GSA Bulletin highlights: New research posted Feb. 4-11, 2011

February 24, 2011

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Keywords: Silicic calderas, Quaternary, Iberian Peninsula, Tsuboi's technique, Colorado River, Gran Desierto dune field, ASTER. Eel River, LiDAR, Franciscan Complex, Spanish Pyrenees, lahars, Semeru volcano, East Java, Carboniferous, Permian, Maritimes Basin, Canada, Atlantic, Pigeon Point conglomerate, California, Noonday Formation, Death Valley, Permian Salado Formation, halotolerant bacteria, Ediacaran

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Silicic calderas in arc settings: Characteristics, distribution, and tectonic controls
Gwyneth R. Hughes and Gail A. Mahood, Stanford University, Dept. of Geological and Environmental Sciences, 450 Serra Mall, Stanford, California 94305-2115, USA. Published online 4 Feb. 2011; doi: 10.1130/B30232.1.

Silicic calderas result from the catastrophic collapse of a magma chamber roof following voluminous and explosive eruption of viscous, silica-rich magma. These types of volcanoes result from the eruption of large, silicic magma chambers in the upper crust. In this compilation study, Gwyneth R. Hughes and Gail A. Mahood of Stanford University examine active volcanic arcs in order to determine how the size and composition of 108 Quaternary silicic calderas relate to crustal and tectonic parameters. In addition, Hughes and Mahood tested whether the spatial distribution of silicic calderas with respect to the volcanic front is different from that of more typical arc volcanoes. They show that volcanic arcs associated with high convergence rates generally have more silicic calderas per 1000 km of arc, though exceptions exist, and that more large silicic eruptions are generally associated with thick, continental crust under local extension. Silicic calderas in continental settings also tend to be distributed over a wider region behind the volcanic front than are more typical arc volcanoes. This study was supported in part by a grant from the U.S. National Science Foundation.

Crustal structure from gravity signatures in the Iberian Peninsula
David Gómez-Ortiz et al., ESCET (Escuela Superior de Ciencias Experimentales y Tecnologia) Área de Geología, Universidad Rey Juan Carlos, 28933 Móstoles, Spain. Published online 4 Feb. 2011; doi: 10.1130/B30224.1,

Through two-dimensional filtering and spectral analysis of gravity data, David Gómez-Ortiz of the Universidad Rey Juan Carlos and colleagues infer the density structure of the Iberian Peninsula's lithosphere (western Europe). The group uses Tsuboi's technique (identical to the equivalent stratum theorem) to map the 3-D Moho interface and find that the characteristic feature of the 3-D Moho geometry is the presence of several lows associated with mountain ranges created by Alpine tectonics.

Eolian dynamics and sediment mixing in the Gran Desierto, Mexico, determined from thermal infrared spectroscopy and remote-sensing data
Stephen Scheidt et al., Dept. of Geology and Planetary Science, University of Pittsburgh, SRCC 200, 4107 O'Hara Street, Pittsburgh, Pennsylvania 15260, USA. Published online 4 Feb. 2011; doi: 10.1130/B30338.1.

The ancestral Colorado River is a major source of quartz-rich sand for the Gran Desierto dune field. Stephen Scheidt of the University of Pittsburgh and colleagues used ASTER thermal infrared remote-sensing laboratory thermal emission spectroscopy of sand samples to measure and validate the surface sand composition and show that feldspar-rich local sources have been previously underestimated as a major component of the dunes. Because of the extensive coverage of samples collected in the past several decades, the laboratory data were spatially interpolated and compared at same spatial scale as the remote-sensing data. This analysis inferred that sand transport occurs from feldspar-rich local sources into the dune field at its margins. The trends in the laboratory data compared well to the remote-sensing retrievals of quarts and feldspar abundance. According to previous studies, the quartz/feldspar ratio is useful for understanding sand provenance, allowing the origin of the Gran Desierto to be placed in better context with respect to other dune fields in the surrounding Mojave and Sonoran deserts. For example, the composition of dunes east of the Pinacate was shown to be higher in potassium feldspar, which is characteristic of other Mojave dune fields. South of the Pinacate, wind mixes quartz-rich sand from the west with feldspar-rich sand sourced from the east. The northern margin of the Desierto is similarly enriched in feldspar from alluvial fans, and the costal sand is influenced by carbonate-rich sand.

Sediment yield, spatial characteristics, and the long-term evolution of active earthflows determined from airborne LiDAR and historical aerial photographs, Eel River, California
Benjamin H. Mackey and Joshua J. Roering, Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Published online 4 Feb. 2011; doi: 10.1130/B30306.1.

Large, glacier-like slow-moving landslides, known as earthflows, dominate erosion and hillslope morphology across much of the Franciscan mélange rock in northern California. Despite their importance in this "melting ice-cream" landscape, the regional-scale erosion rates, spatial characteristics, and long-term evolution of earthflows remain poorly constrained. Benjamin H. Mackey and Joshua J. Roering of the University of Oregon document active earthflows across 220 square kilometers of the main stem Eel River using high resolution digital topography (LiDAR) and historical aerial photographs. By tracking trees growing on the surface of earthflows, they find that 122 earthflow features were active between 1944 and 2006, covering 7.3% of the study area. These active earthflows erode an order of magnitude faster than the catchment average and contribute half of the suspended sediment in the Eel River. Long-term earthflow activity is potentially cyclic, governed by the availability of readily mobilized source material on the upper slopes. This study was supported in part by a grant from the U.S. National Science Foundation.

Decoding downstream trends in stratigraphic grain size as a function of tectonic subsidence and sediment supply
Alexander C. Whittaker et al., Dept. of Earth Science and Engineering, Royal School of Mines, Imperial College, London SW7 2AZ, UK. Published online 4 Feb. 2011; doi: 10.1130/B30351.1.

Geologists have long been interested in the stratigraphic record because it acts as an archive of Earth's history. Unfortunately, getting quantitative information about changing climate or tectonic conditions from studying sedimentary rocks has proven quite difficult. This is because the record is time-averaged, and because reconstruction of the hydraulic processes that led to sediment deposition in the past is often impossible. Alexander C. Whittaker of Imperial College London and colleagues use variation in the size of sediment grains within stratigraphy to provide a new way of decoding the stratigraphic record for tectonic and climatic changes over time. Using a study area in the Spanish Pyrenees, Whittaker and colleagues show that the evolution of this mountain belt more than 30 million years ago led to dramatic changes in the size of sediment entering nearby sedimentary basins. They also demonstrate that the rate at which the grain size of the rocks becomes finer downstream varied significantly. The team uses a simple numerical model to decode these changes, and hence put numbers on tectonic rates and sediment discharges over time. The results are important because they show it is now possible to extract quantitative data on tectonics and climate from physical measurements of sedimentary grain size.

Defining conditions for bulking and debulking in lahars
E.E. Doyle et al., Joint Centre for Disaster Research, Massey University, P.O. Box 756, Wellington 6140, New Zealand. Published online 4 Feb. 2011; doi: 10.1130/B30227.1.

Lahars are among the most destructive and hazardous processes on volcanoes. They commonly begin as erosive, watery floods that entrain sediment and water, transforming into complex mass flows and increasing in volume and discharge by several times downstream, presenting an extreme hazard to low-lying regions. E.E. Doyle of Massey University and colleagues present quantifiable observations of bulking and debulking of rainfall-induced lahars at Semeru volcano, East Java. The use of two closely located instrument sites (510 meters apart) provides a unique and new perspective to better characterize the physical processes of such lahars. Doyle and colleagues concentrate on video footage of these lahars, and calculate flow velocities and volumes at both sites. This enables a quantification of bulking and debulking by lahars along a channel reach, and they define the conditions under which these different behaviors may occur. The results indicate that overall total event estimates of bulking may be misleading because event changes can be significant enough to increase the inundation and associated hazard considerably. These observations provide vital additional data and information for the continuing development and improvement of numerical models, which can be applied to hazard mitigation and analysis tools.

Fluvial response to paleo-equatorial climate fluctuations during the late Paleozoic ice age
Jonathan P. Allen et al., Dept. of Geosciences, 214 Bessey Hall, University of Nebraska, Lincoln, Nebraska 68588-0340, USA. Published online 11 Feb. 2011; doi: 10.1130/B30314.1.

Jonathan P. Allen of the University of Nebraska and colleagues describe strata deposited during the Carboniferous and Permian periods in the Maritimes Basin of Atlantic Canada. During this period of earth history, 300-330 million years ago, Atlantic Canada was located around the equator. The sediments that were deposited during this time period have been interpreted to represent predominantly continental settings. Rocks representing fluvial environments preserve a variety of structures that indicate the rivers experienced prolonged periods of low flow conditions punctuated by intense precipitation events in a strongly seasonal climatic setting. Stratigraphic packages that preserve strongly seasonal river deposits are concentrated into four discrete intervals which can be correlated across the 400-km studied area. This suggests that a coherent regional climate signal is recorded within the strata. These intervals are separated by deposits characteristic of humid and arid climate conditions, and the alternation between these different types of rivers implies several pronounced, long-term changes in precipitation and runoff occurred during the deposition of the studied strata. Furthermore, these intervals coincide with major periods of southern hemisphere glaciation, which suggests that glaciation had a profound and controlling effect on the paleotropical climate of the Atlantic Canada region.

Zircon U-Pb age of the Pescadero felsite: A Late Cretaceous igneous event in the forearc, west-central California Coast Ranges
W.G. Ernst et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA. Published online 11 Feb. 2011; doi: 10.1130/B30270.1.

W.G. Ernst of Stanford University and colleagues dated outcrops of a felsic lava exposed near Pesdacero, California, USA, as 86-90 million years old within a section of sedimentary strata otherwise devoid of such flows. After solidification, the volcanic rock was carried down a subduction zone where it recrystallized at depth, then was exhumed and partially eroded prior to accumulation of the 70-80-million-year-old Pigeon Point conglomerate. Much was happening geologically along this section of the North American continental margin at this time.

The Neoproterozoic Noonday Formation, Death Valley region, California
R. Petterson et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Published online 11 Feb. 2011; doi: 10.1130/B30281.1.

One of the most intense periods of global climate change occurred about 650 million years ago, when most or all of Earth became covered with ice. The recovery from this "icehouse" event to a very warm, "greenhouse" Earth occurred rapidly 635 million years ago, and is marked by a dramatic "rainout" of carbonate on the ocean floor, and a recovery of ocean chemistry to pre-glacial levels; e.g., as indicated by the abundance of the heavy isotope of carbon, C-13. Thus far, the only relatively complete archive of carbon isotopic ratios during this recovery has come from sections in Namibia, leaving some workers to question whether results there are applicable elsewhere on the globe. The data from the Noonday Formation presented by R. Petterson of the California Institute of Technology and colleagues show that the details of both stratigraphic architecture and carbon isotope ratios apparent in Namibia have a close counterpart in the Death Valley region of North America. The correlation reinforces the hypothesis that the measured isotopic variations reflect changes in chemistry of the global ocean, as opposed to post-depositional chemical modification or some other process. It also indicates that the age of deposition of the lower part of the Noonday Formation, previously constrained only to within several hundred million years, is precisely 635-632 million years. This study was supported in part by a grant from the U.S. National Science Foundation.

Synsedimentary dissolution pipes and the isolation of ancient bacteria and cellulose
Robert M. Holt and Dennis W. Powers, Dept. of Geology and Geological Engineering, University of Mississippi, Oxford, Mississippi 38677, USA. Published online 11 Feb. 2011; doi: 10.1130/B30197.1.

Coarse, clear halite from synsedimentary dissolution pipes in the Permian Salado Formation in southeastern New Mexico yielded viable halotolerant bacteria and well-preserved cellulose believed to be Permian in age. Robert M. Holt and Dennis W. Powers of the University of Mississippi show that geologic and hydrologic conditions have isolated these rocks since Permian time. Pipes were dissolved, most likely along cracks created by thermal contraction or desiccation and the boundaries of salt polygons (saucers), on exposed Salado Formation salt-pan surfaces down to the level of the water (brine) table. Macropores developed at the brine level in some horizons. As the water level rose, coarse halite cemented the open space. Millimeter-scale fluid inclusions trapped bacteria, which were probably in a spore state, as well as cellulose. Inclusion water from pipes in some cycles may have isotopic values reflecting Permian meteoric water, while other cycles may show evaporated Permian seawater.

The Shuram and subsequent Ediacaran carbon isotope excursions from southwest Laurentia, and implications for environmental stability during the metazoan radiation
Charles Verdel et al., Dept. of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Published online 11 Feb. 2011; doi: 10.1130/B30369.1.

The appearance of multicellular animals 540 million years ago is accompanied and preceded in the rock record by large variations in the carbon isotope composition of marine sediments. In recent years, two schools of thought have arisen to explain the origin of these isotopic "excursions." The first holds that they are a record of geochemical variability in ancient oceans, while the second holds that they were created during later periods of geochemical alteration. Charles Verdel of the California Institute of Technology and colleagues present new carbon and oxygen isotope data from Ediacaran sediments in the Death Valley region of eastern California and southern Nevada, and find that they match remarkably closely with previously studied stratigraphic sections of equivalent age in Oman and South Australia. Based on this finding, Verdel and colleagues conclude that the isotopic excursions are global in extent and are, indeed, related to geochemical variations in the ocean accompanying the evolution of multicellular animals. Furthermore, they find that the number of excursions has been underestimated in most previous studies, suggesting that the most profound period of evolution in Earth's history occurred during a time of extraordinary geochemical fluctuations in the world's oceans.

Geological Society of America

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