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Isotopic age of the Black Forest Bed, Petrified Forest Member, Chinle Formation, Arizona: An example of dating a continental sandstone
N.R. Riggs, Department of Geology, Northern Arizona University, Flagstaff, Arizona 86001, USA, et al. Pages 1315-1323.
Keywords: isotopic dating, Chinle Formation, Triassic, single-crystal TIMS, SHRIMP.
The Chinle Formation in the Four Corners region is famous for its petrified wood, sweeping landscapes, and dinosaur fossils. The precise age of the formation, however, has long been in dispute because fossil ages are difficult to tie to isotopic ages. This paper presents the result of research into the isotopic age of a layer in the upper Petrified Forest Member of the Chinle Formation, using the isotopes uranium and lead from tiny zircon crystals. A comparison of three methods of U-Pb analysis gives a maximum age of 213 ± 1.5 m.y. for the sampled layer. The zircon became incorporated into sandstone after being carried in ash clouds from volcanoes in eastern California or western Nevada.
Pleistocene hydrogeology of the Atlantic continental shelf, New England
Mark Person, Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA et al. Pages 1324-1343.
Keywords: hydrogeology, glaciology, salinity, paleohydrology.
Salinity levels in offshore aquifers on the Atlantic continental-shelf off New England are relatively fresh and are far out of equilibrium with modern sea-level conditions. Here, we use computer models to evaluate two fluid flow inducing mechanisms which could explain the apparent flushing of these coastal-plain aquifers during the past 2 million years: (1) meteoric recharge during Pleistocene sea-level lowstands and (2) sub ice-sheet recharge during the last continental glaciation. Cross-sectional models of groundwater flow, heat, and solute transport could not reproduce the relatively low-salinity groundwaters observed off Long Island, New York, by using boundary conditions consistent with Pleistocene sea-level fluctuations. Observed salinity conditions were most closely matched by also including the effects of subglacial recharge from the Laurentide Ice Sheet and allowing groundwater to discharge from confined aquifers along submarine canyons near the continental slope. With growing shortages of freshwater supplies in coastal communities globally, freshwater reservoirs on the continental shelf like those found in New England may represent an important yet untapped resource.
Quantitative morphology of a fringing reef tract from high-resolution laser bathymetry: Southern Molokai, Hawaii
C.D. Storlazzi, et al., US Geological Survey, Pacific Science Center, 1156 High Street, Santa Cruz, California 95064-1077, USA. Pages 1344-1355.
Keywords: Fringing reef, reef morphology, spur-and-groove structure, lidar, bathymetry, waves.
High-resolution Scanning Hydrographic Operational Airborne Lidar Survey (SHOALS) laser-determined measurements of water depth were used to define the morphology of the fringing reef off the south coast of Molokai, Hawaii. These data provided a basis for mapping and analyzing morphology of the reef with a level of precision and spatial coverage not attained previously. An extensive fringing coral reef (~40 km) stretches along Molokai's south shore. In total, almost 5000 measurements of coral spur-and-groove height and the distance between adjacent spur crests (wavelength) were obtained along four water depths. The width of the reef was found to exponentially decrease with increasing wave energy. Overall, mean spur-and-groove height and wavelength were shown to be inversely proportional to wave energy. Therefore it appears that waves exert a primary control on both the small- and large-scale morphology of the reef off south Molokai.
Timing and style of Late Quaternary glaciation in northeastern Tibet
Lewis A. Owen, Department of Earth Sciences, University of California, Riverside, California 92521, USA, et al. Pages 1356-1364.
Keywords: Tibet, glaciation, monsoon, cosmogenic radionuclide dating, optically stimulated luminescence dating.
Using newly developing cosmogenic radionuclide surface exposure dating techniques to date Late Quaternary moraines, Lewis Owen and colleagues show that glaciation in NE Tibet is not synchronous with changes in the Northern Hemisphere ice sheets and oceans. Rather glaciation in the high mountains of Asia is strongly controlled by the Asian monsoon. As such, glaciers in Tibet tend to advance more during times of increase monsoon activity that is promoted by increase solar flux in the tropics. This has important implications for modeling and understanding the role of the monsoon under future climate change scenarios, for example, human-induce global warming. The regions adjacent to the Tibetan Plateau are home to approximately 2/3rd of the world's people. Understanding of glacial response and the associated changing hydrology, therefore, has important socioeconomic implications for the people who live in the high mountains and adjacent regions of Asia whose economies are essentially agriculturally based and are dependant upon predictable climatic and hydrological conditions.
The Burnsville fault: Evidence for the timing and kinematics of southern Appalachian Acadian dextral transform tectonics
Charles H. Trupe, Department of Geology and Geography, Georgia Southern University, Statesboro, Georgia 30460-8149, USA, et al. Pages 1365-1376.
Keywords: Structural geology, tectonics, Appalachians, Acadian orogeny, Burnsville fault, strike-slip faults.
The Devonian Acadian orogeny was a major tectonothermal event in the southern Appalachians, but there has been a conspicuous lack of documented deformation structures that can be tied to the Acadian event in this region. New mapping and structural, petrologic, and geochronologic data from western North Carolina show that the contact between the eastern and western Blue Ridge provinces is a Devonian high-grade, dextral strike-slip fault zone, the Burnsville fault. Timing of motion on the Burnsville fault must be younger than 377 Ma, but older than ca. 360 Ma, based on ages of sheared rocks in the fault zone and their cooling history. The Burnsville fault extends ~100 km in northwestern North Carolina and likely correlates with other faults that continue into northwesternmost North Carolina and southwestern Virginia, and southwestern North Carolina. Diachroneity of Acadian clastic wedges, the presence of Silurian-Devonian pull-apart basins, the presence of Devonian high-grade, dextral shearing in the Inner Piedmont, and recent plate reconstructions for the middle Paleozoic, support the interpretation that the late phase of the Acadian orogeny in the southern Appalachians was primarily a dextral transpressional event. The Burnsville fault and the Inner Piedmont formed the boundaries of a dextral transform margin that may have extended the length of the Appalachian orogen.
Pedogenesis and Geochemical Alteration of the Picture Gorge Subgroup, Columbia River Basalt, Oregon
Nathan D. Sheldon, Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA. Pages 1377-1387.
Keywords: Picture Gorge, basalt, paleosols, Oregon, paleoclimate.
Basalt sequences often preserve spectacular brick-red interflow paleosols. Historically, it has been assumed that these ancient soils had been baked and altered to a significant degree and that they would be useless for paleoclimatic and paleoenvironmental reconstructions. This paper presents evidence that these paleosols are often preserved with minimal alteration and that basalt sequences instead may be ideal for preserving paleoclimatic evidence because their parent material does not need to be inferred as with other types of paleosols, and because the emplacement of an overlying flow ensures post-formation erosions is not possible, so the whole soil profile is preserved. This suggests that paleosols preserved in flood basalt sequences such as the Siberian Traps, Deccan Traps, and Columbia River Basalts (as in this study), may provide high-resolution paleoclimatic records of the terrestrial environment during times of mass extinction (Permian-Triassic, Cretaceous-Tertiary) or climatic change (middle Miocene climatic optimum).
Late Holocene earthquakes on the Toe Jam Hill fault, Seattle fault zone, Bainbridge Island, Washington
Alan R. Nelson, Geologic Hazards Team, U.S. Geological Survey, MS 966, P.O. Box 25046, Denver, Colorado 80225, USA, et al. Pages 1388-1403.
Keywords: paleoseismology, earthquake recurrence, reverse faulting, Puget Lowland, fault trenching, earthquake hazards.
New airborne laser mapping combined with detailed stratigraphic studies and radiocarbon dating of earthquake faulting in five backhoe trenches across a newly discovered fault scarp helps define the hazard posed by large earthquakes in the rapidly urbanizing Puget Lowland of Washington State. The scarp, only 15 km west of downtown Seattle, marks the Toe Jam Hill fault, a small back thrust to the 70-km-long Seattle fault. Folded and faulted bedrock, liquefaction features, and forest soil A horizons buried each time that bedrock and glacial sediment were thrust over them, record three, or possibly four, earthquakes between 2500 and 1000 years ago. The most recent earthquake is probably the AD 900-930 earthquake that raised shorelines and triggered a large tsunami in Puget Sound. The amount of vertical deformation during faulting estimated from stratigraphic studies suggests late Holocene earthquake magnitudes near M7, corresponding to fault surface ruptures >36 km long. Radiocarbon dating shows times between large earthquakes as great as 12,000 years to as little as a century or less. Because the Toe Jam Hill fault is a backthrust to the Seattle fault, it may not record every earthquake on the entire Seattle fault. But the earthquake history of the Toe Jam Hill fault is at least a partial proxy for the history of the rest of the Seattle fault zone.
Centennial scale cycles in middle Holocene sea level along the southeastern Swedish Baltic coast
Shi-Yong Yu, GeoBiosphere Science Centre, Department of Geology/Quaternary Science, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden. Pages 1404-1409.
Keywords: Baltic sea-level changes, middle Holocene storminess, NAO, solar forcing, lunar cycle.
Seagrass seeds and stonewort oospores from an ancient lagoon of the Baltic Sea may tell the story of past sea-level fluctuations with a changing tempo and tone. Superimposed on a highstand 6000 years ago, low-frequency oscillations of Baltic sea level may have been a manifestation of tidal actions in the millennial band. After 6000 years before present, sea level was lowered and exhibited centennial-scale fluctuations, probably regulated by the sun through a system most similar to the dipole oscillation of North Atlantic atmosphere.
Brittle structures and their role in controlling porosity and permeability in a complex Precambrian crystalline-rock aquifer system in the Colorado Rocky Mountain Front Range
Jonathan Saul Caine, U.S. Geological Survey, P.O. Box 25046, MS 973, Denver, Colorado 80225, USA, and Stephanie R.A. Tomusiak, Department of Geological Sciences, Campus Box 250, University of Colorado, Boulder, Colorado 80309, USA. Pages 1410-1424.
Keywords: aquifers, Colorado, faults, fractures, groundwater, Precambrian.
Expansion of the Denver metropolitan area has resulted in substantial residential development in the foothills of the Rocky Mountain Front Range. This type of suburban growth, characteristic of much of the semi-arid intermountain west, often relies on groundwater from individual domestic wells and is exemplified in the Turkey Creek Watershed (TCW). The TCW is underlain by complex fractured crystalline bedrock where groundwater resources are poorly understood and concerns regarding groundwater mining and degradation have arisen. As part of a pilot project to establish the limits on the groundwater resource, a field-based geological characterization and numerical modeling study of fractures and faults and their controls on groundwater was initiated. Existing data suggest that groundwater storage, flow, and contaminant transport are primarily controlled by fracture networks. Well permit data and field observations show an aquifer system composed of metamorphic rocks, granitic intrusive rocks, and large fault zones. Computer modeling was used to estimate porosity and permeability and results indicate a complex aquifer system where the upper limits on estimated hydraulic properties suggest limited storage capacity and permeability as compared with many sedimentary rock and surficial deposit aquifers.
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