Pub Date : 2023-11-01DOI: 10.24872/rmgjournal.58.2.57
Oguzhan Kendigelen, Sven Egenhoff, William A. Matthews, Christopher S. Holm-Denoma, Karen R. Whiteley, Virginia Gent, M. Longman, J. Hagadorn
Facies of the Permian Lyons Sandstone are described and interpreted based on analyses of 23 cores from Larimer and Weld counties, Colorado. Here, the Lyons Sandstone consists of very fine- to medium-grained sandstone with minor silt and mudstone interbeds. The unit has five recurrent siliciclastic facies that can be grouped into two facies associations (FA). FA1 consists of (1) high-angle, cross-laminated sandstone (Facies 1; interpreted as eolian dune remnants); (2) low-angle, cross-laminated and horizontally laminated sandstone (Facies 2; interdune); and (3) chaotically bedded to folded sandstone (Facies 3; lower dune flanks). FA2, in contrast, is mainly (4) wavy- to irregularly laminated silty sandstone (Facies 4; wet to damp interdune); and (5) massive to wavy-laminated silt-rich mudstone (Facies 5; ponded water areas between dunes) with minor amounts of high-angle, cross-laminated sandstone (Facies 1) and low-angle, cross-laminated and horizontally laminated sandstone (Facies 2). FA1 is hypothesized to have been produced in an eolian system akin to those that might exist in the dune-dominated portion of an erg, whereas FA2 was deposited in the intermittently wet portion of this eolian system, perhaps along erg margins or in flat dune-adjacent settings that were impacted by the water table. Isopach data suggests that the study area is on the fringe of a larger Lyons system that spans > 100,000 km2, and was deposited close to the Ancestral Rockies—a paleogeography consistent with deposition in erg to erg-margin paleoenvironments. Detrital zircon populations from nearby Colorado Front Range outcrops and from 12 correlative eolian units are dominated by small, well-rounded Paleoproterozoic and Mesoproterozoic grain populations that are remarkably similar between units, signaling a well-mixed system that also received an influx of distally sourced sediment from the Appalachian orogen. Detrital zircon-based maximum depositional ages of the Lyons Sandstone and its equivalents are internally consistent with deposition of the unit during the latest Artinskian to Kungurian.
{"title":"The edge of a Permian erg: Eolian facies and provenance of the Lyons Sandstone in northern Colorado","authors":"Oguzhan Kendigelen, Sven Egenhoff, William A. Matthews, Christopher S. Holm-Denoma, Karen R. Whiteley, Virginia Gent, M. Longman, J. Hagadorn","doi":"10.24872/rmgjournal.58.2.57","DOIUrl":"https://doi.org/10.24872/rmgjournal.58.2.57","url":null,"abstract":"Facies of the Permian Lyons Sandstone are described and interpreted based on analyses of 23 cores from Larimer and Weld counties, Colorado. Here, the Lyons Sandstone consists of very fine- to medium-grained sandstone with minor silt and mudstone interbeds. The unit has five recurrent siliciclastic facies that can be grouped into two facies associations (FA). FA1 consists of (1) high-angle, cross-laminated sandstone (Facies 1; interpreted as eolian dune remnants); (2) low-angle, cross-laminated and horizontally laminated sandstone (Facies 2; interdune); and (3) chaotically bedded to folded sandstone (Facies 3; lower dune flanks). FA2, in contrast, is mainly (4) wavy- to irregularly laminated silty sandstone (Facies 4; wet to damp interdune); and (5) massive to wavy-laminated silt-rich mudstone (Facies 5; ponded water areas between dunes) with minor amounts of high-angle, cross-laminated sandstone (Facies 1) and low-angle, cross-laminated and horizontally laminated sandstone (Facies 2). FA1 is hypothesized to have been produced in an eolian system akin to those that might exist in the dune-dominated portion of an erg, whereas FA2 was deposited in the intermittently wet portion of this eolian system, perhaps along erg margins or in flat dune-adjacent settings that were impacted by the water table. Isopach data suggests that the study area is on the fringe of a larger Lyons system that spans > 100,000 km2, and was deposited close to the Ancestral Rockies—a paleogeography consistent with deposition in erg to erg-margin paleoenvironments. Detrital zircon populations from nearby Colorado Front Range outcrops and from 12 correlative eolian units are dominated by small, well-rounded Paleoproterozoic and Mesoproterozoic grain populations that are remarkably similar between units, signaling a well-mixed system that also received an influx of distally sourced sediment from the Appalachian orogen. Detrital zircon-based maximum depositional ages of the Lyons Sandstone and its equivalents are internally consistent with deposition of the unit during the latest Artinskian to Kungurian.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"52 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139301337","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}
Pub Date : 2023-11-01DOI: 10.24872/rmgjournal.58.2.83
Renee L. Love, Reed S. Lewis, Spencer H. Wood, Dennis M. Feeney, Mark D. Schmitz
Sedimentary deposits north of the western Snake River Plain host Idaho’s first and only producing oil and gas field. They consist of the lower to middle Miocene Payette Formation, the middle to upper Miocene Poison Creek and Chalk Hills Formations, and the Pliocene to lower Pleistocene Glenns Ferry Formation. Using new geochronology, palynomorph biostratigraphy, and geologic mapping, we connect updip surface features to subsurface petroleum play elements. The Payette Formation is a likely main source of the hydrocarbons, and acts as one of the reservoirs in the unnamed basin. Here, we redefine the Payette Formation as 0 to ~3,500 ft (0 to ~1,000 m) of mudstone, with lesser amounts of sandstone overlying and interbedded with the Columbia River Basalt Group and Weiser volcanic field. Index palynomorphs, including Liquidambar and Pterocarya, present in Idaho during and immediately after the middle Miocene climatic optimum, and new U–Pb ages of 16.39 and 15.88 Ma, help establish the thickness and extent of the formation. For the first time, these biostratigraphic markers have been defined for the oil and gas wells. The Poison Creek Formation is sandstone interbedded with mudstone that is ~800–1,800 ft (250–550 m) thick. The Chalk Hills Formation is a tuffaceous siltstone, claystone, and sandstone that is as much as ~4,200 ft (1,280 m) thick. New U–Pb ages are 10.1, 9.04, and 9.00 for the Poison Creek Formation, along with maximum depositional ages of 10.7 to 9.9 Ma for four samples from the Poison Creek Formation. A single U–Pb age of 7.78 Ma was determined from pumice low in the Chalk Hills Formation. Like the Payette Formation, the Poison Creek Formation can be a reservoir, whereas the Chalk Hills Formation acts as a sealing mudstone facies. The overlying sandstone, siltstone, and conglomerate of the Glenns Ferry Formation act as the overburden to the petroleum system in the subsurface, and were important for burial and hydrocarbon maturation. The Glenns Ferry Formation is up to 500 ft (150 m) thick in the study area, as much has been eroded. Whereas the Payette and Poison Creek Formations were deposited during the mid-Miocene climatic optimum amongst and above volcanic flows, the Chalk Hills and Glenns Ferry Formations were deposited within ancient Lake Idaho during an overall increase in aridity and cooling after the mid-Miocene climatic optimum.
{"title":"U–Pb zircon ages, mapping, and biostratigraphy of the Payette Formation and Idaho Group north of the western Snake River Plain, Idaho: Implications for hydrocarbon system correlation","authors":"Renee L. Love, Reed S. Lewis, Spencer H. Wood, Dennis M. Feeney, Mark D. Schmitz","doi":"10.24872/rmgjournal.58.2.83","DOIUrl":"https://doi.org/10.24872/rmgjournal.58.2.83","url":null,"abstract":"Sedimentary deposits north of the western Snake River Plain host Idaho’s first and only producing oil and gas field. They consist of the lower to middle Miocene Payette Formation, the middle to upper Miocene Poison Creek and Chalk Hills Formations, and the Pliocene to lower Pleistocene Glenns Ferry Formation. Using new geochronology, palynomorph biostratigraphy, and geologic mapping, we connect updip surface features to subsurface petroleum play elements. The Payette Formation is a likely main source of the hydrocarbons, and acts as one of the reservoirs in the unnamed basin. Here, we redefine the Payette Formation as 0 to ~3,500 ft (0 to ~1,000 m) of mudstone, with lesser amounts of sandstone overlying and interbedded with the Columbia River Basalt Group and Weiser volcanic field. Index palynomorphs, including Liquidambar and Pterocarya, present in Idaho during and immediately after the middle Miocene climatic optimum, and new U–Pb ages of 16.39 and 15.88 Ma, help establish the thickness and extent of the formation. For the first time, these biostratigraphic markers have been defined for the oil and gas wells. The Poison Creek Formation is sandstone interbedded with mudstone that is ~800–1,800 ft (250–550 m) thick. The Chalk Hills Formation is a tuffaceous siltstone, claystone, and sandstone that is as much as ~4,200 ft (1,280 m) thick. New U–Pb ages are 10.1, 9.04, and 9.00 for the Poison Creek Formation, along with maximum depositional ages of 10.7 to 9.9 Ma for four samples from the Poison Creek Formation. A single U–Pb age of 7.78 Ma was determined from pumice low in the Chalk Hills Formation. Like the Payette Formation, the Poison Creek Formation can be a reservoir, whereas the Chalk Hills Formation acts as a sealing mudstone facies. The overlying sandstone, siltstone, and conglomerate of the Glenns Ferry Formation act as the overburden to the petroleum system in the subsurface, and were important for burial and hydrocarbon maturation. The Glenns Ferry Formation is up to 500 ft (150 m) thick in the study area, as much has been eroded. Whereas the Payette and Poison Creek Formations were deposited during the mid-Miocene climatic optimum amongst and above volcanic flows, the Chalk Hills and Glenns Ferry Formations were deposited within ancient Lake Idaho during an overall increase in aridity and cooling after the mid-Miocene climatic optimum.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"43 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139304226","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}
Pub Date : 2023-06-01DOI: 10.24872/rmgjournal.58.1.39
H. Maher, Emily Persinger
Clastic dikes that occur within the terrestrial, Oligocene White River Group strata at localities throughout the Great Plains typically display internal mud to fine sand layers that are subparallel to the walls. Shrink-swell weathering usually obscures details of the internal layer geometry of the dikes. Recent work in the Slim Buttes area documents internal layer cross-cutting relationships that indicate tens or more of recurrent opening and injection events for thicker individual dikes. Evidence of significant dike-wall modification also exists. Source beds were unobserved despite adequate outcrops. Dikes are enclosed within the Oligocene Brule Formation. Some are truncated at or near the contact with the overlying Miocene Arikaree Group strata, constraining formation timing, whereas others have upper and lower tips within the Brule Formation. Dike strikes test as random in distribution. These dike attributes are consistent with repeated fracture opening and tip propagation from diagenetically driven shrinkage that induced episodic fluid flow which mobilized host-rock sediment (crack-fill instead of crack-seal). Sediment fill is proposed to have come from dike-wall erosion in branching tip regions during propagation events. In general, clastic dikes are polygenetic, and the diagenetically driven, recurrent formation mode evident in the White River Group examples can be considered in addition to standard injection models associated with overpressurized source beds or Neptunian infill.
{"title":"Recurrent fill history of individual clastic dikes in the White River Group at Slim Buttes, South Dakota","authors":"H. Maher, Emily Persinger","doi":"10.24872/rmgjournal.58.1.39","DOIUrl":"https://doi.org/10.24872/rmgjournal.58.1.39","url":null,"abstract":"\u0000 Clastic dikes that occur within the terrestrial, Oligocene White River Group strata at localities throughout the Great Plains typically display internal mud to fine sand layers that are subparallel to the walls. Shrink-swell weathering usually obscures details of the internal layer geometry of the dikes. Recent work in the Slim Buttes area documents internal layer cross-cutting relationships that indicate tens or more of recurrent opening and injection events for thicker individual dikes. Evidence of significant dike-wall modification also exists. Source beds were unobserved despite adequate outcrops. Dikes are enclosed within the Oligocene Brule Formation. Some are truncated at or near the contact with the overlying Miocene Arikaree Group strata, constraining formation timing, whereas others have upper and lower tips within the Brule Formation. Dike strikes test as random in distribution. These dike attributes are consistent with repeated fracture opening and tip propagation from diagenetically driven shrinkage that induced episodic fluid flow which mobilized host-rock sediment (crack-fill instead of crack-seal). Sediment fill is proposed to have come from dike-wall erosion in branching tip regions during propagation events. In general, clastic dikes are polygenetic, and the diagenetically driven, recurrent formation mode evident in the White River Group examples can be considered in addition to standard injection models associated with overpressurized source beds or Neptunian infill.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49389548","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}
Pub Date : 2023-06-01DOI: 10.24872/rmgjournal.58.1.1
J. Hagadorn, A. Bercovici, R. Fleming, Karen R. Whiteley, M. Yusas, T. Lyson, C. Henderson
The first reported Permian (Kungurian to Roadian) palynomorphs are described from Colorado, recovered from bedded gypsum and rare organic-rich shale intercalated in the red siltstone-dominated Lykins and State Bridge Formations. Surprisingly, these units generally lack the taeniate, saccate pollen that typifies most Permian continental rocks elsewhere, yet they contain abundant terrestrially derived palynomacerals, a low-diversity suite of sphaeromorph and acanthomorph acritarchs, and extremely rare non-taeniate, bissacate pollen grains. Acritarchs, known from one stratigraphic interval, are well-preserved and interpreted to represent autochthonous deposition during a marine incursion into the depocenter. This interpretation is consistent with their occurrence in a gray mudstone that is mantled by a mollusk-dominated coquina that bears conodonts, palaeoniscoid scales, and actinopterygian teeth. In contrast, most studied samples are dominated by wood fragments, charcoal, cuticles, and unidentified phytoclasts—all interpreted to represent dispersed plant cuticle and wood of continental origin. Fossils occur in black paper shale, gray fetid calcareous siltstone, and rhythmically bedded gypsum that is closely associated with thin limestone. Palynofacies analyses suggest that non-acritarch palynomacerals were deposited in dysoxic to anoxic waters that received minimal suspended terrigenous input. When combined with sedimentologic information, these non-acritarch fossils are hypothesized to have been deposited in shallow epicontinental lake-like settings that were periodically alkaline, hypersaline, and/or emergent.
{"title":"Palynology of Permian red-bed successions of Colorado and Wyoming and its influence on Laramide strata","authors":"J. Hagadorn, A. Bercovici, R. Fleming, Karen R. Whiteley, M. Yusas, T. Lyson, C. Henderson","doi":"10.24872/rmgjournal.58.1.1","DOIUrl":"https://doi.org/10.24872/rmgjournal.58.1.1","url":null,"abstract":"\u0000 The first reported Permian (Kungurian to Roadian) palynomorphs are described from Colorado, recovered from bedded gypsum and rare organic-rich shale intercalated in the red siltstone-dominated Lykins and State Bridge Formations. Surprisingly, these units generally lack the taeniate, saccate pollen that typifies most Permian continental rocks elsewhere, yet they contain abundant terrestrially derived palynomacerals, a low-diversity suite of sphaeromorph and acanthomorph acritarchs, and extremely rare non-taeniate, bissacate pollen grains. Acritarchs, known from one stratigraphic interval, are well-preserved and interpreted to represent autochthonous deposition during a marine incursion into the depocenter. This interpretation is consistent with their occurrence in a gray mudstone that is mantled by a mollusk-dominated coquina that bears conodonts, palaeoniscoid scales, and actinopterygian teeth. In contrast, most studied samples are dominated by wood fragments, charcoal, cuticles, and unidentified phytoclasts—all interpreted to represent dispersed plant cuticle and wood of continental origin. Fossils occur in black paper shale, gray fetid calcareous siltstone, and rhythmically bedded gypsum that is closely associated with thin limestone. Palynofacies analyses suggest that non-acritarch palynomacerals were deposited in dysoxic to anoxic waters that received minimal suspended terrigenous input. When combined with sedimentologic information, these non-acritarch fossils are hypothesized to have been deposited in shallow epicontinental lake-like settings that were periodically alkaline, hypersaline, and/or emergent.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":"1 1","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"43439149","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}
Pub Date : 2023-06-01DOI: 10.24872/rmgjournal.58.1.19
S. May, K. Bader, Lisa D. Boucher, L. Jacobs, J. Lively, T. Myers, M. Polcyn
We present the first description of Jurassic vertebrate fossils from Texas. The vertebrate specimens were collected from the Upper Jurassic Malone Formation in the Malone Mountains of western Texas. The specimens are fragmentary and not particularly diagnostic, but probably represent elements of plesiosaurians. One specimen is similar to the caudal vertebra of a pliosaurid plesiosaurian, whereas another may be a partial propodial of a small plesiosaurian. Additional bone fragments are not identifiable at this time. These vertebrates were discovered along with abundant plant and invertebrate fossils. Previous studies of the invertebrate fossils indicate a Kimmeridgian to Tithonian age for the Malone Formation, which is consistent with a single grain age of 151±2 Ma from detrital zircon U–Pb geochronology obtained in this study. The Malone Formation was deposited in shallow marine to marginal marine environments along the northern edge of the Chihuahua trough. It is correlative with the La Casita and La Caja Formations of northern Mexico, where similar marine vertebrates have been reported. The Malone Formation is also correlative with the Morrison Formation to the north.
{"title":"A record of Late Jurassic vertebrates from Texas","authors":"S. May, K. Bader, Lisa D. Boucher, L. Jacobs, J. Lively, T. Myers, M. Polcyn","doi":"10.24872/rmgjournal.58.1.19","DOIUrl":"https://doi.org/10.24872/rmgjournal.58.1.19","url":null,"abstract":"\u0000 We present the first description of Jurassic vertebrate fossils from Texas. The vertebrate specimens were collected from the Upper Jurassic Malone Formation in the Malone Mountains of western Texas. The specimens are fragmentary and not particularly diagnostic, but probably represent elements of plesiosaurians. One specimen is similar to the caudal vertebra of a pliosaurid plesiosaurian, whereas another may be a partial propodial of a small plesiosaurian. Additional bone fragments are not identifiable at this time. These vertebrates were discovered along with abundant plant and invertebrate fossils. Previous studies of the invertebrate fossils indicate a Kimmeridgian to Tithonian age for the Malone Formation, which is consistent with a single grain age of 151±2 Ma from detrital zircon U–Pb geochronology obtained in this study. The Malone Formation was deposited in shallow marine to marginal marine environments along the northern edge of the Chihuahua trough. It is correlative with the La Casita and La Caja Formations of northern Mexico, where similar marine vertebrates have been reported. The Malone Formation is also correlative with the Morrison Formation to the north.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48284701","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}
Pub Date : 2022-12-01DOI: 10.24872/rmgjournal.57.2.65
E. Erslev, L. Worthington, M. Anderson, K. Miller
What causes previously stable continental crust in the forelands of Cordilleran orogenic systems to shorten during low-angle subduction? The National Science Foundation/EarthScope Bighorn Project combined seismic imaging of the crust and Moho with kinematic modeling of Laramide (Late Cretaceous–Paleogene) basement-involved deformation to address this question. In north-central Wyoming, asymmetrical ENE-verging upper-crustal folds are highly discordant with broader, N-trending warps in the Moho, indicating crustal detachment. Restorable cross sections of ENE-directed detachment at a depth of ~30 km, combined a smaller component of NNW–SSE shortening due to the east-narrowing shape of the crustal allochthon, can explain the anastomosing network of Laramide basement-cored arches without major deformation of the underlying mantle lithosphere. Thrust-related fold geometries and west-to-east initiation of deformation in the Laramide and Sevier thrust belts point to Cordilleran end-loading from the west. Differences between Laramide (~N65E) and plate (~N25E) convergence directions, along with the fanning of Laramide shortening directions from nearly E–W to the south to NE–SW to the north, indicate slip partitioning during end-loading west of the Rockies. Sub-horizontal detachment with a near-zero critical taper within cratonic crust suggests an extremely weak Laramide detachment zone during deformation. Analogous lower-crustal deformation in subduction forearcs is associated with slow earthquakes and slab dehydration. We hypothesize that low-angle subduction of the Farallon Plate suppressed fluid-consuming melting and corner-flow processes that characterize higher-angle subduction. This allowed subduction-generated fluids to escape upward into the overlying continental lithosphere, causing retrograde metamorphism and increased fluid pressure that facilitated crustal detachment. This hydration-based hypothesis predicts that crustal detachment will accompany major earthquakes in active analog orogens.
{"title":"Laramide crustal detachment in the Rockies: Cordilleran shortening of fluid-weakened foreland crust","authors":"E. Erslev, L. Worthington, M. Anderson, K. Miller","doi":"10.24872/rmgjournal.57.2.65","DOIUrl":"https://doi.org/10.24872/rmgjournal.57.2.65","url":null,"abstract":"\u0000 What causes previously stable continental crust in the forelands of Cordilleran orogenic systems to shorten during low-angle subduction? The National Science Foundation/EarthScope Bighorn Project combined seismic imaging of the crust and Moho with kinematic modeling of Laramide (Late Cretaceous–Paleogene) basement-involved deformation to address this question. In north-central Wyoming, asymmetrical ENE-verging upper-crustal folds are highly discordant with broader, N-trending warps in the Moho, indicating crustal detachment. Restorable cross sections of ENE-directed detachment at a depth of ~30 km, combined a smaller component of NNW–SSE shortening due to the east-narrowing shape of the crustal allochthon, can explain the anastomosing network of Laramide basement-cored arches without major deformation of the underlying mantle lithosphere.\u0000 Thrust-related fold geometries and west-to-east initiation of deformation in the Laramide and Sevier thrust belts point to Cordilleran end-loading from the west. Differences between Laramide (~N65E) and plate (~N25E) convergence directions, along with the fanning of Laramide shortening directions from nearly E–W to the south to NE–SW to the north, indicate slip partitioning during end-loading west of the Rockies.\u0000 Sub-horizontal detachment with a near-zero critical taper within cratonic crust suggests an extremely weak Laramide detachment zone during deformation. Analogous lower-crustal deformation in subduction forearcs is associated with slow earthquakes and slab dehydration. We hypothesize that low-angle subduction of the Farallon Plate suppressed fluid-consuming melting and corner-flow processes that characterize higher-angle subduction. This allowed subduction-generated fluids to escape upward into the overlying continental lithosphere, causing retrograde metamorphism and increased fluid pressure that facilitated crustal detachment. This hydration-based hypothesis predicts that crustal detachment will accompany major earthquakes in active analog orogens.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46740368","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}
Pub Date : 2022-12-01DOI: 10.24872/rmgjournal.57.2.99
L. Brand
Polygonal “cracks” are common in the Coconino Sandstone in Arizona. They have been called desiccation cracks, but several features indicate they are not desiccation cracks. They were never open cracks, but are merely linear depressions, linked to form polygons. They occur only on bounding surfaces, containing almost no clay, and the cracks extend 10 to 15 cm above and below the bounding surfaces. The polygonal patterns continue down from one sandstone lamina to another, for several centimeters. They are persistently continuous across all surfaces within their 20–30 cm vertical range, from the bottomset beds, onto the bounding surface, and continuing into individual cross-beds below the bounding surface. The cracks occur at the Grand Canyon, and are especially numerous and visible in flagstone quarries in the Seligman and Ash Fork area. They occur on some bounding surfaces but not on others, and in some quarries but not in others. The polygonal cracks have been mentioned in passing, but this is the first reported research on these cracks in the Coconino Sandstone. Polygonal cracks have been reported in the Navajo, Page, and Entrada Sandstones, but there are significant differences between these and the Coconino Sandstone cracks, which may indicate differences in their origin.
{"title":"Polygonal linear depressions in the Coconino Sandstone (Permian) of Arizona, and their relevance for interpreting paleoenvironment","authors":"L. Brand","doi":"10.24872/rmgjournal.57.2.99","DOIUrl":"https://doi.org/10.24872/rmgjournal.57.2.99","url":null,"abstract":"\u0000 Polygonal “cracks” are common in the Coconino Sandstone in Arizona. They have been called desiccation cracks, but several features indicate they are not desiccation cracks. They were never open cracks, but are merely linear depressions, linked to form polygons. They occur only on bounding surfaces, containing almost no clay, and the cracks extend 10 to 15 cm above and below the bounding surfaces. The polygonal patterns continue down from one sandstone lamina to another, for several centimeters. They are persistently continuous across all surfaces within their 20–30 cm vertical range, from the bottomset beds, onto the bounding surface, and continuing into individual cross-beds below the bounding surface. The cracks occur at the Grand Canyon, and are especially numerous and visible in flagstone quarries in the Seligman and Ash Fork area. They occur on some bounding surfaces but not on others, and in some quarries but not in others. The polygonal cracks have been mentioned in passing, but this is the first reported research on these cracks in the Coconino Sandstone. Polygonal cracks have been reported in the Navajo, Page, and Entrada Sandstones, but there are significant differences between these and the Coconino Sandstone cracks, which may indicate differences in their origin.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48151721","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}
Pub Date : 2022-12-01DOI: 10.24872/rmgjournal.57.2.117
J. Quick, John-Paul Hogan
We present the results of remote sensing analysis of U.S. Geological Survey digital elevation models, Landsat spectral data, and National Agriculture Imagery Program orthophotos to generate a preliminary geologic map that significantly aided our boots-on-the-ground geologic mapping of the southwest portion of the Three Peaks 7.5ʹ quadrangle in southwest Utah. Sedimentary rocks, intrusive rocks, and a variety of geologic contacts, including unconformities and faults, as well as unconsolidated alluvium are recognized in the study area. We constructed a series of geologic maps using remote sensing data and analysis techniques that are readily available to geoscientists. These techniques include band-ratioing, random forest analysis, and these analyses. Resolution of the resulting geologic maps generated by random forest analysis and principal component analysis were greatly improved by incorporating both the high resolution orthophoto and the 1/3 arc second digital elevation model into the principal component analysis. Our final remotely sensed geologic map integrated results from each technique. We used this remotely sensed geologic map to develop our preliminary plan for the field campaign. We preselected high priority targets (e.g., previously unrecognized units and faults) for in-person field analysis. We also identified highly accessible areas that allowed for efficient use of in-person field time needed for evaluation of large areas covered by relatively homogeneous units. The authors spent 25 days in the field over a seven-week field season, mapping the same area. Here, we compare the remote-sensed geologic maps with the final in-person field checked geologic map and discuss the utility of remote sensing data for detailed geologic field investigations. Preparing a remote sensing geologic map prior to field work has several advantages, including identification of mappable units, recognition of geologic contacts, and selection of priority target areas for direct evaluation of hypothesized field relationships, thereby promoting more efficient geologic mapping.
{"title":"Practical remote sensing data analysis for efficient geological field mapping: An example from the southwest portion of the Three Peaks 7.5ʹ quadrangle, southwest Utah","authors":"J. Quick, John-Paul Hogan","doi":"10.24872/rmgjournal.57.2.117","DOIUrl":"https://doi.org/10.24872/rmgjournal.57.2.117","url":null,"abstract":"\u0000 We present the results of remote sensing analysis of U.S. Geological Survey digital elevation models, Landsat spectral data, and National Agriculture Imagery Program orthophotos to generate a preliminary geologic map that significantly aided our boots-on-the-ground geologic mapping of the southwest portion of the Three Peaks 7.5ʹ quadrangle in southwest Utah. Sedimentary rocks, intrusive rocks, and a variety of geologic contacts, including unconformities and faults, as well as unconsolidated alluvium are recognized in the study area. We constructed a series of geologic maps using remote sensing data and analysis techniques that are readily available to geoscientists. These techniques include band-ratioing, random forest analysis, and these analyses. Resolution of the resulting geologic maps generated by random forest analysis and principal component analysis were greatly improved by incorporating both the high resolution orthophoto and the 1/3 arc second digital elevation model into the principal component analysis. Our final remotely sensed geologic map integrated results from each technique. We used this remotely sensed geologic map to develop our preliminary plan for the field campaign. We preselected high priority targets (e.g., previously unrecognized units and faults) for in-person field analysis. We also identified highly accessible areas that allowed for efficient use of in-person field time needed for evaluation of large areas covered by relatively homogeneous units. The authors spent 25 days in the field over a seven-week field season, mapping the same area. Here, we compare the remote-sensed geologic maps with the final in-person field checked geologic map and discuss the utility of remote sensing data for detailed geologic field investigations. Preparing a remote sensing geologic map prior to field work has several advantages, including identification of mappable units, recognition of geologic contacts, and selection of priority target areas for direct evaluation of hypothesized field relationships, thereby promoting more efficient geologic mapping.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-12-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49609926","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}
Pub Date : 2022-06-01DOI: 10.24872/rmgjournal.57.1.1
C. Holm-Denoma, W. Matthews, Linda K. Soar, M. Longman, J. Hagadorn
We report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of > 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), > 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (< 0.1%; n = 2,927 grains).
{"title":"Provenance of Devonian–Carboniferous strata of Colorado: The influence of the Cambrian and the Proterozoic","authors":"C. Holm-Denoma, W. Matthews, Linda K. Soar, M. Longman, J. Hagadorn","doi":"10.24872/rmgjournal.57.1.1","DOIUrl":"https://doi.org/10.24872/rmgjournal.57.1.1","url":null,"abstract":"\u0000 We report new LA-ICP-MS U–Pb detrital zircon ages and sedimentary petrology of silty to sandy limestones and dolostones, as well as calcareous to dolomitic sandstones of the Devonian–Carboniferous (Mississippian) Chaffee Group. We also report new detrital zircon ages from the late Cambrian Sawatch Quartzite, and a U–Pb zircon crystallization age on a late Mesoproterozoic (1087.9 ± 13.5 Ma) granitoid of underlying basement from the Eagle Basin of northwest Colorado. Grain populations in the Chaffee Group are mostly bimodal. More than 84% of zircons centered around a Paleoproterozoic (ca. 1.78 Ga) mode typical of the Yavapai province that forms much of the basement of Colorado and an early Mesoproterozoic (ca. 1.42 Ga) mode typical of A-type granites that intrude this region. A notable late Mesoproterozoic (ca. 1.08 Ga) mode exists in some Chaffee samples, giving those samples a trimodal detrital zircon age distribution. These bipartite or tripartite detrital zircon age modes exist in Cambrian, Devonian, and Carboniferous strata from paleogeographically adjacent successions, but the correlation between the Chaffee zircons is highest with the region’s basal Cambrian sandstones of the Sawatch Quartzite, Flathead Sandstone, and Ignacio Quartzite, which have similar (ca. 1.08 Ga, 1.43 Ga, 1.70 Ga, respectively) zircon populations, and a paucity of > 1.8 Ga grains. This similarity suggests that most grains in the Chaffee Group derive from recycling of these basal sandstones, and that little sediment was derived directly from thenexposed Precambrian basement highs, from the Wyoming craton to the north, or from Paleoproterozoic arcs and orogens to the west and northeast. Minor Mesoarchean to early Paleoproterozoic (ca. 3.00 to 2.40 Ga) grains exist in the Chaffee Group, an attribute shared by the Late Ordovician Harding Sandstone of Colorado’s Front Range, but that is absent from the region’s underlying Cambrian sandstones—suggesting some recycled mixture of Cambrian and Ordovician sedimentary rocks. No near-depositional age grains are present in the Chaffee Group. The youngest grain is Early Devonian (~417 Ma), > 45 million years (m.y.) older than these strata. Additionally, Paleozoic grains are extremely uncommon (< 0.1%; n = 2,927 grains).","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49655449","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}
Pub Date : 2022-06-01DOI: 10.24872/rmgjournal.57.1.23
J. Hagadorn, Bonita L. Lahey, Linda K. Soar, M. Longman, D. Over, R. Mills
Paired chemostratigraphic and biostratigraphic data suggest that the Devonian–Carboniferous boundary and the Hangenberg extinction event are recorded in the Coffee Pot Member of the Dyer Formation of the White River uplift region of northwestern Colorado. The Hangenberg isotopic excursion interval occurs in biostratigraphically depauperate shallow platform micritic dolostone and limestone representing the aculeatus–?ultimus–kockeli? Zone. The Hangenberg interval strata have δ13Ccarb values up to 7.69‰, and locally contain ooids, stromatolites, and other microbial structures. In three sections, there is a short-lived > 4‰ “pre-Hangenberg” positive excursion that is associated with the presence of detrital dolomite siltstone. The main Hangenberg isotopic signature exists in overlying strata, and is replicated in four different sections of the Dyer Formation across an area ~300 km2. In each section, the excursion interval is preceded by, and in one location is interrupted by, evidence of meteoric diagenesis and/or emergence characterized by macroscopic dissolution features and pronounced negative δ13Ccarb values (to ca. -6‰). Conodont 87Sr/86Sr ratios through the Dyer Formation dip to nearly 0.708 during the onset of the Hangenberg Event, consistent with the brachiopod based 87Sr/86Sr nadir from the same interval in Missouri, France, and Germany. The least-radiogenic trendline through the Dyer 87Sr/86Sr record matches the global minimum 87Sr/86Sr trendline through the Famennian–Tournaisian transition and infills several gaps in the global record.
{"title":"Insights into the Devonian–Carboniferous transition and Hangenberg Event from δ13Ccarb and 87Sr/86Sr chemostratigraphy of shallow platform carbonate strata of northwestern Colorado","authors":"J. Hagadorn, Bonita L. Lahey, Linda K. Soar, M. Longman, D. Over, R. Mills","doi":"10.24872/rmgjournal.57.1.23","DOIUrl":"https://doi.org/10.24872/rmgjournal.57.1.23","url":null,"abstract":"\u0000 Paired chemostratigraphic and biostratigraphic data suggest that the Devonian–Carboniferous boundary and the Hangenberg extinction event are recorded in the Coffee Pot Member of the Dyer Formation of the White River uplift region of northwestern Colorado. The Hangenberg isotopic excursion interval occurs in biostratigraphically depauperate shallow platform micritic dolostone and limestone representing the aculeatus–?ultimus–kockeli? Zone. The Hangenberg interval strata have δ13Ccarb values up to 7.69‰, and locally contain ooids, stromatolites, and other microbial structures. In three sections, there is a short-lived > 4‰ “pre-Hangenberg” positive excursion that is associated with the presence of detrital dolomite siltstone. The main Hangenberg isotopic signature exists in overlying strata, and is replicated in four different sections of the Dyer Formation across an area ~300 km2. In each section, the excursion interval is preceded by, and in one location is interrupted by, evidence of meteoric diagenesis and/or emergence characterized by macroscopic dissolution features and pronounced negative δ13Ccarb values (to ca. -6‰). Conodont 87Sr/86Sr ratios through the Dyer Formation dip to nearly 0.708 during the onset of the Hangenberg Event, consistent with the brachiopod based 87Sr/86Sr nadir from the same interval in Missouri, France, and Germany. The least-radiogenic trendline through the Dyer 87Sr/86Sr record matches the global minimum 87Sr/86Sr trendline through the Famennian–Tournaisian transition and infills several gaps in the global record.","PeriodicalId":34958,"journal":{"name":"Rocky Mountain Geology","volume":" ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2022-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"44236114","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}
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