Regional high-resolution correlations of the Campanian Desert Member of the Blackhawk Formation to the lower Castlegate Sandstone interval, Book Cliffs, Utah–Colorado, show no evidence for a single, throughgoing unconformity separating the amalgamated channel sandstones of the Castlegate from the underlying Blackhawk Formation coastal-plain deposits. This is a facies contact, which interfingers both laterally and vertically. Channel incisions are discrete and mostly confined to individual parasequences. They do not coalesce landward into gigantic valleys as shown in the conventional model but are restricted to the proximal shoreface and are rarely traceable for more than a few kilometers landward. Timelines extend uninterrupted from the shallow marine into the neighboring coastal-plain deposits, connecting flooding surfaces with coals. Shoreface sandstones emerge from the adjacent coastal plain. This has significant implications for modeling in similar clastic settings worldwide. Conventional sequence stratigraphic models of fluviodeltaic systems show radically different stacking patterns compared to those of the high-resolution model presented herein. Each model carries a significantly different prediction regarding the three-dimensional distribution and architecture of reservoir and nonreservoir bodies in a fluviodeltaic system. Hydrocarbon exploration and development activities, such as optimally locating seismic surveys and/or drilling locations, could be impacted if the wrong model is applied. As such, it is recommended that fluviodeltaic systems worldwide should be critically reexamined in light of the high-resolution sequence stratigraphic model presented herein.
Identification and correlation of sequence boundaries are a central tenet in the application of sequence stratigraphy (Posamentier and Vail, 1988; Posamentier et al., 1988; Van Wagoner et al., 1988, 1990). According to conventional models (Van Wagoner et al., 1990), adjacent terrestrial and marine rock packages are usually juxtaposed on either side of the sequence boundary and are generally thought to have no genetic, temporal, or spatial linkage. This leads to fundamental queries on the origin and preservation of adjacent, time-equivalent depositional systems. (1) Where are the comparable deposits for each facies belt? (2) Why were they not deposited nearby? (3) Are single, throughgoing sequence boundaries with concomitant sediment bypass the norm or the exception?
Insights gleaned from studies of the Quaternary Mississippi River system (Blum and Törnqvist, 2000; Blum et al., 2013), Mediterranean Po River Basin (Amorosi et al., 2016, 2017), and other Quaternary systems (Blum and Törnqvist, 2000; Blum and Aslan, 2006) demonstrate genetic, temporal, and spatial linkage is possible between adjoining fluvial coastal-plain and deltaic shoreface deposits during falling sea level (Blum and Törnqvist, 2000; Blum et al., 2013; Amorosi et al., 2016, 2017). These studies challenge the conventional thinking of diachroneity across a third- and fourth-order sequence boundary.
Miall (2016) notes that studying the Pleistocene–Holocene might not be the complete answer for understanding sequence boundaries in the ancient rock record simply because the time frame is short. Experimental studies show incised valleys as composite features, which evolve dynamically over both falling and rising limbs of a base-level cycle (Strong and Paola, 2008, 2010). Holbrook (2010) argues that observations and interpretations of modern and ancient valley-fill deposits need to be combined with experimental work to properly understand the genesis and history of valley systems. Early attempts at integrating flume and field data have revealed that the subaerial component of a sequence boundary (i.e., base of a valley) is not a single, throughgoing surface reflecting significant sediment bypass, nor is it an unconformity or time barrier separating all valley-fill deposits from those that the valley cuts (Holbrook and Bhattacharya, 2012). Collectively, these studies shed uncertainty on the utility of the conventional sequence stratigraphic model in defining the genesis, evolution, and correlation of incised valley fills and their sequence boundaries.
There exists a clear need to critically reexamine the conventional sequence stratigraphic model in the context of a high-resolution, comprehensive outcrop data set. Despite recent experimental and Quaternary-based studies that have highlighted shortcomings and oversimplification of the conventional sequence stratigraphic model, there is little data from the rock record partially because the field-based observations are hampered by poor-quality outcrop. To address these problems, local- to regional-scale laterally continuous and accessible outcrops are needed. Unfortunately, few outcrops can meet these requirements.
Figure 1. Map of key structural features in Utah (modified from Horton et al., 2004). Location of Utah on the United States inset map. CO = Colorado; Mtns = Mountains; UT = Utah; WY = Wyoming.
One of the few areas in the world where this is possible is the Book Cliffs of eastcentral Utah and western Colorado (Figures 1, 2). This classic outcrop belt has 300+ km (187+ mi) of wonderfully exposed braided and meandering fluvial, coastal-plain, estuarine-tidal, and shoreface-to-shelf strata. It has been used over the years as an outdoor laboratory for siliciclastic sequence stratigraphy (Van Wagoner et al., 1990), clastic facies modeling (e.g., Spieker, 1949; Young, 1955, 1957), tectono-stratigraphic relationships in foreland basins (e.g., Fouch et al., 1983; Lawton, 1986; Horton et al., 2004), and applied petroleum-industry analog studies (e.g., Balsley, 1980; Van Wagoner et al., 1990).
Figure 2. Location map showing the entire Book Cliffs regions, from Helper, Utah (UT), to Grand Junction, Colorado (CO). The inset map shows details within the central core of the study area. A total of 45 outcrop measured sections of Desert–Castlegate (gray circles, numbered 1 to 45), 111 well logs nearest to the Book Cliffs outcrop belt (white circles), and 2 Shell cores (black circles) are shown. Depositional dip–oriented cross sections in Figures 8, 9, and 12 are shown (solid black lines and figure numbers) as well as photograph localities in Figures 6, 7, 10, 11, and 13. Distances between adjacent outcrop measured sections are projected orthogonal to the N14°E paleoshoreline trend. Key canyons, creeks, mesas, washes, buttes, and other landmarks are abbreviated. Oversized labels correspond to localities listed in headings (i.e., Helper, DS, BB, CrC, HH, and MR). See legend for full names. Cyn = Canyon; I-70 = Interstate 70; Mt = Mount.
The Desert Member–lower Castlegate Sandstone stratigraphic interval is arguably one of the most intensely studied and visited intervals within the Campanian Book Cliffs (Van Wagoner, 1991, 1995; Nummedal et al., 2001; Young, 2001; Shanley et al., 2003). Conventional sequence stratigraphic interpretations recognize two third-order sequence boundaries (SB) in the Desert–Castlegate interval, Desert-SB (D-SB) and Castlegate-SB (C-SB), and correlate both as single, throughgoing, regional unconformities (Van Wagoner, 1991, 1995; Miall, 1993, 2014, 2016; Yoshida et al., 1996, 1998; McLaurin and Steel, 2000, 2007; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; Hettinger and Kirschbaum, 2002; Hampson, 2010, 2016; Seymour and Fielding, 2013; Hampson et al., 2014; Cross, 2016).
Van Wagoner (1995) also identified six high-frequency sequence boundaries (HFSBs) in the Desert–Castlegate, warranting numerous questions regarding the origin and relationship of HFSBs and third-order SBs. For example, are the third-order D-SB and C-SB an amalgamation of multiple higher-frequency surfaces or are they single, throughgoing unconformities? What does this suggest for a correlation of the laterally adjacent facies belts? Are the Castlegate channels completely unrelated to the Desert–Castlegate shorefaces farther east? Detailed local-to-regional correlations are needed to address these and other fundamental questions regarding the origin and relationship between the channel, coastal-plain, and shoreface deposits in the Desert–Castlegate stratigraphic interval (Miall, 1993, 2014; Adams and Bhattacharya, 2005; Pattison, 2010; Bhattacharya, 2011). The answers should be broadly applicable to modern and ancient nearshore, terrestrial–to–shallow-marine systems worldwide.
Specific objectives of this study are fourfold. (1) Reexamine the lateral and vertical relationships between the Desert Member and lower Castlegate Sandstone, integrating previous work with the new data presented herein. (2) Critically reevaluate the lithostratigraphy and sequence stratigraphy of the Desert–Castlegate interval, paying particular attention to the description, mapping, and correlation of facies belts (i.e., amalgamated fluvial sandstones, coal-bearing coastal-plain deposits, shallow-marine sandstones), and key sequence stratigraphic surfaces (i.e., sequence boundaries, marine flooding surfaces, coal zones). (3) Examine the sedimentology, sedimentary architecture, and sequence stratigraphy of the basal lower Castlegate Sandstone and uppermost Blackhawk Formation, from proximal to distal, looking for evidence of a single, throughgoing unconformity or sequence boundary. (4) Construct a high-resolution sequence stratigraphic framework for the Desert–Castlegate interval that could be used for modeling and prediction in similar clastic settings worldwide.
REGIONAL SETTING AND STRUCTURE
Campanian-aged (Upper Cretaceous) Book Cliffs strata were deposited into the Cordilleran foreland basin through central Utah and western Colorado (Jordan, 1981). This basin developed in response to the Sevier orogeny and was filled with sediments sourced from the Pavant, Canyon Range, and/or Charleston–Nebo thrust belts to the west (Figure 1) (Armstrong, 1968; Lawton, 1985; Horton et al., 2004; DeCelles and Coogan, 2006). Spieker and Reeside (1925, p. 446) note the abrupt introduction of “grits and conglomerates” in the Castlegate Sandstone Member of the Price River Formation and attribute this coarse influx to orogenic movement that increased the coastal-plain gradient and hence the carrying capacity of the streams and rivers. Horton et al. (2004) used a comprehensive seismic, borehole, and outcrop database (i.e., sedimentology, petrology, biostratigraphy) to correlate the timing of active thrusting and sediment influx from the Charleston–Nebo thrust system with the rapid progradation of the Castlegate Sandstone. Collectively, these studies support an orogenic interpretation for the strongly progradational Castlegate Sandstone. Widespread Neogene uplift of the Colorado Plateau lead to the near-horizontal preservation of stratigraphic rock packages across a wide area of eastcentral Utah and western Colorado. As a result, Book Cliffs strata are relatively undeformed with low-angle (<5°) structural dips to the north. Small- to moderate-sized, eastward-flowing rivers transported significant volumes of sediment into the Cretaceous Western Interior seaway, depositing thick successions of sands, silts, and clays. Shorelines were generally oriented north to south with offshore to the east (McGookey et al., 1972).
STUDY AREA AND DATABASE
The study area extends from the westernmost Book Cliffs at Castle Gate, Utah, eastward to their terminus near Grand Junction, Colorado, a distance of approximately 300 km (∼187 mi) (Figure 2). This outcrop belt has some of the best exposures of fluviodeltaic deposits in the world. Numerous side canyons and reentrants dissect the main cliffline, allowing for unparalleled three-dimensional visualization and access. An extensive database, compiled over 25 yr of field work, consists of 45 outcrop measured sections (see Appendices 1 and 2; Datashare 117 at www.aapg.org/datashare), 31,588 high-resolution digital photographs, 6265 paleocurrent measurements, 335 rock samples, 147 thin sections, 111 well logs nearest to the Book Cliffs, and 2 cores (Figure 2). The measured sections integrate high-resolution sedimentological and stratigraphic observations at a millimeter to centimeter scale, allowing outcrop correlation of sedimentary architecture, facies, and surfaces. High-resolution field data were collected at each measured section, supplemented with nearby observations of sedimentary architecture and facies geometries leading to a high-resolution correlation framework. Every canyon, ridge line, and side canyon were traversed and studied from Tusher Canyon to Corral Point, representing the highest density field work in this study (Figure 2). Moderate-density field work (i.e., all canyons accessible by road, with some infill ridge-line hiking between adjacent canyons, supplemented with desert-floor photographs) was carried out from Castle Gate to Gunnison Butte and from Cottonwood Wash to West Salt Creek, Colorado. Low-density field work (i.e., select canyons only; desert-floor photographic database between canyons visited) was conducted from East Salt Creek to Grand Junction, Colorado (Figure 2).
One of the earliest, formal, stratigraphic uses of the term Castlegate was by Spieker and Reeside (1925) who designated the lower part of the Price River Formation as the Castlegate Sandstone Member (Figure 3). At the type section, the Castlegate consists of medium–to–coarse-grained sandstones, associated with lenses of quartz and chert pebbles, little shale, and no coal (Figure 4; Table 1) (Spieker and Reeside, 1925; Spieker, 1931, 1949). The Castlegate Sandstone Member can be recognized in outcrop for more than 200 km (>125 mi) from the Wasatch Plateau to its pinch out in the eastern Book Cliffs, thus forming the “great tongue of the Castlegate sandstone” (Spieker, 1949, p. 70).
Figure 3. Scaled stratigraphic dip–oriented cross section of the Book Cliffs region, Helper, Utah (UT), to western Colorado (CO) (modified from Young, 1955; Cole et al., 1997; Pattison et al., 2007; Pattison, 2010, 2018, 2019). Member (Mb.) boundaries have been extended westward into the undifferentiated Blackhawk Formation (BH Fm.) coastal plain and eastward into the Mancos Shale. The Blackhawk Formation and lower Castlegate Sandstone (C.Ss.) interfinger both laterally and vertically. This is a facies contact, not an unconformity. Key localities are shown along the top. C1–C6 = lower Castlegate Sandstone parasequences 1–6; CH = shoreface-incised channel; Cyn = Canyon; D1–D11 = Desert Member parasequences 1–11; G1–G2 = Grassy Member parasequences 1 and 2; Horse Canyon-N = Horse Canyon North; K1–K4 = Kenilworth Member parasequences 1–4; S1–S3 = Sunnyside Member parasequences 1–3; Ss = Sandstone; VE = vertical exaggeration.
Fisher (1936) expanded the definition of the Castlegate Sandstone to include a significant part of the overlying Price River Formation, thus effectively forming an upper and lower Castlegate Sandstone, the latter of which is equivalent to the original Castlegate Sandstone Member defined by Spieker and Reeside (1925). Fisher et al. (1960) elevated the Castlegate Sandstone to formational status by removing it from the lower part of the Price River Formation and placing it within its own lithostratigraphic formation. The lower Castlegate Sandstone thins to the east and reaches a feather-edge at Corral Point in the eastern Book Cliffs, approximately 30 km (∼19 mi) west of the Utah–Colorado border (Young, 1955).
Figure 4. Type section localities. (A) The eastern Wasatch Plateau–northwestern Book Cliffs region showing location of Castlegate Sandstone (red star), Blackhawk Formation (green star), and Star Point Sandstone (yellow star) type localities (Spieker and Reeside, 1925). Measured outcrop section 1 (purple circle) is shown near Castle Gate. (B) Southcentral Book Cliffs near Green River, Utah. The Desert Member type section (yellow star) is located as per Young (1955). Measured outcrop sections 4–9 are highlighted (purple circles). First appearance of Desert Member parasequences 1–3 (numbered green lines) are plotted. BB = Battleship Butte; BCB = Blue Castle Butte; DS = Desert Siding; GB = Gunnison Butte; GV = Gunnison Valley; I-70 = Interstate 70; LE = Little Elliot; ME = Mount Elliott; MM = Middle Mountain; No. = Number; PRC = Price River Canyon; Sec = Section; SLC = Salt Lake City; TC = Tusher Canyon; TCv = The Cove.
In contrast to the original lithostratigraphic definition at the type section, coal-bearing, mudstone-rich strata, including those deposited in lagoons, estuaries, flood plains, and swamps, are the dominant lithologies in the lower Castlegate Sandstone from Trough Spring Ridge to Horse Heaven, a distance greater than 10 km (>6 mi) (Spieker and Reeside, 1925; Spieker, 1931, 1949; Young, 1955). These lithologies also cap the lower Castlegate Sandstone across a broad swath of the Book Cliffs, from Woodside eastward to Nash Wash, a distance of 60+ km (37+ mi) along depositional dip. The uppermost package of coal-bearing coastal-plain strata is 2–12 m (6–39 ft) thick.
The significance of these observations is threefold. First, the lower Castlegate Sandstone has gradational and interfingered lower and upper boundaries with coal-bearing, mudstone-rich lithologies that are identical to the type section description of the Blackhawk Formation (Figure 4; Table 1). Second, sandstone-dominated lithologies grade eastward into coal-bearing coastal-plain mudstones, which in turn grade eastward into shallow-marine sandstones. Third, all three facies belts (i.e., amalgamated channel sandstones, coal-bearing coastal-plain strata, and shoreface sandstones) comprise the cliff-forming lower Castlegate great tongue of sandstone, not just the amalgamated channel sandstones as per conventional thinking (Figure 3). Bhattacharya (2011) speculates on the missing floodplain fines in the lower Castlegate Sandstone interval and suggests those muds were bypassed via terminal distributary channels into the Mancos sea. Lithostratigraphic and sequence stratigraphic data presented herein show no such missing muds and hence no reason to invoke mud bypass.
Spieker and Reeside (1925) formally named the sandstone, shale, and coal-bearing unit that underlies the lower Castlegate Sandstone as the Blackhawk Formation after the now-abandoned Blackhawk mine (Figure 4; Table 1). It consists of carbonaceous-rich shale, coal, and fine–to–medium-grained sandstones containing no pebbles (Spieker and Reeside, 1925; Spieker, 1931, 1949). Up to one-half of the Blackhawk Formation consists of carbonaceous-rich shale and coal at the type section; the remainder is terrestrial sandstones (Table 1) (Spieker, 1931). The Blackhawk Formation is up to 350 m (1148 ft) thick in the extreme western part of the Book Cliffs and thins progressively eastward (Spieker and Reeside, 1925). A shallow-marine sandstone demarcates the feathered-edge terminus of the Blackhawk Formation in the Corral Point–Cottonwood Canyon region of eastern Utah (Young, 1955).
The Blackhawk Formation grades basinward (eastward) into multiple littoral marine sandstones (Spieker and Reeside, 1925; Spieker, 1931). Young (1955) subdivided the Blackhawk Formation into six members: Spring Canyon, Aberdeen, Kenilworth, Sunnyside, Grassy, and Desert (Figure 3). All six members of the Blackhawk Formation contain coal-bearing lagoonal strata and littoral marine sandstones (Young, 1955, 1957). The former includes lagoonal, estuarine, floodplain, swamp, and lowland deposits that were in close proximity to the sea on a low-relief coastal plain, adjacent to the shallow-marine facies belt (Young, 1955). The Desert Member is the uppermost or youngest unit in the Blackhawk Formation and comprises the shoreface sandstones and overlying coal-bearing strata directly beneath the Castlegate Sandstone (Young, 1955). This member gets its name from a railway siding, which is located 13 km (8.1 mi) north of Interstate 70 on the east side of Highway 6-191, approximately 6 km (∼4 mi) west of Little Elliott Mesa (Figure 4; Table 1).
Although not the focus of this study, the Mancos Shale dominates the Campanian stratigraphy in the eastern Book Cliffs, where it reaches a maximum thickness of approximately 1220 m (∼4000 ft) (Hettinger and Kirschbaum, 2002). Both the lower Castlegate Sandstone and Blackhawk Formation grade eastward into this mudstone belt, whereas tongues of Mancos Shale extend westward partitioning the nearshore stratigraphy (Figure 3). One of the more prominent examples, the Buck Tongue of the Mancos Shale, blankets the top of the lower Castlegate Sandstone across the eastern half of the study area, from Woodside, Utah, to West Salt Creek, Colorado (Figure 3). The Buck Tongue separates the overlying Sego Sandstone from the underlying lower Castlegate Sandstone, forming a wedge-shaped, eastward-thickening geometry, from its zero isopach edge near Woodside to a maximum thickness of 116 m (380 ft) in western Colorado (Young, 1955; Hettinger and Kirschbaum, 2002).
All six members of the Blackhawk Formation were originally defined via recognition and correlation of Mancos Shale tongues that break up the overall progradational stack of shoreface–deltaic sandstones (Young, 1955). Member-bounding flooding surfaces can be tracked into the Mancos Shale mudstone belt via grain size and cementation changes that are manifested as lighter color bands in outcrop (Figure 3; Pattison, 2005). One example is the top of the Grassy Member, which has been correlated for more than 100 km (>62 mi) basinward of its sandstone pinch-out edge, using both outcrop and subsurface data (Hettinger and Kirschbaum, 2002), thus forming an effective datum for high-resolution stratigraphic analyses.
CONVENTIONAL SEQUENCE STRATIGRAPHY
Van Wagoner et al. (1990) and Van Wagoner (1991, 1995) first established a comprehensive sequence stratigraphic framework for the Desert Member–lower Castlegate Sandstone interval (Figure 5A). This work covered a 100+ km (62+ mi) transect through the central and eastern Book Cliffs, from Green River, Utah, to West Salt Creek Canyon, Colorado. Subsequent work extended this framework to encompass all of the western Book Cliffs (Miall, 1993; Olsen et al., 1995; Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; Hettinger and Kirschbaum, 2002; Seymour and Fielding, 2013; Hampson, 2016). Subtle differences are noted between each scheme; however, the fundamental sequence stratigraphic foundation, as originally described by Van Wagoner et al. (1990) and Van Wagoner (1991, 1995), forms the central core of these studies (Figure 5A, B).
Figure 5. Sequence stratigraphy of the lower Castlegate Sandstone (C.Ss.) to upper Blackhawk Formation (Grassy–Desert members). Datum is top lower C.Ss. Thompson Canyon (ThC) is used as a tie point for the two schematic sections. (A) Conventional sequence stratigraphic model of Van Wagoner (1995). (B) Conventional sequence stratigraphic model of Yoshida et al. (1998). BC = Bull Canyon; BzC = Blaze Canyon; C-IVF = Castlegate incised valley fill; C-LSS = Castlegate lowstand sequence set; CSB = Castlegate sequence boundary; C-TST = Castlegate transgressive systems tract; D-HST = Desert highstand systems tract; D-IVF = Desert incised valley fill; D-LST = Desert lowstand systems tract; DSB = Desert sequence boundary; D-TST = Desert transgressive systems tract; G-HSS = Grassy highstand sequence set; HC-N = Horse Canyon North; HFSB = high-frequency sequence boundary; HM = Hatch Mesa; HP = Horse Pastures; IVF = incised valley fill; JT = Jeep Trail; LSSB = Lower Sego sequence boundary; PRC = Price River Canyon; S1-HST = sequence 1–highstand systems tract; S2-HST = sequence 2–highstand systems tract; S2-LST = sequence 2–lowstand systems tract; S2-TST = sequence 2–transgressive systems tract; S3-LST = sequence 3–lowstand systems tract; S3-TST = sequence 3–transgressive systems tract; S4-LST = sequence 4–lowstand systems tract; TC = Tusher Canyon; ThC = Thompson Canyon; WSCr = West Salt Creek.
Van Wagoner (1991, 1995) identified three Castlegate sequence boundaries: C-SB1 to C-SB3. The C-SB1 is the oldest and is restricted to the Blaze Canyon–Bull Canyon region, whereas C-SB3 is the youngest and is only observed at two localities east of the Jeep trail (Figure 5A). The C-SB2 is the most extensive and is correlated for more than 150 km (>94 mi) along depositional dip, from the type section at Castle Gate, Utah, eastward to West Salt Creek, Colorado (Van Wagoner, 1991, 1995; Miall, 1993; Yoshida et al., 1996, 1998; McLaurin and Steel, 2000, 2007; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; Hettinger and Kirschbaum, 2002; Hampson, 2010, 2016; Hampson et al., 2014). The C-SB2 is considered to be the C-SB for this study and is designated as a low-frequency sequence boundary (e.g., Seymour and Fielding, 2013) or a major sequence boundary (Miall, 2014).
In contrast, D-SB is only recognized for approximately 55 km (∼34.4 mi) along depositional dip, from Green River eastward to Pinto Wash. The D-SB is shown to either fade and disappear into the Blackhawk Formation (Yoshida et al., 1996, 1998; Yoshida, 2000) or is cut out by the C-SB toward the west (Hettinger and Kirschbaum, 2002). Eastward, beyond Pinto Wash, most correlations show the D-SB as a dashed line with question marks, suggesting it is a bypass–interfluve surface or a correlative conformity (Figure 5A, B). Van Wagoner (1991, 1995) correlated individual Desert Member channel incisions (i.e., each with 3–10-m [10–33-ft] cuts) as one master bounding surface that cuts 50+ m (164+ ft) of shoreface stratigraphy, forming a single, throughgoing, third-order sequence boundary. At the time, Van Wagoner’s (1991, 1995) correlation and reinterpretation represented a radical shift in thinking, garnering wide support from others that studied these rocks (e.g., Miall, 1993).
Van Wagoner (1995) also has identified three HFSBs in the highstand systems tract (i.e., Grassy highstand sequence set [G-HSS]) that underly the D-SB (Figure 5A). He interprets these HFSBs as a response to small, high-frequency sea-level falls that had no relationship to the generation of the main D-SB. The D-SB and C-SB2 subdivide the Desert–Castlegate stratigraphic interval into one complete sequence with three systems tracts (Figure 5A, B) (Van Wagoner et al., 1990; Van Wagoner, 1991, 1995). The G-HSS and the Castlegate lowstand sequence set (C-LSS) bracket the lower and upper boundaries of this complete sequence (Figure 5A). Although Van Wagoner (1995) does not explicitly number the parasequences, his correlations show at least nine parasequences in the G-HSS and six parasequences in his overlying Desert highstand systems tract (D-HST). The former are equivalent to Desert Member parasequences D3 to D11, whereas the latter are equivalent to lower Castlegate Sandstone parasequences C1 to C6. Both D1 and D2 are identifiable west of Tusher Canyon beyond the study area found in Van Wagoner (1995).
All conventional sequence stratigraphic models show gigantic incised valleys in the upper portions of both the Desert Member and the lower Castlegate Sandstone. Each valley is filled with lowstand to transgressive systems tract deposits (Figure 5A, B). The largest is the Castlegate incised valley fill (C-IVF), which is correlated for approximately 155 km (∼97 mi) along depositional dip (Van Wagoner, 1995; Yoshida et al., 1998). This valley has no defined edges or wings to the north or south and has been correlated using outcrop and subsurface data for at least 130 km (81 mi) along depositional strike. This gigantic C-IVF thickens westward from 33 m (108 ft) at Tusher Canyon to 44 m (144 ft) at Woodside to the northwest and continues uninterrupted as a 40–60-m (131–197-ft)-thick IVF westward to Helper, Utah. At the Castlegate type section, the C-IVF would encompass the entire lower Castlegate Sandstone fluvial package.
Although gigantic valley interpretations are common, not one of these aforementioned studies has clearly demonstrated a valley geometry. A sheetlike geometry would be a more appropriate descriptor given the consistent thickness along depositional strike. Regional isopach maps constructed from outcrop and subsurface data record an eastward-thinning, blanketlike geometry (Hale, 1959; Munger, 1965; Van de Graaff, 1972; Fouch et al., 1983; Hettinger and Kirschbaum, 2002; Seymour and Fielding, 2013). Not one study has defined zero isopach edges to the north or south, which would be a basic requirement for defining a three-dimensional valley geometry.
Critical evaluation of the conventional sequence stratigraphic framework for the Desert–Castlegate interval leads to several fundamental questions, some of which have been raised in earlier studies (Nummedal and Cole, 1993; Pattison, 1994, 2010; Adams and Bhattacharya, 2005; Pattison et al., 2007; Bhattacharya, 2011; Howell et al., 2015, 2018).
1. Where are the time-equivalent fluvial and coastal-plain deposits for the “highstand” shoreface successions: G-HSS and D-HST (Van Wagoner, 1991, 1995) or Sunnyside Member parasequences S1-HST and S2-HST (Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000)?
2. Where are the time-equivalent shoreface successions for the Desert IVF and C-IVF, which would include those formed during D-SB and C-SB time, plus any shorefaces linked to the valley-fill deposits themselves: Desert lowstand systems tract (LST) and C-LSS (Van Wagoner, 1991, 1995) or S2-LST and S3-LST (Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000)?
3. Where are the C-IVF margins along depositional strike?
4. Why is the base of the C-IVF cryptic from Helper to Horse Heaven–Blaze Canyon, a distance of more than 110 km (>69 mi) along depositional dip?
These fundamental questions, plus several other uncertainties regarding the conventional sequence stratigraphic model of the Desert–Castlegate interval, will be addressed below.
ALTERNATIVE SEQUENCE STRATIGRAPHIC FRAMEWORK
Nummedal and Cole (1993) were the first to publish an alternative sequence stratigraphic interpretation for the Desert Member–lower Castlegate Sandstone interval, proposing deposition during falling sea level and no sediment bypass. A similar idea was floated by Pattison (1994) who argued for the genetic, temporal, and spatial linkage of terrestrial and shallow-marine deposits throughout the entire Desert–Castlegate interval. One shortcoming of both studies was the lack of detailed correlations and observations, which was a strength of Van Wagoner’s (1991) model.
More than 10 years passed before alternative ideas were considered again. Adams and Bhattacharya (2005) questioned two aspects of the conventional sequence stratigraphic model. (1) Could some of the Blackhawk Formation shorefaces be part of the falling stage and LSTs and hence related to the cutting of the valleys? (2) Where are all the muddier sediments transported by the Castlegate rivers? Answers were not forthcoming because the data required to address these questions were beyond the scope of their paper.
Van Wagoner (1995) also mentions an alternative stratigraphic framework for the Desert–Castlegate interval, which he incorrectly described as a “lithostratigraphic interpretation.” Both Pattison (2010) and Bhattacharya (2011) argue that Van Wagoner’s (1995) alternative model is sequence stratigraphic rather than lithostratigraphic. Pattison (2010) even went so far as to suggest that Van Wagoner’s (1995) conventional “sequence stratigraphic interpretation” is more lithostratigraphic than sequence stratigraphic because Van Wagoner (1995) lumped similar lithologies together into separate stratigraphic units (i.e., all shallow-marine vs. all channel and coastal plain deposits). Both Pattison (2010) and Bhattacharya (2011) suggest the possibility of temporally linked, laterally adjoining channels and shoreface deposits as an alternative to the conventional model. Recent work by Howell et al. (2015, 2018) finds no evidence for significant sea-level falls in Book Cliffs strata and therefore no third-order sequence boundaries nor IVFs. Detailed local correlations, combined with additional data, are the key for addressing the utility of the conventional versus alternative models (Miall, 2014).
Base of the Lower Castlegate Sandstone
Early work identified an unconformity at the base of the lower Castlegate Sandstone in the Wasatch Plateau but noted that this surface transitioned eastward into a nonerosive contact in the Book Cliffs (Spieker and Reeside, 1925; Spieker, 1931, 1949). This is in complete contrast to conventional sequence stratigraphic–based studies that show the base of the Castlegate Sandstone as a throughgoing unconformity correlatable for 150+ km (93+ mi) throughout the Book Cliffs (Van Wagoner, 1995; Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; Hettinger and Kirschbaum, 2002; Seymour and Fielding, 2013; Hampson, 2016). Such a radical reinterpretation of the base of the lower Castlegate Sandstone triggers a series of fundamental questions.
1. What evidence argued for a radical reinterpretation of this surface from a nonerosive contact to an unconformity in the Book Cliffs?
2. Can the base of the lower Castlegate Sandstone be correlated as one single, throughgoing surface along 150+ km (93+ mi) of Book Cliffs outcrop?
3. Does one specific surface define the base of the lower Castlegate Sandstone or is it better described as an amalgamation of multiple erosional surfaces? How is it recognized?
4. Does a sharp lithological contact within a fluvial sandstone succession necessarily suggest an unconformity of regional scope?
5. Is the scour depth at the base of the lower Castlegate Sandstone greater or equal to the scour depths associated with individual channels?
6. Are lower marker horizons progressively cut out from east to west, thus implying the angular unconformity interpretation as proposed in some studies?
7. Is there additional field data that can shed new insights on the character of the base of the lower Castlegate Sandstone?
Answers to these questions will be critically evaluated below. Key results from previous studies will be integrated with the high-resolution correlations presented herein (Appendices 1 and 2 available in Datashare 117 at www.aapg.org/datashare). Observations are summarized from four regions, which, from west to east, are (1) Wasatch Plateau to Castlegate type section, (2) Helper to Desert Siding, (3) Battleship Butte to Crescent Canyon, and (4) Horse Heaven to County Road 206, Colorado (Figure 2). Specific field characteristics of unconformities (as per Miall 2016) and criteria for cryptic sequence boundary identification (as per Miall and Arush, 2001a) will be assessed against field data collected from the base of the Lower Castlegate Sandstone (Table 2).
Wasatch Plateau to Castlegate Type Section
Spieker and Reeside (1925) identified the Blackhawk–Castlegate contact as an unconformity on the Wasatch Plateau divide between North Gordon Creek and Beaver Creek, approximately 22 km (∼13.8 mi) west-southwest of Castle Gate, Utah, and 6 km (4 mi) south-southeast of Scofield, Utah (Figure 4A) (Table 2). Similarly, Horton et al. (2004) mapped an angular unconformity along the Blackhawk–Castlegate contact at Hjorth Canyon, approximately 55 km (∼34 mi) west-northwest of Helper (Table 2). At Rock Creek, approximately 5 km (∼3 mi) east-southeast of Hjorth Canyon, the Blackhawk–Castlegate contact is no longer an angular unconformity but is sharp and disconformable (Horton et al., 2004). Another 15 km (9 mi) east, the Blackhawk–Castlegate contact is difficult to pinpoint and is marked by dashed lines on the Bennion Creek measured section, which is more than 35 km (>22 mi) northwest of the proximal Book Cliffs outcrops at Helper (Horton et al., 2004). Spieker (1931) suggests that the lapse of time at the Blackhawk–Castlegate contact diminishes eastward through the Wasatch Plateau and vanishes in the western Book Cliffs. Additional work by Spieker (1949) depicts the base of the Price River Formation (i.e., base Castlegate Sandstone Member) as grading eastward from an angular unconformity into a disconformity in the Wasatch Plateau region and then into a gradational contact (i.e., no unconformity) in the western Book Cliffs.
On stratigraphic summary panels in numerous publications, the base of the lower Castlegate Sandstone is plotted as a wavy line representing missing time (Fouch et al., 1983; Miall, 1993, 2014, 2016; Krystinik and DeJarnett, 1995; Van Wagoner, 1995; Yoshida et al., 1996, 1998; McLaurin and Steel, 2000, 2007; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; Hettinger and Kirschbaum, 2002; Taylor and Gawthorpe, 2003; Adams and Bhattacharya, 2005; Hampson, 2010, 2016; Seymour and Fielding, 2013; Hampson et al., 2014). In contrast, others depict the Blackhawk–Castlegate contact as a questionable disconformity or as an amalgamation of sharp, autogenically derived fluvial scour surfaces (Spieker and Reeside, 1925; Spieker, 1931, 1949; Young, 1955, 1957; Van de Graaff, 1972; Lawton, 1986; Chan and Pfaff, 1991; Robinson and Slingerland, 1998; Hajek and Heller, 2012). The type section at Castle Gate, Utah, (Figure 6) has been the focus of numerous studies, and as such, there is a wealth of sedimentological, stratigraphic, architectural, and petrographic data from this region (Chan and Pfaff, 1991; Olsen et al., 1995; Robinson and Slingerland, 1998; Miall and Arush, 2001a, b; McLaurin and Steel, 2007; Hajek and Heller, 2012; Miall, 2014).
Figure 6. Base of the lower Castlegate Sandstone (C.Ss.), type section locality, Castle Gate, Utah. See Figure 2 for photo locations. (A) White arrows point to the base of the stacked and amalgamated channel sandstones. Note that this surface is not at the same level within the field of view. (B) Multiple possibilities for the base of the lower Castlegate Sandstone (i.e., base of amalgamated channel sandstones), all five levels shown with different colored arrows: black, white, red, yellow, and blue. (C) Three different levels for the base of the stacked and amalgamated channel sandstones. (D) Base of the lower Castlegate Sandstone (yellow arrows), as identified by Miall and Arush (2001a, b). Type section. Castle turret on the east side of Highway 6. BH = Blackhawk Formation.
Miall (1993) identified many prominent, laterally extensive bounding surfaces in the lower Castlegate Sandstone. Most are channel scours, but some are interpreted as sequence boundaries, such as surface A, which is interpreted as the regionally correlatable (i.e., 150+ km [93+ mi] basinward) Castlegate sequence boundary (Miall, 1993, 1994, 2014, 2016; Miall and Arush, 2001a, b). Miall and Arush (2001a) discussed three criteria for identifying cryptic sequence boundaries within stacked braided fluvial successions: (1) facies, (2) changes in sequence architecture, and (3) petrology (Table 2). They evaluated these criteria at the type section concluding that there is no definitive evidence suggesting even a cryptic sequence boundary at the base of the lower Castlegate Sandstone at Castle Gate. The 150+ km (93+ mi) regional correlation of surface A (C-SB) seems to contradict this conclusion (Miall, 1993, 1994, 2014, 2016; Miall and Arush, 2001a, b).
Other than the angular unconformity exposures in the Wasatch Plateau south of Scofield (Spieker and Reeside, 1925; Spieker, 1931, 1949) and in the Charleston–Nebo salient 55 km (34 mi) northwest of Helper (Horton et al., 2004), there is no evidence supporting an unconformity interpretation for the base of the lower Castlegate Sandstone in the proximal Book Cliffs. Although the contact between the coal-bearing, mudstone-rich Blackhawk Formation and overlying amalgamated fluvial sandstones of the lower Castlegate Sandstone is sharp, it shifts ±1–2 channel stories throughout the Castle Gate region (Figure 6). Hajek and Heller (2012) noted significant scour and erosional reworking of channel bar tops at Castle Gate but no single, throughgoing erosional surface at the base of the lower Castlegate Sandstone (Table 2). They conclude that the sharpness of this contact is via autogenic channel scouring, not unconformity development. Adams and Bhattacharya (2005) studied the basal contact of the lower Castlegate Sandstone in Salina Canyon, approximately 100 km (∼62 mi) south-southwest of Castle Gate. They observed minor erosion not exceeding the depth of one channel story, further weakening the unconformity interpretation of the Blackhawk–Castlegate contact. In summary, no field criteria corroborate an unconformity nor cryptic sequence boundary interpretation for the base of the lower Castlegate Sandstone at the type section. The Blackhawk–Castlegate boundary is only a sharp facies contact (Table 2).
Helper to Desert Siding
Helper to Desert Siding, Utah, encompasses a very large area of the Book Cliffs, representing approximately 70 km (∼44 mi) of depositional dip distance and 55 km (34 mi) along depositional strike. Some studies show the termination of the base Castlegate Sandstone unconformity in the eastern part of the Wasatch Plateau before reaching the Book Cliffs (Spieker, 1931, 1949; Van de Graaff, 1972; Robinson and Slingerland, 1998; Horton et al., 2004). Most show an unconformity at the base of the lower Castlegate Sandstone from Helper to Desert Siding, either explicitly (Young, 1955, 1957; Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; Hettinger and Kirschbaum, 2002; Hampson, 2010, 2016; Seymour and Fielding, 2013; Miall, 2014) or inferred (Fouch et al., 1983; Lawton, 1986; Franczyk et al., 1990; Chan and Pfaff, 1991; Franczyk and Pitman, 1991; Krystinik and DeJarnett, 1995; Van Wagoner, 1995; McLaurin and Steel, 2000, 2007; Taylor and Gawthorpe, 2003; Adams and Bhattacharya, 2005).
Time gaps vary from study to study. Fouch et al. (1983) indicate a 1.5-m.y. time gap at Price Canyon decreasing to 0.5 m.y. at Sunnyside. Others depict equal time gaps from west to east, with up to 1.6 m.y. of missing time (McLaurin and Steel, 2000). Many studies link the base of the lower Castlegate Sandstone at Castle Gate to the base of the C-IVF farther east, forming one single, throughgoing unconformity. Yoshida et al. (1996, 1998) interpret this surface as a westward-dipping angular unconformity.
Figure 7. Base of the lower Castlegate Sandstone (C.Ss.), Helper to Desert Siding. See Figure 2 for photo locations. (A) Three possible levels for base Castlegate (red, white, and blue arrows), representing the contact between the stacked-amalgamated channel sandstones (i.e., high net-to-gross ratio) and the lower net-to-gross coastal-plain succession of the Blackhawk. Sunnyside region. (B) The base of amalgamated channel sandstones is at the same stratigraphic level (white arrows). Marsh Flat Wash. (C) The base of the Castlegate is difficult to pick at a consistent level because the lower 10–25 m (32–82 ft) has a lower net-to-gross ratio marked by abundant coal- and carbonaceous-shale–bearing strata. Different colored arrows highlight the base of the amalgamated sandstone package, which jumps stratigraphically because of the joining versus splitting of the basal sandstones. The coal zone on top of Desert Member (DM) parasequence (D) D3/D4 is highlighted (likely position). Note the prominent white caps in Grassy Member (GM) parasequence (GPS) 1 and Sunnyside Member (SM) parasequence (S) 3. South of Price River. (D) Excellent example that highlights the multilevel character of the base amalgamated channel sandstone package. See the red versus white arrow levels, and the white versus blue arrow levels for excellent examples. All of the lower DM cycles are channel (CH)–coastal plain facies. Little Elliott Mesa. BH = Blackhawk Formation; FAF = Flat and Foreshore; GM = Grassy Member; IHS = inclined heterolithic strata; K = Kenilworth parasequence; KM = Kenilworth Member; LAS = lateral accretion surface; WC = White Cap.
In contrast, high-resolution local-to-regional correlations presented herein show no evidence for an unconformity at the base of the lower Castlegate Sandstone (Table 2). Mudstone-rich coastal-plain deposits containing single-story channel sandstones are repeatedly overlain by thick, cliff-forming, amalgamated channel sandstones at all localities (Figure 7). A sharp but stratigraphically variable contact marks the boundary between these facies throughout the Helper–Desert Siding region, consistent with observations at Castle Gate. The base of the lower Castlegate Sandstone varies vertically by ±1–2 channel stories. No single, throughgoing erosional surface is noted (Figure 7; Table 2). This is a sharp but conformable facies transition. Parasequences D1 to D3 emerge in the Desert Siding area, which is the type section locality for the Desert Member (Figure 4B) (Young, 1955). Here, the Blackhawk–Castlegate contact is marked by the top of D3, either as the flooding surface above the foreshore–shoreface sandstones or more commonly as a thick and prominent coal bed (Figures 7, 8).
Figure 8. High-resolution stratigraphy of the upper Sunnyside Member–lower Castlegate Sandstone (C.Ss.) interval, Desert Siding (DS), Little Elliott (LE) Mesa, The Cove (TCv), and Middle Mountain (MM) regions. The inset map shows the approximate location of this west-to-east schematic section, location of measured outcrop sections 4–11, and first appearance of Desert Member shoreline sandstones of parasequences 1–5 (numbered green lines). BB = Battleship Butte; BCB = Blue Castle Butte; CH = shoreface-incised channel; D = Desert Member parasequence; FAF = foreshore and flat; GB = Gunnison Butte; GPS = Grassy Member parasequence; GV = Gunnison Valley; ME = Mount Elliott; PRC = Price River Canyon; S3 = Sunnyside Member parasequence 3; TC = Tusher Canyon; WC = White Cap.
Battleship Butte to Crescent Canyon
The Battleship Butte to Crescent Canyon region (Figure 2) covers approximately 40 km (∼25 mi) of mostly depositional dip–oriented strata of the Desert Member–lower Castlegate Sandstone stratigraphic interval. Exceptional three-dimensional outcrops are present and are highly accessible because of the numerous side canyons having four-wheel-drive tracks and roads. From west to east, the lower Castlegate Sandstone becomes muddier, has less amalgamation of fluvial channels (i.e., greater proportion of single -story channels), and begins to contain thick packages of coals and carbonaceous-rich mudstones (Figures 9, 10). Basically, the lower Castlegate Sandstone shows a facies transition into the coastal-plain lithologies that are typical of the Blackhawk Formation (Table 1). Therefore, this region presents the greatest challenge in consistently picking the Blackhawk–Castlegate boundary because of the similarities in coastal-plain lithologies at these mudstone-on-mudstone contacts.
Figure 9. Lateral and vertical facies relationships between the Desert Member (Mb.) (Blackhawk Formation) and the lower Castlegate Sandstone (Ss.). A lower datum is used for all three depositional dip–oriented cross sections (i.e., top of Grassy Mb.). Legend and scales at base. (A) Swasey’s Beach on the Green River to the Coal Canyon region, including measured outcrop sections 9–11. (B) Horse Canyon–South to Trough Spring Ridge. Measured outcrop sections 16–20 are shown. (C) Thompson Pass to Horse Heaven, with measured outcrop sections 23–26. C1–C6 = lower Castlegate Ss. parasequences 1–6; CH = shoreface-incised channel; Cyn = Canyon; D1–D11 = Desert Mb. parasequences 1–11; GPS1–GPS4 = Grassy Mb. parasequences 1–4; N-Side = North Side; VE = vertical exaggeration; W-Side = West Side.
Previous sequence stratigraphic correlations show single, throughgoing unconformities (sequence boundaries) at two levels in this region: the D-SB and C-SB (Van Wagoner et al., 1990; Van Wagoner, 1991, 1995; Miall, 1993, 2014, 2016; Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; McLaurin and Steel, 2007; Hampson, 2010, 2016; Seymour and Fielding, 2013; Hampson et al., 2014). Detailed local correlations presented herein are inconsistent with single, throughgoing unconformity interpretations (Figures 9; Table 2). In fact, multiple incision surfaces are recognized, all of which are correlatable at a parasequence scale. These shoreface-linked, tidal-estuarine channels rarely cut deeper into underlying parasequences (Figures 9, 10). Individual shoreface-incised channels (CHs) do not link vertically across the Desert and Castlegate interval as implied by others. Each channel has well-defined geometries and lateral boundaries and is interpreted as a discrete, high-frequency, parasequence-scale incision event.
Figure 10. Base of the lower Castlegate Sandstone (C.Ss.), Battleship Butte to Crescent Canyon. See Figure 2 for photo locations. (A) Clear and sharp base (yellow arrows) to the amalgamated fluvial sandstones of the lower C.Ss. is noted over a few tens of meters, Long Face South of Tusher Canyon, Northern Bowl. Note that channel stories both merge and split from the amalgamated base (white arrows), clearly indicating that this is a diachronous facies boundary. A geologist is pictured for scale at the cliff top (black circle and arrow). (B) Difficult to pinpoint the base of the lower C.Ss., Browns Wash Canyon, West Bowl. Ten options are shown (i.e., different colored arrows with question marks). A geologist is pictured for scale at the upper right (black oval and arrow). (C) At least four viable options (different colored arrows with question marks) for the base of the lower C.Ss., The Basin, East Canyon. Mustone-filled Desert Member parasequence 9A shoreface-incised channel (D9A-CH). (D) Two different levels for the base of the amalgamated fluvial sandstones (red and blue arrows), Thompson Pass, East Bowl. White Cap (WC) erosion in three Desert parasequences delineates high-frequency incisions at multiple levels. Two geologists are pictured for scale (black circles and arrows) at the cliff top. BH-CP = Blackhawk Formation coastal-plain deposit; C3 = Castlegate parasequence 3; CH = shoreface-incised channel; C.Ss.-CP = Castlegate Sandstone coastal-plain deposit; D = Desert Member parasequence; DM = Desert Member; GPS = Grassy Member parasequence.
Shoreface sandstones of parasequences D4 to D11 thin westward, eventually wedging out into coastal-plain deposits (Figures 9). Flooding surfaces grade westward into prominent coal zones. Thicker coal zones along the tops of the D3, D6, D9, and D11 can be correlated 5–10 km (3.1–6.3 mi) landward (Figures 9). The westward gradation from shoreface to coastal-plain facies, coupled with the correlation of flooding surfaces and coals, confirms a temporal and genetic linkage between these laterally adjacent facies belts (Pattison, 2019).
Horse Heaven Eastern Utah to County Road 206 Western Colorado
The Horse Heaven to County Road 206 (i.e., Mitchell Road) western Colorado region is a large area of the Book Cliffs outcrop belt, approximately 60 km (∼38 mi) both down depositional dip and along depositional strike (Figure 2). Despite the large area, it has the least amount of uncertainty with respect to recognizing and correlating the base of the lower Castlegate Sandstone. The Desert–Castlegate contact is the top of D11, as originally defined by Young (1955). This contact is marked by a 1–3-m (3–10-ft)-thick interbedded stack of coals and carbonaceous shales in the coastal-plain deposits at Christmas Ridge that are correlatable for approximately 15 km (∼9.4 mi) along depositional dip, eventually capping the D11 channel–shoreface package at Horse Heaven (Figures 11, 12). The top D11 flooding surface can be correlated another 50+ km (30+ mi) eastward into the Utah–Colorado border region (Figures 11E). This surface is also recognized on well logs throughout the subsurface of eastern Utah and western Colorado.
Figure 11. Base of the lower Castlegate Sandstone (C.Ss.), Horse Heaven to West Salt Creek–County Road 206 (i.e., old Mitchell Road), Colorado. Desert Member parasequences 1–11 (D1–D11) and lower C.Ss. parasequences 1–6 (C1–C6) are labeled where present. Shoreface-incised channel-fill deposits in the uppermost D11 (D11-CH) and C2 (C2-CH). The top of parasequence D11 and base of parasequence C1 marks the Desert–lower Castlegate contact. See Figure 2 for photo locations. (A) First occurrence locality of C1 and C2 and the C2-CH. Horse Heaven, south face. (B) Approximately 1 km (∼0.6 mi) east of the previous photograph, Horse Heaven, south face–east. Note that C1, C2, and C2-CH are fully developed. The Desert–lower Castlegate contact is demarcated by the thick upper coal zone, as per Pattison (2010). Heterolithic-rich C2-CH completely cuts C1 and C2 in places. (C) D10-D11 and C1–C3, Sagers Canyon, southeast wall. Two nested, large-scale lateral accretion sets observed in the shoreface-incised channel-fill deposit in the uppermost C3 (C3-CH). (D) Farthest basinward deep incision, C4-CH, Strychnine Wash. C4-CH cuts out all of C3 and half of C2. Top D11 marks the base of the C.Ss. (E) Mancos Shale color bands (CB) define the tops of distal D5, D8, D10-D11, and C3, Westwater Creek Canyon. Base of the C.Ss. is defined as the top of the D11 color band, as per Young (1955). Basinward of the pinch-out, feathered edge of the C4-CH. Parasequence boundaries become somewhat cryptic down depositional dip, especially the C2-C3, C3-C4, C4-C5, and C5-C6 contacts. (F) Terminal shoreface deposits for the lower C.Ss., main Fe-ooid accumulation (i.e., belt 2), County Road 206 (Mitchell Road), western Colorado. The photo looks east. A white CB marks the top of the lower C.Ss. in the distance. A geologist is pictured for scale (white circle and arrow).
Prominent, deeply incised, large-scale tidal-estuarine channels are a defining characteristic of the lower Castlegate Sandstone through the western half of this region. Van Wagoner (1991, 1995) correlated at least three nested, incised channels and labeled their bases C-SB1 to C-SB3 (Figure 5A). The middle sequence boundary, C-SB2, demarcates the base of the C-IVF (Van Wagoner et al., 1990; Van Wagoner, 1991, 1995; Miall, 1993, 2014; Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b; Hettinger and Kirschbaum, 2002; Seymour and Fielding, 2013; Hampson, 2016). Many correlate the C-SB2 surface westward as a single, throughgoing unconformity (sequence boundary) for more than 150 km (>94 mi) (Yoshida et al., 1996, 1998; Willis, 2000; Yoshida, 2000; Miall and Arush, 2001a, b: Hettinger and Kirschbaum, 2002; Seymour and Fielding, 2013; Miall, 2014; Hampson, 2016), correlating the C-IVF with the entire lower Castlegate Sandstone package farther to the west (Table 2).
Figure 12. Lateral and vertical facies relationships within the Desert Member–lower Castlegate Sandstone (Ss.) stratigraphic interval. The top of the Grassy Member (GM) is used as a lower datum for all three depositionally dip–oriented cross sections. Legend and scales are at the base. Desert Member parasequences (D1–D11) and lower Castlegate Ss. parasequences (C1–C6) are labeled where visible. (A) Thompson Canyon to Sagers Canyon, including measured outcrop sections 30–33. (B) Strychnine Wash to Big Hole Wash-East, including measured outcrop sections 38–40. (C) West Salt Creek to County Road 206 (Mitchell Road) region, western Colorado. Includes measured outcrop section 45 and both Fe-ooid facies belts. CH = shoreface-incised channel; Cyn = Canyon.
In contrast, high-resolution correlations presented in this study indicate that the C-IVF originates out of the laterally adjacent coal-bearing coastal-plain facies belt, first as the C2-CH (i.e., CH cutting the C2 parasequence) followed by the C3-CH and C4-CH, respectively. One of the most proximal appearances of large-scale tidal-estuarine channel complexes in the lower Castlegate Sandstone is at Thompson Pass where it is nested within a thick package of coastal-plain deposits (Figure 10D). It sporadically crops out through Crescent Canyon and Horse Heaven, eventually forming a continuous sandstone layer (i.e., chrono-sandstone-slab of Pattison, 2010) with the C1 and C2 shoreface sandstones, which are first seen in the eastern part of Horse Heaven. Contrary to most other studies, these Castlegate tidal-estuarine channel fills are interpreted as parasequence scale, CH bodies that may or may not have an HFSB at their base (Pattison, 2018). These tidal-estuarine channel fills originate off of the C2 to C4 flooding surface and hence are labeled C2-CH to C4-CH.
Top of the Lower Castlegate Sandstone
In the eastern half of the study area, the top of the lower Castlegate Sandstone is overlain by the Buck Tongue of the Mancos Shale, making it a sharp and easily identifiable contact. In contrast, in the western half of the study area from Castlegate to Woodside, the channel sandstones of the Sego Sandstone, middle Castlegate, and/or upper Castlegate Sandstone are juxtaposed on top of the lower Castlegate Sandstone fluvial sandstones, rendering this contact indistinguishable. East of Woodside, the uppermost part of the lower Castlegate Sandstone has a remarkably high concentration of carbonaceous-rich mudstone and coal-bearing coastal-plain facies, which contrasts with the sandstone-rich fluvial deposits below (Figure 13). These fine-grained coastal-plain deposits range in thickness from 2 to 12 m (6–39 ft) and extend eastward into the Big Hole Wash region, a distance of more than 70 km (>44 mi) down depositional dip. Hale (1959) briefly notes that carbonaceous shale and lignitic–coaly shale overlies the Castlegate Sandstone over most of the Uinta Basin. Although Young (1955) and Hettinger and Kirschbaum (2002) show these fine-grained deposits capping the top of the lower Castlegate Sandstone on their correlation panels, they provide no discussion. Van Wagoner (1995) includes these deposits in the upper part of the C-IVF.
Figure 13. The top of the lower Castlegate Sandstone (C.Ss.) is marked by yellow arrows in all photos. Dominantly a very low-to-low net-to-gross, coal-bearing, coastal-plain (CP) package, but with rare single-story channel sandstones. The fine-grained package ranges from 2 to 12 m (6–39 ft) thick and rests on top of the amalgamated channel sandstones of the lower C.Ss. White cap (WC) sandstones indicate overlying coals. Buck Tongue (BT) of the Mancos Shale. A layer of orange-brown concretions (orange-brown arrows) occurs 5–8 m (16–26 ft) above the top of the C.Ss., thus forming an excellent regional marker horizon. See Figure 2 for photo locations. (A) Carbonaceous-rich mudstone in uppermost C.Ss., Coal Canyon East. Mudstone-on-mudstone contact (C.Ss.-CP to BT). (B) Dark, carbonaceous-rich mudstones of the lower C.Ss. are overlain by gray mudstones of the BT (Mancos Shale), Horse Canyon–South. Rock hammer straddles the contact, which is demarcated by a brown, Fe-rich bed. (C) Mudstone-on-mudstone contact defines the lower C.Ss.-CP to BT transition, The Basin. Dark carbonaceous-rich mudstones dominate the uppermost part of the lower C.Ss. Shovel (white oval) is approximately 1.1 m (∼3.6 ft) long. (D) Flat-topped cut-bank edge of the wash marks the top of the lower C.Ss., Horse Heaven. Carbonaceous-rich mudstones and white cap sandstones dominate the uppermost C.Ss.
Detailed field work illustrates that the fine-grained coastal-plain deposits in the uppermost lower Castlegate Sandstone have a laterally extensive, sheetlike geometry and are ubiquitous (Figure 13). Interbedded carbonaceous-rich mudstones and coals are the dominant facies, with lesser amounts of very fine–fine-grained, single-story channel sandstones and thin sheetlike siltstones or very fine-grained sandstones. Many sandstones are bleached white via removal of Fe-rich cements as a result of the leaching action of acidic waters sourced from the overlying coal-bearing strata (Spieker, 1931, 1949; Young, 1955). The net-to-gross ratio of this fine-grained package ranges from 5% to 25%, which is considerably less than the 70%–95% net-to-gross ratio of the underlying cliff-forming channel sandstones of the lower Castlegate Sandstone. Most boundaries with the overlying Buck Tongue are carbonaceous mudstone–marine mudstone contacts (Figure 13).
It is speculated that the muddier top of the lower Castlegate Sandstone was tied to lowered fluvial gradients resulting from the onset of the Buck Tongue transgression. This interpretation is consistent with the thin, sand-poor terminal C6 shoreface and the C6B-level Fe-ooid belts in western Colorado. The Fe-ooid belts, terminal shoreface belt, and mudstone-rich coastal-plain deposits are progressively blanketed, from east to west, by marine mudstones during the Buck Tongue transgression.
Figure 14. High-resolution sequence stratigraphic framework. Depositional dip–oriented schematic section is projected along a W14°N–E14°S trend, and all measured sections are projected onto this trend line. Top Grassy Member (GM) is used as a lower datum. BH Fm = Blackhawk Formation; BT = Buck Tongue; C = Castlegate Sandstone parasequence; CP = Corral Point; CPSS = Castlegate parasequence set; C.Ss. = Castlegate Sandstone; D = Desert Member parasequence; DPSS = Desert parasequence set; GPS = Grassy Member parasequence; MFS = maximum flooding surface; PRC = Price River Canyon; Ss. = Sandstone; TB = The Basin; TC = Tusher Canyon, ThC = Thompson Canyon; VE = vertical exaggeration; WSCr = West Salt Creek.
HIGH-RESOLUTION SEQUENCE STRATIGRAPHIC MODEL
The high-resolution sequence stratigraphic model presented herein (Figures 14, 15) was constructed using a comprehensive database acquired and analyzed over the past 25 yr. Seventeen parasequences were identified in the Desert–Castlegate stratigraphic interval: D1 to D11 and C1 to C6. The CHs were present in most parasequences, the widest and deepest of which were along the D4–D6, D11, and C2–C4 levels. Key differences with the conventional sequence stratigraphic model (Van Wagoner, 1995) are summarized below.
Figure 15. Shoreline position curve. The shoreline position curve is the net result of all autogenic and allogenic processes. Basinward versus landward shifts of Grassy Member parasequences 1–4 (G1–G4), Desert Member parasequences 1–11 (D1–D11), and lower Castlegate Sandstone parasequences 1–6 (C1–C6) shorelines are shown. The shifts are based on field measurements of shoreline progradation and shoreline flooding distances for each parasequence. Assumes 5 km (3.1 mi) for the initial and terminal widths of the shoreface–inner shelf facies belt (after Hampson, 2000). No vertical scale is implied because each parasequence is given an equal apparent thickness. In reality, parasequence thickness varies considerably, ranging from 5.1 m (16.7 ft) for G3 up to 63.5 m (208.3 ft) for Kenilworth Member parasequence 1. Shoreline orientations differ slightly from parasequence to parasequence, and therefore, this two-dimensional shoreline position curve may not actually be applicable to any single outcrop locality. It is a compilation diagram only. CPSS = Castlegate parasequence set; Cyn = Canyon; DPSS = Desert parasequence set; MFS = maximum flooding surface.
1. High-resolution CHs occur in most parasequences. Their envelope of incision is generally restricted to the shoreface deposits in which they are housed, locally cutting into underlying parasequences. These higher-frequency erosional surfaces do not coalesce into a third-order sequence boundary. Some are HFSBs. A similar pattern of shoreface-incised tidal-estuarine channel fills is present in all other members of the Blackhawk Formation (Figure 3).
2. All shoreface sandstones emerge out of fine-grained coastal-plain deposits. Points of origin (i.e., localities) have been studied and mapped (Figures 8, 9, 12, 14). Progradational distances are long, whereas flooding surface distances are shorter (Figure 15).
3. All facies belts are temporally, spatially, and genetically linked: (a) amalgamated fluvial sandstones; (b) fine-grained, coal-bearing coastal-plain deposits; and (c) shoreface sandstones with incised tidal-estuarine channels. Lateral facies transitions, from west to east, are (a) → (b) → (c). Vertical transitions, from base to top, are the reverse order: (c) → (b) → (a). Conformable facies contacts are the norm. The application of Walther’s law implies time equivalency of neighboring facies belts.
4. No gigantic incised valleys exist within the Desert–Castlegate interval.
5. Prominent coal beds correlate to marine flooding surfaces, thus linking facies zones (b) and (c) in both time and space.
6. Progradational stacking patterns are the norm for both the nearshore terrestrial and shallow-marine facies belts.
7. Incised channels thin and narrow basinward to a feathered edge, transitioning into shoreface sandstones. No evidence exists for sediment bypass in the shoreface-to-shelf environment.
IMPLICATIONS FOR MODELING
Conventional sequence stratigraphic models of fluviodeltaic systems have radically different stacking patterns and facies relationships (time and space) compared to the high-resolution model presented herein. Selection of the appropriate model has enormous implications for correctly predicting the location and character of adjacent facies belts. Applications to the subsurface may involve (1) predicting the type, size, and location of sandstone reservoirs; (2) determining the three-dimensional architecture of reservoir versus nonreservoir facies; and (3) ascertaining the degree of connectivity versus compartmentalization. Business decisions are likely to be impacted, such as the location of seismic surveys and/or number and location of wells to be drilled. To be successful, the model must be robust, have the least amount of uncertainty, and be correctly applied. Key implications of model selection are discussed below.
Conventional sequence stratigraphic models do not stitch together the laterally adjacent nearshore terrestrial and shallow-marine facies belts in time or space. In the conventional model, a third-order sequence boundary partitions the nearshore terrestrial and shallow-marine deposits, effectively compartmentalizing these neighboring facies belts. A large-scale incised valley fill is an outcome of this model. If a third-order sequence boundary exists, the large valley container would be filled, from base to top, with LST fluvial channels overlain by TST tidal-estuarine channels. The vertical stacking pattern records a proximal-to-distal facies change upward as the valley fills. Timelines within the valley would be much younger than the timelines in the adjoining HST shallow-marine facies belt. There would be no synchroneity in stacking pattern on either side of the third-order sequence boundary. In fact, timelines would either be truncated by the sequence boundary (i.e., HST flooding surfaces) or would onlap onto the sequence boundary (i.e., marker beds in the LST–TST valley fill), neither of which would cut across the third-order sequence boundary.
In contrast, in the high-resolution model presented herein, timelines connect the nearshore terrestrial and shallow-marine facies belts. Thick coal zones correlate to marine flooding surfaces, thus creating synchronous patterns of stacking across these adjoining and time-equivalent facies belts. No third-order sequence boundaries are observed; therefore, no large-scale valley fill exists. All nearshore terrestrial facies have been deposited in a coastal-plain environment directly adjacent to the proximal shallow marine. Patterns of progradation and retrogradation are not only reflected in the shallow-marine deposits but are also mimicked in the adjacent nearshore terrestrial deposits.
Vertical and lateral stacking patterns in each facies belt are also a synchronous match. For example, during shoreline progradation, the low net-to-gross coastal-plain deposits would grade upward into amalgamated fluvial sandstones, recording a distal-to-proximal coastal-plain facies transition as the shoreline moves basinward. The opposite trend would be noted during shoreline retrogradation because amalgamated channel sandstones grade upward into coal-bearing coastal-plain mudstones, driven by the decreased fluvial gradient and concomitant transgression of the sea. Both of these patterns are beautifully displayed in the Desert–Castlegate stratigraphic interval.
Each model predicts radically different lateral and vertical channel stacking patterns, sizes of channels-valleys, and timeline correlations between the adjoining nearshore terrestrial and shallow-marine facies belts. Only one model can be correct per stratigraphic interval and regional locality. The choice of model is critical.
Central to the conventional sequence stratigraphic model is the development of a third-order sequence boundary, concomitant cutting of a large valley, sediment bypass through the valley to feed falling-stage and lowstand deposits, followed by subsequent transgression and valley filling. Laterally adjacent valley-fill and shallow-marine deposits (i.e., those cut by the valley) have no temporal nor spatial connection and are part of different systems tracts. One key prediction from this model would be the occurrence of falling-stage and lowstand-detached (?) sand bodies farther basinward. Applications to subsurface settings may involve business decisions to shoot seismic or drill wells in distal locations to explore for these falling-stage and lowstand reservoir prizes predicted via the conventional model. Evidence pointing toward the conventional model rather than the high-resolution alternative model presented herein would include
- a strongly progradational parasequence stacking pattern with descending regressive shoreline trajectories intersecting depositional slope;
- extensive subaerial erosion and exposure of highstand deposits;
- deep incisions;
- remnant paleosols and roots preserved in the distal shoreface shelf (i.e., exposure surfaces on the shelf during falling sea level);
- widespread lowstand and transgressive lag deposits;
- extensive vadose zone cementation;
- numerous distal shoreface shelf to overlying channel facies contacts; and
- an absence of flat-topped rooted foreshore sandstones in the stack of parasequences cut by the valley, except in interfluve regions at the top of the stack.
None of the aforementioned characterizes the Desert–Castlegate strata; therefore, the high-resolution model is the better fit.
The high-resolution model presented herein has no large-scale sediment bypass, third-order sequence boundaries, or large-scale valley fills. As a result, there is no reason to prospect farther down depositional dip for isolated falling-stage and lowstand deposits because none are expected outside the main area of interest. This model is entirely consistent with the ubiquity of landward-attached fluviodeltaic successions, whereas the conventional model is not. In the high-resolution model presented herein, all sand bodies are attached via the temporal, spatial, and genetic linkage of the nearshore terrestrial and shallow-marine facies belts.
Highstand-attached sandstone tongues are the norm throughout the Cretaceous Western Interior Foreland Basin of North America (McGookey et al., 1972; Krystinik and DeJarnett, 1995). Detached sand bodies do occur but are not as abundant (Bergman and Snedden, 1999). Detachment may be triggered during or after the falling stage, either as a lowstand bypass surface or by transgressive ravinement (Plint, 1988; Ainsworth and Pattison, 1994).
Application of the conventional sequence stratigraphic model sometimes requires the explaining away of lowstand shoreline deposits, literally. For example, Van Wagoner (1995) speculated on the apparent absence of Desert–Castlegate lowstand shorelines in the Book Cliffs region, offering two explanations: (1) lowstand shorelines were much farther east, perhaps in the Denver basin, or (2) lowstand shorelines never existed because rivers dried out and terminated within the confines of the gigantic valley before reaching the shoreline. After an exhaustive and fruitless search for time-equivalent lowstand deposits using hundreds of Piceance Basin well logs, Van Wagoner (1995) settled on the latter explanation. None of the field-based data presented herein support this interpretation. Clearly, the conventional sequence stratigraphic model is not a good fit with the Desert–Castlegate data set.
1. The lower Castlegate Sandstone has a 10+-km (6+-mi)-wide central facies belt of low net-to-gross, coal-bearing coastal-plain deposits that are indistinguishable from the Blackhawk Formation type section lithologies. Original lithostratigraphic work included these facies into the Castlegate great tongue of sandstone as well as the terminal shoreface deposits farther east (Spieker and Reeside, 1925; Spieker, 1931, 1949). Therefore, three facies belts comprise the Lower Castlegate Sandstone: (a) amalgamated fluvial sandstones, (b) coal-bearing coastal-plain mudstones with single-story fluvial channels, and (c) shoreface sandstones. Conformable lateral and vertical facies belt contacts are the norm.
2. The lowermost two-thirds of the amalgamated fluvial sandstones of the lower Castlegate Sandstone grades basinward into low net-to-gross, coal-bearing coastal-plain deposits of the Blackhawk Formation, which in turn pass down depositional dip into Desert Member shoreface parasequences D4 to D11. The uppermost third of this amalgamated fluvial sandstone package also grades basinward into coastal-plain deposits, but these are still part of the lower Castlegate Sandstone, as per original lithostratigraphic work (Spieker and Reeside, 1925; Spieker, 1931, 1949; Young, 1955). This Castlegate great tongue of sandstone continues eastward into Castlegate shoreface parasequences C1 to C6 and eventually into the Mancos Shale mudstone belt.
3. The only definitive evidence for an unconformity at the base of the lower Castlegate Sandstone is in the Wasatch Plateau, west of the Book Cliffs, where an angular unconformity has been recorded at a few localities (Spieker and Reeside, 1925; Horton et al., 2004). In proximal-to-medial Book Cliffs localities, the base of the lower Castlegate Sandstone varies by ±1–2 channel stories. Therefore, there is no single, throughgoing unconformity separating all of the amalgamated channel sandstones of the lower Castlegate Sandstone from the coal-bearing coastal-plain deposits of the Blackhawk Formation. This is a facies contact, which interfingers both laterally and vertically.
4. The uppermost lower Castlegate Sandstone is composed of 2–12 m (6–39 ft) of low net-to-gross, coal-bearing coastal-plain deposits that are indistinguishable from the Blackhawk Formation lithologies observed elsewhere in the Book Cliffs. Most contacts with the overlying Buck Tongue of the Mancos Shale are carbonaceous mudstone to marine mudstone. It is speculated that the fine-grained top of the lower Castlegate Sandstone is associated with decreasing fluvial gradients tied to the onset of the Buck Tongue transgression.
5. The conventional sequence stratigraphic model is not a good fit for the Desert–Castlegate interval for the following three main reasons: (a) a third-order sequence boundary does not separate the nearshore terrestrial facies belt from laterally adjacent shallow-marine belt; (b) neighboring facies belts are within the same systems tract, not different ones as predicted by the conventional model; and (c) large-scale valleys containing all Desert–Castlegate terrestrial facies do not exist.
6. An alternative high-resolution sequence stratigraphic model best explains the Desert–Castlegate field observations summarized herein. The CHs cut the proximal foreshore–shoreface deposits within each parasequence. These channels are discrete, mostly confined to individual parasequences in which they are housed and rarely cut deeper into underlying parasequences. Timelines extend uninterrupted from the shallow marine (i.e., flooding surfaces) into the neighboring coastal-plain deposits (i.e., thick coal zones). Shoreface sandstones emerge from the adjacent coastal plain. Amalgamated channel sandstones grade and interfinger with the carbonaceous mudstone-dominated coastal plain. Sediment bypass does not exist.
7. It is recommended that fluviodeltaic systems worldwide be critically reexamined in the light of the high-resolution sequence stratigraphic model presented herein. Basic differences with the conventional model include the size, shape, and lateral extent of key sequence stratigraphic rock packages (e.g., discrete CHs vs. large-scale IVFs) and key surfaces (e.g., multiple discrete autogenic channel incisions vs. a singular third-order sequence boundary). This in turn gives rise to radically different predictions, such as the presence or absence of detached falling-stage and lowstand shorelines. Applying the most appropriate model to subsurface data sets directly impacts the success of exploration and development activities, such as seismic survey locations, and optimal well placement and numbers.
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I am grateful to Norm Rosen and Dorene B. West for their thorough and thoughtful reviews that prompted greater clarification of key descriptions and interpretations. I also thank AAPG Editor Barry J. Katz and Jory Pacht for their comments. Field data have been collected over the past 25 years. Early funding was generously provided by Shell Research (Koninklijke/Shell Exploration and Production Laboratory), the Netherlands, and Shell (Shell Petroleum Development Company), Nigeria. More recent funding was via Shell Oil Houston and ConocoPhillips Houston research grants and Natural Sciences and Engineering Research Council of Canada Discovery Grant 238532. I am extremely grateful to Jill Stewart, Jenna Phillips, Steve Saban, Jess Peat, Morufu Basiru, and Sean Horner for their dedicated assistance during many long, hard, hot, and gnatty field days. I also thank Huw Williams, Paul Davies, and Doug Stewart for numerous field discussions and help with some of the data collection. And most importantly, I thank Jill, Liam, and Laura for their unfailing love, support, and patience as I lived and worked in my home away from home!
Appendices 1 (45 measured sections) and 2 (high-resolution correlation panel) are available in an electronic version on the AAPG website (www.aapg.org/datashare) as Datashare 117.