Pennsylvanian-Jurassic Sedimentary Basins of the Colorado Plateau and Southern Rocky Mountains

Chapter 7

Pennsylvanian-Jurassic Sedimentary Basins of the Colorado Plateau and Southern Rocky Mountains Ronald C. Blakey Colorado Plateau Geosystems, Carlsbad, CA, United States

Chapter Outline Introduction Location and Geologic Setting Stratigraphic Interval Scope and Organization Precambrian Basement and Its Possible Control on Phanerozoic Deposition Trends and Lineaments Younger Precambrian Sedimentary Basins Phanerozoic Tectonics and Depositional History Early and Middle Paleozoic Pennsylvanian-Permian Triassic Jurassic Cretaceous Cenozoic Source of Voluminous Sand Pennsylvanian-Middle Jurassic Sequence Stratigraphy Introduction Pennsylvanian Permian Triassic Jurassic

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Tectonic Origins of Pennsylvanian-Permian Basins Introduction Yoked Basins Nonyoked Basins Cordilleran Basins Tectonic Setting of Triassic Basins Introduction Moenkopi Shelf Eastern Cordilleran Basin Pre-Shinarump Paleovalleys and Shinarump Deposits Chinle Basin Tectonic Setting of Jurassic Basins Introduction Zuni Sag Utah-Idaho Trough Discussion: Tectonic Evolution and Controls on Deposition Tectonic Sequence of Events Climatic Controls Eustatic Controls Summary Acknowledgments References

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INTRODUCTION Location and Geologic Setting For over 500 million years, the Colorado Plateau and Southern Rocky Mountains, along with adjacent portions of the Central Rocky Mountains, High Plains, and Basin and Range provinces were part of a southwest projection of cratonic North America. This extensive region formed a broad, stable region that lay near sea level. Numerous transgressions and regressions left a widespread and diverse sedimentary rock record from the Neoproterozoic through most of the Cretaceous. Cenozoic tectonics imparted contrasting structural styles across the region, forming the distinct geologic provinces at ­present. This chapter covers the states of Arizona, Utah, New Mexico and Colorado, eastern Nevada, southeastern California,

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and southern Wyoming (Fig. 1). This region is everywhere underlain by Precambrian crystalline basement that was welded onto North America by 1.4 Ga. The veneer of Paleozoic and Mesozoic sedimentary rocks mantles most of the region except where stripped by erosion across Laramide Cenozoic uplifts. Paleozoic and Lower Mesozoic sedimentary rocks were deposited during three distinct tectonic episodes that mark the transition of western North America from a passive margin to an active margin. Early and Middle Paleozoic strata formed on a stable passive margin that succeeded the rifting of North America from Gondwana during the Late Precambrian. Mississippian, Pennsylvanian, and Permian rocks were deposited during a time of transition and significant regional orogenic activity; several sedimentary basins were formed during this interval. Triassic and Jurassic rocks formed in a general back-arc setting behind the rapidly building and evolving Cordilleran Arc (see also Chapter 1). The sedimentary rocks east of the Cordilleran (Wasatch) hingeline (Fig. 1) have experienced minor to major postdepositional orogenic disruption but all lie at or very close to their original site of deposition and no

FIG.  1  Map of Colorado Plateau and southern Rocky Mountains and adjacent regions showing features mentioned in text. (Base DEM from www. geomapapp.org.)



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palinspastic restoration is necessary. Rocks west of the hingeline and south of the Arizona Transition Zone require extensive palinspastic restoration for both Mesozoic shortening and Cenozoic extension. The sedimentary rocks of the region are exposed in a variety of high desert and intermontane settings. On the Colorado Plateau, exposures vary but are dominated by continuous, well-exposed, undeformed outcrops on which three-dimensional studies can be performed. The Southern Rocky Mountains consist of poor to excellent outcrops that display some degree of tectonic deformation and locally discontinuous exposure. Both regions have been regionally uplifted over 2000 m during the Cenozoic. The Colorado River and its tributaries have carved the landscape into its famous scenic landforms during the last 5–10 million years.

Stratigraphic Interval The emphasis of this chapter is the stratigraphic record from the Pennsylvanian through the Middle Jurassic. Late Precambrian and Early and Middle Paleozoic rocks are briefly covered; Cretaceous and Cenozoic rocks of the region are covered in separate chapters of this volume. In general, the Phanerozoic section thickens irregularly westward to over 3000 m across the central and western Colorado Plateau. A wide variety of shallow marine, shoreline, and continental deposits are present, but the region is most famous for its Permian-Jurassic fluvial and eolian strata. Regional correlation is based on published literature and my own stratigraphic work and, in general, tends to conform to nomenclature in use by the US Geological Survey.

Scope and Organization The chapter is organized as follows: It begins with a brief overview of the geologic setting and depositional history. This is followed by a discussion of recent detrital zircon studies and their conclusions regarding the sources of the voluminous sand in Pennsylvanian through Jurassic rocks of the study area. The bulk of the chapter focuses on the bodies of sedimentary rocks and their bounding unconformities—the sequence stratigraphy of the Pennsylvanian through Jurassic sedimentary rock record. The geometry of these deposits and the sedimentary basins thus defined are described and their tectonic setting discussed. The summary is presented as a discussion of the controls on deposition—tectonic, eustatic, and climatic.

PRECAMBRIAN BASEMENT AND ITS POSSIBLE CONTROL ON PHANEROZOIC DEPOSITION Trends and Lineaments The tectonic grain of much of the Colorado Plateau and Southern Rocky Mountains was established during the Proterozoic assembly of southwestern North America (Fig. 2A). The strong SW-NE grain parallels zones of accretion of the Pinal, Mazatzal, Yavapai, and Mojave provinces (Karlstrom and Humphreys, 1998; Whitmeyer and Karlstrom, 2007, Duebendorfer et al., 2006. A SE-NW grain may be related to inherited Precambrian grain as well as wrench tectonics that have affected North America for long periods of time (Stevenson and Baars, 1986). Both trends and the resulting lineaments have had strong controls on Paleozoic and Mesozoic facies distribution and isopach (Blakey, 1988), the distribution of the Ancestral Rockies and associated basins (Stevenson and Baars, 1986; Kluth, 1986), and the Laramide Rocky Mountains (Hoy and Ridgway, 2002).

Younger Precambrian Sedimentary Basins During the Middle and Late Proterozoic (Fig. 2A), a series of sedimentary basins developed across western and southwestern North America. The Apache and Grand Canyon basins developed in the Middle Proterozoic possibly controlled by compressional orogenic events in west Texas and southern New Mexico; the younger Pahrump, Uinta trough, and Belt basins were apparently controlled by the Late Proterozoic rifting of western North America during the breakup of Rodinia (Karlstrom et al., 1999). The effects of these basins on Phanerozoic deposition varied greatly and only a brief review is presented here. The Apache basin was neutral to slightly positive during much of the Paleozoic and Early Mesozoic although the southern margins overlap with the Pennsylvanian-Permian Pedregosa basin. The Grand Canyon basins are mostly on or along the flank of the Kaibab upwarp, a prominent Laramide monoclinal uplift that was generally a positive to neutral area during the Paleozoic. The Uinta trough was an arch during much of the Paleozoic (Ross, 1973) and now forms the core of the Uinta Mountains. The Pahrump and Belt basins evolved into the Cordilleran passive margin during the Paleozoic (Poole et al., 1992).

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FIG. 2  Proterozoic and Early and Middle Paleozoic paleotectonic settings of Southwestern North America. (A) Proterozoic tectonic elements shown on a Neoproterozoic paleogeographic map. Abbreviations: B, basin; P, province; T, trough; ApB, Apache Basin; GCB, Grand Canyon Basin. (B) Late Cambrian; (C) Mississippian.

PHANEROZOIC TECTONICS AND DEPOSITIONAL HISTORY Early and Middle Paleozoic Cambrian through Mississippian sedimentation of the southwestern craton was controlled by several major tectonic elements. The Transcontinental arch formed the backbone of the region from the Late Precambrian into the Mississippian (Fig. 2B and C; see also Chapter 2). Early and Middle Paleozoic strata onlap the arch from west to east and Cambrian and Devonian siliciclastic sediments were derived from the positive structure. Cambrian, Devonian, and Mississippian carbonates were deposited in areas not directly affected by clastic sedimentation or during periods of time when clastic input was reduced. The rapidly subsiding Cordilleran miogeocline developed on the passive, western margin of North America during the Late Precambrian and continued throughout much of the Early and Middle Paleozoic (see Chapter 11). The eastern margin of the miogeocline was the Cordilleran hingeline or Wasatch line—the two terms are used interchangeably. Across this curvilinear feature (Fig. 2A and B) nearshore and shallow marine shelf deposits graded westward into thicker shallow



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marine and offshore marine and continental rise-slope deposits (Poole et al., 1992). The resulting sedimentary package, bounded by the previously described tectonic features, is thin to locally absent east of the hingeline and thickens westward to over 5000 m in western Utah and eastern Nevada. Ordovician and Silurian rocks are absent on the Colorado Plateau and are rare across the Southern Rocky Mountain region. Thin deposits likely blanketed parts of the region but were removed by pre-Devonian erosion. Both systems are represented by extensive carbonate and minor clastic deposition west of the Cordilleran hingeline (Poole et al., 1977). During the Late Devonian, a new tectonic setting developed along the western margin of North America; the Antler arc approached the continent and in latest Devonian and Early Mississippian collided with the continent (Miller et al., 1992b; see also Chapter 11). The Antler foreland basin formed to the east of the collision zone (Poole et al., 1977). The collision between the Antler arc complex and western North America resulted in thrusting of primarily continental slope-rise deposits over the continental shelf strata (Burchfiel et al., 1992). The thickened crust caused adjacent foreland basin development. Upper Devonian and Lower Mississippian siliciclastics filled the rapidly subsiding basin; only muds reached the distal portions of the basin along the western margin of the study area. To the east across most of the Western Interior, vast carbonate deposits formed in clear, tropical seas. Carbonates and clastics intertongue along the Wasatch line region. In response to the Antler foreland basin, a broad forebulge extended along parts of the old Transcontinental arch and Wasatch line as evidenced by onlapping and east-thinning carbonate shoreline deposits (Giles, 1996).

Pennsylvanian-Permian Pennsylvanian and Permian deposits formed in tectonic settings that contrasted with those of the older Paleozoic. The old Transcontinental arch and passive margin elements were broken by dramatically different tectonic elements. To the west, the Antler orogenic belt rose along what had been the previous continental margin (Miller et al., 1992b). To the east, the elements of the Ancestral Rockies, block-like uplifts and adjacent basins, extended from eastern Arizona and southern New Mexico northeastward across the cratonic interior to Nebraska (Kluth, 1986). Between these two orogenic zones lies a broad region of cratonic shelf broken by several major basins; these basins were the sites of some of the thickest Phanerozoic sedimentation in the Western Interior. Pennsylvanian and Permian deposits reflect these tectonic elements as well as the cyclic nature of Late Paleozoic global sea level changes (Soreghan, 1994). Siliciclastic sedimentation was controlled by topography. Conglomeratic units flank orogenic highlands and sandstone was spread over adjacent plains by both river systems and eolian dunes. During sea-level lows, much of the region was blanketed by eolian dune deposits (Blakey et  al., 1988). Most continental deposits and some shallow marine and shoreline deposits consist of ubiquitous redbeds. Carbonates were deposited on marine shelves, especially during sea-level highs; at times, they extended close to orogenic highlands. Basinal deposits include carbonate mudstone, terrigenous mudstone, and locally extensive evaporites. Marine deposits comprise mostly drab tan and gray sedimentary rocks. Shifting shorelines and dynamic tectonic patterns produced complex intercalations of these deposits, especially evident where red and nonred strata intertongue. During the Late Permian, sea level dropped and deposition ceased across most of the region, even in basinal areas. The resulting Permo-Triassic unconformity extends across the region and, although obvious in most places, it can be difficult to locate where Triassic redbeds overlie Permian redbeds.

Triassic Triassic rocks comprise mostly red continental deposits east of the Wasatch line and gray and tan marine carbonate rocks to the west; strata thicken abruptly west of it. The thickening was related to a developing back-arc basin following the Sonoman orogeny (Lawton, 1994; Chapter  11). The Sonoman orogeny involved arc collapse and collision with North America and marked a shift in tectonic patterns (Saleeby and Busby-Spera, 1992; Chapter 11). Following the orogeny, the Cordilleran arc developed along much of western North America and a series of back-arc basins formed across the western margin of the craton. A broad platform, mostly mantled with Triassic fluvial deposits, extended eastward onto the craton. Triassic rivers flowed dominantly toward the northwest (Blakey, 1989; Blakey et  al., 1993). Elements of the Ancestral Rockies uplands persisted in Colorado and shed some detritus into Triassic rivers, but much of the clastic detritus came from the Appalachian-Ouachita Mountains that marked the Pangaean suture (Blakey, 1994; Riggs et al., 1996; Dickinson and Gehrels, 2003). Lower Triassic rocks thicken westward across the platform and then dramatically thicken across the Wasatch line. Upper Triassic rocks were eroded to the west during uplift of the back-arc basin, so their original geometry is difficult to determine. Few Upper Triassic continental rocks are preserved west of the hingeline except in marine basins in western Nevada (Silberling and Roberts, 1962; Saleeby and Busby-Spera, 1992).

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Jurassic The dynamics of Jurassic tectonics of western North America are extremely complex and the subject of much controversy, especially in areas west of the Colorado Plateau. Several phases of the Nevadan orogeny resulted from arc-microcontinent collision with western North America and raised uplands to the west of the Colorado Plateau (Lawton, 1994; Chapter 11). A back-arc or foreland basin, depending on how one interprets the tectonic origin and geometry of the uplands, developed along the Wasatch line (Bjerrum and Dorsey, 1995). Generally referred to as the Utah-Idaho Trough, the basin began subsiding during the Early Jurassic and accumulated over 2000 m of sediment during the Middle Jurassic; subsidence ended during the Late Jurassic (Bjerrum and Dorsey, 1995). Marine deposition dominated the basin center. To the east lay a broad bench that was the site of mostly continental Jurassic sedimentation, in places dominated by eolian deposition (Blakey et al., 1988). On the western margin of the Colorado Plateau, complex intertonguing of marine and continental deposits marks the transition between the two tectonic settings. The greatest change in thickness once again occurs across the Wasatch line. The Cordilleran arc was well established during the Jurassic; south of southern Nevada, the arc was built on continental North America (Andean-style arc) while to the northwest, the arc was built on a complex of accreted terrains, most of which were separated from North America by oceanic crust (Saleeby and Busby-Spera, 1992). By the end of the Middle Jurassic, most of the terrains had fused to North America during the Nevadan orogeny and the arc was Andean in style (Ingersoll and Schweickert, 1986). A major drainage change occurred across the Colorado Plateau in the Middle Jurassic. Triassic and Early Jurassic streams flowed northwest from Pangaean topography to the east; Middle Jurassic streams tapped the arc source to the southwest and flowed to the northeast across the Plateau region (Riggs and Blakey, 1993; Blakey, 1994; Blakey and Parnell, 1995). Northerly winds deflated arid fluvial plains and carried eolian sediment into the arc where eolian sandstone is interbedded with arc volcanics (Busby-Spera, 1988). The last phases of the Nevadan orogeny formed a foreland basin and forebulge-backbulge complex along the western margin of the Colorado Plateau (DeCelles and Currie, 1996). Uplands to the west spawned a major fluvial system that flowed eastward across the Colorado Plateau and Southern Rocky Mountains region and deposited the Morrison Formation (Peterson, 1988b). The Morrison and younger rocks are covered by Miall and Catuneanu (Chapter 9).

Cretaceous Cretaceous deposition across the region occurred in an extensive foreland basin that lay east of the Wasatch line; rocks range from 1 to 5 km thick. West of the Wasatch line, thrusting of the Sevier orogeny developed extensive highlands that shed detritus eastward into the foreland basin. Cretaceous sedimentary rocks are dominantly siliciclastic and range from conglomerate and conglomeratic sandstone along the Sevier front in western Utah to sandstone and mudstone across the central and eastern parts of the region. Stratigraphic sequences clearly document six to seven major transgressive-­ regressive cycles and numerous smaller ones; Maastrictian sedimentation records widespread regressive nonmarine ­deposition (Dyman et al., 1994).

Cenozoic Near the close of Cretaceous deposition, the region was still near sea level; this changed during the Paleogene as Laramide (Rocky Mountain) tectonics resulted in widespread regional uplift. Uplift was punctuated by broad to tight folding, especially monoclinal folding, and sharp, fault-bounded uplift (Miller et al., 1992a). Pennsylvanian through Jurassic rocks that are the focus of this study were stripped from the centers of uplifts but remained mostly buried between and on the flanks of uplifts. In basins of Utah and Colorado, locally thick Paleogene deposits further buried older sedimentary rocks. During the Neogene, integration of the Colorado River system resulted in downcutting and exposure of older sedimentary rocks. The intricate, incised drainage system is responsible for the remarkable exposures of Pennsylvanian through Jurassic rocks for which the region is famous.

Source of Voluminous Sand Pennsylvanian through Jurassic rocks on the Colorado Plateau and adjacent areas contain the greatest concentration of eolian sand in the Geologic record (Kocurek, 2003). Throughout much of that time interval, eastern and northern North America were elevated relative to the Western Interior and especially to the area of study. Logic would dictate that those elevated regions would supply clastic sediment to the study area (see discussion in Riggs and Blakey, 1993) and the welldocumented northerly winds (present coordinates; Parrish and Peterson, 1988) would have been a major factor in transporting sand southward into the Colorado Plateau. The uplifted Ancestral Rocky Mountains exposed Precambrian rocks,



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another potential source of sediment (Mack and Rasmussen, 1984). The evolving Cordilleran arc to the southwest also formed a potential source of sand (Riggs and Blakey, 1993). However, it was not until numerous detrital zircon studies were published in the early 2000s that quantitative and qualitative data carefully documented specific source areas and confirmed the earlier observations (Dickinson and Gehrels, 2003; Gehrels et al., 2011 and references therein; see also May et al., 2013, for analysis farther upwind). Fig. 3 summarizes a large amount of data for the Colorado Plateau and vicinity and provides the basis for the following discussion. Note that all units Middle Permian (Toroweap-White Rim) and younger have moderate peaks at or near their estimated depositional ages. This suggests that provenance areas included coeval magmatic rocks. For Permian rocks, these sources could be either the earliest Cordilleran arc (Riggs et al., 2013) or Appalachian plutons. For Triassic and younger units, grains close to depositional age had to be derived from the Cordilleran arc. Well-documented examples include the Triassic Chinle Formation (Howell and Blakey, 2013), and the Middle Jurassic Page-Carmel (Blakey and Parnell, 1995). All units have substantial peaks that reflect older source terranes. In Fig. 3, these are grouped as Appalachian-Caledonian orogen, Grenville orogen (mostly exposed in Appalachian Mountains), and Mesoproterozoic, Paleoproterozoic, and

FIG. 3  Summary map of detrital zircon data from rocks on Colorado Plateau and vicinity. Vertical axis (not to scale) shows units in study area with detrital zircon data. Colored vertical bands show ages of potential source areas. TA, exposure of Transcontinental Arch (Late Precambrian to Jurassic); ARM, exposure of Ancestral Rocky Mountains; CA, exposure of Cordilleran arc; + symbols, approximate depositional age of given unit; K, Cretaceous arc; J-Tr, Jurassic-Triassic arc; MLP, Late and Middle Permian arc; EP, Early Permian arc. (Modified from Gehrels et al. (2013).)

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Archean rocks that comprise substantial portions of the North American craton. Closest exposures of Precambrian rocks to the study area would have been in the Ancestral Rocky Mountains (exposed from Early Pennsylvanian into the Jurassic) and the Transcontinental arch with portions exposed throughout the Late Precambrian into the Pennsylvanian. Exposures on the Canadian shield likely were continuously exposed throughout the Phanerozoic. The strong peaks in the Permian and Pennsylvanian units likely reflect the uplifted Ancestral Rocky Mountains as well as epeirogenic uplift of the Canadian Shield near the Mississippian-Pennsylvanian boundary (Sloss, 1988, 1996); the Mississippian unit reflects broad exposures on the Transcontinental arch, especially near the end of the Period (Blakey, 2009). The strong Grenville and Appalachian-Caledonian peaks demand an eastern source for much of the sand in Pennsylvanian through Jurassic rock units of the southern Western Interior. Critics of this scenario ask “where are the river deposits that would have transported all of this sand and how did they cross the late Paleozoic seas of the Midcontinent (e.g., Thomas, 2011)?” Perhaps the most important factor in answering this question is the amount of sand that can be transported by eolian processes. Appalachian-sourced rivers, especially those flowing across the northern Midcontinent, would have entered vast arid regions, especially during the Permian, Triassic, and Jurassic (Parrish and Peterson, 1988). As rivers flowed across the broad arid region, their water was lost through evaporation and infiltration. Northeast trade winds (northerly with respect to present coordinates) blew vast amounts of sand southward toward the Colorado Plateau (Riggs and Blakey, 1993; Gehrels et al., 2011, 2013). The rivers may not have flowed as far as the Western Interior (where the widespread rock record would have recorded their presence as coastal deltas) but rather desiccated farther east where post-Pennsylvanian-Permianpre-Cretaceous rocks are rare. Although this portion of the paleogeographic reasoning is based on negative evidence (see discussion in Thomas, 2011), the detrital zircon record lends solid evidence toward this hypothesis (see also Chapter 19).

PENNSYLVANIAN-MIDDLE JURASSIC SEQUENCE STRATIGRAPHY Introduction The following section describes and broadly interprets the strata that comprise Pennsylvanian through Jurassic sedimentary rocks of the Colorado Plateau and Southern Rocky Mountains Region. The section is divided into sequences of accumulation that are bounded by unconformities. Pennsylvanian and Permian sequences (Figs. 4 and 5) (Table 1) are herein given series names or subdivisions of series (i.e., Morrowan; Lower Leonardian). The definition of the sequences and their boundaries follows established patterns of Ross (1973), Blakey and Knepp (1989), Blakey (1996), and Trexler et al. (2004). Lack of sufficient biostratigraphic data in some places makes it difficult to determine whether the series boundary falls exactly within the stratigraphic interval represented by the unconformity. The ages of the sequences and the bounding unconformities must be considered approximate pending further biostratigraphic studies. Triassic sequences are named after subdivisions of regional formations (i.e., Lower Moenkopi Formation) as established by Blakey et al. (1993) and Blakey and Gubitosa (1983). Jurassic sequences are modified from Pipiringos and O’Sullivan (1978) and Blakey (1994) and are also named after lithostratigraphic units. Note that the method of naming sequences and unconformities varies among these various studies. I favor deemphasizing numerals in sequence boundaries, as new work that refines the sequences cannot maintain a parallel system of nomenclature (what can a newly defined unconformity that fits between the J-2 and J-3 be called, for example?). Lithostratigraphy follows established nomenclature across the region although I make no attempt to list every local formational name. This is particularly true for Pennsylvanian and Permian rocks where local partitioning by basins and arches has resulted in a complex array of areally restricted stratigraphic names (Figs. 4 and 5). Each of the sequences discussed in this chapter is illustrated by a paleogeographic map that shows sequence extent, relation to various topographic elements, and general sediment types. As most sequences were deposited over several millions of years, each of these maps should be considered an average representation over the given time interval.

Pennsylvanian Mississippian-Pennsylvanian Boundary The Mississippian-Pennsylvanian unconformity (Fig. 4) is everywhere well developed across the region and is generally easy to pick on moderate to poor rock exposures. In many places, Lower Mississippian gray limestone is overlain by Lower to Middle Pennsylvanian red sandstone and mudstone. Rarely can any angularity be detected at outcrop scale. The contact is locally marked by an irregular erosion surface that may be mantled by weathered cherty limestone debris, bedded conglomerate, and red “tera rosa” pedogenic deposits (Blakey and Knepp, 1989). Along many of the uplifts of the Ancestral Rockies, the sub-Pennsylvanian unconformity is cut down through Devonian and Cambrian rocks and rests on Precambrian

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FIG. 4  Pennsylvanian-Permian time-stratigraphic diagram. Gray shading—unconformities; solid horizontal lines—series boundary, presence of unconformity uncertain; ages: Cordilleran miogeocline column, compiled by Trexler et al. (2004), time scale column from Ogg et al. (2016); other columns compiled from many sources. No vertical or horizontal scale.

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FIG. 5  Geometry of Pennsylvanian and Permian rocks southern Western Interior. (A–D) Restored panel cross-sections showing generalized facies trends and thicknesses. Location of sections shown on E. See Table 1 for formation abbreviations.



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FIG. 5, CONT’D  (E) Summary map of Pennsylvanian and Permian basins uplifts and other features of the study area. Pennsylvanian rocks are absent from most portions of all uplifts. Basins shown in blue shelves and arches shown in tan. Thickness shown in kilometers. (Compiled and averaged from many sources.)

basement. In some sections, especially in Pennsylvanian basins, Pennsylvanian limestone rests on Mississippian limestone. An erosional surface may or may not be evident. Basal coarse deposits, usually limestone- or chert-pebble conglomerate, locally mark the unconformity.

Morrowan Sequence Morrowan strata (Format A of Ross, 1973) are irregularly distributed mostly along the margins of the region (Fig. 6). Ross’s use of the term format is synonymous with sequence as used today. East of the Wasatch line, deposits are mostly less than 100 m thick but in western Utah and eastern Nevada, Bissell (1974) reported up to 500 m of Morrowan strata. Lithology ranges from dominantly carbonate rocks with intercalated mudstone in SE Arizona (Ross, 1973; Blakey and Knepp, 1989) and western Utah (Bissell, 1974) to red sandstone and mudstone and thin carbonate in northwestern Arizona (Blakey, 1990) and northern Colorado and Utah (Johnson et al., 1992). Most deposits are considered to be marine, but most lack detailed modern facies analysis.

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TABLE 1  List of Lithostratigraphic Abbreviations Used on Cross-Sections Jurassic

Ja, Aztec Ss; Jcl, lower Carmel Fm; Jcu, upper Carmel Formation; Je, Entrada Ss; c, Cow Springs Mbr; Jk, Kayenta Fm; Jm, Morrison Formation; b, Brushy Basin Mbr; w, Westwater Mbr; r, Recapture Mbr; s, Salt Wash Mbr; Jmo: Moenave Fm; s, Springdale Ss Mbr; d, Dinosaur Canyon Mbr; Jn, Navajo Ss; Jnu, Nugget Ss; Jp, Page Ss; Jpr, Preuss Fm; Jr., Romana Ss; Js, Stump Fm; Jsc, Curtis and Summerville Fms; Jt, Temple Cap Fm; Jtc, Twin Creek Fm; u, upper part; l, lower part; g, Gypsum Springs Mbr; Jw, Wingate Ss; Jwa, Wanakah Fm Triassic

Trc—Chinle Formation: c, Church Rock Mbr; o, Owl Rock Mbr; p, Petrified Forest Mbr; mb, Moss Back Mbr; b, Monitor Butte Mbr; s, Shinarump Mbr; a, Ankareh Mbr (or Fm); g, Gartra Mbr; Trm—Moenkopi Formation: w, Wupatki Mbr; h, Holbrook and Moqui Mbrs; tp, Timpoweap Mbr; l, Lower Red Mbr; v, Virgin (Ls) Mbr; m, Middle Red Mbr; s, Shnabkaib Mbr; u, Upper Red Mbr; b, Black Dragon Mbr; s, Sinbad (Ls) Mbr; t, Torrey Mbr; m, Moody Canyon Mbr; am, Mahogany Mbr (or mbr Ankareh Fm); Trt, Thaynes Fm; Trdw, Dinwoody Fm; Trw, Woodside Fm Permian

Pc, Coconino Ss; Pcl, Colina Ls; Pcm, Cedar Mesa Ss; Pco, Concha Ls; Pct, Cutler Arkose; Pdc, DeChelly Ss; Pdk, Diamond Creek Fm; Pe, Esplanade Ss; Pep, Epitaph Dol; Per, Earp Fm; Pgf, Gerster, Franson fms; Pgr, Grandeur Fm; pH, Hermit Fm; pHa, Halgaito Fm; Pk, Kaibab Fm; Pkr, Kirkman Fm; Plo, Loray Fm; Ply, Lyons Ss; Pma, Maroon Fm; Poq, Oquirrh Gp; Por, Organ Rock Fm; Pp, Pakoon Ls; Ppl, Plympton Fm; Ppq, Pequop Fm; Pq, Queantoweap Fm; Pr, Rain Valley Fm; Prx, Rex Chert; Ps, Scherrer Fm; Psh, Schnebly Hill Fm; Pt, Toroweap Fm; Pw, White Rim Ss; Pwb, Weber Ss Pennsylvanian

IPb, Bird Spring Gp; IPb, Black Prince Ls; IPbm, Beldon and Minturn fms; IPc, Callville Ls; IPer, Earp Fm; IPfo, Fountain Arkose; IPh, Horquilla Ls; IPh, Honaker Trail Fm; IPm, Manakacha Fm; IPma, Maroon Fm; IPp, Paradox Fm; IPpt, Pinkerton Trail Fm; IPw, Wescogame Fm; IPwa, Watahomigi Fm

Morrowan strata represent the first incursion of Pennsylvanian marine deposits into the Western Interior following significant hiatus and erosion during the Late Mississippian. The unconformity was of short duration in the subsiding Cordilleran miogeocline. The geometry of sedimentary rock distribution in Arizona, Utah, and Colorado may foreshadow negative areas of Ancestral Rockies tectonics but there is no evidence of major uplift this early in the Pennsylvanian (Kluth, 1986).

Unconformity 2 (C-4) Wherever Morrowan strata are present, they are apparently separated from overlying Atokan strata by Unconformity 2. Ross (1973) defined the unconformity in the Pedregosa Basin between the Black Prince Limestone and overlying Horquilla Limestone. In the Grand Canyon, it probably correlates with the unconformity between the Watahomigi and Manakacha formations (McKee, 1982) and Trexler et al. (2004) reported an unconformity at a similar stratigraphic position across north-central Nevada (their C-4). No angularity has been reported and only McKee (1982) has described the surface with any detail. Unconformity 2 (C-4) probably represents a sea-level low, but detailed regional work would be necessary to confirm such an interpretation.

Atokan Sequence Atokan strata (Format B of Ross, 1973) are widely distributed across the region, and are well represented in all Pennsylvanian basins (Fig. 7); they are absent from most intervening, more positive areas. Strata of this interval contain the oldest significant eolian deposits of the Western Interior (Blakey et al., 1988). Although facies type and distribution are variable and locally complex, a common pattern in several basins is quartz sandstone along basin margins and carbonate grainstone surrounding micritic carbonate and terrigenous mudstone in basin centers. Patterns of marine and nonmarine facies also show a circular distribution with marine deposits dominating basin centers. These patterns clearly show that many Pennsylvanian basins were well developed during the Atokan (Kluth, 1986). Arkosic conglomerate along the Central Colorado Trough and in NC Colorado documents early phases of Ancestral Rockies uplift (Casey, 1980; Kluth, 1986).

Unconformity 3 (C-5) Unconformity 3 separates Atokan from DesMoinesian rocks across the study region (Ross, 1973). The unconformity is mainly determined by paleontological studies and can be difficult to pick on outcrop. Defined in the Pedregosa Basin, it probably correlates with unconformity (C-5) at similar stratigraphic positions in Nevada (Trexler et al., 2004).



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FIG.  6  Morrowan paleogeography showing inferred edges of maximum depositional extent (solid black lines). “Molas-type” or “terra rosa” karstrelated deposits are widely scattered outside the areas of Morrowan deposition; see Fig. 5E for names of tectonic and topographic elements. The following pertains to all Pennsylvanian and Permian paleogeographic maps: Sediment types are complex and varied—the sediment shown for given areas shows dominant type. “Mixed” refers to cyclic intercalations of sand, mud, and lime.

Desmoinesian Sequence Desmoinesian strata (Formats C-G of Ross, 1973) are widely distributed across the region. In the Pedregosa Basin, Ross recognized five formats and intervening unconformities, but they are difficult to separate without good fossil control and their regional correlation as individual formats has never been accomplished; Blakey and Knepp (1989) lumped the five formats and that practice is followed here. Present distribution and facies patterns are strongly controlled by the geometry of Pennsylvanian basins (Fig. 8). Each basin has at least 300 m of Desmoinesian strata and the Central Colorado Trough, Paradox Basin, and Oquirrh Basin each contain greater than 1500 m of strata. All basins including the Cordilleran basins display well-developed marginal platform and shelf deposits that wrap around basinal deposits. Marine shelf deposits are dominated by fossiliferous carbonate grainstone and packstone and basin centers contain dark carbonate mudstone and terrigenous mudstone (Johnson et  al., 1992). The Central Colorado Trough and eastern Paradox Basin are rimmed by thick, coarse, arkosic conglomerate that fines abruptly into basin platforms and centers (Casey, 1980; Baars and Stevenson, 1981; Johnson et al., 1992; Karachewski, 1992). Both basins also contain extensive marine evaporite deposits (Wengard and Matheny, 1958; Hite, 1970; Tweto, 1977). Chuck Kluth (personal communication, 2007) has unpublished seismic data coupled with well data that suggest that the northern Central Colorado Trough (Eagle basin) and Paradox Basin were one entity during the Desmoinesian (Fig. 8) and possibly into the Missourian.

328  The Sedimentary Basins of the United States and Canada

FIG. 7  Atokan paleogeography showing inferred edges of maximum depositional extent (solid black lines); Note the intricate pattern of uplifts, basins, and platform-shelf areas; see Fig. 5E for names of tectonic and topographic elements. Ancestral Rocky Mountains consist of Precambrian-cored uplifts; Emery positive area cored by Paleozoic rocks, not Precambrian basement.

Desmoinesian strata were deposited in basins across the southern Western Interior during the first (main) pulse of the Ancestral Rockies (Kluth, 1986). The flanks of basins adjacent to uplifts received influx of immature, coarse, arkosic sediments, locally in excess of 1500 m in some basins.

Unconformity 8 (C-6) Unconformity 8 was defined by Ross (1973) in eastern Arizona where the surface separates Desmoinesian from Missourian strata. The unconformity may correlate with a pre-Missourian unconformity in the Paradox Basin recognized by Welch (1958) and unconformity C-6 in NE Nevada and adjacent Utah (Trexler et al., 2004). The unconformity may be coincident with reduced rates of uplift in the Ancestral Rockies and corresponding lowered rates of subsidence in basins as reported by Kluth (1986).

Missourian Sequence Missourian rocks (Formats H-I of Ross, 1973) are similar in distribution and lithology to those of underlying Desmoinesian rocks. In many basins Missourian rocks show an increase in siliciclastic sedimentation at the expense of carbonate deposition and are commonly banded, cyclic deposits of red, tan, and gray mudstone, sandstone, and carbonate, respectively (Ross, 1973; Blakey and Knepp, 1989). Kluth (1986) recognized that vigorous uplift of the Ancestral Rockies continued



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FIG.  8  Desmoinesian paleogeography showing inferred edges of maximum depositional extent (solid black lines). Desmoinesian and Missourian stages were times of highest Pennsylvanian sea levels; see Fig. 5E for names of tectonic and topographic elements. Ancestral Rocky Mountains consist of Precambrian-cored uplifts; Emery positive area cored by Paleozoic rocks, not Precambrian basement.

into Missourian time (Fig. 9) but at a reduced rate compared to that of Desmoinesian time. Rate of basin subsidence also was reduced. It must be cautioned that comparing rates of sedimentation and subsidence between epochs and sequences should be done with care as duration of different sequences is rarely equal; note also that the entire Middle Pennsylvanian (Desmoinesian-Missourian) is only 5 million years duration on the 2004 timescale, although more recent time scales (Cohen et al., 2013; Ogg et al., 2016) increase this length to 10 million years.

Unconformity 10 Unconformity 10 separates Missourian from Virgilian rocks in SE Arizona (Ross, 1973). Although the length of time missing is not clear, rocks above it commonly show an abrupt increase in sandstone and mudstone (Ross, 1973; Blakey and Knepp, 1989).

Virgilian Sequence Virgilian rocks (Formats J-L of Ross, 1973) comprise widespread siliciclastic deposits across much of the region (Fig. 10). Although carbonate deposition continued in some basin centers and on shelves removed from uplifts, marine, eolian, and fluvial sandstone and associated red mudstone dominate most areas (Loope, 1984; Blakey and Knepp, 1989). Kluth (1986) reported a general continued slowdown of uplift and subsidence during Virgilian time although there remained local areas of vigorous tectonic activity.

330  The Sedimentary Basins of the United States and Canada

FIG. 9  Missourian paleogeography showing inferred edges of maximum depositional extent (solid black lines); see Fig. 5E for names of tectonic and topographic elements. Ancestral Rocky Mountains consist of Precambrian-cored uplifts; Emery positive area cored by Paleozoic rocks, not Precambrian basement.

Permian Pennsylvanian-Permian Boundary The boundary between Pennsylvanian and Permian rocks across the region has generally been recognized as an unconformity, although in many places, the contact is difficult to pick on outcrop (Peirce, 1989). This difficulty exists because of similarity of youngest Pennsylvanian strata with those of the overlying Permian; in many places redbeds rest on redbeds and in other places local conglomerate marks the disconformity (Blakey and Knepp, 1989).

Wolfcampian Sequence Blakey (1996) recognized four Permian sequences and their intervening unconformities across the Colorado Plateau and these Permian elements are used in this chapter (Fig.  4). Wolfcampian strata (the oldest sequence of Blakey, 1996) are widespread across the Colorado Plateau and Southern Rocky Mountain region and are present most places except across the highest Ancestral Rockies uplifts (Fig.  11). Wolfcampian strata comprise heterolithic lithologies that reflect local tectonic and topographic conditions. Thick arkosic deposits are present in the eastern Paradox Basin



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FIG. 10  Virgilian paleogeography showing inferred edges of maximum depositional extent (solid black lines); see Fig. 5E for names of tectonic and topographic elements. Ancestral Rocky Mountains consist of Precambrian-cored uplifts; Emery positive area cored by Paleozoic rocks, not Precambrian basement.

(Campbell, 1980; Mack and Rasmussen, 1984) and thinner arkoses flank other basins. Eolian sandstone is widespread, especially across the Colorado Plateau (Baars, 1962; Blakey, 1996). Carbonates interfinger with both lithologies and dominate deposition in western Utah and adjacent Nevada (Bissell, 1970). The various sedimentary facies accumulated in marine, fluvial, and eolian e­ nvironments (Blakey, 1996, 2002; Langford and Chan, 1989). In many basins of the Ancestral Rockies, especially the Central Colorado Trough, Permian deposits are markedly thinner than underlying Pennsylvanian deposits. This suggests a general slowdown in the Late Paleozoic orogeny (Kluth, 1986); however, rates of sedimentation in parts of the Paradox Basin and in areas of western Utah and eastern Nevada show a continued rapid rate of subsidence and sedimentation (Bissell, 1970; Blakey, 1996). Clearly the Ancestral Rockies orogeny was not finished by Early Permian time.

Unconformity P-sc Unconformity P-sc (Blakey, 1996) marks a break in rapid Wolfcampian sedimentation. In parts of western and northern Colorado, Wolfcampian rocks are the youngest Paleozoic deposits. Elsewhere, Wolfcampian rocks are overlain by unconformity P-sc. The unconformity is planar in many areas and is only confirmed by regional stratigraphic relations

332  The Sedimentary Basins of the United States and Canada

FIG.  11  Wolfcampian paleogeography showing inferred edges of maximum depositional extent (solid black lines). Blue line marks SE margin of Wolfcampian sea; see Fig. 5E for names of tectonic and topographic elements. Ancestral Rocky Mountains consist of Precambrian-cored uplifts.

(Blakey, 1996). The unconformity may correlate with unconformity P-2 in the Cordilleran miogeocline (Trexler et al., 2004). The hiatus may lie within the Wolfcampian or Leonardian or mark the boundary between the two.

Lower Leonardian Sequence Lower Leonardian rocks (the second sequence of Blakey, 1996) are unevenly distributed across the study area. In the Denver, Orogrande, and Holbrook basins, sandstone and red mudstone of eolian, restricted marine, and sabkha origin (Fig. 12) dominate (Blakey et al., 1988). Thicknesses in the Holbrook and Orogrande basins exceed 600 m (Blakey, 1990). In western Utah and Nevada, carbonates are intercalated with quartz sandstone and thickness exceeds 1000 m (Bissell, 1970). Lower Leonardian rocks are apparently absent across much of the central portion of the region. In the southern Colorado Plateau region, the top of the sequence comprises the widespread Coconino-Glorieta eolian complex, probably the largest Late Paleozoic eolian deposit of the Western Interior (Blakey et al., 1988). The increase in sand, especially across northern Arizona and northern New Mexico, was coincident with the withdrawal of marine seas across the Great Plains area (compare Figs. 11 and 12). With seas absent across much of the area east and north of the Colorado Plateau, a larger area of low-lying land allowed increased eolian transport and resulting influx of sand. This change in paleogeography was accompanied by increases in subsidence in the Holbrook and Orogrande basins that facilitated accumulation of thick eolian deposits (Blakey, 1996; Gehrels et al., 2011).



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FIG. 12  Lower Leonardian paleogeography showing inferred edges of maximum depositional extent (solid black lines); see Fig. 5E for names of tectonic and topographic elements.

Unconformity P-tw An unconformity within the Leonardian extends across the region (sub-Toroweap of McKee, 1938; P-tw of Blakey, 1996; P-3 of Trexler et al., 2004). The unconformity is subtle on many outcrops, especially east of the margin of the Toroweap Formation, and is most easily seen by regional stratigraphic patterns (Blakey, 1996). Across the southern and western Colorado Plateau, the unconformity is marked by the marine-flooding surface of the Toroweap marine transgression.

Upper Leonardian Sequence The Upper Leonardian Sequence (third sequence of Blakey, 1996) comprises marine deposits of the Toroweap Formation and coeval eolian facies (Chan, 1989); in both Arizona and Utah, the Toroweap transitions eastward into eolian sandstones of the upper Coconino Sandstone and White Rim Sandstone, respectively. Thin evaporites are present in the Grand Canyon region (Rawson and Turner-Peterson, 1980). The sequence is absent in the Four Corners region and across most of western Colorado (Fig. 13). It is generally less than 100 m thick across the study area; however, it thickens to over 1500 m in parts of the Cordilleran miogeocline (Bissell, 1970). On the Colorado Plateau, the sequence was deposited during a general marine transgression-regression cycle (Rawson and Turner-Peterson, 1980). The sequence apparently lacks coarse material derived from Ancestral Rockies uplifts, suggesting that the mountains were worn down by this time.

334  The Sedimentary Basins of the United States and Canada

FIG.  13  Late Leonardian (Toroweap-White Rim) paleogeography showing inferred edges of maximum depositional extent (solid black lines). The shoreline shown represents the maximum Toroweap transgression; see Fig. 5E for names of tectonic and topographic elements.

Unconformity P-k Unconformity P-k separates the Kaibab Formation from the underlying Toroweap Formation across the western and southern Colorado Plateau (McKee, 1938). In the Cordilleran basin, Trexler et al. (2004) recognized the P-4 unconformity at a similar horizon.

Guadalupian Sequence and Younger Permian Rocks The Guadalupian Sequence (fourth sequence of Blakey, 1996) is present mainly across the western and southeastern portions of the study area where the Kaibab and San Andres formations were deposited (Fig. 14). The sequence likely contains youngest Leonardian as well as Guadalupian rocks (McKee, 1938). Limestone and sandy dolomite of shallow marine and shoreline origin dominate the sequence (Irwin, 1971). The sequence is generally less than 200 m thick except in the Cordilleran miogeocline, where it thickens to over 1000 m (Bissell, 1970). The sequence was deposited during the last major Paleozoic marine transgression-regression cycle across the southern Western Interior (Blakey, 1996). The Kaibab Formation (and related rocks) is the youngest Permian unit across most of the study area. West of the Wasatch line, and in the northern extremities of the study area, younger Permian rocks were deposited in several marine transgressive-regressive cycles (Whalen, 1996).



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FIG. 14  Latest Leonardian and Guadalupian (Kaibab Fm and post-Kaibab) paleogeography showing inferred edges of maximum depositional extent (solid black lines). The shoreline shown represents the maximum Kaibab transgression; see Fig. 5E for names of tectonic and topographic elements.

Triassic Introduction Triassic rocks are generally well exposed across the Colorado Plateau, but less well exposed in the Southern Rocky Mountains. Most Lower and Middle Triassic rocks are assigned to the Moenkopi Formation and most Upper Triassic rocks are assigned to the Chinle Formation (Figs. 15 and 16). Lower Triassic strata display regional geometry similar to that of many Paleozoic strata, with significant thickening westward from the Wasatch line (Fig. 17). Upper Triassic rocks display contrasting geometry and thicken to the SE and NE (Fig. 18).

Unconformity Tr-1 In most places of the southern Western Interior where Triassic rocks rest on Permian rocks, they overlie an obvious unconformity based on both stratigraphic and paleontologic evidence (Stewart et al., 1972b). At the north edge of the study area, the boundary is less certain and locally suggests conformable relations between Permian and Triassic rocks (Stewart et al., 1972b). In the San Rafael Swell, thin limy sandstone deposits lie between undoubted Permian rocks below and Triassic rocks above and contain a molluscan fauna that may be either Permian or Triassic in age (Blakey, 1974). South of these two areas, Unconformity Tr-1 generally separates rocks of Leonardian or Guadalupian age from Lower or Middle Triassic rocks above (Pipiringos and O’Sullivan, 1978; Blakey et al., 1993). The unconformity developed during the Permo-Triassic

336  The Sedimentary Basins of the United States and Canada

FIG. 15  Triassic-Jurassic time-stratigraphic diagram. Gray shaded areas—unconformities. Ages are after Ogg et al. (2016). Numbered unconformities from Pipiringos and O’Sullivan (1978); J-sk and J-sup from Blakey (1994). Unconformity J-1 at least partially equivalent to J-2 (see text). No vertical or horizontal scale.



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FIG.  16  Triassic restored panel cross-sections, southern Western Interior. Sequences and sequence boundaries shown. See Table  1 for formation abbreviations.

sea-level low stand, one of the lowest in the stratigraphic record (Haq et al., 1988; Haq, 1991). Local relief on the surface approaches as much as 100 m in NW Arizona and SW Utah (Nielson and Johnson, 1979). Except for local angular unconformities in the Salt Anticline region, no angular discordance has been reported (Blakey, 1974).

Lower Moenkopi Sequence The oldest Mesozoic sequence in the study area comprises the lower part of the Moenkopi Formation (Blakey et al., 1993); it was deposited across parts of northern Arizona and much of Utah (Fig. 19). The sequence consists of deposits less than 100 m thick along the eastern margin but thickens westward to over 2000 m in NW Utah. As defined herein, the Lower Moenkopi sequence includes strata that lie below the lower massive sandstone of McKee (1954) and coeval Shnabkaib Member; this horizon is coincident or very close to a regional magnetostratigraphic boundary in the Lower Triassic; fossils date the sequence as Lower Triassic (Morales, 1987). The Lower Moenkopi sequence comprises distinct eastern and western lithofacies with an intermediate zone where the two are intercalated. The eastern facies consists of pale to dark reddish-brown mudstone and very fine-grained sandstone. Bedding is generally thin and rhythmic and forms ledgy slopes. Ripple marks, ripple cross lamination, and mudcracks are dominant but a wide range of sedimentary structures indicative of shallow water deposition are present (Blakey, 1974; Dubiel, 1994). The western facies is dominantly gray to tan limestone. Bedding is generally thin and rhythmic, although more massive-weathering units are present. A wide range of carbonate fabric and textures and locally abundant fossils document shallow marine shoreline to offshore marine deposition (Paull and Paull, 1993). The eastern and western facies are complexly intercalated across a zone that trends NNE-SSW across central and western Utah (Blakey et  al., 1993).

FIG. 17  Summary of Lower and Middle Triassic thickness and sub-Triassic paleogeology. East of zero isopach, paleogeology is under Upper Triassic Chinle Formation. Note general paleo-anticlinal SW-NE trend through Ancestral Rockies. (Paleogeology modified from Pipiringos and O’Sullivan (1978).)

FIG. 18  Summary of Upper Triassic thickness. Note how thickness trends contrast markedly with those of Lower and Middle Triassic.



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FIG. 19  Moenkopi Formation lower marine and fluvial sequence paleogeography; see Fig. 17 for names of tectonic and topographic elements. Abrupt western thickening into Cordilleran miogeocline simulates trends in many lower and middle Paleozoic rocks. The 600-m isopach, near the Wasatch line, is shown for reference. Irregular solid line marks approximate south and east extent of preserved deposits.

Carbonate percentage increases westward across the zone and thin carbonate marker beds penetrate eastward into the red sandstone and mudstone. The Lower Moenkopi sequence was deposited on a broad, flat, relatively featureless coastal plain (Blakey, 1974; Ochs and Chan, 1990). The coastal plain was so flat that slight changes in relative sea level caused widespread marine transgression and regression. Fine siliciclastic material was trapped and reworked by low energy coastal environments, especially broad intertidal flats. Carbonate environments persisted in clearer shallow marine environments, especially during weak transgressive episodes, while sand and mud were trapped on the coastal plain (Blakey et al., 1993). The Lower Moenkopi sequence represents a subtle tectonic setting across the Western Interior. Shoreline position, depositional environment, and carbonate versus siliciclastic deposition were controlled by sediment influx from the east and relative sea level during a time of steady subsidence with subsidence rates dramatically increased to the west (Blakey et al., 1993).

Unconformity Tr-lm Unconformity Tr-lm (under the lower massive sandstone) is a widespread but very subtle planar surface that separates similar lithology above and below. The unconformity is difficult to locate on local outcrops but is apparent based on regional trends. The surface overlies strata that dramatically thicken to the west and is overlain by strata that maintain a rather constant thickness across much of the region (Blakey et al., 1993).

340  The Sedimentary Basins of the United States and Canada

Upper Moenkopi Sequence The Upper Moenkopi sequence consists of red sandstone and mudstone that forms ledges and ragged cliffs; the sequence is Middle Triassic based on a widespread vertebrate fauna (Morales, 1987). The sequence is presently recognized across much of northern New Mexico and Arizona and southwestern Utah and parts of adjacent Nevada (Fig. 20). It may be present elsewhere farther north, although it is clearly removed by pre-Chinle erosion in many areas (Blakey, 1974). Unlike the Lower Moenkopi sequence, the Upper Moenkopi sequence does not display a marked westward thickening. Fluvial channel deposits and adjacent flood plain deposits dominate the sequence; the southern and eastern areas of deposition are characterized by perennial meandering stream deposits that grade northward (distally) into ephemeral stream deposits where it is well exposed along the Echo Cliffs from Cameron, to Page, Arizona. In NW Arizona and SW Utah, at least part of the sequence includes mudstone and gypsum coastal plain deposits of the Shnabkaib Member (Blakey et al., 1993). The Upper Moenkopi sequence marks a change in subsidence history of the region. Although streams clearly flowed northwesterly across the region, similar to stream flow below, the lack of northwest thickening suggests that rapid subsidence along and west of the Wasatch line had ended. The sequence may mark a transition between the contrasting isopach patterns of the Lower Moenkopi Formation and overlying Chinle Formation (Figs. 16, 17, and 18).

Unconformity Tr-3 Unconformity Tr-3 is one of the most dramatic Triassic-Jurassic unconformities across the Western Interior (Pipiringos and O’Sullivan, 1978). It separates, with obvious disconformity, the Moenkopi and Chinle formations, except where the

FIG. 20  Moenkopi Formation upper fluvial sequence paleogeography; see Fig. 17 for names of tectonic and topographic elements. Sheet-like deposits across Colorado Plateau region are generally less than 100 m thick.



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former is absent; where the Moenkopi is absent the Chinle overlies rocks as old as Precambrian (Pipiringos and O’Sullivan, 1978). The unconformity is locally angular, especially in the Salt Anticline region near elements of the Ancestral Rockies. Relief on the unconformity is obvious, both at local and regional scales. Documented relief approaches 100 m and 10–20 m is apparent on many outcrops (Blakey, 1974; Blakey et al., 1993). The unconformity represents an obvious change in base level and a change in fluvial regime and style. Both Moenkopi and Chinle streams had NW to N paleoflows but here the similarities end; Moenkopi streams flowed in arid climates and were at least partly ephemeral; except very locally, they lacked any siliceous extrabasinal conglomerate and flowed near base level on broad coastal plains. Chinle streams flowed in humid to semiarid climates and were mostly perennial; deposits contain abundant siliceous extrabasinal conglomerate and show evidence of complex base level changes (Kraus and Middleton, 1987). How these contrasts between fluvial systems were orchestrated by tectonic, eustatic (or other base level changes), or climatic controls has yet to be determined in detail.

Lower Chinle Formation The Chinle Formation is one of the most thoroughly studied units on the Colorado Plateau. The formation was deposited in fluvial systems that varied from meandering to braided to anastomosing; associated lacustrine, paludal, and overbank soil deposits are well preserved (Fig. 21). Abundant paleocurrent data document that north- and northwest-flowing f­ luvial

FIG. 21  Chinle Formation lower fluvial paleogeography; see Fig. 18 for names of tectonic and topographic elements. Note paleovalleys across northern Arizona and southern Utah. Arrows indicate areas where narrow paleovalleys are preserved; incision depths to 75 m are recorded at several locations. Preserved deposits in the Shinarump Member are generally restricted to the vicinity of fluvial systems shown on map. The fluvial systems across northern Utah and SW Wyoming are potentially equivalent to the Shinarump Member. On this and all subsequent maps, the wavy line across Arizona and New Mexico marks southwest extent of Triassic and Jurassic deposits on the Colorado Plateau. The curved line across western Utah marks northwest margin of preserved deposits.

342  The Sedimentary Basins of the United States and Canada

systems were dominant (Stewart et al., 1972a; Blakey and Gubitosa, 1983; Dubiel, 1989). The Chinle was deposited under more humid conditions than those of the underlying Moenkopi, possibly under a monsoonal regime (Dubiel et  al., 1991). Most recent work on the Chinle Formation suggests that this complex fluvial deposit was the result of two or more sequences of deposition with intervening unconformities. However, there is lack of agreement as to where sequences and sequence boundaries are located within the formation (cf. Lucas and Marzolf, 1993). Based on regional observations, I recognize three sequences within the Chinle Formation and two internal, regional unconformities: one at the base of the Moss Back and Sonsela members and one at the base of the Church Rock and Rock Point Members (Fig. 15). Lucas (1993) has proposed a radically different nomenclature system and stratigraphic framework that are not followed here. The Lower Chinle Formation as herein defined includes strata between the basal Chinle unconformity (Tr-3) and a prominent change in sedimentation at a probable unconformity below the Moss Back and Sonsela members. Using this definition of the interval, it includes the Shinarump, Monitor Butte, and lower Petrified Forest members and equivalent units to the east in Colorado and New Mexico. The interval is generally present across the southern two-thirds of the study area and generally thickens to the south. It was deposited by NW to N flowing streams and associated flood basins, lakes, swamps, and pedogenic envoronments (Blakey and Gubitosa, 1983; Dubiel, 1994). Sandstone is mostly coarse-grained to very coarse-grained and commonly is conglomeratic with clasts ranging into the cobble size. The mostly siliceous clasts are variable mixes of quartz, quartzite, and chert, with highly variable (both in composition and percentage) volcanic clasts. Quartz and quartzite are from Precambrian sources, chert is chiefly from Late Paleozoic limestone, and volcanics are from Triassic sources in the Cordilleran arc (Stewart et al., 1972a, 1986). Zircons in some sandstone units document that some streams were sourced by Pangaean highlands (Riggs et al., 1996). Details of the Chinle fluvial systems are well documented (references cited previously and references therein).

Unconformity Tr-sm The base of the Sonsela Sandstone Member and likely coeval Moss Back Member is a regional unconformity (Blakey and Gubitosa, 1983). The scoured base of this large fluvial complex displays several meters of local relief and likely tens of meters of regional relief. The unconformity likely represents a change in fluvial style accompanied by regional base level change. Lupe and Silberling (1985) have related this and other Chinle base level changes to Triassic eustatic events based on tentative correlation of the Chinle to marine rocks in western Nevada. The unconformity may mark the Carnian-Norian stage boundary (Lockley and Hunt, 1994).

Middle Chinle Formation The middle part of the Chinle Formation (Fig. 22) as herein defined includes the Moss Back, Sonsela, Upper Petrified Forest, and Owl Rock members as well as equivalent units in Colorado and New Mexico. The general lithology and sedimentary history are similar to those of the Lower Chinle Formation, although important differences exist. The middle Chinle has a persistent carbonate interval, the Owl Rock Member, which was formed in lacustrine and pedogenic settings (Blodgett, 1988; Dubiel, 1994) and has a higher percentage of intrabasinal carbonate clasts that range into the pebble to cobble range (Blakey and Gubitosa, 1983). This limestone-pebble conglomerate is an important host lithology for Chinle uranium ore deposits. Bentonitic mudstone decreases from south to north across the region. The lithology and depositional environments of the Middle Chinle Formation are well documented in the literature (Blakey and Gubitosa, 1983, 1984; Dubiel, 1989, 1994).

Unconformity Tr-cr Unconformity Tr-cr is marked by a widespread erosional disconformity across the region and a change in fluvial style above. On outcrop the erosional surface ranges from planar to deeply scoured. In the Canyonlands region, scours ranging to 10 m deep are common with overlying strata that contain clasts clearly derived from underlying units.

Upper Chinle Formation The Upper Chinle Formation (Fig. 23) as herein defined includes the Church Rock Member to the north and coeval Rock Point Member to the south. This is the reddest interval in the Chinle and likely represents an overall increase in aridity (Dubiel, 1994). It is also the least understood portion of the Chinle Formation. In much of Utah and Colorado, the interval is marked by ledge-forming sandstone and conglomerate that fine upwards into red mudstone (O’Sullivan, 1970). Both intrabasinal and extrabasinal conglomerate are present. A complex of braided and meandering streams deposited the northern portion of the interval (Blakey and Gubitosa, 1983; Hazel, 1994). The southern portion of the interval is marked by an increase in tabular eolian and sabkha sandstone and silty sandstone (Dubiel, 1994). Once thought to intertongue with the



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FIG. 22  Chinle Formation middle fluvial system paleogeography; see Fig. 18 for names of tectonic and topographic elements. The prominent system flowing NW across the Four Corners region deposited the Moss Back Member; streams draining the Cordilleran arc and flowing NE across the southern Colorado Plateau deposited the Sonsela Member. Whether the systems merged as shown on the map is uncertain. The NE margin of deposits of this interval is very uncertain as younger Chinle fluvial systems truncate the interval in that direction.

overlying Wingate Sandstone (Harshbarger et al., 1957), Nation (1990) has documented a low-angle unconformity that separates similar facies of the two formations.

Jurassic Introduction Jurassic strata are widely exposed across the study area and form the largest areal outcrop of any geologic system across the Colorado Plateau. Because of the areal continuity of Jurassic rocks on outcrop, nomenclature remains relatively consistent across the region (Fig. 15). Jurassic rocks thicken westward into the Utah-Idaho Trough (Fig. 24). Lower Jurassic rocks (Fig. 25) are beveled eastward across the region by the J-2 unconformity (Pipiringos and O’Sullivan, 1978). Middle Jurassic rocks overlap the J-2 surface and extend eastward of the study region (Fig. 26). Early work on Jurassic units focused on description of units, nomenclature, and correlation, but because much of the Colorado Plateau was roadless until the 1950s and field work was difficult, numerous miscorrelations were made. For example, it was later discovered that the type Wingate Sandstone at Fort Wingate, New Mexico (near Gallup) turned out to be Entrada Sandstone, a much younger unit. Fortunately, due to the extensive correct use and correlation of Wingate in Arizona and Utah, the term has prevailed!

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FIG. 23  Chinle Formation upper fluvial paleogeography; see Fig. 18 for names of tectonic and topographic elements. The eolian sandstone and fluvial ephemeral redbed deposits of the Rock Point Member on the SW side of the Four Corners region are generally correlated to fluvial braided and meandering systems preserved in the Church Rock Member to the north; stratigraphic relations between the two members are mostly concealed in the subsurface. The western margin of the interval that was truncated by pre-Jurassic erosion is shown by the dashed line.

The uranium booms of the 1950s and later brought both roads to the region and extensive study of Jurassic rocks, primarily by the US Geological Survey. A culmination of this work resulted in a seminal paper on Triassic-Jurassic rocks of the Western Interior in which regional unconformities were documented and correlated from Montana to Arizona (Pipiringos and O’Sullivan, 1978). These correlations are used in this study (Fig. 15) and, for the most part, are consistent across most of the study area. However, a recent publication that came out as this chapter was being completed (Dickinson, 2018) has challenged some of the regional unconformities and their correlation. Due to the complexity of the topic and the recent appearance of Dickinson’s work, it was not possible for this chapter to challenge, discuss, or revise conclusions based on this recent work.

Unconformity J-0 Unconformity J-0 is a regional unconformity that bevels underlying strata to the southwest (Pipiringos and O’Sullivan, 1978; Blakey, 1994). In SW Arizona and adjacent California, the unconformity truncates Paleozoic strata (Reynolds et al., 1989). Across the study area, the unconformity lacks measurable relief and locally superimposes similar facies, making recognition difficult. In spite of obvious regional truncation and tilting associated with the surface (I am unaware of any locally obvious angular unconformity except in SE California and SW Arizona), stream flow was to the NW above and below the unconformity (Blakey, 1994). The unconformity is believed to mark the Triassic-Jurassic boundary, although some

FIG. 24  Restored panel cross-sections of Jurassic rocks of southern Western Interior. Sequences and unconformities shown. See Table 1 for formation abbreviations. The J-1 and J-2 unconformities may be the same surface in SW Utah; see text for discussion.

FIG. 25  Glen Canyon Group summary isopach map showing area of persistent fluvial-eolian facies changes. (Modified from Blakey et al. (1988).)

346  The Sedimentary Basins of the United States and Canada

FIG. 26  San Rafael Group summary isopach map showing location of major facies changes along the western Colorado Plateau. (Compiled from many sources.)

­paleontological evidence remains highly equivocal (Lockley and Hunt, 1994). A number of paleomagnetic and paleontologic studies have cast doubt on the location of the Triassic-Jurassic boundary and the presence of a J-0 unconformity, at least as defined by Pipiringos and O’Sullivan across the Colorado Plateau (see discussions in Donohoo-Hurley et al., 2010 and Zeigler and Geissman, 2011; Suarez et al., 2017, their Fig. 7).

Lower Glen Canyon Group The Glen Canyon Group represents one of Earth’s greatest fossil desert accumulations. This vast complex of rock was deposited in great interior ergs and by both ephemeral and perennial stream complexes; both the Wingate and Navajo erg sequences contain huge eolian complexes toward the NE part of the study area that are bordered by fluvial complexes to the SW. The area of intertonguing between fluvial and eolian systems is well exposed for both sequences and each occupies generally the same area (Blakey et al., 1988). The Lower Glen Canyon Group includes the Wingate Sandstone and coeval Dinosaur Canyon Member of the Moenave Formation. Eolian deposits of the Wingate intertongue across a 150 km-wide NW trending band with ephemeral fluvial deposits of the Dinosaur Canyon (Blakey et al., 1988; Clemmensen et al., 1989; Blakey, 1994). Together they form a general sheet-like deposit that gradually thickens and thins without obvious trend across the region. Facies patterns, NW stream flow, SE migrating dunes, and patterns of intertonguing (Fig. 27) suggest a complex fluvial-eolian recycling between the two units (Blakey, 1994).

Unconformity J-sk Unconformity J-sk is a widespread disconfomity at the base of the fluvial Kayenta Formation (Nation, 1990; Blakey, 1994). The surface marks an obvious break between eolian deposits below and fluvial deposits above. Local relief of up to 10 m is



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FIG. 27  Lower Glen Canyon Group paleogeography; see Fig. 25 for names of tectonic and topographic elements. Sand and mud of the fluvial Dinosaur Canyon Member of the Moenave Formation grade northeastward into eolian deposits of the Wingate Sandstone; the Whitmore Point Member of the Moenave Formation was deposited in the lake on the western Colorado Plateau.

present where fluvial channels incise cliff-forming eolian sandstone. There is no discernable pattern of truncation or tilting below the surface, which suggests climatic and possibly base level change as the cause. The unconformity formed during the Early Jurassic but is difficult to date more precisely.

Upper Glen Canyon Group The Upper Glen Canyon Group comprises the Kayenta Formation and Navajo Sandstone (Fig. 28). The Navajo Sandstone may be the largest eolian system in the geologic record (Blakey, 1994). The plan view pattern of intertonguing between parts of the Navajo and Kayenta is similar to that of the underlying Wingate-Dinosaur Canyon (Blakey, 1994); however, the cross-sectional geometry of the two is considerably different (Fig. 24). Whereas the Lower Glen Canyon Group has an irregular sheet geometry, the Upper Glen Canyon group thickens significantly to the SW. Because the Upper Glen Canyon Group is everywhere overlain by an erosional surface, it is unknown whether preserved isopach patterns of the sequence reflect primarily westerly increase in subsidence rate, uplift and truncation to the east, or a combination of both (Blakey, 1994). The sequence was deposited during a low eustatic interval (Haq, 1991) and it seems likely that moderate subsidence rates across much of the western portion of the study area may have created a topographic basin below sea level, but without direct connection to the sea. Coeval Kayenta fluvial deposits drained into the basin, the beginnings of the UtahIdaho Trough, and were then deflated by NW winds to partially feed the Navajo ergs (Blakey, 1994; see his Fig. 4). The topographic hole coupled with low sea level isolated the region from all other depositional systems for millions of years

348  The Sedimentary Basins of the United States and Canada

FIG. 28  Upper Glen Canyon Group paleogeography; see Fig. 25 for names of tectonic and topographic elements. Sand and mud of the “silty facies” of the Kayenta Formation grade northeastward into extensive erg deposits of the Navajo Sandstone. Note that eolian dunes are preserved within Cordilleran arc volcanics south and west of the Colorado Plateau.

allowing the Navajo ergs to fill the basin, almost by default. Water from the Kayenta river seeped into the porous sands and maintained a near surface water table during much of Navajo deposition; the high water table trapped eolian sand in the basin, preventing excessive loss of sediment. The high water table is well documented by widespread freshwater carbonates and mass flow sand deposits (Eisenberg, 2003, and references cited therein). Dickinson and Gehrels (2003) demonstrated a dominantly cratonic North American and Appalachian-Pangaean source for the Navajo and other Colorado Plateau erg systems based on zircon ages within eolian sandstone.

Sub-Middle Jurassic (J-1?) Unconformity The top of the Lower Jurassic Navajo Sandstone is everywhere a regional unconformity that cuts down section toward the east. In SW Utah, this unconformity was named the J-1 (Pipiringos and O’Sullivan, 1978). Farther east, a supposedly younger unconformity, the J-2, was recognized (Pipiringos and O’Sullivan, 1978). Recent work has suggested that the two unconformities may be one and the same. Dickinson et al. (2010) demonstrated similar radiometric ages for the Temple Cap Sandstone and the lower Page Sandstone, the former presumably on the J-1 and the latter on the J-2. If both units are the same age, they likely succeed the same unconformity. Kowallis et al. (2001) established a minimum age of 170 Ma for the unconformity based on 39Ar/40Ar ages from intercalated bentonites in the overlying Temple Cap Sandstone. North of the study area, the J-1 is attributed to a Middle Jurassic lowstand (Brenner and Peterson, 1994).



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Temple Cap Sandstone and Lower Page Sandstone The Temple Cap Sandstone is restricted to SW Utah and consists of two facies, red sandy mudstone of sabkha origin and tan cross-bedded sandstone of eolian origin (Peterson, 1994; Sprinkel et al., 2011). Based on stratigraphic position, the unit is early Middle Jurassic (Aalenian) and is related to the first Middle Jurassic transgression into the Western Interior (Peterson and Pipiringos, 1979; Peterson, 1994). The remnant outcrops in SW Utah probably bordered a restricted Jurassic seaway that lay to the NW (Fig. 29). The potentially equivalent lower Page Sandstone, the Harris Wash Member, is exposed across much of SE Utah (Blakey et al., 1996). This unit contains mostly eolian sandstone, although scattered sabkha and possible restricted shallow marine units are also present. Dickinson et al. (2010) obtained age ranges of 171.5–169.5 Ma for the Harris Wash Member at Lake Powell.

Unconformity J-2 The J-2 (and the likely partially equivalent J-1) unconformity is one of the most profound Mesozoic unconformities (Fig. 24) and marks the boundary between the Absaroka and Zuni Sequences. Sloss (1988) speculated that the formation of the J-2 surface was the major controlling event of the current geologic outcrop patterns of central North America. The J-2 can be correlated across the entire Western Interior and into adjacent regions (Pipiringos and O’Sullivan, 1978). Across the study area, the surface cuts down through older strata, resting on Precambrian rocks across elements of the Ancestral Rockies.

FIG. 29  Temple Cap-lower Page paleogeography; see Fig. 26 for names of tectonic and topographic elements. Eolian dune deposits bordered restricted marine and sabkha deposits. The Temple Cap was previously thought to be older than the Page, but detrital zircon studies suggest partial equivalence (see text for details).

350  The Sedimentary Basins of the United States and Canada

Much of the eastward thinning of the Navajo Sandstone may be due to J-2 erosion, although this cannot be determined with certainty until internal correlation of the Navajo is established (Blakey, 1994). Although in negative areas of the northern Western Interior the unconformity only represents 2–3 million years, in most regions more time is represented; the complex paleogeology below the surface must clearly be the result of tectonic warping of much of the craton (Sloss, 1988). Across the study area, the J-2 surface, where cleanly exposed, always documents at least several meters of local relief to local extremes of nearly 50 m in south-central Utah (Blakey et al., 1996).

Page-Lower Carmel The Page Sandstone (post-Harris Wash Member) and coeval parts of the Lower Carmel Formation (Fig.  30) document repeated interactions between coastal eolian systems and interior restricted marine systems (Blakey et al., 1996; Havholm et al., 1993). The sequence was deposited in and along the eastern margin of the rapidly subsiding Utah-Idaho Trough (Blakey et al., 1996; Peterson, 1994), which may reflect the initial stage of Mesozoic foreland basin development across the Western Interior (Bjerrum and Dorsey, 1995). Details of the sedimentary history, relations between marine and eolian events, and eustatic and tectonic controls on deposition were documented by Blakey et al. (1996). A normal marine fauna in the Lower Carmel Formation is Bajocian age (Imlay, 1967, 1980). The interval thickens from a depositional margin across SE Utah to over 200 m in SW Utah; eolian strata to the east are intercalated with and are replaced westward by sabkha and restricted shallow marine redbeds and marine limestone (Figs. 24 and 30). The interval contains abundant volcanic material, ash beds and volcanic grains, which reflect activity in the Cordilleran Arc to the SW (Blakey and Parnell, 1995).

FIG. 30  Page-lower Carmel paleogeography; see Fig. 26 for names of tectonic and topographic elements. Numerous transgressive-regressive deposits mark the lateral transition between the shallow marine-sabkha Carmel Formation and the eolian Page Sandstone.



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Unconformity J-sup A prominent regional unconformity, J-sup (upper surface on the Page Sandstone), separates the lower and upper portions of the Carmel Formation and related rocks (Blakey et al., 1996). The resulting surface is planar to slightly undulatory and in many places is overlain by a prominent bentonite. In some areas, the surface separates eolian sandstone of the Page Sandstone below from red sandstone and mudstone of the Upper Carmel Formation above. Detailed stratigraphic studies demonstrate that the surface cuts down section to the NE with removal of Page Sandstone; east of the depositional to erosional margin of the Page, the surface amalgamates with the J-2 and truncates the Navajo Sandstone. Locally, eolian sandstone of the Page is overlain by eolian deposits in the Upper Carmel Formation. There a crinkled surface or seam of bentonite marks the unconformity. West of the extent of the Page, the Upper Carmel rests directly on the Lower Carmel and the surface is overlain by discontinuous gypsum deposits. In SW Utah granule to pebble volcanic conglomerate lies on or just above the unconformity. Unconformity J-sup probably correlates with a sequence boundary recognized across the Northern Rocky Mountain region (Brenner and Peterson, 1994) and may mark the Bajocian-Bathonian boundary (Imlay, 1980). Regional stratigraphic and sedimentologic patterns suggest that the unconformity was caused by tectonic events, possibly related to events in the Cordilleran arc (Blakey et al., 1996).

Upper Carmel, Entrada The Upper Carmel Formation is a poorly studied but lithologically variable interval that was deposited across much of the Colorado Plateau (Fig. 31). It contains red sandstone and mudstone of fluvial, sabkha, and eolian origin, marine limestone, fluvial volcanic-pebble conglomerate, and restricted marine gypsum (Blakey et al., 1996).

FIG. 31  Upper Carmel paleogeography; see Fig. 26 for names of tectonic and topographic elements. The intricate shoreline of the upper Carmel seaway was bordered by sabkha, dune, and ephemeral stream environments. Deposits of this widespread interval extend eastward into New Mexico and northward into Montana.

352  The Sedimentary Basins of the United States and Canada

The Entrada Sandstone (Fig.  32) overlies the Upper Carmel Formation. Although the contact has not been studied in detail, at many local outcrops it appears to be conformable and gradational. The Entrada Sandstone was deposited in widespread eolian erg complexes, adjacent to and inland from a restricted marine seaway (Peterson, 1994; Crabaugh and Kocurek, 1993). Although the Entrada is as widespread as the older Navajo eolian system, it does not approach the thickness of the Navajo (Fig. 24). Regional facies patterns and stratigraphy of the Entrada are complex and reflect variations in depositional systems, subsidence rates, and height of the water table (Peterson, 1994; Crabaugh and Kocurek, 1993). The Upper Carmel Formation is Bathonian and the Entrada sandstone is Callovian (Imlay, 1980). The sequence thickens westward but is also truncated above by pre-Cretaceous erosion, so original maximum thickness is difficult to determine; the westward thickening was controlled by subsidence in the Utah-Idaho Trough, possibly an early phase of foreland basin development (Bjerrum and Dorsey, 1995). Abundant volcanic material, especially in the Upper Carmel Formation, reflects activity in the Cordilleran Arc (Blakey and Parnell, 1995; Chapman, 1989).

Unconformity J-3 Unconformity J-3 forms a widespread erosional surface across much of the central and northern Colorado Plateau (Pipiringos and O’Sullivan, 1978). The unconformity is locally angular in central Utah, possibly due to soft-sediment deformation in the underlying Entrada Sandstone (Peterson, 1994). Like many of the Mesozoic unconformities of the Western Interior, it is best defined by regional patterns and is commonly difficult to locate on local outcrop, especially where strata underlying the surface are similar to strata above.

FIG. 32  Entrada (pre-J-3) paleogeography; see Fig. 26 for names of tectonic and topographic elements. Eolian deposition extended east of map area; only the few small areas of remnant Ancestral Rockies shown on map were not buried by Entrada deposition.



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Curtis-Summerville The Curtis-Summerville interval is exposed across much of the central and northern Colorado Plateau (Fig. 33); its extent is limited by erosion associated with the overlying J-5 and sub-Cretaceous unconformities (Peterson, 1988a). The sequence was deposited during the last Jurassic marine incursion into the Western Interior, and although sandstone and mudstone dominate the interval, carbonate content increases northward. The Curtis Formation is chiefly fine-grained, light-colored sandstone and sandy limestone and the overlying and partly coeval Summerville Formation comprises red, thin-bedded sandstone and mudstone. The former was deposited in a varied depositional setting of shallow marine and shoreline deposits and the latter was formed in restricted marine and arid coastal plain settings (Peterson, 1994). The Curtis grades southeastward into eolian deposits of the Moab Tongue of the Entrada Sandstone and the Summerville grades southward into eolian and fluvial sandstone of the Romana Sandstone (Caputo and Pryor, 1991; Peterson, 1994). The Curtis-Summerville sequence marks the oldest interval of the study area that received sediment directly from uplifted Paleozoic rocks of the Cordilleran region (Peterson, 1988b), further evidence of uplift and thrusting to the west during the Jurassic (Bjerrum and Dorsey, 1995). Thus, from Early to Late Jurassic, stream deposits reflect a 180-degree shift of stream flow and change in source from Pangaean terranes and the North American craton at the beginning of the Jurassic to Cordilleran terranes at the end of the period (Blakey, 1994).

FIG.  33  Curtis-Summerville paleogeography; see Fig.  26 for names of tectonic and topographic elements. As the Western Interior Seaway spread eastward across Wyoming (Upper Sundance Sea), the study area lay to its south and east; lime, mud, and sand deposits of the Curtis Formation graded SE into coastal plain redbeds and eolian sand.

354  The Sedimentary Basins of the United States and Canada

Unconformity J-5 The J-5 unconformity is another prominent and significant Early Mesozoic sequence boundary, perhaps second only to the J-2 (Pipiringos and O’Sullivan, 1978). The paleogeology below the surface suggests tectonic warping of the western craton. Currie (1998) suggested that the surface was formed by a migrating forebulge system that signaled early phases of thrusting in the Sevier orogenic belt. The J-5 is best understood through regional study as at many places strata under the surface are nearly identical to strata above (Peterson, 1988a). Controversy surrounds the unconformity, especially in the Four Corners region. Regional stratigraphic studies undertaken by the US Geological Survey (see summary in Peterson, 1988a) demonstrated that significant eolian deposits occur on either side of the surface; post-J-5 eolian deposits in the Morrison Formation represent erg sequences formed downwind from semiarid Morrison streams. The Bluff Sandstone, Junction Creek Sandstone, and Recapture Member of the Morrison Formation contain the largest of these eolian deposits. In contrast, Anderson and Lucas (1994) argued that no significant eolian deposits occur above the J-5 and that rather the above units are part of the Entrada and Summerville intervals and the J-5 occurs higher in the section. My own experience with these rocks strongly supports the US Geological Survey correlations; eolian deposition in the Western Interior ended during the deposition of the Morrison, not before.

Morrison Formation and Younger Mesozoic Events The J-5 unconformity marks the top of the interval of study for this chapter; however, a brief summary of subsequent Mesozoic events is presented to place J-5 and older events in a broader context. For a more detailed review with paleogeographic maps, see Turner and Peterson (2004). The Upper Jurassic Morrison Formation clearly defines the beginning of the dominance of westerly derived coarse-grained detritus onto the Colorado Plateau-Southern Rocky Mountain region. The Morrison has been interpreted as a foreland basin deposit to backbulge deposit (DeCelles and Currie, 1996) associated with Sevier thrusting in the Cordilleran region. Significant uplift of the SW margin of the Colorado Plateau followed Morrison continental deposition to form the pre-Cretaceous (K-0) unconformity; the erosional surface that resulted cuts down section rapidly from south-central Utah to SW Utah and removed perhaps 1000 m of older Jurassic deposits (Peterson, 1988a; Blakey, 1989). The complex subsequent Cretaceous deposition filled the rapidly subsiding Rocky Mountain foreland basin with thousands of meters of siliciclastic Cretaceous deposits that were derived from the Sevier Orogenic Belt (Miall and Catuneanu, this volume). The Rocky Mountain foreland basin area was then subjected to Laramide tectonics during latest Cretaceous and Early Tertiary with sharp uplifted blocks and adjacent foreland basins. Given that the youngest Cretaceous deposits formed in fluvial systems that graded to the retreating Western Interior seaway, most of the several kilometers of uplift that affected the Colorado Plateau and Southern Rocky Mountains took place in latest Cretaceous and Cenozoic.

TECTONIC ORIGINS OF PENNSYLVANIAN-PERMIAN BASINS Introduction Pennsylvanian and Permian basins of the southern Western Interior are difficult to classify by modern classification systems. Most modern basin classification is based partly or entirely on origin and geologic setting (Busby and Ingersoll, 1995) and because the basins of the Ancestral Rockies have controversial origin and setting, in relation to Late Paleozoic tectonics (e.g., Ye et al., 1996), they do not readily fit into modern classification. Therefore, I will use a simple scheme to discuss and classify the basins of the Ancestral Rockies region. I divide them into three general types: (1) yoked, (2) nonyoked, and (3) Cordilleran. The general structure and tectonics and sedimentary history of each is summarized in the following sections.

Yoked Basins Yoked basins (Kay, 1951) are immediately adjacent to faulted Ancestral Rocky Mountain cratonic uplifts (Uncompahgre, San Luis, and Front Range uplifts) and their subsidence history and basin fill are closely related (yoked) to the uplift history of the positive elements (Kluth, 1986; Sloss, 1988). These basins display one or more fault-bounded margins and coarse-grained detritus derived from the adjacent uplift. Structural relief with adjacent uplift is measured in thousands of meters and preserved Late Paleozoic deposits approach or exceed 3000 m (Fig. 5). Yoked basins within the study area are the Paradox Basin, Central Colorado Trough-Eagle Basin, Taos Trough, and ancestral Denver Basin. Yoked basins parallel paleotopographic and structural trends of the Uncompahgre, San Luis, and Front Range uplifts, generally elongated NW-NNW and at right angles to the Ouachita-Marathon fold and thrust belt. Hoy and Ridgway (2002) reviewed previous studies and presented new evidence that documented that some Ancestral Rockies faults are thrust faults (see their Fig. 1)



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and that the Central Colorado Trough and Paradox Basin are flexural foreland basins and that parts of the Aphishapa and Sierra Grande uplifts are forebulges. They did not speculate as to plate tectonic style or setting for the Ancestral Rockies. Other workers have speculated on the tectonic setting and causes of the greater Ancestral Rocky Mountains system. In general, these speculations fall into three hypotheses. The first (Kluth and Coney, 1981; Kluth, 1986; Dickinson and Lawton, 2003) argued for a close cause-and-effect relationship between the Ouachita-Marathon and Ancestral Rocky Mountains systems. These workers cited similarities between timing, rates, and distribution of tectonic events, such as the east to west and outboard to inboard migration of Ancestral Rockies events that parallel similar patterns in the OuachitaMarathon system. To further their arguments, they cited the “unique” setting of the Ancestral Rockies; they lay on the SW peninsular projection of SW North America and the Transcontinental arch. Such a precarious position coupled with tectonic events in the Ouachita-Marathon system may have forced SW North America northward relative to the continent as a whole to produce the uplifts and related (yoked) basins of the Ancestral Rockies system (Kluth and Coney, 1981). A second hypothesis (Stevenson and Baars, 1986) cited structural orientation and style, and sedimentary sequences as evidence that the Ancestral Rockies system was part of a continent-wide wrench-fault system, probably related to complex Late Paleozoic tectonic events across southern North America. Some of the basins were cited as large-scale examples of pull-apart basins. The third hypothesis (Ye et  al., 1996) discounted a close tectonic cause and effect between the Ouachita-Marathon system (or the more easterly Appalachian system) and the Ancestral Rockies. They also argued that the tectonic style of the Ouachita-Marathon system (see Chapter 8), a Carpathian-style orogeny rather than continental collision, was unlikely to trigger tectonic events of the Ancestral Rocky Mountains. Carpathian-style orogenies feature a fast-moving upper plate that comprises oceanic and arc crust or thin continental crust and rarely involve extensive metamorphism or basement-involved uplift. To account for the Ancestral Rocky Mountains, they invoked a relationship between a poorly known Andean-style arc that affected northern Mexico during the Desmoinesian and Permian as the triggering mechanism for Ancestral Rocky Mountain deformation. They compared the tectonic causes and settings of the Ancestral Rockies to those of the Laramide Rocky Mountains, involving shallow subduction of vast plates of the paleo-Pacific region beneath western North America. The preceding works and references cited therein demonstrate the wide-ranging opinions as to structural style and tectonic settings of the Ancestral Rockies system. Perhaps the greatest cause for this disparity is that key elements of the Ancestral Rockies were rejuvenated by later Laramide tectonics (see discussion in Hoy and Ridgway, 2002). The sedimentary fill of yoked Ancestral Rockies basins is marked by rapidly deposited sediment and contrasting lateral and vertical facies changes closely tied to adjacent tectonic events of the adjacent yoked uplifts (Hoy and Ridgway, 2002 and references cited therein). Subsidence patterns were complex, locally very rapid, and not completely synchronous from basin to basin; Kluth and Coney (1981) suggested a general progression of time-transgressive events from SE to NW across the region. In general, grain size and siliciclastic content of sedimentary facies reflect proximity and relief of adjacent uplifts. A general model of yoked-basin deposition is presented in Fig. 34.

FIG. 34  Block stratigraphic diagram showing facies, paleotectonic and paleogeographic setting, and depositional systems in a typical Pennsylvanian yoked basin. Subsidence in the basin is believed to be related (yoked) to adjacent fault-bounded uplift. The basin shown is the southern portion of the Taos trough during the Desmoinesian. The bounding fault may have a thrust component (see Hoy and Ridgway, 2002). (Based on Casey (1980).)

356  The Sedimentary Basins of the United States and Canada

Nonyoked Basins Nonyoked basins comprise an important component of the greater Ancestral Rocky Mountains system and they contrast with yoked basins in several important ways. Coarse-grained arkosic sediment is reduced or absent, total preserved thickness of Pennsylvanian and Permian deposits is thinner, subsidence rates were slower, basins lack major faults and folds at their margins, and they lie adjacent to arches or low uplifts rather than major uplifts (Fig. 5). Within the study area, the nonyoked basins are the Sweetwater Trough, Orogrande Basin, Pedregosa Basin, Holbrook Basin, and Grand Canyon Embayment. Generally excellent outcrop and/or subsurface data have yielded many detailed stratigraphic studies of these basins, but, unfortunately, the subtle Late Paleozoic tectonics within the basins make interpretations of mechanisms of basin subsidence difficult to determine. Transpressional, transtensional, and foreland mechanisms have each been invoked (Soreghan, 1994). Sedimentation in nonyoked basins (Fig. 35) reflects the general tectonic setting, especially with regard to basin edge, shelf, slope, or center (Ross, 1973; Blakey and Knepp, 1989). Soreghan (1994) followed models proposed by Heckel (1991) to demonstrate that much of the cyclicity within the Pedregosa and Orogrande basins was strongly controlled by glacioeustacy. Blakey and Middleton (1983) reached similar conclusions for cyclic eolian and sabkha seposits in the Holbrook Basin. Blakey and Knepp (1989) and Blakey (1990) demonstrated the nonsynchronous nature of subsidence between basins in Arizona and that substantial subsidence occurred in some basins well after the generally accepted end of Ancestral Rockies orogeny. For example, the Grand Canyon Embayment underwent maximum rates of subsidence in the Wolfcampian (near the end of Ancestral Rockies uplift); however, the Holbrook Basin only underwent significant subsidence during the Leonardian—no evidence of basin activity was present before or since that time (Blakey, 1990). The Pedregosa Basin received thick Desmoinesian, Missourian, Wolfcampian, and Leonardian sedimentation (Armin, 1987; Blakey and Knepp, 1989). The nonyoked basins are separated from each other by structurally and topographically higher regions generally referred to as arches, uplifts, or upwarps (Kluth and Coney, 1981; Armin, 1987; Blakey and Knepp, 1989), referred to as arches throughout the remainder of this chapter. Pennsylvanian deposits are thin to absent across the arches, commonly pinching out across the structures; Permian deposits are present across each and reflect varying rates of subsidence across the features (Figs. 4 and 5). Sedimentary facies patterns in regions dominated by nonyoked basins and arches are complex and vary rapidly both vertically and laterally (Figs. 4–14). Carbonate and siliciclastic facies and marine and nonmarine environments are present

FIG. 35  Block stratigraphic diagram showing facies, paleotectonic and paleogeographic setting, and depositional systems in a typical Permian nonyoked basin. The basin has no adjacent fault-related block; rather, these basins tend to be bounded by arches or positive areas. The basin shown is the Grand Canyon embayment during the Wolfcampian. (From Blakey (1990, 1996).)



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in all nonyoked basins. In some cases, thin marine marker beds, usually carbonates, extend long distances from basins onto arches and provide key marker beds in otherwise generally nonfossiliferous deposits (Figs. 5 and 14). Eolian dune lithofacies are particularly well represented in regions south and west of the Uncompahgre Uplift and facies patterns associated with these sandstone units are particularly complex (Blakey et al., 1988; Blakey, 1990, 1996).

Cordilleran Basins Cordilleran basins lie to the west of the Ancestral Rocky Mountains but share strong relations to them and are briefly discussed here. Bissell (1970, 1974) provided stratigraphic syntheses of Pennsylvanian and Permian deposits of the eastern Cordilleran region and emphasized the great thickness and extent of Late Paleozoic sedimentation. Jordan and Douglass (1980) and Geslin (1998) emphasized tectonic control on the enormously thick sedimentary accumulation; the complexity of these basins and their sedimentary fill is beyond the scope of this chapter and interested readers are referred to the cited references.

TECTONIC SETTING OF TRIASSIC BASINS Introduction Triassic sedimentary rocks are widespread across the southern Western Interior. Although sedimentary facies are locally similar, overall stratigraphy and tectonic setting contrast sharply with underlying Upper Paleozoic rocks. The Ancestral Rocky Mountains were greatly reduced in relief and area and had relatively minor effects on sedimentary patterns except immediately adjacent to persisting topographic highs (Dubiel, 1994). Many Triassic basins show little relationship to older Ancestral Rockies elements; compare Figs. 5, 17, and 18. Across much of the study area, Lower and Upper Triassic rock mirror each other in thickness trends as the former thicken to the NW and the latter thicken to the SE (Fig. 16).

Moenkopi Shelf Lower and Middle Triassic sedimentation across most of the Colorado Plateau formed on a broad, extensive shelf that extended westward from the remnants of the Ancestral Rockies to the Wasatch line (Fig.  17). Stewart et  al. (1972b), Blakey (1974), Dubiel (1994), and Blakey et al. (1993) have detailed sedimentary patterns and regional stratigraphy of the Moenkopi Formation. Sedimentation began in the Lower Sythian and continued through several depositional sequences into the Middle Triassic (Blakey et al., 1993). At least as viewed from the Colorado Plateau, sedimentation took place in stable and simple tectonic setting, a tectonic lull between more complex settings.

Eastern Cordilleran Basin West of the Colorado Plateau, Lower and Middle Triassic deposits thicken dramatically across the Wasatch line (Paull and Paull, 1993; Blakey et al., 1993) (Fig. 17). Saleeby and Busby-Spera (1992) and Caravaca et al. (2018) related the westward thickening to various tectonic events in the Cordilleran region including the Sonoman orogeny, the Golconda thrust, and Early Mesozoic development of Cordilleran subduction. See Chapter 11 for a thorough discussion of the Sonoman orogeny. The eastern Cordilleran basin is dominated by thick carbonate and dark mudstone deposits (Paull and Paull, 1993; Dubiel, 1994). Several carbonate tongues extend eastward onto the Colorado Plateau as members of the Moenkopi Formation and probably document marine highstands relative to the Colorado Plateau (Blakey et al., 1993).

Pre-Shinarump Paleovalleys and Shinarump Deposits From both the standpoint of lithologic change and amount relief on the surface, the contact between the Lower and Middle Triassic Moenkopi Formation and Late Triassic Chinle Formation marks the most obvious regional unconformity within the study interval. Regional studies have documented that Shinarump deposits occupy paleovalleys and that those deposits are mostly absent in areas between the paleovalleys (Blakey and Gubitosa, 1983). The paleovalleys are of two general types (Fig. 21): (1) broad paleovalleys generally trend NW-N and are tens of kilometers wide and range to a few tens of meters deep. They contain the greatest volume of the conglomeratic very coarse-grained sandstone typical of the Shinarump Member of the Chinle Formation. The light tan to pale brown, cliff-forming rock presents sharp contrast to the underlying redbeds of the Moenkopi Formation. (2) Narrow paleovalleys are generally within the confines of or immediately adjacent to the boundaries of broad paleovalleys (Figs. 21 and 36); they display relief locally exceeding 75 m (Blakey, 1974). The narrow paleovalley fill ranges from conglomeratic sandstone like that in the broad paleovalleys to carbonaceous mudstone and rip-up clast conglomerate (Blakey and Gubitosa, 1983). The architecture of the strata indicates multiple intervals of

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FIG. 36  Panel cross-sections of Shinarump paleovalleys showing sedimentary architecture of valley fill. (A) General panel showing relations between narrow and broad paleovalleys. (B) Narrow paleovalley filled primarily with coarse sandstone and conglomerate based on outcrops in Vermilion Cliffs, Arizona and Circle Cliffs, Utah. (C) Narrow paleovalley filled with heterolithic sandstone, mudstone, and conglomerate based on outcrops in Monument Valley, Utah-Arizona, Circle Cliffs, Utah, and White Canyon, Utah. (Compiled from outcrop photos and field sketches.)

cut and fill with associated lateral accretion, mass wasting, and bedform migration. All aspects of the deposits indicate that streams were confined to narrow, steep valleys and that sediment reworking was a dominant process. In areas where both paleovalleys are present, superposition clearly indicates that narrow paleovalleys are older than broad paleovalleys. The two paleovalley types clearly indicate that two separate episodes of erosion and deposition occurred following the hiatus between the Middle and Upper Triassic. The narrow paleovalley cutting likely represents an abrupt lowering of base level followed by initial Chinle deposition and then a period of extensive valley widening and succeeding broad braided stream aggradation (Blakey and Gubitosa, 1983). However, it is important to note that initial Shinarump deposition, regardless of paleovalley type, contains clasts derived from external sources relative to the Colorado Plateau; therefore, it seems probable that Shinarump rivers were superimposed across the underlying Moenkopi landscape (as opposed to extensive headward erosion) and that these rivers had their sources to areas distal to the study area. Although tempting to relate these events to worldwide sea level lows or significant climate change, tectonic events in the Cordilleran region may have also been responsible for changing base level conditions.

Chinle Basin Upper Triassic sedimentation mainly took place in a broad sedimentary basin that covered much of the Western Interior (Dubiel, 1994). The basin had two general centers, one in NW Colorado and the other in East-central Arizona (Stewart et al., 1972a). Isopach trends suggest that the remnant Uncompahgre uplift acted as a backbone flanked by thicker deposits on either side (Fig. 18). The Chinle Formation is overlain by the J-0 unconformity, which truncates the formation in a W-SW direction; this partly explains the western thinning, opposite to that of the underlying Moenkopi Formation. The Chinle Basin has been described as a back-arc basin by Blakey and Gubitosa (1983), Dubiel (1994), and Lawton (1994) but as indicated previously, modified by post-Chinle tectonics. I suggest that the Chinle was once more extensive and thicker, especially along the western margin of the Colorado Plateau, and stream trends suggest that it probably flowed to the west of the Plateau where deposits are currently absent, except for an outlier in NE Nevada (Dubiel, 1994). Post-Chinle uplift related to back-arc processes probably produced the noticed westward thinning. Early Mesozoic uplift has been documented in SE California and adjacent Arizona (Reynolds et al., 1989; Walker et al., 1983), possibly related to Late Paleozoic-Early Mesozoic truncation of SW North America (Stone and Stevens, 1988).



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Chinle deposition and stratigraphy are complex but all workers agree that deposition occurred in an exclusively continental basin. Streams flowed generally to the NW across the depocenter (Figs. 21–23), but many variations in stream style and flow have been noted (Blakey and Gubitosa, 1983; Dubiel, 1994; Howell and Blakey, 2013).

TECTONIC SETTING OF JURASSIC BASINS Introduction Jurassic sedimentary basins occupy a transition from older tectonic trends described in the previous sections to foreland basin deposition that dominated Cretaceous sedimentary history (Miall and Catuneanu, this volume). In general, Jurassic rocks thicken abruptly westward along the present margin of the Colorado Plateau and are then truncated by sub-Cretaceous or sub-Tertiary unconformities or Cretaceous thrust faults (Fig. 24). Thinning to the SW was more gradual and the past extent of Jurassic rocks in that direction can only be estimated, as the south-facing Jurassic escarpment is today well north of areas of previous deposition (Figs. 25 and 26).

Zuni Sag The Zuni sag is a subtle negative feature better reflected in facies patterns than in isopach trends (Fig. 25). The feature lies at the foot of the Mogollon slope, a controversial upland area (Bilodeau, 1986; Riggs and Blakey, 1993), and trends NW across the SW Colorado Plateau. This trend was the locus of NW-flowing Glen Canyon streams, which flowed into the Utah-Idaho Trough (Blakey, 1994). The Zuni sag is also the site of extensive sabkha deposits in the Entrada Sandstone and a prominent fluvial-eolian facies change in the Morrison Formation (Blakey et al., 1988). The Zuni sag probably was related to back-arc subsidence to the Jurassic Cordilleran arc.

Utah-Idaho Trough The dominant basin during Jurassic sedimentation across the Western Interior was the Utah-Idaho Trough (Imlay, 1980; Peterson, 1994). The basin lies along and west of the Wasatch line (Fig. 25). Jurassic rocks thicken sharply into the trough beginning during Glen Canyon deposition, but especially during deposition of the San Rafael Group (Bjerrum and Dorsey, 1995). The basin margin also marks prominent facies changes from dominantly eolian deposition to the east to dominantly sabkhamarine deposition to the west (Blakey et al., 1988, 1996; Peterson, 1994; Blakey, 1994). Debate continues as to classification and origin of the Utah-Idaho Trough. Bjerrum and Dorsey (1995) carried out a detailed numerical basin analysis to document early foreland basin development as the cause of subsidence. The subsidence was tied to foreland thrusting in early phases of the Nevadan orogeny to the west. Others have argued that true foreland basin subsidence did not initiate until the Early Cretaceous and that the Utah-Idaho Trough was related to dynamic back-arc subsidence (see discussion in Lawton, 1994).

DISCUSSION: TECTONIC EVOLUTION AND CONTROLS ON DEPOSITION Tectonic Sequence of Events Southwestern North America underwent several changes in tectonic setting from the Early Paleozoic through the Mesozoic. The following events are summarized from Marzolf (1990), Miller et al. (1992b), Burchfiel et al. (1992), and Saleeby and Busby-Spera (1992). From Late Precambrian through Devonian, the region was part of a passive margin formed after rifting between North America and Gondwana. The study area was located well within the confines of the North American craton and received chiefly shallow marine and shoreline sedimentation. From the latest Devonian into the Mississippian, an arc collided with the western margin of the continent causing the Antler orogeny and signifying a change in tectonic setting. During the Pennsylvanian, the Ancestral Rocky Mountains formed, although the tectonic setting or cause of this event is not clearly understood. The study area straddled and bordered Ancestral Rocky Mountains uplifts. During the latest Paleozoic and earliest Mesozoic, another arc collided with the west edge of the continent forming the Sonoman orogeny (Chapter 11); events were well west of the study area. During the Jurassic, a series of terranes collided with the western margin of the continent and the configuration of the Cordilleran arc changed. Collectively, these events produced the Nevadan orogeny, also well to the west of the study area. Sometime (or times) between the Pennsylvanian and Late Jurassic, the SW margin of North America was truncated and apparently transported to the SE into Mexico along the Sonoran-Mojave megashear; see Anderson et al. (2005) for multiple views concerning this controversial feature and the chronology of events associated with it. Continued terrane collision and arc reorganization generated the Sevier orogeny throughout the Cordilleran region

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and into the study area. The exact beginning of the Sevier orogeny, Middle Jurassic or Early Cretaceous, remains in debate. The entire western third of the United States was affected by Late Sevier and Laramide (Rocky Mountain) tectonic events from Late Cretaceous through the Eocene. West of the study area, Sevier and Laramide compressional tectonics and Late Cenozoic crustal extension, which is still occurring in some regions, greatly modified the earlier tectonic events that are covered in this chapter. However, on the Colorado Plateau, later events had little effect on earlier events except for uplift, broad folding, and exposure of Late Paleozoic and Mesozoic rocks. In the Southern Rocky Mountains, Laramide structure has overprinted older Ancestral Rocky Mountain structures and has made interpretations of the latter difficult. Devonian through Jurassic tectonic events marginal to the study area had various effects on its sedimentary history, especially in the behavior and flow of rivers. Significant sedimentological change appears in the Early and Middle Pennsylvanian, coincident with Ancestral Rocky Mountains tectonics and late stages of the Antler and related cordilleran tectonic events; following 200 million years of carbonate shelf sedimentation, siliciclastic sedimentation appeared across much of the Colorado Plateau and Southern Rocky Mountains region. Adjacent to the Ancestral Rocky Mountains, coarse arkosic deposits grade laterally into sandstone, mudstone, and carbonate; fluvial paleocurrents radiate from uplifted areas toward adjacent basins. By Late Pennsylvanian and Early Permian, sandstone and mudstone dominated all but the most distal parts of the region; fluvial paleocurrents suggest that rivers drained westward into the Cordilleran seaway and south and southeastward into basins of Texas, and southern New Mexico and Arizona. Carbonate sedimentation returned across the western portion of the study area during the last Permian marine highstands, but fine-grained redbeds and evaporates continued to persist across the eastern portion of the region. Triassic and Early Jurassic river systems of the study area reflect the first-order tectonic grain of North America: rivers flowed west and northwest from the Appalachian continental divide toward the paleo-Pacific Ocean. A sharp increase in grain size of externally sourced clasts is observed in the Upper Triassic Chinle Formation, reflecting changes in source-terrane tectonics, climate, base level, or perhaps all three. In fact, from a pure sedimentologic point of view across the Colorado Plateau, the big change occurred with the initiation of preChinle paleovalleys and continued during subsequent Chinle deposition. The first input of Cordilleran volcanics onto the Colorado Plateau also occurred in Chinle time (Howell and Blakey, 2013). Not coincidentally, this may mark the time when western North America underwent significant tectonic changes following the Sonoman orogeny (Chapter 11). Westward thickening of Lower Triassic marine deposits possibly reflects back-arc or foreland basin tectonics related to the Sonoman orogeny. Back-arc basin development, dynamic subsidence, and possible early foreland basin subsidence are apparent across much of the Colorado Plateau during the Late Triassic and Jurassic (Lawton, 1994; Chapter 11). During the Jurassic, fluvial paleocurrents from the west reflect sources to the southwest and west and are recorded in the Middle Jurassic Carmel Formation and Late Jurassic Summerville and Morrison formations (Blakey et al., 1996; Peterson, 1988a). These changes reflect the evolving and maturing Cordilleran arc and developing Nevadan orogeny to the west.

Climatic Controls Coincident with evolving tectonic conditions across the study area were evolving climatic settings. The following is summarized from Blakey et al. (1988), Peterson (1984, 1988c), Parrish and Peterson (1988), Dubiel (1994), and Blakey (1994, 1996). During the Early and Middle Paleozoic, the region was generally within equatorial zones, although specific climatic indicators such as evaporites, redbeds, and coals are absent. By Pennsylvanian time, most of the study area was within a large tropical desert on the western margins of Pangaea. Widespread eolian sandstone, redbeds, evaporites, and marine faunal elements confirm this setting. An arid setting continued throuout the Permian (redbeds, eolian sandstone, evaporites) and into the Triassic (redbeds, minor evaporites, minor eolian sandstone). However, by Late Triassic Chinle deposition, conditions had become more humid, probably monsoonal as documented by fluvial style and abundant flora. Redbeds and eolian sandstone in the Upper Chinle document a return toward more arid conditions. The entire Early and Middle Jurassic were dominated by arid conditions (widespread eolian deposits, redbeds, evaporites) that probably became more monsoonal in the Late Jurassic during Morrison deposition (flora, stream style, scattered eolian deposits). Morrison paleowind directions also reflect North America’s journey from the tradewind belt into the westerlies belt. Humidity increased until, by Middle Cretaceous, greenhouse conditions dominated the region.

Eustatic Controls Eustatic controls played important roles on deposition of rocks of the study area, and several marine marker beds that penetrate continental sediments document marine highstands relative to the study area. A summary of the stratigraphic record compared to global sea-level charts of Ross and Ross (1988) and Haq (1991) suggests a general correlation (Fig. 37). Soreghan (1994) documented Pennsylvanian eustatic control on deposits in the Pedregosa and Orogrande basins and



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FIG. 37  Chart comparing Pennsylvanian-Jurassic time scale with sea-level curves (Ross and Ross, 1988; Haq, 1991), Sloss sequences (Sloss, 1988), and the stratigraphic sequences and unconformities of this report. Vertical scale in millions of years. On the column, black represents unconformities and yellow depositional sequences. The beginning and ending of sequences is approximated from the multitudes of data used in preparing this report, and are, of course, full of lesser unconformities. Asterisks signify high-water periods on the Colorado Plateau as marked by marine sequences that penetrate the dominantly continental section: (1) Morrowan marine limestone, western Grand Canyon; (2) Atokan marine limestone, western Grand Canyon; (3) Desmoinesian marine carbonates, Mogollon Rim, Arizona; (4) Virgilian marine limestone, western Grand Canyon; (5) Wolfcampian marine limestone (Pakoon Formation), western Grand Canyon and (Elephand Canyon Formation) east-central Utah; (6) Fort Apache Limestone Member, Schnebly Hill Formation, Mogollon Rim; (7) marine limestone, Toroweap Formation (Brady Canyon Member), western Colorado Plateau; (8) marine limestone Kaibab and San Andres formations, western and southern Colorado Plateau; (9) Timpoweap-Sinbad limestones; (10) Virgin limestone, Moenkopi Formation, western Colorado Plateau; (11–12) marine carbonates of Judd Hollow Member (penetrating eolian Page Sandstone), south-central Utah; (13) marine limestone marker, upper Carmel Formation (Paria River Member), south-central Utah; (14) marine sandstone, Curtis Formation, central Utah. Strata between upper Moenkopi Formation and Temple Cap Formation lack marine deposits in the study area.

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Goldhammer et al. (1994) have documented eustatic controls on sedimentation in the Paradox Basin. Blakey and Middleton (1983) documented eustatic controls on eolian-sabkha deposits of Permian age in the Holbrook Basin; the marine marker bed Fort Apache Member of the Schnebly Hill Formation may correspond to a Leonardian global highstand (Blakey, 1990). The moderately thick Triassic continental succession across the region corresponds to global sea-level lows; Lupe and Silberling (1985) related sequences in the Chinle Formation to sea-level cycles in western Nevada. The incredible thickness of Navajo Sandstone in the Utah-Idaho Trough contains no known marine tongues, perhaps because the low Early Jurassic sea level could not flood even the rapidly subsiding continental basin. Middle Jurassic global highstands may be reflected in marine tongues in the Carmel and Curtis formations.

SUMMARY Pennsylvanian through Jurassic sedimentary rocks on the Colorado Plateau, Southern Rocky Mountains, and vicinity comprise one of the most enduring continental-dominated stratigraphic assemblages in the rock record. Mostly well-exposed outcrops supplemented by subsurface data provide the basis for physical correlation; marine incursions of fossiliferous limestone and mudstone aid in age determination. Recent detrital zircon studies have further tightened age assignments, especially in the Jurassic. The interpretations of these rocks yield important information concerning the geologic evolution of Southwest North American and western Pangaea. Unlike most other parts of cratonic North America, the various sedimentary basins were rather short-lived and of diverse origins. Prior to the Pennsylvanian, the region was mostly a broad shelf adjacent to a passive margin. During the Pennsylvanian and Permian, numerous basins developed across the region, mostly in concert with the uplift of the enigmatic Ancestral Rocky Mountains orogen. During the Early Mesozoic, the region was in the back arc of the developing Cordilleran arc. Probable causes of subsidence included back-arc subsidence, dynamic subsidence, and foreland basin subsidence. Throughout the long Late Paleozoic and Early Mesozoic history of the region, source areas changed significantly. Through the Paleozoic until the Early Pennsylvania, sediment was shed off the Transcontinental arch. During the Late Paleozoic into the Early Jurassic, sediment was locally derived from the Ancestral Rockies; large transcontinental rivers and dune systems brought in vast amounts of sediment, mostly sand and wind-blown silt (loess), from uplands in the Appalachians and Canadian Shield. From the Late Triassic into the Cenozoic, the Cordilleran arc shed sediment eastward into the study area. Detrital zircon studies confirm these hypotheses. Finally, this region contains the greatest concentration of protected parklands on Earth. Many of the parks, monuments, and wilderness areas are focused on the colorful, widely exposed, incredibly dissected rocks of Late Paleozoic and Mesozoic age that blanket this region.

ACKNOWLEDGMENTS Although much of this chapter reflects my 35 years of work with the sedimentary rocks on the Colorado Plateau, many of the data and conclusions are based on 35 Master’s theses and I am indebted to these students and their dedication, persistence, and scholarship. Discussions, debates, and collaboration with many other workers have greatly enriched my knowledge of the region; I especially thank Don Baars, Stan Beus, Margie Chan, Becky Dorsey, Lars Clemmensen, Bill Dickinson, Bob Dott, Russ Dubiel, Bill Furnish, Karen Havholm, Phil Heckel, Ray Ingersol, Chuck Kluth, Gary Kocurek, Bart Kowallis, Rip Langford, Tim Lawton, Dave Loope, Andrew Miall, Larry Middleton, Mike Morales, Bill Parker, Rod Parnell, Judy Parrish, Wes Peirce, Pete Peterson, Wayne Ranney, Steve Reynolds, Nancy Riggs, Lynn Soreghan, Lee Stokes, Christine Turner, and Paul Umhoefer. Tim Lawton and Andrew Miall provided formal reviews of the first edition of this manuscript and David Houseknecht provided careful review of the present manuscript.

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Baars, D.L., and Stevenson, G.M., 1981, Tectonic evolution of the Paradox Basin: Utah and Colorado, in Wiegand, D.L., ed., Geology of the Paradox Basin: RMAG, Denver, Rocky Mountain Association of Geologists, Field Conference Guidebook, RMAG, p. 23–31. Bilodeau, W.L., 1986, The Mesozoic Mogollon highlands, Arizona: An Early Cretaceous rift shoulder: Journal of Geology, v. 94, p. 724–735. Bissell, H.J., 1970, Realms of Permian tectonism and sedimentation in western Utah and eastern Nevada: Bulletin of the American Association of Petroleum Geologists, v. 54, p. 285–312. Bissell, H.J., 1974, Tectonic control of Late Paleozoic and Early Mesozoic sedimentation near the hinge line of the Cordilleran miogeosynclinal belt, in Dickinson, W.R., ed., Tectonics and Sedimentation: SEPM, Special Publication 22, p. 83–97. 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Blakey, R.C., Peterson, F., and Kocurek, G., 1988, Late Paleozoic and Mesozoic eolian deposits of the Western Interior of the United States: Sedimentary Geology, v. 56, p. 3–125. Blakey, R.C., Basham, E.L., and Cook, M.J., 1993, Early and Middle Triassic paleogeography, Colorado Plateau and vicinity, in Morales, M., ed., Aspects of Mesozoic Geology and Paleontology of the Colorado Plateau: Museum of Northern Arizona, Bulletin 59, p. 13–26. Blakey, R.C., Havholm, K.G., and Jones, L.S., 1996, Stratigraphic analysis of eolian interactions with marine and fluvial deposits, middle Jurassic Page Sandstone and Carmel Formation, Colorado Plateau, USA: Journal of Sedimentary Research, v. 66, p. 324–342. Blodgett, R.H., 1988, Calcareous Paleosols in the Triassic Dolores Formation: Southwestern Colorado, Boulder: Geological Society of America, Special Paper 216, p. 103–121. 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