||2 May 2006
GSA Release No. 06-17
May / June Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - The May-June issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several newsworthy items. Topics include: discovery of Permian-Triassic boundary section in the U.S.; analysis of the Permian-Triassic boundary in Antarctica; evidence of long periods of surface water flow at Mars' Eberswalde delta; new maps of submarine canyons; and sea-level change and subsidence along the U.S. mid-Atlantic margin.
Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns, . 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.
Non-media requests for articles may be directed to GSA Sales and Service, .
- Geochronology and stratigraphy of late Pleistocene lake cycles on the southern Bolivian Altiplano: Implications for causes of tropical climate change
Christa Placzek, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, et al. Pages 515-532.
- Keywords: lakes, U-Th, climate, Bolivia, insolation, ENSO.
- Large, deep lakes approaching the size of modern Lake Baikal and Lake Michigan once occupied basins on the arid southern Bolivian Altiplano, which is now covered by parched salt pans. We sampled and mapped lake sediments to determine the age and extent of these ancient lakes. We found that large lakes once occupied the same basin at two different times, between 14,100-18,100 and 98,000-120,000 years ago. Smaller lakes filled the basins between 11,000-13,000, between 20,000-24,000, around 46,000 and between 80,000-95,000 years ago. Such major changes in lake size seem to demand equally large shifts in ancient climate, which we think may be linked to changes moisture content of the Amazon basin and changes in the well-known El Niņo/La Niņa cycle in the adjacent Pacific Ocean.
- Migration of the Pee Dee River system inferred from ancestral paleochannels underlying the South Carolina Grand Strand and Long Bay inner shelf
Wayne E. Baldwin, U.S. Geological Survey, St. Petersburg, Florida 33701, USA, et al. Pages 533-549.
- Keywords: Atlantic Coastal Plain, continental shelf, paleochannels, coastal sedimentation, sea-level changes, paleogeographic maps.
- Onshore boreholes and nearshore, high-resolution, seismic-reflection data have allowed for detailed mapping of a complex network of paleochannels underlying the lower coastal plain and inner shelf of northeastern South Carolina. Seven paleochannel groups within this network have been interpreted to represent distinct generations of the ancestral Pee Dee River system. These paleochannels outline a prolonged southwestward migration of this major Piedmont-draining river between the Late Pliocene and present. Our article discusses inferred ages for the individual paleochannel groups and a temporal framework for the overall migration, the relative importance of the mechanisms that forced the migration, comparisons between the Pee Dee paleochannels and those mapped at other sites along the U.S. Atlantic margin, and the implications of this migration on the long term evolution of the northeastern South Carolina coast.
- Transgressive oversized radial ooid facies in the Late Jurassic Adriatic Platform interior: Low-energy precipitates from highly supersaturated hypersaline waters
Antun Husinec, Croatian Geological Survey, Sachsova 2, HR-10000 Zagreb, Croatia, and J. Fred Read, Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA. Pages 550-556.
- Keywords: radial ooids, low energy, platform-interior parasequences, Late Jurassic, Adriatic Platform.
- The paper describes unusual oolitic facies that form bases of many parasequences in parts of the Late Jurassic platform, Croatia. The radial, oversized irregular ooids, some of which are broken, were previously interpreted as vadose origin, but appear to be characteristic of low energy settings and hypersaline, highly supersaturated waters with low Mg/Ca ratios, that typified the interior of the Late Jurassic platform. They also occur elsewhere in Tethys, but appear to be an unusual facies at other times in geologic history.
- Quantitative geomorphology of the Mars Eberswalde delta
Lesli J. Wood, Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78713-8924, USA. Pages 557-566.
- Keywords: Mars, delta, geomorphology, Eberswalde, channel, planetary geology.
- The Mars Eberswalde Delta, formed in six stages, is comprised of both tongue- and leaf-shaped lobes with distinct channels, meander belts, and accretionary beds. Measurements of sinuosity, channel width, meander belt width and meander lengths enable comparison to Earth deltas. Overlapping geometries and incised lobes suggest long periods of surface water flow and deltaic deposition, possibly into a standing body of water.
- Quantification of the effects of eustasy, subsidence, and sediment supply on Miocene sequences, mid-Atlantic margin of the United States
James V. Browning, Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08854, USA, et al. Pages 567-588.
- Keywords: Miocene, sequence stratigraphy, Delaware, New Jersey, eustasy.
- Backstripping quantifies the roles of global sea level change, sediment supply, and subsidence on the U.S. mid-Atlantic margin. Comparison of the timing of Miocene sequences between New Jersey and Delaware shows that global sea level change is the primary influence on sequence development. However, excess subsidence in Delaware relative to New Jersey in the early Miocene is attributed to local flexural response to the progradation of sequences offshore of Delaware in the Miocene leading to preservation of thicker sequences.
- Morphologies of knickpoints in submarine canyons
Neil C. Mitchell, School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, Wales. Pages 589-605.
- Keywords: accretionary prisms, tectonically active continental slopes, submarine canyon morphology, stream bed erosion.
- Submarine canyons cut deep into the underwater slopes of the continents have been mapped with modern echo-sounders. The many narrow beams of these sounders are now able to reveal the seabed's shape in great detail. The new maps show the canyon floors are steep in places, much like rapids and waterfalls found in mountain streams. Some rapids are up-stream (towards land) of faults, a feature that has also been observed in some streams. These characteristics are interpreted to mean that canyon floors can be eroded in a similar way to rivers; although there are no rivers deep underwater, the seafloor is carved by powerful flows of suspended rock and mud.
- The transition from explosive to effusive eruptive regime: The example of the 1912 Novarupta eruption, Alaska
Nancy K. Adams, Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA, et al. Pages 620-634.
- Keywords: lava domes, plugs, vesicles, microtextures, bubble collapse, decoupled degassing, Novarupta 1912.
- The shift from explosive to effusive silicic volcanism seen in many historical eruptions reflects a change in the style of degassing of erupted magma. This paper focuses on such a transition during the largest eruption of the 20th century, the 1912 eruption of Novarupta. The transition is recorded in a dacite block bed which covers an elliptical area of 4 km2 around the vent. Approximately 700 studied blocks fall into four main lithologic categories: (1) pumiceous, (2) dense, (3) flow-banded dacites, and (4) welded breccias. Textural analyses of the blocks indicate portions of the melt underwent highly variable degrees of outgassing. Vesicle populations show features characteristic of bubble coalescence and collapse. A decrease in measured vesicularity and increased evidence for bubble collapse compared with pumice from earlier Plinian episodes mark the transition from closed- to open-system degassing. Block morphology and textures strongly suggest the magma was first erupted as a relatively gas-rich lava dome/plug but incomplete outgassing led to explosive disruption. Heterogeneous degassing of ascending magma began in Plinian Episode III and resulted in instability during Episode IV dome growth and a (series of) Vulcanian explosion(s). Modeling of the dynamics of explosion initiation and ejecta dispersal indicates that a significant concentration in gas is required to produce the explosions responsible for the observed block field dispersal. The amount of gas available in the hot pumiceous dome material appears to have been inadequate to drive the explosion(s), therefore external water most likely contributed to the destruction.
- Dome formation and extension in the Tethyan Himalaya, Leo Pargil, northwest India
Rasmus C. Thiede, Institut fuer Geowissenschaften, Universitaet Potsdam, Postfach 601553, Potsdam 14415, Germany, et al. Pages 635-650.
- Keywords: Himalaya, Tibet, extension, dome, geochronology, exhumation.
- Rocks in the Himalaya originally formed at depths of ~20-30 km during the collision between India and the Asian continent occur today at surface and form domal culminations in excess of 5000 m elevation .These domes constitute an irregular chain of structures that are bounded by normal faults within the northern Himalaya and southern Tibet. Several of the domes are characterized by ongoing normal faulting, which is ultimately responsible for transferring or exhuming these rocks from middle crustal levels to the surface. The former depth of these rocks is documented by the presence of the minerals kyanite and garnet, which attest to a mid-crustal origin. However, the mechanisms and timing of these exhumation processes is the subject of a long-standing, ongoing debate. Importantly, the faults along the periphery of some of these domes appear to be closely linked with tectonically active structures that generate earthquakes, which documents that these domes are actively forming. In addition, these faults pose significant hazards for communities and infrastructure in these high-elevation regions. In our study we used a combination of geological field mapping, structural analysis and thermochronology in the western flank of the Leo Pargil dome, northwestern Indian Himalaya, in an attempt to unravel its evolution during the ultimate 15 million years. By using 40Ar/39Ar thermochronology on the mineral mica and the fission track method on the mineral apatite we were able to constrain the ascent of the domal rocks from original temperatures of 350 °C in the middle crust to 60 °C, revealing a three-stage cooling pathway since the middle Miocene. 40Ar/39Ar white mica ages of 16 to 14 Ma suggest a first phase of rapid cooling and provide minimum estimates for the onset of dome exhumation. Apatite fission track ages between 10 and 8 Ma suggest that ductile fault displacement, typical for deformation processes at greater depth in the Earth's crust, had ceased by then. In addition, rock samples dated with the fission track method indicate that the rate of cooling had decreased significantly by this time. We interpret this to indicate decreased fault displacement along the Leo Pargil shear zone and a possible transfer of deformation to the adjacent Kaurik-Chango normal fault system between 10 and 6 Ma. Interestingly, apatite fission track ages ages from lower elevations of the domal structure suggest accelerated cooling since the Pliocene. Based on the overall geologic conditions of the region, this phenomenon cannot be related to pure fault displacement, and therefore may reflect more pronounced regionally distributed and erosion-driven exhumation. We speculate that this may be related to the uplift of the Himalayan mountain ranges to elevations were the effects of global climate change associated with an increasingly stormy and erosive climate could impact the mountain belt.
- Cenozoic exhumation and deformation of northeastern Tibet and the Qinling: Is Tibetan lower crustal flow diverging around the Sichuan Basin?
Eva Enkelmann, Geowissenschaften, Technische Universität Bergakademie, D-09599 Freiberg, Germany, et al. Pages 651-671.
- Keywords: Tibet, Qinling, plateau growth, fission track, structural geology.
- Apatite fission-track thermochronology and structural data elucidate the exhumation and deformation history of the Paleozoic-Mesozoic Qinling orogen, located northeast of the Tibetan Plateau. In particular, we examine the extent to which the Qinling were affected by the rising plateau. The northern and eastern Qinling show slow continuous cooling and predominantly strike slip deformation since the Cretaceous. In contrast, in the southwestern Qinling, rapid Late Cenozoic cooling initiated a few million years later than in the eastern Tibetan Plateau. Active sinistral and dextral strike-slip faults delineate an area of eastward rock flow and bound the area of rapid cooling outlined by apatite fission-track thermochronology. The data can be interpreted to indicate that lower crustal flow causes active plateau uplift in this area, or alternatively, northeastern Tibet may be growing eastward faster in the western Qinling than the entire South China Block is extruding to the east.
- Miocene to present activity along the Red River fault, China, in the context of continental extrusion, upper-crustal rotation, and lower-crustal flow
Lindsay M. Schoenbohm, Department of Geological Sciences, Ohio State University, Columbus, Ohio 43210, USA, et al. Pages 672-688.
- Keywords: Red River fault, Xianshuihe-Xiaojiang fault system, Dali fault system, southeast Asia, extrusion, lower-crustal flow.
- In this paper we present new mapping which illuminates the history of displacement and deformation along the Red River fault in China, which runs from near the Tibetan Plateau southeast toward the South China Sea. We find a minimum of 40 km dextral slip along the fault, ~25 km of which post-dates river incision, surface uplift and the development of a diffuse, regional fault system accommodating rotation around the Eastern Himalayan syntaxis. This suggests a significant change in crustal conditions on the southeast margin of the Tibetan plateau in Pliocene time, possibly the result of the lower crustal flow.
- Late-stage evolution of the Chemehuevi and Sacramento detachment faults from apatite (U-Th)/He thermochronometry-Evidence for mid-Miocene accelerated slip
Timothy J. Carter, School of Earth Sciences, The University of Melbourne, 3010, Victoria, Australia, et al. Pages 689-709.
- Keywords: metamorphic core complexes, Basin and Range Province, low-angle normal faults, (U-Th)/He, low-temperature thermochronology.
- The southwestern U.S. has been an area of profound crustal stretching, up to more than 100% in some areas, over approximately the last 25 million years. This phenomenon has led to the transport of rocks from deep within the crust to the surface along enigmatic low-angle faults. Through the application of the highly sensitive temperature-dependant (U-Th)/He isotope dating method to apatite, a calcium phosphate mineral found in the rocks of the Chemehuevi and Sacramento Mountains, new insights have been gained into the timing and rate at which these ranges were transported towards the surface. The new data indicate that movement towards the surface occurred in two phases, starting about 23 million years ago at a relatively slow rate, and then speeding up about 15 million years ago to much faster rates. This unusual pattern has also been observed in apatite (U-Th)/He data from the nearby Harcuvar Mountains, thus suggesting a regional change in fault movement rates about 15 million years ago. We suggest that this is the result of a change in thermal conditions beneath the earth's crust, related to complex interactions between the North American and Pacific tectonic plates farther west.
- Plate-scale stress fields driving the tectonic evolution of the central Ouachita salient, Oklahoma and Arkansas
Amy E. Whitaker, Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA, and Terry Engelder. Pages 710-723.
- Keywords: joints, Ouachita fold-and-thrust belt, stress fields, fracture, Arkoma Basin, pinch zone.
- Two stages of regional cross-fold jointing reflect the rearrangement of a lithospheric-plate-scale stress field during the Ouachita (i.e., Alleghanian) orogeny. These joints overprint northward-verging thrust-related folds in the Ouachita foreland. Stage-one jointing during the Ouachita orogeny is associated with a stress field recorded by an Atokan and earlier regional joint set whose strike is NNE. A later regional joint set, representing the second stage of jointing during the Ouachita orogeny, affects rocks in the Arkoma foreland as well as in the fold belt. Stage-two jointing records a stress field associated with NNW-directed closure during the final stage of plate convergence and persisted past the Desmoinesian.
- Regional-scale assessment of a sequence-bounding paleosol on fluvial fans using ground-penetrating radar, eastern San Joaquin Valley, California
George L. Bennett V, Department of Geological Sciences, Michigan State University, East Lansing, Michigan 48824-0001, USA, et al. Pages 724-732.
- Keywords: fluvial sequence stratigraphy, ground-penetrating radar, fluvial fans, Quaternary, San Joaquin Valley.
- Ground penetrating radar (GPR) was used to image the shallow stratigraphic horizons across a broad area of three fluvial fans in the San Joaquin Valley of California. Approximately 190 km of GPR surveys provided insight into the spatial distribution of sequence bounding paleosols that influence the amount of water recharging aquifers across these fans. These paleosols, which were deposited during past interglacial periods, form important stratigraphic markers for understanding both the stratigraphic development of fans and the hydrogeologic character of fan deposits. This research showed that GPR is an effective technique for imaging shallow paleosols since these clay-rich units attenuate the GPR signal. These surveys showed that the paleosols are laterally-extensive; however, discrete erosional breaks are present in areas where paleochannels existed during glacial outwash periods that reactivated the fan. Differences between the fans, due to local tectonic subsidence rates under each fan, were also observed. This research has improved our understanding of the controls on fluvial deposition in the eastern San Joaquin Valley.
- A Permian-Triassic boundary section at Quinn River Crossing, northwestern Nevada, and implications for the cause of the Early Triassic chert gap on the western Pangean margin
Erik A. Sperling, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA, and James C. Ingle, Jr. Pages 733-746.
- Keywords: Permian-Triassic mass extinction, Humboldt County, Nevada, chemostratigraphy, chert, anoxia.
- The Permian-Triassic mass extinction, which occurred approximately 250 million years ago, was the most devastating biological crisis of all time, with 80-95% of species becoming extinct. Unlike the more recent Cretaceous-Tertiary extinction, which saw the demise of the dinosaurs circa 65 million years ago, stratigraphic sections preserving evidence of this event are scarce. This study presents new data from the Quinn River Formation, northwestern Nevada, suggesting that the Formation contains the first complete Permian-Triassic boundary section recognized in the United States, and one of the few sections worldwide preserving strata that were deposited in a deep-water depositional environment. Geochemical and sedimentological data from the Quinn River Formation indicate that the deep ocean became anoxic and possibly sulfidic prior to the extinction. Although the links between this major oceanographic change and the extinction are not yet understood, the new Quinn River locality holds the promise to shed light on this critical interval in earth history.
- Permian-Triassic boundary in the central Transantarctic Mountains, Antarctica
James W. Collinson, Department of Geological Sciences and Byrd Polar Research Center, The Ohio State University, Columbus, Ohio 43210, USA, et al. Pages 747-763.
- Keywords: Antarctica, boundary, Glossopteris, Lystrosaurus, Permian, Triassic.
- The Permian-Triassic boundary, perhaps marking the world's greatest extinction of plants and animals 252 million years ago, occurs within a sequence of river deposits in the central Transantarctic Mountains, now 500 kilometers from the South Pole. These deposits are unique in that they contain both a well-preserved Late Permian flora and an Early Triassic reptile and amphibian fauna that lived in a polar region (65°S). This boundary interval, representing on the order of 200,000 years, recorded the demise of Permian forests and the earliest appearance of Triassic reptiles and amphibians in a polar region. The reptiles are closely related to those of the same age in mid-latitudinal (40°S) South Africa, suggesting that they may have migrated from there. The migration of these animals into Polar Regions suggests unusual warming, perhaps related to CO2 emissions from massive volcanic eruptions in Siberia that subsequently led to large releases of methane gas from ocean floors, all about the time of the boundary. A two-step extinction in Antarctic floras and unrelated floras in East Greenland brackets Carbon isotope evidence for a massive release of methane gas. The two-step extinction in widely separated parts of the supercontinent Pangea supports the hypothesis by others that a global event, perhaps harmful mutations caused by enhanced UV radiation, may have played an important role in the extinction of plants.
To view abstracts for the GSA BULLETIN, go to www.gsajournals.org.
Representatives of the media may obtain a complimentary copy of any GSA BULLETIN article by contacting GSA Director of Communications, .
Other non-media requests for articles should be directed to
GSA Sales and Service, , 1-888-443-4472.
The Geological Society of America
3300 Penrose Place - PO Box 9140
Boulder, CO 80301-9140, USA
To unsubscribe from GSA's media distribution list, notify Ann Cairns.