||7 January 2008
GSA Release No. 08-02
January / February Media Highlights:
The Geological Society of America Bulletin
Boulder, CO, USA - Geology topics of interest include exceptional microfossil preservation from sediments of coastal Tanzania and Eocene-Oligocene global climate and sea-level changes.
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- A Paleogene calcareous microfossil Konservat-Lagerstätte from the Kilwa Group of coastal Tanzania
Paul R. Bown et al., Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT, UK. Pages 3-12.
- Bown et al.'s paper describes the exceptional, and arguably unique, microfossil preservation that they have found in Paleogene (32 to 57 million years old) sediments of coastal Tanzania. These clay-rich sediments were targeted as they contain glassy-looking planktonic foraminifera (fossil shells of zooplankton), which despite their great age, are comparable in appearance to well-preserved modern shells.
- Keywords: calcareous nannofossils, foraminifera, preservation, Lagerstätte, Paleogene, diversity.
- Eocene-Oligocene global climate and sea-level changes: St. Stephens Quarry, Alabama
Kenneth G. Miller et al., Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08854, USA. Pages 34-53.
- A major climate change occurred spanning the end of the Eocene Epoch (circa 34 million years ago). Miller et al.'s study of St. Stephens Quarry, Alabama, and New Jersey coreholes establishes the relationships among ice volume, cooling, oxygen isotopes, and continental margin deposition across this critical climate transition. A latest Eocene cooling even (circa 33.8 million years ago) was followed by an earliest Oligocene (circa 33.5 million years ago) ice-volume and cooling even that lowered sea-level and formed a major erosional surface (sequence boundary) during the early stages of the global sea-level fall.
- Keywords: Eocene-Oligocene, sea level, climate, ice volume, Alabama, sequence stratigraphy, icehouse, greenhouse.
- Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States
M. Elliot Smith et al., Department of Geology, Sonoma State University, Rohnert Park, California 94928, USA. Pages 54-84.
- Smith et al.'s study uses high-precision 40Ar/39Ar geochronology of volcanic ash layers to reconstruct the history of a series of ancient lakes that formed between the ranges of the Rocky Mountains while they were being formed. The results indicate that lakes occupied the central Rocky Mountains for 8 million years, from 53 to 45 million years ago, during a time of elevated global temperatures. The strata that were deposited from these lakes represent the famous Green River Formation, and contain a wealth of fauna, flora, and chemical information that can now be directly compared to the marine record and other continental deposits. Local geography, tectonics, and volcanism played important roles in determining the biological and chemical conditions in the Green River Formation lakes.
- Keywords: Ar-Ar, Absaroka, Uinta Basin, Piceance Creek Basin, land-mammal ages, lake-type, Laramide.
- Watershed reconstruction of a Paleocene-Eocene lake basin using Sr isotopes in carbonate rocks
E.H. Gierlowski-Kordesch et al., Geological Sciences, Ohio University, 316 Clippinger Laboratories, Athens, Ohio 45701-2979, USA. Pages 85-95.
- The watershed of a 60 million-year-old lake system in Utah is reconstructed using geochemical methods that, today, are normally used to trace surface and underground water movement. Strontium isotopes from waters draining the source rocks in the projected ancient uplands are traced to the limestones representing the ancient lake sediments. Two major river inputs were identified for the lakes of the Flagstaff Formation located in the central portion of the Wasatch Plateau of central Utah. The lakes during Flagstaff time alternated from fresh to saline then back to fresh, depending on local tectonic movements restricting flow out of the lake basin.
- Keywords: lacustrine, Flagstaff Formation, nonmarine carbonates, paleohydrology, Sevier Belt, Utah.
- Remote sensing and geochemistry for detecting hydrocarbon microseepages
Shuhab D. Khan and Sarah Jacobson, University of Houston, Geosciences, 4800 Calhoun Rd., 312 S&R 1, Houston, Texas 77204-5007, USA. Pages 96-105.
- Khan and Jacobson present the idea that alteration zones caused by hydrocarbon-induced microseepages can be identified remotely. They have used Hyperion hyperspectral remote sensing data to map the alteration zones and confirmed their results by field, mineralogical, geochemical and carbon isotope studies. This work can serve as a beginning to exploration methods used in detecting hydrocarbon microseepages.
- Keywords: hyperspectral remote sensing, hydrocarbon microseepages, surface Alteration, spectroscopy, Patrick Draw, carbon isotope.
- Geochemical evidence for airborne dust additions to soils in Channel Islands National Park, California
Daniel R. Muhs et al., U.S. Geological Survey, MS 980, Box 25046, Federal Center, Denver, Colorado 80225, USA. Pages 106-126.
- There is an increasing awareness that dust plays important roles in climate change, biogeochemical cycles, nutrient supply to ecosystems, and soil formation. In Channel Islands National Park, California, soils are clay-rich Vertisols or Alfisols and Mollisols with vertic properties. The soils are overlain by silt-rich mantles that contrast sharply with the underlying clay-rich horizons. Silt mantles contain minerals that are rare or absent in the volcanic rocks that dominate these islands. Immobile trace elements (Sc-Th-La and Ta-Nd-Cr) and rare-earth elements show that the basalt and andesite on the islands have a composition intermediate between upper-continental crust and oceanic crust. In contrast, the silt fractions and, to a lesser extent, clay fractions of the silt mantle have compositions closer to average upper-continental crust and very similar to Mojave Desert dust. Island shelves, exposed during the last glacial period, could have provided a source of eolian sediment for the silt mantles, but this is not supported by mineralogical data. Muhs et al. hypothesize that a more likely source for the silt-rich mantles is airborne dust from mainland California and Baja California, either from the Mojave Desert or from the continental shelf during glacial low stands of sea. Although average winds are from the northwest in coastal California, easterly winds occur numerous times of the year when "Santa Ana" conditions prevail, caused by a high-pressure cell centered over the Great Basin. The eolian silt mantles constitute an important medium of plant growth and provide evidence that abundant eolian silt and clay may be delivered to the eastern Pacific Ocean from inland desert sources.
- Keywords: Silt mantles, dust, eolian, soils, marine terraces, volcanic rocks, California, Channel Islands National Park, Santa Cruz Island, Santa Barbara Island, Anacapa Island, mineralogy, geochemistry, trace elements, rare earths, Santa Ana winds.
- Where do floodplains begin? The role of total stream power and longitudinal profile form on floodplain initiation processes
Vikrant Jain et al., University of Texas at San Antonio, Department of Earth & Environmental Sciences, One UTSA Circle, San Antonio, Texas 78249, USA. Pages 127-141.
- Analysis of landscape patterns and transitions in landscape processes is a subject of ongoing analysis in geomorphology. Jain et al.'s paper examines one key transition in fluvial systems — the initiation of floodplain formation. This transition reflects a shift from dominantly erosion and sediment transport to dominantly deposition by processes operating outside a channel. The authors examine this relationship on the basis of available energy in the system (total stream power) and the form of longitudinal profiles. The total stream power plot derived along longitudinal profiles represented by a second order exponential curve has a bimodal pattern. In most cases, floodplains begin to form in a "transition zone" characterized by a "trough area" within the bimodal stream power distribution. Indirect controls such as basin geology and accommodation space also influence the initiation and pattern of floodplains. Systematic quantifiable relationships provide powerful tools that aid prediction of river character and behavior. The methods developed in this study provide a readily transferable methodological framework with which to assess the controls under which differing river processes operate.
- Keywords: total stream power; longitudinal profile; floodplain formation; bedrock channels; upper Hunter catchment, Australia.
- Coupling of rock uplift and river incision in the Namche Barwa-Gyala Peri massif, Tibet
Noah J. Finnegan et al., Cornell University, Earth and Atmospheric Sciences Department, Snee Hall, Ithaca, New York 14853, USA. Pages 142-155.
- Can a river erode enough material to deform Earth's crust? Yes, suggests recent analysis of topographic, satellite, and geochemical data from the Yarlung Tsangpo River gorge in southeastern Tibet. Finnegan et al. identify a strong correlation between spatial patterns in contemporary erosion rates and rock uplift rates over the geologic past, indicating crustal deformation in this corner of the Himalaya is controlled by the ferocious incision of the Yarlung Tsangpo/Brahmaputra River. Their findings support the intriguing hypothesis that changes in erosion rates, even from one watershed to another, may profoundly influence where and how fast mountains are uplifted in continental collision zones.
- Keywords: river incision, rock uplift,climate, tectonics and landscape evolution, eastern Himalayan syntaxis, Namche Barwa.
- Evolution of the offshore western Gulf of Corinth
Rebecca E. Bell et al., National Oceanography Centre, Southampton, University of Southampton Waterfront Campus, European Way, Southampton SO14 3ZH, United Kingdom. Pages 156-178.
- Scientists at the National Oceanography Centre, Southampton, have imaged a series of active faults offsetting the seabed of the Gulf of Corinth, which shed new light on the processes involved in the early stages of continental crust break-up. The Gulf of Corinth, NW of Athens in central Greece, is an area where Earth's crust has been extending over a geologically short time period of less than 5 million years. The analysis of high-quality seismic reflection data and high-resolution swath bathymetry has led to the discovery that five active fault systems exist offshore in the western part of the Gulf. Their activity, based on the orientation and offset of stratigraphic horizons and the subsidence of lowstand sedimentary deltas, indicates that the offshore western Gulf has undergone a two-stage tectonic evolution. The activity and orientation of the key border faults controlling extension switched around 0.4 million years ago. Bell et al. show that the total geodetic extension rate across the western Gulf can be explained by activity on these faults together with those onshore, without the need for enhanced slip on a low-angle structure. These findings could lead to a re-assessment of how cracks propagate during continent separation and where earthquakes in this area could possibly occur.
- Keywords: Gulf of Corinth, normal fault, seismic reflection, seismic stratigraphy, rift evolution.
- Contrasting early and late Mesozoic petrotectonic evolution of northern California
W.G. Ernst et al., Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Pages 179-194.
- Belts of rock (i.e., terranes) that make up the central and western Klamath Mountains and the western Sierra Foothills consist of basaltic (oceanic) crust and deep-sea sedimentary strata. These oceanic terranes formed adjacent to the North American continental margin and were stranded against it by dominantly strike-slip motion 385-155 million years ago. In marked contrast, the Sierran granitoid batholith plus associated volcanic rocks and derived erosional debris filled the Great valley forearc and Coast Ranges trench. These quartzofeldspathic assemblages reflect nearly head-on subduction of the Farallon oceanic plate 155-85 million years ago.
- Keywords: transpression, transtension, convergence, terrane assembly, northern California, Mesozoic plate tectonics.
- Fluid flow due to the advance of basin-scale silica reaction zones
Richard J. Davies et al., CeREES (Centre for Research into Earth Energy Systems), Department of Earth Sciences, Durham University, Science Laboratories, South Road, Durham, DH1 3LE, UK. Pages 195-206.
- Earth scientists are interested in how the earth recycles elements and compounds in the form of solids, fluids, and gases. The hydrological cycle, in particular, is a well-known system with which most school children are familiar. Davies et al. look at what happens to the water that is trapped in the sediment that is deposited in the world's oceans. As the sediment is buried, it compacts and the fluid is forced out. Davies et al. have found that some chemical reactions that occur as sediment is buried are extremely effective at pumping the trapped water upwards out of Earth's crust and back into the hydrological cycle. They found that sediment rich in silica (located in the Arctic, Antarctic, and Equatorial regions) passes through a chemical reaction boundary after it has been buried to approximately 500 meters. As it passes through this chemical reaction boundary, the rate of water expelled from the sediment is extremely high — four times higher than for sediment that does not go through the reaction. The chemical reactions Davies et al. study occur at a particular temperature across entire ocean basins, therefore the pumping of fluid is a basin-wide phenomena that occurs at a specific level in the sedimentary strata of Earth's upper crust.
- Keywords: reaction zone, opal-A, opal-CT, fluid flow, diagenesis.
- Quantifying heterogeneity in variably fractured sedimentary rock using a hydrostructural domain
Megan J. Surrette and Diana M. Allen, Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada. Pages 225-237.
- Permeability is an important parameter for determining groundwater flow and sustainability. However, estimating the permeability of fractured rock is difficult due to the heterogeneous and often complex fracture networks found at regional scales. In this study, Surrette and Allen tested a hypothesis that different "domains" or "regions" of rock could be identified based on their respective structural (or fracture) characteristics, and that these would give rise to different "hydrostructural domains" based on their self-similar hydraulic properties, such as permeability and porosity. They tested this hypothesis for the fractured sedimentary rocks found on Mayne Island in southwest British Columbia, Canada. Three potential domains were identified in the fractured turbidite sequences: highly fractured interbedded mudstone and sandstone (IBMS-SS), less fractured sandstone (LFSS), and a fault and fracture zone (FZ) domain based on fracture intensity (spacing). Fracture data, measured in outcrop, were used to create discrete fracture network (DFN) models for different field sites, and equivalent permeability estimates for the bulk fractured rock mass were determined using a groundwater flow model. Statistically significant differences were found between fracture characteristics for different domains. Also, the IBMS-SS and FZ domains were found to have a greater relative potential permeability (approximately 10-13 m2) and porosity compared to the LFSS domain (approximately 10-14 m2), due largely to their greater fracture intensities (closer fracture spacing). The possibility of increased infiltration rates within FZ domain, coupled with a high storage potential relative to the other domains, suggests that fault and fracture zones are likely areas of recharge.
- Keywords: Faults, fractures, aquifers, groundwater, discrete fracture network modeling.
- Rheological implications of heterogeneous deformation at multiple scales in the Late Cretaceous Sierra Nevada, California
Eric Horsman et al., Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, Wisconsin 53706, USA. Pages 238-255.
- Relative motion of tectonic plates stresses rocks near plate margins, producing features likes faults and folds. Some rocks accommodate more of this deformation than others due to inherent strength variation between different kinds of rock. Studying how ancient examples of tectonic folding and faulting are related to rock types allows us to better understand modern tectonic processes. Horsman et al. studied rocks in California's Sierra Nevada Mountains, which record the development of an ancient volcanic arc due to relative plate motions. Their work demonstrates how various rock types record different amounts of deformation at the centimeter, meter, and kilometer scales. One important conclusion is that no rock type was more than about 10 times stronger than any other. Therefore, important modern structures, like folds and faults, may form and develop due to modest rock strength variations.
- Keywords: Rheology, strain, structural analysis, Sierra Nevada, viscosity.
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