Boulder, Colo., USA - On 27 Feb. and 6 Mar. 2014, GSA Bulletin published 11 articles online ahead of print, including two that are open access: "O2 constraints from Paleoproterozoic detrital pyrite and uraninite" and "Sediment transfer and deposition in slope channels: Deciphering the record of enigmatic deep-sea processes from outcrop." Other articles cover geological features in the Alps; the Appalachians; Death Valley; India; the Himalaya; the Columbia River Basalt Province; San Simeon, California; Kaua'i, Hawai'i; and artic Canada.
GSA Bulletin articles published ahead of print are online at http://gsabulletin.
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O2 constraints from Paleoproterozoic detrital pyrite and uraninite
J.E. Johnson et al., Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. Published online ahead of print on 27 Feb. 2014; http://dx.
The rise of oxygen is the most important environmental change in the history of our planet. One of the first clues indicating that Earth's atmosphere once lacked oxygen for the first half of its history was the observation of oxygen-sensitive detrital grains in sedimentary rocks. The curious presence of detrital pyrite and uraninite grains became one of the core pieces of evidence that the early Earth had significantly less oxygen. In this paper, J.E. Johnson and colleagues present observations of the youngest significant detrital pyrite and uraninite in the geological record. These deposits constrain the rise of oxygen to younger than 2.415 billion years ago. They also developed a new mathematical model that combines physical transport processes with chemical weathering to quantify O2 concentrations prior to the rise of oxygen. The model constrains the ancient atmosphere at 2.4 billion years ago to having less than ~ 10-5 atm of O2 -- a vanishingly trace amount. Altogether, their results reveal that these small redox-sensitive detrital grains have considerable value as one of the most sensitive and robust indicators of an ancient Earth, essentially devoid of oxygen.
Quantifying sediment supply at the end of the last glaciation: Dynamic reconstruction of an alpine debris-flow fan
S. Savi et al., University of Bern Institute of Earth Sciences Baltzersstrasse 1+3 Bern, Bern 3012, Switzerland. Published online ahead of print on 27 Feb. 2014; http://dx.
This paper by S. Savi and colleagues quantifies the rates of erosion and sediment supply derived from a small mountain basin during a time span that covers the last interglacial period (the Holocene, from about 12,000 years before present until today). Techniques such as seismic survey, stratigraphic analysis, radiocarbon dating and cosmogenic nuclides are used to reconstruct the history of the sediment supplied from the Zielbach catchment (central-eastern Italian Alps). The combined results from these different approaches reveal an extremely fast erosion and sedimentation immediately after the end of the last glaciation (rates up to 30 mm per year). This fast sedimentation was responsible for the formation of the large Zielbach fan, which seems to be in place since that time. Following, the sediment supply decreased following a trend generally known as "paraglacial cycle." The data, demonstrate that sediment discharge and surface erosion was high following the retreat of the Alpine glaciers, and then decreased to the values observed today. These trends, however, have been modified by climate changes and human activities, indicated by sedimentation peaks.
Fault gouge dating in the Southern Appalachians, USA
J.S. Hnat and B.A. van der Pluijm (corresponding author), Department of Geological Sciences, University of Michigan, 1100 N. University Ave., Ann Arbor, Michigan 48109-1005, USA. Published online ahead of print on 27 Feb. 2014; http://dx.
The Southern Appalachians is among the longest and best studied mountain ranges in the world, yet critical questions about absolute timing of thrust belt evolution remain. Using Ar geochronology of clay in fault rock, J.S. Hnat and B.A. van der Pluijm dated the age of major thrust faults in Tennessee and Virginia. Their results show that thrusts were active in the same time window (276-280 million years ago) instead of progressively younger faulting toward the foreland. In addition to constraining the absolute timing of deformation, this resolves contradictory observations of cross-cutting fault relationships and supports the model of an internally-stressed orogenic foreland wedge for the Southern Appalachians.
Geochronologic and stratigraphic constraints on the Mesoproterozoic and Neoproterozoic Pahrump Group, Death Valley, California: A record of the assembly, stability, and breakup of Rodinia
Robert C. Mahon et al., Idaho State University Department of Geosciences, 921 S. 8th Ave. Stop 8072, Pocatello, Idaho 83209-8072, USA. Published online ahead of print on 27 Feb. 2014; http://dx.
This paper presents new age constraints on the Pahrump Group - a globally relevant sequence of sedimentary rocks in Death Valley National Park and the surrounding region. Robert C. Mahon and colleagues present results suggesting the age of the upper part of the Crystal Spring Formation (lowermost unit of the Pahrump Group) is some 300 million years younger than previously inferred. Based on this new data, new stratigraphic nomenclature is applied, elevating the upper Crystal Spring Formation to the Horse Thief Springs Formation. This new understanding significantly clarifies correlation of stratigraphic sequences across the southwestern United States, including correlations to rocks in the Grand Canyon, Uinta Mountains and Wasatch Range, and southeastern Idaho.
Detrital zircons in basement metasedimentary protoliths unveil the origins of southern India
Diana Plavsa et al., Tectonics, Resources and Exploration (TRaX), School of Earth and Environmental Sciences, University of Adelaide, SA 5005, Australia. Published online ahead of print on 27 Feb. 2014; http://dx.
In the quest of understanding the evolution of our planet, geologists often delve into the past in order to constrain the distribution of the continental plates, amalgamation and dispersal of supercontinents as well as their effect on the climate and the evolution of life on Earth in general. Approximately 550 million years ago, a large ocean (the Mozambique Ocean) existed between the Indian craton and the continental fragments now making up most of Madagascar and East Africa. The closure of the Mozambique Ocean resulted in the formation of a Himalayan-scale orogen (the East African Orogen), whose roots are now exposed in East Africa, Madagascar, South India, Sri Lanka and Antarctica. In this study, Diana Plavsa and colleagues focus on a relatively small part of this orogen now exposed at the southernmost tip of India. In order to better constrain the timing and the location of the Mozambique Ocean suture, they looked at the provenance of metasedimentary rock packages from either side of this lithosphere-scale structure. Sources akin to the Indian and East African basement rocks were found in metasedimentary rocks on either side of the suture respectively, thus enabling Plavsa and colleagues to better constrain its location and the evolution of the supercontinent Gondwana.
Sediment transfer and deposition in slope channels: Deciphering the record of enigmatic deep-sea processes from outcrop
Stephen M. Hubbard et al., Department of Geoscience, University of Calgary, Calgary, Alberta, T2N 1N4, Canada Published online ahead of print on 6 March 2014; http://dx.
The processes within submarine channels are difficult to directly monitor. Therefore, the breadth of sedimentary processes active in these systems is poorly understood. In this contribution by Stephen M. Hubbard and colleagues, the stratigraphy of channel-form bodies in outcrop are evaluated in order to provide insight into the sequence and relative magnitude of events related to erosion, sediment bypass, and deposition within channels that crosscut the seascape. Commonly overlooked evidence of protracted sedimentary bypass and erosion is recorded in fine-grained deposits locally preserved within channelized stratigraphy, along with a breadth of scours or internal channel-form stratal surfaces. The lifecycle of a submarine channel is dominated by sedimentary bypass, yet the products of shorter-lived channel filling and abandonment bias the stratigraphic record. Submarine channels serve as the primary conduits of sediment and contaminant transfer from land to the deep sea, and their deposits host important energy resources in the stratigraphic record.
Switch from thrusting to normal shearing in the Zanskar shear zone, NW Himalaya: Implications for channel flow
Melanie A. Finch et al., Monash University, School of Geosciences, Wellington Road, Clayton, VIC 3800, Australia. Published online ahead of print on 6 March 2014; http://dx.
This paper contributes to current discussion of whether the Himalayan front comprises rocks extruded from under Tibet by the channel flow model. Melanie A. Finch and colleagues investigated the Zanskar Shear Zone in NW India. This is part of a 2000 km normal fault that runs parallel to the Himalayan front and controls the evolution of the mountain chain. In Zanskar, rocks record first an intense period of mountain building and thickening of the continental crust. This early record is overprinted by the Zanskar Shear Zone, which records the opposite: a period of normal movement and extension. Here we could also constrain the timing of the switch from thickening to extension by dating granites crystallized during the switch. Finch and colleagues found that the switch from crustal thickening to extension occurred between 26 and 24 million years ago (Ma) and that by ~20 Ma, crustal extension had ended, so that extension lasted a maximum of six meters per year. The channel flow model requires several tens of millions of years of activity along the normal fault, and prolonged crustal melting. Their findings do not support this model, but rather supports the hypothesis that extension developed to relax a mountain chain that had become too high and the crust too thick.
Pyroclastic edifices record vigorous lava fountains during the emplacement of a flood basalt flow field, Roza Member, Columbia River Basalt Province, USA
Richard J. Brown et al., Department of Earth Sciences, Durham University, Science Labs, Durham, DH1 3LE, UK. Published online ahead of print on 6 March 2014; http://dx.
The vents in the ground where the largest and longest lived volcanic eruptions on Earth (flood basalt eruptions) issued from have proved elusive. This research investigates the best-preserved examples of the vents and volcanic cones of a 1300 cubic kilometers flood basalt eruption in the Columbia River Flood Basalt Province in the northwestern United States. Geological mapping of the volcanic cones indicates that the eruptions that occurred along the more than 180 km-long fissure were unusually vigorous for basaltic eruptions. The eruptions generated tall lava fountains that constructed broad shallow-sloped cones composed predominantly of welded pyroclastic material. These contrast with scoria cones typically developed during small volume basaltic eruptions. The evidence strengthens ideas that flood basalt eruptions are capable of injecting climate-altering gases high into the atmosphere above vigorous fountains and plumes.
Low-temperature blueschist-facies mafic blocks in the Franciscan mélange, San Simeon, California: Field relations, petrology, and counterclockwise P-T paths
Estibalitz Ukar and Mark Cloos, Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. Published online ahead of print on 6 March 2014; http://dx.
From the abstract: "Dozens of mafic blueschist blocks are found in the Franciscan mélange, which is well exposed along more than six kilometers of nearly continuous sea cliffs and wave-cut benches near San Simeon, California, USA. Thirty-four blocks were studied to discover all the varieties in this classic locality of mélange. ... Pieces of the underplated blueschist terrane were probably detached from the hanging wall as slabs that boudinaged and reboudinaged while entrained in shale-matrix mélange upwelling from depths of at least 15 km."
Knickpoint formation, rapid propagation, and landscape response following coastal cliff retreat at the last interglacial sea-level highstand: Kaua'i, Hawai'i
Benjamin H. Mackey et al., Department of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch, New Zealand. Published online ahead of print on 6 March 2014; http://dx.
The Na Pali Coast of the island of Kaua'i, Hawai'i, has spectacular sea cliffs incised by deep valleys, and has long attracted the interest of geologists looking to study how streams and waterfalls incise into rock. We revisited this classic location and used cosmogenic nuclide exposure dating to track the retreat rate of a 40 m tall waterfall in Ka'ula'ula Valley. The waterfall is migrating much faster than previous estimates, and has moved nearly 4 km upstream from the coast over the past 120 thousand years. The initiation time of this waterfall corresponds with the last interglacial, when sea levels were 6-10 m higher than today. Combining the exposure dating with topographic analysis and modeling of Kauai's subsidence in relation to sea level fluctuations, Benjamin H. Mackey and colleagues propose that the waterfall was formed by sea cliff erosion during the last interglacial. Waterfalls or knickpoints are primarily thought to form during periods of sea level fall, but they argue cliff erosion during periods of high sea level may be an important process of waterfall formation on ocean islands and other steep coasts.
Numerical constraints on degassing of metamorphic CO2 during the Neoproterozoic Franklin large igneous event, Arctic Canada
Peter I. Nabelek et al., University of Missouri, Columbia, Missouri 65211, USA. Published online ahead of print on 6 March 2014; http://dx.
Some large episodic climatic changes during Earth's history have been attributed to massive extrusions of flood basalts and coeval intrusions of sills into underlying sedimentary basins. These Large Igneous Provinces (LIPs) have been implicated, on the one hand, as taking CO2 from the atmosphere by chemical weathering of basalts, and on the other hand, as increasing the carbonic gas budget through CO2- and CH4-producing metamorphism of sedimentary rocks next to sills. Gabbroic sills of the Franklin Large Igneous Province intruded the Neoproterozoic sedimentary strata of the western Arctic about 720 million years ago at the onset of the global Sturtian glaciation. Using computer simulations, Peter I. Nabelek and colleagues demonstrate that the efficiency by which the metamorphic CO2 was added to the atmosphere was highly dependent on the permeability of the sedimentary strata. At the probable permeability, the contribution of CO2 to the atmosphere from the vicinity of one very large sill would have been only a few ppm during the centuries long metamorphic episode caused by the sill. During the several million years long Franklin igneous event, the low permeability of the sedimentary rocks would have inhibited a rapid increase in the amount of metamorphic CO2 in the atmosphere.