Boulder, Colo., USA – GSA Bulletin postings for February cover the sculpting of Earth's surface as seen in the Pyrenees; facies architecture in Washington State, USA; atmospheric circulation recorded in the Permian Maroon Formation; preglacial fluvial gorges and valleys; banded iron formations; paleosols in Wapadsberg Pass, South Africa; Ediacaran fossils from Mistaken Point, Newfoundland; geologic forensics and sedimentary fingerprints; catastrophic outburst floods recorded in the Tibetan Plateau; the Sudbury impact structure, Canada; and observations at the Sanctuary of Zeus.
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Volumetric budget and grain-size fractionation of a geological sediment routing system: Eocene Escanilla Formation, south-central Pyrenees
Nikolas A. Michael et al., Department of Earth Sciences and Engineering, Imperial College London, South Kensington Campus, Exhibition Road, London, SW7 2AZ, UK. Posted online ahead of print on 3 Feb. 2014; http://dx.doi.org/10.1130/B30954.1. Open Access.
The sculpting of the Earth's surface by the processes of erosion generates particulate sediment that is transported into regions of subsidence, where it accumulates over long periods of time. Nikolas Michael and colleagues constrain the rates of erosion, transport and deposition of a system where the source regions of sediment can be confidently connected to depositional sinks. They present new results from a geological example, about 40 million years old, on the southern flank of the Pyrenean mountain chain in northern Spain. Understanding the budget of sediment from source regions to depositional sinks helps geologists to make predictions of the architecture of sedimentary rocks and of the variation in size of the sediment grains -- key factors in the exploitation of oil, gas, water and mineral resources from beneath Earth's surface.
Facies architecture of a continental, below-wave-base volcaniclastic basin: The Ohanapecosh Formation, Ancestral Cascades arc (Washington, USA)
Martin Jutzeler et al., Centre of Excellence in Ore Deposits, School of Earth Sciences, University of Tasmania, Private Bag 79, Hobart TAS 7001, Australia. Posted online ahead of print on 3 Feb. 2014; http://dx.doi.org/10.1130/B30763.1.
The Ohanapecosh Formation (32-26 million years old) consists of voluminous volcaniclastic facies that were deposited in the Ancestral Cascades, in Washington State, USA. Absence of shoreline-related facies suggests accumulation in a deep continental basin. Various volcanic sources, mostly andesitic and basaltic, are at the origin of the formation. The thickest beds are derived from entrance of pyroclastic flows into water, whereas other beds imply underwater resedimentation, shallow-water eruptions and inter-eruptive background sedimentation.
Abrupt and high-magnitude changes in atmospheric circulation recorded in the Permian Maroon Formation, tropical Pangaea
M.J. Soreghan et al., School of Geology and Geophysics, University of Oklahoma, Norman, OK, 73019, USA. Posted online ahead of print on 3 Feb. 2014; http://dx.doi.org/10.1130/B30840.1
The formation and evolution of the Pangaean supercontinent 300 million years ago led to fundamental changes in Earth's climate. The rise of a mountain belt that straddled the equator impacted rainfall patterns around the tropics and altered atmospheric circulation. But constraining the details of past atmospheric circulation from geologic data has been challenging. In this paper, M.J. Soreghan and colleagues utilize a novel suite of sedimentologic and geochemical data to constrain how wind patterns changed over the western edge of the Pangaean supercontinent. Measuring changes in grain size, magnetic character, and provenance of sediment over a 12 m section of preserved wind-deposited silt (loess) in central Colorado, the authors show that both the magnitude and direction of winds changed repeatedly over time periods (tens and hundreds of thousands of years) similar in scale to those that have regulated glacial cycles in Earth's recent past. Fundamental changes in the magnitude and direction of winds, and attendant changes in precipitation, suggests a mega-monsoonal circulation was established by early Permian time.
Buried preglacial fluvial gorges and valleys preserved through Quaternary glaciations beneath the eastern Laurentide Ice Sheet
Patrick Lajeuesse, Centre d'études nordiques and Département de géographie, Université Laval, Québec, Quebec, G1V 0A6, Canada. Posted online ahead of print on 3 Feb. 2014; http://dx.doi.org/10.1130/B30911.1.
The results presented in this paper by Patrick Lajeuesse reveal for the first time that many large river and lake valleys of northeastern North America are underlain by deep bedrock inner gorges and V-shaped valleys. Geological evidence provided by excavation and drilling work at many dam sites as well as seismic data collected on lakes and at river mouths indicates that these deep bedrock trenches were not eroded by glaciers, but were rather incised by streams or rivers that flowed before the ice ages when sea level was much lower than today. These valley profiles share striking similarities with those of many fjord-lakes of other glaciated landscapes, suggesting that many other valleys previously interpreted as glacial in origin may in fact enclose preserved preglacial fluvial channels at their bottom. Because they have survived glacial erosion beneath the Laurentide Ice Sheet through successive glaciations, valleys of northeastern North America have a great potential for containing a long term record of sedimentation and environmental change. During glacial episodes, these fluvial valleys also formed preferential pathways that influenced glacial ice flow and the delivery of sediments to ocean basins.
Replacement origin for hematite in 2.5 Ga banded iron formation: Evidence for postdepositional oxidation of iron-bearing minerals
Birger Rasmussen et al., Department of Applied Geology, Curtin University, Kent St, Bentley WA 6102, Australia. Posted online ahead of print on 3 Feb. 2014; http://dx.doi.org/10.1130/B30944.1.
Banded iron formations (BIFs) are widely used to infer early microbial processes and the composition of the ancient ocean. Those inferences are based on assumptions of the mineralogy of the original sediment and how it was deposited, topics that have been hotly debated for over 100 years. Current models widely assume that hematite in BIFs was derived directly from the primary precipitation of iron oxides, thereby implying that part of the water column contained dissolved oxygen prior to the appearance of atmospheric oxygen ~2.32 billion years ago. Our results from a classic type-section of 2.5-billion-year-old BIF provide evidence that hematite is not "primary" but formed by the replacement of other iron-bearing minerals after burial, challenging existing models. We argue that hematite growth was caused by large-scale infiltration of surface-derived fluids carrying dissolved oxygen immediately after the Great Oxidation Event. A secondary origin for hematite raises the possibility that the primary precipitates were iron-rich clays rather than iron oxides, and also precludes the use of hematite in BIF to infer the composition of the ancient ocean and to constrain biological processes.
Latest Permian paleosols from Wapadsberg Pass, South Africa: Implications for Changhsingian climate
Robert A. Gastaldo et al., Department of Geology, Colby College, Waterville, Maine 04901, USA. Posted online ahead of print on 13 Feb. 2014; http://dx.doi.org/10.1130/B30887.1.
Atmospheric carbon dioxide in the latest Permian, prior to the largest mass extinction in Earth's history nearly 251 million years ago, has been suggested to have ranged in concentration from 4x to 12x that of the modern world. Today, atmospheric pCO2 is close to 400 parts per million (ppm). Ancient soils (paleosols) are the most common depositional environment in the Karoo Basin of South Africa, where rocks that transition across the mass extinction event are reported to be exposed. One boundary section outcrops at Wapadsberg Pass in the Eastern Cape Province. In this study by Robert Gastaldo and colleagues, several of the ancient soils are characterized by soil nodules, most of which formed below the water table that existed across the landscape. But, some soil nodules developed where carbonate cements formed in equilibrium with atmospheric gas concentrations. Calculations based on geochemical analyses of stable carbon isotopes, preserved in carbonate cements in these latest Permian soil nodules, and original organic matter, indicate that latest Permian carbon dioxide ranged from an estimated low concentrations of 500 to 900 ppm to high concentrations of 1300 to 1900 ppm, nearly five times that of current values, prior to the mass extinction event.
Volcanosedimentary paleoenvironments of Ediacaran fossils in Newfoundland
Gregory J. Retallack, Department of Geological Sciences, University of Oregon, Eugene, 97302 USA. Posted online ahead of print on 13 Feb. 2014; http://dx.doi.org/10.1130/B30892.1.
Ediacaran fossils from Mistaken Point, Newfoundland, did not live in the darkness of the deep ocean, as has been widely assumed, but in intertidal and coastal soils. New evidence for a terrestrial habitat comes from ungraded crystal and lapilli tuffs, volcanic bombs and block-and-ash flows. The Mistaken Point Formation is geochemically comparable with convergent continental margin sequences, rather than deep sea oceanic sediments. Beds formerly interpreted as turbidites, were instead paleosols, tsunamites, and seismites. Such coastal habitats support interpretation of the Ediacaran fossils of Newfoundland as fungi, lichens, and microbial colonies, rather than animals.
Provenance variability along the Early Ordovician north Gondwana margin: Paleogeographic and tectonic implications of U-Pb detrital zircon ages from the Armorican Quartzite of the Iberian Variscan belt
J. Shaw et al., School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Road, Victoria, BC, V8P 5C2, Canada. Posted online ahead of print on 13 Feb. 2014; http://dx.doi.org/10.1130/B30935.1.
How was Earth's most recent supercontinent constructed? While Earth Scientists agree that all of Earth's continents came together to form Pangea 350 million years ago, there is little consensus with regard to how and why. The role of the Iberian Peninsula remains particularly controversial. An ancient mountain range spanning Spain and Portugal records a continental collision involved in the formation of Pangea, but what exactly was colliding with what? Was Iberia its own small continent derived from South America? A part of continental Africa? In this study, Ph.D. candidate Jessica Shaw and her colleagues at the universities of Victoria, British Columbia, and Salamanca, Spain, used geologic forensics to address this controversy. By age-dating large numbers of zircon grains contained within sedimentary rocks, one can construct a sedimentary "fingerprint" unique to a particular time and region. Shaw and her colleagues discovered a perfect match between 470 million year old Iberian sandstones and equal-aged sandstones from northeast Africa and Arabia. These findings show that Iberia formed a portion of an Atlantic-style African passive margin that faced north towards the ocean whose closure heralded the collisions that fused Pangea. Still unknown: when and how Iberia separated from Africa.
Ice dams, outburst floods, and glacial incision at the western margin of the Tibetan Plateau: A >100-k.y. chronology from the Shyok Valley, Karakoram
Dirk Scherler et al., Institute of Earth and Environmental Sciences, University Potsdam, 14476 Potsdam, Germany. Posted online ahead of print on 13 Feb. 2014; http://dx.doi.org/10.1130/B30942.1.
Catastrophic outburst floods from large glacier-dammed lakes have repeatedly occurred at the end of the last ice age, causing significant environmental and even global climatic changes. Most of these events are tied to the large continental ice sheets that covered much of North America and Northern Europe during the Pleistocene epoch. Smaller, but similar catastrophes may also have occurred in many of the Earth's glaciated mountain ranges, but traces are difficult to find. Dirk Scherler and colleagues found evidence in the Karakoram Range, northern India, that a more than 180-km-long valley glacier blocked one of the largest tributaries of the Indus River. Field mapping and cosmogenic-nuclide dating allowed the authors to reconstruct a chronology that documents repeated damming and outburst flooding, which likely occurred throughout much of the Quaternary period, i.e., the last ~2 million years. Intriguing similarities between their study site in the eastern Karakoram and areas farther downstream the Shyok and Indus rivers suggest that in this part of the world, ice dams and catastrophic outburst floods were common phenomena during the Quaternary and may have contributed to the deep incision of the western margin of the Tibetan Plateau.
Preservation of detrital shocked minerals derived from the 1.85 Ga Sudbury impact structure in modern alluvium and Holocene glacial deposits
Olivia A. Thomson et al., University of Puerto Rico Geology Department of Geology University of Puerto Rico Call Box 9000 Mayaguez, Puerto Rico 00681-9000. Posted online ahead of print on 13 Feb. 2014; http://dx.doi.org/10.1130/B30958.1.
Meteorite impacts can cause dramatic changes in global climate, biology, surface geomorphology, and also concentrate economic metals. Recreating the terrestrial impact history is thus important to many fields of Earth science. However, many impact structures have eroded away with time, and are no longer preserved in the geologic record as intact structures. Some minerals in impact-shocked rocks preserve a diagnostic microstructural record of impact deformation at the grain-scale, and survive as sand grains in sediment long after the impact structure erodes. Here, Olivia Thomson and colleagues report the occurrence of detrital shocked zircons and quartz in modern alluvium and Holocene glacial deposits eroded from the giant 1.85 billion-year-old Sudbury impact structure in Ontario, Canada. The shocked sand grains were identified using optical and electron microscopy to identify and document high-pressure shock deformation evidence on the surfaces and interiors of individual grains. One conclusion from this study is that zircons with shock microstructures can survive the process of erosion and sedimentary transport nearly two billion years after an impact event. Detrital zircons are ubiquitous in siliciclastic rocks of all ages, which means that shocked zircons eroded from older impact structures (i.e., impacts in the Archean or Hadean) should be preserved in younger sediments.
Quasi-flexural folding of pseudo-bedding
George H. Davis, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Posted online ahead of print on 13 Feb. 2014; http://dx.doi.org/10.1130/B30963.1.
This paper presents a set of observations and discoveries that author George Davis made at the Sanctuary of Zeus archaeological site in Arcadia, Greece. The sanctuary contains a beautifully exposed and highly folded limestone formation of Cretaceous age. Though superficially the folding appears to be standard layer flexure in response to tectonic stress, Davis recognized that the mechanism of the folding is a hybrid never before fully recognized or documented. Standard layer flexure was prohibited because the apparent bedding in the limestone is not bedding at all, but rather "pseudo-bedding" that developed when the limestone was being buried and loaded (by younger formations) from above. Bedding-like surfaces, known as primary stylolites, developed where limestone dissolved away through "pressure dissolution." Continuous bedding surfaces are largely absent, and thus when tectonic stresses were applied, only limited flexural-slip took place. The slip deficit normally accommodated by flexure had to be accommodated by tectonically-induced pressure dissolution, which created an amazing array of odd geological structures marked by the combination of faulting, folding, and interpenetration of rock layers. Davis believes that these types of structures are not unique to Arcadia, but will be found in all mountain belts that contain deformed pseudo-bedded limestones.
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