Boulder, CO, USA - BULLETIN papers examine carbon-14 dating of marine mud fossils in Ireland that suggests high ice-sheet sensitivity to small climate changes; formation of Valles Marineris, Mars; a buried fossil forest in the Gold Hill Loess, Alaska; a 20-meter-high salt pillar near the Dead Sea; how shrimp affect groundwater flow in the Biscayne aquifer; a possible emerging natural gas play in the Appalachian Basin; and banded iron formations exposed by the Agouron South African Drilling Project.
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Age, provenance, and tectonic setting of Paleoproterozoic quartzite successions in the southwestern United States
James V. Jones III et al., Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. Pages 247-264.
Thick (greater than 2 km), compositionally pure successions of ultramature quartz sandstone (i.e., orthoquartzite) occur among 1.8-1.6 billion year old (Ga, or Giga annum) rock exposures throughout North America and elsewhere around the world. These distinctive successions have potential to elucidate numerous aspects of Proterozoic (2.5-0.5 Ga) earth systems. In this study by Jones et al., new field research in the southwestern United States combined with analytical work involving multiple-age dating techniques helps to constrain deposition and deformation of quartzites exposed in southern Colorado and northern New Mexico and provides new insights into the sedimentary character and tectonic setting of these remarkable rocks.
Cosmogenic 10Be chronology of the last deglaciation of western Ireland and implications for sensitivity of the Irish Ice Sheet to climate change
Jorie Clark et al., School of Environmental Science, University of Ulster, Coleraine, County Londonderry BT52 1SA, UK. Pages 3-16.
Accelerator mass spectrometry carbon-14 dates of fossiliferous marine mud identify two re-advances of the Irish Ice Sheet from the north and central lowlands of Ireland into the northern Irish Sea Basin during the Killard Point stadial at around 16.5 thousand years before present (B.P.), with subsequent deglaciation occurring by around 15.0-15.5 thousand years B.P. Killard Point stadial moraines have been mapped elsewhere in Ireland but have previously remained undated. Clark et al. report sixteen 10-beryllium (Be) surface exposure dates that constrain the age of retreat of the Killard Point stadial ice margin from western Ireland. Eight 10Be dates from the Ox Mountains (13.9-18.1 thousand years ago) indicate that final deposition of the moraine occurred at 15.6 plus or minus 0.5 thousand years ago. Eight 10Be dates from Furnace Lough (14.1-17.3 thousand years ago, mean age of 15.6 plus or minus 0.4 thousand years ago) are statistically indistinguishable from the Ox Mountain samples, suggesting that the moraines were deposited during the same glacial event. Given the agreement between the two age groups, and their common association with a regionally significant moraine system, we combine them to derive a mean age of 15.6 plus or minus 0.3 thousand years ago. This age is in excellent agreement with the timing of deglaciation from the Irish Sea Basin (at or older than 15.3 plus or minus 0.2 thousand years B.P.) and suggests the onset of near-contemporaneous retreat of the Irish Ice Sheet from its maximum Killard Point stadial limit. A reconstruction of the ice surface indicates that the Irish Ice Sheet reached a maximum surface elevation of about 500 m over the central Irish Lowlands during the Killard Point stadial, suggesting a high sensitivity of the ice sheet to small changes in climate.
Continental-scale salt tectonics on Mars and the origin of Valles Marineris and associated outflow channels
David R. Montgomery et al., Quaternary Research Center, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA. Pages 117-133.
A synthesis of deformation patterns within and around the Thaumasia Plateau, Mars, points to a new interpretation for regional deformation and the origin of Valles Marineris and associated outflow channels. Montgomery et al. state that geothermal heating and topographic loading of extensive buried deposits of salts and/or mixtures of salts, ice, and basaltic debris would allow for weak detachments and large-scale gravity spreading. They propose that the generally linear chasmata of Valles Marineris reflect extension, collapse, and excavation along fractures radial to Tharsis, either forming or reactivated as part of one lateral margin of the Thaumasia gravity-spreading system. The other, dextral, lateral margin is a massive splay of extensional faults forming the Claritas Fossae, which resembles a trailing extensional imbricate fan. Topographic observations and previous structural analyses reveal evidence for a failed volcanic plume below Syria Planum that could have provided both thermal energy and topographic potential for initiating regional deformation, either intrusively through inflation or extrusively through lava flow and/or ash fall emplacement. Higher heat flow during Noachian time, or geothermal heating due to burial by Tharsis-derived volcanics, would have contributed to flow of salt deposits, as well as formation of groundwater from melting ice and dewatering of hydrous salts. Their hypothesis provides a unifying framework to explain perplexing relationships between the rise of the Tharsis volcanic province, deformation of the Thaumasia Plateau, and the formation of Valles Marineris and associated outflow channels.
Late Pliocene Dawson Cut Forest Bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska
T.L. Pw et al., Department of Geology, Arizona State University, Tempe, Arizona 85287, USA. Pages 294-320.
Thick, wind-blown deposits (loess) in the Fairbanks region of interior Alaska are a rich source of information on past climates and environments during the last three million years. Pw et al. find that a buried forest bed in the lower part of this loess record contains fossil tree remains, pollen, tree-ring characteristics, and carbon isotopic signatures that are very similar to the modern boreal forest of central Alaska. Associated volcanic ash beds suggest that this buried forest bed is about two million years old, so that the northern boreal forest of northwestern North America, as we know it today, likely has a very long history.
U-Pb ages (3.8-2.7 Ga) and Nd isotope data from the newly identified Eoarchean Nuvvuagittuq supracrustal belt, Superior Craton, Canada
Jean David et al., GEOTOP-UQAM-McGill and Dept. des Sciences de la Terre et de l'Atmosphere, Universite du Quebec a Montreal, C.P. 8888, Succursale Centre-Ville, Montreal, Quebec H3C 3P8, Canada. Pages 150-163.
The geologic record of the early Earth is sparse due to the continual reworking of Earth's surface by geological processes. A few remnants of early crust (greater than 3.7 billion years ago) do remain, but are often highly deformed and the original rock features are often obliterated. David et al.'s study of the Nuvvuagittuq volcano-sedimentary sequence in the northeast Superior Province of northern Quebec, Canada, indicates an age of about 3.8 billion years old. The sequence contains recognizable volcanic and sedimentary units including a felsic schist dated at 3817 16 million years ago. Tonalites that envelop the sequence yield an age of 3661 4 million years ago and Neodymium-isotope data from these tonalites suggest that they formed from the melting of the volcano-sedimentary package. This old, crustal sequence may be a fragment of the Earth's earliest continental crust.
Postorogenic shoshonitic rocks and their origin by melting underplated basalts: The Miocene of Limnos, Greece
Georgia Pe-Piper et al., Department of Geology, Saint Mary's University, Halifax, Nova Scotia B3H 3C3, Canada. Pages 39-54.
Potassium-rich volcanic rocks, termed shoshonites, are a common feature of many mountain belts, including the Absaroka Range of Wyoming. Precisely how such volcanic rocks form and their relationship to mountain building processes is disputed. Unusual features of 20-million-year-old shoshonite volcanoes in the island of Limnos, Greece, demonstrate that the magma was largely derived by the melting of the lower part of the Earth's crust and Pe-Piper et al. argue that this process is of general applicability to this rock type. Formation of such volcanoes, thus, depends on a suitable source of heat from the Earth's mantle during mountain building processes.
Formation and dating of a salt pillar in Mount Sedom diapir, Israel
Amos Frumkin, Department of Geography, The Hebrew University of Jerusalem, 91905 Jerusalem, Israel. Pages 286-293.
The formation of a 20-meter-high salt pillar in Mount Sedom, Dead Sea area, was analyzed and dated in the study by Frumkin, and these results were complemented by measurement of present uplift rate because the pillar is a part of the actively rising Sedom diapir. Contrary to earlier assumptions, which stated that the salt pillar was formed by direct rainfall, the observed solutional notches and morphology of the neighboring chasm indicate that the salt pillar is due to a karstic cave that collapsed. The uppermost part of the cave still carried an underground ephemeral stream about 4000 years ago, as indicated by six carbon-14 dates of wood washed into the highest cave level. The sudden collapse that left the isolated salt pillar was probably triggered by a catastrophic earthquake. The measured uplift rate, 9.3 3.5 millimeters per year, shows that when the pillar formed, it was much closer to the base of Mount Sedom.
Magnetostratigraphy of the Ludlow Member of the Fort Union Formation (Lower Paleocene) in the Williston Basin, North Dakota
D.J. Peppe et al., Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. Pages 65-79.
To determine the geomagnetic polarity stratigraphy and the duration and age of the Ludlow Member of the Fort Union Formation (Lower Paleocene), Peppe et al. constructed a 325 meter composite lithostratigraphic section of the Upper Cretaceous Hell Creek Formation and the Lower Paleocene Ludlow and Tongue River Members of the Fort Union Formation in the Little Missouri River valley of North Dakota, USA. They analyzed paleomagnetic samples from nine of the logged sections. The principal magnetic carrier in the Ludlow Member sediments is likely titanomaghemite, as indicated by predominantly irreversible thermomagnetic curves measured from sandstone, siltstone, and carbonaceous shale samples. Peppe et al. infer that the magnetization of the samples is primary because the characteristic directions are consistent with those of the Paleocene of North America. By extrapolating the measured sediment accumulation rate from the Cretaceous-Tertiary (K-T) boundary to the top of C28n and then to the top of the Ludlow Member, they estimate the duration of the member to range from 2.31 to 2.61 million years. This is the first estimate for the duration and age of the Ludlow Member, and it can be used as an important tool for interpreting rates of biotic recovery after the K-T extinction.
Paleogene landscape evolution of the central North American Cordillera: Developing topography and hydrology in the Laramide foreland
Steven J. Davis et al., Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Pages 100-116.
From the chemical signature of lake and river waters preserved in rocks between roughly 50 and 40 million years ago, Davis et al. have reconstructed the hydrologic environment of basins formed during uplift of the modern Rocky Mountains. Further, they show that the varied environments in the basins (e.g. freshwater lakes, saline lakes, and river systems) are largely the result of changes in the sources and amounts of water flowing to the basins. Finally, they suggest that there may have been a large-scale pattern to the hydrologic changes that occurredmoving gradually from north to south, and mirroring the pattern of volcanism going on at nearly the same time to the west in what is now Idaho and Nevada.
Neogene sediment structures in Bounty Trough, eastern New Zealand: Influence of magmatic and oceanic current activity
G. Uenzelmann-Neben et al., Alfred-Wegener-Institute fur Polar- und Meeresforschung, Postfach 120161, 27515 Bremerhaven, Germany. Pages 134-149.
New seismic reflection profiles have been interpreted by Uenzelmann-Neben et al. to shed more light on the Neogene deposition in Bounty Trough, an aborted rift characterizing the eastern New Zealand continental margin. Two major processes influencing the deposition were identified. Magmatic activity led to the formation of basement highs, which deform the sedimentary sequences up to early and middle Miocene. The origin of the magmatism unfortunately cannot be solved with the new data presented by Uenzelmann-Neben et al. It is difficult to say whether the magmatic activity is an on-going process. Drape structures in the youngest sedimentary unit argue against this. The Oligocene-Miocene represents a period of major change. Bottom current activity then took over as the most important depositional process. Strong cooling events in the late Miocene resulted in modification in the oceanographic regime east of New Zealand. This led to the formation of channels, sediment drifts, and sediment waves. At least since the Miocene, bottom current activity has been the dominating depositional process.
Prominence of ichnologically influenced macroporosity in the karst Biscayne aquifer: Stratiform "super-K" zones
Kevin J. Cunningham et al., U.S. Geological Survey, 3110 SW 9th Avenue, Fort Lauderdale, Florida 33315, USA. Pages 164-180.
Macroporosity related to geometrically complex arrangements of tunnels and shafts created by thalassinidean shrimp, which burrow in carbonate sediment beneath the sea floor, strongly affects where groundwater flows within Pleistocene limestone in substantial parts of the Biscayne aquifer, southeastern Florida. The macroporous zones represent an alternative pathway for focused groundwater flow that differs considerably from standard karst flow-system paradigms, which describe groundwater movement through fractures and cavernous dissolution features. X-ray computed tomography scans of samples of this type of macroporosity and surrounding limestone matrix were used by Cunningham et al. in lattice Boltzmann computer simulations that calculated its permeability. Results show that burrow-related permeability is as high as 3.5 times 10 to the sixth Darcies, about 2 to 5 orders of magnitude higher than the reported permeabilities for Jurassic "super-K" zones of the giant Ghawar oil field, Saudi Arabia. The highly permeable macroporous zones of the Biscayne aquifer provide well-documented examples of ichnologically-influenced macroporosity for comparative analysis of the origin and evolution of similar zones in other carbonate aquifers, as well as petroleum reservoirs.
Enhanced fracture permeability and accompanying fluid flow in the footwall of a normal fault: The Hurricane fault at Pah Tempe hot springs, Washington County, Utah
Stephen T. Nelson et al., Department of Geological Sciences, Brigham Young University, S-389 ESC, Provo, Utah 84602, USA. Pages 236-246.
The Pah Tempe hot springs near Hurricane, Utah, release large quantities of warm, CO2-laden water directly into the Virgin River. Geochemical analysis by Nelson et al. of these waters reveals several important features. First, they have circulated to great depths (more than 3 miles) and entered the subsurface anciently under colder climate conditions. This long and deep circulation has been enabled by fractures associated with the Hurricane Fault, a major active structure near the western margin of the Colorado Plateau. CO2 gas quantities released are quite large, including trains of CO2 bubbles rising through the Virgin River. However, the large majority of CO2 is released to the surface in the dissolved state, predominantly as carbonic acid and bicarbonate.
The paradox of minibasin subsidence into salt: Clues to the evolution of crustal basins
Michael R. Hudec et al., Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station, Box X, Austin, Texas 78713-8924, USA. Pages 201-221.
Subsidence of sedimentary minibasins into salt is paradoxical, in that subsidence begins while minibasin fill is less dense than the fluid salt substrate. How can a light minibasin sink into a dense fluid? Hudec et al. identify five mechanisms to explain this anomalous subsidence, which are functions of tectonic environment, regional bathymetry, and sedimentation rate. Subsidence mechanisms control minibasin bathymetry, which, in turn, controls sediment-fill patterns. Furthermore, salt-withdrawal minibasins are in many ways simple analogs for crustal basins, so minibasin subsidence can provide insight into problems in crustal dynamics.
Tracking the burial and tectonic history of Devonian shale of the Appalachian Basin by analysis of joint intersection style
Gary G. Lash and Terry Engelder, Dept. of Geosciences, State University of New York-College at Fredonia, Fredonia, New York 14063, USA. Pages 265-277.
Lash and Engelder address the timing and chronology of fracturing of Middle and Upper Devonian black shale deposits, including the Marcellus Shale, exposed on the Appalachian Plateau of western New York State. Specifically, the case is made that these rocks were hydraulically fractured during the Alleghanian Orogeny as a consequence of the transformation of organic matter to hydrocarbons close to, or at peak burial depth. These results are especially important in that they suggest that the organic-rich shale, an emerging natural gas play in the Appalachian Basin, carries the fractures, which enhance the permeability of these otherwise tight hydrocarbon source rocks, at present depths of 6000-7000 feet.
An iron shuttle for deepwater silica in Late Archean and early Paleoproterozoic iron formation
Woodward W. Fischer et al., Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. Pages 222-235.
It has been known for almost a century that banded iron formations (BIFs) are an enigmatic rock type unique to discrete intervals in the Precambrian, and are, in some fashion, related to the redox state of the oceans and atmosphere. Previous interest in BIFs largely concerned the source and oxidation of the iron. However these deposits contain equal or more silica, suggesting that the elemental cycles of iron and silica were coupled in a fashion unseen in younger deposits, and furthermore that a better understanding of this coupling might offer unique insights in the origin of these rocks. The study by Fischer et al. used material from a well-preserved about 2.7-2.5 billion year old carbonate platform succession collected as part of the Agouron South African Drilling Project. With a particular focus on the silica cycle and using field geology, petrography, and carbon isotopes, they have assembled a conceptual model for BIF deposition that demonstrates mechanistically how an active Late Archean biological redox cycle of iron can explain the environmental, textural, mineralogical, geochemical, and paleontological features of BIFs. Because the Archean fossil record is notably poor, this hypothesis for BIF deposition is important because, if correct, it adds significant understanding about Late Archean biology and the environmental theater for the coming oxygen revolution.
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