Boulder, Colo., USA New GSA Bulletin postings include studies of the geomorphic impact of 19th-century placer mining along the Fraser River, British Columbia; seafloor images around Ross Island, obtained by the Swedish research vessel Oden from 2007-2011; a foray into the fossil record of early Tertiary mammal evolution in Africa via magnetostratigraphic analyses of exposed fossiliferous sequences in Algeria; and a new contribution to the growing volume of published geoscience research for southeastern Mongolia.
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Paleogeographic reconstruction of a late Paleozoic arc collision zone, southern Mongolia
Matthew J. Heumann et al., ConocoPhillips, Houston, Texas, USA. Posted online 8 June 2012; doi: 10.1130/B30510.1.
Matthew J. Heumann and colleagues present the first published collection of detrital zircon geochronology results from southeastern Mongolia as well as a detailed field investigation for two areas within the region, Bulgan Uul and Nomgon. Results show the first radiometric age constraints on Permian strata for Bulgan Uul and Nomgon and provide strong evidence for these deposits having once been part of the same ocean basin, which is now bisected and offset in a left-lateral sense across the East Gobi Fault zone. This paper contributes to the growing volume of published research for southeastern Mongolia, which until the late 1990s had largely gone unnoticed in the geologic community. It also adds to the understanding of the tectonic and depositional history for the region, while highlighting many of the inconsistencies in previous tectonic models for central Asia.
Placer mining along the Fraser River, British Columbia: The geomorphic impact
Andrew D. Nelson and Michael Church, Dept. of Geography, The University of British Columbia, 1984 W. Mall, Vancouver, British Columbia, Canada V6T 1Z2. Posted online 8 June 2012 (now part of the July 2012 issue); doi: 10.1130/B30575.1.
Andrew D. Nelson and Michael Church show that 19th-century placer gold mining along the Fraser River discharged about 60 million cubic meters of tailings into the river. These tailings were flushed from mines distributed along the river between Hope and Quesnell through the steep part of the river upstream of Hope on a time scale of 50 to 100 or more years. Downstream of Hope, placer tailings appear to have accumulated into a zone of instability and aggradation that persists in the river to the present day. This observation suggests that geomorphic processes, from the 20th century to the present in the lower gravel-bed reach of Fraser River, are influenced by historical mining along the river. Nelson and Church conclude that researchers who study ongoing processes should consider the historical context of those processes in order to understand whether the processes they observe represent normal conditions or are in fact transient adjustments to perturbation. They note that managers along the Fraser River and elsewhere who may use historical estimates and past observations of sediment transport rates should be wary of the assumption that historical rates represent normal conditions that will continue into the future.
Coupled U-Pb dating and Hf isotopic analysis of detrital zircon of modern river sand from the Yalu River (Yarlung Tsangpo) drainage system in southern Tibet: Constraints on the transport processes and evolution of Himalayan rivers
J.Y. Zhang et al., Structural Geology Group, China University of Geosciences (Beijing), Beijing 100083, China. Posted online 8 June 2012; doi: 10.1130/B30592.1.
The Yalu River (i.e., the Yarlung Tsangpo), flowing eastward along the north side of the Himalayan Range, carries a large quantity of materials eroded from the Tibetan plateau and the Himalayan Range to the Bay of Bengal. This study by J.Y. Zhang and colleagues uses the age and geochemistry of zircon grains in river sand to quantify the relative contribution of sediments to sediment loads in the river from different regions of the Himalaya and Tibet. Comparison of zircon grains in river sand and those in sediments deposited 3 to 10 million years ago in the eastern Himalaya suggests that the Yalu River flowed over the Himalayan crest during this time period. This result suggests that the geometry of the Yalu River course has not been stable, varying drastically with time as a result of the rapid uplift of the Himalaya and recent climate changes.
Caldera structure, amount of collapse, and erupted volumes: The case of Bolsena caldera, Italy
V. Acocella et al., Dip. Scienze Geologiche Universit Roma Tre, Italy. Posted online 8 June 2012; doi: 10.1130/B30662.1.
Calderas are common subcircular depressions in volcanic areas, resulting from the emptying of an underlying magma chamber during an eruption. However, the exact structure and mechanism of formation of calderas, as derived from eruptions, are still poorly understood. The approx. 20-km-wide Bolsena Caldera in Italy formed between 0.2 and 0.6 million years ago. The largely preserved structural rim and the availability of subsurface data make Bolsena ideal to investigate caldera structure in relation to the subsidence and erupted volume. In this paper, V. Acocella and colleagues use remote sensing, field analysis and available subsurface data to highlight two features not found in other calderas: (A) there are subsidence-related structural variations along the caldera rim because the observed deformation depends upon the amount of collapse of the caldera; and (B) there is significant (~200 m) and prolonged (approx. 200,000 years after the last eruption) post-eruptive subsidence, possibly related to the activity of the nearby Latera Caldera (west rim of Bolsena), suggesting a common magmatic reservoir.
Ice-flow switching and East/West Antarctic Ice Sheet roles in glaciation of the western Ross Sea
Sarah L. Greenwood et al., Dept. of Geological Sciences, Stockholm University, 10691 Stockholm, Sweden. Posted online 21 June 2012; doi: 10.1130/B30643.1.
Understanding the long-term behavior of the Antarctic ice sheets is critical in providing a context for recently observed changes in ice flow. Ice discharged from East and West Antarctica meets in the western Ross Sea. This, therefore, is a key region for unraveling the past fluctuations in behavior of the two continental ice sheets. Images of the seafloor around Ross Island, obtained by the Swedish research vessel Oden from 2007-2011, reveal glacial landforms molded on the continental shelf by previous expansion and retreat of thick continental ice. These landforms document several different ice flow regimes, which Sarah L. Greenwood and colleagues decipher in this GSA Bulletin paper. Groups of landforms associated with ice-sheet retreat indicate localized re-advance responses to the loss of regional ice from the Ross Sea and point to the importance of Ross Island in stabilizing the march of retreat during the last deglaciation. Earlier landform assemblages conflict with the pattern of retreat, and their contrasting configurations indicate that in this region different outlets of the West and East Antarctic ice sheets did not operate in phase. Rather, the contributions from different source areas and the preferred drainage routes from the western Ross Sea shifted and switched throughout the last glacial period.
Chronology of the Eocene continental deposits of Africa: Magnetostratigraphy and biostratigraphy of the El Kohol and Glib Zegdou Formations, Algeria
Pauline Coster et al., Institut International de Paloprimatologie, Palontologie Humaine: Evolution et Paloenvironnements (iPHEP), UMR-CNRS 6046, Universit de Poitiers UFR SFA, 40 avenue du Recteur Pineau, F-86022 Poitiers cedex, France. Posted online 21 June 2012; doi: 10.1130/B30565.1.
The fossil record of early Tertiary mammal evolution in Africa is poor. Insufficient age constraints and discrepancies in paleontological interpretations of early African mammal localities have led, over the years, to very different estimates of their correlation and chronology. Pauline Koster and colleagues conducted magnetostratigraphic analyses, associated with biostratigraphic studies, in the fossiliferous sequences exposed in the northwestern Hammadas of the Saharan Platform in the Glib Zegdou area and in the Saharan Atlas at the El Kohol locality (Algeria) to further define the age of these Eocene continental deposits. The placement of the Algerian localities into a consistent chronological framework constitutes considerable advancement to achieve biostratigraphic correlation of the Paleogene Afro-Arabian mammal localities. These new results, which provide the first reliable continental references for the early Paleogene of Afro-Arabia, are crucial for our understanding of the early Tertiary emergence, diversification and paleobiogeographic history of Afro-Arabian mammals.
Permian-Carboniferous arc magmatism and basin evolution along the western margin of Pangea: Geochemical and geochronological evidence from the eastern Acatln Complex, southern Mexico
Moritz Kirsch et al., Universidad Nacional Autnoma de Mxico (UNAM), Centro de Geociencias, Campus Juriquilla, 76230 Quertaro, QRO, Mexico. Posted online 21 June 2012; doi: 10.1130/B30649.1.
In Late Paleozoic paleogeographic reconstructions, the Mixteca terrane of southern Mexico has been placed either within or peripheral to Pangea, which bears on the Pangea-A versus Pangea-B controversy. Moritz Kirsch and colleagues provide major- and trace element geochemical, Sm-Nd isotopic, as well as LA-ICP-MS U-Pb geochronological data from the Acatln Complex, Mexico, that document the development of a Carboniferous-Permian arc system along the western margin of the supercontinent adjacent to a subducting part of the paleo-Pacific ocean. The data support a Pangea-A solution and characterize arc development along the periphery of Pangea, while Pangea was amalgamating, and therefore pertain to the transfer of subduction zones from the interior of Pangea to its periphery. The incorporation of data both from arc-related igneous rocks as well as spatially proximal, contemporary metasedimentary rocks also documents the relationship between basin formation and plutonism in a peripheral orogen.
Eocene vegetation and ecosystem fluctuations inferred from a high-resolution phytolith record
Lauren A. Miller et al., University of Michigan, Geological Sciences, 1100 N. University Ave., Ann Arbor, Michigan 48109, USA. Posted online 21 June 2012; doi: 10.1130/B30548.1.
Vegetation community structure has a fundamental control on ecosystem function, both locally (e.g., productivity) and at regional to global scales (e.g., albedo, carbon storage). Understanding ecosystem variability in the geologic past provides models for how plant communities will respond to future climatic change. Nathan D. Sheldon and colleagues document a high-resolution paleovegetation record from Montana during a global greenhouse climate (middle Eocene) using plant silica microfossils (phytoliths) and other, aquatic, biosilica (e.g., diatoms, sponge spicules) collected from fossil soils (paleosols). Phytoliths form within plant tissue and are diagnostic at or below family level for many types of plants, including palms and grasses. Data presented by Sheldon and colleagues show that Montana was dominantly forested in the Eocene, with palm and Costus-type phytoliths, indicating paratropical conditions. Local to regional disturbance events led to the short-lived proliferations of grasses more than 15 million years before their rise to ecological dominance. Costus-type phytoliths co-occurred with aquatic biosilica but were inversely correlated with grasses, suggesting that they inhabited open, wet microhabitats, such as forest gaps and stream margins. The fluctuations between open and closed habitats suggest dynamic vegetation community structure, even at a time of relative climatic stability.
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