January-February 2011 GSA Bulletin highlights

(Press-News.org) Boulder, CO, USA - The Jan.-Feb. 2011 GSA Bulletin focuses on river geomorphology; submarine landslides and submarine uplift; the Sangamon paleosol in the Lower Mississippi Valley; the nature and formation of basins, plateaus, cratons, and mountains around the world, including continent building, plate tectonics, and subduction zones, and magmatism; charcoal accumulation rates and teleconnections among regional climates; zircon dating of Amazon River sand; the Messinian salinity crisis; and characteristics of the Sierra Madera impact structure.

Keywords: Sandy River, Sangamon paleosol, Budva Basin, submarine landslides, Green River Basin, western Tethys, Alborz mountains, Peloritani Mountains, Great Divide Basin, Purana basins, Xunhua and Linxia basins, Ellis Bay Formation, Hirnantian Isotopic Carbon Excursion, STEEP study, granite, charcoal accumulation rates, Macquarie Island, Mexican volcanic arc, Amazon River, Central Asian Orogenic Belt, Zuccale fault, Messinian salinity crisis, Sierra Madera impact structure

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Magnitude and timing of downstream channel aggradation and degradation in response to a dome-building eruption at Mount Hood, Oregon
Thomas C. Pierson et al., U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98684, USA. Pages 3-20.

Although it has been demonstrated that large explosive eruptions in tropical climatic settings with heavy rainfall can result in dramatic downstream geomorphic and sedimentation effects due to extreme sediment loading of rivers (such as at Mount Pinatubo in 1991), there has been no documentation of magnitudes and timing of such effects in rivers impacted by smaller and different types of eruptions in non-tropical climatic settings. This study by Thomas C. Pierson of the Cascades Volcano Observatory and colleagues documents to what degree downstream river channels can be affected in a temperate climatic setting about 70 km downstream from the source volcano following a moderate-size dome-building eruption. The bed of the lower Sandy River was raised vertically at least 23 m within about a decade of the start of the eruption in 1781, but more than half a century was required for it to incise back down to its present bed profile. A similar amount of channel aggradation occurring today would have a devastating impact on communities in the Sandy River valley, flooding and burying homes, businesses, roads, and other infrastructure. The study has demonstrated that relatively small eruptions can, under the right circumstances, cause localized river channel aggradation on a par with the Mount Pinatubo disaster far downstream from the source volcano.



Age, genesis, and paleoclimatic interpretation of the Sangamon/Loveland complex in the Lower Mississippi Valley, U.S.A.
Helaine W. Markewich et al., U.S. Geological Survey, 3039 Amwiler Road, Suite 130, Atlanta, Georgia 30360, USA. Pages 21-39.

For more than a century, the Sangamon paleosol, an ancient soil formed during the last major interglacial warm period, has been a recognized and studied component within glacially-derived geologic deposits in the central United States including the Upper Mississippi and Lower Missouri River valleys. Field studies by Helaine W. Markewich of the U.S. Geological Survey and colleagues demonstrate that a prominent reddish paleosol occurs in silt-rich deposits of the Lower Mississippi Valley from southernmost Illinois to northwestern Mississippi. Compositional, pedologic, micromorphologic, stratigraphic, and age data indicate that this paleosol is, wholly or in part, time equivalent to the Sangamon paleosol recognized in the central United States and underwent at least two soil-forming periods, the first from about 130 to 90 thousand years ago and a second from about 74 to 58 thousand years ago. Their data suggest that the Sangamon paleosol in the Lower Mississippi Valley formed when the regional paleoclimate was warm to hot with a wider range in temperature, precipitation, and evapotranspiration than present; had seasonal to decadal or longer periods of drought; and had southward trends of increasing temperature and precipitation and decreasing seasonality and variation in annual to decadal precipitation. In the Lower Mississippi Valley, the Sangamon paleosol is both an important stratigraphic marker and paleoclimatic indicator.



A biocalcification crisis at the Triassic-Jurassic boundary recorded in the Budva Basin (Dinarides, Montenegro)
Alenka E. Crne et al., Ivan Rakovec Institute of Palaeontology, ZRC SAZU, Novi trg 2, SI-1000 Ljubljana, Slovenia. Pages 40-50.

The mass extinction at the Triassic-Jurassic boundary (TJB) is one of the five major biotic crises of the Phanerozoic and coincides with volcanic activity in the Central Atlantic Magmatic Province. Data from the deep-water environment of the Budva Basin (Dinarides, Montenegro) provide evidence of an abrupt termination of carbonate deposition across the TJB linked to the perturbation of the global carbon cycle. A sudden decrease of carbonate deposition in the Budva Basin can be best explained by a biocalcification crisis (i.e., crisis of skeletal carbonate-producing organisms), resulting in reduced shedding of shallow-water carbonate into the deep-water basin. Alternatively, increased dissolution of carbonate would also result in reduced carbonate deposition in deep-water environments. According to Alenka Crne of the Ivan Rakovec Institute of Palaeontology, Slovenia, and colleagues, both non-exclusive scenarios support the hypothesis of ocean acidification due to increased CO2, SO2 and CH4 fluxes.



Tectonic and sedimentary evolution of the frontal part of an ancient subduction complex at the transition from accretion to erosion: The case of the Ligurian wedge of the Northern Apennines, Italy
Francesca Remitti et al., Dipartimento di Scienze della Terra, Universita di Modena e Reggio Emilia, Modena, Italy. Pages 51-70.

Plate subduction is an extremely dynamic process occurring with great amounts of released energy. Ninety percent of global seismicity is liberated during subduction. This process is also responsible for a great deal of slope instability, triggering huge submarine landslides that represent a geo-hazard as well as a sudden change of environmental conditions. In this work, Francesca Remitti of the Universita di Modena e Reggio Emilia, Italy, and colleagues report an extensive submarine mass wasting event (affecting an area of about 200 x 20 km) that occurred during subduction and changed the paleogeographic setting of the Northern Apennines of Italy in the Early Neogene (about 22 million years ago). They were able to find a causal link between the outsized slope instability of the Apennine margin and the contemporaneous tectonic changes occurring at the frontal part of the subduction system. The Apennine system, in fact, was shifting from being characterized by frontal accretion (a process nowadays occurring in southwest Japan) to frontal erosion (a process nowadays occurring in Pacific Central America).



Paleogeographic reconstruction of the Eocene Idaho River, North American Cordillera
Lauren M. Chetel et al., BP America, 200 Westlake Park Blvd., Houston, Texas 77079, USA. Pages 71-88.

Eocene Lake Gosiute in southwestern Wyoming was progressively filled in by volcaniclastic sediment between 49.6 and 47.0 million years ago. The source of this material has long been thought to have been the Absaroka volcanic province, immediately north of the greater Green River Basin. Lead isotope compositions of sandstone from this interval, however, are consistent with derivation from the Challis volcanic field. The 40Ar/39Ar ages of single detrital K-feldspar crystals from greater Green River Basin sandstones are nearly identical to 40Ar/39Ar eruptive ages for volcanic rocks of the Challis volcanic field (49.8-45.5 million years old), but Lauren M. Chetel of BP America and colleagues also identify Mesozoic and Proterozoic crystals that are consistent with cooling ages for rocks that were likely exposed in the Idaho segment of the North American Cordillera during the Eocene. Most of these rocks are traversed by or are up-gradient of a major Eocene paleovalley in central Idaho. The sudden appearance of Challis-derived sediment in the greater Green River Basin indicates that a major river, here named the Idaho River, connected the interior of the North American Cordillera to the greater Green River Basin. This connection requires 500 km to reach from central Idaho to the greater Green River Basin. The Idaho River probably carried detritus that was stripped from distant uplifted mountains above the active Challis volcanic field as far south as the Piceance Creek Basin, suggesting a total length of at least 1000 km. Middle Eocene metamorphic core complexes in the northern Cordillera likely produced a major highland in the headwaters of the Idaho River, which generated river systems that drained both eastward into the Wyoming foreland and westward into the Oregon Coast ranges.



Tectonic history of the western Tethys since the Late Triassic
Antonio Schettino and Eugenio Turco, Universita degli Studi di Camerino, Dipartimento di Scienze della Terra, Via Gentile III da Varano, 62032 Camerino (MC), Italy. Pages 89-105.

In a study, Antonio Schettino and Eugenio Turco of the Universita degli Studi di Camerino, Italy, illustrate the tectonic history of the western Tethys since the late Triassic through a set of computer-generated plate reconstructions. The plate motions model, which has been constrained by the Atlantic plate kinematics and on land geologic evidence, defines thirteen tectonic phases, spanning the time interval from the late Ladinian (230 million years ago) to the present. The tectonic evolution of the western Tethys area described in this article is compatible with both global-scale plate kinematics and geological constraints from on-land data observed across the present-day mosaic of displaced terranes surrounding the Mediterranean region.



Arabia-Eurasia continental collision: Insights from late Tertiary foreland-basin evolution in the Alborz Mountains, northern Iran
Paolo Ballato et al., Institut fur Erd- und Umweltwissenschaften and DFG Leibniz Center for Surface Process and Climate Studies, Universitat Potsdam, Karl-Liebknecht-Strasse 24, 14476 Potsdam, Germany. Pages 106-131.

Continental collision occurs at tectonic plate convergent margins and results in the suturing of continents. The collision between the Arabian and Eurasian plate produced high mountains, such as the Caucasus, the Zagros, and the Alborz mountains, and elevated plateaus, such as the Turkish-Iranian plateau, across a wide area in the Middle East. In this paper, Paolo Ballato of Universitat Potsdam and colleagues bring new geologic data from the Alborz mountains of northern Iran to constrain the timing and the mechanisms of the Arabia-Eurasia continental collision. Their data suggest the occurrence of a two-stage collision process involving "soft" collision starting from ca. 36 Ma followed by "hard" collision starting from ca. 20 Ma. During the "soft" collision, plate convergence was mainly accommodated by subduction of Arabia beneath Eurasia, while during the "hard" collision the plate convergence was mainly accommodated by crustal shortening and thickening that led to the development of high mountains similar to those of present day.



Shallow burial and exhumation of the Peloritani Mountains (NE Sicily, Italy): Insight from paleothermal and structural indicators
Luca Aldega et al., Dipartimento di Scienze Geologiche, Universita "Roma Tre," Largo San Leonardo Murialdo 1, 00146 Roma, Italy. Pages 132-149.

Clay minerals and organic matter dispersed in sediments undergo diagenetic and very low-grade metamorphic reactions in response to sedimentary and/or tectonic burial. The reactions are irreversible under diagenetic and anchizonal conditions, so that exhumed sequences generally retain indices and fabrics indicative of their maximum thermal maturity and burial. In particular, mixed layered clay minerals (I-S) and vitrinite reflectance coupled with apatite fission track data are widely used to investigate levels of diagenesis and time of exhumation in the sedimentary portions of the orogens but they are poorly integrated with classical stratigraphic and structural data. In this paper, Luca Aldega of the Universita Roma Tre and colleagues provide an extensive data set of clay mineralogy, thermo-chronology, organic petrography, and field based structural analysis to identify a strategy to detect the amount of out-of-sequence compressive reactivation in orogenic systems followed by late stages of exhumation. This research allowed them; (1) to reconstruct the burial evolution of the Stilo-Capo D'Orlando sedimentary basin (northeast-Sicily, Italy) during Oligocene-Miocene times; (2) to record a decrease in paleo-geothermal gradient values which marked the evolution of the basin from a fore-arc to a thrust-top setting during the convergence-collision process between the Calabria-Peloritani Arc and the African plate; and (3) to quantify the tectonic overburden that has been totally removed by extensional tectonics and erosion since the late Miocene. This research contributes to kinematic reconstructions in Sicily and can also have an impact on the understanding of the burial and exhumation processes of other segments of the Alpine orogenic system.



Climate-induced formation of a closed basin: Great Divide Basin, Wyoming
Paul L. Heller et al., Dept. of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA. Pages 150-157.

This study by Paul L. Heller of the University of Wyoming and colleagues evaluates the possible roles of tectonic activity versus climate change as a cause for formation of the Great Divide Basin, a large closed basin that sits along the U.S. Continental Divide in Wyoming. Modeling the impact of erosion and the observed minor faulting in the region demonstrates that basin closure could have been accomplished primarily by removal of material by the Platte River drainage nearby, but beyond the limits of, the Great Divide Basin. Isostatic rebound due to ongoing river erosion outside of the basin can account for a few tens of meters of uplift along the northeastern margin of the basin, forming a lip that closed the basin. Limited precipitation within the basin area precludes the ability of rivers draining the basin to keep pace within this modest uplift. Hence, climate played the overarching control in formation of this large closed basin in southern Wyoming.



Constraints on the development of Proterozoic basins in central India from 40Ar/39Ar analysis of authigenic glauconitic minerals
James E. Conrad et al., U.S. Geological Survey, MS-999, Menlo Park, California 94025, USA. Pages 158-167.

A number of sedimentary basins in central India, known as the Purana basins, formed following the assembly of the Indian subcontinent. These basins host sequences of sedimentary rocks that were deposited in alluvial and shallow marine environments, but have proven difficult to date accurately since they lack fossils and volcanic rocks are minor components. Based on sparse data, the Purana basins have been considered Neoproterozoic in age. James E. Conrad of the U.S. Geological Survey and colleagues present new 40Ar/39Ar dates of glauconitic minerals found in certain levels of these sedimentary sequences that give ages ranging from 1566 to 1686 Ma, indicating an older Mesoproterozoic age for the sedimentary rocks. Recognition of this older age for the Purana basins has important implications for studies of regional tectonics and Mesoproterozoic oceans.



Stable isotope evidence for topographic growth and basin segmentation: Implications for the evolution of the NE Tibetan Plateau
Brian G. Hough et al., Dept. of Earth & Environmental Sciences, University of Rochester, Rochester, New York 14627, USA. Pages 168-185.

Records of rock type, magnetic orientation, and stable isotope composition from the Xunhua and Linxia basins, located along the Tibetan Plateau's northeastern margin, suggest that topography in the intervening Jishi Shan mountain range began to develop between 16 and 11 million years ago. Changes in local climate patterns resulting from the evolution of local topography are tracked through comparison of stable isotope compositions of sediments on both sides of the Jishi Shan. Similarity of isotopic compositions is interpreted to reflect the presence of integrated basins whereas distinct isotopic compositions reflect separate basin histories. Originally similar (from 20.3-16 million years ago), the oxygen isotope trends in Xunhua and Linxia become distinctly different between about 16 and 11 million years ago, indicating separation of the adjacent basins. The difference in isotope trends stems from a large increase in oxygen isotope values in the Xunhua basin, whereas those in Linxia remain relatively constant. This pattern is interpreted as an increase in the aridity of the downwind Xunhua basin and suggests the development of a rain shadow behind the growing Jishi Shan. The development of a rain shadow and its associated aridity contrast to the wetter upwind side of the growing range highlights the importance of evaporative enrichment in this extremely continental setting and explains the presence of anomalously positive oxygen isotope values observed in modern rainfall. The findings presented by Brian G. Hough of the University of Rochester and colleagues add to a growing body of evidence for deformation along the Plateau's north and northeastern margins in the middle to late Miocene.



Chitinozoan biostratigraphy of a new Upper Ordovician stratigraphic framework for Anticosti Island, Canada
Aicha Achab et al., Institut National de la Recherche Scientifique, Centre Eau Terre Environnement, 490, rue de la Couronne, Quebec, QC, G1K 9A9, Canada. Pages 186-205.

Chitinozoans are marine, organic-walled microfossils of uncertain origin, known from the Ordovician to the Devonian. Because of their rapid evolution, they are regarded as a high-resolution stratigraphic tool for correlating and dating sedimentary strata. This study by Aicha Achab of Canada's Centre Eau Terre Environnement and colleagues reveals that an important part of the Ellis Bay Formation at the eastern end of Anticosti Island is not coeval with strata of the Ellis Bay at the western end of the island because it contains chitinozoans characteristic of the older Vaureal Formation. Based on these findings, new east-west correlations are proposed. These correlations are corroborated by geochemical, sedimentological, and sequence stratigraphic data, and bring to light the need for revising the Upper Ordovician stratigraphy of Anticosti. The comparison of chitinozoan assemblages from the Ellis Bay Formation, with coeval Upper Ordovician successions from other regions in the world, suggests that the formation is of Hirnantian age. An important glaciation occurred during the Hirnantian. The geochemical signature of this glaciation is recognized worldwide and it is known as the Hirnantian Isotopic Carbon Excursion (HICE). The work of Achab and colleagues has determined that, on Anticosti Island, the HICE begins just below the base of the Hirnantian, and has 3 peaks, the last being the most important. These results are in agreement with the data from reference sections in China and Scotland.



Application of LiDAR to resolving bedrock structure in areas of poor exposure: An example from the STEEP study area, southern Alaska
Terry L. Pavlis, Dept. of Geological Sciences, University of Texas at El Paso, El Paso, Texas 79968, USA; and Ronald L. Bruhn. Pages 206-217.

High resolution digital terrain models from airborne laser ranging, or LiDAR, have begun to see widespread use in the geosciences, but, to date, most applications have emphasized geological hazard assessments (e.g. earthquake studies, landslides, etc.) or studies of surface processes (e.g. geomorphology of coastal regions, rivers, etc.). Terry L. Pavlis of the University of Texas at El Paso and Ronald L. Bruhn of the University of Utah show that LiDAR terrain models have a potentially wider use in applications for resolving bedrock geology where outcrops are sparse, but where small-scale topography reflects the subtle expression of underlying bedrock structure. They emphasize the utility of shaded relief images prepared from the terrain model with a variety of simulated sun lighting angles as a tool for analyzing the geology as well as 3-D visualization of the structure. In this study, real time GPS, with a field Geographic Information System for mapping, also proved invaluable when evaluating the origins of features that were observed on shaded relief imagery prior to field work. Analysis of a subset of the LiDAR data indicates that bare-ground models in areas of open forest produce the best results when compared to aerial photography in which the terrain is virtually invisible in the forest shadows. Based on their experience in this project, Pavlis and Bruhn envision a need to reconsider the workflow of many field geology mapping projects to optimize the integration of field mapping with use of remotely sensed imagery and LiDAR elevation models.



The contribution of crustal anatexis to the tectonic evolution of Indian crust beneath southern Tibet
Jess King et al., Department of Earth Sciences, University of Hong Kong, Hong Kong SAR, China. Pages 218-239.

When continents collide to form mountains, the deformed crust invariably melts, producing granites. Just north of the Himalaya, in the Sakya region of southern Tibet, Jess King of the University of Hong Kong and colleagues discovered granites intruded in two distinct episodes after India-Asia collision. The two suites of granite have distinct geochemical and structural signatures that carry crucial implications for the tectonic evolution of the region. Early granites, dated between 28 and 23 million years old, may have helped to weaken the mid- to lower-crust, triggering the ductile flow predicted by some thermo-mechanical models of Himalayan growth during the Oligocene. Later granite plutons, dated between 15 and 9 million years old, were emplaced during uplift of thickened crust to shallow levels ( END