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Lithofacies control in detrital zircon provenance studies: Insights from the Cretaceous Methow basin, southern Canadian Cordillera Kathleen DeGraaff-Surpless, Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA, et al. Pages 899–915.
Keywords: provenance, zircon, Methow Basin, Canadian Cordillera, Baja BC hypothesis, Kolmogorov-Smirnov test.
Age distributions of sand-sized zircon grains from samples of sedimentary strata can provide extraordinarily detailed information about the evolution, tectonic history, and paleogeography of a sedimentary basin. By identifying a spectrum of grain ages that approximates the age distribution of geologic units within a source region, zircon geochronology can identify specific source regions for a basin, and may help characterize spatial and temporal variations in sediment supply to a basin. This investigation demonstrates that the degree of homogenization of zircon age distributions is directly related to the depositional environment, and that details of basin evolution not evident from sediment composition may be recorded by changes in sediment age spectra. In addition, the composite age spectrum for the Methow basin indicates that the basin most likely developed in close proximity to the southern Canadian Cordillera, providing evidence against large-scale translation of the Methow basin from a southern source region.
Stratigraphic investigations of carbon isotope anomalies and Neoproterozoic ice ages in Death Valley, California Frank A. Corsetti, Department of Earth Sciences, University of Southern California, Los Angeles, California, 90089-0740, USA, and Alan Jay Kaufman, Department of Geology, University of Maryland, College Park, Maryland 20742-4211, USA. Pages 916–932.
Keywords: chemostratigraphy, Death Valley, Earth history, glaciation, Neoproterozoic, stratigraphy, carbon isotopes.
Death Valley is a geologic wonderland visited by many geologists and non-geologists every year. Given the extreme temperatures during most of the year (summer temps exceed 125 ºF), it is easy to overlook the fact that ancient glacial deposits indicative of one of the coldest times in Earth history (snowball Earth) are underfoot. In this article, we document glacial deposits and geochemical markers deposited during snowball Earth times (ca. 750–600 million years ago), and discuss how the Death Valley data may (or may not) fit with current snowball Earth hypotheses.
Trace element chemistry of individual apatite phenocrysts as a tool for fingerprinting altered volcanic ash beds: Assessing interbed and intrabed variation at local and regional scales George H. Shaw, Geology Department, Union College, Schenectady, New York 12308, USA. Pages 933–942.
Keywords: K-bentonite, Gaspé, Tioga, correlation, apatite, rare earth elements.
This article reports chemical analyses of single microscopic crystals of the mineral apatite found in layers of altered volcanic ash. Apatite crystals contain small amounts of rare earth elements, which differ in concentration depending upon the chemistry of the magma blown from a volcano during an explosive eruption. The differences in chemistry may be used to "fingerprint" the ash layer, allowing it to be traced from place to place. Because different layers contain different concentrations of these trace elements, they can be distinguished from one another. One of the major points in this study is to demonstrate that where multiple layers occur in a given location (i.e., where there were a number of eruptions through a period of time) the chemical signatures are different enough to tell them apart. A second major point is the demonstration that the trace element concentrations in apatites in a single layer are the same over distances of a few hundred kilometers. A critical factor in this study is the ability to analyze several (~20) crystals from each layer in order to be able to calculate statistical properties for demonstrating similarity or difference.
Hydrothermal and tectonic activity in northern Yellowstone Lake, Wyoming Samuel Y. Johnson, U.S. Geological Survey, MS 966, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA, et al. Pages 954–971.
Keywords: Yellowstone Lake, seismic reflection profiles, hydrothermal processes, explosive eruptions, extension faults, earthquake hazards.
Seismic reflection data from northern Yellowstone Lake document the distribution, size, geometry, and relative age of geothermal features, including the world's largest known hydrothermal explosion craters, asymmetric crater ejecta, and vents and domes of variable size and concentration. Recognition of submerged shoreline terraces completes a postglacial record of "heavy breathing" of the Yellowstone caldera, an essential context for understanding past and future caldera behavior. Documentation of the Lake Hotel fault zone provides important information for local to regional earthquake hazard assessment.
Geochronology of the Mesoproterozoic State Farm gneiss and associated Neoproterozoic granitoids, Goochland terrane, Virginia Brent E. Owens, Department of Geology, College of William and Mary, Williamsburg, Virginia 23187, USA, and Robert D. Tucker, Department of Earth and Planetary Sciences, Washington University, St. Louis, Missouri 63130, USA. Pages 972–982.
Keywords: geochronology, State Farm gneiss, Goochland terrane, Neoproterozoic, Mesoproterozoic.
This paper focuses on the ages and chemical characteristics of two main types of rocks in the Goochland Terrane, an isolated block of Precambrian continental crust in the east-central part of Virginia. We present the first precise age determinations (by radiometric methods) for these rocks, as well as the first report of their major and trace element compositions. The State Farm gneiss forms the core area of the terrane, and its age ranges from ca. 1023 to ca. 1046 million years old. These ages are similar to other parts of ancient North America, and the Goochland Terrane may thus represent an isolated fragment of the continent. Its chemical composition indicates that it was predominantly granitic prior to metamorphism. We also document the presence of a series of younger granitic plutons within the State Farm gneiss. These rocks intruded the gneiss during the time interval from ca. 588 to 654 million years ago. They have a distinctive chemical composition that suggests they were emplaced during continental rifting at this time. These rocks therefore represent intrusion of magmas associated with late Precambrian rifting of eastern North America. This rifting ultimately led to the formation of the Iapetus Ocean.
Paleogeography of Miocene Western Amazonia: Isotopic composition of molluscan shells constrains the influence of marine incursions H.B. Vonhof, Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands, et al. Pages 983–993.
Keywords: Miocene, Amazonas, paleosalinity, paleohydrology, isotope geochemistry.
In this manuscript the strontium isotope composition of fossil shells from the heart of the Amazon Basin is used to unravel the history of marine incursions in Miocene Western Amazonia. The data show that, for most of the middle–late Miocene, Western Amazonia was a freshwater environment, dominantly fed by Andean runoff, with occasional contribution of cratonic freshwater. Evidence for a marine incursion is documented in a single (narrow) stratigraphic interval only, in which maximum aquatic salinty ranges are calculated to have been oligohaline to mesohaline. Although these results show that some marine influence was evident, they are clearly at odds with the previously postulated existence of a middle Miocene seaway across the South American continent.
Neogene glacial record from the Sirius Group of the Shackleton Glacier region, central Transantarctic Mountains, Antarctica Michael J. Hambrey, Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK, et al. Pages 994–1015.
Keywords: Neogene, glacial record, Sirius Group, Antarctica, Shackleton Glacier, Transantarctic Mountains.
Unraveling the early glacial history of Antarctica is crucial if we are to understand what will happen to the present ice sheet as global temperatures rise. This record is best preserved on the continental shelf and in a number of inland sites. A paper by Hambrey and others concerning some of the latter sites demonstrates how the past nature of the Antarctic ice sheet can be inferred from detailed field investigations of preserved glacial sediments. High in the Transantarctic Mountains of Antarctica in latitude 85ºS, ancient glacial deposits are preserved along the flanks of a major outlet from the East Antarctic ice sheet, the Shackleton Glacier. These deposits are critical for examining the nature of former ice flow from the interior to the coast, and indicate a quite different regime from that of the present day. Their age is uncertain, but they predate the onset of a stable ice sheet ca. 3 million years ago. Resting on the most extensive ancient glacially grooved and striated pavement so far reported from the continent are up to 100 m of glacial sediment. These deposits indicate a dynamic ice sheet associated with much warmer and wetter conditions than those of today, with abundant evidence of river and lake sedimentation, as well as the growth of woody vegetation. Even the topography was different--definitely flatter and probably much lower, so ice flow was much more uniform in this region, and preceded major faulting and uplift.
Neogene volcanism at the front of the central Mexican Volcanic Belt: Basaltic andesites to dacites, with contemporaneous shoshonites and high-TiO2 lava Discussion: Ignacio S. Torres-Alvarado and Surendra P. Verma, Centro de Investigación en Energía, UNAM, Privada Xochicalco s/n, Centro, Temixco, 62580 Morelos, Mexico.
Keywords: Mexican Volcanic Belt, Central American volcanic arc, OIB-type volcanism, subduction, rifts.
Reply: Dawnika L. Blatter, Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA, et al. Pages 1020-1024.
Keywords: andesite, basaltic andesite, dacite, Mexican Volcanic Belt, shoshonite, central Mexico.
Neogene volcanism in the Zitacuaro-Valle de Bravo region of the central Mexican Volcanic Belt includes three distinct types of lava. Arc-front samples are dominated by basaltic andesites and shoshonites with high concentrations of fluid-mobile elements and pre-eruptive water. Toward the back of the arc, samples are predominantly dry, high-TiO2 lavas. Along the broad central axis of the arc are mainly typical calc-alkaline andesites and dacites.
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