Geology of the Intermountain West https://giw.utahgeology.org/giw/index.php/GIW <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> Utah Geological Association en-US Geology of the Intermountain West 2380-7601 Basin-range uplift and canyon cutting in 3 million years, Kingston Canyon, Piute County, southwestern Utah https://giw.utahgeology.org/giw/index.php/GIW/article/view/96 <p>The Sevier Plateau is a gently east-tilted, block-faulted range in the High Plateaus transition zone of southwestern Utah. Part of this range underwent basin-range deformation, including at least 6000 feet (1800 m) of uplift, in a 3-million-year time span. The north-south range, whose southern end is just north of Bryce Canyon National Park, was uplifted and tilted by the Sevier fault zone along the western side. Kingston Canyon is a deep, east-west antecedent canyon that cut through the range and maintained itself during uplift. The deformation took place between 8 and 5 Ma, constrained by isotopic dating of pre-uplift rhyolite flows (8 Ma), now exposed on the crest of the range, and a post-canyon-cutting rhyolite dome (5 Ma), now in the bottom of Kingston Canyon. This episode of uplift and canyon cutting represents the most closely constrained example known in the Great Basin and adjacent transition zone of main-phase uplift by basin-range faults.</p> Peter Rowley Robert Biek David Hacker Copyright (c) 2022 Utah Geological Association https://creativecommons.org/licenses/by-sa/4.0/ 2022-02-03 2022-02-03 9 1 11 10.31711/giw.v9.pp1-11 An unconformity in the Pole Creek area (Sevier Plateau) west of Antimony, western Garfield County, Utah, and its bearing on the Sevier gravity slide https://giw.utahgeology.org/giw/index.php/GIW/article/view/97 <p>Pole Canyon is cut into the western backslope of the Sevier Plateau of southwestern Utah, a gently-tilted, block-faulted range that extends north from Bryce Canyon National Park through the eastern part of the Marysvale volcanic field. The canyon exposes a spectacular angular unconformity that separates brecciated, intensely deformed, and steeply dipping Eocene to Oligocene sedimentary and volcanic rocks below, from gently east-dipping Miocene volcanic rocks above. Although identified in 1968 by the senior author, it took renewed geologic mapping in 2015 by all three authors to discover that the rocks below the unconformity were deformed by gravity sliding. We named it the Sevier gravity slide, one of the largest terrestrial landslides on Earth. The ages of the volcanic rocks above and below the unconformity constrain the age of sliding at between 25.8 and 23.1 Ma; later dating elsewhere put the slide movement at between 25.2 and 25.1 Ma.</p> Peter Rowley Robert Biek David Hacker Copyright (c) 2022 Utah Geological Association https://creativecommons.org/licenses/by-sa/4.0/ 2022-02-03 2022-02-03 9 13 24 10.31711/giw.v9.pp13-24 A site bearing on the origin of iron deposits in the Iron Springs Mining District, Iron County, Utah https://giw.utahgeology.org/giw/index.php/GIW/article/view/98 <p>The discovery of the origin of iron in the Iron Springs mining district of southwestern Utah is a story of unconventional thinking based on detailed geologic mapping. This district, for many years the largest iron producer in the West, owes its resources to emplacement of three Miocene laccoliths of quartz monzonite porphyry. A visit to the geosite, in the outer part of one of them, The Three Peaks laccolith, reveals evidence of magma emplacement and mineralization of the overlying host rock. This outcrop formed by upward and outward bulging during intrusion of a rapidly congealing, crystal-rich magma. The pluton was emplaced remarkably close to the surface, about 1.2 miles (2 km) depth, and the ferromagnesian phenocrysts became unstable and broke down (deuteric alteration), releasing iron molecules into the hydrothermal solutions. As the magma solidified, subvertical extension joints formed. The radial joints in particular, oriented perpendicular to the intrusive contacts, allowed the iron-rich solutions to escape into the concordant upper contact of a pure limestone about 280 feet (85 m) thick. This limestone is the Co-op Creek Limestone Member of the Carmel Formation (Middle Jurassic). The joints tapped the solidifying crystal mush adjacent to the joints. The iron in the solutions replaced some or most of the Co-op Creek Limestone Member, creating huge ore bodies of hematite.</p> Peter Rowley David Hacker Robert Biek Copyright (c) 2022 Utah Geological Association https://creativecommons.org/licenses/by-sa/4.0/ 2022-02-03 2022-02-03 9 25 37 10.31711/giw.v9.pp25-37 A Review of charophytes of the Cleveland-Lloyd Dinosaur Quarry at Jurassic National Monument in the upper part of the Morrison Formation (Late Jurassic), Emery County, Utah, USA https://giw.utahgeology.org/giw/index.php/GIW/article/view/101 <p>The Cleveland-Lloyd Dinosaur Quarry at Jurassic National Monument in central Utah has been extensively studied for nearly 80 years. During this time, studies have heavily focused on the taphonomy, depositional setting, and potential behavioral inferences of the most dominant vertebrate taxon at the quarry, Allosaurus fragilis. However, despite their importance for paleoecological interpretations, microfossils from the quarry, such as charophytes and ostracods, have been conspicuously absent from any detailed discussion in the literature. Here we present a review of the known taxa of charophytes from the Cleveland-Lloyd Dinosaur Quarry and test the variability of abundance and taphonomic conditions throughout the quarry deposit. Our results indicate that significant differences in charophyte abundances exist in the lower and upper parts of the quarry, and a wide variance of taphonomic conditions is present in charophyte gyrogonites in the uppermost contact with the overlying carbonate bed. These results support prior interpretations of the Cleveland-Lloyd Dinosaur Quarry as an ephemeral pond and bring further attention to the importance of microfossils in paleoecological reconstructions.</p> Joseph Peterson Jonathan Warnock Jason Coenen Charles Bills Mateo Denoto Copyright (c) 2022 Geology of the Intermountain West https://creativecommons.org/licenses/by/3.0/us/ 2022-05-01 2022-05-01 9 39 47 10.31711/giw.v9.pp39-47 The lithostratigraphic Tidwell Member of the Morrison or Summerville Formations (Upper Jurassic)—who, what, where, when? https://giw.utahgeology.org/giw/index.php/GIW/article/view/103 <p><span data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;The Tidwell Member, a lithostratigraphic unit on the Colorado Plateau, has variously been referred to the Upper Jurassic Morrison Formation or the Upper Jurassic Summerville Formation. Its authorship has been ascribed to U.S. Geological Survey geologists Robert B. O'Sullivan or Fred Peterson. Because Peterson and O'Sullivan have different type sections, a resolution to authorship is needed. Both authors meet the minimum requirements for a new stratigraphic unit as given by the 1983 North American Stratigraphic Code under which Peterson and O'Sullivan operated. However, both authors also operated under the more restrictive Stratigraphic Nomenclature in Reports of the U.S. Geological Survey, which makes it clear that O'Sullivan used Tidwell Member informally, whereas Peterson made a formal proposal. Thus, Peterson is the rightful author and the type section is at Shadscale Mesa, Emery County, Utah. To resolve the issue of which formation the Tidwell Member belongs, its strata were examined across the northern Colorado Plateau in context with both the underlying marine Summerville Formation and the overlying terrestrial Salt Wash Member of the Morrison Formation. Meters-thick gypsum beds, one criterion for including the Tidwell Member in the Summerville, was found to be mostly restricted to the west side of the Colorado Plateau. Numerous low-angled anticlines at the top of the Summerville Formation are truncated beneath this gypsum, thus the gypsum beds cannot be part of the Summerville Formation. The unconformity marks the J-5 unconformity. Detailed analysis of the gypsum beds shows a complex origin indicative of smaller playa lakes rather than broad coastal sabkhas. The gypsum beds show an interfingering relationship with the overlying and lateral interbedded thin sandstone-siltstone-mudstone-limestone facies assemblage of the Tidwell Member. This relationship is interpreted as localized gypsum playa lakes that formed at the terminus of prograding fluvial fans from the Elko highlands to the west, with possible lesser contribution from the growing fluvial fan exiting from the Grand Canyon bight near the present-day Arizona-Nevada border. An isopachous map of the Tidwell Member supports this interpretation, and also indicates additional, less significant source areas to the southeast and east of the northern Colorado Plateau. A widespread tabular sandstone at the base of the Tidwell Member lateral to the gypsum facies known as Bed A, is interpreted to have originated mostly as a sand sheet analogous to the Selima Sand Sheet of the eastern Sahara or the sand sheet of the Gran Dieserto in Sonora, Mexico. The source of the sand is from the fluvial fans to the west. Similarities between the lithofacies of the contemporaneous Tidwell and Ralston Creek Members of the Morrison Formation indicates deposition under similar environmental conditions. This supports the inclusion of the Ralston Creek Member in the Morrison Formation as previously suggested, rather than as a separate formation.&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 Tidwell Member, a lithostratigraphic unit on the Colorado Plateau, has variously been referred to the Upper Jurassic Morrison Formation or the Upper Jurassic Summerville Formation. Its authorship has been ascribed to U.S. Geological Survey geologists Robert B. O'Sullivan or Fred Peterson. Because Peterson and O'Sullivan have different type sections, a resolution to authorship is needed. Both authors meet the minimum requirements for a new stratigraphic unit as given by the 1983 North American Stratigraphic Code under which Peterson and O'Sullivan operated. However, both authors also operated under the more restrictive Stratigraphic Nomenclature in Reports of the U.S. Geological Survey, which makes it clear that O'Sullivan used Tidwell Member informally, whereas Peterson made a formal proposal. Thus, Peterson is the rightful author and the type section is at Shadscale Mesa, Emery County, Utah. To resolve the issue of which formation the Tidwell Member belongs, its strata were examined across the northern Colorado Plateau in context with both the underlying marine Summerville Formation and the overlying terrestrial Salt Wash Member of the Morrison Formation. Meters-thick gypsum beds, one criterion for including the Tidwell Member in the Summerville, was found to be mostly restricted to the west side of the Colorado Plateau. Numerous low-angled anticlines at the top of the Summerville Formation are truncated beneath this gypsum, thus the gypsum beds cannot be part of the Summerville Formation. The unconformity marks the J-5 unconformity. Detailed analysis of the gypsum beds shows a complex origin indicative of smaller playa lakes rather than broad coastal sabkhas. The gypsum beds show an interfingering relationship with the overlying and lateral interbedded thin sandstone-siltstone-mudstone-limestone facies assemblage of the Tidwell Member. This relationship is interpreted as localized gypsum playa lakes that formed at the terminus of prograding fluvial fans from the Elko highlands to the west, with possible lesser contribution from the growing fluvial fan exiting from the Grand Canyon bight near the present-day Arizona-Nevada border. An isopachous map of the Tidwell Member supports this interpretation, and also indicates additional, less significant source areas to the southeast and east of the northern Colorado Plateau. A widespread tabular sandstone at the base of the Tidwell Member lateral to the gypsum facies known as Bed A, is interpreted to have originated mostly as a sand sheet analogous to the Selima Sand Sheet of the eastern Sahara or the sand sheet of the Gran Dieserto in Sonora, Mexico. The source of the sand is from the fluvial fans to the west. Similarities between the lithofacies of the contemporaneous Tidwell and Ralston Creek Members of the Morrison Formation indicates deposition under similar environmental conditions. This supports the inclusion of the Ralston Creek Member in the Morrison Formation as previously suggested, rather than as a separate formation.</span></p> Kenneth Carpenter Copyright (c) 2022 Geology of the Intermountain West https://creativecommons.org/licenses/by/3.0/us/ 2022-05-27 2022-05-27 9 49 114 10.31711/giw.v9.pp49-114 Geomorphic and tectonic development of Swan Valley, southeast Idaho, since the eruption of the Basalt of Antelope Flat—new 40Ar/39Ar, geochemical, and paleomagnetic data https://giw.utahgeology.org/giw/index.php/GIW/article/view/104 <p><span data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;Swan Valley is a graben in eastern Idaho that formed by extension along the Grand Valley and Snake River faults and preserves a record of explosive rhyolitic volcanism sourced from the Yellowstone Plateau and Heise volcanic fields, as well as locally sourced basaltic lavas. The Pleistocene Basalt of Antelope Flat intersected the South Fork of the Snake River and generated a temporary hyaloclastite dam. Previous workers proposed that the lava dam allowed for the accumulation of water that led to the generation of paleo-Swan Lake. Although lacustrine deposits from paleo-Swan Lake have not been described or mapped, several-meters thick intercalated hyaloclastites and pillow lavas require the interaction between continued volcanism and standing water. In this work, we present new geochemical, geochronologic, and paleomagnetic data to reinterpret the eruptive history of the basalts within the valley, estimate the volume and duration to fill paleo-Swan Lake, and calculate incision rates of the Snake River through Quaternary basalts. Using geochemical and paleomagnetic data, we reinterpret deposits previously mapped as the Basalt of Antelope Flat as three temporally distinct units. The duration to fill paleo-Swan Lake is calculated as 12 to 20 years. An absence of lacustrine deposits, shorelines, or other indicators of a lake environment led us to propose that shallow, marshy, wetland conditions existed locally to produce hydrovolcanic deposits characteristic of the Basalt of Antelope Flat. We report a new 40Ar/39Ar age of 904 ± 11 ka (2σ) for the Basalt of Antelope Flat, which we use to determine an average incision rate of 0.014 cm/yr for the Snake River\r\nthrough Quaternary basalts. Our multi-method approach provides updated constraints to the eruptive and geomorphological history of southeastern Idaho.&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}">Swan Valley is a graben in eastern Idaho that formed by extension along the Grand Valley and Snake River faults and preserves a record of explosive rhyolitic volcanism sourced from the Yellowstone Plateau and Heise volcanic fields, as well as locally sourced basaltic lavas. The Pleistocene Basalt of Antelope Flat intersected the South Fork of the Snake River and generated a temporary hyaloclastite dam. Previous workers proposed that the lava dam allowed for the accumulation of water that led to the generation of paleo-Swan Lake. Although lacustrine deposits from paleo-Swan Lake have not been described or mapped, several-meters thick intercalated hyaloclastites and pillow lavas require the interaction between continued volcanism and standing water. In this work, we present new geochemical, geochronologic, and paleomagnetic data to reinterpret the eruptive history of the basalts within the valley, estimate the volume and duration to fill paleo-Swan Lake, and calculate incision rates of the Snake River through Quaternary basalts. Using geochemical and paleomagnetic data, we reinterpret deposits previously mapped as the Basalt of Antelope Flat as three temporally distinct units. The duration to fill paleo-Swan Lake is calculated as 12 to 20 years. An absence of lacustrine deposits, shorelines, or other indicators of a lake environment led us to propose that shallow, marshy, wetland conditions existed locally to produce hydrovolcanic deposits characteristic of the Basalt of Antelope Flat. We report a new 40Ar/39Ar age of 904 ± 11 ka (2σ) for the Basalt of Antelope Flat, which we use to determine an average incision rate of 0.014 cm/yr for the Snake River <br>through Quaternary basalts. Our multi-method approach provides updated constraints to the eruptive and geomorphological history of southeastern Idaho.</span></p> Stacy Henderson Tiffany Rivera Peter Lippert Brian Jicha Copyright (c) 2022 Geology of the Intermountain West https://creativecommons.org/licenses/by/3.0/us/ 2022-06-08 2022-06-08 9 115 130 10.31711/giw.v9.pp115-130