GSA Bulletin: Rock avalanches, ancient weather, astronomical clocks, anoxia, and volcanism

(Press-News.org) Boulder, Colo., USA – Highlights from GSA Bulletin articles published online on 20 March through 1 April 2014 include a discussion of a catastrophic rock avalanche in the Atlas Mountains of Morocco 4,500 years ago and that village situated there now; evidence of rain and humidity in ancient soils in the western United States; a contribution to the on-going EarthTime initiative, which is working to refine and calibrate deep time geochronometers; and a call for intensive field studies in volcanic areas.

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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Catastrophic rock avalanches in a glaciated valley of the High Atlas, Morocco: 10Be exposure ages reveal a 4.5 ka seismic event
Phillip D. Hughes et al., School of Environment, Education and Development, The University of Manchester, Manchester M13 9PL, UK. Published online 20 Mar. 2014; http://dx.doi.org/10.1130/B30894.1. Photo Available.

A huge, catastrophic rock fall occurred 4,500 years ago in the high Atlas Mountains, Morocco. The timing of this event has been determined using exposure-age dating. The collapse of the northwest face of Mount Aksoual, which reaches a height of nearly 13,000 ft (4000 m), caused large-scale landscape change. the rock fall represents one of the biggest recorded in Africa and sits below a cliff face 6500 ft (2000 m) high, close to an active tectonic fault. Today, a village sits precariously on this huge mass of boulders.

Early to Middle Ordovician back-arc basin in the southern Appalachian Blue Ridge: Characteristics, extent, and tectonic significance
James Tull et al., Florida State University, Earth Ocean and Atmospheric Science, Tallahassee, Florida 32306-4520, USA. Published online 20 Mar. 2014; http://dx.doi.org/10.1130/B30967.1.

This paper by James Tull and colleagues links the stratigraphy and tectonic history of a large segment of the southern Appalachian Blue Ridge and adjacent areas in Alabama, Georgia, and North and South Carolina to a common tectonic setting: formation within an Ordovician (about 480 to 460 million years ago) proto-North American (Laurentian) back-arc basin. The tectonic setting suggested by this study indicates that the Taconic (early to middle Ordovician time) orogeny in the southernmost Appalachians differs from that in the northern Appalachians, and began as an extensional accretionary orogen along the outer margin of the Laurentian continent, rather than resulting from an exotic (non-Laurentian) arc collisional setting.

Analogue modeling of positive inversion tectonics along differently oriented pre-thrusting normal faults: An application to the Central-Northern Apennines of Italy
Alessandra Di Domenica et al., Dipartimento di Ingegneria e Geologia, Università degli Studi "G. d'Annunzio" Chieti-Pescara, Campus Universitario Madonna delle Piane, Via dei Vestini, 31-66013 Chieti Scalo (CH), Italy. Published online 20 Mar. 2014; http://dx.doi.org/10.1130/B31001.1.

Alessandra Di Domenica and colleagues created a sandbox experiment to test the influence of preexisting discontinuities on foreland fold-and-thrust systems development. The model provide innovative results concerning the development of a chain, in terms of thrusts geometry and tectonic styles, highlighting the role exerted by coexisting differently oriented inherited faults. The experimental setup design was based on field data and observations collected in the Central-Northern Apennine chain (Italy).

Multiproxy approach reveals evidence of highly variable paleoprecipitation in the Upper Jurassic Morrison Formation (western United States)
Timothy S. Myers et al., Roy M. Huffington Dept. of Earth Sciences, Southern Methodist University, Dallas, Texas 75275, USA. Published online 20 Mar. 2014; http://dx.doi.org/10.1130/B30941.1.

Geochemical analyses of ancient soils in the western United States provide estimates of rainfall and humidity during deposition of the Morrison Formation in the Late Jurassic, approximately 150 million years ago. Weathering indices, used as proxies for rainfall, indicate that precipitation was highly variable within the Morrison depositional basin, with estimates ranging from 50 to 1200 mm per year and averages around 800 mm per year. These estimates of ancient precipitation indicate that aridity decreased over time, and there was an abrupt transition from relatively dry southern environments to wetter northern environments. Humidity regimes inferred from geochemical proxies range from semiarid to humid, suggesting somewhat wetter conditions than the precipitation estimates. The inferred paleo-precipitation patterns do not match the modern latitudinal distribution of rainfall that arises from zonal atmospheric circulation.

Integrating 40Ar/39Ar, U-Pb, and astronomical clocks in the Cretaceous Niobrara Formation, Western Interior Basin, USA
Bradley B. Sageman et al., Dept. of Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, Illinois 60208, USA. Published online 20 Mar. 2014; http://dx.doi.org/10.1130/B30929.1.

This study by Brad Sageman and colleagues develops and applies new methods for refinement of the geologic time scale to a key Late Turonian through Early Campanian (Late Cretaceous, about 80 to 90 million years ago) succession in the Western Interior Basin of North America. Integration of new high-precision radioisotope dates using both Ar-Ar and U-Pb systems, and floating astrochronologies developed from spectral analysis of the chalk and marl beds of the Niobrara Formation, produce revised estimates for these age boundaries. Sageman and colleagues introduce a new method of estimating uncertainties due to geologic correlation and combines these with astrochronologic uncertainties and those associated with radioisotopic methods to achieve comprehensive total uncertainty estimates for the Stage boundary ages. The co-occurrence of U-Pb and Ar-Ar data from the same ashes allows confirmation of the recently revised age for the Fish Canyon monitor mineral (28.201 million years old) and thereby makes a contribution to the on-going EarthTime initiative, which is working to refine and calibrate deep time geochronometers.

Testing the astronomical time scale for oceanic anoxic event 2, and its extension into Cenomanian strata of the Western Interior Basin (USA)
Chao Ma et al., Department of Geoscience, University of Wisconsin–Madison, 1215 W Dayton Street, Madison, Wisconsin 53706, USA. Published online 27 Mar. 2014; http://dx.doi.org/10.1130/B30922.1.

Late Cretaceous strata of the Western Interior Basin, USA, preserve astronomically influenced sedimentation and abundant volcanic ash beds, providing a remarkable opportunity to develop integrated astronomical and radioisotopic time scales. Chao Ma and colleagues used X-ray fluorescence (XRF) core scanning to develop a new elemental data set for cyclostratigraphic investigation of Cenomanian/Turonian strata, including the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member. 40Ar/39Ar ages from ashes in three biozones, including a new age from the uppermost Lincoln Limestone Member, provide geochronologic constraints for the cyclostratigraphic analysis. Ma and colleagues note that results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the USGS #1 Portland core. They note that identification of an astronomical signal in the underlying Hartland Shale Member permits extension of the Western Interior Basin astrochronology into the earlier Cenomanian, prior to Oceanic Anoxic Event 2 (OAE2). High rates of sedimentation in the Angus core during the interval of OAE 2 initiation allow recognition of a strong precessional control on bedding development. As a consequence, the new results provide a rare high-resolution chronometer for the onset of OAE 2, and the timing of proposed hydrothermal trace metal enrichment as observed in the XRF data.

Landscape modification in response to repeated onset of hyperarid paleoclimate states since 14 Ma, Atacama Desert, Chile
Teresa E. Jordan et al., Department of Earth & Atmospheric Sciences, Cornell University, Ithaca, New York 14853-1504, USA. Published online 1 Apr. 2014; http://dx.doi.org/10.1130/B30978.1.

The Atacama Desert of western South America is an extreme environment that spans from the western flank of the Andes Mountains to the Pacific coast. Where annual rainfall is less than 5 mm, there is no plant life, and water almost never flows either in abandoned stream channels or on hill sides. Given millions of years of time to act, the Atacama Desert processes that shape the landscape and develop soils create a mixture of materials on the desert floor that are unique. By studying the record of landforms and soils in a valley in which sediments are trapped, authors Teresa E. Jordan and colleagues demonstrate that the Atacama Desert has been hyperarid for 12 million years. Yet, like all other places on Earth, the climate changed repeatedly. Whereas the growth of height of the Andes Mountains may have set in place a necessary condition to initiate extreme aridity, it is likely that changes in the temperature of the Pacific Ocean set the pace for the repeated climate shifts of the last six million years.

Age and eruptive center of the Paeroa Subgroup ignimbrites (Whakamaru Group) within the Taupo Volcanic Zone of New Zealand
D.T. Downs et al., School of Environment, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand. Published online 1 Apr. 2014; http://dx.doi.org/10.1130/B30891.1.

The Whakamaru Group ignimbrites are the result of the largest rhyolitic eruption within New Zealand and one of the largest eruptions of the past million years. This study by D.T. Downs and colleagues demonstrates that this eruptive episode was more prolonged and complex than previously documented. There were at least two distinct eruptive episodes from geographically separate vents. The distribution and locations of vent derived lithic clasts within the younger part of the Whakamaru Group (termed the Paeroa Subgroup) indicates that these ignimbrites erupted in non-energetic manner, and from a linear vent zone source. This Paeroa linear vent zone coincides with the preset-day Paeroa Fault, and eruptions are speculated to have occurred in a fissure style. No caldera has been identified for such a large eruption, and collapse maybe concealed as fault related movement and/or collapse in adjacent calderas. Downs and colleagues note that the results of this study demonstrate the need for intensive field studies in volcanic areas, and that structural controls may play an as yet unknown role in controlling vent locations and eruptive styles.

Mid-Miocene rhyolite volcanism in northeastern Nevada: The Jarbidge Rhyolite and its relationship to the Cenozoic evolution of the northern Great Basin (USA)
Matthew E. Brueseke et al., Department of Geology, Kansas State University, 108 Thompson Hall, Manhattan, Kansas 66506, USA. Published online 1 Apr. 2014; http://dx.doi.org/10.1130/B30736.1.

This study of the Jarbidge Rhyolite by Matthew E. Brueseke and colleagues focuses on understanding the occurrence and cause of widespread mid-Miocene felsic volcanism in the northern Great Basin, USA. Physical characteristics, whole rock geochemistry, 40Ar/39Ar geochronology, and oxygen isotope data indicate that at least 500 cubic kilometers of primarily phenocryst-rich rhyolite lavas erupted 15 to 16.1 million years ago in the vicinity of the Jarbidge Mountains (Nevada). Brueseke and colleagues interpret the distribution of the Jarbidge Rhyolite in northeastern Nevada to reflect an intimate association with temporally and spatially coincident extension that resulted from the rapid collapse of the Nevadaplano high plateau approx. 17 to 16 million years ago, rather than the Yellowstone hotspot and its interaction with North America.

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