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 Fallout tuffs in the Eocene Duchesne River Formation, northeastern Utah—ages, compositions, and likely source https://giw.utahgeology.org/giw/index.php/GIW/article/view/68 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;Thin fallout tuffs are common in the terrestrial deposits of the Eocene Duchesne River Formation on the flanks of the Uinta Mountains of eastern Utah. Their ages and compositions provide new insight into the tectonic events and magmatic history of the western Cordillera and provide important constraints on the Cenozoic land mammal chronology. Whole-rock compositions of the volcanic ash show that they underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. However, immobile element ratios and abundances, such as Zr/Ti, La/Nb, and Y are typical of rhyolites formed in a subduction-related setting. Glass shards preserved in one sample all had SiO2 values >75%, typical of high-silica rhyolite. Preserved phenocrysts in the ash beds include quartz, sanidine, plagioclase, and biotite with variable amounts of accessory zircon, apatite, titanite, and allanite. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of chemical compositions suggest a genetic relationship to three or four separate eruptions. Sanidine compositions from five samples range from Or73 and Or79. Only one sample had preserved plagioclase with compositions ranging between An22 – An49. Allanite from the ash beds has lower total rare earth elements (REE) concentrations than allanite from other well-studied rhyolites. Titanite in one sample has lower concentrations of REE, Fe, and Al than expected of rhyolites and is probably detrital.\n\nPlagioclase and sanidine from two different tuff beds near the middle of the Duchesne River Formation yielded analytically indistinguishable 40Ar/39Ar ages of 39.47 ± 0.16 Ma and 39.36 ± 0.15 Ma, respectively. These dates, along with the compositional data seem to limit the eruptive source for these fallout tuffs to the northeast Nevada volcanic field, one of the few volcanically active regions of western North America at the time. These new radiometric ages, along with stratigraphic relations and previously published ages for tuffs in the Bishop Conglomerate (which unconformably overlies the Duchesne River Formation), constrain the timing of late Laramide uplift in the region from 39 to about 37 Ma and post-Laramide epeirogenic uplift from 34 Ma to 30 Ma. Finally, the ages also provide additional evidence that the Duchesnean North American Land Mammal Age ended in the Eocene, which was originally named and defined from the Duchesne River Formation.&quot;}" data-sheets-userformat="{&quot;2&quot;:15107,&quot;3&quot;:{&quot;1&quot;:0},&quot;4&quot;:{&quot;1&quot;:2,&quot;2&quot;:14281427},&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}">Thin fallout tuffs are common in the terrestrial deposits of the Eocene Duchesne River Formation on the flanks of the Uinta Mountains of eastern Utah. Their ages and compositions provide new insight into the tectonic events and magmatic history of the western Cordillera and provide important constraints on the Cenozoic land mammal chronology. Whole-rock compositions of the volcanic ash show that they underwent post-emplacement argillic alteration, typical of a wetland/floodplain depositional setting. However, immobile element ratios and abundances, such as Zr/Ti, La/Nb, and Y are typical of rhyolites formed in a subduction-related setting. Glass shards preserved in one sample all had SiO2 values &gt;75%, typical of high-silica rhyolite. Preserved phenocrysts in the ash beds include quartz, sanidine, plagioclase, and biotite with variable amounts of accessory zircon, apatite, titanite, and allanite. Biotite compositions have Fe/(Fe+Mg) ratios typical of calc-alkaline igneous rocks and clusters of chemical compositions suggest a genetic relationship to three or four separate eruptions. Sanidine compositions from five samples range from Or73 and Or79. Only one sample had preserved plagioclase with compositions ranging between An22 – An49. Allanite from the ash beds has lower total rare earth elements (REE) concentrations than allanite from other well-studied rhyolites. Titanite in one sample has lower concentrations of REE, Fe, and Al than expected of rhyolites and is probably detrital.<br><br>Plagioclase and sanidine from two different tuff beds near the middle of the Duchesne River Formation yielded analytically indistinguishable 40Ar/39Ar ages of 39.47 ± 0.16 Ma and 39.36 ± 0.15 Ma, respectively. These dates, along with the compositional data seem to limit the eruptive source for these fallout tuffs to the northeast Nevada volcanic field, one of the few volcanically active regions of western North America at the time. These new radiometric ages, along with stratigraphic relations and previously published ages for tuffs in the Bishop Conglomerate (which unconformably overlies the Duchesne River Formation), constrain the timing of late Laramide uplift in the region from 39 to about 37 Ma and post-Laramide epeirogenic uplift from 34 Ma to 30 Ma. Finally, the ages also provide additional evidence that the Duchesnean North American Land Mammal Age ended in the Eocene, which was originally named and defined from the Duchesne River Formation.</span></p> Michael Jensen Bart Kowallis Eric Christiansen Casey Webb Michael Dorais Douglas Sprinkel Brian Jicha Copyright (c) 2020 Utah Geological Association https://creativecommons.org/licenses/by/4.0/legalcode 2020-03-18 2020-03-18 7 1 27 10.31711/giw.v7.pp1-27 An unusually diverse northern biota from the Morrison Formation (Upper Jurassic), Black Hills, Wyoming https://giw.utahgeology.org/giw/index.php/GIW/article/view/69 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;The Little Houston Quarry in the Black Hills of Wyoming contains the most diverse vertebrate fauna in the Morrison Formation (Upper Jurassic) north of Como Bluff and the second-most diverse in the entire formation, after Reed’s Quarry 9. The deposit was an occasionally reactivated abandoned river channel, in interbedded green mudstone and laminated green-gray siltstone above a channel sandstone. The dinosaur material is densely distributed and is disarticulated to articulated, with several associated skeletons. The biota contains charophytes, horsetails, a possible seed fern, possible conifers, gastropods, two types of unionoid bivalves, diplostracans (“conchostracans”), a malacostracan, ray-finned fish, lungfish, a frog, salamanders, two types of turtles, rhynchocephalians, a lizard, choristoderes, two types of crocodyliforms, a pterosaur, Allosaurus and several types of small theropods including Tanycolagreus? and probable dromaeosaurids, numerous Camarasaurus and a diplodocine sauropod, a stegosaur, the neornithischian Nanosaurus, and the mammals Docodon, Amblotherium, and a multituberculate. Among these taxa, one of the unionoid bivalves, an atoposaurid crocodyliform, and the species of Amblotherium, which appear to be new and unique to the locality so far. The Docodon material may represent the first occurrence of D. apoxys outside of its type area in Colorado. Additionally, small, unusual theropod tooth types reported here may represent the first Late Jurassic occurrence of cf. Richardoestesia in North America and a possible abelisauroid, respectively.&quot;}" data-sheets-userformat="{&quot;2&quot;:15107,&quot;3&quot;:{&quot;1&quot;:0},&quot;4&quot;:{&quot;1&quot;:2,&quot;2&quot;:14281427},&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 Little Houston Quarry in the Black Hills of Wyoming contains the most diverse vertebrate fauna in the Morrison Formation (Upper Jurassic) north of Como Bluff and the second-most diverse in the entire formation, after Reed’s Quarry 9. The deposit was an occasionally reactivated abandoned river channel, in interbedded green mudstone and laminated green-gray siltstone above a channel sandstone. The dinosaur material is densely distributed and is disarticulated to articulated, with several associated skeletons. The biota contains charophytes, horsetails, a possible seed fern, possible conifers, gastropods, two types of unionoid bivalves, diplostracans (“conchostracans”), a malacostracan, ray-finned fish, lungfish, a frog, salamanders, two types of turtles, rhynchocephalians, a lizard, choristoderes, two types of crocodyliforms, a pterosaur, Allosaurus and several types of small theropods including Tanycolagreus? and probable dromaeosaurids, numerous Camarasaurus and a diplodocine sauropod, a stegosaur, the neornithischian Nanosaurus, and the mammals Docodon, Amblotherium, and a multituberculate. Among these taxa, one of the unionoid bivalves, an atoposaurid crocodyliform, and the species of Amblotherium, which appear to be new and unique to the locality so far. The Docodon material may represent the first occurrence of <em>D. apoxys</em> outside of its type area in Colorado. Additionally, small, unusual theropod tooth types reported here may represent the first Late Jurassic occurrence of cf. Richardoestesia in North America and a possible abelisauroid, respectively.</span></p> John Foster Darrin Pagnac ReBecca Hunt-Foster Copyright (c) 2020 Utah Geological Association https://creativecommons.org/licenses/by/4.0/legalcode 2020-03-21 2020-03-21 7 29 67 10.31711/giw.v7.pp29-67 Rhyolite ignimbrite boulders and cobbles in the Middle Jurassic Carmel Formation of Utah and Arizona—age, composition, transport, and stratigraphic setting https://giw.utahgeology.org/giw/index.php/GIW/article/view/70 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;A stratigraphic layer containing rhyolite cobbles and boulders in the Middle Jurassic Carmel Formation of southern Utah represents a singular, unusual event in the otherwise low-energy sedimentation of this formation. A laser-fusion, single-crystal 40Ar/39Ar age of 171.73 ± 0.19 Ma obtained from sanidine in one of the clasts is about 8 m.y. older than a zircon U-Pb age obtained on a fallout tuff from the sediments surrounding the clasts (163.9 ± ~3.3 Ma). The volcanic clasts are poorly-welded rhyolite ignimbrites that may have been deposited as much as 200 km from the eruptive center, perhaps along pre-existing valleys. The tuff deposits then remained in place for several million years during which time they were subjected to weathering, alteration, and perhaps topographic inversion, creating mesas capped with tuff underlain by soft Middle Jurassic silt and mud. Triggered by unusual rainfall or earthquakes, debris flows carried the clasts a few 10s of kilometers from their outcrops to the depositional site. Earlier work proposed that the Middle Jurassic arc was a low-standing, arc-graben. If this was the case, then the tectonic setting was likely similar to the modern Central American arc in the vicinity of Nicaragua where tuffs erupted from a low-standing arc deposited onto an adjacent highland and were then eroded by streams flowing to the east onto a fluvial plain that is near the sea.&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 stratigraphic layer containing rhyolite cobbles and boulders in the Middle Jurassic Carmel Formation of southern Utah represents a singular, unusual event in the otherwise low-energy sedimentation of this formation. A laser-fusion, single-crystal 40Ar/39Ar age of 171.73 ± 0.19 Ma obtained from sanidine in one of the clasts is about 8 m.y. older than a zircon U-Pb age obtained on a fallout tuff from the sediments surrounding the clasts (163.9 ± ~3.3 Ma). The volcanic clasts are poorly-welded rhyolite ignimbrites that may have been deposited as much as 200 km from the eruptive center, perhaps along pre-existing valleys. The tuff deposits then remained in place for several million years during which time they were subjected to weathering, alteration, and perhaps topographic inversion, creating mesas capped with tuff underlain by soft Middle Jurassic silt and mud. Triggered by unusual rainfall or earthquakes, debris flows carried the clasts a few 10s of kilometers from their outcrops to the depositional site. Earlier work proposed that the Middle Jurassic arc was a low-standing, arc-graben. If this was the case, then the tectonic setting was likely similar to the modern Central American arc in the vicinity of Nicaragua where tuffs erupted from a low-standing arc deposited onto an adjacent highland and were then eroded by streams flowing to the east onto a fluvial plain that is near the sea.</span></p> Bart Kowallis Douglas Sprinkel Eric Christiansen Skylor Steed David Wheatley Copyright (c) 2020 Geology of the Intermountain West https://creativecommons.org/licenses/by/4.0/legalcode 2020-04-21 2020-04-21 7 69 96 10.31711/giw.v7.pp69-96 Hydraulic modeling and computational fluid dynamics of bone burial in a sandy river channel https://giw.utahgeology.org/giw/index.php/GIW/article/view/71 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;An oval recycling flume with live-beds (moveable) of medium and very coarse grained sands were used to explore the process of bone burial as a precursor to fossilization. Two-dimentional computation fluid dynamics was used to visualize and interpret the flow turbulence around bones. Results show that a water mass approaching and passing a static bone (obstruction) is subjected to flow modification by flow separation, flow constriction, and flow acceleration producing complex flow patterns (turbulence). These complex patterns include an upstream high-pressure zone, down flows, and vortices (with flow reversal near the bed) causing bed shear stress that produce bed erosion. Downstream of the bone, the water mass undergoes flow deceleration, water recirculation (turbulence eddies), flow reattachment, low-pressure zone (drag), and sediment deposition. Scour plays a crucial role by undercutting bone on the upstream side and may cause the bone to settle into the bed by rotation or sliding. Scour geometry is determined by bone size and shape, approaching flow velocity and angle to flow, flow depth, bed topography, and bed friction. Drag on the downstream side of the bone causes scoured sediment deposition, but burial by migrating bed forms is the most important method of large bone burial. Bone may be repeatedly buried and exposed with renewed scour. However, each episode of scour may lower the bone deeper into the bed so that it essentially buries itself. No difference in these effects were noted between experiments using fine or coarse grain sizes. This experimental work is then used to interpret the possible history of bone burial in the Upper Jurassic Morrison Formation on the bone wall inside the Quarry Exhibit Hall at Dinosaur National Monument, Utah.&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}">An oval recycling flume with live-beds (moveable) of medium and very coarse grained sands were used to explore the process of bone burial as a precursor to fossilization. Two-dimentional computation fluid dynamics was used to visualize and interpret the flow turbulence around bones. Results show that a water mass approaching and passing a static bone (obstruction) is subjected to flow modification by flow separation, flow constriction, and flow acceleration producing complex flow patterns (turbulence). These complex patterns include an upstream high-pressure zone, down flows, and vortices (with flow reversal near the bed) causing bed shear stress that produce bed erosion. Downstream of the bone, the water mass undergoes flow deceleration, water recirculation (turbulence eddies), flow reattachment, low-pressure zone (drag), and sediment deposition. Scour plays a crucial role by undercutting bone on the upstream side and may cause the bone to settle into the bed by rotation or sliding. Scour geometry is determined by bone size and shape, approaching flow velocity and angle to flow, flow depth, bed topography, and bed friction. Drag on the downstream side of the bone causes scoured sediment deposition, but burial by migrating bed forms is the most important method of large bone burial. Bone may be repeatedly buried and exposed with renewed scour. However, each episode of scour may lower the bone deeper into the bed so that it essentially buries itself. No difference in these effects were noted between experiments using fine or coarse grain sizes. This experimental work is then used to interpret the possible history of bone burial in the Upper Jurassic Morrison Formation on the bone wall inside the Quarry Exhibit Hall at Dinosaur National Monument, Utah.</span></p> Kenneth Carpenter Copyright (c) 2020 Geology of the Intermountain West https://creativecommons.org/licenses/by/4.0/legalcode 2020-04-30 2020-04-30 7 97 120 10.31711/giw.v7.pp97-120 Methane emissions from muds during low water-level stages of Lake Powell, southern Utah, USA https://giw.utahgeology.org/giw/index.php/GIW/article/view/72 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;The Glen Canyon Dam, along the Colorado River in Page, Arizona, was completed in 1963, creating the Lake Powell reservoir which spans the Arizona-Utah border. The water levels of Lake Powell peaked in 1983 and have declined since, releasing overlying pressure on the underlying sediment. In general, water levels experience seasonal highs and lows, with punctuated periods of considerable and steady decreases (1987 to 1993, 1999 to 2005, and 2011 to 2014) and less dramatic recoveries (1993 to 1999 and 2005 to 2011). This release of overpressure coupled with increasing pore pressures due to biological methane production has created mud volcanoes, structures along the shoreline made of cavities that allow fluid and gas to rise to the surface and escape. Although these sedimentary structures have been assessed using geophysical techniques and excavation to characterize their morphologies and fracture propagation, limited chemical data has been reported on the inputs and products of these gas- and fluid-escape features. \nThis research investigates the relative proportions of methane (CH4), carbon dioxide (CO2), and air (unseparated nitrogen [N2] and oxygen [O2]) gas released, the variability of these proportions through time, and how these gases formed in the subsurface. The field site is along the Lake Powell near Hite, Utah. Three gas samples were collected from mud volcanoes along the delta in July 2014, whereas 21 samples were collected in July 2015 and were analyzed via gas chromatography (GC). The GC analyses from 2014 and 2015 have a mean CH4 concentration of 81.47 ± 9.29 percent of volume (% v/v) and 32.40 ± 15.31% v/v, respectively. In May 2016, 50 samples from 25 vents were collected and analyzed via GC for bulk composition, and 11 of which were analyzed by isotope ratio mass spectrometry (IRMS) for carbon and hydrogen isotope content of CH4. The 2016 GC analysis detected average relative concentrations for CH4, CO2, and air of 74.51 ± 14.08% v/v, 2.82 ± 3.76% v/v, and 22.67 ± 14.28% v/v, respectively. Gas compositions from individual vents varied over the three-day sampling timeframe in the summer of 2016 including CH4 decreases of up to 66% v/v and increases of up to 38% v/v. IRMS signatures of samples collected in 2016 indicate the gasses are in part generated during microbial respiration through hydrogenotrophic and acetoclastic methane production.&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 Glen Canyon Dam, along the Colorado River in Page, Arizona, was completed in 1963, creating the Lake Powell reservoir which spans the Arizona-Utah border. The water levels of Lake Powell peaked in 1983 and have declined since, releasing overlying pressure on the underlying sediment. In general, water levels experience seasonal highs and lows, with punctuated periods of considerable and steady decreases (1987 to 1993, 1999 to 2005, and 2011 to 2014) and less dramatic recoveries (1993 to 1999 and 2005 to 2011). This release of overpressure coupled with increasing pore pressures due to biological methane production has created mud volcanoes, structures along the shoreline made of cavities that allow fluid and gas to rise to the surface and escape. Although these sedimentary structures have been assessed using geophysical techniques and excavation to characterize their morphologies and fracture propagation, limited chemical data has been reported on the inputs and products of these gas- and fluid-escape features. <br>This research investigates the relative proportions of methane (CH4), carbon dioxide (CO2), and air (unseparated nitrogen [N2] and oxygen [O2]) gas released, the variability of these proportions through time, and how these gases formed in the subsurface. The field site is along the Lake Powell near Hite, Utah. Three gas samples were collected from mud volcanoes along the delta in July 2014, whereas 21 samples were collected in July 2015 and were analyzed via gas chromatography (GC). The GC analyses from 2014 and 2015 have a mean CH4 concentration of 81.47 ± 9.29 percent of volume (% v/v) and 32.40 ± 15.31% v/v, respectively. In May 2016, 50 samples from 25 vents were collected and analyzed via GC for bulk composition, and 11 of which were analyzed by isotope ratio mass spectrometry (IRMS) for carbon and hydrogen isotope content of CH4. The 2016 GC analysis detected average relative concentrations for CH4, CO2, and air of 74.51 ± 14.08% v/v, 2.82 ± 3.76% v/v, and 22.67 ± 14.28% v/v, respectively. Gas compositions from individual vents varied over the three-day sampling timeframe in the summer of 2016 including CH4 decreases of up to 66% v/v and increases of up to 38% v/v. IRMS signatures of samples collected in 2016 indicate the gasses are in part generated during microbial respiration through hydrogenotrophic and acetoclastic methane production.</span></p> Margariete Malenda Thomas Betts Wendy Simpson Michael Wizevich Edward Simpson Laura Sherrod Copyright (c) 2020 Geology of the Intermountain West https://creativecommons.org/licenses/by/4.0/legalcode 2020-05-20 2020-05-20 7 121 136 10.31711/giw.v7.pp121-136 The Morrison Formation and its bounding strata on the western side of the Blanding basin, San Juan County, Utah https://giw.utahgeology.org/giw/index.php/GIW/article/view/73 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;In 2016 and 2017, the Utah Geological Survey partnered with the U.S. Bureau of Land Management to conduct a paleontological inventory of the Morrison Formation south and west of Blanding, Utah, along the eastern margin of the Bears Ears National Monument. The Morrison in this region is critical to understanding Upper Jurassic stratigraphy across the Colorado Plateau because it is the type area for the Bluff Sandstone, Recapture, Westwater Canyon, and Brushy Basin Members of the Morrison Formation, which are the basis for nomenclature in New Mexico and Arizona\nas well. Researchers have disagreed about nomenclature and correlation of these units, which transition northward in the study area into the Tidwell, Salt Wash, and Brushy Basin Members. Numerous vertebrate localities make inclusion of the Bluff Sandstone and Recapture Members in the Middle Jurassic San Rafael Group, as suggested by some previous workers, unlikely. The Salt Wash Member does not separate the Bluff Sandstone and Recapture Members at Recapture Wash, but sandstone lenses of Salt Wash facies occur higher in northern Recapture exposures. Northward, along the outcrop belt east of Comb Ridge, the Bluff-Recapture interval thins, interlenses, and pinches out into the Tidwell and lower Salt Wash, with the main lower sandstone interval of the Westwater Canyon merging northward into the upper Salt Wash Member.\nThe partly covered, 1938 type section of the Brushy Basin Member is identified along Elk Mountain Road at the southern end of Brushy Basin. We describe a detailed, accessible Morrison Formation reference section about 11.2 km (7 mi) to the south along Butler Wash. There, 81.68 m (268 ft) of Brushy Basin Member is well exposed along a road between the top of the Westwater Canyon Member and the base of the Lower Cretaceous Burro Canyon Formation. We informally call the upper sandstone bed(s) of the Westwater Canyon Member that cap mesas and benches in the region\n“No-Mans Island beds.” Smectitic mudstones between the No-Mans Island beds and the main sandstone body of the Westwater Canyon suggest that the Salt Wash-Brushy Basin contact to the north may be somewhat older than the base of the Brushy Basin Member as originally defined in its type area. Determining whether the No-Mans Island beds pinch out to the north or are removed by erosion below the regional basal Brushy Basin paleosol requires further research. Several significant fossil vertebrate and plant sites have been documented in the Brushy Basin type area. Newly identified volcanic ashes provided zircons for U-Pb ages of 150.67 ± 0.32 Ma from near the top of the Brushy Basin Member and of 153.7 ± 2.1 Ma and 153.8 ± 2.2 Ma for two zircons in lower part of Recapture Member. At the top of the Brushy Basin Member, ferruginous paleosols commonly overlying conglomeratic sandstone are speculated to be of Early Cretaceous age (detrital zircon age pending) and are assigned herein to the Yellow Cat Member of the Burro Canyon Formation. These iron-rich paleosols suggest wetter climatic conditions during the Jurassic-Cretaceous transition in the Blanding basin.&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}">In 2016 and 2017, the Utah Geological Survey partnered with the U.S. Bureau of Land Management to conduct a paleontological inventory of the Morrison Formation south and west of Blanding, Utah, along the eastern margin of the Bears Ears National Monument. The Morrison in this region is critical to understanding Upper Jurassic stratigraphy across the Colorado Plateau because it is the type area for the Bluff Sandstone, Recapture, Westwater Canyon, and Brushy Basin Members of the Morrison Formation, which are the basis for nomenclature in New Mexico and Arizona as well. Researchers have disagreed about nomenclature and correlation of these units, which transition northward in the study area into the Tidwell, Salt Wash, and Brushy Basin Members. Numerous vertebrate localities make inclusion of the Bluff Sandstone and Recapture Members in the Middle Jurassic San Rafael Group, as suggested by some previous workers, unlikely. The Salt Wash Member does not separate the Bluff Sandstone and Recapture Members at Recapture Wash, but sandstone lenses of Salt Wash facies occur higher in northern Recapture exposures. Northward, along the outcrop belt east of Comb Ridge, the Bluff-Recapture interval thins, interlenses, and pinches out into the Tidwell and lower Salt Wash, with the main lower sandstone interval of the Westwater Canyon merging northward into the upper Salt Wash Member.</span></p> <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;In 2016 and 2017, the Utah Geological Survey partnered with the U.S. Bureau of Land Management to conduct a paleontological inventory of the Morrison Formation south and west of Blanding, Utah, along the eastern margin of the Bears Ears National Monument. The Morrison in this region is critical to understanding Upper Jurassic stratigraphy across the Colorado Plateau because it is the type area for the Bluff Sandstone, Recapture, Westwater Canyon, and Brushy Basin Members of the Morrison Formation, which are the basis for nomenclature in New Mexico and Arizona\nas well. Researchers have disagreed about nomenclature and correlation of these units, which transition northward in the study area into the Tidwell, Salt Wash, and Brushy Basin Members. Numerous vertebrate localities make inclusion of the Bluff Sandstone and Recapture Members in the Middle Jurassic San Rafael Group, as suggested by some previous workers, unlikely. The Salt Wash Member does not separate the Bluff Sandstone and Recapture Members at Recapture Wash, but sandstone lenses of Salt Wash facies occur higher in northern Recapture exposures. Northward, along the outcrop belt east of Comb Ridge, the Bluff-Recapture interval thins, interlenses, and pinches out into the Tidwell and lower Salt Wash, with the main lower sandstone interval of the Westwater Canyon merging northward into the upper Salt Wash Member.\nThe partly covered, 1938 type section of the Brushy Basin Member is identified along Elk Mountain Road at the southern end of Brushy Basin. We describe a detailed, accessible Morrison Formation reference section about 11.2 km (7 mi) to the south along Butler Wash. There, 81.68 m (268 ft) of Brushy Basin Member is well exposed along a road between the top of the Westwater Canyon Member and the base of the Lower Cretaceous Burro Canyon Formation. We informally call the upper sandstone bed(s) of the Westwater Canyon Member that cap mesas and benches in the region\n“No-Mans Island beds.” Smectitic mudstones between the No-Mans Island beds and the main sandstone body of the Westwater Canyon suggest that the Salt Wash-Brushy Basin contact to the north may be somewhat older than the base of the Brushy Basin Member as originally defined in its type area. Determining whether the No-Mans Island beds pinch out to the north or are removed by erosion below the regional basal Brushy Basin paleosol requires further research. Several significant fossil vertebrate and plant sites have been documented in the Brushy Basin type area. Newly identified volcanic ashes provided zircons for U-Pb ages of 150.67 ± 0.32 Ma from near the top of the Brushy Basin Member and of 153.7 ± 2.1 Ma and 153.8 ± 2.2 Ma for two zircons in lower part of Recapture Member. At the top of the Brushy Basin Member, ferruginous paleosols commonly overlying conglomeratic sandstone are speculated to be of Early Cretaceous age (detrital zircon age pending) and are assigned herein to the Yellow Cat Member of the Burro Canyon Formation. These iron-rich paleosols suggest wetter climatic conditions during the Jurassic-Cretaceous transition in the Blanding basin.&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}"><br>The partly covered, 1938 type section of the Brushy Basin Member is identified along Elk Mountain Road at the southern end of Brushy Basin. We describe a detailed, accessible Morrison Formation reference section about 11.2 km (7 mi) to the south along Butler Wash. There, 81.68 m (268 ft) of Brushy Basin Member is well exposed along a road between the top of the Westwater Canyon Member and the base of the Lower Cretaceous Burro Canyon Formation. We informally call the upper sandstone bed(s) of the Westwater Canyon Member that cap mesas and benches in the region “No-Mans Island beds.” Smectitic mudstones between the No-Mans Island beds and the main sandstone body of the Westwater Canyon suggest that the Salt Wash-Brushy Basin contact to the north may be somewhat older than the base of the Brushy Basin Member as originally defined in its type area. Determining whether the No-Mans Island beds pinch out to the north or are removed by erosion below the regional basal Brushy Basin paleosol requires further research. Several significant fossil vertebrate and plant sites have been documented in the Brushy Basin type area. Newly identified volcanic ashes provided zircons for U-Pb ages of 150.67 ± 0.32 Ma from near the top of the Brushy Basin Member and of 153.7 ± 2.1 Ma and 153.8 ± 2.2 Ma for two zircons in lower part of Recapture Member. At the top of the Brushy Basin Member, ferruginous paleosols commonly overlying conglomeratic sandstone are speculated to be of Early Cretaceous age (detrital zircon age pending) and are assigned herein to the Yellow Cat Member of the Burro Canyon Formation. These iron-rich paleosols suggest wetter climatic conditions during the Jurassic-Cretaceous transition in the Blanding basin.</span></p> James Kirkland Donald DeBlieux ReBecca Hunt-Foster John Foster Kelli Trujillo Emily Finzel Copyright (c) 2020 Geology of the Intermountain West https://creativecommons.org/licenses/by/4.0/legalcode 2020-06-04 2020-06-04 7 137 195 10.31711/giw.v7.pp137-195 High-quality casts of the missing holotype of Petalodus ohioensis Safford 1853 (Chondrichthyes, Petalodontidae) at the Field Museum of Natural History and their bearing on the validity and priority of the species https://giw.utahgeology.org/giw/index.php/GIW/article/view/81 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;The validity of the chondrichthyan species Petalodus ohioensis Safford 1853, has long been in doubt due to the poor quality of the published figures and the unknown whereabouts of the holotype. That situation changed with the discovery of casts of the holotype in the collections of the Yale Peabody Museum of Natural History. The quality of the casts is poor, but sufficient to establish P. ohioensis as a valid species and as a senior synonym of P. alleghaniensis Leidy 1856. Recently, casts of the holotype of much better quality were found in the collections of the Field Museum of Natural History, accompanied by documentation indicating that they were likely obtained directly from Safford by O.P. Hay in 1896. The Field Museum casts clearly show the bands of ridges at the base of the crown on the labial and lingual sides, which are not visible on the Yale Peabody Museum casts.&quot;}" data-sheets-userformat="{&quot;2&quot;:15233,&quot;3&quot;:{&quot;1&quot;:0},&quot;10&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 validity of the chondrichthyan species Petalodus ohioensis Safford 1853, has long been in doubt due to the poor quality of the published figures and the unknown whereabouts of the holotype. That situation changed with the discovery of casts of the holotype in the collections of the Yale Peabody Museum of Natural History. The quality of the casts is poor, but sufficient to establish P. ohioensis as a valid species and as a senior synonym of P. alleghaniensis Leidy 1856. Recently, casts of the holotype of much better quality were found in the collections of the Field Museum of Natural History, accompanied by documentation indicating that they were likely obtained directly from Safford by O.P. Hay in 1896. The Field Museum casts clearly show the bands of ridges at the base of the crown on the labial and lingual sides, which are not visible on the Yale Peabody Museum casts.</span></p> Wayne Itano Kenneth Carpenter Copyright (c) 2020 Utah Geological Association https://creativecommons.org/licenses/by/4.0/legalcode 2020-07-28 2020-07-28 7 197 203 10.31711/giw.v7.pp197-203 Paleontology of Bears Ears National Monument (Utah, USA) https://giw.utahgeology.org/giw/index.php/GIW/article/view/82 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;Bears Ears National Monument (BENM) is a new landscape-scale national monument in southeastern Utah, jointly administered by the Bureau of Land Management and the U.S. Forest Service as part of the National Conservation Lands system. As initially designated in 2016, BENM encompassed 1.3 million acres of land with exceptionally fossiliferous rock units. Subsequently, in December 2017, presidential action reduced BENM to two smaller management units (Indian Creek and Shash Jáá). Although the paleontological resources of BENM are extensive and abundant, they have historically been under-studied. Herein we summarize prior paleontological work within the original BENM boundaries to provide a more comprehensive picture of the known paleontological resources, which are used to support paleontological resource protection. The fossil-bearing units in BENM comprise a nearly continuous depositional record from aproximately the Middle Pennsylvanian Period (about 310 Ma) through the middle of the Cretaceous Period (about 115 Ma). Pleistocene and Holocene deposits are known from unconsolidated fluvial terraces and cave deposits. The fossil record from BENM provides unique insights into several important paleontological intervals of time including the Carboniferous-Permian icehouse-greenhouse transition and evolution of fully terrestrial tetrapods, the rise of the dinosaurs following the end-Triassic mass extinction,and the response of ecosystems in dry climates to sudden temperature increases at the end of the last glacial maximum.&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}">Bears Ears National Monument (BENM) is a new landscape-scale national monument in southeastern Utah, jointly administered by the Bureau of Land Management and the U.S. Forest Service as part of the National Conservation Lands system. As initially designated in 2016, BENM encompassed 1.3 million acres of land with exceptionally fossiliferous rock units. Subsequently, in December 2017, presidential action reduced BENM to two smaller management units (Indian Creek and Shash Jáá). Although the paleontological resources of BENM are extensive and abundant, they have historically been under-studied. Herein we summarize prior paleontological work within the original BENM boundaries to provide a more comprehensive picture of the known paleontological resources, which are used to support paleontological resource protection. The fossil-bearing units in BENM comprise a nearly continuous depositional record from aproximately the Middle Pennsylvanian Period (about 310 Ma) through the middle of the Cretaceous Period (about 115 Ma). Pleistocene and Holocene deposits are known from unconsolidated fluvial terraces and cave deposits. The fossil record from BENM provides unique insights into several important paleontological intervals of time including the Carboniferous-Permian icehouse-greenhouse transition and evolution of fully terrestrial tetrapods, the rise of the dinosaurs following the end-Triassic mass extinction,and the response of ecosystems in dry climates to sudden temperature increases at the end of the last glacial maximum.</span></p> Robert J. Gay Adam K. Huttenlocker Randall B. Irmis M. Allison Stegner Jessica Uglesich Copyright (c) 2020 Utah Geological Association https://creativecommons.org/licenses/by/3.0/us/legalcode 2020-08-29 2020-08-29 7 205 241 A tale of two breccia types in the Mississippian Leadville Limestone of Lisbon and other fields, Paradox Basin, southeastern Utah https://giw.utahgeology.org/giw/index.php/GIW/article/view/83 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;Two types of breccia are found in the Mississippian Leadville Limestone, Paradox Basin, southeastern Utah: (1) breccia associated with karstification and (2) breccia created by natural hydrofracturing, i.e., “autobreccia.” Breccia associated with sediment-filled cavities is relatively common throughout the upper one-third of the Leadville Limestone in Lisbon and other Paradox Basin oil and gas fields. These cavities and/or cracks are relat-ed to karstification of exposed Leadville during Late Mississippian time. Infilling of cavities by detrital carbon-ate and siliciclastic sediment occurred before deposition of the Pennsylvanian Molas Formation or Hermosa Group. Transported material consists of poorly sorted detrital quartz and feldspar grains, chert fragments, as well as clasts of carbonate and clay. Carbonate muds infilling karst cavities are very finely crystalline non-po-rous dolomites. Post-burial brecciation, caused by natural hydrofracturing, is also quite common within Leadville reser-voirs at Lisbon and other fields. Brecciation created an explosive-looking, pulverized rock, an “autobreccia” as opposed to a collapse breccia. Clasts within autobreccias remained in place or moved very little. Dolomite clasts are commonly surrounded by solution-enlarged fractures partially filled with coarse rhombic and late saddle dolomites. Areas between clasts exhibit good intercrystalline porosity and microporosity or are filled by dolomite cements. Intense pyrobitumen lining of pores was concurrent with, or took place shortly after, brecciation. The presence of zebra dolomites and zebra vugs attest to high temperatures associated with natural hydrofracturing. Rimmed microstructures or stair-step fractures are present, reflecting shear and explosive flu-id expulsion from the buildup of pore pressure. Abundant pyrobitumen makes porous breccias and dolomites look like black “shales.” Post-burial breccias are associated with the best reservoir development at Lisbon field. Outcrop analogs for both breccia types are present in the stratigraphically equivalent Mississippian section along the south flank of the Uinta Mountains in northeastern Utah. Based on field observations, a key com-ponent for autobrecciation is the presence of an underlying aquifer that serves as a conduit for hydrothermal fluids. Large volumes of water throughput are required to produce brecciation and the amount, type, and gen-erations of dolomite present at Lisbon field. We propose a model where convection cells bounded by base-ment-rooted faults transfer heat and fluids possibly from crystalline basement, Pennsylvanian evaporites, and Oligocene igneous complexes. Post-burial brecciation often results in the formation of large, diagenetic hydro-carbon traps. &quot;}" data-sheets-userformat="{&quot;2&quot;:513,&quot;3&quot;:{&quot;1&quot;:0},&quot;12&quot;:0}">Two types of breccia are found in the Mississippian Leadville Limestone, Paradox Basin, southeastern Utah: (1) breccia associated with karstification and (2) breccia created by natural hydrofracturing, i.e., “autobreccia.” Breccia associated with sediment-filled cavities is relatively common throughout the upper one-third of the Leadville Limestone in Lisbon and other Paradox Basin oil and gas fields. These cavities and/or cracks are relat-ed to karstification of exposed Leadville during Late Mississippian time. Infilling of cavities by detrital carbon-ate and siliciclastic sediment occurred before deposition of the Pennsylvanian Molas Formation or Hermosa Group. Transported material consists of poorly sorted detrital quartz and feldspar grains, chert fragments, as well as clasts of carbonate and clay. Carbonate muds infilling karst cavities are very finely crystalline non-po-rous dolomites. Post-burial brecciation, caused by natural hydrofracturing, is also quite common within Leadville reser-voirs at Lisbon and other fields. Brecciation created an explosive-looking, pulverized rock, an “autobreccia” as opposed to a collapse breccia. Clasts within autobreccias remained in place or moved very little. Dolomite clasts are commonly surrounded by solution-enlarged fractures partially filled with coarse rhombic and late saddle dolomites. Areas between clasts exhibit good intercrystalline porosity and microporosity or are filled by dolomite cements. Intense pyrobitumen lining of pores was concurrent with, or took place shortly after, brecciation. The presence of zebra dolomites and zebra vugs attest to high temperatures associated with natural hydrofracturing. Rimmed microstructures or stair-step fractures are present, reflecting shear and explosive flu-id expulsion from the buildup of pore pressure. Abundant pyrobitumen makes porous breccias and dolomites look like black “shales.” Post-burial breccias are associated with the best reservoir development at Lisbon field. Outcrop analogs for both breccia types are present in the stratigraphically equivalent Mississippian section along the south flank of the Uinta Mountains in northeastern Utah. Based on field observations, a key com-ponent for autobrecciation is the presence of an underlying aquifer that serves as a conduit for hydrothermal fluids. Large volumes of water throughput are required to produce brecciation and the amount, type, and gen-erations of dolomite present at Lisbon field. We propose a model where convection cells bounded by base-ment-rooted faults transfer heat and fluids possibly from crystalline basement, Pennsylvanian evaporites, and Oligocene igneous complexes. Post-burial brecciation often results in the formation of large, diagenetic hydro-carbon traps. </span></p> Thomas C. Chidsey David Eby Douglas Sprinkel Copyright (c) 2020 Utah Geological Association https://creativecommons.org/licenses/by/3.0/us/legalcode 2020-08-29 2020-08-29 7 243 280 New social insect nests from the Upper Jurassic Morrison Formation of Utah https://giw.utahgeology.org/giw/index.php/GIW/article/view/84 <p><span style="font-size: 11pt; font-family: Calibri,Arial; font-style: normal; color: #000000;" data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of the Upper Jurassic Morrison Formation near Green River, Utah. At least seven distinct nests are visible in the locality horizon, identifiable at the outcrop scale by loci of anastomosing, and orthogonally connected hor-izontal burrows and vertical shafts. A boulder-sized block from the in situ horizon has eroded and rolled downhill, revealing the ventral aspect of the nest, showing a view of the overall nest architecture. Burrow and shaft clusters are organized into mega-galleries which have branching arms and ovate, bulbous cham-bers. The organization of distinct trace morphologies is consistent with ethological complexity of the social insects. A small sample was collected and analyzed by serial sectioning and petrographic thin sectioning to observe small-scale morphological features. Centimeter-scale analysis shows chamber, gallery, and burrow walls have complex topography. Pebble-sized, hollow, ellipsoid features are distributed throughout the up-permost facies of the nest and have undergone complete silicification of their outer surfaces. The ellipsoids share similarity with pellet structures made of mud or carton produced by modern termites. This trace fossil assemblage suggests it is possible that termites had acquired subterranean nesting behavior, and mud or carton utilization in nest construction in seasonally arid habitats by the Late Jurassic.&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}">This paper reports a new assemblage of social insect ichnofossils from the Brushy Basin Member of the Upper Jurassic Morrison Formation near Green River, Utah. At least seven distinct nests are visible in the locality horizon, identifiable at the outcrop scale by loci of anastomosing, and orthogonally connected hor-izontal burrows and vertical shafts. A boulder-sized block from the in situ horizon has eroded and rolled downhill, revealing the ventral aspect of the nest, showing a view of the overall nest architecture. Burrow and shaft clusters are organized into mega-galleries which have branching arms and ovate, bulbous cham-bers. The organization of distinct trace morphologies is consistent with ethological complexity of the social insects. A small sample was collected and analyzed by serial sectioning and petrographic thin sectioning to observe small-scale morphological features. Centimeter-scale analysis shows chamber, gallery, and burrow walls have complex topography. Pebble-sized, hollow, ellipsoid features are distributed throughout the up-permost facies of the nest and have undergone complete silicification of their outer surfaces. The ellipsoids share similarity with pellet structures made of mud or carton produced by modern termites. This trace fossil assemblage suggests it is possible that termites had acquired subterranean nesting behavior, and mud or carton utilization in nest construction in seasonally arid habitats by the Late Jurassic.</span></p> Elliott Armour Smith Mark A. Loewen James I. Kirkland Copyright (c) 2020 Utah Geological Association https://creativecommons.org/licenses/by/3.0/us/legalcode 2020-08-29 2020-08-29 7 281 299 10.31711/giw.v7.pp281-299 G.K. Gilbert and the Bonneville shoreline https://giw.utahgeology.org/giw/index.php/GIW/article/view/85 <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 Bonneville shoreline, the highest, and second-most prominent shoreline of Pleistocene Lake Bonneville in Utah, Nevada, and Idaho, has been thought for many years to have formed during a period of\nprolonged overflow (500 to 1000+ years) and lake-level stability prior to the Bonneville flood. That traditional idea was initially promoted by G.K. Gilbert during the 1870s before he spent over a decade on field work related to Lake Bonneville. During Gilbert’s field work, his observations led him to a different interpretation of how the Bonneville shoreline developed, and by the time his final report on Lake Bonneville was published in 1890, he was no longer promoting the idea of prolonged overflow. Instead he thought of the Bonneville shoreline as a geomorphic record of the highest level attained by the transgressing lake in the closed basin; the shoreline marks the boundary between lacustrine-dominated landforms below and fluvial-dominated landforms above. For over 120 years after Gilbert’s (1890) monograph was published, researchers ignored his interpretation, and assumed (but did not present supporting evidence), that Lake Bonneville had overflowed for a prolonged period prior to the Bonneville flood while the Bonneville shoreline developed. Re-examination of the geomorphology of the Bonneville shoreline, the stratigraphy of Lake Bonneville deposits, the geomorphology of the overflow area, and the history of Lake Bonneville, shows that Gilbert’s 1890 interpretation is consistent with observations. Considering this, to accurately interpret the history of Lake Bonneville the Bonneville shoreline should be viewed as the level the lake had reached in the closed basin when its transgression ceased and it began to spill into the Snake River drainage basin.&quot;}" data-sheets-userformat="{&quot;2&quot;:513,&quot;3&quot;:{&quot;1&quot;:0},&quot;12&quot;:0}">The Bonneville shoreline, the highest, and second-most prominent shoreline of Pleistocene Lake Bonneville in Utah, Nevada, and Idaho, has been thought for many years to have formed during a period of<br>prolonged overflow (500 to 1000+ years) and lake-level stability prior to the Bonneville flood. That traditional idea was initially promoted by G.K. Gilbert during the 1870s before he spent over a decade on field work related to Lake Bonneville. During Gilbert’s field work, his observations led him to a different interpretation of how the Bonneville shoreline developed, and by the time his final report on Lake Bonneville was published in 1890, he was no longer promoting the idea of prolonged overflow. Instead he thought of the Bonneville shoreline as a geomorphic record of the highest level attained by the transgressing lake in the closed basin; the shoreline marks the boundary between lacustrine-dominated landforms below and fluvial-dominated landforms above. For over 120 years after Gilbert’s (1890) monograph was published, researchers ignored his interpretation, and assumed (but did not present supporting evidence), that Lake Bonneville had overflowed for a prolonged period prior to the Bonneville flood while the Bonneville shoreline developed. Re-examination of the geomorphology of the Bonneville shoreline, the stratigraphy of Lake Bonneville deposits, the geomorphology of the overflow area, and the history of Lake Bonneville, shows that Gilbert’s 1890 interpretation is consistent with observations. Considering this, to accurately interpret the history of Lake Bonneville the Bonneville shoreline should be viewed as the level the lake had reached in the closed basin when its transgression ceased and it began to spill into the Snake River drainage basin.</span></p> Charles G. Oviatt Copyright (c) 2020 Utah Geological Association https://creativecommons.org/licenses/by/4.0/legalcode 2020-11-22 2020-11-22 7 300 320 10.31711/giw.v7.pp300-320