|12 Feb. 2013
GSA Release No. 13-09
The historic river crossing of Lee's Ferry marks an important and heavily debated geologic transition from the Canyonlands of the Colorado Plateau to the steep Grand Canyon.
See related article by J. Pederson, http://dx.doi.org/10.1130/G34051.1.
The Geological Society of America Journal Geology: Dynamic Geoscience
Boulder, Colorado, USA – New Geology science posted online ahead of print on 7 Feb. 2013 draws on data from several sites in the U.S., as well as work in Christchurch, New Zealand, Argentina, South Australia, Japan, the southeastern Pacific, South Africa, and Mars. Tectonics, flooding, carbon storage, fossils, earthquakes, aeolian transport, and volcanoes are discussed. Brief highlights follow.
1. The sedimentary fingerprint of the 2011 Mississippi River flood across the Louisiana coast
2. Evidence of extinction in the Confusion Range in western Utah
3. Global CO2 storage
4. Liquefaction in Christchurch, New Zealand, during a series of major earthquakes between 2010 and 2011
5. Classical geological structures develop at Earth's surface are due to relatively rapid processes in Earth's mantle.
6. Lee's Ferry, Arizona, and the Colorado Plateau bull's-eye
7. The Black Sea chemocline
8. Reinterpretation of data regarding subduction and exhumation in west-central Argentina
9. Environmental preferences of fossil assemblages of the Ediacara Biota
10. Metamorphic pressure in the Sambagawa belt, Japan
11. Phase transitions in quartz
12. The need for seismic hazard assessment for low strain rate areas to be revisited
13. The Pukapuka volcano chain
14. Evidence of dune and ripple migration over a dune field on Mars with wind direction estimates
15. Salmon habitat potential in the Oregon Coast Range
16. Field work in South Africa about the zigzag fossil burrow known as Treptichnus pedum
17. Earthquake swarms and Redoubt Volcano
Detailed highlights are provided below. GEOLOGY articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary GEOLOGY articles by contacting Kea Giles at the address above. Non-media requests for articles may be directed to GSA Sales and Service, . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published.
Tracking sedimentation from the historic A.D. 2011 Mississippi River flood in the deltaic wetlands of Louisiana, US
Nicole S. Khan et al., Sea Level Research, Dept. of Earth and Environmental Science, University of Pennsylvania, 240 South 33rd Street, Philadelphia, Pennsylvania 19104, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33805.1.
Management and restoration of the Mississippi River deltaic plain (southern United States) and associated wetlands require a quantitative understanding of sediment delivery during large flood events, past and present. Nicole S. Khan and colleagues investigate the sedimentary fingerprint of the 2011 Mississippi River flood across the Louisiana coast (Atchafalaya Delta, Terrebonne, Barataria, and Mississippi River Delta basins) to assess spatial patterns of sedimentation and to identify key indicators of sediment provenance. The sediment deposited in wetlands during the 2011 flood was distinguished from earlier deposits based on biological characteristics, primarily absence of plant roots and increased presence of centric (planktonic) diatoms indicative of riverine origin. By comparison, the lithological (bulk density, organic matter content, and grain size) and chemical (stable carbon isotopes of bulk organic matter) properties of flood sediments were nearly identical to the underlying deposit. The findings of this study not only provide insight into how large-scale river floods influence wetland sedimentation, they lay the groundwork for identifying previous flood events in the stratigraphic record.
Chemostratigraphy indicates a relatively complete Late Permian to Early Triassic sequence in the western United States
Matthew R. Saltzman and Alexa R.C. Sedlacek, School of Earth Sciences, The Ohio State University, Columbus, Ohio 43210, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33906.1.
The Permian-Triassic mass extinction is the largest in Earth's history. In the United States, the record of this critical geobiologic interval was long thought to be absent at a major sedimentary hiatus. Matthew R. Saltzman and Alexa R.C. Sedlacek present new evidence that a more continuous record of the Permian-Triassic boundary extinction event in shallow marine, Bahamas-like carbonate platform deposits of the Confusion Range in western Utah that represent the ancient western margin of Pangaea. This discovery is important because study of the causes of the Permian-Triassic extinction have come almost exclusively from the ancient Tethys region (localities in the Italian Alps, south China, Iran, Pakistan and others). The controversy over the likely causes is in large part a debate over the interpretation of the associated carbon isotope excursion, which is present in western Utah based on our integrated carbon, strontium and sedimentologic studies.
CO2 sequestration in a UK North Sea analogue for geological carbon storage
Niklas Heinemann et al., Ikon GeoPressure, Stockton Road, Durham DH1 3LE, UK. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33835.1.
Niklas Heinemann and colleagues examine a natural rock that has held carbon dioxide for millions of years, and discover that only 11% plus or minus 8% of the total carbon dioxide has reacted with the host rock. As only a low percentage has reacted after millions of years, then it is likely that very little reaction will occur in the thousands of years of storage which are needed for reduce anthropogenic climate change. This lack of reaction is not a problem for geological carbon storage, but means that such reaction cannot necessarily be relied upon to trap the carbon dioxide as solid mineral phases.
Recurrent liquefaction in Christchurch, New Zealand, during the Canterbury earthquake sequence
Mark C. Quigley et al., Dept. of Geological Sciences, University of Canterbury, Christchurch 8014, New Zealand. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33944.1.
Repeated liquefaction in Christchurch, New Zealand, during a series of major earthquakes between 2010 and 2011 resulted in the national government classifying almost 6000 residential properties in eastern Christchurch at a loss of over US$800 million. The lead author of this study lived in one of these properties throughout the earthquake sequence and witnessed 10 successive liquefaction episodes in his backyard (literally). The areal extent and thickness of sand ejected to the surface (i.e., "sand blows") during each one of these episodes was mapped and measured and compared to the seismological attributes of the well-recorded earthquakes, recorded by three seismometers within 3 km of the study site. This is the perhaps the world's best observational record of liquefaction. The seismologic threshold for causing liquefaction was determined, providing crucial data for liquefaction susceptibility studies at this and other analogous sites around the planet. New empirical relationships were derived to enable paleoseismologists to deduce the relative seismological characteristics (i.e., peak ground acceleration) of earthquakes from the relative thicknesses and/or areal extents of liquefaction-induced sand blows. This study improves the assessment of liquefaction potential and recognition of liquefaction phenomena from the geologic record globally.
Instability of a lithospheric step beneath western North Island, New Zealand
Tim Stern et al., Institute of Geophysics, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G34028.1.
This study shows that classical geological structures develop at Earth's surface that are due to relatively rapid processes in Earth's mantle. If good stratigraphic control exists, values for the viscosity of Earth's upper mantle can be estimated from this type of study. Tim Stern and colleagues highlight geological observations from western North Island, New Zealand, that show a migrating zone of uplift and subsidence. The amplitude of the uplift-subsidence is ~4 km, the wavelength is ~ 300 km, and it developed on a time scale of about 10 million years. Stern and colleagues show that this surface deformation is most readily explained by a mantle instability, or delamination, that started from a strike-slip zone with an element of tension (or what we call a transtensional zone). They develop a numerical model based on dimensional variables that allows us to test for the most important conditions to trigger an instability in this back environment and find that the key parameters are a low viscosity lower crust, and a ratio of crustal to mantle viscosity of about five and an absolute upper mantle viscosity of about 1020 Pa s.
Colorado River chronostratigraphy at Lee's Ferry, Arizona, and the Colorado Plateau bull's-eye of incision
Joel L. Pederson et al., Dept. of Geology, Utah State University, Logan, Utah 84322, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G34051.1.
The historic river crossing of Lee's Ferry marks an important and heavily debated geologic transition from the steep Grand Canyon downstream to the broader Canyonlands of the central Colorado Plateau upstream. New data from the spectacular suite of river terraces here allow Joel L. Pederson and colleagues to reconstruct the recent geologic history of the Colorado River. Results indicate a strong response to climate drivers that is superimposed upon an overall incision rate of ~350 meters per million years (~35 cm per thousand years). This very well-constrained and surprisingly healthy incision rate is greater than that recorded downstream in Grand Canyon, yet is somewhat slower than rates upstream in the central Colorado Plateau, revealing an overall bull's-eye pattern of rapid incision in the central Colorado Plateau. This pattern does not match recently proposed sources of uplift along the southwestern flank of the plateau. Instead, Pederson and colleagues suggest this incision bull's-eye is primarily set by isostatic rebound from the deep and broad erosion of weak rocks in the central plateau, which is in turn driven by the propagating response to the geologically recent integration of the Colorado River off the plateau in the Grand Canyon region.
Establishment of euxinic conditions in the Holocene Black Sea
Sebastian Eckert et al., Mikrobiogeochemie, Institut für Chemie und Biologie des Meeres (ICBM), Carl von Ossietzky Universität, Postfach 2503, D-26111 Oldenburg, Germany. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33826.1.
The Black Sea forms the largest ocean basin on Earth, which is completely devoid of oxygen below 150-200 m water depth. About 9,000 years ago -- during the Holocene sea level rise -- Mediterranean seawater spilled into the Black Sea fresh-water lake via the Bosporus. The inflowing dense seawater created a physical and chemical boundary, the chemocline, separating oxic surface water from hydrogen sulfide-bearing deep water. The speed and nature of the post-glacial seawater inflow remains a matter of debate, with the most drastic scenario envisioning a rather catastrophic flooding of the Black Sea shelf and presumed dramatic consequences for the coastal inhabitants of that time. Based on coeval geochemical signatures in several sediment cores retrieved from different locations in the Black Sea, Eckert et al. were able to reconstruct the vertical migration of the Black Sea chemocline. They found that, contrary to the great flood hypothesis, the chemocline rose rather slowly and inundated the shelf in at least two distinct steps since the last glacial.
Exhumation history of the Andean broken foreland revisited
Federico M. Dávila, CICTERRA-CONICET, Universidad Nacional de Cordoba, Cordoba, Argentina; and Andrew Carter. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33960.1.
In west-central Argentina, to 400-800 km from the Peru-Chile Trench, subduction is shallow. Such tectonic scenarios are appropriated to produce increases in rock uplift and exhumation, where thrusting and dynamic forces work together. Nevertheless, a prevalence of pre-Cenozoic apatite fission-track cooling ages suggests that there would have been such link in Argentina. Reinterpretation of published AFT exhumation data (more than 320 ages) using more realistic lower geothermal gradient values allows for substantial exhumation and explains the lack of Cenozoic exhumation ages across the foreland. Flat subduction would have removed the mantle wedge, a significant source of heat flow into the crust.
How well do fossil assemblages of the Ediacara Biota tell time?
James G. Gehling, South Australia Museum and University of Adelaide, North Terrace, Adelaide, South Australia 5000, Australia; and Mary L. Droser, Dept. of Earth Sciences, University of California, Riverside, California 92521, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33881.1.
Fossils of the Precambrian Ediacara Biota hold the key to understanding the origin and evolution of early animal life on this planet. Abundant fossils of these soft-bodied organisms occur globally and span 40 million years. These fossils have traditionally been loosely grouped into three associations interpreted to have temporal significance -- with certain fossils representing older rocks and others representative of younger times. Research reported here demonstrates that actually these extinct organisms had very strong environmental preferences and that these associations do not reflect time or the relative age of these fossils but rather the environmental preference. Fossils from all three assemblages occur in rocks representing four different environments in a single geographic area west of the Flinders Ranges in South Australia. Only two fossils occur in all four environments; the majority of the taxa occur in only one or sometimes two environments. Thus, surprisingly, Earth’s earliest marine ecosystem was very well differentiated into distinct communities, much as communities are today. Furthermore, the absence of a particular type of organism may not indicate that it has gone extinct but rather may indicate that the preferred environment was not preserved.
Paleo-mantle wedge preserved in the Sambagawa high-pressure metamorphic belt and the thickness of forearc continental crust
Mutsuki Aoya et al., Institute of Geology and Geoinformation, National Institute of Advanced Industrial Science and Technology (AIST), Central 7, Tsukuba 305-8567, Japan. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33834.1.
Mantle rocks, such as peridotite and serpentinite, are rarely exposed on the Earth's surface. In a case where mantle rocks are distributed in deeply subducted and metamorphosed regions, there are two possible origins for mantle rocks: lower part (footwall-side) or upper part (hanging-wall-side) of the subduction boundary. This footwall-hanging-wall problem has been rarely conclusive. In a classic high-pressure metamorphic belt, the Sambagawa belt, Japan, it has been revealed by wide-ranging field studies that mantle-rock bodies are common in the higher-pressure (deeper) part of the belt but are nearly absent in the low-pressure (shallower) part. If the mantle rocks originated in the footwall, they should be metamorphosed together with the subducted material, and their distribution should not show any correlation with metamorphic pressure. The restricted distribution, therefore, is strong evidence for hanging-wall mantle-wedge origin of the mantle rocks, indicating that subducted oceanic plate can transport substantial amount of hanging-wall material to the Earth's surface. Metamorphic pressure of the boundary region between the mantle-rock present and absent areas is known to be 8.0 to 9.5 kbar, and the corresponding depth (~30 to 35 km) represents the thickness of the continental crust in the Cretaceous Sambagawa subduction zone
Effect of phase transitions on seismic properties of metapelites: A new high-temperature laboratory calibration
Alba S. Zappone, Institute for Process Engineering, Swiss Federal Institute of Technology, Zurich 8092, Switzerland, and Philip M. Benson. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33713.1.
Phase transitions are a common phenomenon in all materials, including geological minerals. Of these minerals, arguably the most important in crustal rock-forming processes is quartz. Alba S. Zappone and colleagues report new experimental laboratory measurements of the seismic properties of a rock that occurs abundantly in the middle continental crust (metapelite), presenting evidence for phase transitions using elastic wave velocity methods. They argue that well-calibrated data relating the Vp to pressure and temperature on rocks representative of the middle crust are therefore the key to using this method as a diagnostic tool for determining temperature conditions at depth.
Erosion-induced isostatic rebound triggers extension in low convergent mountain ranges
P. Vernant et al., Géosciences Montpellier, CNRS-Université Montpellier 2, Montpellier 34095, France. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33942.1.
Earthquakes occur even in places where no deformation related to horizontal tectonic plate motion is expected. Among the regions where no present-day horizontal deformation can be quantified to the level of accuracy of GPS measurements (i.e., 0.3 mm/yr), the Alps and the Pyrenees mountain ranges (Western Europe) or the New Madrid area (USA) can be cited. In these areas, the causes for the recorded seismicity are puzzling to earth scientists. Here, P. Vernant and colleagues show that erosion of topography can lead to vertical motions, which in turn can create very low horizontal deformations and earthquakes. This implies that seismic hazard assessment for low strain rate areas needs to be revisited, because erosion-related earthquakes could increase seismic hazard.
Non-hotspot volcano chains produced by migration of shear-driven upwelling toward the East Pacific Rise
Maxim D. Ballmer et al., Dept. of Geology and Geophysics, School of Ocean and Earth Sciences and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33804.1.
Most volcanism on Earth reflects plate-tectonic processes, occurring along the boundaries between the moving plates. Volcanism occurring within plate interiors is instead typically explained by deep-rooted "plumes" that transport hot material upward to feed stationary "hotspots" of volcanism. As the plate moves above them, these hotspots produce chains of volcanoes, such as the Hawaiian Islands. One volcano chain, with the exotic name "Pukapuka," submerged beneath the southeastern Pacific, lacks a characteristic hotspot feature: Its source of volcanism is not stationary. Instead, it has been moving eastward rather quickly. By comparing computer simulations with ship-based observations of the chemistry, volumes, and ages of Pukapuka lavas, Ballmer and coauthors demonstrate a new mechanism for migrating intraplate volcanism. They show that the massive injection of hot material by multiple plumes producing many of the Polynesian island chains also spreads out beneath the Pacific plate. As it spreads, it breaks up into multiple fingers, which melt at their tips to sustain volcanism. The Pukapuka volcano chain is formed by one such finger, which has traversed the Pacific plate and recently emerged beneath the neighboring Nazca Plate. These findings are important for understanding our planet's interior dynamics and how they produce volcanism.
Pervasive aeolian activity along rover Curiosity’s traverse in Gale Crater, Mars
Simone Silvestro et al., Carl Sagan Center, SETI Institute, 189 North Bernardo Avenue, Suite 100, Mountain View, California 94043, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G34162.1.
Simone Silvestro and colleagues present evidence of dune and ripple migration over the dune field that lies along Curiosity’s traverse to Aeolis Mons in the NASA Mars Science Laboratory (MSL) landing site. They further estimate wind directions within the dune field through analysis of dune and ripple morphologies and atmospheric modeling and measured a ripple migration rate of 0.66 m/Earth year and dune migration rate of 0.4 m/Earth year in the southwest dune field sector. Ripple and dune morphologies indicate a bidirectional wind regime with winds mainly coming from the ENE and from the NW. These findings show that the MSL landing site is an active environment with the action of the wind being the major agent of landscape modification in the current atmospheric setting. Their constraints on the wind regime provide the unique opportunity to use ground measurements from MSL to test the accuracy of winds predicted from orbital data and make the first ground observations of a known active environment on Mars.
Controls on valley width in mountainous landscapes: The role of landsliding and implications for salmonid habitat
C. May et al., Dept. of Biology, James Madison University, Harrisonburg, Virginia 22807, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33979.1.
Most of the highly productive habitat for salmon occurs in low gradient streams with broad valleys, yet there is limited understanding of what controls the width of valleys in mountainous landscapes. C. May and colleagues used high-resolution topographic data in the Oregon Coast Range to explore controls on valley width and couple these findings with models of salmon habitat potential. The first step in this inquiry was to determine how valley floor width varies with drainage area in a catchment that exhibits relatively uniform ridge-and-valley topography sculpted primarily by shallow landslides and debris flows. In this steep and highly dissected terrain, valleys get systematically wider as one moves downstream according to a simple mathematical equation. This drainage area-valley width relation was our baseline for comparing valley width in a neighboring catchment with large deep-seated landslides. In this terrain, anomalously wide valleys tend to occur upstream of ancient large landslides. According to habitat potential models for coho salmon, broad valley segments associated with deep-seated landsliding result in a larger proportion of the channel network with the capacity to provide productive habitat. Because landslides in this area are structurally controlled, our findings suggest a strong link between geologic properties and aquatic habitat realized by geomorphic processes.
Environmental tolerance and range offset of Treptichnus pedum: Implications for the recognition of the Ediacaran-Cambrian boundary
Luis A. Buatois et al., Dept. of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon SK S7N 5E2, Canada. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G33938.1.
New research by an international team of scientists led by the University of Saskatchewan has revealed novel information about the most important transition in the history of life: the Ediacaran-Cambrian boundary. The Ediacaran-Cambrian boundary (about 541 million years ago) represents a major divide in the history of the biosphere. Ediacaran biotas were dominated by soft-bodied organisms that are now considered for the most part to be unrelated to modern metazoans. On the other hand, the Cambrian witnessed the rapid development of almost all modern groups of animals, including the rise of skeletal faunas, a major evolutionary event known as the Cambrian explosion. Field work undertaken by this research team in South Africa has shown that the fossil burrow used to place the position of this boundary (a zigzag burrow known as Treptichnus pedum) is found in sediments that were laid down in a wide range of shallow-marine settings, and is present not only in deposits that formed offshore in several meters of water, but also in intertidal deposits right at the shoreline. The use of this burrow (most likely produced by soft-bodied worms known as priapulids) to delineate the Ediacaran-Cambrian boundary has been controversial because many scientists believed its presence was restricted to certain environments, therefore reducing its utility as a general fossil guide. To establish the broad environmental tolerance of this form, the research team combined detailed field work with conceptual and methodological tools employed in the recently established field of Stratigraphic Paleobiology. This study provides a solid base to the paleontologic criteria employed to establish the Ediacaran-Cambrian boundary, a topic that is receiving huge attention within the paleobiological and geological community because of its implications for the early evolution of life.
Using repeating volcano-tectonic earthquakes to track post-eruptive activity in the conduit system at Redoubt Volcano, Alaska
Helena Buurman et al., Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, Alaska 99775-7320, USA. Posted online 7 Feb. 2013; http://dx.doi.org/10.1130/G34089.1.
Redoubt Volcano erupted for 3 months in 2009, generating spectacular ash clouds and producing a large lava dome that remains in place today. Both this eruption and the last eruption in 1989 were preceded by bursts of earthquake activity -- known as earthquake swarms-- that provided scientists with early warning of the imminent eruption. When more earthquake swarms occurred around the vent in the year after the eruption, the alert levels were raised in anticipation of eruptive activity. No eruption occurred, however, so the earthquakes were dismissed as having occurred either within the cooling lava dome or in the summit glaciers rather than being generated by the volcano. Helena Buurman and colleagues show that deeper earthquakes identical to those recorded during the 2009 eruption were reactivated during these earthquake swarms. Because these deeper earthquakes were recorded during eruptive activity, their presence after the eruption suggested that the magmatic system at Redoubt was still active. An active gas vent on the lava dome also closed after the last of the earthquake swarms, providing further evidence that although the volcano had not been erupting, the system had remained open and active for nearly a year before finally closing -- much longer than previously thought.