Boulder, CO, USA – The February issue of Geosphere, published by the Geological Society of America, is now available online. Geology topics of interest include: revised dating of Grand Canyon lava flows; Missouri's Reelfoot Rift and its impact on the central Mississippi River Valley; LIDAR mapping of active surface faults in Houston, TX; a paleo-continental divide in northeastern Nevada; and a cybergeology approach to geologic mapping. Two articles address north-central Nevada's Caetano caldera.
Large-magnitude Miocene extension of the Eocene Caetano caldera, Shoshone and Toiyabe Ranges, Nevada
Joseph P. Colgan et al., U.S. Geological Survey, Menlo Park, California 94025, USA.
Keywords: Basin and Range Province, Miocene, extension tectonics, calderas.
The late Eocene Caetano caldera formed about 34 million years ago during eruption of the Caetano Tuff (described in a companion paper by John and others in this issue of Geosphere). Remnants of the caldera are presently exposed in a series of north-trending, east-tilted (about 40 degrees), fault-bounded blocks that crop out across 40 km (east-west) of the Shoshone and Toiyabe Ranges in north-central Nevada. Restored geologic cross-sections indicate that the caldera was originally about 12-18 km (north-south) by 20 km (east-west) and has therefore been stretched to about twice its original width. The authors interpret Miocene sedimentary rocks exposed between these fault blocks to represent material shed from rising mountain ranges into adjacent basins while the faults were moving, indicating that deformation began about 16 million years ago and continued until 10-12 million years ago. These older basins and ranges were broken up by younger, locally active faults that formed the modern basins and ranges seen in north-central Nevada today. It is likely that Miocene faulting and tilting was not confined to the former caldera, but also affected surrounding Paleozoic rocks-and, potentially, large, nearby Carlin-type gold deposits.
History of Quaternary volcanism and lava dams in western Grand Canyon based on lidar analysis, 40Ar/39Ar dating, and field studies: Implications for flow stratigraphy, timing of volcanic events, and lava dams
Ryan Crow et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA.
Keywords: Grand Canyon region, Uinkaret, basalt flows, lava dams, volcanic history.
John Wesley Powell wrote in 1895: “...what a conflict of water and fire there must have been [in western Grand Canyon]! Just imagine a river of molten rock running down over a river of melted snow.” Over 110 years later, a synthesis of new and existing dates on these lava flows shows that many are significantly younger than initially thought and all are less than 725 thousand years old. The geochronology data indicates four major episodes when lava flows either erupted into the canyon or flowed over the rim into it: 725-475 thousand years ago (ka), 400-275 ka, 225-150 ka, and 150-75 ka. These flows formed lava dams in western Grand Canyon that had dramatic impact on the Colorado River. This paper presents light detection and ranging (lidar) data to establish the elevations of the tops and bottoms of basalt flow remnants along the river corridor. These data show the original extent of now-dissected intra-canyon flows and aid in correlation of flow remnants. From 725 to 475 ka, volcanism built a high edifice within Grand Canyon in the area of the Toroweap fault, with dike-cored cinder cones on both rims and within the canyon itself. These large-volume eruptions helped drive the far-traveled basalt flows which flowed down-canyon over 120 km. A second episode of volcanism, from 400 to 275 ka, built a 215-m-high dam along the Hurricane fault, about 15 km downstream. The ca. 200 and 100 ka flows (previously mapped as Gray Ledge) were smaller flows and lava cascades that entered the canyon from the north rim between the Toroweap and Hurricane faults. The combined results suggest a new model for the spatial and temporal distribution of volcanism in Grand Canyon in which composite lava dams and edifices were generally leaky in proximal areas. Available data suggest that the demise of volcanic edifices may have involved either large outburst-flood events or normal fluvial deposition at times when the river was established on top of basalt flows. These data highlight complex interactions of volcanism and fluvial processes in this classic locality.
Reelfoot rift and its impact on Quaternary deformation in the central Mississippi River valley
Ryan Csontos et al., University of Memphis, Ground Water Institute 300 Engineering, Memphis, TN 38125, USA.
Keywords: Reelfoot rift, Mississippi embayment, New Madrid seismic zone, Mississippi River alluvium, geomorphology.
This article presents research into the Reelfoot rift within the Mississippi embayment of the central United States. The project is timely and relevant to a wide audience in light of the seismic risk and increased interest in petroleum potential within the Mississippi embayment. The northern end of the rift near the town of New Madrid, Missouri, was the site of the great 1811-1812 New Madrid earthquakes, and it remains the most seismically active area east of the Rocky Mountains. This research utilizes existing and new data sets which better define the structure and stratigraphy within the Reelfoot rift and presents them as a 3-D model for interpretation and visualization. This allows the generation of a unique picture and new understanding of the central Mississippi River Valley. Quaternary reactivation of Precambrian basement faults is of wide international interest in developing a better understanding of rift systems and their earthquake threat. Within the Mississippi embayment it is important to understand the source of the current and historic seismicity. Delineation of basement structures within the Reelfoot rift is also of interest to the petroleum industry, because the authors have mapped basement blocks that may help define petroleum targets. This work substantially improves the interpretation of the geologic history, formation, and development of the Reelfoot rift and its impact on Quaternary deformation.
Automated extraction of data from text using an XML parser: An earth science example using fossil descriptions
Gordon B. Curry et al., Digital Geosciences Laboratory, Dept of Geographical and Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, Scotland, UK.
This paper describes a method of automating the computerization (digitization) of important sections of earth science information that is currently only available in printed text. At the present time, full digitizing of printed information requires manual entry of data into a database, which is slow, unrewarding, and likely to introduce mistakes. This paper demonstrates a method of automatically digitizing descriptions of fossil species, which are an immense source of information on the history of life on the planet. These descriptions are written in a very regular way, to such an extent that they can be read by computers using new software (a parser) that creates markers, or tags, around segments of text. Once tagged in this way, the information can be analyzed much more thoroughly that was previously possible. The digitized information is also much more complete than was previously available, as the entire species description is tagged, including all the features of the fossil, its stratigraphic distribution, and geographic location.
Lidar mapping of faults in Houston, Texas, USA
Richard M. Engelkemeir and Shuhab D. Khan, Department of Geosciences, University of Houston, Houston, Texas 77204-5007, USA.
Keywords: lidar, Houston, faults, subsidence
This paper uses LIDAR for mapping active surface faults in Houston, Texas. These faults result in damages to houses, pipelines, roads and other constructions. Accurate mapping of their locations therefore aids in hazard mitigation.
Ash-flow tuffs and paleovalleys in northeastern Nevada: Implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin
Christopher D. Henry, Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557.
Keywords: paleogeography, extension, Eocene, ash-flow tuff, Nevada
The distribution of distinctive volcanic and sedimentary rocks indicates that northeastern Nevada 40 million years ago was a high plateau, possibly 4 kilometers (13,000 feet) high, incised with deep valleys. Rivers in the valleys on opposite sites of a “paleo-continental divide” that ran approximately north-south slightly west of Elko, Nevada, drained either westward to the Pacific Ocean or eastward to large basins in Utah. The high plateau probably resulted from thickening of the crust following a long period of folding that ended about 60 million years ago. The plateau was probably similar to parts of the modern Andes Mountains of South America or Tibet.
Magmatic and tectonic evolution of the Caetano Caldera, north-central Nevada: A tilted, mid-Tertiary eruptive center and source of the Caetano Tuff
David A. John et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA.
Keywords: calderas, ash-flow tuff, magma resurgence, Basin and Range Province, extensional tectonics
The Caetano caldera in north-central Nevada formed during a supervolcano eruption of greater than 1100 cubic kilometers (greater than 270 cubic miles) of Caetano Tuff about 33.8 million years ago. The caldera formed by when the roof of the magma chamber collapsed as the Caetano Tuff erupted from the chamber. Collapse left an ovoid depression, a caldera, about 20 kilometers long by 12 to 18 kilometers wide and up to 1 kilometer deep. Due to younger faulting and tilting (see companion paper by Colgan and others in the same issue of Geosphere), the caldera was broken into several blocks that expose caldera features from the former surface to a depth of more than 5 kilometers. This extraordinary three-dimensional view allows a far more detailed analysis of caldera formation and evolution than is available for almost any other caldera in the world. Reconstruction of the Caetano caldera also constrains nearby Carlin-type gold deposits, presently the largest producing gold deposits in the United States, to have formed at depths of less than or equal to 1 kilometer.
Outcrop fracture characterization using terrestrial laser scanners: Deep-water Jackfork sandstone at Big Rock Quarry, Arkansas
Mariana I. Olariu et al.; John F. Ferguson, corresponding author, Geosciences Department, University of Texas at Dallas, P.O. Box 830688, Mail Station FO21, Richardson, Texas 75083-0688, USA.
Keywords: cluster analysis, fractures, turbidite, outcrop, lidar, laser scanner, laser.
A new way of doing geology is being invented that is sometimes called "cyber geology." This paper is a pioneering example of this new approach to geologic mapping. Laser scanners and GPS are used to produce high-resolution (approaching one sample per square centimeter) maps of the outcrop of the Jackfork sandstone in the Big Rock Quarry near Little Rock, Arkansas. A three-dimensional virtual model of the outcrop is produced on a computer from this very large (millions of samples) data set. A processing scheme has been implemented that automatically identifies distinct surfaces and their orientations from the three-dimensional point cloud of sample points. These surfaces can be interpreted in terms of fractures and bedding planes in the Jackfork sandstone. The large number of virtual strikes and dip measurements permit a statistical characterization of the fracture orientations as well as an analysis of the special variability of the fractures in different locations within the quarry.
A prominent geophysical feature along the northern Nevada rift and its geologic implications, north-central Nevada
David A. Ponce and J.M.G. Glen, MS989, U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94025, USA.
Keywords: gravity and magnetic anomalies, northern Nevada rift, epithermal gold deposits, Battle Mountain-Eureka mineral trend, Basin and Range, Nevada.
The origin and character of a prominent large-scale geophysical feature in north-central Nevada is considered. This crustal-scale fault is coincident with the western margin of the northern Nevada rift—a mid-Miocene rift that includes mafic dike swarms and associated volcanic rocks and is partly coincident with the central part of the Battle Mountain-Eureka mineral trend. Geophysical evidence suggests that the northern Nevada rift partly followed this pre-existing feature in north-central Nevada. If the crustal fault along the northern Nevada rift and a previously inferred crustal fault along the central part of the Battle Mountain-Eureka mineral trend are in fact the same feature, deposits at a greater distance from the crustal feature associated with the northern Nevada rift at Battle Mountain could be explained by post-emplacement tectonic events. In any case, these large-scale crustal features are important to understanding the metallogeny, tectonics, magmatism, and water resources of the Great Basin.
Identification of quartz and carbonate minerals across northern Nevada using ASTER thermal infrared emissivity data—Implications for geologic mapping and mineral resource investigations in well-studied and frontier areas
Barnaby W. Rockwell and Albert H. Hofstra, U.S. Geological Survey, Box 25046, MS 973, Denver Federal Center, Denver, Colorado 80225, USA.
Keywords: remote sensing, ASTER, Thermal infrared, quartz, carbonate
Outcrops and detritus composed of quartz and carbonate (calcite and dolomite) were identified and mapped across a 400 km by 400 km area of northern Nevada, USA, using thermal infrared data collected by the ASTER sensor aboard the EOS Terra satellite platform. This paper shows that such data can be used to generate accurate and cost-effective maps of these minerals at regional to local scales. Well-mapped rock types include: thick sequences of quartz sandstone (or quartzite) and conglomerate, bedded radiolarian chert, rhyolite, and diatomite as well as thick sequences of dolomite, limestone, and marble. Alluvial fan surfaces, sand dunes, and beach deposits composed of quartz and/or carbonate are prominent map features. Also detected were small hot spring silica sinter and travertine deposits in geothermal areas and quartz deposited from ancient hydrothermal systems that formed large deposits of gold and silver.
Gondwanan/peri-Gondwanan origin for the Uchee terrane, Alabama and Georgia: Carolina zone or Suwannee terrane(") and its suture with Grenvillian basement of the Pine Mountain window
Mark G. Steltenpohl et al., Department of Geology and Geography, Auburn University, Auburn, Alabama 36849, USA.
Keywords: Uchee terrane, Carolina zone, Gondwana, peri-Gondwana, southern Appalachians
In 1964, an exploratory petroleum well was drilled through one mile of Gulf Coastal Plain sediments in southeastern Alabama. The retrieved core included Silurian to Devonian sedimentary rocks that surprisingly contain fossils provincial to proto-Africa (i.e., Gondwanan), rather than proto-North America (Laurentian), setting off a scientific debate about how these rocks got there. At the same time, discoveries made in studies of the ocean floor were being formulated into the sea-floor spreading mechanism that would explain why and how continents drift around the surface of Earth, the missing link that had escaped Alfred Wegner’s conceptualization of continental drift. The Suwannee terrane, as the Alabama rocks became known, figured prominently in J. Tuzo Wilson’s 1966 article in Nature (“Did the Atlantic close and reopen"”), which led Kevin Burke to later coin the term “Wilson Cycle.” This orphaned block of Gondwanan crust is now known to extend in the subsurface beneath southern Alabama, Georgia, and South Carolina. A collage of deformed and metamorphosed fragments of ancient volcanic island arcs, known collectively as Carolinia (i.e., Carolina Superterrane), form the most eastern exposures of the Appalachian orogen and also project to depth beneath the Atlantic coastal plain. These arc terranes formed in an ancient ocean peripheral to western Gondwanaland prior to its climactic Appalachian collision with Laurentia that consolidated the supercontinent Pangaea. Due to the lack of fossils and reliable isotopic dates within these terranes, the nature and timing of their docking with Laurentia are controversial; hence, one of the most significant events in Appalachian history is also one of the least understood. Steltenpohl and others report new isotopic and structural information on the poorly known Uchee terrane in Alabama that bears on the problem. The Uchee terrane occupies a particularly critical tectonic position, being sandwiched between Laurentian continental basement exposed in the Pine Mountain window and Gondwanan crust of the overlying, albeit buried, Suwannee terrane. Steltenpohl and coauthors confirm that the Uchee is an “exotic” peri-Gondwanan arc terrane -- not part of Laurentia as was previously believed -- and thus provide a new puzzle piece that helps to constrain models for plate tectonic development of the Pangaean suture.
Late Cenozoic paleogeographic evolution of northeastern Nevada: Evidence from the sedimentary basins
Alan R. Wallace, et al., U.S. Geological Survey, MS 176, Mackay School of Earth Sciences and Engineering, University of Nevada, Reno, Reno, Nevada 89557, USA.
Keywords: sedimentary basins, tectonics, geomorphology, Nevada, Miocene, Pliocene, gold, Humboldt River.
New geologic and dating studies along a 200-km-long transect across northeastern Nevada has led a better understanding of how the landscape has changed over the past 20 million years (m.y.). The studies focused on four sedimentary basins along the transect (Chimney, Ivanhoe, Carlin, and Elko, from west to east) because they record events, such as the formation of mountains and erosion by streams, that produced the landscape changes over time. The landscape was a subdued, eroding upland until about 16 m.y. ago, when volcanic eruptions in the western half of the transect and faulting in the eastern half dammed streams and formed the sedimentary basins. These basins gradually filled up and partially buried adjacent highlands. The dams for the western three basins failed after about 2 m.y., and the streams integrated into the beginnings of the Humboldt River drainage system. The Elko basin remained isolated until about 10 m.y., at which point it began to drain to the west into the Humboldt system. The river system flowed westward into northwestern Nevada, where active faulting was forming deepening valleys. As a result, the basin sediments in the upstream areas have been eroding and carried to the new downstream basins. This erosion has re-exposed the pre-basin highlands, so some modern ranges are fairly old rather than young as previously thought. Similarly, major mineral deposits in the upstream areas gradually have been exposed, weathered, or eroded, whereas those in downstream areas have been covered up. This concept is very important for the evaluation of and exploration for mineral deposits in the region.
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