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Contact: Kea Giles
Geological Society of America

Geosphere covers Mexico, the Colorado Plateau, Russia, and offshore New Jersey

Geosphere articles posted online Feb. 21, 2014

Boulder, Colo., USA – New Geosphere postings cover using traditional geochemistry with novel micro-analytical techniques to understand the western Trans-Mexican Volcanic Belt; an investigation of mafic rock samples from a volcanic field near Yampa, Colorado, travertine deposits in the southeastern Colorado Plateau of New Mexico and Arizona; a study "Slushball Earth" rocks from Karelia, Russia, using field and micro-analytical techniques; and an addition to the "The History and Impact of Sea-level Change Offshore New Jersey" special issue.

Abstracts for these and other Geosphere papers are available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of Geosphere articles by contacting Kea Giles at the address above.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geosphere in articles published. Contact Kea Giles for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Bedding attitudes as a sequence stratigraphy proxy: A case study from borehole images, Integrated Ocean Drilling Program Expedition 313, Site M28
Christophe Basile ISTerre, Université Joseph Fourier, Grenoble, France; and and Donald Monteverde, Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA. Published online 21 Feb. 2014, http://dx.doi.org/10.1130/GES00856.1.
Special issue: Results of IODP Exp313: The History and Impact of Sea-level Change Offshore New Jersey.

Sequence stratigraphy describes the relationship between sea-level variations and the organization of sedimentary bodies. Among various types of observations, it commonly uses the geometry of sedimentary layers at kilometer scale on seismic sections. This article is the first attempt to similarly use the geometry of sedimentary bedding at a meter scale. It shows that at all scales the orientation of sedimentary beds can be used as a stratigraphic tool related to sedimentation processes and to sea level.

The origin of intraplate magmatism in the western Trans-Mexican Volcanic Belt
Beatriz A. Díaz-Bravo et al. (Corresponding author: Arturo Gómez-Tuena: tuena@geociencias.unam.mx or tuena@unam.mx), Centro de Geociencias, Universidad Nacional Autónoma de México, Querétaro 76230, México. Published online 21 Feb. 2014, http://dx.doi.org/10.1130/GES00976.1.

In this contribution we document the chemical characteristics of an unusual set of volcanic rocks from western Mexico. Erupted in one of the most complex tectonic settings on the planet, and with compositions that are more akin to oceanic islands than to convergent margins, these rocks provide a unique opportunity to understand the compositional underpinnings upon which a continent rests and grows. By combining traditional geochemistry with novel micro-analytical techniques, we characterize the pristine heterogeneous nature of the mantle source below Mexico and the petrologic and tectonic conditions that promote its melting.

40Ar/39Ar geochronology, isotope geochemistry (Sr, Nd, Pb), and petrology of alkaline lavas near Yampa, Colorado: Migration of alkaline volcanism and evolution of the northern Rio Grande rift
M.A. Cosca et al., U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA. Published online 21 Feb. 2014, http://dx.doi.org/10.1130/GES00921.1.

M.A. Cosca and colleagues investigated mafic rock samples from a small volcanic field near Yampa, Colorado, using geochemical and isotopic techniques. Their results indicate that these volcanic rocks are melt products of the subcontinental lithospheric mantle that ascended rapidly to the surface between 4.5 and 6 million years ago. The ages of the volcanic rocks become younger toward the southwest and form part of a regional pattern of volcanism closely associated with late Cenozoic extension that is migrating toward the northeast margin of the Colorado Plateau. The youngest (Quaternary) volcanic rocks within this migrating sequence of volcanism coincide with the western margin of the Rio Grande rift in Colorado, and projection of this volcanism defines the incipient northern segment of the rift that includes the Leucite Hills, Wyoming.

U-series geochronology of large-volume Quaternary travertine deposits of the southeastern Colorado Plateau: Evaluating episodicity and tectonic and paleohydrologic controls
A. Priewisch et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Published online 21 Feb. 2014, http://dx.doi.org/10.1130/GES00946.1

Large-volume travertine deposits in the southeastern Colorado Plateau of New Mexico and Arizona, USA, occur along the Jemez lineament and Rio Grande rift. These freshwater carbonates reflect locations where mantle-derived carbon dioxide and deeply sourced groundwater were conveyed to the surface along faults and discharged through springs. U-series dating shows that large-volume (2.5 cubic kilometers) deposition took place episodically 700� thousand years ago, 350� thousand years ago, and 100󈞔 thousand years ago. These pulses of travertine formation coincide with regional basaltic volcanism, which supplied the excess CO2. Landscape erosion rates show that travertine deposits developed on local topographic highs that required high hydraulic head. Large-volume travertine deposition was thus controlled by both tectonic activity and past hydrologic conditions. By analogy to the active Springerville-St. Johns CO2 gas field in Arizona, the large volumes and similar platform geometries of travertine deposits in this study are interpreted to represent extinct CO2 gas reservoirs that leaked large volumes of deeply derived CO2 to Earth's surface system.

Field and microanalytical isotopic investigation of ultradepleted in 18O Paleoproterozoic "Slushball Earth" rocks from Karelia, Russia
I.N. Bindeman et al., Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA. Published online 21 Feb. 2014, http://dx.doi.org/10.1130/GES00952.1.

Geologic records on the composition of Earth's ancient atmosphere, hydrosphere, and meteoric precipitation are rare and are typically derived from isotopic and chemical materials that resided near the surface. There is still enigmatic connection between the first appearance of oxygen in the air 2.2-2.4 billion years ago and three to four pan-global glaciation events that preceded it, each lasting millions of years. Here I.N. Bindeman and colleagues describe rocks with the lowest known values of oxygen isotopes (delta-18O down to -27.3 per mil), mapped individually and regionally, and now extending 450 km along the Paleoproterozoic Belomorian complex in Karelia, Russia. The protolith of these rocks, now represented by nearly-gem quality rubies, carries isotopic signatures of glacial ice and is interpreted to have formed by subglacial hydrothermal alteration inside of intracontinental rift zones, which affected tens of cubic kilometers of rocks. As Karelia was located near equatorial latitudes during Paleoproterozoic, ultra-depleted in delta-18O waters outside of polar regions provide first independent evidence for a moderately glaciated, so-called "slushball," Earth climate between 2.45 and 2.23 billion years ago, in which low- or mid-latitude, midsize continents were covered with glaciers while the ocean remained at least partially unfrozen to allow for intracontinental isotopic distillation in a large temperature gradient. This paper provides detailed micro-analytical and geochronologic investigation in an attempt to constrain the timing and mechanism of isotopic depletion and tie them to Paleoproterozoic glaciations.

Tectonic evolution of the Tualatin basin, northwest Oregon, as revealed by inversion of gravity data
D.K. McPhee et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Published online 21 Feb. 2014, http://dx.doi.org/10.1130/GES00929.1.

The Tualatin basin, west of Portland (Oregon, USA), is one of several basins within the Puget-Willamette lowland, a structural and topographic trough between the Coast Range and Cascade volcanic arc. The Puget-Willamette lowland is seismically active and home to most of the Oregon and Washington State populations and major cities. New gravity measurements show that the Tualatin basin is characterized by a gravity low, which reflects the large density contrast between basin fill and the surrounding basement rock, composed primarily of Siltetz terrane. Modeling of the gravity data shows that the Tualatin basin is about 6 km deep and contains several kilometers of sediment that pre-date the emplacement of Miocene Columbia River Basalt Group (CRBG) around 15 million years ago. In addition, the new three-dimensional geometry of the Tualatin basin presented here shows that the basin is bounded by linear faults along the southwest and northeast margins, suggesting a pull-apart origin. The opening of the basin may have been augmented by north-northwest extension in the Coast Range during the late Eocene. The present geometry of the Tualatin basin may imply stronger ground shaking along basin edges from seismicity in the region.



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