santa Rosa Formation is the basal Chinle-Dockum unit, and subsequently it was buried beneath hundreds of meters of younger Triassic Chinle-Dockum strata capped by a continuous blanket of Middle Jurassic Entrada Sandstone that extended all the way to the Oklahoma panhandle before burial under Cretaceous strata of the interior seaway (William Dickinson, 2015, personal commun.).

Another possibility is a reworking of the Devonian Temple Butte Sandstone, exposed in northern Arizona and southeast Nevada (Dickinson and Gehrels, 2003). This unit exhibits a minor population of grains derived from the Amarillo-Wichita uplift (Dickinson and Gehrels, 2003). However, the unit is also buried beneath thousands of meters of strata and does not have the quantity of ca. 525 Ma grains to explain the amount present in the Wanakah Formation.

Therefore, the McClure Mountain syenite is the most likely source for these 512–539 Ma grains because of its proximity to the basin and because it was likely exposed during the Middle Jurassic. Denudation of the McClure Mountain syenite could also explain the influx of the 512–539 Ma zircons and the observed K-feldspar and biotite; neither reworking of the Santa Rosa Formation nor reworking of Temple Butte Sandstone would explain the lithological change that is observed.

The Wanakah Formation (like the Tidwell Member) was likely deposited in a large hypersaline lacustrine environment that was episodically desiccated (as evidenced by paleosols, lack of marine fossils, and thick gypsum deposits, some of which were reworked into cross-bedded sandstone). Sandstones are commonly topped by micritic algal mats and are interpreted as shorelines or shallowing water deposition that allowed for growth of mats (Fig. 3). Although this was a lake system, sedimentological evidence such as thick gypsum deposits (>7 m thick) as well as paleosols with calcite concretions suggest that the climate was arid.

The Entrada Sandstone represents an eolian depositional system centered around 30°N paleolatitude (Kocurek, 1981; Crabaugh and Kocurek, 1993). The contact between the Entrada Sandstone and the Wanakah Formation is likely at the scour surface between these units rather than at the shale-sandstone contact above, as evidenced by a marked difference in age probability distribution and lithological change (Figs. 4 and 5; supplemental QEMSCAN data [see ^{footnote 1}]). A minor unconformity likely exists at this newly defined contact; however, the scour surface probably represents the onlapping of a lake over a dune field rather than the J-5 unconformity. The age probability distribution in Marker Bed A shows a continuation of the McClure Mountain syenite denudation progressing throughout time and suggests that the Wanakah Formation and the Tidwell Member of the Morrison Formation are conformable in Escalante Canyon.

A period of stream capture and drainage reorganization during the Middle Jurassic could explain the observed change in detrital zircon U-Pb ages that accompanied the transition from eolian sedimentation of the Entrada Sandstone to hypersaline lacustrine sedimentation of the Wanakah Formation and the Tidwell Member of the Morrison Formation. The abrupt appearance of sediments sourced from the McClure syenite, along with the creation of lakes, is consistent with the capture of a river system and an accompanying increase in discharge and water-table rise in southwest Laurentia (Smith et al., 1989; Ashley et al., 2004; Carroll et al., 2008; Moore and Eckardt, 2012).

Stream capture and drainage reorganization can be triggered by a variety of mechanisms, including tectonism, base-level changes, headward erosion, and lake spillover (Brocklehurst and Whipple, 2002; Schoenbohm et al., 2004; Brook et al., 2006; Clark et al., 2006; Oskin and Burbank, 2007; House et al., 2008; Crossey et al., 2015). It is possible that a tectonic event simply uplifted the southwestern flank of the Ancestral Front Range, exposed the syenite, and caused this influx of McClure Mountain syenite grains; however, uplift alone cannot account for the accompanying shift in depositional environment that is recorded in the rocks (Fig. 6). Climate change could explain both the change to a wetter depositional environment and the stream capture event. However, the hypersaline lake system suggests the climate remained arid, so climate change dramatic enough to create a stream capture likely did not occur. Therefore, a tectonic event such as uplift of the southwest Ancestral Front Range that triggered a stream capture, diverted a major drainage into the basin, and denuded the McClure Mountain syenite is the most plausible interpretation (Fig. 6).

These new data presented here suggest the following conclusions:

• (1) The clastic sediments in these paleolakes (Wanakah Formation, Tidwell Member) were sourced from the McClure Mountain syenite.

• (2) The J-5 unconformity is not present in western Colorado; the Wanakah Formation and the Tidwell Member are conformable, as are the Entrada Sandstone and the Wanakah Formation.

• (3) The contact between the Entrada Sandstone and the Wanakah Formation is at the scour surface in Escalante Canyon (rather than at the sandstone-shale contact, as typically interpreted), so the basal lithofacies in the Wanakah Formation is a sandstone.

This new interpretation explains the changes in depositional environments of southwestern Laurentia during the Middle to Late Jurassic. Drainage reorganization and stream capture resulted in a shift from eolian to hypersaline lacustrine, which lasted from Wanakah Formation through Tidwell Member deposition (Fig. 6). This lake system and drainage basin likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. This study illustrates how U-Pb detrital zircon geochronology can be used to illuminate drainage patterns and landscape evolution, not just on Laurentia during the Middle Jurassic, but in any basin with similar changes in depositional environments.

Funding for this research was provided by a grant from the Unconventional Energy Center for Applied Research at Colorado Mesa University and the American Chemical Society Petroleum Research Fund (both to Potter-McIntyre). We gratefully acknowledge the participation of students at Colorado Mesa University, especially Thomas Spain, for help with field research. The manuscript was strengthened with thoughtful reviews by William R. Dickinson, Michael Wagreich, and Glenn R. Sharman.