Castlewood Canyon is one of the most distinctive landforms on the Colorado plains—a geomorphology that developed as Cherry Creek and its precursors incised into the Eocene Wall Mountain Tuff and overlying Castle Rock Conglomerate (CRC). Outcrops of the CRC in Castlewood Canyon State Park (CCSP) contain boulders of the Wall Mountain Tuff that are up to two meters in diameter, and the conglomerate itself is composed of large (up to 0.5 m), diverse clasts of Precambrian granite, gneiss, quartzite, and other lithologies eroded from the Colorado Front Range that is 25 km to the west and as much as 100 kilometers to the northwest. These clasts and other evidence suggest transport and deposition by a sequence of flood events. Such flooding events, albeit smaller in scale, continue to occur in modern times, including a catastrophic flood caused by the failure of the Castlewood Dam in 1933, and a canyon-scouring event in 2023. These events and the geologic history of this canyon are described in this paper, illustrating that nature, mild though it may be for millennia, is still shaping the Castlewood Canyon system.

Hydrous amorphous silica (aka opal) is a common cement in the Upper Eocene Castle Rock Conglomerate (CRC) of the southwestern Denver Basin. Petrographic study of standard thin sections indicates that this opal forms from 5% to as much as 40% of any given sample. It also commonly occurs as a precursor to fibrous length-fast chalcedony, a crystalline form of quartz cement. Similar opal cement apparently derived from shards of volcanic glass is even more common in the subjacent Wall Mountain Tuff, a welded deposit of volcanic ash that was the most likely source of silica in the opal cement in the CRC. This paper provides a first-of-its-kind attempt to quantify the amount of opal cement in selected samples of the CRC based on X-ray diffraction (XRD) analysis. Because opal is amorphous, its abundance cannot be quantified using standard XRD techniques, but experimenting with heating indicates that it is possible to convert powdered amorphous opal to a crystalline form of silica in less than 48 hours at a temperature of 550°C. Comparison of pre- and post-heat treatment XRD diffractograms thus provides a potential tool for quantifying opal content in the CRC and other opal-cemented samples. This new analytical technique and its results are described in this paper.

The Castle Rock Conglomerate is one of Colorado’s most iconic, youngest, and coarsest grained rock units. It is also one of the hardest sedimentary rocks in Colorado and forms prominent buttes in the southwestern Denver Basin. Yet the reasons for its induration and resistance to weathering have not previously been investigated. Sedimentologic observations paired with sedimentary petrology indicate that much of the unit is comprised of a planar-bedded to cross-bedded, mostly poorly sorted, angular to subrounded assemblage of quartz, K-feldspar, quartzite, and unusually large volcanic rock fragments along with some plagioclase and mica flakes. The largest volcanic rock fragments are up to ∼2 m in size and composed of the immediately subjacent Wall Mountain Tuff of late Eocene age. Sedimentary rock fragments and well-rounded quartz grains are rare. Together these features suggest a diverse and relatively proximal provenance for the unit. Pervasive opaline cement coats most grains, and locally exhibits pendant features typical of vadose precipitation. These opal cements formed prior to any grain compaction and indicate early silica precipitation at shallow burial depths. Where the primary pores were not completely cemented by the opal, most were later filled with length-fast chalcedony cement. We hypothesize that cementation of the conglomerate began soon after deposition as weathering of the Wall Mountain Tuff and weathering of clasts of the tuff within the conglomerate, yielded ground water super-saturated with silica. These fluids initially catalyzed precipitation of common opal (hydrous amorphous silica) and later fostered precipitation of length-fast chalcedony. Together, these cements created a silica-cemented “concrete” much more resistant to weathering than any carbonate-cemented sandstone, and much harder than man-made calcite-cemented concrete found in many sidewalks and roadways.

ABSTRACT Facies of the Permian Lyons Sandstone are described and interpreted based on analyses of 23 cores from Larimer and Weld counties, Colorado. Here, the Lyons Sandstone consists of very fine- to medium-grained sandstone with minor silt and mudstone interbeds. The unit has five recurrent siliciclastic facies that can be grouped into two facies associations (FA). FA1 consists of (1) high-angle, cross-laminated sandstone (Facies 1; interpreted as eolian dune remnants); (2) low-angle, cross-laminated and horizontally laminated sandstone (Facies 2; interdune); and (3) chaotically bedded to folded sandstone (Facies 3; lower dune flanks). FA2, in contrast, is mainly (4) wavy- to irregularly laminated silty sandstone (Facies 4; wet to damp interdune); and (5) massive to wavy-laminated silt-rich mudstone (Facies 5; ponded water areas between dunes) with minor amounts of high-angle, cross-laminated sandstone (Facies 1) and low-angle, cross-laminated and horizontally laminated sandstone (Facies 2). FA1 is hypothesized to have been produced in an eolian system akin to those that might exist in the dune-dominated portion of an erg, whereas FA2 was deposited in the intermittently wet portion of this eolian system, perhaps along erg margins or in flat dune-adjacent settings that were impacted by the water table. Isopach data suggests that the study area is on the fringe of a larger Lyons system that spans > 100,000 km2, and was deposited close to the Ancestral Rockies—a paleogeography consistent with deposition in erg to erg-margin paleoenvironments. Detrital zircon populations from nearby Colorado Front Range outcrops and from 12 correlative eolian units are dominated by small, well-rounded Paleoproterozoic and Mesoproterozoic grain populations that are remarkably similar between units, signaling a well-mixed system that also received an influx of distally sourced sediment from the Appalachian orogen. Detrital zircon-based maximum depositional ages of the Lyons Sandstone and its equivalents are internally consistent with deposition of the unit during the latest Artinskian to Kungurian.

Just as the world’s population, knowledge in general, and the Rocky Mountain Association of Geologists (RMAG) have changed in major ways during the past 100 years, so too has the study and interpretation of carbonate rocks and reservoirs. The RMAG, a century old in 2022, has evolved from just 50 original charter members who held the organization’s first “meeting” in 1922, to its approximately 1800 members today. Thus, RMAG’s publications have helped document the evolution of carbonate rock studies, particularly those in the Rocky Mountain region. Key contributions have been made through RMAG’s hundreds of luncheon talks, through its quarterly technical publication, The Mountain Geologist, initiated in 1964, and the exceptionally comprehensive Geologic Atlas of the Rocky Mountain Region, published 50 years ago in 1972. In addition, since 1953 the RMAG has published field guides and symposia volumes focused on specific basins, types of reservoirs, and structural geology among other things. Many of these books contain papers focused on carbonate rock units in the Rockies. Analysis of the papers published in The Mountain Geologist each year from 1964 through 2021 reveals that a fairly consistent 10 to 15% of that journal’s articles each year deal directly with some aspect of carbonate rocks. Earlier papers in the 1960s and 1970s dealt mainly with outcrop studies and the correlation of specific carbonate units based on measured sections or the use of fossils to define facies and biostratigraphic units. During the 1980s emphasis shifted to refining carbonate depositional models and focusing more on carbonate diagenesis through detailed petrographic studies, isotopic analyses, cathodoluminescence, and scanning electron microscopy. The 1990s brought a shift to papers focused more on specific carbonate hydrocarbon reservoirs ranging from the peritidal dolomites in Cottonwood Creek Field to relatively deepwater Waulsortian mudmounds in the Mississippian Lodgepole Formation and the “basinal” chalks of the Niobrara Formation. The focus of carbonate studies shifted again in the early 2000s to the use of 3-D seismic data to better understand specific carbonate reservoirs and the increased interpretation of carbonate deposits within the context of sequence stratigraphy. The tools used to study carbonate rocks expanded even further over the past decade with more refined isotopic data, improved SEM studies, and the use of elemental data obtained with X-ray fluorescence analyses. No doubt the next decade will bring even more improvements in data collection methods and the interpretation of depositional and diagenetic processes that have impacted all Rocky Mountain carbonate deposits.

ABSTRACT We report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of > 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), > 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (< 0.1%; n = 2,927 grains).

ABSTRACTPaired chemostratigraphic and biostratigraphic data suggest that the Devonian–Carboniferous boundary and the Hangenberg extinction event are recorded in the Coffee Pot Member of the Dyer Formation of the White River uplift region of northwestern Colorado. The Hangenberg isotopic excursion interval occurs in biostratigraphically depauperate shallow platform micritic dolostone and limestone representing the aculeatus–?ultimus–kockeli? Zone. The Hangenberg interval strata have δ13Ccarb values up to 7.69‰, and locally contain ooids, stromatolites, and other microbial structures. In three sections, there is a short-lived > 4‰ “pre-Hangenberg” positive excursion that is associated with the presence of detrital dolomite siltstone. The main Hangenberg isotopic signature exists in overlying strata, and is replicated in four different sections of the Dyer Formation across an area ~300 km2. In each section, the excursion interval is preceded by, and in one location is interrupted by, evidence of meteoric diagenesis and/or emergence characterized by macroscopic dissolution features and pronounced negative δ13Ccarb values (to ca. -6‰). Conodont 87Sr/86Sr ratios through the Dyer Formation dip to nearly 0.708 during the onset of the Hangenberg Event, consistent with the brachiopod based 87Sr/86Sr nadir from the same interval in Missouri, France, and Germany. The least-radiogenic trendline through the Dyer 87Sr/86Sr record matches the global minimum 87Sr/86Sr trendline through the Famennian–Tournaisian transition and infills several gaps in the global record.

Big Sandy and Clinesmith oil fields are located about five miles apart along the boundary between Woodson and Wilson counties in southeast Kansas. They are located on the Pennsylvanian Cherokee Platform and were discovered almost 60 years apart in 1923 and 1982 respectively. Both fields produce from Desmoinesian lower Bartlesville sandstone reservoirs at shallow depths (1,100 to 1,200 ft) from reservoirs that have been interpreted as “shoestring sandstones.” However, if Big Sandy is restricted just to the area of Section 23 and the southeast quarter of Section 22, T26S, R14E where it offers the best wireline log control, the two fields have different orientations. Big Sandy has a southwest/northeast trend almost perpendicular to Clinesmith Field, which trends from north-northwest to southward. Big Sandy Field has a more elliptical shape with a length-to-width ratio of 3:1, vs. 10:1 for the very linear Clinesmith Field. Another major difference between the two fields is that the gamma-ray logs in and along the Clinesmith reservoir trend generally have a fairly well-defined fining-upward trend characteristic of a fluvial channel system. Big Sandy logs, in contrast, show much more variability from well to well. Petrographically the two fields have characteristics that are both consistent and strikingly different. Both reservoirs have a consistent very-fine to fine sand grain size with fair to well-sorted grains. Both also have texturally immature grains that are angular to subrounded with nearly identical compositions of abundant quartz, and associated plagioclase, biotite, muscovite, plant debris, and metamorphic rock fragments. Thus, the Bartlesville Sandstone in both fields had the same nearby sediment source terrain comprised mainly of granites and metamorphic rocks. Each field also produces mainly from primary interparticle porosity in sandstones with loosely packed grains where total porosity locally exceeds 20%. A difference is that, although both fields contain common shale clasts, those in Big Sandy Field tend to be much larger and occur with common siderite (iron carbonate) nodules. Siderite is rare in Clinesmith Field although a few small nodules occur in the associated floodplain shales, along with carbonaceous partings. Also present in Big Sandy’s reservoir are other iron-bearing minerals such as glauconite, pyrite, and chlorite, although all of these are far less common than the siderite. Such an abundance of iron-rich minerals can occur in marginal marine environments such as estuaries where a reducing diagenetic environment forms just below the sediment/water interface. In contrast, well-oxygenated fluvial systems generally contain any iron in highly oxidized forms such as hematite and limonite. From these observations, it appears that Big Sandy’s reservoir interval was deposited in an estuarine (coastal marine) setting with diverse localized depositional environments whereas the Clinesmith reservoir represents a nearly straight, south-flowing fluvial channel and adjacent floodplain, with no marine influence on deposition.

The Yuma Arch is herein proposed as a major structural element forming the northeast margin of the Denver Basin. This previously little-recognized regional geological feature has significance in exploration for oil in this area of the basin. It was initially uplifted during the Paleozoic, but is largely masked by late Cretaceous - early Tertiary tectonic events. Limited pre-Cretaceous exploration drilling has not defined the precise location of the axis of the arch, but subsurface studies show that it occupies most of Yuma, Sedgwick and Phillips counties, Colorado, and Deuel County, Nebraska. The immediate west flank of the arch lies in parts of Washington and Logan counties, Colorado, and most significantly in Cheyenne County, Nebraska, where initial Paleozoic exploration has been focused around the discovery of Bird Oil Field in 1982, a small but unique, over-full, domal trap. Preservation of several cores from Pennsylvanian boreholes in Cheyenne County, Nebraska, facilitated study of cyclical Missourian-Virgilian carbonate reservoirs that provide significant future potential objectives for exploration on the underexplored arch. These cores also provided key biostratigraphic data that helped to clarify the correlation of classical Pennsylvanian strata. Studies of Paleozoic source rocks and migration pathways were conducted over the central and northeastern Denver Basin. These studies indicate that the immediate west flank and the crest of the Yuma Arch provide a significant regional catchment for oil in Paleozoic strata, and should be a target for future exploration. Processing and interpretation of aeromagnetic data provided a means to characterize the Precambrian crystalline core of the arch, and to define areas for seismic imaging of mobile basement block margins that appear to have initiated and sustained the growth of low-relief Paleozoic structural traps. This study reinforces the significant decoupling between pre-salt (Precambrian and Pennsylvanian) and post-salt (Cretaceous and Tertiary) geological processes in this area.

Integration of lithologic and wireline log data allows subdivision of the Upper Cretaceous Niobrara Formation into stratigraphic units that provide a framework for predicting both hydrocarbon source and reservoir potential of the formation throughout the Rocky Mountain region of the western United States. Regional mapping of these Niobrara subdivisions also provides a basis for interpreting the nature of deposition in the Western Interior Seaway during a time of relatively high sea level. Water depth, climate, proximity to the thrust belt bordering the seaway on the west, paleobathymetry, current circulation patterns, and sea level fluctuations all played a role in shaping the nature and distribution of Niobrara lithologies. The Niobrara and stratigraphically equivalent formations to the west contain a spectrum of lithologies including chalks, marls, shales, and sandstones. Coccolith-rich fecal pellets, probably formed by pelagic copepods that thrived during times of high sea level as water circulated through the seaway, provide a distinctive feature of the formation. These chalk pellets occur even in some siliciclastic-rich intervals in the western part of the seaway. Hydrocarbon production comes from three major lithologies: microporous and fractured coccolith- and planktonic foraminifer-rich limestones present mainly in the eastern part of the seaway; fractured sand-rich facies, mainly in the western and southwestern parts of the seaway; and fractured marls and shales, mainly in the central part of the seaway. Black shales, some of which are quite rich in coccoliths and chalk fecal pellets, provide the major hydrocarbon source rocks. Similar shaly beds also provide sealing facies over more chalky or sandy reservoir intervals. Organic richness in the source intervals generally increases from less than 1% in the siliciclastic-rich western facies to more than 7% in the eastern clastic sediment-starved facies. Thermal maturity of these source beds varies regionally on the basis of burial depth and local heat flow.

The Upper Cretaceous Niobrara Formation and its stratigraphic equivalents contain good to excellent source rocks capable of generating significant volumes of hydrocarbons where the formation has achieved adequate thermal niatunty. The hydrocarbon source rock potential of the Niobrara varies stratigraphically and geographically. Wireline log data, lithologic descriptions, and analytical data including liock-Eval pyrolysis analyses provide an understanding of the distribution of source potential within the Niobrara. Lithologies exhibited by the N~obrara and stratigraphically equivalent formations in the western portion of the Western Interior Seaway inclucle chalks, marts, shales, and sandstones. Marly and shaly units are the major hydrocarbon source rocks, although the chalky lithologies also provide some source potential. Total organic carbon (TOC) content as high as 8% has been measured In the east-central portion of the Seaway in s~l~ciclast~c sediment-starved facies. Westward and nonhwarcl, the influx of siliciclastic sediments dilutes organic matter. TOC v:tlues, averaging 1 to 2%: are significantly lower to the west :mcl north than in the Denver Basin and Great Plains area. In die Niobrara, the most common kerogen is Type 11, although in the Piceance Basin, a ~nis of Type I1 ancl Ill is present. Laramicle structural modification of the sti~dy area has been the prinia~y control on thermal maturity in the formation. The Niobrara is Immature in the shallow portion of basins ancl has begun to generate gas only in the deepest basinal locations with elevated heat flow (e.g., the W~ttenberg Field area in the Denver. 13asin). Local heat flow variations resulting from volcmic xtivity have had an impact on source rock maturity in limited areas such as the Raton Basin. Timing of generation, as with maturation, is primarily controllecl by Laraniide sedimentation. One-cliniensional maturity mocleling calibl-ated w~th measu~-ecl thermal maturity from 11, and T,,,,, valc~es can be i~secl to predict timing, relative volumes, and composition for hydrocarbons generated in many of the Laramide basins.

Glick Field, located in Kiowa and Comanche counties of southern Kansas, was discovered in 1957 and has produced more than 362 BCF from Mississippian Osage chert commonly referred to as the Chat. Other Chat reservoirs in Kansas and Oklahoma produce mainly from mixed chert and dolomite beneath the pre-Pennsylvanian unconformity, but Glick Field's reservoir is dominated by chert containing abundant sponge spicules. Glick Field is partly a stratigraphic trap with production ending where the spiculitic facies pinches out into tight limestones to the south and west which provide a lateral seal. Updip, to the northeast, the productive facies is truncated by the unconformity. Reworked chert conglomerates overlying the spiculitic reservoir at the unconformity also produce some gas. The spiculitic chert forming the reservoir was deposited below storm wavebase and grades laterally in all directions into echinoderm and brachiopod-rich skeletal wackestones and lime mudstones. Even where completely silicified, these associated limestones are tight. Thus, the reservoir is an in situ oval-shaped complex of internally brecciated sponge mats and bioherms capped in part by the chert conglomerate. The spiculitic chert contains up to 50% porosity in molds after sponge spicules, matrix micropores, and vugs connected in part by fracture and breccia porosity. Distribution of the sponge bioherms which form the reservoir facies was partly controlled by a subtle change on the shallow Mississippian carbonate shelf from clean skeletal limestones southward into shaly (and probably more anoxic) carbonates known locally as the Cowley Facies. The sponge bioherms formed most commonly just updip from this boundary which can be mapped across southern Kansas. Thus, lithologic mapping provides a potential exploration tool with which to find other stratigraphically trapped spiculitic reservoirs in the area.