Ancestral Front Range Research Articles

Concurrence of Late Triassic lithostratigraphic, radioisotopic, and biostratigraphic data support a Carnian age for the Popo Agie Formation (Chugwater Group), Wyoming, USA

The Upper Triassic Popo Agie Formation of the Chugwater Group of Wyoming, northeastern Utah, and northwestern Colorado has been an enigmatic unit since its definition and is commonly excluded from large-scale studies of continental Upper Triassic strata in the western USA. Lithostratigraphic correlations of Popo Agie Formation outcrops are documented from west-central Wyoming through northeastern Utah and northwestern Colorado, which demonstrates the presence of the Popo Agie Formation throughout this region. Unique detrital zircon age distributions have led previous workers to hypothesize a paleodrainage connecting basal units of the Dockum in west Texas and eastern New Mexico, USA, with the Gartra Grit, a basal unit of the Popo Agie in northeastern Utah. Biostratigraphically informative taxa such as parasuchid phytosaurs in the absence of leptosuchmorph phytosaurs support an assignment of the Gartra Grit and Popo Agie Formation to the Otischalkian estimated holochronozone. We present the first detrital zircon age distributions for the Popo Agie Formation. Multidimensional scaling analysis of zircon populations shows that the Popo Agie samples are similar to other Upper Triassic units surrounding the Ancestral Uncompahgre Highlands, Central Colorado Trough, and the Ancestral Front Range. Additionally, we present the first maximum depositional ages (youngest population) for the Popo Agie Formation at a location where the top of the ochre unit is no older than 225 ± 4 Ma, and the upper purple to ochre transition is no older than 230 ± 5 Ma. By leveraging existing biostratigraphy, regional lithostratigraphy, and new radioisotopic ages we temporally constrain the Popo Agie Formation, enabling us to integrate the upper Chugwater Group, Chinle Formation, and Dockum Group strata into a testable Late Triassic chronostratigraphic framework for the western USA. The consilience of data support a Carnian age for the majority (if not entirety) of the Popo Agie Formation, making this—and equivalent strata in the Dockum Group (i.e., Camp Springs Conglomerate, and strata of the Tecolotito and Los Esteros members of the Santa Rosa Formation)—among the oldest continental Late Triassic stratigraphic units in the western USA.

Detrital zircon geochronology and provenance of the Middle to Late Jurassic Paradox Basin and Central Colorado trough: Paleogeographic implications for southwestern Laurentia

AbstractMiddle to Upper Jurassic strata in the Paradox Basin and Central Colorado trough (CCT; southwestern United States) record a pronounced change in sediment dispersal from dominantly aeolian deposition with an Appalachian source (Entrada Sandstone) to dominantly fluvial deposition with a source in the Mogollon and/or Sevier orogenic highlands (Salt Wash Member of the Morrison Formation). An enigmatic abundance of Cambrian (ca. 527–519 Ma) grains at this provenance transition in the CCT at Escalante Canyon, Colorado, was recently suggested to reflect a local sediment source from the Ancestral Front Range, despite previous interpretations that local basement uplifts were largely buried by Middle to Late Jurassic time.This study aims to delineate spatial and temporal patterns in provenance of these Jurassic sandstones containing Cambrian grains within the Paradox Basin and CCT using sandstone petrography, detrital zircon U-Pb geochronology, and detrital zircon trace elemental and rare-earth elemental (REE) geochemistry. We report 7887 new U-Pb detrital zircon analyses from 31 sandstone samples collected within seven transects in western Colorado and eastern Utah. Three clusters of zircon ages are consistently present (1.53–1.3 Ga, 1.3–0.9 Ga, and 500–300 Ma) that are interpreted to reflect sources associated with the Appalachian orogen in southeastern Laurentia (mid-continent, Grenville, Appalachian, and peri-Gondwanan terranes). Ca. 540–500 Ma zircon grains are anomalously abundant locally in the uppermost Entrada Sandstone and Wanakah Formation but are either lacking or present in small fractions in the overlying Salt Wash and Tidwell Members of the Morrison Formation. A comparison of zircon REE geochemistry between Cambrian detrital zircon and igneous zircon from potential sources shows that these 540–500 Ma detrital zircon are primarily magmatic. Although variability in both detrital and igneous REE concentrations precludes definitive identification of provenance, several considerations suggest that distal sources from the Cambrian granitic and rhyolitic provinces of the Southern Oklahoma aulacogen is also likely, in addition to a proximal source identified in the McClure Mountain syenite of the Wet Mountains, Colorado. The abundance of Cambrian grains in samples from the central CCT, particularly in the Entrada Sandstone and Wanakah Formation, suggests northwesterly sediment transport within the CCT, with sediment sourced from Ancestral Rocky Mountains uplifts of the southern Wet Mountains and/or Amarillo-Wichita Mountains in southwestern Oklahoma. The lack of Cambrian grains within the Paradox Basin suggests that the Uncompahgre uplift (southwestern Colorado) acted as a barrier to sediment transport from the CCT.

Detrital U-Pb zircon and 40Ar/39Ar muscovite geochronology from Middle Pennsylvanian strata in the Anadarko Basin, Texas Panhandle, USA

The Late Mississippian-Permian Anadarko Basin formed in Texas and Oklahoma, USA as the result of inversion of Neoproterozic and Cambrian rift structures. Subsidence was driven by flexural loading of the Amarillo-Wichita Uplift, and this uplift may represent the easternmost element of the Ancestral Rocky Mountains system. The northwestern part of this basin has generally been interpreted to have been filled by sediment derived from the Ancestral Front Range Uplift, ~475 km to the northwest during the early stages of basin filling. We test this model using U-Pb detrital zircon and 40Ar/39Ar detrital muscovite results from three subsurface samples of the Morrow B sandstone in the northwestern part of the Anadarko Basin. We provide a new maximum depositional age of 310.9 ± 4.9 Ma that indicates the age of the Morrow B to be no older than late Atokan to early Desmoinesian Age (Moscovian), ~10 Myr younger than previously interpreted. In contrast to some previous interpretations, we propose that the most likely source for the sediment in the Morrow B is the Amarillo Uplift to the south. Detrital zircon and detrital muscovite data have age peaks at 900–1300 Ma, 1370 Ma and 1600–1800 Ma corresponding to derivation from Grenville, Granite-Rhyolite and Yavapai-Mazatzal basement provinces, respectively. A dominant detrital zircon peak at 1370 Ma suggests that Mesoproterozoic granites in the Amarillo Uplift were exposed by Middle Pennsylvanian time, and detritus eroded from the Amarillo Uplift dominated the lowstand sediment budget of the Texas Panhandle during this time; small volumes of sediment were likely sourced from the Ancestral Front Range. This study presents the first detrital geochronology data from the subsurface Anadarko Basin and the first detrital muscovite data from late Paleozoic southwestern Laurentia. The results presented here highlight the interpretive power of combined detrital zircon and muscovite datasets.

Detrital Zircon Geochronology of Pennsylvanian-Permian Strata in Colorado: Evidence for Appalachian-Derived Sediment and Implications for the Timing of Ancestral Rocky Mountains Uplift

Pennsylvanian-Permian time in north-central Colorado corresponds with uplift of the Ancestral Front Range and deposition of the Fountain, Ingleside, and Lyons Formations along its flanks. In southwestern Colorado, deposition of the Molas and Hermosa Formations along the flanks of the Uncompahgre Highlands largely represents Pennsylvanian time. We present new detrital zircon U-Pb geochronology data for the Ingleside and Lyons Formations in north-central Colorado and the Molas and Hermosa Formations in southwestern Colorado to understand sediment provenance and dispersal patterns. We determined U-Pb ages using LA-ICPMS on 120-150 zircon grains from five sandstone samples collected from shallow marine and eolian facies within the Ingleside, Lyons, Molas, and Hermosa Formations. All sandstone samples display a mixed Laurentian derivation, with age populations that record local and distal sediment sources. All samples also contain between 5% and 10% concordant Paleozoic-age zircon grains ranging from 330–490 Ma, coinciding with high magmatic flux during the Taconic and Acadian orogenies in the Appalachian orogen. Ultimate derivation from the Appalachians are also interpreted for zircon age populations ranging from 500-750 Ma and 1000-1300 Ma that likely originated from Pan-African and Grenville terranes respectively. This study detects the earliest documented appearance of Paleozoic zircons along the northern Ancestral Front Range, corresponding to deposition of the lower Ingleside Formation. We compare our data along the Front Range to previous detrital zircon studies from the underlying Fountain Formation to conclude that the Fountain-Ingleside transition was accompanied by a decrease in locally sourced detrital zircons, most likely marking the cessation of Ancestral Front Range uplift. Conversely, deposition across the Molas-Hermosa contact in southwestern Colorado was accompanied by an increase in locally-sourced detrital zircon grains, most likely marking the initiation of the Uncompahgre uplift.

Middle Jurassic landscape evolution of southwest Laurentia using detrital zircon geochronology

The Middle Jurassic Wanakah Formation in western Colorado is a poorly understood unit in terms of depositional environment and absolute age of deposition; however, a refined interpretation of the provenance has important implications for understanding the landscape evolution of southwestern Laurentia during Mesozoic rifting of the supercontinent Pangea and the opening of the Gulf of Mexico. This study presents the first U/Pb age dating of detrital zircons from the Middle to Late Jurassic Entrada Sandstone, Wanakah Formation, and Tidwell and Salt Wash Members of the Morrison Formation. Detrital zircon geochronology results show a marked increase in ca. 523 Ma grains (compared to most Mesozoic sediments on the Colorado Plateau) that begins abruptly in the Wanakah Formation and continues into the basal Marker Bed A of the Tidwell Member of the Morrison Formation. U/Pb ages and petrography suggest that the Wanakah Formation was sourced, in large part, from the McClure Mountain syenite on the southwestern flank of the Ancestral Front Range. This abrupt change in provenance occurred due to stream capture and drainage reorganization that input a large amount of water into the basin and caused a shift in depositional environment from the eolian Entrada Sandstone to the hypersaline lake environments of the Wanakah Formation and the Tidwell Member. Additionally, stratigraphic, petrological, and detrital zircon analyses suggest that the contact between the Wanakah Formation and the Tidwell Member of the Morrison Formation is conformable, and the previously interpreted J-5 unconformity is likely not present in western Colorado. The stream capture and drainage reorganization that created the lake system recorded in the Wanakah Formation and the Tidwell Member likely evolved into the major fluvial system that deposited the Salt Wash Member of the Morrison Formation. The evolution of paleodrainages and provenance are important to understand because they help to constrain landscape evolution across southwestern Laurentia, and these insights can help to illuminate the influence of tectonic and sediment controls on depositional environment.

Proposing an Entirely Pennsylvanian Age for the Fountain Formation through New Lithostratigraphic Correlation along the Front Range

The Fountain Formation is an important unit recording ancestral Rocky Mountain tectonics and paleogeography along the Front Range urban corridor. The age of the formation constrains timing of uplift of the ancestral Front Range and Ute Pass uplift and subsidence of adjacent basins. Yet, age models for the Fountain Formation are crude and varied. Specifically, available biostratigraphic data suggest an entirely Pennsylvanian age for the Fountain Formation, however historical lithostratigraphic assignment of the Lyons Formation atop the Fountain Formation south of Lyons, Colorado allows for a significant early Permian component of deposition. New stratigraphic and sedimentologic data recorded from the Ingleside Formation and the Lower Permian unit at Manitou Springs, Colorado demonstrate: 1) a conformable contact between the upper Fountain Formation and Lower Permian strata at both localities and 2) close grain size and framework mineralogy comparisons at both localities. These data suggest that the Lower Permian at Manitou Springs, Colorado best correlates to the Ingleside Formation, rather than the previously mapped Lyons Formation. This newly proposed lithostratigraphic correlation aligns with the available biostratigraphic data that the Fountain Formation is a Pennsylvanian unit with little to no Lower Permian component. An entirely Pennsylvanian age for the Fountain Formation indicates that active uplift of the ancestral Front Range and delivery of first-cycle arkose had ceased by the latest Pennsylvanian.

Late Paleozoic tectonics and paleogeography of the ancestral Front Range: Structural, stratigraphic, and sedimentologic evidence from the Fountain Formation (Manitou Springs, Colorado)

Sedimentologic, stratigraphic, and structural data indicate that the Fountain Formation at Manitou Springs consists of three tectonostratigraphic units, herein termed the lower, middle, and upper Fountain Formation. Both the lower and middle Fountain Formation were deposited in a fan-delta setting adjacent to the active ancestral Ute Pass fault. The lower is thin and predominantly characterized by depositional stability as recorded by abundant well-developed, deeply rooted paleosols. The middle Fountain Formation reflects fan progradation and highland rejuvenation as a result of increased activity of the ancestral Ute Pass fault. This activity was likely the result of a basinward splay of the ancestral Ute Pass fault with reverse dip-slip motion as indicated by (1) exhumation of the lower Paleozoic Sawatch Formation and (2) fault-parallel folding. The upper Fountain Formation reflects deposition in a NW-SE–trending, braided-river system and appears to postdate movement on the ancestral Ute Pass fault. Lithostratigraphic correlation allows for a Latest Pennsylvanian–Early Permian age for the upper Fountain Formation, thus constraining cessation of the ancestral Ute Pass fault to Middle to Late Pennsylvanian time. The Fountain Formation was deposited within a NW-SE–oriented structural trough (i.e., Woodland Park trough) that separated the ancestral Front Range into a northern block (i.e., ancestral Front Range block) and a southern block (i.e., Ute Pass uplift). The Woodland Park trough was bounded on its southern margin by the ancestral Ute Pass fault, which was active throughout deposition of the lower two Fountain tectonostratigraphic units, during which time sediments were shed northward across the ancestral Ute Pass fault into a marine environment. By the time of deposition of the upper Fountain Formation, movement on the ancestral Ute Pass fault had ceased or dramatically decreased. During this time, the Fountain Formation records predominantly axially oriented, braided-stream deposition. Comparison of the timing and kinematic history of the ancestral Ute Pass fault with other documented reverse and strike-slip faults of the ancestral Rocky Mountains suggests that: (1) cessation of faulting within the ancestral Rocky Mountains displays a slight east-to-west younging, but the crude age resolution of adjacent basin fill also allows for the possibility of relatively synchronous cessation of faulting, and (2) kinematics of all faults examined are consistent with a NE‑SW–oriented maximum horizontal compressional stress field. These inferences contrast with the proposed Laurentian–Gondwanan diachronous closure models for the ancestral Rocky Mountains, which require rotation of the maximum horizontal stress orientation through time and pronounced east-to-west younging of faulting. Rotation of the horizontal stress field may be recorded in multistage, kinematic histories of faults, but multiple faults require study to assess this model. Age constraints on post-tectonic units of the ancestral Rocky Mountains, such as the upper Fountain Formation tectonostratigraphic unit defined herein, are poor and further refinement would greatly aid understanding of the kinematic timing and tectonic driving force of these ancient mountains.

Dynamics of a Transgressive Prodeltaic System: Implications for Geography and Climate Within a Pennsylvanian Intracratonic Basin, Colorado, U.S.A.

Detailed sedimentological analysis of a prodeltaic turbidite unit within the Pennsylvanian Minturn Formation provides insight into the nature of fan-delta deposition, and allows reconstruction of paleogeographic and paleoclimatic conditions within a tectonically active cratonic basin. The formation, a thick succession of fan-delta deposits, exhibits a complex stratigraphic architecture of alluvial, fluvial, and deltaic fithofacies that were deposited in the Central Colorado Basin (CCB), U.S.A., a fault-bounded depression within the Ancestral Rockies. An ~ 20 to 35-m-thick, regionally extensive, unconformity-bounded prodelta unit was deposited during a large eustatic sea-level rise that temporarily interrupted coarsegrained fluvial-deltaic deposition. The unit consists of dark-green shale and sandstone event beds with tool marks produced by abundant plant debris. The sandstone event beds were previously interpreted as turbidites because they contain current ripples, parallel lamination, and sole marks consistent with deposition from turbidity currents in the absence of storm-generated surface gravity waves. We have found, however, that proximal prodeltaic deposits contain evidence for combined oscillatory and unidirectional flow (i.e., asymmetric hummocky cross stratification (HCS), quasi-planar lamination, and combined-flow ripple stratification), indicating deposition under large waves in relatively shallow water. Paleohydraulic calculations indicate that formation of the HCS beds requires a fetch of at least 75 km to generate the necessary wave periods. Given the position of the CCB in the lee of the ancestral Front Range relative to the prevailing trade winds, and the narrow seaway to the south, storms either tracked across the Cordilleran seaway from the west or formed in situ within the CCB. The former hypothesis is consistent with paleoclimate models that purport the development of summer monsoonal circulation patterns over western equatorial Pangea during the late Paleozoic, and regional observations of southeastward eolian bedform migration directions.

Syndepositional thrust-related deformation and sedimentation in an Ancestral Rocky Mountains basin, Central Colorado trough, Colorado, USA

Pennsylvanian–Permian synorogenic deposits (Minturn and Sangre de Cristo Formations) of the Central Colorado trough record an interplay of deformation and sedimentation in an Ancestral Rocky Mountains basin. The Central Colorado trough was a north-trending basin bordered by basement-involved highlands of the Uncompahgre uplift on the west and the Ancestral Front Range and Apishapa uplift on the east. Stratigraphic data show that the Central Colorado trough was an asymmetric basin in which coarse-grained sediments were deposited adjacent to the Sand Creek–Crestone thrust fault system of the Uncompahgre uplift. These deposits pinch out eastward against the Apishapa uplift along the eastern margin of the basin. Lithofacies analysis shows that the Central Colorado trough was filled by fan-delta, fluvial-delta, and turbidite deposits of the Middle Pennsylvanian Minturn Formation and by alluvial-fan, braided-stream, and meandering-stream deposits of the Upper Pennsylvanian–Permian Sangre de Cristo Formation. Geologic mapping has identified three syndepositional structures in the strata of the Central Colorado trough that indicate Pennsylvanian–Permian shortening: (1) the Gibson Peak growth syncline in the footwall of the Crestone thrust fault, which formed by syndepositional rotation of the Crestone Conglomerate Member of the Sangre de Cristo Formation during thrust displacement; (2) the Sand Creek thrust fault, which cuts the lower part of the Crestone Conglomerate Member but is covered by younger deposits of the Crestone Conglomerate Member; and (3) an intraformational angular unconformity in the Sangre de Cristo Formation that separates folded strata from overlying less deformed strata. All three structures indicate general east-west shortening during deposition. We interpret the Central Colorado trough as a flexural basin on the basis of syndepositional thrust-related structures, basin asymmetry, and lithofacies distribution. Displacement on the east-verging Sand Creek–Crestone thrust fault system appears to have controlled uplift of the central part of the Uncompahgre uplift and also subsidence in the adjacent basin. The Apishapa uplift, located along the eastern margin of the basin, is interpreted as a possible flexural forebulge related to crustal loading of thrust sheets along the western margin of the Central Colorado trough. Geologic mapping, subsurface data, and identification of syndepositional structures has also led to a better understanding of post-Paleozoic deformation of Pennsylvanian–Permian strata of the Central Colorado trough and the structural evolution of the present Sangre de Cristo Mountains.

Effects of Sedimentation, Tectonics, and Glacio-Eustasy on Depositional Sequences, Pennsylvanian Minturn Formation, North-Central Colorado

The Pennsylvanian Minturn Formation was deposited adjacent to the Ancestral Front Range on the eastern margin of the actively subsiding Central Colorado Basin during a time of high-frequency eustatic sea level changes attributed to glaciations in the Southern Hemisphere. In the 100 km2 study area near McCoy, Colorado, up to nine depositional cycles composed of marine and nonmarine deposits are recognized. Three tectonic blocks were subsiding at different rates in the study area. Detailed correlation of measured sections along well-exposed outcrop belts has shown that seven of these cycles extend continuously across the field area, which includes two delta complexes. Two cycles are absent on the central block. The study area is well suited for examining the role of local tectonic vs. eustatic processes in controlling the distribution of sedimentary deposits. In the study area, cycles are bounded by erosional surfaces with as much as 43 m of relief. These surfaces are inferred to be sequence boundaries and are overlain by braided fluvial conglomerates inferred to be incised valley-fill deposits. Marine flooding surfaces overlie the conglomerates. The ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received January 17, 1996; revised manuscript received October 18, 1996; final acceptance March 20, 1997. 2Department of Geological Sciences, University of Colorado at Boulder, Campus Box 250, Boulder, Colorado 80309-0250. Present address: Department of Geology, University of Colorado at Denver, Campus Box 172, P.O. Box 173364, Denver, Colorado 80217-3364. I thank the members of my doctoral dissertation committee at the University of Colorado (Ted Walker, Martin Lockley, Alan Scott, David Budd, and John Pitlick) for their advice and guidance during the course of my research. Financial support was provided by the Rocky Mountain Section of SEPM, the Colorado Mountain Club, Union Pacific Resources, and the Department of Geological Sciences at Colorado University-Boulder. Charles Ross provided fusulinid identifications and biostratigraphic zonation. I thank Paul Weimer (Colorado University-Boulder) for advice and encouragement, and Keith Miller (Kansas State University) for carefully reviewing a preliminary draft of this paper. AAPG reviewers William Devlin, Romeo Flores, Denise Stone, and Elected Editor Kevin Biddle provided many suggestions that greatly improved the final manuscript. I also thank the residents of McCoy, Bond, and Copper Spur, Colorado, for granting access to their property, and for all their kindness while I was working in the field.

Fluvial Transitions and Paleogeography in Upper Part of Maroon Formation (Pennsylvanian and Permian), Northwestern Colorado: ABSTRACT

Six detailed lateral profiles (as much as 7 m thick and 50 m long) of fluvial channel sandstone bodies were measured at different locations in the northern part of the Eagle basin, northwestern Colorado. Contrasts in the grain size, internal geometry, and architecture of the sandstone bodies reveal significant facies changes across the basin. Proximal bodies that formed closest to the basin-margin ancestral Uncompahgre and Front Range uplifts consists of massive cross-bedded and flat-bedded coarse-grained to conglomeratic sandstone deposited mainly channel scour and fill and by vertical and lateral accretion on sand-gravel bars in low-sinuosity rivers. Medial sandstone bodies consist mostly of cross-bedded and flat-bedded fine to coarse-grained sandstone that was also deposited in low-sinuosity rivers on sand flats and in sandy bars. Bankfull discharge in proximal and medial channels was probably about 400 to 1000 m/sup 3//sec. The most distal sandstone body is composed of flat- and ripple-laminated, very fine to fine-grained sandstone that probably was deposited in a distributary area, in part by sheet floods. Overall facies changes indicate down-system decreases in competence and depth of flow. Paleocurrent data indicate that Maroon rivers draining the ancestral Uncompahgre uplift flowed northeastward across most of the basin before merging withmore » rivers draining the ancestral Front Range uplift. The depositional axis of the basin was therefore probably strongly skewed to the northeast during late Maroon time.« less

Depositional Systems of Fountain Formation and Its Basinal Equivalents, Northwestern Denver Basin, Colorado: ABSTRACT

The objective of this study is to provide a better understanding of the depositional systems of the Pennsylvanian Fountain Formation in north-central Colorado. The study area is bounded by T4N, T11N, R66W, and R70W, encompassing portions of the foothills outcrop belt and the Denver basin. The sedimentary sequence observed in surface exposures displays little vertical variation. It is composed of vertically stacked, fining-upward, gravel to siltstone and mudstone cycles containing trough and planar cross-beds, horizontal beds, root structures, and nodular limestone. This succession represents deposition in Donjek-type braided streams and abandoned channel-fill sequences, and the development of soil horizons on a subaerial alluvial fan or alluvial plain. In the subsurface, the vertical succession begins with a basal coarse-grained 140-ft (43-m) thick interval that is identical on the rocks found in End_Page 857------------------------------ surface outcrops to the west. This interval is overlain by 410 ft (125 m) of red shale, siltstone, sandstone, and fossiliferous limestone that grades eastward into black organic shale and limestone. Capping the sequence is a 500-ft (152 m) interval of red shale, siltstone, sandstone, gypsum- and anhydrite-bearing dolomites and fossiliferous limestones that interfinger with typical Fountain coarse-grained terrigenous clastics. This vertical succession of Fountain rocks in the subsurface suggests the following sequence of depositional systems from base to top: alluvial fan and braided alluvial plain, fan deltas, and small interfan embayments that grade eastward into a normal-salinity marine shoreface and an offshore hypersaline carbonate shelf. Normal salinity marine conditions were probably maintained in the parallic zone by the influx of ancestral Front Range runoff. End_of_Article - Last_Page 858------------

Evidence for Eolian Origin of Cross-Stratification in Sandstone of the Casper Formation, Southernmost Laramie Basin, Wyoming

The Casper Formation of Pennsylvanian and Permian age in the southernmost Laramie Basin contains large trough cross-stratification for which both eolian and subaqueous origins have been proposed. To determine which is the most reasonable interpretation of origin, information concerning depositional processes was obtained from a study of associated sedimentary structures and settling velocities of light and heavy minerals. Two important characteristics of sandstone units in the Casper Formation strongly suggest an eolian origin: (1) The cross-stratified units typically contain low-crested, locally truncated ripple marks that nearly everywhere are oriented with their crests parallel to the dip of the cross-strata, and (2) there are evenly spaced lag grains along bedding surfaces. Other characteristics of the sandstone units are compatible with an eolian environment, although their origin is more ambiguous. For example, low-dipping cross-strata, often cited as evidence against an eolian origin, are similar to the internal stratification of eolian dunes where moisture prohibits development of large avalanche faces. Moreover, large-scale contemporaneous deformation of sand, although commonly attributed to subaqueous gliding, probably requires more cohesion than would be present in saturated subaqueous sand. Moist eolian conditions may provide this cohesion. Finally, settling velocities of associated light and heavy minerals indicate deposition from eolian suspension. Therefore, I conclude that sandstone units in the Casper Formation of the southernmost Laramie Basin were deposited as a coastal dune field that bordered the ancestral Front Range uplift and migrated laterally in response to a fluctuating sea level.

Petrology and Paleoenvironments, Robinson Member, Minturn Formation (Desmoinesian), Eagle Basin, Colorado

The Robinson Member is one of eight carbonate members of the 6,000-ft (1,824 m), predominantly sandstone, Minturn Formation (Desmoinesian) in the study area west of the Gore Range in central Colorado. The Robinson Member consists of three limestone units (lower, middle, and upper) interstratified with marine and nonmarine sandstone, siltstone, and conglomerate. Total thickness of the Robinson Member ranges from 200 to 400 ft (60-120 m). Several fusulinid biozones can be recognized in the Robinson Member. They are parallel with and equivalent to the lower, middle, and upper limestones. Fusulina pristina and Fusulina curta do not appear to range above the lower Robinson. Fusulina rockymontana and Wedekindellina coloradoensis are present throughout the Robinson. Fusulina truncatulina has been found only in the upper Robinson and Fusulina plattensis is limited to the upper Robinson and overlying Elk Ridge Member. The areal distribution and the thickness of four time-transgressive facies which make up each of the Robinson limestone units imply that the late Paleozoic Eagle basin was an asymmetric depositional basin with a relatively deep marginal trough along the east side. The basin was bounded on the east by the Ancestral Front Range highland and on the southwest by an Ancestral Uncompahgre highland. From east to west the limestone units contain (1) an oolite facies composed of superficial oolites, pseudo-oolites, and oncolites in microspar or micrite; it is interpreted to be a shallow-water, high-energy deposit on the east edge of the marginal trough; (2) a tubular Foraminifera facies composed mainly of tubular Foraminifera micrite; (3) a phylloid algae facies of biomicrite containing abundant Archaeolithophyllum, Eugonophyllum, fusulinids, and Komia; and interpreted to be a deeper water deposit near the middle of the trough; (4) a stromatolite facies of stromatolites, laminated micrite, and vuggy ostracod-bearing intramicrudite, which indicates quiet-water to supratidal deposition near the western margin of the trough. Small Solenopora bioherms are present locally in this facies The stromatolite facies grades westward into and is interbedded with siltstones and evaporites of the Eagle Valley Evaporite near the center of the Eagle basin. The overall patterns of the limestone facies and sandstone beds indicate intervals of rapid nondepositional marine transgression, followed by relatively slow regression in which a series of limestone facies was deposited. A typical sequence of lithofacies in the Robinson limestones from bottom to top is: argillaceous micrite, phylloid algae biomicrite, tubular Foraminifera biomicrite, and oosparite and/or oomicrite. Bioherms as thick as 80 ft (24 m) are in the phylloid algae and stromatolite facies. Nonphylloid algae appear to be the major components of these bioherms. However, recrystallization and dolomitization make recognition of the alga types difficult. Diagenesis of the limestones involves recrystallization, silicification, dolomitization, and intrastratal solution.

Clay Mineralogy of Pennsylvanian Redbeds and Associated Rocks Flanking Ancestral Front Range of Central Colorado

During Late Pennsylvanian time a major landmass, the Ancestral Front Range, extended north and south through central Colorado. Terrigenous sediments in the adjacent basins were, for the most part, derived from this highland. These sedimentary rocks are predominantly redbeds which have been interpreted to be of primary detrital origin, and the conclusion has been drawn that the red pigment (hematite) originated in lateritic soils which covered the highland. Accordingly, because modern laterites are formed only in areas of tropical climates, it has been concluded by various authors that the climate in the area of the Ancestral Front Range during Pennsylvanian time was warm and humid. The close association of redbeds with evaporites in the Eagle basin on the west side of the Front Range suggests an arid climate. Hence, two conflicting lines of evidence exist concerning the Pennsylvanian climate. This study was undertaken to determine what information concerning this problem might be obtained from a study of the clay mineralogy of the rocks which were derived from the Ancestral Front Range. Inasmuch as most of the clay-size material in the rocks probably was derived from the soils of the source area, and because climate has a dominant effect on the types of clay minerals formed in soils, it seems likely that the nature of the clays would offer important clues regarding the source-area climate. Clay-mineral analyses were made of samples from the Molas regolith, the Belden Formation, and the Minturn Formation on the west flank of the Ancestral Front Range; and from the pre-Fountain regolith, the Glen Eyrie Formation, and the Fountain Formation on the east flank of the Ancestral Front Range. Two major types of clay-mineral assemblages occur in these rocks. The Molas regolith, and the Belden and Glen Eyrie Formations contain illite, montmorillonite, mixed-layer illite-montmorillonite, and significant amounts of detrital koalinite. On the basis of this assemblage it is concluded that the climate in which these clay minerals formed was temperate with moderate rainfall. The pre-Fountain regolith (east flank of the Ancestral Front Range), and the Minturn and Fountain Formations, co tain illite, mixed-layer illite-montmorillonite, chlorite, mixed-layer chlorite-vermiculite, and subordinate amounts of authigenic kaolinite. On the basis of this clay-mineral assemblage it is concluded that the climate in which these clay minerals formed was semiarid to arid.

Petrology of Minturn Formation, East-Central Eagle County, Colorado

The Minturn Formation accumulated along the east side of the Eagle (Maroon) basin from detritus stripped from the ancestral Front Range uplift. Sedimentation began in early Des Moines time and continued through Middle and Late (?) Pennsylvanian until a wedge of clastic sediments as thick as 6,000 feet formed adjacent to the uplift in the area of Minturn, Colorado. The Minturn consists of coarse arkose and conglomerate in outcrops along the basin margin. The formation thins toward the basin center, and the coarse sediments gradually grade laterally into finer-grained arkose, siltstone, mudstone, and shale. West of the junction of Gore Creek and Eagle River the finer-grained sediments interfinger with lenses of evaporite which project outward from the basin center. Lovering and Mallory (1962) designated the evaporite-bearing sequence the Eagle Valley Evaporite. Some carbonate beds are intercalated with the detrital sediments in the upper three-fourths of the formation. Tweto (1949) gave member status to seven of the more persistent carbonate units in the Pando area. These members also crop out in the Minturn area along Eagle River and Gore Creek. Only the Jacque Mountain and Robinson Limestone Members extend into the area of evaporite interbedding. The other carbonate members appear to pinch out or change facies laterally in the eastern part of the area. Limestone beds along the basin margin contain faunal assemblages that indicate normal-marine, open-shelf deposition. The fauna of the limestone beds decreases gradually toward the basin center; there the limestone consists of algal stromatolites and wave-fragmented intraclasts, indicating very shallow water or possibly an intertidal environment in that area. Petrographic studies show a general lack of grain-supported textures, widespread occurrence of micrite, and a corresponding scarcity of sparry calcite cement in essentially all of the limestone. Most fossils do not show evidence of abrasion. These factors suggest that the limestone beds in the marginal trough formed in relatively quiet water. Local zones of agitation are indicated, however, by oolite beds in the White Quail and Ja que Mountain Limestone Members. Most sandstone bodies in the upper part of the Minturn consist of moderately to poorly sorted arkose and impure arkose. Dominant clay minerals in the sandstone and associated shale and mudstone are illite with some mixed-layer clay minerals and chlorite. Kaolinite and montmorillonite are absent or extremely sparse in most sediments. Petrographic and field evidence suggests that the Minturn Formation accumulated in a narrow trough adjacent to the mobile, ancestral Front Range uplift. This trough apparently was connected with an epeiric sea on the northwest and possibly on the southeast. Fluvial sediments accumulated along the shoreward edge of this trough while shallow-marine arkose, shale, and other rock types formed farther seaward. Conditions favorable for limestone deposition recurred periodically. The central part of the basin had a relatively higher elevation than the marginal trough through much of the time of Minturn deposition and served as a shallow evaporite pan. Water of normal-marine salinity occupied the marginal trough during recurring periods but became distinctly shoal toward this central area of evaporite deposition. This is indicated by the presence of algal stromatolites and wave-fragmented intraclasts in limestone of the western part of the area. The absence of marine fossils in these limestone beds is attributed to unfavorable conditions of salinity and temperature associated with this extreme shallowing.

Geologic History Of Piceance Creek-Eagle Basins

The Eagle basin occupies a part of the Maroon trough that began to develop in Early Pennsylvanian time. The trough extended across northwestern Colorado and was bounded by the positive elements of the ancestral Front Range uplift on the northeast and the Uncompahgre uplift on the southwest. The early sediments are clastics, carbonates, and evaporites deposited during cyclic marine transgressions and regressions in the narrow trough. The evaporites are of the basin-center type according to Sloss (1958), but are strongly affected by contemporaneous clastic deposition. The growth of the Front Range in Morrowan time and the uplift of the Uncompahgre in Desmoinesian time contributed large volumes of clastic sediments to the trough during the regressions; these sediments are in erbedded with the evaporites. The coarse clastics from the ancestral Front Range and Uncompahgre uplift continued to influence strongly the sedimentary types of the Maroon basin throughout Triassic and Jurassic deposition, the predominant type being redbeds. In Cretaceous time, the area was inundated by a sea in which the sediments being deposited were black shale, marine sandstones, bars, and beach deposits. During Late Cretaceous time, the entire basin became a shallow sea with paludal and lagoonal sediments, including coal, shale, and underclay that were interbedded with shoreline sandstones in response to fluctuations of the sea. Some of the older structural features, such as the Douglas Creek arch, as well as the Uncompahgre uplift, began to influence sedimentation during Late Cretaceous time, as evidenced by thinning in the Mancos Shale over the arch. After prolonged sedimentation, the Eagle basin was uplifted at the end of Mesozoic time, and folded and faulted during the Laramide orogeny. The Sawatch Range, White River uplift, and the Uinta Mountains came into being in successive stages of the orogeny, and separated the Maroon basin into the present tectonic basins, namely, the Piceance basin, Carbondale basin, Axial basin, Coyote basin, and Sand Wash basin. Many of the present-day structural features were formed during the long period of orogeny from Late Cretaceous to early Tertiary time. The types of sediments have been very important in determining the reservoirs for the accumulation of oil and gas after the Laramide orogeny. Virtually all of the oil and gas in the Paleozoic in the Eagle basin has been produced from the Weber Sandstone. It is a facies having some permeability and effective porosity that was, at the same time, sufficiently brittle to fracture. The intergranular porosity and permeability alone in this formation are insufficient in many of the fields to support commercial rates of production. Thus, in addition to the types of sediments, folding and fracturing have been all-important in establishing substantial reserves in the Paleozoic reservoirs. The same control of accumulations by fracturing and sedimentary type applies to Cretaceous and Tertiary reservoirs in the Piceance basin, though folding was not a prime prerequisite to accumulation. All important Cretaceous and Tertiary oil and gas fields are related to the local improvement of porosity and permeability greater than that normal in the sediments encasing the reservoirs. In contrast to the normal relationships of gas, oil, and water found in the Paleozoic reservoirs, many of these relationships are reversed in the Cretaceous and Tertiary reservoirs. The relationships are similar to those in the San Juan basin of New Mexico and may have a similar explanation. More realistically, the explanation may be the indigenous nature of the oil and gas, where the only migration has been to local reservoirs in juxtaposition with the source beds.

Pennsylvanian and Lower Permian of Northern Denver Basin, Colorado, Wyoming, and Nebraska

Permo-Pennsylvanian sedimentation in the northern Denver basin was influenced by the positive areas of Frontrangia (ancestral Front Range) and the Cambridge arch. Near these sources, coarse clastics interfinger with shales and limestones of the basin. The Cambridge arch was first covered by marine Pennsylvanian sediments during Desmoinesian time. Frontrangia contributed coarse arkosic sediments from Morrowan into Leonardian time. A northwest trend of Frontrangia and the Cambridge arch is indicated by isopach and lithofacies data. The Transcontinental arch was a positive pre-Morrowan feature which also was very slightly positive during Desmoinesian and Missourian time and at the end of the Pennsylvanian Period. The Hartville, Wyoming, area was basinal until uplifted near the end of Wolfcampian time as indicated by truncated Wolfcampian sediments. lsopach maps indicate that the Las Animas arch was mildly positive during Morrowan, Atokan, and Desmoinesian time. On the east flank of the Laramie Mountains the Casper Formation includes sediments of Atokan through Wolfcampian Age. Sediments of questionable Morrowan Age may be present in the basal Casper or in underlying tongues of the Fountain. At the type locality strata as young as Leonardian are believed to be present in the upper Casper. The Ingleside Formation is the Colorado equivalent of the Casper and near the Wyoming-Colorado boundary line also includes sediments of Atokan through Wolfcampian Age. South of the Wyoming-Colorado boundary line basal Ingleside intertongues with the Fountain Formation and at the type locality the Ingleside is entirely Wolfcampian. The Hartville Formation is divided into seven members ranging in age from Atokan to Leonardian, inclusive. Wolfcampian fusulinids occur 70-80 feet below the subsurface shale marker in Cheyenne County, Nebraska. The Satanka Formation consists of the predominantly red sediments that lie between the Casper or Ingleside Formation below and the Opeche Shale or the Lyons Sandstone above. Lateral equivalents of the Satanka are the uppermost Casper of the type locality, the Cassa Member of the Hartville Formation, the middle part of the Lyons Sandstone at the type locality and rocks of Leonardian Age in northwestern Kansas. Lithofacies maps of Pennsylvanian and early Permian sediments indicate decreasing clastic ratios in progressively younger strata. Pre-Desmoinesian clastics decrease in particle-size toward the center of the Denver basin, whereas in sediments deposited following burial of the Cambridge arch, during Desmoinesian time, the clastic particles become smaller eastward across the basin. The present structure of the northern Denver basin is abruptly asymmetrical, much steeper on the west than on the east side, and slightly elongate in a northerly direction. The deepest part of the basin lies only 18-20 miles east of the Permo-Pennsylvanian outcrop. Isopach maps of the Pennsylvanian and Lower Permian Series show relatively greater subsidence in the center of the basin than on the flanks, and also indicate a general southward tilting. About one well per 400 square miles has penetrated the Lower Permian-Pennsylvanian section. These objectives in the northern Denver basin must be considered largely unexplored.

Pennsylvanian and Lower Permian of Northern and Eastern Wyoming

Deposition and distribution of Lower Pennsylvanian in Wyoming were affected by positive areas east and west of the state and in northeastern (Black Hills region) and southeastern (Laramie Mountain region) Wyoming. During Middle and Upper Pennsylvanian, uplift on the west became more intense and great thicknesses of clastics were deposited. The sea retreated eastward, and western and northern Wyoming became land, remaining so throughout Lower Permian. During that time a large part of southeastern Wyoming was a subsiding area. Vigorous uplift of the ancestral Front Range began in Middle Pennsylvanian and exerted influence on Wyoming sedimentation into Lower Permian. The Laramie Mountain region was a northward prong of this element. The Black Hills are reflected by thinning of post-Osagian-pre-Desmoinesian deposits. Continuous deposition apparently occurred in the Hartville region from Morrowan(?) to Leonardian with development of a Wolfcampian anhydrite basin. Evidences of these uplifts are revealed from study of the Amsden, Tensleep, Casper, and Hartville formations. It is suggested that the Tensleep be accorded formational status and include all units below the Permian redbeds and above the Amsden cherty carbonate rocks. The Casper formation of the Laramie Mountains is tentatively divided into lower, middle, and upper divisions approximating Middle and Upper Pennsylvanian and Lower Permian. Sandstone units were derived from erosion of the Ancestral Rockies and the Tensleep of northern and western Wyoming.

GEOLOGY OF THE McCOY AREA, EAGLE AND ROUTT COUNTIES, COLORADO

The McCoy area lies in the transition zone between the ancestral Front Range Highland and the Central Colorado Basin. The Precambrian crystallines, here mostly granite, are overlain by sedimentary rocks of Cambrian age. Limestones overlying the Cambrian rocks and previously mapped as Ordovician are assigned a Mississippian age. The Pennsylvanian McCoy formation is re-defined to include over 3500 feet of coarse arkosic sandstones and grits with interbedded shales and limestones. A Walchia bed is recognized as a member of the formation. Red siltstones overlying the McCoy formation conformably are defined as the State Bridge siltstone and are either Pennsylvanian or Permian. Above the siltstone are thin representatives of some of the Mesozoic formations to the west: the Shinarump, Chinle, Entrada, Morrison, Dakota, Benton, and Niobrara formations. The sedimentary formations were deformed during the Laramide orogeny into open asymmetrical folds and then faulted. The mosaic pattern of the faults is interpreted to mean deformation due to vertical forces rather than horizontal. A monzonite stock was intruded in Laramide times, after the faulting. Following prolonged erosion, extensive basalt and andesite flows were poured out in miocene time. After another period of erosion volcanic activity broke forth again. River terraces and three erosion surfaces are recognized. The Black Mountain surface upon which the Miocene lavas were spread was warped before the Sheephorn surface developed. The Blacktail surface was formed some 500 feet below and into this the Colorado River has cut another 500 feet.