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> en-US giw@utahgeology.org (Douglas Sprinkel) paulinkenbrandt@gmail.com (Paul Inkenbrandt) Fri, 13 Aug 2021 16:59:37 -0600 OJS 3.1.2.4 http://blogs.law.harvard.edu/tech/rss 60 Carbonate mound springs of the Upper Jurassic Morrison Formation of central Montana and their paleoclimatic significance for the northern foreland basin https://giw.utahgeology.org/giw/index.php/GIW/article/view/89 <p>Recent investigations of the Upper Jurassic Morrison Formation in central Montana resulted in the discovery of 105 small (&lt; 3 m diameter) carbonate buildups in close geographic and stratigraphic proximity. The buildups are divided into two groups by dominant mineralogic composition: siderite versus calcium carbonate. The buildups are found in five distinct spatial clusters and are distributed in association with, and in alignment to, regional Jurassic-aged structural lineaments. The buildups are distributed stratigraphically between 40 to 52 m above the base of the Morrison Formation. Interpretation of electrical resistivity tomography surveys indicates that additional buildups are present in the subsurface. Carbonate-rich<br>groundwater migrated up fractures to the capillary fringe or the surface. The siderite buildups formed in the near subsurface capillary fringe, whereas the carbonate mounds are subartesian mound spring tufa deposits. The bulk rock negative δ18O and δ13C values demonstrate the buildups were produced by meteoric waters in a continental setting with the groundwater having a short residence time in the subsurface. The presence of the subsurface buildups and mound spring tufa deposits scattered throughout a 12-m portion<br>of the Morrison section indicates that the region experienced extended periods of increased precipitation.</p> Dean Richmond, John Pigott, Richard Lupia, Michael Behm, David Hein Copyright (c) 2021 Utah Geological Association https://creativecommons.org/licenses/by/3.0/us/ https://giw.utahgeology.org/giw/index.php/GIW/article/view/89 Localized bank collapse or regional event? https://giw.utahgeology.org/giw/index.php/GIW/article/view/91 <p>This study presents a detailed synopsis of the sedimentological and structural features displayed within an underdescribed enigmatic facies observed in the basal Lower Jurassic Kayenta Formation of the Colorado Plateau. The facies comprises pebble to cobble-sized clasts of fine to medium-grained crossbedded sandstone with mud-draped and deformed foresets, as well as clasts of parallel-laminated but highly contorted siltstone and mudstone, supported in a silty to sandy matrix. The deposits are internally deformed and show both ductile and brittle structures in close spatial proximity, with a consistent and pervasive westdirected sense of shear. The facies occurs consistently within the same approximate stratigraphic interval, at or near the base of the Kayenta Formation. It is, however, observed only at four localities, distributed in a crudely linear arrangement parallel to the Utah-Idaho trough, despite extensive studies of outcrops of the same stratigraphic interval widely distributed across both Utah and Arizona. This study interprets the depositional processes as that of a partially subaerial debris flow with depositional events perhaps taking place during the waning period after ephemeral stream activity. The clast morphology and composition suggests a local source for the sediment entrained within the flow, and a limited transport distance. All of these observations are difficult to reconcile with the consistency of the stratigraphic interval in which the facies occur, or with the regional distribution of preserved examples. Consequently, this study discusses the potential for a common and time-equivalent triggering mechanism across all examples, which may have regional significance in the Jurassic evolution of the region.</p> Charlotte Priddy, Amy Regis, Stuart Clarke, A. Leslie, Thomas Dodd Copyright (c) 2021 Utah Geological Association https://creativecommons.org/licenses/by/3.0/us/ https://giw.utahgeology.org/giw/index.php/GIW/article/view/91 Fri, 17 Dec 2021 00:00:00 -0700 The Upper Cretaceous Rock Springs Formation of northwest Colorado https://giw.utahgeology.org/giw/index.php/GIW/article/view/94 <p><span data-sheets-value="{&quot;1&quot;:2,&quot;2&quot;:&quot;The Upper Cretaceous Rock Springs Formation of the Mesaverde Group in northwestern Colorado, southwestern Wyoming, and northeastern Utah is composed of fluvial, deltaic, and marine sediments that record the regression of the Western Interior Seaway during the Early to Middle Campanian. Contemporaneous deposits are present along the eastern and southeastern margins of the Greater Green River Basin in Wyoming, but correlation across the basin is challenging. Analysis of a small (1-km-long), understudied outcrop in northwestern Colorado assists in bridging that gap. The outcrop consists of distal and proximal deltaic deposits, overlain by distributary-channel complexes within delta-plain deposits. Correlation panels based on subsurface wireline logs and outcrop gamma-ray profiles show that the deposits are younger than lithostratigraphically equivalent strata of the Rock Springs Formation in Utah and Wyoming. Regional nomenclature is introduced for the area, and it is shown that these deposits differ from better-documented, older Rock Springs Formation deposits in Utah and Wyoming by having a higher net sandstone percentage due to the presence of substantial distributary-channel complexes. This study benefits subsurface exploration efforts in the Greater Green River Basin by providing outcrop analogs of reservoir distribution and quality.&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 Upper Cretaceous Rock Springs Formation of the Mesaverde Group in northwestern Colorado, southwestern Wyoming, and northeastern Utah is composed of fluvial, deltaic, and marine sediments that record the regression of the Western Interior Seaway during the Early to Middle Campanian. Contemporaneous deposits are present along the eastern and southeastern margins of the Greater Green River Basin in Wyoming, but correlation across the basin is challenging. Analysis of a small (1-km-long), understudied outcrop in northwestern Colorado assists in bridging that gap. The outcrop consists of distal and proximal deltaic deposits, overlain by distributary-channel complexes within delta-plain deposits. Correlation panels based on subsurface wireline logs and outcrop gamma-ray profiles show that the deposits are younger than lithostratigraphically equivalent strata of the Rock Springs Formation in Utah and Wyoming. Regional nomenclature is introduced for the area, and it is shown that these deposits differ from better-documented, older Rock Springs Formation deposits in Utah and Wyoming by having a higher net sandstone percentage due to the presence of substantial distributary-channel complexes. This study benefits subsurface exploration efforts in the Greater Green River Basin by providing outcrop analogs of reservoir distribution and quality.</span></p> Stephen Phillips, Hudson Hudson Copyright (c) 2022 Geology of the Intermountain West https://creativecommons.org/licenses/by/3.0/us/ https://giw.utahgeology.org/giw/index.php/GIW/article/view/94 Mon, 17 Jan 2022 13:09:57 -0700