Boulder, Colo., USA Two new themes: "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California" and "The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes " add new content to Geosphere's already dynamic lineup. Also in the April 2013 issue: themes "ANDRILL McMurdo Ice Shelf (MIS) and Southern McMurdo Sound (SMS) Drilling Projects" and "Exploring the Deep Sea and Beyond," plus three individual articles not associated with a specific theme.
1. Theme introduction;
2. Theme introduction;
3. Alteration minerals, assemblages, and textures in a McMurdo Ice Shelf core;
4. Submarine landslides;
5. Waterford Formation, Karoo Basin, South Africa;
6. Cretaceous carbon isotope fluctuations in northern Japan; and
7. Application of terrestrial laser scanning for measuring Earth-surface features.
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Introduction: Geodynamics and consequences of lithospheric removal in the Sierra Nevada, California
Craig H. Jones and Jason B. Saleeby, Department of Geological Sciences and CIRES, University of Colorado, Boulder, Colorado 80309, USA. Issue: April 2013. Originally posted online 6 March 2013; open access at http://dx.doi.org/10.1130/GES00907.1. Themed issue: "Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California."
The work addressing the issues in this new themed issue (Geodynamics and Consequences of Lithospheric Removal in the Sierra Nevada, California) falls broadly into four categories: (1) geophysical and geochemical work constraining the modern lithospheric structure of the region; (2) geological, petrological, and geochemical work constraining the lithospheric structure prior to about 12 million years ago; (3) geological, sedimentological, and geochronological work bearing on the evolution of uplift and subsidence; and (4) numerical and analytical modeling seeking to relate the geological observations to deep-seated processes.
Introduction: The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes
Myron G. Best (Eric H. Christiansen, corresponding author) et al., Dept. of Geological Sciences, Brigham Young University, Provo, Utah 84602, USA. Issue: April 2013. Originally posted online 6 March 2013; open access at http://dx.doi.org/10.1130/GES00870.1. Themed issue: "The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes."
Thirty-six to eighteen million years ago, some 200 colossal explosive volcanic eruptions broadcast tens of thousands of cubic miles of volcanic ash across what is now western Utah, Nevada, and eastern California. This unusually intense burst of explosive eruptions or "ignimbrite flareup" (Latin ignis, meaning fire, and nimbus, meaning cloud) included some of the largest explosive eruptions known on Earth. At least thirty of the eruptions came from "super volcanoes." where more than 250 cubic miles of magma were ejected. Individual layers of ash emplaced in the course of these eruptions are found as much as 100 miles from their sources and are as much as 300 feet thick. As a result of geologically instantaneous withdrawals of such huge volumes of magma from the crust, collapse depressions, or calderas, formed over the evacuated magma chambers. The largest was 35 miles across and 3 miles deep -- as big as the better known Yellowstone caldera. These calderas are no longer evident in the landscape because of obliteration by millions of years of faulting and erosion, but the vast ignimbrite layers still reveal the enormity of these events.
Alteration of volcanic deposits in the ANDRILL AND-1B core: Influence of paleodeposition, eruptive style, and magmatic composition
Alessio Di Roberto et al., Istituto Nazionale di Geofisica e Vulcanologia, sezione di Pisa, Via della Faggiola, 32-56126 Pisa, Italy. Issue: April 2013. Originally posted online 6 March 2013; http://dx.doi.org/10.1130/GES00812.1 Themed issue: "The ANDRILL McMurdo Ice Shelf (MIS) AND Southern McMurdo Sound (SMS) Drilling Projects"
Alessio Di Roberto and colleagues studied alteration minerals, assemblages, and textures in a 175-m-thick volcanic sequence found between 759.32 and 584.19 m below seafloor within the 1285-m-long ANDRILL (Antarctic Geological Drilling project) McMurdo Ice Shelf core (MIS AND-1B). They identified three main alteration zones through the application of different analytical methods (optical and scanning electron microscopy, electron microprobe, and X-ray diffraction). Alteration zoning is guided by the texture of the volcanic deposits, which is in turn determined by the eruptive style, transport mechanisms, and paleodepositional conditions. In particular, alteration reflects the evolution of paleodepositional conditions from submarine or shallow water to subaerial due to the growth of a nearby volcanic edifice. The general alteration trend is also influenced by the contribution of volcanogenic sediments derived from the reworking of silica-rich pyroclasts from earlier volcanic activity.
Estimation of submarine mass failure probability from a sequence of deposits with age dates
Eric L. Geist et al., U.S. Geological Survey, 345 Middlefield Road, MS 999, Menlo Park, California 94025, USA. Issue: April 2013. Originally posted online 18 March 2013; http://dx.doi.org/10.1130/GES00829.1. Themed issue: Exploring the Deep Sea and Beyond.
Submarine landslides present a significant hazard to communications cables, offshore energy development, and to coastal regions via the generation of tsunamis. One of the critical components of assessing hazards posed by submarine landslides is determining the probability of occurrence. For an empirical determination of probability, there are very few places in the world where a sequence of submarine landslides have been individually dated. However, in 2005, cores from two Integrated Ocean Drilling Project (IODP) sites in the Ursa Basin, northern Gulf of Mexico penetrated through a sequence of deposits left by landslides. This study develops a methodology to estimate the probability of submarine landslides from a sequence of dated deposits, such as those found in the Ursa Basin.
Contrast in the process response of stacked clinothems to the shelf-slope rollover
George E.D. Jones et al., Stratigraphy Group, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L69 3GP, UK. Issue: April 2013. Originally posted online 6 March 2013; http://dx.doi.org/10.1130/GES00796.1.
The shelf edge rollover represents a critical zone in understanding the timing and processes involved in the transfer of sediment and organic carbon from continents to oceans. Subsurface datasets provide exceptional understanding of the long term development of a margin, and modern basin margin studies are invaluable in studying both down dip and across strike variability. However, both suffer from a lack of high-resolution stratigraphic and sedimentological detail that permit process-based interpretations to be made. This gap in data coverage can be filled by the use of large-scale ("seismic-scale") outcrop based studies where the transition from the continental shelf to continental slope can be identified and characterized. This study concentrates on the different process response of deltas to shelf edge rollover zones in two successive exhumed clinothems from the lower Waterford Formation, Karoo Basin, South Africa. Their depositional architecture demonstrates that the delivery of sediment to deep water settings is governed by parameters other than the presence and proximity of a fluvial supply point, which is heavily advocated by current models for shelf construction.
Carbon isotope stratigraphy of terrestrial organic matter for the Turonian (Upper Cretaceous) in northern Japan: implications for ocean-atmosphere delta-13C trends during mid-Cretaceous climatic optimum
Go-Ichiro Uramoto et al., Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Monobe B200, Nankoku, Kochi 169-8502, Japan. Issue: April 2013. Originally posted online 18 March 2013; http://dx.doi.org/10.1130/GES00835.1.
Carbon isotope data of terrestrial organic matter obtained from the Cretaceous successions in northern Japan elucidate detailed carbon isotope fluctuations for the Turonian (Cretaceous) supergreenhouse in this region of East Asia. Correlation of terrestrial carbon isotope curve within northern Japan reveals three positive carbon isotope events in the Turonian. These carbon isotope events are correlated with previously documented marine carbon isotope events in Europe. Our correlations documented the marked carbon isotope variations occurred near simultaneously in the ocean-atmosphere-biosphere system in the Turonian. In addition, global correlation of Turonian marine and terrestrial carbon isotope events identifies changes in the isotopic difference between terrestrial and marine carbon isotope data, and are interpreted to reflect changes in atmospheric pCO2 levels, and climate driven stresses of humidity and soil processes. In earlier stages of Turonian, the isotopic differences are increased. Elevated atmospheric pCO2, and increased humidity and soil processes in enhanced greenhouse conditions during mid-Turonian are interpreted to enlarge the isotopic differences. In later stages of Turonian, the isotopic difference is constant level, and the lowering of atmospheric pCO2 or decrease of climate stress which related to the diverse paleoclimatic cooling is interpreted to have restored the ocean-atmosphere carbon isotope trends.
Quantifying the effects of terrestrial laser scanner (TLS) settings and survey configuration on land surface roughness measurement
Owen W. Brown, Department of Geography, University of Lethbridge, Lethbridge, Alberta T1K 3M4, Canada; and Chris H. Hugenholtz, Department of Geography, University of Calgary, Calgary, Alberta T2N 1N4, Canada. Issue: April 2013. Originally posted online 18 March 2013; http://dx.doi.org/10.1130/GES00809.1.
The application of terrestrial laser scanning (TLS) for measuring Earth surface features is increasing. However, TLS surveys require users to choose and specify certain properties of the scan (i.e., resolution, height, distance, number of scan positions), often with limited understanding of how these properties affect the accuracy of the data. Owen W. Brown and Chris H. Hugenholtz present results from an experiment that quantifies the effects of different scan settings and survey configurations on the measurement of centimeter-scale surface roughness. The main goal is to provide quantitative evidence to help guide and optimize field-based surface roughness measurements involving TLS data. The experiment involved an array of artificial roughness elements placed on an asphalt surface, similar to the approach of using inverted buckets in boundary layer experiments to simulate a rocky or sparsely vegetated surface with smooth interspaces. The independent variables consisted of laser point spacing, number of scan positions, and the height and distance of the scanner relative to the roughness array. The dependent variables were roughness element height, data occlusion, relative vertical accuracy, the root mean square height of the cup array, and the relative roughness of the asphalt surface. Two roughness patterns were tested, isotropic and anisotropic. Results show that when the laser point spacing was greater than the size of the individual roughness elements, their calculated height was between 32% and 73% below their actual height, but with a smaller spacing the calculated height was either equivalent to their actual height or only slight lower. Therefore, before a TLS survey is undertaken, manual measurements of roughness elements should be used to determine the size of the smallest roughness elements of interest, thus guiding the selection of laser point spacing.
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