||3 July 2007
GSA Release No. 07-28
July / August Media Highlights:
The Geological Society of America Bulletin
Boulder, CO, USA – Geology topics of interest include new insights into Permian-Triassic boundary events and unraveling the mystery of North America's sheet sandstones.
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- Erosional truncation of uppermost Permian shallow-marine carbonates and implications for Permian-Triassic boundary events
Jonathan L. Payne et al., Department of Geological & Environmental Sciences, Stanford University, Stanford, California, 94305, USA. Pages 771-784.
- Keywords: extinction, carbonate platform, carbon cycle, microbialite, calcification
- The end-Permian catastrophe eliminated a greater proportion of marine animal diversity than any other mass extinction in the geological record. The cause(s) of the event are incompletely understood, but the stratigraphic distributions of fossil species and associated radiometric age constraints indicate that extinction was geologically rapid, occurring in less than 500 ky. On many carbonate platforms, fossiliferous uppermost Permian beds are abruptly overlain by microbial buildups. Shallow-marine carbonate strata exposed in South China, Japan, and Turkey are each characterized by abrupt transitions from fossiliferous uppermost Permian beds to microbial buildups. In each case, uppermost Permian strata are erosionally truncated. However, biostratigraphy and carbon isotope stratigraphy indicate that little time is missing at the boundary. Moreover, petrographic observations and isotope data reveal no positive evidence for subaerial exposure associated with the episode of erosional truncation. Hence, it appears that erosion occurred in a submarine environment. An episode of submarine carbonate dissolution induced by rapid, massive CO2 release from a sedimentary carbon reservoir could explain the coincidence of erosion with the selective extinction of heavily calcified marine invertebrate genera and the onset of a large negative carbon isotope excursion. Thermal metamorphism and decarbonation of coal and carbonate rocks on the Siberian Craton during intrusion of Siberian Trap magma may account for massive carbon release at this time.
- Evidence of an abrupt environmental disruption during the mid Paleocene biotic event (Zumaia Section, W. Pyrenees)
Gilen Bernaola et al., Departamento de Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Universidad del País Vasco, Bilbao, Spain. Pages 785-795.
- Keywords: Mid Paleocene Biotic Event, calcareous nannofossils, planktic foraminifera, benthic foraminifera, Zumaia, Pyrenees.
- During the Mid Paleocene Biotic Event (MPBE) an abrupt environmental disruption occurred in the photic zone and at the seafloor. Calcareous nannoplankton, planktonic foraminifer and benthonic foraminifer assemblages at Zumaia section (Western Pyrenees) experienced a rapid and remarkable transformation. The major calcareous plankton assemblage changes suggest a shift from relatively cooler mesotrophic to warmer, more oligotrophic conditions, indicating a disturbed environment due to the warming of the ocean. Benthic foraminifer assemblages were also significantly affected by the MPBE, where diversity of the assemblages and buliminids show net decline and the low food and opportunistic taxa increase in abundance. The reorganization of the planktic ecosystem possibly involved changes in the food flux (type, quantity) to the seafloor, thus triggering changes in the benthic communities. The MPBE was a short-lived event. According to the counting of limestone/marl couplets, which have demonstrated to be the stratigraphic expression of precession cycles throughout the Zumaia section, the MPBE lasted for about 52-53 kyr, with the core of the event representing about 10-11 kyr. The Zumaia section is the first land-based locality in which the MPBE is recognized and described in detail. Due to its expanded character and excellent paleontological record, this section may prove to be a global reference section for the study of this short-lived event.
- Formation of amphitheatre-headed valleys by waterfall erosion after large-scale slumping on Hawai'i
Michael P. Lamb et al., University of California, Berkeley, Earth and Planetary Science, Berkeley, CA 94720-4767, USA. Pages 805-822.
- Keywords: Amphitheater, Sapping, plunge pool, waterfall, knickpoint, Hawaii
- In this article, we investigate the origin of Kohala amphitheatre-headed valleys in Hawai'i, which are the most widely cited terrestrial analogs for Martian valleys in basalt. We argue that valley head erosion is dominated by waterfall plunge pools contrary to previous interpretations that called upon erosion from spring water (i.e. seepage erosion or groundwater sapping) and propose that the valleys formed following a large flank collapse of Kohala volcano, the Pololū Slump. As dominant streams poured over this headscarp as waterfalls, vertical plunge pool erosion and undercutting caused upstream propagation of knickpoints, eventually producing amphitheatre-headed valleys. A simple expression is presented for upslope headwall propagation by vertical waterfall erosion based on abrasion by impacting sediment particles in plunge pools.
- Basin-wide magnetostratigraphic framework for the Bighorn Basin, Wyoming
William Clyde et al., University of New Hampshire, Department of Earth Sciences, James Hall, 56 College Road, Durham, New Hampshire, 03824-3589, USA. Pages 848-859.
- Keywords: magnetostratigraphy, Paleocene, Eocene, Bighorn Basin, mammals, plants.
- New paleomagnetic data from the Bighorn Basin in Wyoming are used in this article to correlate between multiple stratigraphic sections in different parts of the basin. Fossil and geochemical information from these varied locations in the basin can now be stratigraphically integrated and compared to similar records from elsewhere in the world. These new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as Biohorizon B. Biohorizon B is now constrained to occur close to the position of a significant carbon and oxygen isotope excursion that has been recorded in deep marine sediment cores. This suggests a potential link between global changes in climate and mammalian turnover at this time. These new data also suggest that the basin started to form earlier, and subsided more rapidly, in the northwestern part of the basin, which suggests that the Beartooth Uplift played a major role in basin formation.
- High-resolution sequence stratigraphy of lower Paleozoic sheet sandstones in central North America: The role of special conditions of cratonic interiors in development of stratal architecture
Anthony C. Runkel et al., Minnesota Geological Survey, St.Paul, Minnesota 55114-1057, USA. Pages 860-881.
- Keywords: stratigraphy, sheet sandstones, subsidence, cratonic interior, Cambrian, Ordovician.
- This article provides important new insight into the origin of "sheet sandstones," common but puzzling formations in the central part of North America and on other continents. These 500-million-year-old formations are well known to geologists for being unusually pure (composed almost entirely of quartz) and for their thin, widespread, blanket-like distribution. Our ten-year study has removed much of the long-standing mystery about how such sheets of sand were laid down across vast expanses of the continents.
- Geological records of the Lhasa-Qiangtang and Indo-Asian collisions in the Nima area of central Tibet
Paul Kapp et al., University of Arizona, Department of Geosciences, Tucson, AZ 85721, USA. Pages 917-932.
- Keywords: Tibet, plateau, thrust belt, Indo-Asian collision, suture zone, basin development
- The vast, internally drained region of the Tibetan plateau interior is the focus of some of the most provocative concepts in continental tectonics today, yet our understanding of its geological evolution and uplift history remains poor. This article presents geological and geochronologic data from the Nima basin area along the Bangong suture zone, located near the center of the Tibetan plateau. The data demonstrate that the suture zone was significantly shortened and uplifted above sea level during Early to mid-Cretaceous collisional orogenesis and subsequently reactivated during the mid-Tertiary. Mid-Tertiary reactivation is recorded by a well dated (late Oligocene – early Miocene) and very thick (~4000 m) succession of syncontractional nonmarine strata that was deposited at elevations in excess of 4.5 km based on paleoelevation results presented elsewhere. Oligo-Miocene reactivation of the Bangong suture zone was coeval with reactivation of the Indus-Yarlung suture zone to the south and immediately predated shear zone development within the Himalayan Main Central Thrust sheet. These timing relations suggest that mid-Tertiary suture zone reactivation in Tibet may have driven the Himalayan thrust belt into a supercritical state, leading to the subsequent southward emplacement/extrusion of the Main Central Thrust sheet. Collectively, the results suggest that mountain building initiated in Tibet prior to the Indo-Asian collision and that the Himalayan thrust belt is merely the southern part of a much larger Himalayan-Tibetan composite orogenic system.
- Validation of tectonic reconstructions by crustal volume: New Zealand through the Cenozoic
R.A. Wood and V.M. Stagpoole, Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand. Pages 933-943.
- Keywords: tectonic models, plate tectonics, sediment budget, mass balance, New Zealand
- Plate tectonic processes often destroy evidence of the geologic history of a region. Analysis of the interaction of the major components of the mass balance--sedimentation, paleogeography, growth of a crustal root, and non-rigid deformation--offers a means to test first-order plate reconstructions. In this paper, we balance total rock volume through time and quantitatively test hypotheses about the tectonic evolution of the Pacific-Australian plate boundary in the New Zealand region since 65 Ma, including areas for which there is little or no geologic record. Although the solutions are non-unique, the uncertainties in each factor can be estimated and used to constrain the models. The plate reconstructions in this article are consistent with present-day knowledge of New Zealand geology, and are used to make testable predictions about the deformation process.
- Late Paleozoic Tectonic History of the Ertix Fault in the Chinese Altai and its Implications for the Development of the Central Asian Orogenic System
Stephanie M. Briggs et al., University of California, Los Angeles, Earth and Space Sciences, Los Angeles, California, 90095-1567, USA. Pages 944-960.
- Keywords: Ertix, Irtysh, central Asia, magmatism, Xinjiang, deformation
- This article provides data about the timing of magmatism, deformation, and faulting in northwestern China as Asia was being tectonically assembled about 450 to 250 million years ago. The authors have established this deformation and faulting to be younger than previously recognized and hypothesize about the connection between deformation studied and the regional tectonics at that time.
- Geology and thermochronology of Tertiary Cordilleran-style metamorphic core complexes in the Saghand region of central Iran
Charles Verdel et al., California Institute of Technology, Geological and Planetary Sciences, Pasadena, California 91125, USA. Pages 961-977.
- Keywords: metamorphic core complex, extensional tectonics, thermochronology, Iran, Alpine-Himalyan orogen, Arabia-Eurasia collision
- The collision of the Arabian and Eurasian plates about 10 to 20 million years ago in what is now Iran is the youngest continent-continent collision on Earth. Approximately 20 million years prior to the collision, a pulse of volcanism occurred across Iran. We present new field observations along with geochronologic and thermochronologic data which indicate that at least 30 km of crustal extension took place in central Iran at the same time as the volcanic pulse. This phase of extension created geologic features referred to as metamorphic core complexes, which have also been recognized in the western United States and throughout the Alpine-Himalayan mountain chain. Our data also refine the timing of the Arabia-Eurasia collision.
- Dating polyphase deformation across low-grade metamorphic belts: An example based on 40Ar/39Ar muscovite age constraints from the southern Quebec Appalachians, Canada
Sébastien Castonguay et al., Geological Survey of Canada, GSC-Québec, Quebec City, Quebec G1K 9A9, Canada. Pages 978-992.
- Keywords: Quebec Appalachians, 40Ar/39Ar geochronology, polyphase deformation, deformation age, greenschist facies metamorphism, dynamic recrystallization.
- This article uses field-based structural and metamorphic observations and the 40Ar/39Ar isotopic dating method to propose a refined model for the tectonic evolution of the Appalachian belt of southern Quebec during the Early Paleozoic. The 40Ar/39Ar dating of single-grains of muscovites, a type of white mica found in the metamorphosed rocks and in the rock fabric of faults and shear zones, has allowed the authors to establish the precise timing of mountain-building processes. The results show that these processes, such as thrust faulting, extensional faulting, and metamorphism have occurred from ca. 456 Ma to ca. 390 Ma in different places and time periods across the Sutton Mountains of southern Quebec.
- Early Pliocene change of deposition style in the Cape Basin, southeastern Atlantic
Estella Weigelt and Gabriele Uenzelmann-Neben, Alfred Wegener Institute for Polar and Marine Research, Bremen 27616, Germany. Pages 1004-1013.
- Keywords: Cape Basin, Seismostratigraphy, Upwelling, ODP Leg 175, Closure American Seaway, Early Pliocene
- This article presents a high-resolution seismostratigraphic model for the Cape Basin by the incorporation of reflection seismic records with drill site information. Many effects of climate change, ocean currents, and tectonics on sedimentary deposition can be detected in seismic reflection patterns. A marked increase in seismic reflection amplitudes since the Early Pliocene (ca. 4.4 Ma) indicates a reorganisation in the depositional system associated with enhanced upwelling as a consequence of the closure of the Central American Seaway.
- Petrotectonic evolution and melt modeling of the Peñon Blanco arc, central Sierra Nevada foothills, California
Cameron A. Snow, Apache Corporation, 2000 Post Oak Blvd., Suite 100, Houston, Texas, 77056-4400, USA. Pages 1014-1024.
- Keywords: California, volcanic arcs, Cordilleran margin, Jurassic, tectonics, geochemistry.
- Continental growth and element transfer from Earth's mantle to its crust occurs in volcanic arcs associated with subduction zones. The authors present geochemical data from the Penon Blanco Arc of California's Sierra Nevada Foothills. The ca. 200 Ma volcanic rocks of the Penon Blanco Arc record a rich history of near-trench volcanism. Modeling of the data suggests that these rocks formed from a small percentage of partial melting, which when coupled with the fact that they are quite extensive, implies that near-trench volcanism could rapidly fractionate the mantle with respect to trace elements.
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