In this new, open-access GSA Bulletin article, Christopher A-L. Jackson and colleagues enter this debate, critiquing the geological and geophysical evidence forwarded in support of the two main competing genetic models. Their study suggests that much of this evidence is not diagnostic of either model and that a revised model is required. Although their results are unlikely to be universally accepted, they at least will stimulate ongoing debate regarding the origin of this enigmatic structure.
FEATURED ARTICLE Understanding passive margin kinematics: A critical test of competing hypotheses for the origin of the Albian Gap, Santos Basin, offshore Brazil C. A-L. Jackson et al., Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, University Station, Austin, Texas, USA. Published online ahead of print on 19 May 2015; http://dx.doi.org/10.1130/B31290.1. This paper is OPEN ACCESS online.
Other GSA BULLETIN articles (see below) cover such topics as 1. The catastrophic gravitational collapse at Volcan Parinacota; 2. Two papers concerning the Cretaceous-Paleogene (K-Pg) mass extinction; and 3. Fluid pressure cycles in the Tellaro Detachment, Italy.
GSA BULLETIN articles published ahead of print are online at http://gsabulletin.gsapubs.org/content/early/recent; abstracts are open-access at http://gsabulletin.gsapubs.org/. Representatives of the media may obtain complimentary copies of articles by contacting Kea Giles.
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Early Holocene collapse of Volcán Parinacota, Central Andes, Chile: Volcanological and paleohydrological consequences B.R. Jicha et al., University of Wisconsin, Madison, Wisconsin, USA. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/B31247.1.
Volcán Parinacota, located in the Chilean Andes, suffered a catastrophic gravitational collapse that produced an approx. six cubic kilometer debris avalanche, which is about three times larger than the 1980 Mount St. Helens debris avalanche. Although the eruptive history of Parinacota before and after the collapse is well known, previous age limits span about 8 to 20 thousand years ago (ka). New cosmogenic 10Be surface-exposure dates from boulders atop the deposit indicate that the avalanche occurred at 8.8 +/- 0.5 ka. These data require a voluminous initial phase of post-collapse volcanism and a reevaluation of the widely held view that Lago Chungará formed when the debris avalanche blocked the Paleo-Lauca River. Authors Brian R. Jicha and colleagues suggest that volcano collapse did not result in the formation of Lago Chungará, but instead led to a major expansion of a preexisting closed basin. Thus, the new age allows the interplay between volcano collapse, its eruption rate, and its geomorphic and paleohydrologic consequences to be more fully understood.
Potential K-Pg tsunami deposits in the intra-Tethyan Adriatic carbonate platform section of Hvar (Croatia) T. Korbar et al., Croatian Geological Survey, Zagreb, Croatia. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/B31084.1.
This paper by Tvrtko Korbar and colleagues reports potentially the first case of a tropical carbonate platform sedimentary succession recording the K-Pg event, including relatively thick inner-platform tsunami deposit, which provides a new constraint for modeling both the western Tethyan paleogeography and the catastrophic aftermath of the Chicxulub asteroid impact at the Cretaceous-Paleogene boundary.
A paleoclimatic and paleoatmospheric record from peatlands accumulating during the Cretaceous-Paleogene boundary event, Western Interior Basin, Canada Rhodri M. Jerrett et al., School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/B31166.1.
The mass extinction at the Cretaceous-Paleogene (K-Pg) boundary (66 million years ago) was one of the most severe in Earth' history, yet the precise nature of climate change at that time is not fully resolved. In this study, Rhodri M. Jerrett and colleagues approach the problem by analyzing the composition of a coal that spans the K-Pg boundary. The Ferris-Nevis coal represents a fossilized peatland that accumulated during the K-Pg extinction event in Alberta and Saskatchewan. By analyzing the preservation style of the constituent plant material (macerals) and the isotopic composition of the seam, it was possible to reconstruct changes in climate and atmospheric composition in western Canada through the K-Pg boundary. Jerrett and colleagues find that centennial- to millennial-scale cycles in temperature and/or rainfall and atmospheric composition, similar to those occurring in the last several thousand years, were occurring before and after the extinction event. However, there is no evidence for dramatic changes in climate immediately before or at the extinction itself.
A paleoclimatic and paleoatmospheric record from peatlands accumulating during the Cretaceous-Paleogene boundary event, Western Interior Basin, Canada Rhodri M. Jerrett et al., School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/B31166.1.
The mass extinction at the Cretaceous-Paleogene (K-Pg) boundary (66 million years ago) was one of the most severe in Earth' history, yet the precise nature of climate change at that time is not fully resolved. In this study, Rhodri M. Jerrett and colleagues approach the problem by analyzing the composition of a coal that spans the K-Pg boundary. The Ferris-Nevis coal represents a fossilized peatland that accumulated during the K-Pg extinction event in Alberta and Saskatchewan. By analyzing the preservation style of the constituent plant material (macerals) and the isotopic composition of the seam, it was possible to reconstruct changes in climate and atmospheric composition in western Canada through the K-Pg boundary. Jerrett and colleagues find that centennial- to millennial-scale cycles in temperature and/or rainfall and atmospheric composition, similar to those occurring in the last several thousand years, were occurring before and after the extinction event. However, there is no evidence for dramatic changes in climate immediately before or at the extinction itself.
Stratigraphy, structure, and volcano-tectonic evolution of Solfatara maar-diatreme (Campi Flegrei, Italy) Roberto Isaia et al., Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli Osservatorio Vesuviano, Via Diocleziano 328, 80124 Napoli, Italy. Published online ahead of print on 7 May 2015; http://dx.doi.org/10.1130/B31183.1.
We performed a multidisciplinary investigation on the Solfatara volcano within the Campi Flegrei (Tyrrhenian coast, southern Italy). This research involved a volcanological survey, deposit facies reconstruction, and structural and geophysical analyses. The Solfatara volcano is characterized by a crater cut into earlier volcanic deposits, a small rim of ejecta and a deep structure (down to 2-3 km). A new geological map and cross sections show a complex architecture of different volcano-tectonic features including scoria cones, lavas and crypto-domes, feeder dikes, pipes, ring and regional faults, and explosive craters. Fault and fracture analyses, Electrical Resistivity Tomography investigations suggest that the Solfatara area is dominated by a maar-diatreme evolution. The type volcanism that occurred in past at the Solfatara area should be considered as a future eruption scenario at Campi Flegrei.
Paleoelevation records from lipid biomarkers: Application to the tropical Andes Veronica J. Anderson et al., Jackson School of Geosciences, University of Texas at Austin, Austin, Texas, USA. Published online ahead of print on 19 May 2015; http://dx.doi.org/10.1130/B31105.1.
New results from two novel lipid biomarker-based proxies help constrain the late Cenozoic surface elevation history of the Eastern Cordillera in the tropical northern Andes of Colombia. Previous well-known studies have suggested rapid latest Miocene to Pliocene (6 to 3 million years ago [Ma]) uplift on the basis of an abrupt shift in pollen species assemblages within sedimentary basin fill of the elevated Bogotá plateau. Reconstructed temperatures from one of these biomarker-based proxies show a more gradual cooling trend from ca. 7.6 Ma to present, consistent with less than 1000 m of elevation gain since latest Miocene-Pliocene time and in agreement with geologic evidence for accelerated shortening and exhumation at this time. These results suggest that the Bogotá plateau may have uplifted more gradually than previous works had proposed, which has implications for the evolution of unique tropical alpine biomes native to the Eastern Cordillera.
Mineral reactions associated with hydrocarbon paleo-migration in the Huincul High, Neuquén Basin, Argentina A.L. Rainoldi et al., Consejo Nacional de Investigaciones Científicas y Técnicas, Centro Patagónico de Estudios, Metalogenéticos, Argentina. Published online ahead of print on 19 May 2015; http://dx.doi.org/10.1130/B31201.1.
This article describes the bleaching of a Cretaceous red-bed sequence produced by the migration of hydrocarbons and related fluids. The objective of this contribution is to determine the geochemical and physical processes responsible for the discoloration of the red bed outcrops in Huincul High and their relationship to red bed bleaching at the basin scale in the Neuquén Basin, the most important hydrocarbon producer in Argentina. Ana L. Rainoldi and colleagues focus on the diagenetic reactions involved during the alteration and in the role of clay mineral authigenesis as an alternative means of demonstrating that reducing pore waters have migrated through a sedimentary unit in absence of precursor hematite cement.
Fluid pressure cycles, variations in permeability and weakening mechanisms along low-angle normal faults: The Tellaro Detachment, Italy Luca Clemenzi et al., Natural and Experimental Tectonics Research Group (NEXT), University of Parma, Parma, Italy. Published online ahead of print on 19 May 2015; http://dx.doi.org/10.1130/B31203.1.
Rock mechanics theories predict that active extensional faults should be steeper than 30-40 degrees. Nevertheless, extensional faults dipping less than 30 degrees have been described worldwide and are named Low-Angle Normal Faults (LANF) or misoriented normal faults. The existence of LANFs is explained by two contrasting models: (i) those proposing that such faults were active at optimal angles, than were passively rotated to low-angle attitude after their activity; (ii) those proposing that specific conditions can promote the activity of misoriented faults. This paper describes a LANF in the Northern Apennines (Italy) which was active in mid-late Miocene times (10 to 6 million years ago) and is now visible along the Tyrrhenian shoreline. Our results show that such fault was already dipping less than 30 degrees during its main activity, and was only affected by minor subsequent passive rotation. The activity of this misoriented fault was promoted by specific mechanical conditions in the fault zone, namely elevated fluid pressure and low differential stress. The presence of the LANF influenced the circulation of subsurface fluids. In particular, fault-related processes created a network of fluid conduits and barriers, which was very variable over space and time and promoted or prevented subsurface fluid circulation.
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