https://giw.utahgeology.org/giw/index.php/GIW/issue/feed Geology of the Intermountain West 2023-10-18T08:38:03-06:00 Douglas Sprinkel giw@utahgeology.org Open Journal Systems <p>The Geology of the Intermountain West is an open-access journal published by the Utah Geological Association providing authors a digital option for rapid publication of research on the geology of Utah and surrounding areas.</p> https://giw.utahgeology.org/giw/index.php/GIW/article/view/120 The Kanarra fold-thrust structure—the leading edge of the Sevier fold-thrust belt, southwestern Utah: 2023-01-27T09:05:15-07:00 William Chandonia williamjchandonia@gmail.com John Hogan jhogan@mst.edu <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;The multiple origins proposed for the Kanarra anticline in southwestern Utah as a drag-fold along the Hurricane fault, a Laramide monocline, a Sevier fault-propagation fold, or a combination of these processes, serve to muddy its tectonic significance. This in part reflects the structural complexity of the exposed eastern half of the fold. The fold evolved from open and up-right to overturned and tight, is cross-cut by multiple faults, and was subsequently dismembered by the Hurricane fault. The western half of the fold is obscured because of burial, along with the hanging wall of the Hurricane fault, beneath Neogene and younger sediments and volcanics. We present the results of detailed bedrock geologic mapping, and geologic cross sections restored to Late Cretaceous time (prior to Basin and Range extension), to demonstrate the Kanarra anticline is a compound anticline-syncline pair inextricably linked with concomitant thrust faulting that formed during the Sevier orogeny. We propose the name Kanarra fold-thrust structure to unambiguously identify the close spatial and temporal association of folding and thrusting in formation of this prominent geologic feature. We identify a previously unrecognized thrust, the Red Rock Trail thrust, as a forelimb shear thrust that was in a favorable orientation and position to have been soft-linked, and locally hard-linked, with the thrust ramp of the basal detachment to form a break thrust. The east verging Red Rock Trail thrust is recognized by a distinctive cataclasite in the Lower Jurassic Navajo Sandstone. The hanging wall of the Red Rock Trail thrust is displaced eastward over the Middle Jurassic Carmel Formation and Upper Cretaceous formations and can be traced for at least 27 km and possibly farther. We contend the Kanarra fold-thrust structure unambiguously defines the leading edge of the Sevier fold-thrust belt\nin southwestern Utah. Stratigraphic relationships in the southern and northern part of the Kanarra fold-thrust structure constrain its development between the early and late Campanian (about 84 to 71 Ma) but possibly younger. In southwestern Utah, initial movement along the Iron Springs thrust at about 100 Ma (Quick and others, 2020) and subsequent eastward advancement of the Sevier deformation front to the Red Rock Trail thrust at about 84 to 71 Ma coincided with well-documented magmatic flare ups in the Cordilleran arc in the hinterland of the Sevier fold-thrust belt. This temporal relationship between magmatic flare ups and thrusting is consistent with a close correspondence between arc-related processes and episodic foreland deformation.&quot;}" data-sheets-userformat="{&quot;2&quot;:15105,&quot;3&quot;:{&quot;1&quot;:0},&quot;11&quot;:4,&quot;12&quot;:0,&quot;14&quot;:{&quot;1&quot;:2,&quot;2&quot;:0},&quot;15&quot;:&quot;Calibri&quot;,&quot;16&quot;:11}">The multiple origins proposed for the Kanarra anticline in southwestern Utah as a drag-fold along the Hurricane fault, a Laramide monocline, a Sevier fault-propagation fold, or a combination of these processes, serve to muddy its tectonic significance. This in part reflects the structural complexity of the exposed eastern half of the fold. The fold evolved from open and up-right to overturned and tight, is cross-cut by multiple faults, and was subsequently dismembered by the Hurricane fault. The western half of the fold is obscured because of burial, along with the hanging wall of the Hurricane fault, beneath Neogene and younger sediments and volcanics. We present the results of detailed bedrock geologic mapping, and geologic cross sections restored to Late Cretaceous time (prior to Basin and Range extension), to demonstrate the Kanarra anticline is a compound anticline-syncline pair inextricably linked with concomitant thrust faulting that formed during the Sevier orogeny. We propose the name Kanarra fold-thrust structure to unambiguously identify the close spatial and temporal association of folding and thrusting in formation of this prominent geologic feature. We identify a previously unrecognized thrust, the Red Rock Trail thrust, as a forelimb shear thrust that was in a favorable orientation and position to have been soft-linked, and locally hard-linked, with the thrust ramp of the basal detachment to form a break thrust. The east verging Red Rock Trail thrust is recognized by a distinctive cataclasite in the Lower Jurassic Navajo Sandstone. The hanging wall of the Red Rock Trail thrust is displaced eastward over the Middle Jurassic Carmel Formation and Upper Cretaceous formations and can be traced for at least 27 km and possibly farther. We contend the Kanarra fold-thrust structure unambiguously defines the leading edge of the Sevier fold-thrust belt<br>in southwestern Utah. Stratigraphic relationships in the southern and northern part of the Kanarra fold-thrust structure constrain its development between the early and late Campanian (about 84 to 71 Ma) but possibly younger. In southwestern Utah, initial movement along the Iron Springs thrust at about 100 Ma (Quick and others, 2020) and subsequent eastward advancement of the Sevier deformation front to the Red Rock Trail thrust at about 84 to 71 Ma coincided with well-documented magmatic flare ups in the Cordilleran arc in the hinterland of the Sevier fold-thrust belt. This temporal relationship between magmatic flare ups and thrusting is consistent with a close correspondence between arc-related processes and episodic foreland deformation.</span></p> 2023-01-25T20:18:02-07:00 Copyright (c) 2023 Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW/article/view/122 The concrete Diplodocus of Vernal—a cultural icon of Utah 2023-03-17T19:45:27-06:00 Michael Taylor dino@miketaylor.org.uk Steven Sroka stevesroka@utah.gov Kenneth Carpenter kenneth.carpenter-1@colorado.edu <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;Although many casts have been made of the Carnegie Museum’s iconic Diplodocus, initially in plaster and more recently in various plastics, one stands alone as having been cast in concrete. This skeleton, made from the original Carnegie molds starting in 1956–1957, was unveiled at the Utah Field House of Natural History in Vernal, Utah, in 1957, and stood outside the museum for three decades. The fate of the molds after this casting is uncertain. The concrete Diplodocus was the museum’s icon for 32 years until the weather damage became too great. The cast was then taken down and repaired, and fresh molds made from it by Dinolab in Salt Lake City. From these molds, a new replica was cast in water-expanded polyester and\nmounted inside the Field House. This cast was moved to the Field House’s new location in 2004 and was remounted in the atrium, but the old concrete cast could not be easily remounted and was instead transferred to the Prehistoric Museum at Price, Utah. It has, however, yet to be remounted there, as it awaits a new building for the museum. Meanwhile, the new molds have been used to create more Diplodocus casts\nthat are mounted in Japan and elsewhere, and have also furnished missing parts of the iconic rearing Barosaurus skeleton in the atrium of the American Museum of Natural History in New York City. Thus, the concrete Diplodocus of Vernal has become one of the most influential of all Diplodocus specimens, second only to the Carnegie original.&quot;}" data-sheets-userformat="{&quot;2&quot;:15105,&quot;3&quot;:{&quot;1&quot;:0},&quot;11&quot;:4,&quot;12&quot;:0,&quot;14&quot;:{&quot;1&quot;:2,&quot;2&quot;:0},&quot;15&quot;:&quot;Calibri&quot;,&quot;16&quot;:11}">Although many casts have been made of the Carnegie Museum’s iconic Diplodocus, initially in plaster and more recently in various plastics, one stands alone as having been cast in concrete. This skeleton, made from the original Carnegie molds starting in 1956–1957, was unveiled at the Utah Field House of Natural History in Vernal, Utah, in 1957, and stood outside the museum for three decades. The fate of the molds after this casting is uncertain. The concrete Diplodocus was the museum’s icon for 32 years until the weather damage became too great. The cast was then taken down and repaired, and fresh molds made from it by Dinolab in Salt Lake City. From these molds, a new replica was cast in water-expanded polyester and mounted inside the Field House. This cast was moved to the Field House’s new location in 2004 and was remounted in the atrium, but the old concrete cast could not be easily remounted and was instead transferred to the Prehistoric Museum at Price, Utah. It has, however, yet to be remounted there, as it awaits a new building for the museum. Meanwhile, the new molds have been used to create more Diplodocus casts that are mounted in Japan and elsewhere, and have also furnished missing parts of the iconic rearing Barosaurus skeleton in the atrium of the American Museum of Natural History in New York City. Thus, the concrete Diplodocus of Vernal has become one of the most influential of all Diplodocus specimens, second only to the Carnegie original.</span></p> 2023-02-20T19:20:46-07:00 Copyright (c) 2023 Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW/article/view/124 Snow drought and monsoon floods—hydrological extremes in the Cedar Valley watershed during water year 2021, southwestern Utah 2023-03-17T20:55:42-06:00 Erich Mueller richmueller@suu.edu Garrett Sudweeks garrett@kairosgeodetics.com Shadrach Ashton shadashton@gmail.com Micah Olson olsonmicah86@gmail.com <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;Water year 2021 was a year of extremes in the Cedar Valley watershed of southern Utah, with snow drought resulting in extremely low snowmelt runoff of Coal Creek and intense monsoon rainfall resulting in several floods in different parts of the valley. Winter snow accumulation was depressed throughout southern Utah, perhaps due to La Nina conditions affecting winter storm trajectories. Coal Creek, the principal stream providing surface water to Cedar Valley, typically receives most of its annual flow from snowmelt runoff, but in 2021 had a peak snowmelt discharge 15% to 25% of that recorded in the previous two years and the third lowest snowmelt runoff on record. Following this extremely low snowmelt runoff period, more than 10 floods of Coal Creek occurred following monsoon storms in July and August that exceeded the 2021 snowmelt peak. Additionally, several thunderstorms produced rainfall rates in exceedance of the 100-year event and induced flooding within Cedar City and the town of Enoch. Flood inundation modeling using HEC-RAS and high-resolution topographic data showed good agreement with field and\npubic-survey data on the high-water stage during the Enoch flooding, and demonstrated that the flooding was likely exacerbated by the low topography and limited drainage potential in flooded areas. Whereas the monsoon storms improved soil moisture and helped alleviate drought conditions, they also resulted in urban flooding and did little to replenish the regional water supply.&quot;}" data-sheets-userformat="{&quot;2&quot;:15105,&quot;3&quot;:{&quot;1&quot;:0},&quot;11&quot;:4,&quot;12&quot;:0,&quot;14&quot;:{&quot;1&quot;:2,&quot;2&quot;:0},&quot;15&quot;:&quot;Calibri&quot;,&quot;16&quot;:11}">Water year 2021 was a year of extremes in the Cedar Valley watershed of southern Utah, with snow drought resulting in extremely low snowmelt runoff of Coal Creek and intense monsoon rainfall resulting in several floods in different parts of the valley. Winter snow accumulation was depressed throughout southern Utah, perhaps due to La Nina conditions affecting winter storm trajectories. Coal Creek, the principal stream providing surface water to Cedar Valley, typically receives most of its annual flow from snowmelt runoff, but in 2021 had a peak snowmelt discharge 15% to 25% of that recorded in the previous two years and the third lowest snowmelt runoff on record. Following this extremely low snowmelt runoff period, more than 10 floods of Coal Creek occurred following monsoon storms in July and August that exceeded the 2021 snowmelt peak. Additionally, several thunderstorms produced rainfall rates in exceedance of the 100-year event and induced flooding within Cedar City and the town of Enoch. Flood inundation modeling using HEC-RAS and high-resolution topographic data showed good agreement with field and pubic-survey data on the high-water stage during the Enoch flooding, and demonstrated that the flooding was likely exacerbated by the low topography and limited drainage potential in flooded areas. Whereas the monsoon storms improved soil moisture and helped alleviate drought conditions, they also resulted in urban flooding and did little to replenish the regional water supply.</span></p> 2023-03-17T20:00:45-06:00 Copyright (c) 2023 Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW/article/view/125 The March 2020, Mw 5.7 Magna, Utah, earthquake—documentation of geologic effects and summary of new research 2023-03-24T21:16:42-06:00 Adam Hiscock adamhiscock@utah.gov Emily Kleber ekleber@utah.gov Adam McKean adammckean@utah.gov Ben Erickson benerickson@utah.gov Greg McDonald gregmcdonald@utah.gov Richard Giraud rich124swe@gmail.com Jessica Castleton jessicacastleton@utah.gov Steve Bowman stevebowman@utah.gov <p>The March 18, 2020, Mw 5.7 Magna earthquake was the largest earthquake in Utah since the 1992 ML 5.8 St. George earthquake. The Magna earthquake occurred in the northwest corner of the Salt Lake Valley, home to 1.2 million people. Immediately following the earthquake, the Utah Geological Survey organized teams to collect perishable field data on the geologic effects of ground shaking near the epicenter, as well as establish a web-based digital clearinghouse to collect, distribute, and archive data related to the earthquake. This earthquake also coincided with the beginning of the COVID-19 global pandemic, which added extra challenges to our earthquake response. Teams used a small, unmanned aircraft system to obtain aerial photos and videos of geologic effects to supplement ground-based reconnaissance. The observed geologic effects of ground motions from the Magna earthquake include liquefaction in the form of sand boils, tension cracks, lateral spreading, and localized subsidence. No primary surface fault rupture was observed. The areas with the highest observed concentration of liquefaction features were close to the shore of Great Salt Lake and near the epicenter, northeast of the town of Magna. Photos and other documentation of the geologic effects associated with this earthquake are critical in helping to understand the hazards associated with moderate magnitude earthquakes in the Wasatch Front region. The earthquake sequence and associated geologic effects were well documented, due to the proximity to a major metropolitan area and the mainshock and aftershocks occurring within the densest part of the Utah Regional Seismic Network. In the two years since the earthquake, numerous studies have been published documenting and interpreting data to characterize the Magna event and discuss how new data add to what is known about seismic hazards along the Wasatch Front.</p> 2023-03-24T13:40:47-06:00 Copyright (c) 2023 Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW/article/view/126 Potential drilling hazards for wells targeting the Cane Creek shale, Pennsylvanian Paradox Formation, Paradox fold and fault belt, southeastern Utah and southwestern Colorado 2023-03-24T21:16:35-06:00 Thomas Chidsey tomchidsey@gmail.com <p>The Cane Creek shale of the Pennsylvanian Paradox Formation represents a major target for oil and gas in the Paradox fold and fault belt of the northern Paradox Basin of southeastern Utah and southwestern Colorado. Early exploration and development attempts resulted in blowouts due to unexpected gas-bearing intervals and casing collapses caused by salt flowage in the Paradox Formation. These problems represent some of the types of drilling hazards that could be expected when planning Cane Creek wells. Horizontal drilling first used in the early 1990s changed the Cane Creek shale play from one of mostly drilling failures to a more successful commercial play. Depending on the location, exploratory Cane Creek wells may penetrate a section that ranges in age from Cretaceous through Pennsylvanian. Drilling in the region often encounters a wide variety of lithologies (carbonates, shale, mudstone, sandstone, and evaporites) and associated potential hazards that may include: (1) swelling clays, (2) high porosity-permeability or fractured zones resulting in lost circulation or excessive mudcake buildup, (3) “kicks” due to the influx of reservoir fluid (oil, water, or gas) into the wellbore, (4) uranium-rich zones, (5) washouts, (6) hole deviation, sticking, and other well-integrity problems, (7) chert, and (8) overpressured intervals. In addition, natural carbon dioxide, which flows from the partially human-made Crystal Geyser near some Cane Creek wellsites, represents an unusual drilling hazard if encountered in the northernmost part of the fold and fault belt. Using the lessons learned from the recently completed research well, State 16-2 (renamed the State 16-2LN-CC, API No. 43-019-50089, after the horizontal leg was drilled), and other wells in the region, drilling engineers and operators can better plan for potential hazards when exploring for hydrocarbons in the Cane Creek shale or deeper targets (Mississippian Leadville Limestone and Devonian Elbert Formation) in the fairly remote, relatively sparsely explored Paradox fold and fault belt. The goal is to de-risk wells, lower expenses, and mitigate problems before they occur. The expected results are safer and more successful drilling of wells to the Cane Creek shale and deeper reservoirs ultimately leading to additional commercial hydrocarbon discoveries in the region.</p> 2023-03-24T14:03:59-06:00 Copyright (c) 2023 Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW/article/view/127 Utah geosite—the Salina Canyon unconformity, a classic example of missing time 2023-05-10T19:59:03-06:00 Shelley Judge sjudge@wooster.edu Emmett Werthmann emmett.werthmann@wri.org Cristina Millan millan.2@osu.edu Michael Braunagel braunagel.2@buckeyemail.osu.edu Erica Maletic maletic.2@osu.edu <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #0e0d0c;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;Salina Canyon, Utah, reveals a spectacular angular unconformity along an east-west transect through the southern part of the Wasatch Plateau. This region of Utah is well known as the eastern extent of Sevier orogenesis, but it also includes subsequent extensional overprinting. Earliest descriptions of this unconformity were published by Dutton (1880) and Spieker (1946, 1949), and work continues today. Field relationships expose many classic stratigraphic and sedimentologic features of erosional surfaces. Due to the geometry of the progressive unconformity onto the topographic high of the Sanpete-Sevier Valley antiform, the angular discordance of strata results in a gap in time of greater than 107 million years in the west, decreasing toward the east to about 39 million years and finally to less than 17 million years. Paleosols and small-scale channels/scours with infilled basal conglomerates are also prominent along the unconformity, as are several mine adits. Because of its abundant geologic features, the Salina Canyon unconformity is a superb teaching and learning space for geoscientists and outdoor naturalists.&quot;}" data-sheets-userformat="{&quot;2&quot;:15105,&quot;3&quot;:{&quot;1&quot;:0},&quot;11&quot;:4,&quot;12&quot;:0,&quot;14&quot;:{&quot;1&quot;:2,&quot;2&quot;:920844},&quot;15&quot;:&quot;Calibri&quot;,&quot;16&quot;:11}" data-darkreader-inline-color="">Salina Canyon, Utah, reveals a spectacular angular unconformity along an east-west transect through the southern part of the Wasatch Plateau. This region of Utah is well known as the eastern extent of Sevier orogenesis, but it also includes subsequent extensional overprinting. Earliest descriptions of this unconformity were published by Dutton (1880) and Spieker (1946, 1949), and work continues today. Field relationships expose many classic stratigraphic and sedimentologic features of erosional surfaces. Due to the geometry of the progressive unconformity onto the topographic high of the Sanpete-Sevier Valley antiform, the angular discordance of strata results in a gap in time of greater than 107 million years in the west, decreasing toward the east to about 39 million years and finally to less than 17 million years. Paleosols and small-scale channels/scours with infilled basal conglomerates are also prominent along the unconformity, as are several mine adits. Because of its abundant geologic features, the Salina Canyon unconformity is a superb teaching and learning space for geoscientists and outdoor naturalists.</span></p> 2023-05-09T21:30:06-06:00 Copyright (c) 2023 Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW/article/view/128 The John Wesley Powell Fossil Track Block—theropod tracks with ornithopod-like morphology from the Early Jurassic Navajo Sandstone, Glen Canyon National Recreation Area, Utah-Arizona 2023-10-10T08:26:47-06:00 Andrew Milner arcmilner@gmail.com Vincent Santucci vincent_santucci@nps.gov John Wood jack_wood@nps.gov Tylor Birthisel tbirthisel@nhmu.utah.edu Erica Clites eclites@gmail.com Martin Lockley martin.lockley@ucdenver.edu <p><span data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;A large fallen block of Early Jurassic Navajo Sandstone located at Lake Powell, within Glen Canyon National Recreation Area, south-central Utah, displays natural casts of vertebrate tracks. The footprints occur on at least three track-bearing horizons preserved on and between stromatolitic sandstone beds. Two large, parallel trackways, plus a third, divergent trackway, on the main track layer (MTL) superficially resemble ornithopod footprints; however, they were produced by large-sized theropod dinosaurs, rather\nthan ornithischians, and we identify these as Eubrontes. \nSmall coelophysoid theropod tracks (Grallator) are the most common vertebrate ichnofossils on all track-bearing horizons, with approximately 50 footprints preserved on the MTL, six on the highest surface, and three on thinner float slabs stratigraphically lower in section. An additional 12 tracks in three trackways of Anchisauripus size occur on the MTL, but they superficially resemble Kayentapus in having wider divarication angles than typical Anchisauripus. The MTL also preserves at least five closely associated tetradactyl footprints that we identify as cf. Brasilichnium. A nearby, smaller fallen block preserves distinct Batrachopus tracks, which are rare in eolian environments. \nThe microbial (possibly endoevaporitic) mats and stromatolitic horizons on which the animals had walked produced a distinct ichnomorphologic variation because of substrate consistency and the elastic properties of the mats, resulting in differential compaction of the bedding surfaces. Lithic compaction of the finer-grained sediments between denser, more resistant sandstone beds pre- and/or post-lithification resulted in additional deformation of the tracks, followed by natural erosion. We interpret these natural cast footprints on the MTL as possible transmitted tracks. The track-bearing, microbial-mat surfaces represent interdunal pooling of water, probably during periods of increased precipitation and/or rising water tables during wet seasons.&quot;}" data-sheets-userformat="{&quot;2&quot;:15105,&quot;3&quot;:{&quot;1&quot;:0},&quot;11&quot;:4,&quot;12&quot;:0,&quot;14&quot;:{&quot;1&quot;:2,&quot;2&quot;:0},&quot;15&quot;:&quot;Calibri&quot;,&quot;16&quot;:11}">A large fallen block of Early Jurassic Navajo Sandstone located at Lake Powell, within Glen Canyon National Recreation Area, south-central Utah, displays natural casts of vertebrate tracks. The footprints occur on at least three track-bearing horizons preserved on and between stromatolitic sandstone beds. Two large, parallel trackways, plus a third, divergent trackway, on the main track layer (MTL) superficially resemble ornithopod footprints; however, they were produced by large-sized theropod dinosaurs, rather<br>than ornithischians, and we identify these as Eubrontes. <br>Small coelophysoid theropod tracks (Grallator) are the most common vertebrate ichnofossils on all track-bearing horizons, with approximately 50 footprints preserved on the MTL, six on the highest surface, and three on thinner float slabs stratigraphically lower in section. An additional 12 tracks in three trackways of Anchisauripus size occur on the MTL, but they superficially resemble Kayentapus in having wider divarication angles than typical Anchisauripus. The MTL also preserves at least five closely associated tetradactyl footprints that we identify as cf. Brasilichnium. A nearby, smaller fallen block preserves distinct Batrachopus tracks, which are rare in eolian environments. <br>The microbial (possibly endoevaporitic) mats and stromatolitic horizons on which the animals had walked produced a distinct ichnomorphologic variation because of substrate consistency and the elastic properties of the mats, resulting in differential compaction of the bedding surfaces. Lithic compaction of the finer-grained sediments between denser, more resistant sandstone beds pre- and/or post-lithification resulted in additional deformation of the tracks, followed by natural erosion. We interpret these natural cast footprints on the MTL as possible transmitted tracks. The track-bearing, microbial-mat surfaces represent interdunal pooling of water, probably during periods of increased precipitation and/or rising water tables during wet seasons.</span></p> 2023-10-09T19:26:46-06:00 Copyright (c) 2023 Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW/article/view/129 Stratigraphy, sedimentology, and paleoclimatic proxies of the Upper Jurassic Morrison Formation of central Montana: Geology of the Intermountain West 2023-10-18T08:38:03-06:00 Dean Richmond drrichmond@ou.edu <p>Since the discovery of dinosaurs in the late 19th century, the Upper Jurassic Morrison Formation has received considerable scholarly attention. However, the formation in central Montana needed to be sufficiently investigated. Recent dinosaur excavations from exposed Morrison strata on the southern flank of Spindletop dome near the town of Grass Range, Montana, prompted a review of the formation’s geology, paleobotany, and paleoclimate. A review of historical stratigraphic measurements of the formation in Montana reveals a considerable variance in measured thicknesses. Stratigraphic measurements and regional log data indicate that the formation averages 71 m thick across central Montana. A new regional isopach map of the formation from well-log data illustrates a broad distributive fluvial system that migrated from the southwest toward the northeast. The formation is divided into two informal facies in the study area: lower and upper depositional facies. The lower depositional beds represent the mud-rich distal-most distributive fluvial facies that overlies the stranded muddy tidal flat of the Swift Formation. An increased sandstone:mudstone ratio and small isolated fluvial channel and crevasse splay beds indicate that the upper depositional beds represent the slow progression of the distributive fluvial system. However, a review of the regional field stratigraphy and well-log data did not provide a regional correlatable facies change to warrant subdividing the formation into members.<br>The stratigraphic positions and climatic interpretations for lithologic, faunal, and floral paleoclimatic proxies are specified. The compilation of climate proxy data from central Montana demonstrates that the climate in this region was wetter than in southern parts of the Morrison foreland basin. The climatic proxies signify that the environmental conditions were variable during the Late Jurassic in central Montana, displaying changing temperatures with mesic and xeric intervals of unknown duration.</p> 2023-10-17T16:49:09-06:00 Copyright (c) 2023 Geology of the Intermountain West