May-June GSA Bulletin media highlights

04/21/05

Boulder, Colo. – The May-June issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several potentially newsworthy items. Topics include new developments regarding the Monterey hypothesis and mapping of landslide-prone terrain across the Oregon coast range.

Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at acairns@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for additional information or assistance.

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Phosphogenesis and organic-carbon preservation in the Miocene Monterey Formation at Naples Beach, California--The Monterey hypothesis revisited
Karl B. Föllmi, Institut de Géologie, Université de Neuchâtel, Neuchâtel, Switzerland, et al. Pages 589–619.
Keywords: Monterey Formation, Naples Beach, organic carbon preservation, phosphogenesis, Monterey hypothesis, Miocene.

The Monterey hypothesis is one of the first hypotheses proposed in which links are drawn between changes in ocean circulation, nutrient distribution, primary productivity rates in ocean surface waters, organic carbon storage such as in the Monterey Formation of California, and climate change during the Miocene. Our work on the Monterey Formation of the Naples Beach section near Santa Barbara, California--a classical locality of this formation--allows us not only to develop a model of organic matter preservation and concomitant phosphate accumulation, but also to show that organic carbon preservation rates are rather low and not compatible with the Monterey hypothesis as such. We propose a corollary of the Monterey hypothesis, in which major sinks of organic matter are located in continental and near-shore sediments.

Geochemical correlation in deltaic successions: A reality check
Colin P. North, Malcolm J. Hole, and Daniel G. Jones, Department of Geology and Petroleum Geology, University of Aberdeen, King's College, Aberdeen AB24 3UE, Scotland, UK. Pages 620–632.
Keywords: correlation, geochemistry, deltas, sequence stratigraphy, Cretaceous, Book Cliffs.

Geologists need to be able to trace equivalent layers of sediments over long distances. This is especially important when trying to produce water and hydrocarbon resources from ancient deltas deep under the Earth. Much time may pass between accumulation of each layer, during which sea level may rise or fall, and the shape of the delta changes considerably. In this way, impermeable fine-grained mudstones end up deposited on top of highly permeable sandstones. This means fluids underground will flow only parallel to the layers, not across them. Layer correlation is often done by matching the fossils they contain. But in many ancient delta successions, the fossils have been dissolved out as the sediments were buried. To get around this problem, it has been suggested that the bulk chemistry of the sediment may provide a kind of fingerprint to match layers. This method has been tried by others for subsurface successions, but has never before been tested where the layer correlation can be checked visually. We have now done such a test of the method, using the extensive Cretaceous-age rock outcrops in the Book Cliffs of SE Utah, USA. In our test, we found that the chemistry of individual layers is highly variable over short distances, making them indistinct from those above and below. Furthermore, the way the chemistry varies corresponds more closely to the amount of clay in the sediment than with position in the delta or sediment age, obscuring any variation from one layer to the next, which might be used for layer-matching. We conclude that, for successions of delta sediments like those in our investigation, whole-rock chemistry is unreliable as a means of correlation.

Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera
John W. Shervais, Department of Geology, Utah State University, Logan, Utah 84322-4505, USA, et al. Pages 633–653.
Keywords: ophiolite, age, CRO, cordillera, tectonics.

New zircon U/Pb dates show that the Coast Range ophiolite (CRO) in northern California formed ca. 165–172 Ma, in the fore-arc of an ancient island arc complex. New 40Ar/39Ar dates on basaltic glass show that this ancient forearc collided with an oceanic spreading center ca. 164 Ma. Radiolarians extracted from chert lenses intercalated with the basal indicate that the sedimentary strata range in age from Bathonian near the base of the complex to late Callovian or early Kimmeridgian in the upper part. These cherts preserve evidence of a major faunal change from relatively small-sized, polytaxic radiolarian faunas to very robust, oligotaxic, nassellarian-dominated faunas that indicate changes in oceanic circulation.

These data show that the CRO formed prior to the Late Jurassic Nevadan orogeny, probably by rapid forearc extension above a nascent subduction zone. We infer that CRO formation ended with the collision of an oceanic spreading center ca. 164 Ma. We further suggest that the "classic" Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation, and with a subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for Late Jurassic arc collision in the Sierra foothills, or a backarc origin for the CRO.

Characterizing structural and lithologic controls on deep-seated landsliding: Implications for topographic relief and landscape evolution in the Oregon Coast Range, USA
Joshua J. Roering, Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA; et al. Pages 654–668.
Keywords: deep-seated landslides, landscape evolution, Oregon Coast Range, slope stability, Tyee Formation, relief.

Mountainous regions are often subject to large landslides that transfer large amounts of soil and sediment to rivers and pose a significant hazard. Landslides may ultimately limit the height of mountains, although this notion is difficult to test given the complexity of most mountainous landscapes. We developed a novel topography-based method for mapping landslide-prone terrain across the Oregon Coast Range. Our results reveal that a large portion of the region has been subject to large (km-scale) slope failures, although the timing and triggering mechanisms are poorly constrained. The pattern of fossil landslide features reflects variations in the underlying bedrock, and our calculations suggest that the height of mountains is depressed in areas with a high proportion of large landslides. Particular units in the stack of sedimentary bedrock that underlies the Coast Range are more prone to landsliding. Given continued tectonic forcing of the Coast Range over the next one to two million years, these slide-prone units will become more prevalent such that large landslides and thus shorter mountains will dominate large sections of the region.

Integrating seismic reflection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine Mountain window, USA
John H. McBride, Department of Geology, Brigham Young University, Provo, Utah 84602-4606, USA, et al. Pages 669–686.
Keywords: Appalachians, seismic reflection, tectonics, décollement, faulting.

A ubiquitous feature of hundreds of kilometers of deep seismic profiles over the southern Appalachians is a shallow crustal subhorizontal reflection package beneath the exposed core of the orogen. This feature is generally interpreted to track the position of the master Appalachian décollement; however, beneath the Pine Mountain Window of western Georgia, controversy has surrounded the issue of whether the décollement continues beneath the window or has been exhumed to near the present-day surface. Reprocessing of deep seismic reflection data across the window indicates that a band of reflectivity interpreted as the décollement can in fact be traced beneath the window. Our results suggest that the window exposes an allochthonous basement duplex or horse-block thrust upward from the south along the Late Proterozoic rifted continental margin.

Contrasting structural histories of the Salmon River belt and Wallowa terrane: Implications for terrane accretion in northeastern Oregon and west-central Idaho
Keith D. Gray and John S. Oldow, Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA. Pages 687–706.
Keywords: structure, tectonics, Wallowa terrane, Salmon River belt, Cordilleran Arc, Mesozoic.

Metamorphic rocks exposed in the Salmon River and Snake River drainages of west-central Idaho and northeastern Oregon form a boundary zone, known as the Salmon River belt, separating Mesozoic and Paleozoic units of North America and the accreted terranes of the continental borderland farther west. The Salmon River belt experienced metamorphism and deformation in the mid-Jurassic before the Wallowa terrane of northeastern Oregon was attached to the North American continent and records a history like that of other parts of the indigenous North American magmatic arc system. The Salmon River belt together with the volcanic arc terranes of northeastern Oregon were involved in late Cretaceous deformation associated with the Western Idaho shear zone.

Variations in exhumation level and uplift rate along the oblique-slip Alpine fault, central Southern Alps, New Zealand
Timothy A. Little, School of Earth Sciences, Victoria University of Wellington, Wellington, New Zealand, et al. Pages 707–723.
Keywords: oblique convergence, oblique ramping, exhumation, thermochronology, Southern Alps, Alpine fault.

Geophysical and geological data from the Southern Alps are used to explore the relationship between plate motions and crustal structure on the geomorphology, exhumation state, and deformation style of rocks uplifted along a major oblique-slip fault. A ~50 km-long segment of the Southern Alps has a higher uplift rate and more relief than surrounding regions, as is expressed by radiometric ages (cooling ages) that define a pattern of ages that increase away from the western side of the central Southern Alps. Higher uplift rates near the Alpine fault throughout the central Southern Alps may be related to an increase in the velocity of points to the east of the fault, associated with a strengthening of the crust and a narrowing of the range, a steeper inclination of the Alpine fault, and a boost in velocity resulting from deformation in that fault's eastern block. New age determinations from this block (Ar/Ar technique applied to the mineral hornblende) indicate that rocks transported to the surface from >500 °C since 6 Ma are confined to a 20 km-long bulge at the southern end of the central Southern Alps. Outside this region there has been <70 km of convergence on the Alpine fault plate boundary structure. We conclude that the Alpine fault is curved at depth and steepens in dip by 15°–20° as it enters the Southern Alps from the south to enhance local rates of rock uplift near Franz Josef and Fox Glaciers and to cause seismic locking of that historically quiescent fault.

Catastrophic emplacement of the Heart Mountain block slide, Wyoming and Montana, USA
Edward C. Beutner, Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA, and Gregory P. Gerbi, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA. Pages 724–735.
Keywords: Heart Mountain fault, fluid pressure, carbonate dissociation, Wyoming, Eocene, Absaroka Supergroup, Montana.

The Heart Mountain block slide, which is by far the largest subareal rock slide known on Earth, has been an enigma since its discovery over 100 years ago. Located just east of Yellowstone Park, this huge slide formed ca. 48 Ma as a volcanic complex collapsed, probably as a consequence of a volcanic explosion. Though other volcanic collapses have occurred, none has led to sliding of this magnitude, as the front of the slide advanced ~50 km into the adjacent basins. An unusual rock found along the slide surface, which is in carbonate rock below the volcanics, contains evidence of the mechanism that permitted such extensive sliding on a surface of very low (<2°) slope. This rock, composed largely of very small carbonate and volcanic rock grains in a carbonate matrix, contains accreted grains identical to those found in fallout from volcanic eruption clouds and impact ejecta clouds as well as in diatremes. In all of these settings, the accreted grains have been demonstrated or hypothesized to form in a gaseous suspension, and we believe that this is the secret of the very large displacement of the Hearth Mountain fault. Frictional heating along the sliding surface led to dissociation of carbonate, producing CO2 as the suspending medium and providing a gas cushion, which reduced friction along the fault to near zero. The rock that formed from this suspension of fine grains contains sedimentary-like features, as one would expect if the rock formed by settling of moving particles from suspension.

Carbonate dilation breccias: Examples from the damage zone to the Dent Fault, northwest England
Jon P.T. Tarasewicz, et al., Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Pages 736–745.
Keywords: Fault breccia, coseismic, damage zone, cementation, Variscan.

Earthquake faulting causes rock masses to slip past each other, but also fractures rocks bordering the fault. These fault damage zones are permeable, and allow deep-seated fluids--sometimes carrying economically important minerals or hydrocarbons--to migrate upward in the crust. New observations from the Dent Fault in northwest England confirm suspicions that such permeability may not last long. It may be sealed up by mineral cements in the tens to hundreds of years between one large earthquake and the next.

Evolution of the Cordilleran orogen (southwestern Alberta, Canada) inferred from detrital mineral geochronology, geochemistry, and Nd isotopes in the foreland basin
Gerald M. Ross, Geological Survey of Canada–Calgary, Calgary, Alberta T2L 2A7, Canada, et al. Pages 747–763.
Keywords: Cordillera, orogen, foreland, provenance, tectonics, isotopes.

Mountain belts are some of the most impressive geomorphic features on the surface of the Earth. The Canadian Cordillera is known best for its spectacular national parks (Banff, Jasper) and skiing (Lake Louise) but less well known is the unique value of this mountain range as a laboratory for understanding how mountain ranges form. In their article for GSA Bulletin, Gerald Ross and colleagues have used chemical and isotopic fingerprints of sand grains contained in ancient river sediments to reconstruct how the Canadian Cordilleran mountain belt grew over the last 150 m.y. Ross and colleagues examined sandstones and mudstones deposited by rivers draining the ancestral mountain belt in southwestern Alberta and looked specifically for key indicators that could be used to reconstruct how this part of the Canadian landmass formed. Several key findings include (1) the Rocky Mountains may have looked very similar to the present mountains between 150–140 Ma; (2) the mountain belt was affected by a long period of erosion (nearly 25 m.y., between 140 and 115 Ma) that dramatically reduced topography; and (3) the mountain belt was mantled by sediment, including volcanic ash, from the Coast Mountains and was exhumed to its present morphology sometime after 50 Ma. This study has shown that insights about tectonic processes on Earth can be derived by the analysis of seemingly innocuous grains of sand.

Fold-thrust belt evolution expressed in an internal thrust sheet, Sevier Orogen: The role of cataclastic flow
Zeshan Ismat, Department of Earth and Environment, Franklin and Marshall College, Lancaster, Pennsylvania, 17603, USA, and Gautam Mitra, Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York, 14627, USA. Pages 764–782.
Keywords: cataclastic flow, fold-thrust belt, Sevier, energy, folding.

Mountain belts typically form by shortening and thickening of crustal rocks; much of the shortening takes place by folding, a process of bending of sedimentary rock layers, and thrust faulting, a process in which older, deeper rocks are carried on top of younger, shallower rocks. How this deformation takes place within mountain belts depends on various physical conditions, such as temperature and pressure. At greater depths within the earth, the pressure and temperature increases and rocks deform more plastically. At shallower depths (~=12 km), rocks tend to fracture and behave in a more "brittle" manner. Because of poor preservation and lack of suitable methods for recognizing and studying these rocks, brittle deformation (or cataclasis) is often considered to be a minor component to the overall evolution of a mountain belt and is therefore ignored. In the Canyon Mountains, central Utah, the cataclasis is well preserved, and we developed a systematic method to analyze the fracture sets that developed from this type of deformation. Based on our work in the Canyon Mountains, we have shown that this cataclasis is a key player in the overall evolutionary history of mountain belts as a whole, and should not be ignored.

Rates and time scales of clay-mineral formation by weathering in saprolitic regoliths of the southern Appalachians from geochemical mass balance
Jason R. Price, et al., Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824-1115, USA. Pages 783–794.
Keywords: clays, rates, mass balance, Appalachians, regolith.

Rates of clay formation in western North Carolina soils have been calculated from stream water chemistry. These rates of clay-mineral formation have been used to determine the time needed for a 5% change in relative clay abundance in the soil; this corresponds to the "response time" of the clay mineral to, for example, a change in climate. The 5% change in relative clay abundance is the smallest change that can generally be detected using available analytical techniques. Response times range from tens of thousand to hundreds of thousands of years. Extrapolating the North Carolina clay formation rates to other southern Appalachian soils, the time required to form measured clay abundances ("production times") range from two thousand years to two million years, with mean values ranging from fifty thousand years to one million years. The results of this study are consistent with arguments that the best resolution of climate change in marine clay-rich sediments is approximately one to two million years.

Dating offset fans along the Mojave section of the San Andreas fault using cosmogenic 26Al and 10Be
A. Matmon, U.S. Geological Survey, Menlo Park, California 94025, USA, et al. Pages 795–807.
Keywords: San Andreas fault, offset alluvial fans, cosmogenic isotopes, slip rate, Little Rock Creek.

Analysis of cosmogenic 10Be and 26Al in samples collected from exposed boulders and from buried sediment from offset fans along the San Andreas fault near Little Rock, California, yielded ages ranging from 16 to 413 ka, which increase with distance from their source at the mouth of Little Rock Creek. In order to determine the age of the relatively younger fans, erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established. Boulder erosion rate, ranging between 17 and 160 mm k.y.–1, was estimated by measuring 10Be and 26Al concentrations in nearby bedrock outcrops. Since the boulders on the fans represent the most resistant rocks in this environment, we used the lowest rate for the age calculations. Monte Carlo simulations were used to determine ages of 16 ± 5 ka and 29 ± 7 ka for the two younger fan surfaces.

Older fans (>100 ka) were dated by analyzing 10Be and 26Al concentrations in buried sand samples. The ages of the three oldest fans range between 227 ± 242 ka and 413 ± 185 ka. Although fan age determinations are accompanied by large uncertainties, the results of this study show a clear trend of increasing fan ages with increasing distance from the source near Little Rock Creek, and provide a long-term slip rate along this section of the San Andreas fault. Slip rate along the Mojave section of the San Andreas fault for the past 413 k.y. can be determined in several ways. The average slip rate calculated from the individual fan ages is 4.2 ± 0.9 cm yr–1. A linear regression through the data points implies a slip rate of 3.7 ± 1.0 cm yr–1. A most probable slip rate of 3.0 ± 1.0 cm yr–1 is determined by using a chi-squared test. These rates suggest that the average slip along the Mojave section of the San Andreas fault has been relatively constant over this time period. The slip rate along the Mojave section of the San Andreas fault, determined in this study, agrees well with previous slip rate calculations for the Quaternary.

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