Boulder, Colo. - The January-February issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics include new insights into Neoproterozoic climate change from rocks in the Grand Canyon and a new hypothesis concerning solar heating and physical weathering of Earth's surface.
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Distal fluvio-lacustrine volcaniclastic resedimentation in response to an explosive silicic eruption: The Pliocene Mushono Tephra Bed, central Japan
Kyoko Kataoka, Department of Geosciences, Osaka City University, Osaka 558-8585, Japan, and Department of Ground Hazards, Research Institute for Hazards in Snowy Areas, Niigata University, Niigata 950-2181, Japan, et al. Pages 3-17.
Keywords: distal setting, explosive silicic eruption, fluvio-lacustrine, resedimentation, volcaniclastic sediment.
The present article reports the volcaniclastic resedimentation in fluvio-lacustrine setting away from eruptive source area. The depositional and environmental response to reworking of voluminous volcaniclastic material is assessed based on a detail facies analysis of the Mushono Tephra Bed, central Japan.
Tectonic implications of fault-scarp-derived volcaniclastic deposits on Macquarie Island: Sedimentation at a fossil ridge-transform intersection?
Nathan R. Daczko, Department of Geological Sciences and Institute for Geophysics, Jackson School of Geosciences, University of Texas, Austin, Texas 78712, USA and GEMOC ARC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia, et al. Pages 18-31.
Keywords: sedimentology, fault scarps, ridges, transform faults, Macquarie Island.
This article uses unique rock exposures on sub-Antarctic Macquarie Island in the Southern Ocean to detail how sedimentary rocks form in the deep ocean in a tectonically active area.
High-resolution δ13C stratigraphy of the Chuar Group (~770-742 Ma), Grand Canyon: Implications for mid-Neoproterozoic climate change
C.M. Dehler, Department of Geology, Utah State University, Logan, Utah 84322-4505, USA, et al. Pages 32-45.
Keywords: Neoproterozoic, Grand Canyon, C-isotope record, Chuar Group, lithostratigraphy, shale geochemistry, climate change.
The Chuar Group is a series of layered rocks exposed solely in Grand Canyon, Arizona, and is one of the best dated and best preserved geologic records from Precambrian time. The Precambrian part of geologic time represents 89% of Earth History -- from its birth, 4.5 billion years ago, to the onset of complex life about 543 million years ago ("Cambrian Explosion"). The Chuar Group is known to represent the time period between about 770 and 742 million years ago and records (in the limestones, mudstones, and sandstones) climate changes that correspond to major fluctuations in the carbon cycle -- an order of magnitude greater than global carbon cycle fluctuations of today. Other researchers have attributed similar Precambrian fluctuations in carbon to mega-icehouse and mega-hothouse conditions on Earth -- conditions unlike any that we could fathom on Earth today, such as the world's oceans being entirely frozen, even in equatorial regions ("snowball Earth hypothesis"). Research by Dehler and team suggest that much milder climate fluctuations could have produced these changes in the carbon cycle, and that the main controlling factor was the placement of all major continents in low latitudes, which would have affected where and how carbon was sequestered through changes in precipitation, sediment burial rates, and equatorial ecosystem productivity.
Episodic arc migration, crustal thickening, subduction erosion, and magmatism in the south-central Andes
Suzanne Mahlburg Kay, INSTOC, Cornell University, Ithaca, New York 14853, USA; et al. Pages 67-88.
Keywords: Andes, Southern Volcanic Zone, magmatism, subduction erosion, thick crust, geochemistry, isotopes, Miocene.
The south-central Chilean Andes have long been cited as a site of continental growth and the adjacent margin as a potential site where crust has been removed as the oceanic plate has subducted beneath the continent. The purpose of this paper is to argue that an ~85-km-wide east-west strip of the Andean margin near 34¢XS latitude was lost in two strong episodes of forearc subduction erosion in the last 20 million years. Approximately 50 km of loss is proposed in an episode between ~20 and 16 million years ago, and an additional ~ 35 km in another between ~8 and ~3 million years ago. Evidence for loss of this crust comes from the eastward displacement of the frontal volcanic arc front at these times, and chemical and isotopic evidence for recycling of small amounts of the removed continental margin in the erupted magmas. At the same time, important contractional deformation resulted in shortening and thickening of the continental crust beneath and to the east of the uplifting magmatic arc front. The resultant changes in the geometry of the continental crust and mantle lithosphere due to this shortening are also reflected in the chemistry and isotopic ratios of the erupted magmas. The combined loss and shortening of the crust in this east-west transect of the Andean margin appears to have reduced the width of the continent by as much as 170 km in the last 20 million years.
Trench-parallel shortening in the Northern Chilean Forearc: Tectonic and climatic implications
Richard W. Allmendinger, Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA, et al. Pages 89-104.
Keywords: forearc, Andes, Chile, deformation, geomorphology, tectonics.
The plate boundary between the subducted Nazca plate and western South America is responsible for some of the largest earthquakes in recorded history. We describe young deformation features in the Coastal Cordillera of northern Chile, which overlies the seismogenic zone between the Nazca plate and South America. These features show that shortening has occurred parallel to the Coastal Cordillera since the late Miocene and is continuing today. We suggest that coupling between the Nazca plate and South America, in combination with the curvature of the plate boundary, has produced this shortening. The relationship between the fault scarps and small abandoned river valleys in the Coastal Cordillera shows that there was more moisture in the now hyperarid Atacama Desert prior to the Pliocene.
Structural evolution of fault zones in sandstone by multiple deformation mechanisms: Moab fault, southeast Utah
Nicholas C. Davatzes et al., Department of Geological and Environmental Science, Stanford University, Stanford, California 94305-2115, USA. Pages 135-148.
Keywords: deformation band, joint, Moab fault, mechanics, stress, sheared joint.
Fault zones at shallow depths less than ~10 km are complex zones of localized shear typically composed of many different types of structures. In sandstone offset by the Moab fault in SE Utah, systematic variation in the types of structures comprising the fault zone and their distribution are related to the geometry of the fault and its loading history. Because the type and distribution of structures in the fault zone influence the strength of a fault and its effect on the movement of fluids in the subsurface, our results predict systematic variations in these parameters during fault evolution.
GIS as an aid to visualizing and mapping geology and rock properties in regions of subtle topography
Kevin Belt, Environmental Science Graduate Program, Oklahoma State University, Stillwater, Oklahoma 74078, USA, and Stanley T. Paxton, School of Geology, Oklahoma State University, Stillwater, Oklahoma 74078, USA. Pages 149-160.
Keywords: GIS, topography, sedimentology, stratigraphy, Oklahoma, Permian.
With the prospect of global warming and increased instances of drought, much preventative benefit could be derived from better regional documentation of the geological controls on surface erosion and topography. The need for this documentation is particularly acute in regions with a historical record of drought and erosion, such as occurred in the U.S Southwest and high plains in the 1930s. Toward this end, we evaluated relationships between bedrock geology and topography in north-central Oklahoma through the use of Geographic Information Systems (GIS). Quantifying the fabric of a landscape has always been a challenge because of the large number of measurements required to describe the topography. GIS helps to expedite the process with digital data sets that contain a high density of measurements over large geographic areas. This high data density is particularly valuable in areas with subtle topographic relief. Maps, produced from our analysis, reveal that subtle topographic relief in north-central Oklahoma is controlled by (1) bedrock types and (2) the presence of previously unrecognized regional fracture sets.
Physical weathering in arid landscapes due to diurnal variation in the direction of solar heating
L.D. McFadden, Department of Earth and Planetary Science, University of New Mexico, Albuquerque, New Mexico 87131, USA, et al. Pages 161-173.
Keywords: physical weathering, desert soils, fractures, cracks, desert pavement, thermal stresses.
It is common knowledge that solid materials sitting on the surface of the earth physically break down over time into smaller and smaller pieces. For example, rocks become sediment, asphalt and concrete pavements crack, and monuments and buildings inevitably disintegrate. In this paper, we present data supporting a new hypothesis for how rocks crack: when factors such as preexisting weaknesses are excluded, an anomalously large fraction of cracks on the surfaces of exposed boulders in the southwestern deserts of the United States are aligned in an approximately north-south orientation. We propose that these north-south "meridional" cracks are caused by stresses produced by the recurrent east-west heating and cooling of the rock by the sun as it crosses the sky. Our dataset of crack orientations (n = ~700) provides convincing evidence for a new basic hypothesis that links physical weathering at Earth's surface with changing diurnal solar heating. In addition to individual rocks exposed on desert surfaces, this mechanism of cracking is potentially significant in other climates and on other planets, as well as for man-made structures and materials.
Field measurements of incision rates following bedrock exposure: Implications for process controls on the long profiles of valleys cut by rivers and debris flows
Jonathan D. Stock, Department of Earth and Planetary Science, University of California, Berkeley, California, USA, et al. Pages 174-194.
Keywords: geomorphology, erosion, neotectonics, rivers, weathering.
Stock and others measured extreme rates of river incision into rock over the course of a decade in Pacific Rim rivers. In these sites, rivers are now cutting into bedrock at cm/year rates that approach those of horizontal plate motion, or fingernail growth. They argue that although these rates depend on the strength of the rock in a predictable way, their occurrence results from human disturbances to the landscape, including logging and engineering. If these rates continue, they could convert some floodplains into small gorges, removing valuable fish and other wildlife habitat.
Spatial and temporal patterns in fluvial recovery following volcanic eruptions: Channel response to basin-wide sediment loading at Mount Pinatubo, Philippines
Karen B. Gran and David R. Montgomery, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195, USA. Pages 195-211.
Keywords: Pinatubo, sediment transport, sediment supply, fluvial, volcanoes.
One of the largest eruptions of the twentieth century occurred in June 1991 at Mount Pinatubo, Philippines. The eruption dramatically affected the landscape, filling some river valleys with as much as 200 meters of loose sand and gravel. While some rivers have recovered, others remain choked with sediment more than a decade following the eruption. Our research follows the recovery of these rivers from wide, multi-channel, and sediment-laden to narrow, single-channel rivers capable of sustaining aquatic ecosystems of pre-eruption complexity. The time-scale for recovery depends not just on the amount of eruptive sediment, but also on where that sediment is stored, whether on hillslopes, in valleys, or in large terraces formed during the eruption or in post-eruption lahar flows.
The influence of riparian vegetation on stream width, Eastern Pennsylvania, USA
Nicholas E. Allmendinger, Department of Geosciences and Natural Resource Management, Western Carolina University, Cullowhee, North Carolina 28723, USA, et al. Pages 228-243.
Keywords: riparian vegetation, floodplains, bank erosion, hydraulic geometry, width.
The goal of our paper is to explain why streams with forested riparian zones are wider than streams with non-forested riparian zones. At all of our sites, curving channels migrate laterally, with bank erosion occurring at the outside bank and compensating sediment deposition occurring at the inside bank. At non-forested sites, rates of lateral migration are high. Bank sediments are easily eroded, and dense grasses on active floodplains promote deposition. At forested sites, rates of lateral migration are low. Dense roots create banks that are difficult to erode, and the low density of grasses on the inside banks inhibits deposition. Previous observations in the literature suggest that width is inversely correlation with the resistance of banks to erosion. Our data suggest the opposite: channels are wider in forested sites where banks are difficult to erode. This paradox is resolved when the influence of grasses on inside bank deposition is included: channels are narrow where deposition is enhanced by grasses, and channels are wide where few grasses are available to promote deposition. Our results suggest that depositional and erosional processes must be considered together to explain the widths of alluvial streams.
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