Minturn Formation Research Articles

Tubular Calcareous Nodules from the Pennsylvanian Minturn Formation of Colorado

Unusual tubular calcareous nodules occur in reddish-brown siltstones in the upper 150 m of the Pennsylvanian Minturn Formation in northwestern Colorado. These nodules have an inner core extending the length of the specimens and consisting of sparry calcite and/or siliciclastic material. This core is surrounded by several concentric zones of crudely laminated micritic limestone with variously shaped small cavities filled with clear calcite. The outer parts of the nodules generally consist of clotted micrite with some siliciclastic material incorporated near the contact with the surrounding sediment. Several nodules were later fractured, some with the cracks partially filled with micrite and others filled with sparry calcite. These nodules may be microbialites that encrusted small reed-like plants that grew in a salt marsh, a shallow coastal lake, or a pool on a muddy tidal flat. These plants later decayed allowing access of siliciclastic sediment into the hollow core.

Systematics and occurrences of Edestus (Chondrichthyes) worldwide and new occurrences from Colorado and Texas

The species of Edestus Leidy (Chondrichthyes, Edestidae) are divided into two groups: those close to Edestus minor Newberry and those close to Edestus heinrichi Newberry and Worthen. The occurrences of Edestus worldwide (North America, Britain and Russia) are reviewed. In North America, Edestus occurs in the Illinois Basin and in the western United States, including South Dakota, Colorado and Texas, but not in marine beds of the same age in the Appalachian Basin. The absence of Edestus in the Appalachian Basin supports the idea that Edestus required access to the open sea, even if its remains are sometimes found in shallow or marginal marine deposits. The chronostratigraphic range of Edestus is from the middle Atokan through the Desmoinesian (Middle Pennsylvanian). All Atokan and lower Desmoinesian occurrences are from the E. minor group. The ranges of the E. minor group and the E. heinrichi group overlap in the upper Desmoinesian. We report two teeth of E. minor from the latest Atokan, in the Minturn Formation at McCoy, Eagle County Colorado. This is the first report of the genus from the southern Rocky Mountain region. We report some teeth of Edestus from Texas for the first time.

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.

Deposits from Wave-Influenced Turbidity Currents: Pennsylvanian Minturn Formation, Colorado, U.S.A.

Turbidity currents generated nearshore have been suggested to be the source of some sandy marine event beds, but in most cases the evidence is circumstantial. Such flows must commonly travel through a field of oscillatory flow caused by wind-generated waves; little is known, however, about the interactions between waves and turbidity currents. We explore these interactions through detailed process-oriented sedimentological analysis of sandstone event beds from the Pennsylvanian Minturn Formation in north-central Colorado, U.S.A. The Minturn Formation exhibits a complex stratigraphic architecture of fan-delta deposits that developed in association with high topographic relief in a tectonically active setting. An ~20–35-m-thick, unconformity-bounded unit of prodelta deposits consists of dark green shale and turbidite-like sandstone beds with tool marks produced by abundant plant debris. Some of the sandstone event beds, most abundant at distal localities, contain reverse-to-normal grading and sequences of sedimentary structures that indicate deposition from waxing to waning flows. In contrast, proximal deposits, in some cases less than a kilometer away, contain abundant beds with evidence for deposition by wave-dominated combined flows, including large-scale hummocky cross-stratification. We interpret the majority of these event beds as a record of deposition from hyperpycnal flows, i.e., turbidity currents generated directly from highly concentrated river plumes, which accelerated and decelerated in response to a rising and falling flood discharge. Additional support for this interpretation includes the following: (1) a variety of sole marks including flute and gutter casts, as well as tool marks made by relatively large (up to tens of centimeters across) woody debris (i.e., groove, prod, and chevron marks); (2) consistent unimodal orientation of sole marks and abundant ripple cross-stratification, which indicate strong downslope-directed flow; (3) a well documented sedimentological framework for the formation of fan-delta deposits adjacent to nearby highlands; and (4) plant fossils typical of middle- to high-elevation habitats that are abundant in the turbidite beds but absent in underlying and overlying shoreline and marginal marine deposits, which have a separate floral assemblage. Differences in grain sizes, vertical stratification sequences, and bed thicknesses between outcrops are interpreted to result from the spatial distribution of wave effects, the time history of hyperpycnal flows, and the interaction of these processes. The latter varied both spatially and temporally and produced a wide range of bed types, which are incorporated into a new conceptual model for storm-influenced hyperpycnal flows.

Sedimentology and sequence stratigraphy of fan-delta and river-delta deposystems, Pennsylvanian Minturn Formation, Colorado

Fan-delta and river-delta strata of the middle Pennsylvanian Minturn Formation were deposited in the Central Colorado trough and are well exposed in the Sangre de Cristo Mountains. Proximal fan-delta strata were deposited in lowstand, transgressive, and highstand systems tracts, whereas distal fan-delta strata were deposited in lowstand, highstand, and forced regressive systems tracts. Fan-delta strata were deposited adjacent to coeval westward-dipping thrust faults along the western margin of the Central Colorado trough. Inferred high subsidence rates caused by thrust loading along the western margin of the basin created accommodation space closer to the basin margin and resulted in localized aggradation of fan-delta deposits.Proximal river-delta strata were deposited in transgressive, highstand, and early regressive systems tracts, whereas distal river-delta strata were deposited during all stages of sea level change. River-delta deposystems are interpreted to have formed in parts of the basin that experienced relatively less subsidence associated with Pennsylvanian thrust loading. Lower subsidence rates associated with river-delta deposystems resulted in progradation into more distal parts of the basin.Results of our study point out that lateral changes in depositional systems, related to local variation in tectonic subsidence, may produce significant along-strike differences in the sequence stratigraphic framework of flexural basins. Our analysis also shows that potential reservoir facies in coeval fan-delta and river-delta deposystems form at different times and in different parts of the basin during sea level fluctuation.

Ctenacanthus and other chondrichthyan spines and denticles from the Minturn Formation (Pennsylvanian) of Colorado

Chondrichthyan spines and dermal denticles are reported from the Middle Pennsylvanian Minturn Formation, Eagle County, Colorado. The most common element is a dorsal finspine referred to Ctenacanthus buttersi St. John and Worthen, 1883. Some of the specimens are more complete distally than the holotype and only previously figured specimen of C. buttersi. Less common remains include a dorsal finspine referred to Acondylacanthus nuperus St. John and Worthen, 1883, a smooth-ribbed dorsal finspine close to “Ctenacanthus” furicarinatus Newberry, 1875, a spine fragment probably referrable to Physonemus sp., and two large-noded dorsal finspines probably referrable to two different species of Bythiacanthus. Dermal denticles are referred to Petrodus patelliformis M'Coy, 1848. Ctenacanthus buttersi finspines and some large cladodont teeth, referred to “Symmorium” occidentalis (Leidy), 1859, may belong to the same species. This conjecture is based mainly on the relative abundances of chondrichthyan teeth found at the same locality.

CTENACANTHUS AND OTHER CHONDRICHTHYAN SPINES AND DENTICLES FROM THE MINTURN FORMATION (PENNSYLVANIAN) OF COLORADO

Chondrichthyan spines and dermal denticles are reported from the Middle Pennsylvanian Minturn Formation, Eagle County, Colorado. The most common element is a dorsal finspine referred to Ctenacanthus buttersi St. John and Worthen, 1883. Some of the specimens are more complete distally than the holotype and only previously figured specimen of C. buttersi. Less common remains include a dorsal finspine referred to Acondylacanthus nuperus St. John and Worthen, 1883, a smooth-ribbed dorsal finspine close to “Ctenacanthus” furicarinatus Newberry, 1875, a spine fragment probably referrable to Physonemus sp., and two large-noded dorsal finspines probably referrable to two different species of Bythiacanthus. Dermal denticles are referred to Petrodus patelliformis M'Coy, 1848. Ctenacanthus buttersi finspines and some large cladodont teeth, referred to “Symmorium” occidentalis (Leidy), 1859, may belong to the same species. This conjecture is based mainly on the relative abundances of chondrichthyan teeth found at the same locality.

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.

Middle Pennsylvanian, late Atokan-early Desmoinesian echinoderms from an intermontane basin, the Central Colorado Trough

Middle Pennsylvanian, Atokan and Desmoinesian, crinoids and echinoids are described from the Belden and Minturn Formations of the Central Colorado Trough. The echinoderms from the Minturn Formation were living in an active intermontane trough setting. They are found in two environments, on the flanks of phylloid algal reefs and in patches on abandoned delta lobes on the gently sloping substrate. Faunas are dominated by cromyocrinids and are similar to stable platform Terrigineous Facies Belt faunas of the Midcontinent.New species described areDicromyocrinus beldenensis, Neoprotencrinus rockensis, andSciadiocrinus wipsorum. Goleocrinus, and the columnal generaBlothronagma, Cyclocaudex, andPlatyplateiumare reported for the first time from the Minturn Formation. The columnal taxaBlothronagma cinctutumandFloricyclus angustimargoare believed to be cromyocrinids.B. cinctutummay be synonomous withSynarmocrinus molasensis, andFloricyclus angustimargomay be synonymous withUlocrinus rockymontanus.

Geology and geochemistry of the Kokomo mining district, Colorado

The ores of the Kokomo mining district, Colorado, are Ag-Pb-Zn massive sulfide replacement deposits (mantos) controlled by northeast-striking, high-angle faults and hosted by limestone beds in the Pennsylvanian Minturn Formation. The Minturn Formation is as much as 1,830 m thick and consists of interbedded nonmarine, coarse clastic sedimentary rocks and thin marine limestones. The sedimentary section is greatly expanded by Cretaceous-Tertiary porphyritic granodiorite and quartz monzonite sills, dikes, and stocks.Hydrothermal alteration is dominated by skarns in calcareous sedimentary rocks centered on the Tucker Mountain area. Manto orebodies occur to the south in an updip direction distal to the skarns. Intrusive rocks throughout the Kokomo district have been affected by propylitic alteration. Argillic and sericitic alteration of sedimentary and intrusive rocks is restricted to areas immediately around the mantos and along faults that served as conduits for hydrothermal solutions. Mantos are surrounded by halos of siderite-manganosiderite, hydrothermal dolomite, and/or jasperoid in the limestone.Massive sulfide replacement ore exhibits banded and tabular textures very similar to ores at Leadville and Gilman, Colorado. Ore deposition occurred in the general sequence: jasperoid,; iron sulfides (pyrrhotite, marcasite, and pyrite) + siderite, quartz, high iron sphalerite, ferroan dolomite, galena, chalcopyrite and sulfosalts, barite, ferroan calcite, and calcite. Fluid inclusion analyses of quartz and carbonate gangue minerals indicate that temperatures averaged 350 degrees C during the early stages of ore deposition and decreased to about 160 degrees C during the late stages. Ore fluid salinities also decreased through the ore deposition event from 4.6 to 18 O and delta 18 C values compared to diagenetic dolomite due to isotopic exchange between limestones and hydrothermal fluids. Carbonate gangue minerals within the ore zone exhibit lower delta 18 O and delta 13 C values, and these values yield a calculated fluid delta 18 O (sub H 2 O) = 5.0 to 6.9 per mil, within the range for magmatic water. Sulfur isotope values of sulfide minerals range from -0.6 to + 0.8 per mil. The narrow range of delta 34 S values around 0 per mil suggests a magmatic sulfur source.The fission track age of the Kokomo ore system is about 40 Ma. Fission track dates are younger to the north and indicate a thermal center around Tucker Mountain. A porphyritic rhyolite plug on Tucker Mountain, described as a Pennsylvanian volcanic feature in previous literature, yields a fission track age of 39.5 Ma.The Kokomo stable isotope values, fluid inclusion homogenization temperatures, and salinities are comparable to data from other Leadville-type (magmatic hydrothermal) carbonate-hosted sulfide replacement deposits in the Colorado mineral belt, including Leadville and Gilman.

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.

Petrology, geochemistry, provenance, and alteration of Pennsylvanian-Permian arkose, Colorado and Utah

Research Article| December 01, 1994 Petrology, geochemistry, provenance, and alteration of Pennsylvanian-Permian arkose, Colorado and Utah PETER C. VAN DE KAMP; PETER C. VAN DE KAMP 1Cornelis Corporation, 1750 Cabernet Lane, St. Helena, California 94574-1604 Search for other works by this author on: GSW Google Scholar BERNARD E. LEAKE BERNARD E. LEAKE 2Department of Geology and Applied Geology, The University of Glasgow, Glasgow, G12 8QQ, United Kingdom Search for other works by this author on: GSW Google Scholar GSA Bulletin (1994) 106 (12): 1571–1582. https://doi.org/10.1130/0016-7606(1994)105<1571:PGPAAO>2.3.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PETER C. VAN DE KAMP, BERNARD E. LEAKE; Petrology, geochemistry, provenance, and alteration of Pennsylvanian-Permian arkose, Colorado and Utah. GSA Bulletin 1994;; 106 (12): 1571–1582. doi: https://doi.org/10.1130/0016-7606(1994)105<1571:PGPAAO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Late Paleozoic and modern feldspathic clastic sediments of the Colorado Rockies were derived from dominantly igneous and meta-igneous Precambrian basement rocks. Average composition, expressed as Gazzi-Dickinson QFL (quartz-feldspar-lithicfragments) is 36:64:0 in modern stream sands, for the Cutler Formation is 54:46:0, for the Minturn Formation is 57:43:0, and for the Fountain Formation is 51:49:0. No major mafic components are present in the provenance as indicated by low Co, Cr, and Ni abundances, although higher Fe and Mg abundances in Cutler Formation rocks indicate derivation from somewhat more mafic provenance than the Fountain and Minturn Formation rocks. Fountain Formation rocks are more altered by predepositional weathering, diagenesis, and Holocene weathering than their equivalents in the Minturn and Cutler Formations. Thus plagioclase is albitized and clay altered or absent in the Fountain Formation, whereas it is abundant and much less altered in the Minturn and Cutler rocks. Fountain Formation sandstone in the Steamboat Mountain section lacks remaining detrital plagioclase but contains authigenic albite as overgrowths on, and replacement of, detrital K-feldspar and as pore-filling crystals. In the Eldorado Springs section there is extensive epithermal potassic alteration in the Fountain Formation from the detrital assemblage quartz + plagioclase + K-feldspar + biotite + muscovite to the assemblage quartz + K-feldspar + authigenic adularia + illite/muscovite + kaolinite. K and Rb have been metasomatically added to these rocks with concomitant removal of Ca, Na, Co, Cr, Cu, Ni, Ga, Th, U, Zn, Ba, Ce, La, and Y during passage of hydrothermal fluids, apparently in the Tertiary. These fluids were probably heated by deep-seated magma adjacent to the Colorado Lineament and/or range front faults that served as geothermal water conduits toward the surface. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Eustatic Versus Tectonic Controls on Depositional Sequences in the Pennsylvanian Minturn Formation of North-Central Colorado: ABSTRACT

Eustatic Versus Tectonic Controls on Depositional Sequences in the Pennsylvanian Minturn Formation of North-Central Colorado: ABSTRACT

Wewokella and other sponges from the Pennsylvanian Minturn Formation of north-central Colorado

The calcareous heteractinid sponge Wewokella solida Girty, 1911, is reported from Colorado for the first time. Triactine-based skeletons are well preserved and dermal and gastral layers are composed of smaller spicules than those in the main wall of the many specimens. A fragment of an unnamed demosponge, possibly related to Heliospongia Girty, 1908, and fragments of root tufts occur with Wewokella in the Middle Pennsylvanian Minturn Formation near McCoy, in Eagle County, Colorado.

A paleomagnetic investigation of the Minturn Formation, Colorado: A study in establishing the timing of remanence acquisition

One hundred sixty-five oriented hand samples were collected from 55 sites in the Minturn Formation located between Wellsville and Howard, Colorado, in an attempt to locate the base of the Permo-Carboniferous Reversed Superchron (PCRS) in western North America. After the samples were demagnetized using thermal and alternating field cleaning techniques, the normal polarity magnetizations (which marks the base of the Permo-Carboniferous Reversed Superchron (PCRS)) were not found in the Minturn Formation. Reversed polarities were consistent throughout the section studied. A conglomerate test was conducted on syndepositional rip-up clasts taken from the formation. The test indicates that a post-depositional, secondary component of magnetization has been acquired by the clasts and remains stable up to temperatures of 610–640°C. However, the high temperature components for three of the four test sites show random directions indicating that a primary remanent magnetization was acquired shortly after deposition. The conglomerate test demonstrates that a secondary Permian direction can be successfully isolated from a primary Pennsylvanian one revealed at blocking temperatures above 650°C. In this study, the two components were isolated using step-wise thermal demagnetization. The results of the study indicate that the base of the PCR superchron lies stratigraphically below the Minturn Formation, probably in Early Atokan or Morrowan age sediments.

Structural Control on Distribution of Sedimentary Facies in the Pennsylvanian Minturn Formation of North-Central Colorado: ABSTRACT

Structural Control on Distribution of Sedimentary Facies in the Pennsylvanian Minturn Formation of North-Central Colorado: ABSTRACT

Genetic Sequences of the Lower Minturn Formation (Middle Pennsylvanian), Southeastern Eagle Basin, Colorado: ABSTRACT

Genetic Sequences of the Lower Minturn Formation (Middle Pennsylvanian), Southeastern Eagle Basin, Colorado: ABSTRACT

A newPernopecten(Bivalvia: Pectinacea) from the Pennsylvanian Gothic Formation of Colorado

NORTH AMERICAN Pennsylvanian localities from which the scallop Pernopecten has been reported include sites in Iowa, Missouri, Nebraska, Texas, and Ohio, among others (Newell, 1937; Hoare, 1961; Hoare et al., 1979). Pernopecten has not been reported from any states west of the Rocky Mountains previously. Emmons (1886, p. 70) and later Girty (1903, p. 419) referred doubtfully to a specimen of the Mesozoic pectinacean Entolium? sp. from the Carboniferous Weber grits (=Minturn Formation?) of the Leadville, Colorado, region. The similarity of Entolium to Pernopecten suggests that this specimen may have been a Pernopecten, but the specimen was not figured, was apparently not catalogued, and is not available for study. A single right valve of a morphologically distinct form of Pernopecten was collected from a fossiliferous horizon within the Taylor River Limestone Member of the Gothic Formation in Gunnison County, Colorado.

Oil-Impregnated Outcrops and Their Relationship to Petroleum Generation in Late Paleozoic Eagle Basin, Northwestern Colorado: ABSTRACT

Oil-impregnated outcrops of Mississippian and Pennsylvanian rocks are present in widely separated areas of the Eagle basin. Along the northeast margin of the fault-bounded Eagle basin, oil-impregnated arkosic sandstones of the lower Minturn Formation (Desmoinesian) are interbedded with black, organic-rich shales that provided a local source for the oil. Algal mounds in the lower Minturn Formation in this area contain oil residue. Oil residue is also present in the Mississippian Leadville formation in several parts of the basin, both in paleokarst-related vugs and intergranular porosity in oolite grainstones. The Leadville formation was sourced by overlying organic-rich shales of the Belden Formation (Morrowan-Atokan). Under Pennsylvanian-Permian arkosic sandstones near the southwestern margin of the basin are not adjacent to potential source rocks. The Schoolhouse Tongue of the Weber Sandstone is heavily stained in many areas over its 50 mi of outcrop exposure. Numerous arkosic sandstones in the underlying Maroon Formation are also sained. Faults bounding intrabasinal horst blocks were conduits for upward oil migration, and the horsts may have provided the trap. Oil is also present in overlying Triassic and Jurassic sands in this area, supporting the theory of vertical migration up faults. Oil residue is also present in the Maroon Formationmore » 40 mi to the southeast in the Crystal River Valley. Secondary porosity from dissolution of calcite cement preceded oil emplacement. Beaching of normally red-brown sandstones is ubiquitous with the oil residue and probably represents reduction and partial removal of iron by acidic, reducing fluids generated from the Belden Shale.« less

The Paleoenvironmental Significance of the Nearshore Curvolithus Ichnofacies

Newly discovered Curvolithus-rich trace-fossil assemblages in fan-delta deposits of the Pennsylvanian Minturn Formation of Colorado are remarkably similar to several other Carboniferous deltaic ichnofaunas, as well as those from Jurassic fan-delta environments and other marginal-marine deposits with freshwater influence. The Minturn trace fossils comprise the type assemblage of the Curvolithus ichnofacies, dominated by infaunal locomotion-feeding activity in environments characterized by relatively rapid deposition of sandy sediment exceeding physical reworking. The ichnofacies also occurs in sandy, low-energy, open-shelf environments. Thus, the ichnofacies is a paleo-energy and paleo-depositional rate indicator, and we predict that its occurrence in strongly localized deltaic and nearshore sandy facies recurs throughout the Phanerozoic.