Rocky Mountain Geology最新文献

{"title":"Diagenesis of sandstones and carbonates of the Ignacio Quartzite and McCracken Sandstone Member of the Elbert Formation, southwestern Colorado, U.S.A.","authors":"E. McBride","doi":"10.2113/GSROCKY.51.2.69","DOIUrl":"https://doi.org/10.2113/GSROCKY.51.2.69","url":null,"abstract":"Syndepositional events that affected fluvial, estuarine, and shallow-marine sediments of the units studied in southwest Colorado—the McCracken Sandstone Member of the Elbert Formation and the Ignacio Quartzite—included bioturbation, local formation of glaucony grains and phosphate crusts, local carbonate pedogenesis, and possibly silcrete pedogenesis. Although the sediments were ultimately buried up to approximately 4 km, shallow burial was the site of: (1) reddening of feldspathic sand and shale of the Ignacio Quartzite; (2) initial loss of porosity in both units by compaction of locally abundant clay clasts and clay drapes; and (3) initial partial cementation by overgrowths of feldspar and microquartz crystals. During deeper burial, the quartzarenite sands of the McCracken Sandstone Member of the Elbert Formation underwent compaction to porosity values below that typical of stable packing patterns of rigid grains followed by complete cementation by megaquartz overgrowths to produce highly indurated sandstones. Silica derived from intergranular pressure dissolution in local sandstone beds and during stylolite formation was inadequate to provide the amount of silica present in the cement. Much silica apparently was imported in fluids expelled from adjacent sedimentary basins. Such fluids were also responsible for bleaching red beds in the Ignacio Quartzite, dolomitizing limestone in the McCracken Member, and introducing small amounts of oil, some of which survives as pyrobitumen. Values of stable carbon and oxygen isotopes in dolostones in the McCracken indicate that dolomitization was accomplished by hot fluids. Kaolinite—an alteration product of K-feldspar, illite, and trace amounts of barite and sphene—formed during intermediate to deep burial.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"51 1","pages":"69-80"},"PeriodicalIF":0.0,"publicationDate":"2016-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.51.2.69","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68314031","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":"Memorial for Paul L. Heller (September 16, 1952-July 6, 2016)","authors":"B. Foreman, E. Hajek, R. Lynds, M. McMillan, C. Paola","doi":"10.2113/GSROCKY.51.2.81","DOIUrl":"https://doi.org/10.2113/GSROCKY.51.2.81","url":null,"abstract":"Paul Heller in 2011. With no warning, and far too early, we have lost our friend, mentor, and colleague Paul Heller. Paul was part of the New York Jewish diaspora of the 1950s and ’60s, working his way west via Western Washington University (M.S., 1978). He completed his Ph.D. at the University of Arizona (1983) with Bill Dickinson, and ultimately landed at the University of Wyoming, where he was a member of the faculty from 1983–2016. Having caught the bug for sedimentation and tectonics, Paul emerged as one of the leaders of the next wave of tectonic sedimentologists, who focused on how new insights on basin mechanics could be used to understand stratigraphic patterns. Some of his earliest work involved deep-marine sedimentation, springing from fieldwork in the Pacific Northwest, and culminating in an important paper proposing the submarine-ramp model for sandy turbidites. Paul also was a pioneer of the idea of isotopic provenance, using isotopes as fingerprints of the origin of sediments. But perhaps Paul's most influential early work was that on the interplay of lithospheric flexure and sedimentation. He proposed the idea, widely accepted now but controversial at the time, that mountain building in foreland basins might be marked distally not by gravels but rather by fine-grained sediments, if the increasing load creates accommodation faster than it can be filled with deposits. Paul and colleagues synthesized much of this new quantitative approach in a basin analysis short course offered through the American Association of Petroleum Geologists. It found another expression through Paul's central and inspirational involvement in the development of experimental stratigraphy. The interplay of tectonic and surface processes remained central to Paul's work, up through his most recent publication on the dynamic topography of the North American Western Interior. But of course basins record more than tectonics, and …","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"51 1","pages":"81-84"},"PeriodicalIF":0.0,"publicationDate":"2016-01-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.51.2.81","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68314121","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":"Testing the Bannock detachment breakaway: Negative results support moderate- to high-angle splay system and domino-style fault block rotation along the Valley fault, southern Portneuf Range, southeastern Idaho, U.S.A.","authors":"Joshua A. Keeley, D. Rodgers","doi":"10.2113/GSROCKY.50.2.119","DOIUrl":"https://doi.org/10.2113/GSROCKY.50.2.119","url":null,"abstract":"New geologic mapping, kinematic analysis, and tephrochronologic age correlations were completed in the southern Portneuf Range of southeast Idaho to characterize the Valley fault, a large-offset normal fault interpreted as the breakaway for the regionally extensive Bannock detachment system. The Valley fault separates ∼11.8–7.57 Ma Salt Lake Formation from underlying Neoproterozoic to Cambrian Brigham Group and Cambrian rocks. Three-point problems indicate that the Valley fault strikes north–northwest (NNW) and dips an average of 9° west–southwest (WSW), in stark contrast to previous work, which interpreted the fault as steeply west-dipping. Footwall strata strike NNW and dip 37 ± 11° east–northeast (ENE). Measured bedding-to-fault cutoff angles are, therefore, ∼46 ± 11°. Hanging-wall strata strike NNW and dip 20 ± 5° ENE adjacent to the fault and steepen progressively down -section to ∼70 ± 10° ENE due to the Valley fault's minor listricity and multiple hanging-wall splays. Beneath the Miocene strata, Cambrian hanging-wall bedrock strike NNW and dip 55 ± 10° ENE.  Adding a 9° Valley fault dip to these bedding dips yields cutoff angles of 29° to 80° for Miocene strata and 64° for hanging-wall bedrock. Top-to-the-west, dip-slip offset across the Valley fault and its hanging-wall splays—using the Miocene unconformity as a marker—is 13.7 ± 1 km. The older, subhorizontal Mine Hollow fault has 1,000 ± 300 m top-to-the-west normal offset.\u0000\u0000The initial dip of the Valley fault is interpreted to have been steeper than its present 9° dip. If footwall strata were subhorizontal prior to extension, as indicated by reconstruction of the Miocene unconformity, then the Valley fault had an initial dip of 46 ± 11° WSW based on measured bedding-to-fault angles. Combined with the modern low dip of the Valley fault, these data are interpreted to indicate that the Salt Lake Formation accumulated as growth strata adjacent to the active Valley fault, while the fault as well as footwall and hanging-wall strata tilted northeast through time. The Valley fault's mildly listric shape, the progressive domino-style tilting, and the large amount of total slip suggest that the Valley fault is a Basin-Range normal fault rather than a low-angle breakaway fault for the Bannock detachment system. The preferred model for extension in the region is ∼7 km of uplift and exhumation during two phases of moderate- to high-angle domino-style fault block rotation. First, the Valley fault splays and Mine Hollow fault accommodated 5 km of extension between 11.8 to 9.16 Ma; then the Valley fault accommodated 10 km offset between 9.16 to <7.57 Ma (likely as recent as 5–4 Ma). Our regional cross section and restoration suggest that both the Clifton fault to the west (Bannock detachment) and the Valley fault are two separate faults, each with 13–15 km offset. Reconstruction of these Miocene faults shows that the Mesozoic Paris thrust fault has a preextensional footwall flat length of 30 km, simila","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"50 1","pages":"119-151"},"PeriodicalIF":0.0,"publicationDate":"2015-09-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.50.2.119","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313501","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":"Strata-bound vein array in the basal Pierre Shale, Lake Francis Case, South Dakota, U.S.A.","authors":"H. Maher, S. Ferguson, Ryan Korth, J. Marshall, Laura Pickett","doi":"10.2113/GSROCKY.50.2.153","DOIUrl":"https://doi.org/10.2113/GSROCKY.50.2.153","url":null,"abstract":"A distinctive strata-bound vein array occurs in the basal Pierre Shale exposed along the shores of Lake Francis Case, a reservoir on the Missouri River in south-central South Dakota. Typically 2–4 meters in thickness, the array consistently outcrops over a >50-km distance, a significant areal footprint. Ash layers define the upper and lower bounds of the vein array. Two, suborthogonal, preferred directions of vertical veins (northeast and southeast strike) define a regional pattern. By volume, vertical veins comprise 1–2% of the rock. Thinner, more discontinuous, and irregular horizontal veins also occur. Comparisons between array orientations and the joint/vein pattern in the immediately underlying marls of the top of the Niobrara Chalk identify distinct differences. Traverse data suggest that the vein arrays are characterized by uniform horizontal extension. Vertical veins in the array are typically 1–2 centimeters thick and contain massive jarosite, selenite, and fibrous gypsum. The abundance of jarosite and fibrous gypsum distinctly correlates with position in the weathering profile, and these phases are interpreted as due to replacement of original selenite during modern weathering. However, for initial vein array formation, the following suggests that they are not related to modern weathering and formed at depth: (1) a lack of correlation of vein width/frequency with position in the weathering profile; (2) the regional extent; (3) the consistent preferred orientations; (4) the uniform horizontal extension; and (5) the coarse-grained character of the selenite. The consistent strike pattern suggests influence of a regional stress field. The mechanism/timing of vein array formation is unclear. Formation due to diagenetic processes, which are especially significant in mud rocks, would explain the strata-bound character and isotropic horizontal strain and is considered most likely. Formation during glacial loading is one intriguing possibility. Localization of the vein array may be due to the organic-rich character of the host Burning Brule Member of the Sharon Springs Formation.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"50 1","pages":"153-165"},"PeriodicalIF":0.0,"publicationDate":"2015-09-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.50.2.153","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313618","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":"Heavy minerals in the late Eocene sandstone of Medicine Pole Hills, southwestern North Dakota","authors":"J. Webster, A. J. Kihm, Aaron A. Klingbeil","doi":"10.2113/GSROCKY.50.1.1","DOIUrl":"https://doi.org/10.2113/GSROCKY.50.1.1","url":null,"abstract":"Late Eocene sandstone capping hills and small buttes in the Medicine Pole Hills in southwestern North Dakota has been correlated with the Chalky Buttes Member of the Chadron Formation. Heavy-mineral analysis was used to assess this correlation. Optical microscopy and chemical analysis using a scanning electron microscope/energy dispersive spectrometer (SEM/EDS) system were used to determine abundances and compositions of heavy minerals separated from 0.25–0.30 mm fractions of nine samples from seven depositional units (353–602 grains per sample). The 0.063–0.125 mm fraction was also studied for one of these samples. Samples ranged from coarse- to medium- to fine-grained poorly consolidated sandstone. Total heavy mineral recovery (0.4–14.1%) generally corresponded with the grain-size distribution of samples, with higher recoveries in coarser samples. Opaque grains (ferruginous aggregates, Fe-Ti oxides, and leucoxene) comprised 0.8–15.6 percent of heavy mineral grains. Non-opaque grains were dominated by amphibole (24.4–52.2%) and epidote-clinozoisite (10.6–38.1%), with variable diopside (0–51.1%). Generally less abundant minerals included garnet, apatite (bone fragments), biotite, and muscovite. One sample was biotite-rich (36.5%). Chemical compositions of heavy minerals showed no significant stratigraphic variations, suggesting the nature of the sources did not vary. Diopside, biotite, and approximately 1/3 of the amphibole grains were interpreted as derived from Tertiary volcanic sources. The majority of the amphibole grains (tremolite and blue-green hornblende), epidote, and garnet were derived from Precambrian metamorphic sources. The relative contribution of volcanic sources was variable (19–64%), being highest in those samples with more diopside and (with the exception of the biotite-rich sample) highest in those samples with higher overall heavy-mineral abundances. Individual heavy-mineral abundances also correlated fairly well with grain size. Overall, variations in heavy minerals were explained by variable volcanic source contributions and variations in grain-size distributions of the samples. Despite the variability found, cluster analysis showed that the samples from Medicine Pole Hills were consistently different than the generally much more stable heavy-mineral assemblages found in the Chalky Buttes Member. They were also consistently different when volcanic heavy minerals were ignored. The differences could not be explained by variable volcanic contributions, grain-size effects, and/or differences in heavy mineral stabilities alone. They require differences in provenance, suggesting that the sandstone of Medicine Pole Hills should not be correlated with the Chalky Buttes Member, or, at the very least, it should be considered a distinct facies of the Chalky Buttes Member.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"50 1","pages":"1-29"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.50.1.1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313279","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":"Late Laramide tectonic fragmentation of the eastern greater Green River Basin, Wyoming","authors":"J. Lillegraven","doi":"10.2113/GSROCKY.50.1.30","DOIUrl":"https://doi.org/10.2113/GSROCKY.50.1.30","url":null,"abstract":"Today9s greater Green River Basin is limited to the southwestern quarter of Wyoming. From late in the Cretaceous into late Paleocene time, however, sedimentary accumulations within that basin continued uninterruptedly much farther to the east, connecting areas now occupied by the isolated Hanna, Carbon, Pass Creek, and Laramie basins. Field-based research resulted in three contiguous geologic maps that focus on modern basin margins and boundaries among those eastern elements. Analyses of derived cross sections and restored stratigraphic columns suggest that active subsidence and rapid sedimentary accumulation persisted with only minor interruptions until very late in the Laramide Orogeny. That led to a generally symmetrical north–south cross-sectional configuration of the original Hanna Basin, with its true depositional axis set well south of its apparent position of today. The Hanna Basin9s present strong asymmetry developed only secondarily. That basin9s modern configuration reflects Paleogene influences of: (1) late Laramide (early Eocene and probably younger) basement-involved contractional tectonics and associated uplifts; (2) out-of-the-basin thrusting passively responding to stratigraphic crowding; (3) prodigious syntectonic erosion; and (4) resulting basin fragmentation. North–south dimension of the late Paleocene (i.e., pre-fragmentation) greater Hanna Basin sedimentary sequence was roughly twice that of today, and near-sea-level topographic conditions persisted until late Eocene time. As expected, remnants of basin margins universally show major thinning of stratigraphic sequences. Principal thinning was from tectonic causes, however, exhibiting erosional angular unconformities only rarely. Out-of-the-basin, younger-on-older faulting (in which fault planes cut down-section) accompanied by massive erosion was the rule at all basin margins. Uplift of Simpson Ridge Anticline postdated deposition of upper Paleocene strata in direct continuity between what is now the separated Hanna and Carbon basins. The basement-involved fault system responsible for westward relative tectonic transport (and ca. 8 km of elevation) by Simpson Ridge also led to raising the attached Carbon Basin. Original Hanna Formation of the Carbon Basin was beveled away by erosion and soon thereafter became replaced by shallow-slope sliding of a long-runout allochthon, the Carbon Basin Klippe. The klippe9s original site of deposition probably was to the northeast, above what later became Flat Top Anticline. Uplift of Flat Top and Simpson Ridge anticlines was essentially synchronous (latest Paleocene or, more probably, early Eocene), establishing a lengthy, faulted-synclinal separation of the Hanna/Carbon Basin from the Laramie Basin. That syncline also bifurcated Simpson Ridge Anticline into western and eastern segments. A second allochthon, the Dana Klippe, rests upon southern parts of the Hanna Syncline (of Hanna Basin). That klippe9s site of deposition probably was abov","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"50 1","pages":"30-118"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.50.1.30","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313446","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 provenance of Pennsylvanian to Permian sandstones from the Wyoming craton and Wood River Basin, Idaho, U.S.A.","authors":"P. Link, R. Mahon, L. Beranek, E. Campbell-Stone, R. Lynds","doi":"10.2113/GSROCKY.49.2.115","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.2.115","url":null,"abstract":"Pennsylvanian rocks of the northern U.S. Rocky Mountains are mature quartzose sandstones. This paper uses detrital zircon geochronology on seven samples from the Wood River Formation, Tensleep Sandstone, and Weber Sandstone to determine if these sandstones have a common provenance, representing eastern Laurentian and Appalachian sand reworked within shallow-marine and eolian environments from the Wyoming craton westward to the Pioneer thrust plate of south-central Idaho. Our work suggests that this continental sand blanket was mixed with local sources on the south in the Yavapai-Mazatzal provinces of the Ancestral Rocky Mountains and in samples from the western Cordilleran thrust belt in south-central Idaho. In total, these Pennsylvanian sandstones contain a broad spectrum of detrital zircon U-Pb ages including, from old to young: A) minor Archean-age (3300–2550 Ma) populations; B) Paleoproterozoic (2000–1600 Ma), Mesoproterozoic (1470–1350 Ma), and major “Grenvillian” (1250–950 Ma) populations; and C) Cryogenian- to Ediacaran-age (665–565 Ma) and minor Paleozoic (495–410 Ma) populations. We interpret these detrital zircon ages to represent provenance mainly from the Appalachian mountain belt of eastern North America; however, central Appalachian versus northern Appalachian derivation is not clearly distinguished. The Weber Sandstone from the north flank of the Uinta Mountains in northeast Utah contains a strong 1700–1640 Ma age population derived from the Yavapai-Mazatzal provinces in the adjacent Ancestral Rocky Mountains. The shallow-marine Hailey Member of the Wood River Formation in south-central Idaho yields a population of >1800 Ma detrital zircons reworked from the uplifted Mississippian Copper Basin Formation. Both the Hailey and Wilson Creek Members of the Wood River Formation contain unique 640–490 Ma grains that may represent provenance from the Big Creek-Beaverhead plutonic belt of east-central Idaho and/or eastern Klamath terrane in the Klamath Mountains of northwest California and southwest Oregon. These new data support published models for Pennsylvanian–Permian transport of siliciclastic sediment with sources mainly from the North American craton, north of the Ancestral Rocky Mountains, into continental margin basins.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"115-136"},"PeriodicalIF":0.0,"publicationDate":"2014-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.2.115","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312760","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":"Strain localization in the Spanish Creek mylonite, Northern Madison Range, southwest Montana, U.S.A.","authors":"Joshua E. Johnson, D. P. West, C. Condit, K. Mahan","doi":"10.2113/GSROCKY.49.2.91","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.2.91","url":null,"abstract":"The Spanish Creek mylonite zone formed within multiply deformed granitic orthogneisses in the Northern Madison Range in southwestern Montana. An integrated approach incorporating fieldwork, microstructural analysis, electron backscatter diffraction (EBSD), and U-Pb zircon geochronology was utilized to characterize this mylonite zone and place its evolution into a broader regional context. The mylonitic rocks are characterized by a well-developed L-S fabric and a variably porphyroclastic character. U-Pb zircon geochronology yielded a magmatic crystallization age of 2824 ± 47 Ma (2σ) for the granitic protolith of the porphyroclastic Spanish Creek mylonite, indicating that the mylonitic deformation could have occurred during either a late Archean tectonic event or the late Paleoproterozoic Big Sky Orogeny, potentially activated and then reactivated with each successive event. Three distinct structural realms defined within the study area on the basis of structural observations and EBSD data reveal differences in the degree of mylonitic deformation. Two models are herein proposed to explain these significant differences, which are manifest in deformation microstructures and CPO fabrics between structural realms. In the first model, the intrusion and mylonitization of the granitic protolith for the rocks of the third structural realm were broadly coeval during a late Archean tectonic event, allowing for high-temperature mylonitic deformation. In an alternate model, the mylonitic deformation significantly postdated the ∼2.8 Ga intrusion of the granitic protolith, with factors aside from temperature influencing the nature of the deformation. Strain localization provides a viable explanation for the observed differences in the intensity of deformation between structural realms.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"91-114"},"PeriodicalIF":0.0,"publicationDate":"2014-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.2.91","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313646","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":"Reservoir stratigraphic heterogeneity within the Lower Cretaceous Muddy Sandstone in the Powder River Basin, northeast Wyoming, U.S.A.: Implications for carbon dioxide sequestration","authors":"Majie Fan","doi":"10.2113/GSROCKY.49.2.167","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.2.167","url":null,"abstract":"The Muddy Sandstone in the Powder River Basin (PRB), northeast Wyoming, is a promising reservoir for CO 2 sequestration because: (1) existing wells for hydrocarbon production can be used for CO 2 injection when a field is depleted; and (2) data are available to assess the ability and capacity to trap CO 2 . Here, I provide new data and compile published results to: (1) characterize four oil and gas fields (the Amos Draw, Kitty, Hilight, and Sand Dunes fields) in the PRB with respect to lithofacies, sedimentary environment, sandstone composition, sand-body geometry, and porosity and permeability; and (2) assess the controls on reservoir heterogeneity and the CO 2 sequestration potential. Five lithofacies are recognized based on core description and log responses. They are interpreted as offshore, lower–middle wave-dominated shoreface, weathering zone, fluvial incised-valley, and tide-influenced estuarine depositional environments. The Muddy Sandstone contains predominantly quartz, with total quartz higher than 70 percent of the total framework grains. The percentage of feldspar is generally less than 5 percent, except for the Rozet Member in the Amos Draw Field, which is up to 22 percent. Sandstone petrographic examination also shows that the Muddy Sandstone can be divided into four groups based on the relative abundance of pore space, carbonate cement, and matrix. Sandstone with high porosity up to 23 percent is found in the shoreface lithofacies in the Amos Draw and Hilight fields and is also found in the estuarine lithofacies in the Kitty Field. The incised-valley lithofacies is of particularly low porosity due to high matrix content and carbonate cementation. The measured porosity in the sandstone varies between 1 percent and 23 percent, and the permeability is generally less than 10 millidarcys (mD). The variation of porosity is consistent with the observation in thin sections. XRD results show that the pore-filling clay minerals include kaolinite, chlorite, illite, and smectite. Core and well log correlation show that sandstone formed in lower–middle shoreface environments is laterally extensive and of uniform thickness, whereas sandstone of fluvial and estuarine origins is more variable in lateral extent and thickness. Based on examination of lithofacies, sandstone geometry, and thin section petrography, I suggest that the best reservoir interval for CO 2 sequestration in the Amos Draw Field is the lower Rozet Member, in the Sand Dunes and Hilight fields is the Springen Ranch Member, and in the Kitty Field is the Ute Member. Variables examined in this study provide important inputs for calculating CO 2 capacity potential and predicting chemical reactivity after CO 2 injection. Reservoir quality of the Muddy Sandstone is highly heterogeneous, and the complexity may be attributed to a combination of depositional environment, history of relative sea-level change during deposition, and type and extent of diagenetic alteration. The Muddy Sandsto","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"167-190"},"PeriodicalIF":0.0,"publicationDate":"2014-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.2.167","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68313522","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":"Bryozoa of the uppermost Gerster Limestone (late Wordian, Permian), Medicine Range, northeastern Nevada, U.S.A.","authors":"E. H. Gilmour","doi":"10.2113/GSROCKY.49.2.137","DOIUrl":"https://doi.org/10.2113/GSROCKY.49.2.137","url":null,"abstract":"Abundant bryozoans occur in the uppermost Gerster Limestone (Permian) in a saddle directly west of the main Medicine Range section of the Gerster Limestone of northeastern Nevada. The stratigraphic position of this shaly limestone is confirmed by the conodont Merrilina praedivergens (late Wordian), which occurs elsewhere in the uppermost Gerster Limestone. Three new genera and five new species of bryozoans have been discovered in this shaly limestone. Subsequent analyses identified the new genera as Autospinifera n. gen., Utgaardostylus n. gen., and Wyseotrypa n. gen. The new species are Autospinifera rossae n. sp., Utgaardostylus stylata n. sp., Wyseotrypa parallela n. sp., Dyscritella triangulara n. sp., and Dyscritellina laminata n. sp. Other species of bryozoans present were previously described from the Russian Far East, Murdock Mountain Formation of the Leach Mountains of northeastern Nevada, Phosphoria Formation of eastern Idaho, and Gerster Limestone of the Medicine Range section of Nevada. This bryozoan fauna represents a late middle-Permian boreal fauna.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"49 1","pages":"137-165"},"PeriodicalIF":0.0,"publicationDate":"2014-09-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSROCKY.49.2.137","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68312825","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}