|10 Feburary 2009
GSA Release No. 09-04
Director - GSA Communications & Marketing
March-April 2009 GSA Bulletin Media Highlights
Boulder, CO, USA - March-April GSA Bulletin studies include new findings on Mars; flood-driven sedimentation effects on a Kauai coral reef; New Mexico fossil soils and Earth's ancient atmosphere; the rocks of Mount Everest, including the famous Yellow Band carbonate; evidence that sand grains from the ancestral Appalachian belt now make up the scenic cliffs of Arches, Canyonlands, Capitol Reef, and Zion National Parks on the Colorado Plateau; and metamorphic rocks along the deepest river gorge in the world.
Highlights are provided below. Representatives of the media may obtain complementary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA Bulletin in articles published. Contact Christa Stratton for additional information or assistance.
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Supply and dispersal of flood sediment from a steep, tropical watershed: Hanalei Bay, Kauai, Hawai'i, USA
A.E. Draut et al., U.S. Geological Survey, 400 Natural Bridges Drive, Santa Cruz, California 95060, USA. Pages 574-585.
River floods bring substantial quantities of sediment from steep coastal watersheds into the ocean. Whereas in temperate climates flood sediment tends to be rapidly dispersed away from shore as waves rework it, in tropical regions flood sediment can stay near shore for months before strong enough wave energy arrives to remove it. The study by Draut et al. of a coral-reef-lined bay on the island of Kauai, Hawaii, showed that when floods are followed by low wave energy, thousands of tons of mud remained near shore for months after the flood events. When large amounts of sediment (and pollutants that also are carried by flood waters) stay in the coastal zone for weeks to months, this has potentially harmful consequences for coral reefs, which need sediment-free surfaces and clear water to be productive and to reproduce. Because climate change and urbanization are likely to increase the amount of flood sediment entering the tropical ocean, it is important to understand how flood sediment could impact coastal ecosystems, especially during seasons when wave energy will not be great enough to redistribute sediment offshore.
Identifying watershed-scale barriers to groundwater flow: Lineaments in the Canadian Shield
Tom Gleeson and Kent Novakowski, Civil Engineering Dept., Queen’s University, Kingston, Ontario K7L 3N6, Canada. Pages 333-347.
Bedrock aquifers are important drinking water sources and waste repositories. However, the fracture patterns which control groundwater flow in bedrock aquifers are poorly understood and difficult to predict. Gleeson and Novakowski examine lineaments, which are large fracture zones that are often controversially assumed to be conduits of groundwater flow. Lineaments in the Canadian Shield are examined using remote sensing, fracture mapping, drilling, hydraulic characterization and numerical simulation. Lineaments in this setting are reinterpreted as barriers, instead of conduits, to groundwater flow, which may affect how water is managed and regulated in bedrock aquifers.
Slip rate of the western Garlock fault, at Clark Wash, near Lone Tree Canyon, Mojave Desert, California
Sally F. McGill et al., Dept. of Geological Sciences, California State University, San Bernardino, 5500 University Parkway, San Bernardino, California 92407-2397, USA. Pages 536-554.
Field mapping and radiocarbon dating of an ephemeral stream channel that is offset about 66 m left laterally across the western Garlock fault indicate that the fault has been slipping at an average rate of about 7.6 mm/year over the past 10,000 years or so. This is comparable to other slip rates measured farther east along the fault over a similar time period, but it is faster than the rate at which left-lateral elastic strain has been accumulating across the fault in recent decades. The relatively high long-term slip rate along the western Garlock fault reported by McGill et al. favors the conjugate fault model over other models that have been proposed for the fault.
Seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modern soils from central New Mexico, USA
D.O. Breecker et al., Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Pages 630-640.
Calcium carbonate in soils, sometimes referred to as caliche or calcrete, occurs in a variety of forms from soft, round nodules to hard, cemented layers. It is commonly found in modern soils from the western United States and other areas of the world with relatively dry climates. In many areas of the world, soil carbonate can also be found in ancient soils. These ancient “fossil soils” were originally formed at the surface of the Earth but were subsequently buried for thousands or millions of years, only to be returned to the surface again by erosion in recent times. Such fossil soils provide an excellent archive of Earth’s ancient climate because the soil carbonate in them preserves a record of the environmental conditions prevailing during its formation. However, in order to make accurate interpretations about past climate from soil carbonate in fossil soils, scientists need to know when during the year soil carbonate forms and therefore what portion of seasonal variability soil carbonate records. In fact, seasonal variations in the soil environment can be larger than many of the climate change events in Earth’s past, so understanding exactly what conditions soil carbonate records is necessary to accurately characterize ancient climates. By studying modern soils in central New Mexico, Breecker et al. determined that calcium carbonate does not form during average growing season conditions as previously assumed, but instead forms when soils are warm and very dry. Breecker et al. therefore concluded that soil carbonate in fossil soils provides a perspective on an ancient climate that is biased toward drought-like conditions. This finding requires that many previously published climate interpretations be reconsidered. In particular, the findings indicate that the concentration of carbon dioxide in Earth’s ancient atmosphere may have been far lower than previously thought.
Deformation band clusters on Mars and implications for subsurface fluid flow
Chris H. Okubo et al., Lunar and Planetary Laboratory, The University of Arizona, 1541 East University Boulevard, Tucson, Arizona 85721, USA. Pages 474-482.
The study by Okubo et al. presents high-resolution imagery that reveals unprecedented lines of evidence for the presence of deformation band clusters in layered sedimentary deposits in the equatorial region of Mars. Clusters of deformation bands, consisting of many hundreds of individual subparallel bands, can act as important structural controls on subsurface fluid flow in terrestrial reservoirs and evidence of diagenetic processes are often preserved along them. Similar to terrestrial examples, evidence of diagenesis in the form of light- and dark-toned discoloration and wall rock induration is recorded along many of the deformation band clusters on Mars. The identification of deformation band clusters on Mars is a key to investigating the migration of fluids between surface and subsurface reservoirs in the planet’s vast sedimentary deposits.
Quaternary glaciation of Muztag Ata and Kongur Shan: Evidence for glacier response to rapid climate changes throughout the Late Glacial and Holocene in westernmost Tibet
Yeong Bae Seong et al., Dept. of Geology, University of Cincinnati, Cincinnati, Ohio 45221, USA; corresponding author: Lewis Owen. Pages 348-365.
Seong et al. define the timing and extent of past glaciation for two massifs, Muztag Ata and Kongur Shan, in western Tibet using remote sensing, geomorphic mapping, and 10-Be terrestrial cosmogenic nuclide surface exposure dating. They show that in this region glaciation has changed progressively over the last few hundred thousand years from one that produced ice caps to one that produced less extensive and more deeply entrenched valley glaciers. Furthermore, glaciers in this region have advanced at least ten times since around 17,000 years ago (Late-glacial and Holocene). This, they argue, reflects quasi-periodicity fluctuations on millennial time scales suggesting that glaciers in western Tibet likely respond to Northern Hemisphere climate oscillations (rapid climate changes), with minor influences from the south Asian monsoon. Seong et al. provide the first well-defined glacial geologic evidence to suggest that glaciers in western Tibet respond to rapid climate changes on millennial timescales throughout the Late-glacial and Holocene. Their findings have important implications for testing models for global climate change and predicting future climate change.
The paleo-ice stream in Vestfjorden, north Norway, over the last 35 k.y.: Glacial erosion and sediment yield
J.S. Laberg et al., Dept. of Geology, University of Tromsø, N-9037 Tromsø, Norway. Pages 434-447.
The behavior and stability of modern ice sheets is of major concern due to the advent of global warming. The nature of the subglacial deformable till layer is of importance for understanding the location and stability of the ice streams as well as understanding ice sheet configuration and topography. The areas influenced by the Fennoscandian Ice Sheet are easily accessible and high-quality seismic and swath bathymetry data have been acquired from this area where the history of analogue to modern ice streams including their deformable till was studied by Laberg et al.
A criteria-based methodology for determining the mechanism of transverse drainage development, with application to southwestern United States
John Douglass et al., Dept. of Geography, Paradise Valley Community College, 18401 N. 32nd Street, Phoenix, Arizona 85032, USA. Pages 586-598.
Rivers flow down hill, but can sometimes end up cutting across what would seem to be obstructive barriers like mountains or highlands. These transverse drainages are (surprisingly) quite common, and over the last few hundred years, geoscientists have figured out four ways in which these mountain-crossing rivers develop: antecedence, superimposition, overflow, and piracy. Antecedence requires the river to be flowing across the uplifting mountain. Superimposition necessitates that the river was flowing across the mountain while it was buried in some weak bedrock or sediment. Overflow simply means the river was ponded by the mountain structure for a period of time in a lake, and eventually spilled across the lowest divide. Piracy means the river flowed to some other location before its channel was redirected along a new shorter course. Each of these mechanisms requires markedly different landscape conditions to generate these anomalous mountain-crossing river courses. Douglass et al. developed a set of criteria to more accurately determine which of the four mechanisms is responsible for a transverse drainage. The most famous example, Grand Canyon, was one of the field sites. Based on our research, Grand Canyon likely formed via the overflow mechanism, but further work is required to better substantiate this hypothesis.
Stratigraphic correlation of Cambrian-Ordovician deposits along the Himalaya: Implications for the age and nature of rocks in the Mount Everest region
Paul M. Myrow et al., Dept. of Geology, Colorado College, Colorado Springs, Colorado 80903, USA. Pages 323-332.
The depositional age and nature of rocks high up on Mount Everest are poorly known because of difficult access and limited recovery of diagnostic fossils. Detailed study of Cambrian and Ordovician (542-443 million years ago) sedimentary rocks from along an about 1100 km length of the Himalaya has allowed for correlation of specific formations from northern India to Tibet. The rocks, which are metamorphosed and variably deformed, are found in association with extensive major fault systems of the Himalaya that formed in response to continental collision between India and Asia. Myrow et al. show that, despite their deformation, the rocks locally preserve sedimentary textures and primary stratigraphy that indicate a coherency of ancient depositional systems for Cambrian deposits along much of the Himalayan margin at this time. Their correlation allows, for the first time, identification of precise depositional ages of several units in the Everest region, in particular the famous Yellow Band carbonate and directly underlying strata, which are both shown to be late Middle Cambrian in age. Highly fractured Ordovician rocks cap Mount Everest. Their correlations indicate that the base of the summit pyramid of Everest, the foot of the “Third Step,” is composed of a 60-meter-thick, white-weathering layer that records the buildup of ancient microbes on the sea floor at this time. This deposit crops out only 70 meters below the summit of Mount Everest.
Constraints on the metamorphic evolution of the eastern Himalayan syntaxis from geochronologic and petrologic studies of Namche Barwa
Amanda L. Booth et al., Dept. of Geological and Environmental Sciences, Building 320, Stanford University, Stanford, California 94305, USA. Pages 385-407.
The deepest river gorge in the world is located at the eastern edge of the Himalayan mountain belt, where the Tsangpo River slices through two of the tallest peaks in the region, Namche Barwa and Gyala Peri. Here, intense erosion by the Tsangpo exposes about 7000 m of actively deforming metamorphic rocks and very young granites. Booth et al. report the age of metamorphism is also very young (3-10 million years old), using uranium-lead and thorium-lead dating of the metamorphic minerals monazite and titanite. The rocks themselves have experienced temperatures and pressures of 700-900 degrees Celsius and 10-14 kilobars, corresponding to depths in the crust of up to 40 kilometers. Exhumation of high-grade metamorphic rocks over a relatively short time period, spatially correlated with erosion by the Tsangpo, suggests these features are produced by feedbacks between tectonic and surficial processes.
U-Pb ages of detrital zircons in Jurassic eolian and associated sandstones of the Colorado Plateau: Evidence for transcontinental dispersal and intraregional recycling of sediment
William R. Dickinson et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 408-433.
In the study by Dickinson et al. tiny zircon sand grains show that the eolian sandstones deposited in Jurassic deserts, but now forming scenic cliffs in multiple national parks of the Colorado Plateau (Arches, Canyonlands, Capitol Reef, Zion), are composed of sand grains derived largely from the ancestral Appalachian belt on the opposite side of the continent at a time when opening of the Atlantic Ocean was just beginning to separate North America from Africa as the supercontinent Pangea broke up. Zircon crystals contain minuscule amounts of uranium, which converts over time to lead by radioactive decay at a known rate. The uranium/lead ratio in zircon sand grains can accordingly be used to establish the time that each zircon sand grain first formed as part of a zircon crystal in parent granite bedrock. As no process in the sedimentary realm can change the uranium/lead ratio in zircon, uranium/lead ratios in detrital zircons form a faithful record of the ages of the granite bedrock from which the zircon grains were eroded. The ages of the detrital zircons in the eolian sandstones of the Colorado Plateau show unequivocally that their ultimate derivation was mainly from the Appalachian belt, with only minor contributions of zircon from bedrock lying farther west in North America. Sediment was transported across the low-lying interior of the continent from headwaters in Appalachian highlands by a transcontinental paleoriver system, then picked up off riverine floodplains by paleowinds that blew sand southward into the vast Jurassic sand seas that covered more than 100,000 square miles of the present Colorado Plateau and adjoining areas. This pattern of sediment delivery over long distances persisted for almost 50 million years during Jurassic time. Only the detrital zircons in the eolian sands have survived to tell the tale, for the paleoriver courses of interior North America have long since been destroyed by erosion over the 150 million years since desert sands covered the site of the Colorado Plateau.
Paleomagnetism of mid-Paleozoic subduction-related volcanics from the Chingiz Range in NE Kazakhstan: The evolving paleogeography of the amalgamating Eurasian composite continent
Natalia M. Levashova et al., Geological Institute, Academy of Sciences of Russia, Pyzhevsky Lane, 7, Moscow, 109017, Russia. Pages 555-573.
The tectonic and paleogeographic evolution of the Ural-Mongol belt between the cratons of Baltica, Siberia, and Tarim is key to the formation of the Eurasian composite continent during Paleozoic time, but the views on this complicated process remain disparate and sometimes controversial. A study by Levashova et al. of three volcanic formations of mid-Silurian, Lower to Middle Devonian, and Middle Devonian age from the southwestern boundary of the Chingiz Range (northeast Kazakhstan) yields what are interpreted as primary paleomagnetic directions that help clarify the evolution of the belt. A single-polarity characteristic component in mid-Silurian andesites yields a positive intraformational conglomerate test, whereas dual-polarity prefolding components are isolated from the two Devonian collections. These new data can be evaluated together with previously published paleomagnetic results from Paleozoic rocks in the Chingiz Range, and allow them to establish with confidence the polarity of each result, and hence to determine the hemisphere in which the area was located at a given time. Levashova et al. conclude that northeast Kazakhstan was steadily moving northward, albeit with variable velocity, crossing the equator in Silurian time. These new paleomagnetic data from the Chingiz Range also agree with and reinforce the hypothesis that the strongly curved volcanic belts of Kazakhstan underwent oroclinal bending between Middle Devonian and Middle Permian time. A comparison of the Chingiz paleolatitudes with those of Siberia shows, insofar as the sparse data allow, similarities between the northward motion of the Chingiz unit and that of Siberia, which imposes important constraints on the evolving paleogeography of the Ural-Mongol belt.
Petrogenesis and structure of the Buck Creek mafic-ultramafic suite, southern Appalachians: Constraints on ophiolite evolution and emplacement in collisional orogens
Virginia Peterson et al., Dept. of Geology, Grand Valley State University, Allendale, Michigan 49401, USA. Pages 615-629.
The Buck Creek mafic-ultramafic suite is among the larger and more lithologically diverse of the fragmented ultramafic exposures in the southern Appalachian Blue Ridge. This ophiolite fragment is unusual in its preservation of dry, high-pressure-temperature conditions, with evidence for localized hydration while the rocks were near peak metamorphism. Integration of detailed field, petrological, structural, and geochemical observations, by Peterson et al., points to the origin of these rocks in a mid-ocean ridge setting. This provides constraints on their emplacement during the early assembly of the southern Appalachian Blue Ridge, and contributes to the understanding of ophiolite emplacement in collisional orogens.
View the complete table of contents for the current issue of GSA Bulletin, or review abstracts for these articles at http://gsabulletin.gsapubs.org/current.dtl.