GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 8 2021 © 2021 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to GIW@utahgeology.org. THE UPPER CRETACEOUS ROCK SPRINGS FORMATION OF NORTHWEST COLORADO—A PREVIOUSLY UNDESCRIBED KEY DELTAIC OUTCROP Stephen P. Phillips and Samuel M. Hudson GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Production Cover Design and Desktop Publishing Douglas A. Sprinkel Cover Drone photograph of the north side of Vermillion Creek. The Upper Cretaceous Rock Springs Formation is cen- tered in the photograph. The top of the Rock Springs For- mation is just right of center within a prominent valley. Above the Rock Springs Formation is the gray-colored Trail Member of the Ericson Sandstone, followed by the brown colored Rusty zone of the Ericson Sandstone. The Rusty Member also forms a prominent valley. The Can- yon Creek Member of the Ericson Sandstone is in the up- per right-hand corner of the photograph. The horizontal distance across the center of the photograph is approxi- mately 500 m. i Become a member of the UGA to help support the work of the Association and receive notices for monthly meetings, annual field conferences, and new publi- cations. Annual membership is $20 and annual student membership is only $5. Visit the UGA website at www.utahgeology.org for information and membership application. The UGA board is elected annually by a voting process through UGA members. However, the UGA is a volunteer-driven organization, and we welcome your voluntary service. 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Foster Utah Field House of Natural History State Park Museum 435.789.3799 eutretauranosuchus@ gmail.com Editors GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 8 2021 45 ABSTRACT The Upper Cretaceous Rock Springs Formation of the Mesaverde Group in northwestern Colorado, southwestern Wyoming, and northeastern Utah is composed of fluvial, deltaic, and marine sediments that record the regression of the Western Interior Seaway during the Early to Middle Campanian. Contempo- raneous deposits are present along the eastern and southeastern margins of the Greater Green River Basin in Wyoming, but correlation across the basin is challenging. Analysis of a small (1-km-long), understudied outcrop in northwestern Colorado assists in bridging that gap. The outcrop consists of distal and proximal deltaic deposits, overlain by distributary-channel complexes within delta-plain deposits. Correlation pan- els based on subsurface wireline logs and outcrop gamma-ray profiles show that the deposits are younger than lithostratigraphically equivalent strata of the Rock Springs Formation in Utah and Wyoming. Region- al nomenclature is introduced for the area, and it is shown that these deposits differ from better-document- ed, older Rock Springs Formation deposits in Utah and Wyoming by having a higher net sandstone per- centage due to the presence of substantial distributary-channel complexes. This study benefits subsurface exploration efforts in the Greater Green River Basin by providing outcrop analogs of reservoir distribution and quality. The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Stephen P. Phillips1 and Samuel M. Hudson1 1Department of Geological Sciences, Brigham Young University, S389 Eyring Science Center, Provo UT, USA 84602; the.geologsist@outlook.com; sam.hudson@byu.edu Citation for this article. Phillips, S.P., and Hudson, S.M., 2021, The Upper Cretaceous Rock Springs Formation of northwest Colorado—a previously undescribed key deltaic outcrop: Geology of the Intermountain West, v. 8, p. 45–71, https://doi.org/10.31711/giw.v8.pp45-71. © 2021 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to GIW@utahgeology.org. INTRODUCTION The Upper Cretaceous Rock Springs Formation of the Mesaverde Group in northwestern Colorado, southwestern Wyoming, and northeastern Utah re- cords an overall regression of the Cretaceous Western Interior Seaway during the Early to Middle Campan- ian (e.g., Roehler, 1993; Plink-Björklund, 2008, 2019; Rudolph and others, 2015). It represents an early-stage sedimentary wedge sourced from the Sevier orogen- ic fold and thrust belt (Armstrong and Oriel, 1986) and is composed of fluvial, deltaic, shallow marine, and deep marine deposits (e.g., Roehler, 1993; Plink- Björklund, 2008, 2019; Rudolph and others, 2015). The Rock Springs Formation has been studied in detail via outcrops around the Rock Springs uplift and along the northern flank of the Uinta Mountains (e.g., Roehler, 1993; Plink-Björklund, 2008, 2019; Rudolph and oth- ers, 2015). Outcrop of the Rock Springs Formation is also present in a small, tectonically complex exposure in northwest Colorado along Vermillion Creek (Sears, 1924, 1925; Hale, 1950) (figure 1). This outcrop has nev- er been studied in detail and its relationship to other outcrops has not been well-established. It is commonly 46 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Figure 1. Caption is on the following page. 47 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 bypassed via subsurface correlation panels that tie the Rock Springs uplift to outcrops on the edge of the Sand Wash Basin, Colorado, even though it represents a stra- tegic tie-point along that transect (Hale, 1955; Douglass and Blazzard, 1961; Keith, 1965; Miller, 1977; Kiteley, 1983; Roehler, 1987; Luo and Nummedal, 2010). The aims of this paper are to present a detailed description of the sedimentology and architecture of the outcrop, interpret depositional settings based on that descrip- tion, and utilize this information to facilitate a compar- ison to subsurface geophysical data. Additionally, this paper aims to utilize this information to enable compar- isons between outcrops on the north flank of the Uinta Mountains and the east flank of the Rock Springs uplift (figure 1). A better understanding of the depositional history of this outcrop can inform future studies that utilize subsurface correlation and sequence stratigraphic interpretation. Future paleogeographic reconstructions will also benefit from a better understanding of shoreline position of the Rock Springs Formation deposits through time. Additionally, the Rock Springs Formation has a rich history of economic significance, mainly through the mining of coal in the Rock Springs uplift (e.g., Andrew, 1965; Roehler, 1986; Scott and others, 1995). However, oil and gas accumulation within the Rock Springs Formation is potentially significant, yet unknown, largely because of sparse testing in much of the Greater Green River Basin (Finn and Johnson, 2005; Johnson and others, 2005). This work can inform future exploration efforts through detailed description of facies distribution and reservoir geometry/connectivity in the subsurface of the Washakie and Sand Wash Basins. GEOLOGIC SETTING Study Area The Rock Springs Formation is present in a small tectonically complex exposure on the southern edge of the Vermillion Basin in northwestern Colorado (e.g., Sears, 1924; Hale, 1950; Burger, 1965; Ritzma, 1965, 1971) and is divided into two key outcrop areas by Ver- million Creek—the NW outcrop area and the SE out- crop area (figure 1C). In total, the outcrop stretches for less than 5 km in a northwest-southeast direction with about 1 km of high-quality outcrop (figure 1). The Ver- million Basin is a modern-day basin or topographic low drained by Vermillion Creek and its tributaries (e.g., Nightingale, 1930; Gras, 1955) and is located within the Greater Green River Basin (Rigatti and others, 2007) (figure 1). The Greater Green River Basin is further di- vided into several sub-basins and uplifts (figure 1). The outcrop area is located along a northern bound- ing fault of the Uinta Mountains called the Sparks fault (figures 1 and 2). The Sparks fault is a thrust fault with a sinistral component (Weber, 1971; Johnson and Ander- sen, 2009) and a history of movement during the late Paleocene to early Eocene (Ritzma, 1965, 1971; Weber, 1971; Bradley, 1995). Faulting and associated folding was followed by erosion and the unconformable depo- sition of younger Eocene deposits above older rocks (Ritzma, 1965; Bradley, 1995). This is highlighted in the Vermillion Creek area by an angular unconformity that juxtaposes shallowly dipping (2°to 10°) deposits of the Wasatch and Green River Formations against vertical to nearly vertical Cretaceous rocks (Sears, 1924; Ritzma, 1965) (figure 2). Figure 1 (figure is on the previous page). Location maps of various scales. (A) Map of the Greater Green River Basin showing the location of the Rock Springs Formation and equivalent rocks. Red dashed box indicates the extent of map shown in B. Basemap attribution: ESRI, USGS, and NOAA. Abbreviations: WFTB = Wyoming fold and thrust belt, HB = Hanna Basin, WRM = Wind River Mountains, SBA = Sandy Bend arch, GM = Granite Mountains, MA = Moxa arch, RSU = Rock Springs uplift, VB = Vermillion Basin, GRB = Green River Basin, UM = Uinta Mountains, GDB = Great Divide Basin, WA = Wam- sutter arch, WB = Washakie Basin, CR = Cherokee Ridge, SWB = Sand Wash Basin, RU = Rawlins uplift, SM = Sierra Madre, PR = Park Range. Outcrop pattern in Wyoming is from Love and Christainsen (1985). (B) Map of the study area showing the extent of the Rock Springs Formation outcrop, available well data, and surface to subsurface correlation lines for figure 8. The location of C is also indicated. (C) Satellite image of the Vermillion Creek area showing the location of the Rock Springs For- mation outcrop examined in the study. Satellite imagery attribution: ESRI, Maxar, GeoEye, Earthstar Geographics, CNES/ Airbus DS, USDA, USGS, AeroGRID, IGN, and the GIS User Community. Cross section is shown on figure 9. 48 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Additional outcrops of the Rock Springs Formation are located in three distinct areas from the Vermillion Creek area in the Vermillion Basin: Clay Basin is 45 km to the northwest, the Glades is 60 km to the northwest, and the Rock Springs uplift is 50 km to the north (figure 1). Rocks of a similar age, but different lithostratigraphic nomenclature, are also present 105 km east of the Ver- million Creek area on the east edge of the Washakie Ba- sin (Lynds and Slattery, 2017) (figure 1). The Laramide-age Rock Springs uplift began to form around 79 Ma (Lynds and Xie, 2019). The center of the uplift is cored by the older Baxter and Blair For- mations (Schultz, 1920) and the Rock Springs Forma- tion is present around this anticlinal structure in beds that dip away from the center of the uplift (Yourston, 1955) (figure 1). The Glades and Clay Basin outcrops Figure 2. Overview photo of the southeast outcrop (vantage point is from the northwest outcrop). The nearly vertical Creta- ceous rocks are annotated as well as the Sparks fault and the nearly horizontal Tertiary Green River and Wasatch Formations. 49 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 are located on the north flank of the Uinta Mountains and dip northward into the subsurface (Roehler, 1986) (figure 1). Additional data for the Rock Springs Forma- tion is available in the subsurface between these out- crop areas via borehole geophysical logs from oil and gas wells (figure 1). Stratigraphy The name “Rock Springs Formation” has its origins in a report by Schultz (1920) where coal-rich continen- tal deposits of the Rock Springs uplift were termed the “Rock Springs coal group.” Sears (1925) first used the name “Rock Springs Formation” and Burger (1965) es- tablished the eastern flank of the Rock Springs uplift as the type locality for the formation. The Rock Springs Formation has since been subdivided into a series of members or tongues named after various ranches and physiographic features in the Rock Springs uplift. In ascending order, these members are: the Chimney Rock Tongue (Hale, 1950), Black Butte Tongue (Hale, 1950), Brooks Sandstone Tongue (Smith, 1961), Coul- son Shale Tongue (Smith, 1961), McCourt Sandstone Tongue (Smith, 1961), and Gottsche Tongue (Roehler, 1965; Smith, 1965) (figure 3). We propose a new mem- ber, the Vermillion Creek Tongue, for the Rock Springs Formation. The Vermillion Creek Tongue overlies the Gottsche Tongue and unconformably underlies the Trail Member of the Ericson Sandstone. The McCourt Sandstone and Gottsche Tongues are also present in the Vermillion Creek area. The Rock Springs Formation is underlain by the Blair Formation and unconformably overlain by the Trail Member of the Ericson Sandstone (Sears, 1925; Roehler, 1965; Miller, 1977) (figure 3). To- gether, the Blair Formation, Rock Springs Formation, Ericson Sandstone, and younger Almond Formation make up the Mesaverde Group in the Rock Springs up- lift area (Yourston, 1955) (figure 3). Chimney Rock Tongue The basal member of the Rock Springs Formation is called the Chimney Rock Tongue and consists of an upward coarsening, sandy marine wedge that pinches out into marine mudstone to the southeast of the Rock Springs uplift. It also transitions into continental de- posits to the northwest. The Chimney Rock Tongue has been interpreted as a wave-dominated deltaic system by many workers (Hale, 1955; Roehler, 1978; Hendricks, 1983; Levey, 1985; Løseth and others, 2006; Plink- Björklund, 2008). The main sandstone wedge consists of proximal and distal delta-front deposits that grade into prodeltaic deposits in a basinward direction (Hale, 1955; Roehler, 1978; Hendricks, 1983; Levey, 1985; Løseth and others, 2006; Plink-Björklund, 2008). Land- ward of the main sandstone wedge, delta-plain depos- its are composed of various depositional environments such as fluvial, lagoonal, bay, estuarine, lacustrine, and swamp (Hale, 1950, 1955; Douglass and Blazzard, 1961; Jacka, 1970). Other workers have favored a shoreface Figure 3. Stratigraphic columns for the eastern Rock Springs uplift, Vermillion Creek area in the Vermillion Basin, and the eastern Washakie Basin. The study interval is highlight- ed with a red box. Columns and age data are modified from Lynds and Slattery (2017) except for the Vermillion Creek column. Absolute ages for the Vermillion Creek Tongue are not known, therefore, this column is simply placed in its rel- ative position based on stratigraphic relationships. 50 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 centric terminology (Roehler, 1990; Purwanto, 2002; VanHolland, 2005; Rudolph and others, 2015). Black Butte Tongue The Chimney Rock Tongue is overlain by, and tran- sitional with, marine mudstone deposits of the Black Butte Tongue (Hale, 1950). The Black Butte Tongue weathers recessively forming distinct valleys between the Chimney Rock Tongue and overlying sandy tongues of the upper Rock Springs Formation. It is generally in- terpreted as a marine prodeltaic deposit (Roehler, 1965, 1978; Kirschbaum, 1985, 1986) or lower shoreface to offshore deposits (Rudolph and others, 2015). These sediments are thought to have been deposited within an embayment bordered by swampy deltaic plains (Hale, 1955). The tongue forms an important subsurface mark- er that can be correlated over great distances (more than 100 km) and is used to subdivide the lower sandy parts (Chimney Rock Tongue) from the upper sandy parts (Brooks and McCourt Sandstone Tongues) of the Rock Springs Formation (Hale, 1955; Douglass and Blazzard, 1961; Weichman, 1961; Miller, 1977; Hendricks, 1983; Kiteley, 1983; Roehler, 1985, 1987; Luo and Nummedal, 2010; Rudolph and others, 2015). Brooks Sandstone Tongue The Brooks Sandstone Tongue is one of two major upward-coarsening marine sandstone units of the up- per Rock Springs Formation that pinch out into marine mudstone in a southeastward direction. Deposits of the Brooks Sandstone Tongue transition into continental deposits in a northwestward direction, and the member is not recognized in the far north of the Rock Springs uplift. Equivalent rocks in the north remain undivid- ed. It has not been studied individually, but has been considered in larger scale studies of the Rock Springs Formation and Mesaverde Group (e.g., Roehler, 1965; Hendricks, 1983; Rudolph and others, 2015). It is in- terpreted as a wave-dominated deltaic system by some authors (Hale, 1955; Douglass and Blazzard, 1961; Roe- hler, 1978; Hendricks, 1983; Kirschbaum, 1985; Levey, 1985; Uroza, 2008) and as a shoreface system by others (e.g., Rudolph and others, 2015). Coulson Shale Tongue The Brooks Sandstone Tongue is overlain by, and transitional with, marine mudstone deposits of the Coulson Shale Tongue. The Coulson Shale Tongue weathers recessively despite having thin sandstone beds, including a regional sandstone marker bed that lies just above the Brooks Sandstone Tongue on the east flank of the Rock Springs uplift, southern Green River Basin, and Clay Basin (Rudolph and others, 2015). The Coulson Shale Tongue has been interpreted as offshore marine (Roehler, 1978; Rudolph and others, 2015) or prodeltaic (Roehler, 1983, 1985, 1986). Similar to the Black Butte Tongue, it forms an important subsur- face marker that can be correlated over great distances (more than 100 km). The Coulson Shale Tongue is used to separate sandy deposits of the Brooks and McCourt Sandstone Tongues in the subsurface (Hale, 1955; Dou- glass and Blazzard, 1961; Weichman, 1961; Miller, 1977; Hendricks, 1983; Kiteley, 1983; Roehler, 1985, 1987; Luo and Nummedal, 2010; Rudolph and others, 2015). McCourt Sandstone Tongue The McCourt Sandstone Tongue is the second (up- per) of two major upward-coarsening marine sand- stone units of the upper Rock Springs Formation that pinch out into marine mudstone in a southeastward direction. Deposits of the McCourt Sandstone Tongue transition into continental deposits in a northwestward direction, and the member is not recognized in the far north of the Rock Springs uplift. Equivalent rocks in the north remain undivided. The McCourt Sandstone Tongue has been studied in detail (Roehler, 1986; Uro- za, 2008) as well as in larger-scale studies of the Rock Springs Formation and Mesaverde Group (e.g., Roehler, 1965; Hendricks, 1983; Kirschbaum, 1986, 1989; Ru- dolph and others, 2015). It has been interpreted as both a deltaic system (Roehler, 1983, 1986; Uroza, 2008) and a shoreface system (Roehler, 1986; Rudolph and others, 2015) on the east flank of the Rock Springs uplift and in the Glades area. In the vicinity of the city of Rock Springs, Wyoming, it has been interpreted as a mix- ture of wave-dominated deltaic and shoreface deposits (Kirschbaum, 1986, 1989). 51 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Gottsche Tongue The uppermost member of the Rock Springs Forma- tion on the Rock Springs uplift is the Gottsche Tongue because the upper part of the Rock Springs Formation has been cut out by an unconformity (see figure 3). It is chiefly composed of mudstone with varying amounts of lenticular sandstones and isolated coal beds (Roehler, 1983, 1990). Though early workers commonly com- bined the Gottsche Tongue with the Ericson Sandstone (e.g., Hale, 1955; Douglass and Blazzard, 1961), they are easily differentiated because the Ericson Sandstone has nearly 100% sandstone and conglomerate, and is gener- ally coarser in grain size than the Gottsche Tongue. The Gottsche Tongue is interpreted as being deposited in freshwater marshes and swamps developed on coastal plains (Roehler, 1983, 1990). Small distributary-chan- nel and splay deposits are locally present and soils are generally poorly drained due to permanent saturation (Roehler, 1990). Between the Rock Springs uplift, Clay Basin, and Vermillion Creek area, the deposits of the Gottsche Tongue transition into fully marine delta-front deposits. Nomenclatural Issues Due to lithologic and environmental changes in a northwestward direction, the nomenclature as out- lined becomes problematic. Moving landward, sand- stone tongues (Chimney Rock, Brooks, and McCourt Sandstone Tongues) transition into rocks of continen- tal origin that lack significant marker beds, and mud- stone tongues (Black Butte and Coulson Shale Tongues) pinch out completely. In this northernmost area, work- ers commonly refer to these rocks as “the main body of the Rock Springs Formation,” as “continental deposits,” as “the coal bearing facies,” or something similar (Hale, 1950; Douglass and Blazzard, 1961; Roehler, 1965). Some workers have interpreted the outcrops in the Vermillion Creek area as part of the Rock Springs For- mation (Burger, 1965; Kiteley, 1983). However, the no- menclature from the Rock Springs uplift has not been introduced to the Vermillion Creek area, possibly due to correlation issues. Historically, most workers have tended to focus on outcrop to the west, north, and east of Vermilion Basin and neglect to include outcrop in- formation from this locality (Hale, 1955; Douglass and Blazzard, 1961; Keith, 1965; Weichman, 1961; Miller, 1977; Kiteley, 1983; Roehler, 1987; Luo and Nummedal, 2010). We introduce a new nomenclatural scheme for the Vermillion Creek area in the Vermillion Basin based on correlation and facies analysis. Vermillion Creek Tongue The Vermillion Creek Tongue (a new member and stratotype) is named for Vermillion Creek, which flows through the outcrop area on the southern edge of Vermillion Basin. The type section begins at 40°50'5.84" N., 108°40'34.94" W., and ends at 40°50'7.22" N., 108°40'29.43" W. However, the entire outcrop area, which stretches from approximately 40°50'13.58" N., 108°40'44.04" W. to 40°49'47.13" N., 108°40'1.76" W., can be considered as a type locality. Due to the limited surface exposures of the outcrop belt, a type log (Shell Creek Unit 6, API no. 0508106438) is presented alongside the type measured section for subsurface reference (figure 4). Additionally, the landward extent of the unit is shown on figure 1. The eastward extent of the unit beyond the outcrop is unknown due to sparse well- log data, and uncertain correlation in that direction. The Rock Springs Formation in the Vermillion Creek area consists of only two tongues: the Gottsche Tongue and the newly named Vermillion Creek Tongue. The Gottsche Tongue can be readily distinguished from the Vermillion Creek Tongue by color and bioturbation. The Gottsche Tongue has a distinct brown color and contains iron-stained burrows and concretions. The Vermillion Creek Tongue is colored various shades of gray to white at its base. The specific sedimentological characteristics of this new tongue are presented in following sections of this paper, and a type section is located to the northwest of Vermillion Creek (figure 4). In short, the Vermillion Creek Tongue differs from the Gottsche Tongue by being composed of a single sand- rich progradational package of deltaic and nearshore delta-plain deposits, whereas the Gottsche Tongue is composed of more landward mud-dominated delta- plain deposits. It is also clearly a younger progradational package separated from the Gottsche Tongue by a flooding surface in the Vermillion Creek area (figure 52 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Figure 4. Comparison between the Vermillion Basin NW measured type section from the NW outcrop area and outcrop gamma-ray curve (measurements every 0.5 m) with the nearest along-strike (northeast) subsurface well log. Both coarsen- ing-upwards successions that are present at the outcrop are also present in the subsurface. Additionally, a typical (incorrect) gamma-ray/lithostratigraphic pick for the base of the Ericson Sandstone is shown in blue. The true base of Ericson pick should be made much higher in the well log based on comparison with the outcrop gamma-ray curve. This highlights the usefulness of surface to subsurface ties in the vicinity of the Vermillion Creek area. 53 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 4). The Vermillion Creek Tongue’s westward extent is limited by erosion prior to deposition of the Ericson Sandstone as well as the transition from nearshore deposits to more landward facies, characteristic of the Gottsche Tongue. Age Constraints The Rock Springs Formation was deposited during the Campanian (Roehler, 1986) from 81.71 Ma (Scaph- ites hippocrepis II zone) at its base to approximately 80 Ma (Baculites maclearni zone) at the upper erosional unconformity (Luo and Nummedal, 2010) (figure 3). Parts of the Haystack Mountains Formation at the east- ern edge of the Washakie Basin are time-equivalent to the Rock Springs Formation, including parts of the Rock Springs Formation that were eroded prior to deposition of the Ericson Sandstone (Luo and Nummedal, 2010) (figure 3). Lynds and Slattery (2017) published a help- ful correlation of Upper Cretaceous strata from Wyo- ming that summarizes radiometric ages and ammonite biozones. The entire marine part of the Rock Springs Formation in the Rock Springs uplift is younger than the Hatfield Sandstone Member of the Haystack Moun- tains Formation (Lynds and Slattery, 2017); howerver, some older sandstone units of the Haystack Mountains Formation (Cow Creek Sandstone, O’Brien Spring Sandstone, Bolten Ranch, and Tapers Ranch Sandstone Members) are equivalent to the marine tongues of the upper Rock Springs Formation (Lynds and Slattery, 2017) (figure 3). No age data is available for the Vermil- lion Creek area. Surface to Subsurface Correlation Several workers have attempted subsurface correla- tion to try and tie outcrops of the Rock Springs uplift with outcrop on the edges of the Greater Green River Basin (Hale, 1955; Hallock, 1960; Douglass and Blaz- zard, 1961; Weichman, 1961; Keith, 1965; Miller, 1977; Hendricks, 1983; Kiteley, 1983; Roehler, 1985, 1987; Irwin, 1986; VanHolland, 2005; Luo and Nummedal, 2010; Rudolph and others, 2015). Several of these cor- relations pass very near outcrops of the Rocks Springs Formation in the Vermillion Creek area (Miller, 1977; Kiteley, 1983; Irwin, 1986; Roehler, 1987; Luo and Nummedal, 2010). In spite of the close proximity, these correlations did not incorporate the outcrop data. Tectonic Setting The Rock Springs Formation was deposited in a foreland basin (Wiltschko and Dorr, 1983; DeCelles, 1994; Liu and Nummedal, 2004; Luo, 2005). The thrust belt was active in the Campanian causing rapid subsid- ence in the foreland basin and the creation of significant accommodation (Wiltschko and Dorr, 1983). However, subsidence was not only controlled by flexure near the thrust front, but must also have had a dynamic man- tle flow component (Liu and Nummedal, 2004; Liu and others, 2005) as evidenced by rapid and widespread subsidence all across Wyoming, not just in the foredeep depozone (Liu and others, 2005). The boundary between the Rock Springs Formation and the overlying Ericson Sandstone is a widespread angular unconformity (Smith, 1961; Roehler, 1965; Miller, 1977). The length of time represented by the unconformity is approximately 1.5 million years (Mill- er, 1977; Kiteley, 1983) (figure 3). During a cessation of thrusting, isostatic rebound and associated erosion took place prior to deposition of the Ericson Sandstone (Liu and Nummedal, 2004; Liu and others, 2005). De- formation progressed eastward through time resulting in the incorporation of foredeep sediments within the thrust belt and erosion of older foredeep sediments, in- cluding part of the Rock Springs Formation (DeCelles, 1994; Liu and Nummedal, 2004; Liu and others, 2005). While it is clear that a foreland basin was present, the basin was likely subdivided or broken by Laramide deformation, which complicates the standard fore- land basin model (DeCelles, 2004; Rudolph and oth- ers, 2015). Laramide deformation probably began after deposition of the Rock Springs Formation (as early as 79 Ma; Gosar and Hopkins, 1969; Lynds and Xie, 2019), but a pre-Laramide deformational history may have had a local positive effect on subsidence (Hagen and others, 1985). The overall highly progradational nature of the Rock Springs Formation is generally controlled by high sedi- ment supply derived from the growth and erosion of the thrust belt, coupled with lower accommodation due to 54 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 slowing subsidence rates (Gill and Cobban, 1973; Hen- dricks, 1983; Kiteley, 1983; DeCelles, 1994; Hudson and others, 2019). Generally, sedimentation rates were equal to or higher than rates of subsidence (Hendricks, 1983; Kirschbaum, 1986; Devlin and others, 1993; Rudolph and others, 2015). However, during deposition of the transgressive Black Butte and Coulson Shale Tongues, subsidence either increased briefly, or eustatic sea level rose, causing widespread flooding and the deposition of significant marine mudstone over more proximal facies (Hendricks, 1983; Kiteley, 1983; Roehler, 1990; Rudolph and others, 2015). METHODS To better understand the variability of deposits in the field area, field data were collected from outcrops on both sides of Vermillion Creek (NW outcrop area and SE outcrop area; figure 1), including photographs and two measured sections. Measured sections include a graphic log with observations of texture, sedimenta- ry structure, bedding, bioturbation, and geomorphic expression. Sections were located based on accessibil- ity. Facies were then defined by grain size, sedimenta- ry structure, and/or bioturbation (table 1; figure 5) and subsequently grouped into associations. Additionally, one scintillometer (Radiation Solutions Inc. RS-230 BGO Super-Spec handheld gamma-ray spectrometer) survey was undertaken to measure the natural radioac- tivity of the rock along the same path as the measured section in the NW outcrop area (figures 1 and 4) . Mea- surements were taken every half meter. The resulting gamma-ray profile was then used to correlate units from the outcrop with subsurface gamma-ray curves from boreholes north of the outcrop. Additional out- crop gamma-ray profiles and measured sections were obtained for the east flank of the Rock Springs uplift and for Clay Basin to facilitate correlations between major outcrop areas (figure 1). A gamma-ray cutoff of 65 API units was used for net sandstone calculations on well-log data using IHS Petra. To map the vertical and lateral architectural ele- ment organization within the outcrop, additional pho- tographs were taken using a DJI Phantom 4 Pro drone and used to create a photogrammetric model of the main part of the outcrop (figure 6). Agisoft Metashape Professional was used to develop the photogrammetric model, and interpretations were made using LIME soft- ware (Buckley and others, 2019). Finally, to aid environmental interpretations, sam- ples were collected from mudstone beds throughout the section for pyrolysis. Fresh samples were obtained by digging holes beyond the surface exposure. Whole rock pyrolysis was performed using the HAWK Workstation (Wildcat Technologies). Samples were crushed with a mortar and pestle to 40 mesh size, then combusted to measure evolved hydrocarbons, CO, CO2, and to- tal carbon (table 2). A standard total organic carbon/ carbonate carbon (TOC+CC) method was used where the sample was heated from 300°C to 850°C at a rate of 25°C per minute during the pyrolysis stage, and from 300°C to 850°C at a rate of 25°C per minute during the oxidation stage (Espitalié and others, 1977; Peters and Cassa, 1994). FACIES ANALYSIS Facies Association 1 (FA1) – Delta Front FA1a Distal Delta Front – Description FA1a deposits are composed of heterolithic very fine grained sandstone and silty shale (10% to 60% sand- stone) (figure 5A). Sandstone beds increase in thick- ness from laminated (< 1 cm) to thick bedded (30 to100 cm) upsection. They are structureless, low to high-an- gle cross-stratified, planar cross-stratified, hummocky cross-stratified, or wave-rippled (figure 5B) and contain ubiquitous terrestrial organic matter. Laminated parts (both sandstone and mudstone) of the association have no discernible burrowing. When beds reach cm-scale, Chondrites (figure 7A), Planolites, Schaubcylindrichnus freyi, and Cylindrichnus (figure 7B) burrows are present with a maximum bioturbation index (BI) of 3 (Reineck 1963; Taylor and Goldring, 1993). Bioturbation intensi- ty is highly variable on the bed scale. Thicker sandstone beds may contain Palaeophycus (figures 7C and 7D), Thalassinoides, Chondrites, Planolites, Rosselia rotatus (fig- ure 7E), Cylindrichnus, Neonereites multiserialis (figure 7F), Intrites (cf., Menon and others, 2017) (figure 7C), and tool marks (figure 7C), especially along both basal and 55 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 upper bedding planes (figure 7B; BI up to 3). Minor (cm- scale) load features are common on the base of sandstone beds. The shale beds are laminated and fissile to chippy. The association is recessive, but locally, exceptionally well exposed due to the vertical attitude of the bedding. Where exposed, beds appear to maintain thickness laterally. Table 1. Facies in the Rock Springs Formation. Facies Code Facies Name Sedimentary Structures, Textures, and Additional Notes Bedding Facies Association Ss1 Trough cross-stratified sandstone Very fine to medium grained; variable cementation; commonly contains carbonaceous debris; Ophiomorpha, Thalassinoides; high and low-angle foresets Thin to medium FA1 FA2 FA3 Ss2 Asymmetric ripple cross-stratified sandstone Fine to medium grained; variable cementation; Skolithos Very thin to thin FA2 Ss3 Symmetric ripple cross-stratified sandstone Very fine to fine grained; variable cementation; commonly contains carbonaceous debris; Skolithos Very thin to thin FA2 Ss4 Planar cross-stratified sand- stone Very fine to fine grained; variable cementation; indistinct horizontal burrows Thin to medium FA1a Ss5 Hummocky to swaley cross-stratified sandstone Fine grained; variable cementation; contains carbonaceous debris; Ophiomorpha Medium FA1a Ss6 Bioturbated or rooted sandstone Very fine to medium grained; variable cementation; bioturbation has nearly to completely destroyed original bedding and sedimentary structures; Ophiomorpha, Thalassinoides, Planolites, Chondrites, Chondrites, Schaubcylindrichnus freyi, Cylindrichnus, Paleophycus, Rosselia rotatus, Neonereites multiserialis Medium or obscured FA2 Ss7 Soft-sediment deformed sandstone Medium grained; variable cementation; commonly contains carbonaceous debris Very thin to thick FA1 FA2 Ss8 Structureless sandstone Very fine to medium grained; variable cementation; Ophiomorpha Medium or obscured FA1 FA2 FA3 M Silty mudstone Structureless to laminated; nodular or chippy to fissile; Chondrites Laminated FA1 FA3 L Pebble lag Pebbles are composed of mud clasts that typically weather leaving voids along basal surfaces; Apectoichnus longissimus in woody debris NA FA2 C Coal and carbonaceous mudstone Contains carbonaceous debris; true coal exhibits well-developed cleats Laminated to medium FA3 56 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 FA1b Proximal Delta Front – Description FA1b deposits are composed of very fine to medium grained, medium bedded (10 to 30 cm), low- to high-angle trough cross-stratified, structureless, or soft-sediment deformed sandstone. Bioturbation is composed of locally abundant, but typically sparse, Ophiomorpha (figure 7G) or Palaeophycus burrows (BI = 1 to 2). Both basal and upper contacts are sharp. The unit is laterally continuous across the entire outcrop. Interpretation and Depositional Setting We interpret the deposits of FA1 as marine-delta- ic. Several key criteria for a deltaic deposit are outlined in the literature: (1) a direct link to feeding rivers can be identified, (2) significant progradation occurred in- dicating a fluvial sediment source, (3) deposits are up- ward-shallowing, (4) seaward-dipping clinoforms are present, and (5) the deposit is lobate or elongate as a shoreline protuberance (e.g., Kirschbaum, 1989; Roe- hler, 1990; Bhattacharya, 2006, 2010). No direct link to feeding rivers could be established for the lowermost deposits exposed in the Vermillion Creek area possibly due to wave reworking of the delta front, and also pos- sibly due to limited spatial outcrop exposure. However, these deposits are overlain by delta-plain and distrib- utary-channel deposits (figure 6), which do indicate a direct link to a fluvial source, at least for younger de- posits (e.g., Bhattacharya, 2006, 2010). The abundance of terrestrial organic matter is also likely derived from a fluvial source on or near a delta. Additionally, the presence of Intrites structures and soft-sediment defor- mation indicate high sedimentation rates potentially associated with fluvio-deltaic sedimentation (e.g., Bhat- tacharya, 2006, 2010; Menon and others, 2017). Progra- dation can be established to be at least 15 km downdip Figure 5. Selected facies from the Vermillion Creek area. (A) Interbedded sandstone and silty mudstone which includes facies M, Ss1, Ss3, Ss4, and Ss7. (B) Symmetrical rippled sandstone (Ss3). (C) Trough cross-stratified sandstone (Ss1). (D) Asymmetrical rippled sandstone (Ss2). (E) Small-scale soft-sediment deformed sandstone (Ss6). (F) Large-scale soft- sediment deformed sandstone (Ss6) within distributary channel deposits. Note that F has been rotated to horizontal (top is to the right) to highlight that this slump feature occurred at the base of the sandstone. 57 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Figure 6, part 1. Virtual outcrop interpretations for the NW outcrop area northwest of Vermillion Creek. (A) Aerial view of the NW outcrop looking straight down from above. Solid yellow line represents measured section segments and dashed yellow line represents section offsets. (B) Line drawings of the NW outcrop indicating important surfaces and channel complexes (1 through 3). (C) Facies map of the NW outcrop indicating various depositional environments. Refer to figure 1 for location of outcrop area. 58 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Figure 6, part 2. Virtual outcrop interpretations for the SE outcrop area southeast of Vermillion Creek. (D) Oblique aerial view of the SE outcrop. Solid yellow line represents measures section segments and dashed yellow line represents section offsets. (E) Line drawings of the SE outcrop indicating important surfaces and channel complexes (4 through 6). (F) Facies map of the SE outcrop indicating various depositional environments. Refer to figure 1 for location of outcrop area. 59 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 (southeast) via subsurface correlation, but to the east and southeast of the outcrop, well data is too sparse for confident correlation. Progradation may be up to 27 km based on available well data. For comparison, the Mc- Court Sandstone Tongue can be correlated at least 70 km in the subsurface with outcrop control over much of that distance. The McCourt Sandstone Tongue has been interpreted as deltaic by many authors, and meets all of the aforementioned criteria for deltaic deposits (Roe- hler, 1983, 1986; Uroza, 2008). The deposits of FA1 do meet the criteria that the deposits be upward-shallow- ing. This is evidenced by the increase in more proximal facies over more distal facies. However, other systems also exhibit upward shallowing of facies such as shore- Table 2. Pyrolysis data in stratigraphic order with sample DP-L near the top of the Rock Springs Formation and sample PS1-1 near the base. See figure 8 for sample locations. Sample ID S1 (mg/g) S2 (mg/g) S3 (mg/g) PI TOC (%) Tmax (°C) HI OI Kerogen Type Facies Association DP-L 3.33 51.04 11.24 0.06 10.26 424 497 109 II FA3 DP-K 1.1 16.17 4.59 0.06 5.96 425 271 77 II/III FA3 DP-J 0.77 14.95 5.93 0.05 5.89 423 253 100 II/III FA3 DP-I 0.38 1.01 0.69 0.28 0.87 412 115 79 III FA3 DP-H 0.73 1.1 0.75 0.4 1.06 409 103 71 III FA3 DP-G 0.24 0.43 0.75 0.36 0.72 409 59 103 III FA3 DP-F 7.84 110.72 19.02 0.07 17.02 423 650 111 I FA3 DP-E 3.52 27.77 8.51 0.11 10.1 418 274 84 II/III FA3 DP-D 1.84 45.93 23.35 0.04 15.28 425 300 152 II FA3 DP-C 2.06 10.89 1.93 0.16 3.58 425 303 53 II FA3 DP-B 0.3 1.26 0.6 0.19 0.74 429 169 80 III FA3 DP-A 2.99 17.2 2.77 0.15 4.41 426 390 62 II FA3 DP-A-EQ 1.37 19.61 4.45 0.07 6.1 430 321 72 II FA3 DP-7 0.44 0.43 0.56 0.51 0.59 363 71 94 III FA3 DP-6 0.46 0.45 0.55 0.51 0.58 394 78 95 III FA3 DP-5 0.36 0.45 0.58 0.45 0.71 417 62 81 III FA3 DP-4 0.33 0.36 0.34 0.48 0.35 316 102 97 III FA3 DP-3 0.65 1.01 0.96 0.39 1.32 418 76 72 III FA3 DP-2 0.35 0.39 0.5 0.47 0.49 369 79 102 III FA3 DP-1 0.36 0.77 0.62 0.32 0.94 412 81 65 III FA3 PS2-7 0.28 1.62 0.94 0.15 1.49 427 108 62 III FA1a PS2-6 1.18 1.92 1.31 0.38 1.65 415 116 79 III FA1a PS2-5 0.33 0.98 0.92 0.25 1.16 415 84 79 III FA1a PS2-4 0.52 1.49 1.24 0.26 1.49 417 99 82 III FA1a PS2-3 0.46 1.32 0.98 0.26 1.38 416 95 70 III FA1a PS2-2 0.44 1.19 0.82 0.27 1.28 423 92 64 III FA1a PS2-1 0.21 1.35 1.05 0.14 1.15 428 116 91 III FA1a PS1-4 0.15 0.27 0.32 0.35 0.33 428 80 97 III FA1a PS1-3 0.25 0.49 0.44 0.33 0.45 427 109 97 III FA1a PS1-2 0.09 0.19 0.29 0.32 0.27 434 71 105 III FA1a PS1-1 0.19 0.77 1.02 0.2 0.84 428 92 121 III FA1a 60 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 face systems and bayhead deltas. The outcrop is not ex- tensive enough to identify any clinoformal surfaces, and we cannot prove deltaic geometry with available data. Deltaic environments are commonly stressed due to high turbidity and lowered salinity (e.g., Howard and Frey, 1973; Wightman and others, 1988; Pemberton and Wightman, 1992; Bhattacharya, 2006, 2010). The rela- tively low BI in the Vermillion Creek area more closely matches the expectations for deltaic shorelines rather than shoreface systems that are distant from active del- taic processes. Furthermore, the trace fossil assemblage for deposits of FA1 (Rosselia rotatus, Planolites, Schaub- cylindrichnus freyi, Cylindrichnus, Thalassinoides, and Chondrites) are referable to the Rosselia ichnofacies of sandy delta-front deposits (MacEachern and Bann, 2020). The variable, and generally low, BI are charac- teristic features of this ichnofacies (MacEachern and Bann, 2020). The variable bioturbation intensity at the bed scale is another feature common to the Rosselia ich- nofacies, and may indicate variable physico-chemical stresses common in deltaic environments (MacEachern and Bann, 2020). The presence of Neonereites multise- rialis is a noteworthy departure from expectations for deltaic sediments. Examples from the literature suggest Figure 7. Selected photographs of trace fossils. (A) Chondrites burrows (Ch). (B) Cylindrichnus burrow (Cy). (C) Palaeophycus burrows (P), tool marks (T), and Intrites (i). (D) Palaeophycus burrows (P) and Ophiomorpha burrows (O). (E) Rosselia rotatus burrow (R). (F) Neonereites multiserialis traces (N). (G) Ophiomorpha burrows (O) within the proximal delta-front sandstone. (H) Apectoichnus longissimus borings (A). (I) Heavily bioturbated distributary channel sandstone. All traces are Ophiomorpha (O). 61 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 that these traces are confined to deep water flysch (e.g., Pickerill, 1991; Cherif and Naimi, 2021) or lower shore- face to offshore transition deposits (e.g., Bouchemla et al., 2021). The noted presence of Neonereites multiseri- alis may be the first instance of that trace in delta-front sediments. Architecture FA1a deposits are overlain by FA1b deposits, and there are two repeated successions of this stacking pat- tern (figure 6). The lowermost succession is obscured at its base by Quaternary sediments (figure 6), so its as- sociation with underlying sediments cannot be ascer- tained at the outcrop. However, correlations to subsur- face boreholes via wireline logs indicate relatively high gamma-ray readings suggestive of shale deposits (figure 4). The second succession is overlain by deposits of FA3 (delta plain; figure 6). Facies Association 2 (FA2) – Distributary Channel Description FA2 deposits are dominantly composed of fine to medium grained sandstone with minor interbeds of mudstone. Beds range from thin to thick bedded. The dominant sedimentary structure is unidirectional trough cross-stratification (figure 5C), but asymmetri- cal rippled (including climbing ripples; figure 5D) and soft-sediment deformed sandstone (figures 5E and 5F) is common. Sandstone can sometimes appear structure- less. Symmetrical ripples are present at the tops of some units. Internal mud-clast lags are also present and car- bonaceous debris is common throughout. Apectoichnus longissimus (figure 7H) and Skolithos burrows are pres- ent as well as locally abundant Ophiomorpha (figure 7I). The BI is highly variable from 0 up to 4. Basal contacts are sharp and scoured. Master bedding surfaces extend from the top to the base of each unit (figure 6). Individ- ual sandstone bodies pinch out laterally and have a len- soidal geometry except in cases where the edges of the bodies are obscured. The association can be underlain or overlain by additional FA2 (distributary channel) de- posits or by FA3 (delta plain) deposits (figure 6). Interpretation and Depositional Setting These deposits are interpreted as distributary-chan- nel deposits near and behind the delta front. Master bedding surfaces that extend from the top to the base of individual channel bodies are interpreted as accretion surfaces and may include lateral and/or downstream components (figure 6). The lensoidal geometry of these deposits coupled with the presence of distinct accre- tion surfaces suggests a fluvial origin (e.g., Miall, 2010). However, the presence of Ophiomorpha and brack- ish-water Apectoichnus (previously Teredolites) bur- rows suggests close proximity to marine conditions in distributary channels (e.g., Bromley and others, 1984; Gingras and others, 2004, 2005; Donovan, 2018; King and others, 2020). Architecture FA2 deposits (distributary channel) are arranged in mul- tistory and multilateral channel complexes with intervening FA3 deposits (delta plain). There are a total of six complex- es, and no attempt has been made to correlate complexes between the northwest and southeast outcrops. Complexes 1, 2, 3, and 5 exhibit both multistory and multilateral archi- tectures (figure 6). Individual complexes range from 2 to 6 stories. Complex 4 and 6 suffer from poor exposure making identification of story surfaces difficult (figure 6). However, for complex 4, variable dip directions for interpreted mas- ter bedding surfaces indicates higher complexity than what is apparent from the virtual outcrop data (figure 6). It is likely that all six complexes are multistory and multilateral. Facies Association 3 (FA3) – Delta Plain Description FA3 deposits are composed of silty mudstone, silty carbonaceous mudstone (figure 5F), coal (figure 5F), and isolated, very thin (1 to 3 cm) to medium (10 to 30 cm) beds of very fine to fine-grained carbonaceous sandstone. Sandstone beds can be trough cross-strati- fied or structureless. Carbonaceous debris and wood fragments are common throughout. The association is usually overlain and underlain by FA2–distributary channel deposits (figure 6). However, it also overlies de- posits of FA1b–proximal delta front (figure 6). 62 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Interpretation and Depositional Setting These deposits are interpreted collectively as depos- its of the delta-plain environment. The abundance of carbonaceous material, coal beds, and lack of marine bioturbation suggests a continental origin. Thin sand- stone deposits are likely crevasse splays (e.g., Allen, 1964; Miall, 2010) and coal beds indicate the presence of swampy conditions on the delta plain (Roehler, 1983, 1990). PYROLYSIS Thirty-one samples were collected from mudstone beds along the measured section in the NW outcrop area in order to better understand kerogen types within various units (figure 8). Pyrolysis data shows variability in organic matter type and richness throughout the sec- tion. Data from samples representing delta-front strata (FA1) indicate a relatively low hydrogen index (HI; 50 to 200) typical of type III, terrestrially sourced kerogen (table 2; figure 8). All samples contain coal flecks that are sometimes visible with the naked eye, and always visible with the aid of a hand lens (10x). Low HI values in delta-front deposits are likely due to significant ter- restrial input from fluvial point sources on a delta (e.g., Meilijson and others, 2020). Samples taken from delta-plain strata (FA3) exhibit a wider range of HI (59 to 650; table 2; figure 8). Sam- ples with the highest HI values correspond with the highest TOC (table 2). These samples contain signifi- cant carbonaceous material, that is visible to the naked eye, yet have HI values that indicate a mixed type II/ III kerogen. This suggests that FA3 deposits consist of a mix of humic and algal kerogens. While not common, coal-bearing strata can contain mixed kerogen types (e.g., Fleet and Scott, 1994). Peat swamps can contain algal matter via marine flooding (Bagge and Keeley, 1994) or non-marine lacustrine facies variation (Par- nell, 1988; Powell and Boreham, 1994; Thompson and others, 1994). Both scenarios are possible for the FA3 deposits in the Vermillion Creek area. With HI values as high as 650 (sample DP-F), there is also the possi- bility that some samples contain some type I kerogen. Type I kerogens are algal and distinctly lacustrine in origin, suggesting lacustrine influence. Coal and lacus- trine deposition often take place in close proximity to each other, with ponded waters commonly occurring in a poorly drained delta-plain environment, and can be gradational through time forming two separate end members of a depositional continuum (Fleet and Scott, 1994). CORRELATION Two correlation panels have been created to tie the sections in the Vermillion Creek area to the Rock Springs uplift and Clay Basin (figure 9). In attempting to correlate between these outcrops, it was first neces- sary to determine how the section in the Vermillion Creek area compared to the closest available borehole wireline data along depositional strike (figure 4). Of interest is the interpretation of the basal contact of the Trail Member of the Ericson Sandstone. This contact has historically been located at the base of a significant increase in net sandstone and grain size, as determined by the abundance of low gamma-ray response in wire- line logs (e.g., Kiteley, 1983; Roehler, 1987) (figure 4); a purely lithostratigraphic interpretation. Our outcrop gamma-ray measurements indicate that the uppermost Rock Springs Formation deposits also have this same log response, and we have interpreted the basal ero- sional unconformity of the Ericson Sandstone as much higher in the section than previous workers (Kiteley, 1983; Roehler, 1987) (figure 4). Correlation between major outcrops was aid- ed by the use of high-confidence picks that could be matched to outcrop gamma-ray curves (figure 9). These high-confidence picks include the Rock Springs Forma- tion/Ericson Sandstone contact, the flooding surface of the Coulson Shale Tongue, and minor flooding surfac- es within and above the McCourt Sandstone Tongue (figure 9). We also make the assumption that the Eric- son Sandstone members are relatively isopachous over short distances (about 10 km or less). Over larger dis- tances (many tens of km), the Ericson Sandstone thick- ens gradually eastward (figure 9). Thicknesses of the Ericson Sandstone in the Vermillion Creek area were measured from satellite imagery with the assumption that the deposits are nearly vertical, similar to the Rock Springs Formation in the area. 63 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Figure 8. Hydrogen Index (HI) shown with the Vermillion Basin NW section from the NW outcrop area. HI values are typ- ically indicative of type III, terrestrial kerogen or type II/III mixed terrestrial/marine or possibly lacustrine kerogen*. The abundance of type III kerogen suggests a heavy fluvial source common in deltaic settings. Note that measured section data does not extend as low as gamma-ray data due to outcrop quality. Holes were dug through cover to obtain the lowermost gamma-ray measurements. Refer to figure 4 for explanation of symbols. 64 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 Our correlation panels indicate that almost the en- tire section preserved at the Vermillion Creek area is younger than the Rock Springs Formation at both the Rock Springs uplift and Clay Basin (figure 9). Anoth- er notable feature is the apparent thinning of the Gott- sche Tongue between the Vermillion Creek area and Clay Basin (figure 9), indicating that the basal Eric- son Sandstone erosional unconformity removed more Figure 9. Surface to subsurface correlations. (A) Correlation from the east flank of the Rock Springs uplift (Pretty Water Creek Pipeline) to the Vermillion Creek type section. Prominent sandstones of the McCourt Sandstone Tongue pinch out in a southeastward direction. Upward-coarsening marine sandstones of the Vermillion Creek area are partly equivalent to the Gottsche Tongue of the Rock Springs uplift. Note the presence of a second coarsening-upward marine sandstone, labeled as “Marine Marker,” which is the lower part of the Vermillion Creek Tongue. The basal Ericson Sandstone unconformity removes progressively more of the Rock Springs Formation to the west. Also note that the outcrop gamma ray for the Pretty Water Creek Pipeline location is shifted away from the neighboring well log (S.P. Fed. 14-20 well) for ease of viewing. The original position is still shown behind the S.P. Fed. 14-20 well. (B) Correlation from the Clay Basin area to the Vermillion Creek outcrop. Prominent sandstones of the McCourt Sandstone Tongue pinch out in a southeastward direction. Upward- coarsening marine sandstones of the Vermillion Creek area are partly equivalent to the Gottsche Tongue of the Clay Basin area. Again, note the presence of a second coarsening-upward marine sandstone, labeled as “Marine Marker,” which is the lower part of the Vermillion Creek Tongue. The basal Ericson Sandstone unconformity removes progressively more of the Rock Springs Formation to the west. VCT = Vermillion Creek Tongue. See figure 1 for location of cross sections. 65 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 of the Rock Springs Formation in the Clay Basin area, possibly due to increased uplift (e.g., Smith, 1961; Roe- hler, 1965; Miller, 1977; Uroza, 2008). The correlation panels also indicate that the Gottsche Tongue of the Rock Springs uplift and Clay Basin transitions into lat- erally continuous marine deposits approximately 16 km southeastward from the southernmost Rock Springs uplift outcrop (figure 9). Overlying this basal marine Gottsche Tongue is a progradational package of deltaic, delta plain, and distributary-channel complexes herein defined as the Vermillion Creek Tongue. A type section for this new tongue is presented in figure 4. DISCUSSION Comparison With Nearby Areas Deposits of the sandy marine tongues of the Rock Springs Formation at the Rock Springs uplift and Clay Basin meet the established criteria for deltaic deposits, namely: (1) a direct link to feeding rivers can be iden- tified, (2) significant progradation occurred indicating a fluvial sediment source, (3) deposits are upward-shal- lowing, (4) seaward-dipping clinoforms are present, and (5) the deposit is lobate or elongate as a shoreline protuberance (e.g., Kirschbaum, 1989; Roehler, 1990; Bhattacharya, 2006, 2010). Marine shoreline deposits exposed at the base of the Rock Springs Formation in the Vermillion Creek area are also interpreted as del- taic deposits. However, there are a few key differences. Outcrops in the Vermillion Creek area contain a signifi- cantly higher biodiversity (eight or more ichnogenera) than deltaic deposits of the other areas (an average of three different ichnogenera per location). Additionally, the total thickness of each of the FA1 sequences in the Vermillion Creek area is just over 20 m whereas similar deposits elsewhere are 40 m or more thick. The delta-plain deposits of the Gottsche Tongue at the Rock Springs uplift and Clay Basin and the delta- plain interval in the Vermillion Creek Tongue are not time-equivalent (figure 9). However, both intervals record the final preserved marine influence of each area until flooding events associated with the Upper Campanian Almond Formation occurred. There are some similarities between locations. Thin, discontinuous coal beds are common in all areas, as well as thin sandy splay deposits. Teredolites or Apectoichnus bored logs in channel lag deposits were found in the Rock Springs uplift and in the Vermillion Creek area. Brackish-water burrows such as these, suggest close proximity to the sea in distributary channels (e.g., Bromley and others, 1984; Gingras and others, 2004, 2005; King and others, 2020). The differences between the outcrops in the Ver- million Creek area and other areas are readily apparent. The Gottsche Tongue has a significantly lower net sand- stone percentage (29% average near the Rock Springs uplift, n = 28) than the upper deposits of the Vermillion Creek Tongue (62% average near Vermillion Creek, n = 16). However, in spite of a low net sandstone percent- age, multistory and multilateral deposits do exist in the Gottsche Tongue, though single story channel deposits are more common. Stacking Patterns and Implications for Subsurface Exploration The Vermillion Creek area exhibits an overall pro- gression from fully marine deltaics to marine-influ- enced distributary-channels and delta-plain deposits. This progression is consistent with the progradation of a deltaic system having more proximal facies associa- tions systematically overlying more distal facies asso- ciations (e.g., Coleman and Prior, 1982; Bhattacharya, 2006, 2010). The stacked distributary-channel complexes are separated by laterally continuous delta-plain deposits suggesting periodic abandonment and reoccupation of the Vermillion Creek area by distributary channels. Periods of abandonment correlate with incidences of higher HI, suggesting marine or lacustrine influence. The fact that marine influence (marine trace fossils) is present in at least three of the six distributary-chan- nel complexes, including the uppermost complexes, suggests aggradation of the shoreline. Contemporane- ous shoreline deposits were likely near the Vermillion Creek area for the entirety of the deposition of the dis- tributary-channel complexes. Wireline data is sparse and correlation is difficult east of the Vermillion Creek area. While difficult to prove time-equivalency, coars- ening upward successions are present east of the Ver- 66 The Upper Cretaceous Rock Springs Formation of Northwest Colorado—A Previously Undescribed Key Deltaic Outcrop Phillips, S., and Hudson, S.M. Geology of the Intermountain West 2021 Volume 8 million Creek area that may be time-equivalent to the delta-plain and distributary-channel deposits. The interpretation that contemporaneous shoreline deposits are likely present to the east of the Vermillion Creek area has implications for oil and gas explora- tion activities in the nearby Sand Wash and Washakie Basins. Marine sandstone reservoirs may have greater prospectivity than channelized facies due to larger lat- eral extents. Oil and gas accumulation within reservoirs of this type, east of the Vermillion Basin, are sparsely tested, and may be significant (Finn and Johnson, 2005; Johnson and others, 2005). The type of delta has an in- fluence on the geometry of subsurface reservoirs (e.g., Slatt, 2006). It is unknown what type of delta deposits are present east of Vermillion Creek. However, wave-dom- inated deltas typically have fewer distributary channels than their fluvial-dominated counterparts, and can become entrenched (e.g., Bhattacharya, 2006, 2010). The fact that the percentage of distributary-channel deposits in the Rock Springs Formation is significant- ly higher in the Vermillion Creek area than the Rock Springs Formation elsewhere may suggest a slight shift toward fluvial dominance, but also may be due to fluc- tuating accommodation. Distributary channels display high-sedimentation rates as indicated by significant soft-sediment deformation in channelized deposits and high sedimentation rates can be an indicator of fluvial dominance (e.g., Bhattacharya, 2006, 2010). CONCLUSIONS The Rock Springs Formation of the Vermillion Creek area in northwestern Colorado is composed of marine-deltaic, distributary-channel, and delta-plain sediments representing progradation to aggradation of the shoreline. The Rock Springs Formation outcrops in the Vermillion Creek area differs from Rock Springs Formation outcrops elsewhere in Utah and Wyoming by having thinner delta-front successions and having a higher net sandstone (distributary channels) within delta-plain deposits. The Rock Springs Formation in the Vermillion Creek area is younger than Rock Springs Formation elsewhere in Utah and Wyoming. It also preserves the youngest available outcrop of the Rock Springs Forma- tion described in literature and is an important data point between outcrops of the Rock Springs uplift or Clay Basin and contemporaneous deposits on the east and southeast edges of the Greater Green River Basin. Additionally, a new member of the Rock Springs For- mation has been defined and has been named the Ver- million Creek Tongue. Future subsurface exploration for hydrocarbons east of the Vermillion Basin is likely to encounter shoreline deposits that are time-equivalent to the dis- tributary-channel complexes of the Vermillion Creek Tongue. This geographic area is not well tested, and may be significant. ACKNOWLEDGMENTS This work was funded by ConocoPhillips. We are grateful to Joe Phillips (Occidental Petroleum Corpora- tion), Jonathon Sevy (Brigham Young University), Chel- sea Jolley (Marathon Oil), Riley Brinkerhoff (Wasatch Energy Management), and Peter Pahnke (Wexpro) for their help in the field. 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