Research Article| December 01, 1973 Reinterpretation of the Boundary between the Cosumnes and Logtown Ridge Formations, Amador County, California ROBERT V. SHARP; ROBERT V. SHARP 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar WENDELL A. DUFFIELD WENDELL A. DUFFIELD 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information ROBERT V. SHARP 1U.S. Geological Survey, Menlo Park, California 94025 WENDELL A. DUFFIELD 1U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (12): 3969–3976. https://doi.org/10.1130/0016-7606(1973)84<3969:ROTBBT>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation ROBERT V. SHARP, WENDELL A. DUFFIELD; Reinterpretation of the Boundary between the Cosumnes and Logtown Ridge Formations, Amador County, California. GSA Bulletin 1973;; 84 (12): 3969–3976. doi: https://doi.org/10.1130/0016-7606(1973)84<3969:ROTBBT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Recent detailed geologic mapping in the Sierran foothills reveals that rocks previously included in the Jurassic Amador Group must be redefined. The term “Amador Group” was applied by Taliaferro and Clark to a section of epiclastic metasedimentary rocks (the Cosumnes Formation) and the seemingly conformable overlying metavolcanic rocks (the Logtown Ridge Formation).New structural and stratigraphic evidence indicates that at their type localities on the banks of the Cosumnes River the boundary between the two formations should be relocated about 610 m downsection from the position shown by Clark. This change removes all known paleontological control on the age of the Cosumnes Formation.Structural relations show that the type Cosumnes and Logtown Ridge Formations are in fault contact at the Cosumnes River. Rocks of the Cosumnes Formation are now grouped with a complex unit of megabreccia that includes other strata previously termed the “western belt” of the Calaveras Formation. The megabreccia formed, at least partly, sometime between late Paleozoic and Late Jurassic times, but rocks in the megabreccia, including the Cosumnes Formation, could be older than late Paleozoic.The term “Amador Group” is herein abandoned. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The authors propose, based on reconnaissance geology studies and interpretation of landforms as depicted by Landsat Thematic Mapper (TM) images combined with digitized topography, that the Quezaltenango basin of Guatemala is part of a caldera. The Quezaltenango basin is an elliptical depression, about 12 by 25 km and about 500 m deep. The proposed Xela Caldera extends beyond the basin more than 10 km to the north. The geomorphological features of the area that are typical of a geologically young large-scale caldera include bounding walls that have steep interior and gentle exterior slopes; broad flat areas at the base of the walls; at least one large block, about 3 by 12 km, that only partly floundered as the caldera collapsed; resurgence of a younger volcanic dome, flow and small-scale caldera complex (last active in 1818); younger volcanoes located along the structural margin of the major caldera (one of which is currently active) lobate features on the caldera margins that may indicate a multiple sequence of eruptions; and an active, high-temperature geothermal system. The valley is coincident with a gravity low. Extensive ash-flow tuff sheets that have no identified source are located north of the caldera, and may be the outflowmore » deposits. The Xela caldera is similar in size to the Atitlan caldera, which lies about 50 km southeast of Quezaltenango. The Xela Caldera, if confirmed by future studies, may contain undiscovered geothermal resources, may present a significant geologic hazard to the more than 400,000 people who occupy the Quezaltenango valley, and may be a new member of the list of magmatic systems that have the capability to change global climate for several years.« less
accurately portray a dome that forms from multiple effusive events that occur over years to decades.The dacite lava domes of historic age at Mount St. Helens, Washington, and Santiaguito, Guatemala, are examples of this type.Taylor Creek Rhyolite and structurally similar domes of endogenous origin perhaps are most amenable to theoretical analysis, because each apparently had a liquid core that buffered a dome or flow toward endogenous growth throughout its formation.However, time-related changes in fluid-and solid-material properties that occur during lava-dome growth can complicate accurate theoretical portrayal of even these structurally simple bodies.
Replicate 40 Ar/ 39 Ar analyses of sanidine from seven Oligocene (28.2 Ma) rhyolite flows using a continuous laser dating system routinely yield analytical precisions of less than 0.45% (1s) on samples smaller than 0.5 mg. This is as good or better than has been obtained by any other method even though the sample mass is several orders of magnitude smaller. The system has the potential of resolving differences in apparent age of as little as 0.2% for high K/Ca samples as young as a few Ma. The analyses suggest that the seven volcanic units analyzed were erupted in an interval of about 0.1 m.y. or less.
Thirty‐eight separate domes and flows of phenocryst‐poor, high‐silica rhyolite of similar major element chemical composition were erupted over the past 1 m.y. from vents arranged in a crudely S ‐shaped array atop a granitic horst in the Coso Range, California. Most of the extrusions are probably less than about 0.3 m.y. old. The area is one of Quaternary basaltic volcanism and crustal extension. The central part of the rhyolite field is characterized by high heat flow, low apparent resistivity, and substantial fumarolic activity indicative of an active geothermal system. The immediate source of heat for the surficial geothermal phenomena is probably a silicic magma reservoir that may still contain molten or partially molten material at a depth of at least 8 km beneath the central part of the rhyolite field. Outlying rhyolite extrusions probably reflect the presence of feeder dikes emanating from the reservoir beneath the central region. Azimuths of dikes appear to be parallel to the regional tectonic axis of maximum horizontal compression, analogous to some dike‐fed flank eruptions on basaltic shields and andesitic stratovolcanoes. The areal extent of a magma reservoir and the present total heat content of the silicic magma system at Coso may be less than was previously estimated. However, the area is still considered to be one of significant geothermal potential.
