Emerson Palmer

Joshua Tree National Park is located in the Colorado and Mojave Deserts of southeastern California. Data from granitic samples of Creataceous and Jurassic age were collected from the Palms and White Tank Plutons and combined with data from previously collected samples. Satellite imagery shows possible geochemical zoning in all the plutons within the area. Color index data was collected in the field and geographically correlated to see if this zoning could be recognized in hand sample. This being the case, fresh samples were then collected, made into thin sections, and geochemically analyzed using X-ray Flourescence. The Palms pluton shows geochemical zonation that can indeed be observed remotely as well as in hand sample. The White Tank pluton has been found to be geochemically zoned, and can now be divided into two to three distinct plutons.; the White Tank can no long e be designated as a single pluton.

Kenny Brown

This study aims to examine the genetic relationship between felsic and voluminous basaltic lavas in mid-ocean ridge settings. Salton Sea rhyolites erupted through sediments blanketing the intersection of the East Pacific Rise and the San Andreas fault zone. The Torfajökull volcanic complex is located near the Mid-Atlantic Ridge, at the intersection of the active East Rift Zone and the Southeastern Zone. Chemical analyses reveal that the samples include rhyolite, dacite, and trachydacite. Torfajökull samples have 63-71% wt. silica, high amounts of alumina (13-15% wt.) and low phosphorous (<0.1% wt.) and Sr (80-120 ppm). Salton Sea rhyolite is comparatively silica rich (74%), has lower alumina and Sr, and higher K/Na. Loss on ignition to 700C indicates that all dacites and rhyolites contain <0.4% wt. H2O.

     The Salton Sea rhyolite is a series of five < 1 km3 domes. Zircon Th-U ages are predominantly 17 ka, identical to a ~16 ka K-Ar whole rock age (Muffler and White, 1969). Inherited zircons of Jurassic age suggest this rhyolite represents small volume partial melts of Jurassic granitic basement, or deltaic sediments derived from Jurassic basement rocks exposed on the northeastern shoulder of the Salton trough. Torfajökull is host to several postglacial (<10 ka) rhyolite eruptions; we focused on three flows from the Domadalshraun vent, which erupted ~130 km3 of dacite and rhyolite between 2 and 8 ka (McGarvie, 1985). Th-U ages of zircon from two samples are entirely 22-60 ka, ca. 20-50 ka older than eruption ages; the earliest flow has relatively older zircons. The earliest flow is more homogeneous, felsic, and has more An4-7 phenocrysts and matrix grains, whereas later flows are more heterogeneous, mafic, have more calcic plagioclase phenocrysts, and include more abundant xenocrysts (An30-70) and mafic to intermediate inclusions. These observations suggest that the flows erupted from a large volume, thermally and compositionally zoned Domadalshraun magma chamber formed at least 60 ka.

Philip Chapman

Iceland is battling a soil erosion problem throughout the country due to a lack of significant vegetation cover, steep terrain, high rainfall, and frequent volcanic activity. The goal of this study was to estimate Iceland's soil erosion by calculating the revised universal soil loss equation (RUSLE) using a geographic information system (GIS). The results were compared to a 1998 field study of Iceland's soil erosion by the Agricultural Research Institute of Iceland and the Icelandic Soil Conservation Service. Remote sensing data were used to derive estimates for spatially distributed soil loss equation factors. Of these factors, two—the conservation practice and soil erodibility—were held constant for the entire country and thus not used. The cover management factor was computed using a 1 kilometer resolution Greenland and Iceland land cover map from 2000 published by the European Commission Joint Research Centre. The rainfall erosivity factor was estimated using annual rainfall averages from 1971-1993 that were obtained from the Icelandic Meteorology Office. These points were encoded onto a map of Iceland and projected into contours. The slope steepness and slope length factors were estimated using a digital elevation model (DEM) from the ETOPO2 Global 2' Elevation data set. The cover management, rainfall erosivity, and topography factors were converted to a raster projection so that maximum RUSLE factors could be calculated to display predicted areas of high erosion. The individual RUSLE factor predictions were overlain to predict Iceland's areas of maximum potential soil erosion. When compared to published field data, the GIS- generated RUSLE underpredicted the observed soil erosion for Iceland, indicating that the maximum factor combination is not driving soil erosion in Iceland. It appears that the land cover of Iceland is driving the soil erosion because the maximum potential soil erosion map for land cover appears to correlate well with the observed soil erosion.

Christy Carter

Andrew Wolf

Mike Smith

Codirector of the Big Horn Basin Collection Team and Site near Worland, Wyoming.  Current research focuses on developing a early Eocene mammalian taxonomic analysis for a field site in the Big Horn Basin, Wyoming.

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Nicole Fohey

Elizabeth A. Anker

This study relates a suite of basaltic dikes, the Angeles Crest dike swarm, in the San Gabriel Mountains in southern California to its mantle source. The dikes cut a 218 Ma pluton and are cut by 85 Ma dikes and a 75 Ma pluton. They may be contemporaneous with the Independence dike swarm, intruded around 148 Ma and extending over 600km from the east-central Sierra Nevada to the southern Mojave Desert. Hornblende is abundant in these dikes and petrography indicates that it is present as both an igneous mineral phase and a metamorphic alteration product, so the magma that generated these dikes was hydrous and the dikes have a more recent metamorphic overprint. Silica concentrations range from 44 to 52%; basalt types range from slightly undersaturated (in silica) alkali basalt to quartz tholeiite (slight silica oversaturation). 87Sr/86Sr ranges from 0.7046 to 0.7058, and 143Nd/144Nd ranges from 0.51244 to 0.51266. These rocks are isotopically and elementally enriched relative to the mid-ocean ridge basalts (MORB) that were subducting under this region in the Mesozoic, are depleted relative to Mesozoic batholithic rocks, and do not possess typical arc geochemical affinities. There is a weak positive correlation between 143Nd/144Nd and alkalis. Adding rocks of local crustal composition to increase alkali content depresses 143Nd/144Nd ratios and increases silica, a relationship not seen in these rocks. It may be possible to derive the Mesozoic pluton and dike signatures through periodic enrichment and depletion in slab-derived fluids of an enriched subcontinental lithosphere. This geochemical model requires that the mantle wedge in subduction zones does not convect.

Heather M. Barr

 with JEWETT, David G., U. S. Environmental Protection Agency, National Risk Management Research Laboratory, Subsurface Protection and Remediation Division, Ada, OK. 74820

This study examines the experimentally determined hydraulic conductivity values of porous media cores with conductivity values calculated using empirical equations based on grain size distribution. This work is part of a larger investigation of the heterogeneity of physical, chemical, and biological properties of subsurface porous media at a U.S. Department of Energy field site. A series of porous media cores were collected from a Quaternary shoreface sand deposit located near Mappsville, Virginia, on the Delmarva Peninsula. Falling-head permeameter tests were conducted to determine the experimental hydraulic conductivity values of the cores. Hydraulic conductivity values ranged from 7.90 x 10 -3 cm/s to 3.40 x 10 -2 cm/s. Sieve analyses also were conducted for the same cores and the weight of each grain size fraction was recorded and the mean grain size, median grain size, effective grain size, and sorting were calculated and graphed. The core samples vary from well-sorted, fine-grained sand to a moderately well sorted medium-grained sand. Hydraulic conductivity values were determined using the effective grain sizes and the empirical equations developed by: 1) Hazen, 2) Shepherd, and 3) the USBR. Results from the empirical equations were statistically compared to the experimental measurements. Sum of squares due to deviation (SSD) results indicate that the Shepherd method generates conductivity values closest to the experimental values (SSD = 9.30 x 10 -4), followed by the USBR method (SSD = 1.26 x 10 -2), and then the Hazen method (SSD = 1.03 -1). The Shepherd method provided the best results in this study, perhaps because it applies to a greater domain of sediment samples (grains size: silt to gravel; textural maturity: immature to mature) and thus allows for more variation in its coefficients ("C" and "j"). This study demonstrates that empirical hydraulic conductivity values calculated using grain size data can be used as surrogates for experimentally measured values. However, care must be exercised in selecting the appropriate empirical equation.

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Christin M. Blazek

Water control structures are used to control freshwater pulses into estuarine systems. The purpose of this study is to determine if water control structures influence channel morphology and sediment characteristics. A graphical representation has been been used to illustrate the channel morphology of four estuaries in the southwest Florida area: Henderson Creek, Faka-Union Canal, Blackwater RIver, and Cocohatchee Canal. Henderson, Faka-Union, and Cocahatchee easch are controlled by a different type of water control structure (WCS). Blackwater has no WCS and has been used as a control for comparison. In addition to WCS vs. no WCS, channel morphology has been compared above and below each WCS and during the wet and dry seasons. Sediment characteristics have been determined for each of the areas along transects and through time. These data are being used to show how each type of WCS affects the sedimentology within the channel as well as how channel morphology responds to the presence of a WCS.

Rosalice H. Buehrer

Fractional crystallization, an important magma evolutionary process, can be described in mathematical terms using the Rayleigh fractionation law which, coupled with geochemical data, allows for the development of petrogenetic models. Such a model is calculated for the Granite Mountains pluton, a Triassic pluton in southern California, demonstrating the extent to which fractional crystallization can be applied to the major and trace element data to predict the original composition of the parent magma. For a fractionation range between 0.1 and 0.3, the model supports fractional crystallization with negligible, if any, rock assimilation or magma mixing. This is consistent with a magma chamber located in a tectonic setting initially fairly inactive at the time of intrusion and cooling, and supports the prediction of a single magma source for the formation of the Granite Mountains rocks, to be tested by future isotope ratio studies.

Eric A. Flodin

with WOODEN, J.L., U.S. Geological Survey, Menlo Park CA, 94025

COLEMAN, D.S, Dept. of Earth Sciences, Boston University, Boston MA, 02215

The Early Proterozoic Baldwin Gneiss comprises a metamorphosed magmatic arc intrusive suite in the central San Bernardino Mountains of southern California, and represents the western limit of the Mojave crustal province. Protoliths of the intrusive suite range from tonalite and quartz monzodiorite to granite, with minor amphibolite and trondhjemite. Associated meta-sedimentary rocks include quartz wackes and impure quartzites. Peak metamorphic conditions reached middle-upper amphibolite facies. In the Baldwin Lake type area, U/Pb zircon geochronology yields intrusive ages of ~1780-1680 Ma. The gneisses strike NW with moderate (35&Mac176;-55&Mac176;) SW dips, and down-dip mineral lineations consistently trend WSW. Microscopic shear indicators, including asymmetrical d and s porphyroclasts, asymmetric pressure shadows, elongate recrystallized quartz ribbons, and s-c fabrics yield west-vergent shear with a lesser component of flattening.

Regional relationships suggest that Baldwin gneiss correlates with rock units now structurally above shelf metasediments of Joshua Tree terrane [Eagle Mountains assemblage of Powell (1993)]. If these correlations are confirmed by work in progress, these structural observations imply west-vergent juxtaposition of the margin of the Mojave province with an unidentified western craton between ca. 1680 and 1189 Ma.

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Natasha M. Kramer

with JACOBSON, C.E., Dept. of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011, cejac@iastate.edu

GROVE, M., Dept. of Earth and Space Sciences, University of California, Los Angeles, CA 90024

The Pelona, Orocopia and Rand schists are latest Cretaceous-Paleocene, high P/T ensimatic terranes that structurally underlie the Mesozoic arc in central and southern California. The tectonic setting of protolith deposition is poorly understood. Many workers have suggested correlation with either Franciscan trench sediments or Great Valley forearc turbidites, derived from an arc source. Mukasa and others (1984), however, dated a metadiorite in schist that they interpreted to require pre-Middle Jurassic deposition, and to exclude these lithostratigraphic correlations.

     We are testing these models using U-Pb ages of detrital zircons from a northwest-southeast transect of schist bodies at Blue Ridge, the Sierra Pelona and the Gavilan Hills. Pb/U ages range from 73 to 1750 Ma; 48-88% of grains are ~125 Ma. Precise 206Pb*/238U ages of this youngest population of grains indicate maximum depositional ages of 81 Ma at Blue Ridge, 76 Ma for Sierra Pelona, and 73 Ma for the Gavilan Hills. We conclude (1) schists were derived from the recently extinct Cretaceous arc, (2) underwent tectonic burial ~10-15 Ma later, during the Laramide orogeny, and (3) these terranes are younger than 80 Ma Rand Schist to the north, suggesting southward younging of Laramide orogenesis in the magmatic arc.

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Michael Potter

with, Michael, WOODEN, Joseph, L., STEWART, John H., and COLEMAN, Drew S.

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