Rocky Mountain Geology最新文献

{"title":"Interpreting stratigraphic relationships and Laramide structural history of the northeastern margin of the Hanna Basin (Wyoming): Meniscoessus (Mammalia, Multituberculata) exposes its faults","authors":"W. Clemens, J. Lillegraven","doi":"10.2113/GSROCKY.48.2.143","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.2.143","url":null,"abstract":"Linkage of paleontological and geological discoveries provides new opportunities to strengthen interpretations of paleogeographic evolution of the Rocky Mountains9 deepest structural basin. We report discovery in the northeastern Hanna Basin (south-central Wyoming) of a lower first molar of Meniscoessus cf. M. robustus , an advanced form of multituberculate mammal known only from the North American Western Interior in Upper Cretaceous local faunas of the Lancian North American Land Mammal Age. It aids in dating patchy outcrops of the Ferris Formation, overlain and covered laterally by significantly younger, thrust-emplaced Hanna Formation. The specimen documents a member of the M. robustus species group, also recovered from more southwesterly strata of the Ferris Formation in the Hanna Basin. The fossil-bearing strata were deposited close to ancient sea level but are tectonically overturned and bounded above and below by what originally were north-vergent thrust faults. We present a new geologic map (scale 1:24,000) including two representative cross sections. Using an interpretive cross-sectional evolutionary model, we propose that the Hanna Basin, until late in Laramide orogenesis, had a markedly more extensive northern existence in the upland areas now occupied by the Freezeout Hills and southern Shirley Mountains. Local Laramide orogenic history in the mapped area is dominated to the north by development of at least 10 kilometers of Cretaceous–early Eocene structural relief across Archean granitic rocks. Those ancient rocks today form the NNE–SSW-oriented, axial core of the asymmetrical Shirley Mountains Anticline. Completion of the Shirley Mountains9 uplift postdated deposition of almost the entire stratigraphic sequence now exposed along the northern Hanna Basin. North-vergent, out-of-the-basin thrust faults developed in response to crowding initiated by the much larger, south-vergent, basement-involved thrust complex known as the Shirley Fault. Those out-of-the-basin thrust faults had mostly bedding-parallel planes of displacement. But they commonly cut stratigraphically down -section during basin-margin deformation, thus placing younger strata of the hanging walls onto older strata of the footwalls. These thin-skinned, younger-on-older fault relationships today exhibit steeply basinward-dipping to overturned strata. The faulting led to greatly thinned stratigraphic sections when juxtaposed against basin-margin, mountainous uplifts expressing oppositely vergent, basement-involved thrust-fault systems. These kinds of down-section thrust faults probably will become recognized as common expressions of basin subdivision along steeply dipping, basin-margin strata throughout the Rocky Mountain province. Furthermore, several occurrences of this phenomenon appear to have been long-misinterpreted as depositional/erosional angular unconformities. Such recognition demands re-thinking of the areas9 geologic histories. Complexities of erosional history","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"10 1","pages":"143-167"},"PeriodicalIF":0.0,"publicationDate":"2013-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.2.143","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311780","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Silicified layers within the Paleogene volcaniclastic Brian Head Formation, southern Utah: Insights into the origin of silicified beds in nonmarine strata","authors":"T. Schinkel, M. Wizevich","doi":"10.2113/GSROCKY.48.2.125","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.2.125","url":null,"abstract":"The Brian Head Formation represents the first widespread volcanism in the Tertiary of southwestern Utah. In the Casto Canyon area, about 20 km north of Bryce Canyon National Park, silicified beds are found within the upper part of the formation, an ∼200-m-thick sequence of volcaniclastic sandstone, bentonitic mudstone, and thin discontinuous micrite limestone beds. The sequence is primarily of fluvial origin, and the limestones were deposited in associated freshwater wetland environments. Silicified layers are typically associated with the limestone beds. Three types of silicified beds were recognized: thin (mm–cm scale), thick (up to 1.3 m thick), and silicified root mats. Petrographic analyses revealed a paragenetic sequence that consists of: (1) microcrystalline calcite (micrite); (2) spar calcite, locally replacing micrite; (3) non-fibrous microcrystalline quartz, including widespread replacement of spar and micrite; and (4) chalcedony. Stable isotopic ratios of carbon (δ 13 C from 0 to −2 per mille [‰]) and oxygen (δ 18 O from 25 to 33‰) in the calcite indicate precipitation in meteoric water. Calcite precipitation likely occurred in a palustrine setting shortly after burial, possibly in a semiarid climate. Isotope ratios of oxygen (δ 18 O from 12.7 to 29.3‰) in the microcrystalline quartz are compatible with precipitation by 80–150°C microcrystalline quartz-bearing fluids. Because the petrographic data indicate that the microcrystalline quartz mineralization post-dates the calcite, it follows that elevated-temperature fluids were also of groundwater origin. Subsurface elevated-temperature fluids, possibly associated with volcanism of the Marysvale volcanic complex, dissolved microcrystalline quartz from abundant glass shards in the volcaniclastic unit. Subsequent cooling of fluids caused dissolution of spar and micrite within limestone beds and the precipitation of microcrystalline quartz, thus forming the silicified layers of the Brian Head Formation.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"48 1","pages":"125-141"},"PeriodicalIF":0.0,"publicationDate":"2013-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.2.125","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312175","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The Nathrop Domes, Colorado: Geochemistry and petrogenesis of a topaz rhyolite","authors":"Daniel Wegert, D. Parker, M. Ren","doi":"10.2113/GSROCKY.48.1.1","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.1.1","url":null,"abstract":"The four Nathrop Domes (∼30 Ma) are located near Buena Vista, Colorado, within the Arkansas Valley Graben segment of the Rio Grande Rift. The domes are largely composed of sparsely to moderately porphyritic, flow-banded rhyolite, with local vitrophyric margins. Phenocrysts include sanidine, plagioclase, quartz, biotite, and Fe–Ti oxides. Bright red Mn-garnet locally occurs in vapor-phase cavities. Two-feldspar and two-oxide temperatures assuming 1 kb pressure were, respectively, 670°C and 653°C. All four domes erupted rhyolite of essentially identical major-element chemistry, although with substantial trace-element variations, high incompatible trace-element contents (Rb up to 364 ppm; Nb up to 67 ppm), and extreme depletion of Ba, Sr, P, Eu, and Ti. These depletions are consistent with fractionation of observed phenocrysts. A rare-earth element diagram shows parallelism of the rhyolite plots, with sloping light REE values, flat heavy REE values, and a prominent, negative Eu anomaly. The narrow range of major-element compositions within the Nathrop Domes’ rhyolites precludes any fractionation modeling amongst them. Nonetheless, their elevated Rb/Sr ratios (as great as 75–120) strongly suggest that these magmas have undergone extensive fractional crystallization. Neodymium isotope analysis shows ∊ Ndt values of −10.1 for a Bald Mountain sample and −13.9 from a Precambrian granite sample (t = 29 Ma). The similarity of the ∊ Ndt of the Bald Mountain rhyolite and the Precambrian granite suggests that the Nathrop rhyolite magmas were initially formed through partial melting of Precambrian rocks. Neodymium Crustal Index (NCI) calculations were performed using the Precambrian granite to estimate the contribution of crustal sources, assuming lithospheric mantle contributions of basalt with ∊ Nd of 0 to 4, or asthenospheric mantle contributions with ∊ Nd of 5 or greater. The resulting calculations indicate NCI values of 0.727 to 0.789, assuming lithospheric mantle, and 0.799 assuming an asthenospheric mantle contribution with ∊ Nd = 5. Thus, 72.7 to 79.9 percent of the Nd present in the Nathrop rhyolite sample is likely from crustal sources, depending upon what type of mantle contribution was involved. The Nathrop rhyolites may represent the earliest phase of magmatism associated with the northern segments of the Rio Grande Rift.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"48 1","pages":"1-14"},"PeriodicalIF":0.0,"publicationDate":"2013-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.1.1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312035","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Detrital zircon geochronology from Cenomanian-Coniacian strata in the Bighorn Basin, Wyoming, U.S.A.: Implications for stratigraphic correlation and paleogeography","authors":"S. May, G. Gray, L. Summa, N. R. Stewart, G. Gehrels, M. Pecha","doi":"10.2113/GSROCKY.48.1.41","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.1.41","url":null,"abstract":"A high-flux, Late Cretaceous magmatic event in the western United States has been tested as a zircon source for high-resolution chronostratigraphic correlation in coeval sedimentary rocks in northwest Wyoming. Thirteen samples of Cenomanian–Coniacian sandstone in the Bighorn Basin yielded more than 1200 U/Th/Pb detrital zircon ages from the Mowry Shale, the Frontier Formation, and the Cody Shale. In addition, two individual clast ages were obtained from a conglomerate located near the top of the Frontier Formation. These formations are dominated by detrital zircon grains that yield paleontologically constrained depositional or near-depositional ages. Each sample has a minimum of 22 grains comprising the youngest age peak. Individual youngest peak ages range from 99.4 to 87.7 Ma, spanning Cenomanian through Middle Coniacian time (Gradstein et al., 2012). Three of four stratigraphic sections yield samples with minimum age peaks that young upward, are consistent with available paleontological control, and suggest an age resolution of one–two million years despite an estimated analytical error of 2 percent (+/− 2 Ma for 100 Ma samples). An age reversal at the top of the fourth section demonstrates that recycling of older sediments into younger beds can be an important control on the age of zircon populations, even during intervals of sediment accumulation dominated by first-cycle zircons from an active magmatic arc. The presence of nearly depositional age volcanic cobbles at the top of the Frontier Formation implies rapid erosion and transport of coarse material from a volcanic source eastward into the foreland basin. The new detrital zircon data, in conjunction with available paleontological constraints, provide a framework for detailed stratigraphic correlation.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"48 1","pages":"41-61"},"PeriodicalIF":0.0,"publicationDate":"2013-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.1.41","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312092","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"South Fork Fault as a gravity slide: its break-away, timing, and emplacement, northwestern Wyoming, U.S.A.: COMMENT","authors":"T. Hauge","doi":"10.2113/GSROCKY.48.1.63","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.1.63","url":null,"abstract":"Clarey's (2012) model for South Fork (SF) thrusting contains major errors as regards timing of emplacement, number of emplacement events, magnitude of displacement, and geometry of the SF allochthon. A model better supported by data: (1) has SF thrusting taking place before local emplacement of the Heart Mountain (HM) allochthon, rather than after; (2) has emplacement of the SF allochthon by multiple events rather than by a single catastrophic event; (3) envisions only gradual changes in the magnitude of displacement along strike of the SF thrust system, rather than abrupt doubling of displacement across tear faults; (4) regards the SF allochthon as segmented by tear faults only where it has moved across footwall lateral ramps, not in its hinterland; and (5) recognizes that the fault viewed by Clarey (2012) as a break-away to the SF system is instead a fault within the HM allochthon.\u0000\u0000Clarey's (2012) claim that SF thrusting postdated emplacement of the HM allochthon is based on his assertion that the HM detachment and overlying allochthon are folded above the SF frontal ramp, both on his section A–A′ and near the Castle fault. This argument is disproven by the geologic map of Pierce and Nelson (1969), which presents a much more complete picture of relevant relationships than is shown in Clarey (2012). Pierce and Nelson's (1969) cross section A–A′ is drawn where the preserved HM allochthon and the SF frontal ramp are in …","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"48 1","pages":"63-65"},"PeriodicalIF":0.0,"publicationDate":"2013-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.1.63","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312130","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"U-Pb zircon age constraints on two episodes of Paleoproterozoic magmatism and development of the Grizzly Creek shear zone, White River Uplift, western Colorado, U.S.A.","authors":"James V. Jones, C. Shaw, Joseph L. Allen, T. Housh","doi":"10.2113/GSROCKY.48.1.15","DOIUrl":"https://doi.org/10.2113/GSROCKY.48.1.15","url":null,"abstract":"New U-Pb zircon ages from Precambrian exposures in the White River Uplift of western Colorado reveal at least two episodes of Proterozoic granitic magmatism and help to constrain the age of the 1-km-thick Grizzly Creek shear zone. Gneissic granite exposed along Mitchell Canyon northwest of Glenwood Springs, crystallized at 1765±9 Ma and is the oldest igneous unit recognized in the area. The gneissic foliation is defined by alternating layers of biotite and elongated pink K-feldspar up to 5 cm in size, and the fabric strikes west-northwest and dips moderately to steeply north. Cross-cutting relationships with gneissic country rocks and other igneous units exposed in adjacent drainages were not observed. However, deformation and metamorphism are inferred to be between the age of intrusion (ca. 1765 Ma) and the age of younger, unfoliated, coarse-grained to K-feldspar megacrystic granite exposed near the mouth of No Name Canyon, which crystallized at 1743±8 Ma. This younger granite is cut by the Grizzly Creek shear zone to the north and only contains a locally developed magmatic foliation south of the shear zone. Foliated to mylonitic, fine-grained biotite granite exposed in the hanging wall of the shear zone along No Name Canyon crystallized at 1745±10 Ma, suggesting that it might be related to coarse-grained granite exposed in the shear zone footwall. These new ages define two granitic magmatic events in this area at ca. 1765 and 1745 Ma and provide a maximum age of deformation in the Grizzly Creek shear zone of 1743 Ma. Similarities in the orientation, structural style, and kinematics between the Grizzly Creek shear zone and other well-documented structures in the region raise the possibility that the shear zone records multiple episodes of both Paleoproterozoic and Mesoproterozoic (ca. 1.4 Ga) deformation, in which case the younger events would have occurred in the absence of local magmatism. Thus, the Grizzly Creek shear zone might represent a kinematic link between major crustal shear zones to the north and south throughout crustal assembly and stabilization in southern Laurentia, but details of the timing and kinematic relationships between these structures remain uncertain.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"48 1","pages":"15-39"},"PeriodicalIF":0.0,"publicationDate":"2013-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.48.1.15","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312085","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Evolution of fracture porosity and permeability during folding by cataclastic flow: Implications for syntectonic fluid flow","authors":"Zeshan Ismat","doi":"10.2113/GSROCKY.47.2.133","DOIUrl":"https://doi.org/10.2113/GSROCKY.47.2.133","url":null,"abstract":"The Canyon Range Syncline, central Utah, folded and continued to tighten by cataclastic flow, where fracture-bound blocks, defined by a distributed network of mesoscale (outcrop) fracture sets slid past each other. Thin zones of microscale cataclasite coat many of the fracture-bound blocks9 surfaces. Different generations of fracture sets used to accommodate cataclastic flow have been unraveled using crosscutting relationships and are used to track different stages of the syncline9s folding history. Many of the fracture sets preserve evidence for fluid flow (such as iron-oxide precipitates) at different stages of folding. The number of generations of quartzite and iron-oxide cataclasite zones preserved along the mesoscale fractures within the Canyon Range Syncline is used here, in conjunction with mesoscale crosscutting relationships, to develop a three-dimensional kinematic model for fracturing and potential fluid flow during folding. This study shows that there is no relationship between porosity and permeability with degree of deformation, i.e., amount of folding. Also, slight lithological variations play a large role in the geometry of the interconnected fracture network.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"47 1","pages":"133-155"},"PeriodicalIF":0.0,"publicationDate":"2012-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.47.2.133","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311693","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A late Eocene (Chadronian) mammalian fauna from the White River Formation in Kings Canyon, northern Colorado","authors":"Karen J. Lloyd, J. Eberle","doi":"10.2113/GSROCKY.47.2.113","DOIUrl":"https://doi.org/10.2113/GSROCKY.47.2.113","url":null,"abstract":"Several decades (1940s–2011) of prospecting and collecting of mammalian fossils from late Eocene strata of the White River Formation in Kings Canyon, a high-altitude (∼2,500 m; 8,200 ft) paleovalley incised into the Medicine Bow Mountains near the Wyoming–Colorado border, have produced a significant faunal assemblage that includes at least 14 species (in 13 families). Research by others suggests that Kings Canyon was at least at its present-day elevation during late Eocene time. Study of the fossil vertebrates, therefore, provides a rare glimpse into a late Eocene, high-altitude mammalian fauna. Here, we describe the Kings Canyon fauna that includes: the lagomorph Megalagus ; the rodents Ischyromys , Cylindrodon, Pelycomys , and Paradjidaumo ; the carnivore Hesperocyon ; the artiodactyls Archaeotherium , Bathygenys , Poabromylus, Pseudoprotoceras , and Leptomeryx ; and the perissodactyls Mesohippus , Megacerops , and Rhinocerotidae. The mammalian fauna corroborates the Chadronian (late Eocene) age of the White River Formation in Kings Canyon suggested by others. The co-occurrence of Megalagus brachyodon , Leptomeryx speciosus , and Pseudoprotoceras longinaris suggests a middle to late Chadronian age. Several taxa reported here represent geographic range extensions. Leptomeryx speciosus and Poabromylus are extended south from Wyoming, while the discovery of Bathygenys alpha at Kings Canyon represents the first known occurrence of this species in Colorado (otherwise known from Wyoming, Montana, and Texas). Ranges of Cylindrodon , Hesperocyon , and Archaeotherium are extended slightly westward from northeastern Colorado. Similar to the late Chadronian-aged Florissant fauna from central Colorado, geographically and faunally the Kings Canyon fauna appears to be near the border between the Great Plains and Rocky Mountain Provinces defined by others. Also, like Florissant, the Kings Canyon fauna seems consistent with weakened faunal provinciality during Chadronian time.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"47 1","pages":"113-132"},"PeriodicalIF":0.0,"publicationDate":"2012-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.47.2.113","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311241","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Petrology of the Eureka Quartzite (Middle and Late Ordovician), Utah and Nevada, U.S.A.","authors":"E. McBride","doi":"10.2113/GSROCKY.47.2.81","DOIUrl":"https://doi.org/10.2113/GSROCKY.47.2.81","url":null,"abstract":"The Eureka Quartzite is a sheet-like quartzarenite up to 200-m thick that was deposited on the eastern shelf of the Cordilleran miogeocline from Canada to California. It is the only sandstone lithosome from the Middle Cambrian through Devonian succession in the Great Basin and is remarkable in its purity of detrital and authigenic quartz, scarcity of bedding, and heterogeneity of both grain packing and quartz cement abundance. Sand sources ranged from the Peace River Arch in Canada to the Transcontinental Arch in mid-continental North America. The Eureka represents a third-order regressive–transgressive stratigraphic sequence, although whether the regression formed in response to eustasy or epirogenic uplift of western North America is unresolved.\u0000\u0000The near absence of detrital clay, body fossils, and subaerial features in addition to the presence of herringbone cross-beds indicate that the Eureka was deposited in intertidal and shallow subtidal environments except for minor eolian deposits. Bioturbation destroyed most primary stratification, although discrete burrows are rare. Textural features of quartz (superb roundness, bean grain shape, crescentic impact scars) indicate a prolonged episode of eolian abrasion prior to marine deposition.\u0000\u0000Regionally the detrital composition is 99.5 percent detrital monocrystalline quartz, 0.5 percent K-feldspar and carbonate allochems, and a trace of heavy minerals. From 2 percent to 4 percent feldspar and carbonate allochems that were initially present have been replaced by quartz during burial. The chief authigenic phases are quartz overgrowths with minor calcite (now dolomite) and illite. Spheroidal and amoeboid calcite-cemented concretions up to 3 cm in diameter formed at shallow burial depths, but all carbonate in the concretions has been leached in outcrop. The heterogeneity of grain compaction and amount of quartz cement resulted in beds that range from semifriable to sedimentary quartzites in the same outcrop. Compaction by the combination of grain rearrangement, pressure dissolution, and grain fracturing generated anomalously low intergranular volumes that average 14 percent in Nevada and 21 percent in Utah.\u0000\u0000The normal evolution of microquartz overgrowths ( 10 μm) during cementation was retarded; consequently, microquartz and mesoquartz cement (80 percent) dominate over macroquartz (20 percent), and they are the only cements in the least-cemented beds and laminations. Illite co-precipitated with microquartz and impeded quartz cementation by coating quartz crystal faces. Despite reaching temperatures >135° C for ∼100 million years, much of the Eureka, especially in Utah, remains incompletely cemented and retains porosity of ∼2 percent. The chief cause of cement heterogeneity appears to be authigenic illite abundance. Pressure dissolution of quartz at shale beds and clay drapes that formed stylolites was the most likely major source of silica for quartz cement.\u0000\u0000Invasion by hydrocarbons and hydrogen sul","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"47 1","pages":"81-111"},"PeriodicalIF":0.0,"publicationDate":"2012-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.47.2.81","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311894","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Triassic fossils found stratigraphically above ‘Jurassic’ eolianites necessitate the revision of lower Mesozoic stratigraphy in Picket Wire Canyonlands, south-central Colorado","authors":"A. Heckert, E. J. Sload, S. Lucas, B. Schumacher","doi":"10.2113/GSROCKY.47.1.37","DOIUrl":"https://doi.org/10.2113/GSROCKY.47.1.37","url":null,"abstract":"The recent discovery of Triassic tetrapod fossils in the Picket Wire Canyonlands of southeastern Colorado necessitates large-scale modification of the currently accepted stratigraphy of the area. The bone-bearing strata lie stratigraphically above a thick (∼80 meter [m]) eolianite historically identified as the Middle Jurassic Entrada Sandstone. The identifiable fossils include teeth and bone fragments of Late Triassic tetrapods, including metoposaurs, phytosaurs, and aetosaurs, recovered from thin (m-scale) discontinuous channels of limestone-pebble conglomerate deposited in a high-energy fluvial environment. Metoposaur bones consist of characteristically textured dermal bone fragments of the skull and pectoral elements, as well as a tooth. Phytosaur fossils consist of type C and B teeth, skull and jaw fragments, and some osteoderms. Aetosaurs are represented by several distinctive osteoderms, including some with evidence of prominent eminences and anterior bars. All identifiable tetrapods pertain to taxa known only from strata of Late Triassic age elsewhere, but none constrains the age of the fossil assemblage more precisely, although the assemblage is similar to lower Chinle Group assemblages of Carnian age (Otischalkian–Adamanian). The two most reasonable solutions to the discovery of Late Triassic index fossils stratigraphically above “Jurassic” beds are that the Triassic strata of this area have been mistakenly correlated with the Middle Jurassic Entrada Sandstone, or else the fossils are reworked into dramatically younger (Middle to Upper Jurassic) beds. The conglomerates are lithologically dissimilar from other Jurassic units regionally, but similar to Upper Triassic conglomerates of Wyoming (Gartra Formation) and New Mexico (Cobert Canyon Bed). Therefore, we consider the fossils to be in Upper Triassic strata. New lithostratigraphic data, including a composite measured section from the Picket Wire Canyonlands—as well as analysis and correlation of newly measured sections and others in the literature from south-central Wyoming, Colorado, Oklahoma, and New Mexico—suggest that the eolianite below the bone-bearing horizon and the finer clastic strata directly beneath the eolianite are best correlated to the Red Draw Member of the Jelm Formation. We correlate the bone-bearing conglomerates with the Cobert Canyon Bed at the base of the Chinle Group, described by previous authors as limestone and lithic-pebble conglomerate underlying the Travesser Formation in northern New Mexico. The gypsiferous and clastic strata overlying the conglomerates and below the Morrison Formation, ∼30 m higher in Picket Wire Canyon, are referred to the Middle Jurassic Ralston Creek (= Bell Ranch) Formation, a correlative of the Summerville Formation. These correlations extend the known distribution of Jelm Formation strata southeastward from north-central Colorado and south-central Wyoming and highlight the need for a major, modern restudy of this unit.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"47 1","pages":"37-53"},"PeriodicalIF":0.0,"publicationDate":"2012-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.47.1.37","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68311667","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}