Topics include
1. Sonograms of Earth.
2. Study of an 1138-m-long drill core, representing the last 20 million years of glacial history.
3. Greenhouse-icehouse oscillations as a natural consequence of plate tectonics operating in the presence of continental masses.
4. An alternative hypothesis for the origin of Grand Canyon (Arizona, USA).
5. Determination of spring-water origins and pathways in the Cuatrociénegas Basin, Mexico.
6. Ancient interactions between the ice sheet and the ocean at the Ross Sea continental shelf.
7. The misconception about the evolution of the northern Rio Grande Rift, Gore Range, Colorado.
8. Solving the debate over Death Valley.
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.
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Pleistocene sequence stratigraphy of the shallow continental shelf, offshore New Jersey: Constraints of Integrated Ocean Drilling Program Leg 313 core holes
Kenneth G. Miller et al., Dept. of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/GES00795.1.
The Pleistocene epoch (the past 2.55 million years) was marked by large (>100 m) sea-level rises and falls that controlled deposition and erosion of sediment. Geologists' understanding of the relationship between sea level and the sediment record has been limited by the ability to recognize and date Pleistocene packages of sediments called sequences that are bracketed by sea-level falls. In this paper, Kenneth G. Miller and colleagues integrate data from core samples obtained by Integrated Ocean Drilling Program Expedition 313 with new seismic stratigraphic data ("sonograms of the Earth") to interpret Pleistocene sea-level changes on the inner to middle continental shelf and the Hudson shelf valley. Improved age control allows recognition and dating of six Pleistocene sequences. Miller and colleagues suggest that sequences were preserved only during peak high global sea-level events except for a few low stand deposits preserved in eroded (incised) valleys. Incised valleys document more southerly courses of the paleo-Hudson Valleys compare to the modern.
Lithostratigraphy determined from downhole logs in the AND-2A borehole, southern Victoria Land Basin, McMurdo Sound, Antarctica
Sabine Hunze et al. (Thomas Wonik, corresponding author), Leibniz Institute for Applied Geophysics (LIAG), Stilleweg 2, D-30655 Hannover, Germany. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/GES00774.1.
During the 2007-2008 austral spring season, the ANDRILL Southern McMurdo Sound Project recovered an 1138-m-long core, representing the last 20 million years of glacial history. An extensive downhole logging programme was successfully carried out. The aim of these analyses was to use detailed interpretation of the downhole logs to obtain a description of the lithologies and their specific physical properties that is independent of the core descriptions. Sabine Hunze and colleagues use statistical analyses to establish an independent lithological column and to identify boundaries of change in sediment composition, provenance, and/or environmental conditions, and the uranium content in the downhole logs to determine hiatuses. The main purpose of this paper is to provide important new constraints on lithostratigraphy (Plio-Pleistocene sediment composition and paleoenvironment) that have general bearing for understanding the climatic evolution of the Victoria Land Basin within the West Antarctic Rift. Some remarkable results could be achieved from the downhole logging data of AND-2A borehole although the boundary conditions for interpretation were far from ideal: (1) there is no great variability in the lithology of the AND-2A core, (2) the cementation occur over various lithology and changes the physical parameters of each lithology significantly. All results presented in this paper show the benefit of downhole logging for the overall understanding of the ANDRILL geological setting.
Continental arc-island arc fluctuations, growth of crustal carbonates, and long-term climate change
Cin-Ty A. Lee et al., Dept. of Earth Science, MS-126, Rice University, 6100 Main Street, Houston, Texas 77005, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/GES00822.1.
The Cretaceous to early Paleogene (50 to 140 million years ago) was characterized by a greenhouse baseline climate, driven by elevated concentrations of atmospheric CO2. Hypotheses for the elevated CO2 concentrations invoke an increase in volcanic CO2 production due to higher oceanic crust production rates, higher frequency of large igneous provinces, or increases in pelagic carbonate deposition, the last leading to enhanced carbonate subduction into the mantle source regions of arc volcanoes. However, these are not the only volcanic sources of CO2 during this time interval. Cin-Ty A. Lee and colleagues show that ocean-continent subduction zones, manifested as a global chain of continental arc volcanoes, were as much as 200% longer in the Cretaceous and early Paleogene than in the late Paleogene to present, when a cooler climate prevailed. They suggest that greenhouse-icehouse oscillations are a natural consequence of plate tectonics operating in the presence of continental masses, serving as a large capacitor of carbonates that can be episodically purged during global flare-ups in continental arcs. Importantly, they note that if the global crustal carbonate reservoir has grown with time, as might be expected because platform carbonates on continents do not generally subduct, the greenhouse-driving potential of continental arcs would have been small during the Archean, but would have increased in the Neoproterozoic and Phanerozoic after a significant reservoir of crustal carbonates had formed in response to the evolution of life and the growth of continents.
Rejection of the lake spillover model for initial incision of the Grand Canyon, and discussion of alternatives
William R. Dickinson, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/GES00839.1.
Almost 150 years after John Wesley Powell first ran its rapids, geologists still cannot agree about the origin of the Grand Canyon of Arizona. All agree that the canyon was cut by the Colorado River, but why the river cut the canyon exactly where it did and when it did is fiercely debated. This paper by William R. Dickinson considers and rejects the hypothesis that incision of the Grand Canyon was initiated by spillover of water from a supposedly deep lake that formed in north-central Arizona east of the Kaibab-Coconino Plateau and was filled by inflow of water from the upper Colorado River in Utah. Consideration of the morphology and history of the Colorado River drainage system as a whole supports the alternative hypothesis that an ancestral Miocene Colorado River cut a shallow canyon through the Kaibab-Coconino Plateau but exited into the Virgin River drainage north of the mouth of the present Grand Canyon. Subsequent headward erosion upstream from the Grand Wash Cliffs was capable of carving the lower Grand Canyon to capture the ancestral Colorado River near the geographic center of the modern Grand Canyon, thereby integrating the courses of the upper and lower Colorado Rivers for the first time near the Miocene-Pliocene time boundary some five million years ago. Thereafter, river flow along its present course deepened and widened the full Grand Canyon.
Identifying origins of and pathways for spring waters in a semiarid basin using He, Sr, and C isotopes: Cuatrociénegas Basin, Mexico
B.D. Wolaver et al., Bureau of Economic Geology, The University of Texas at Austin, 10100 Burnet Road, Austin, Texas 78758, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/GES00849.1.
This study by B.D. Wolaver and colleagues presents the first dissolved noble gas and He isotopic data from northeastern Mexico. Helium, carbon, and strontium isotopes are used to infer spring sources in a water-stressed area. Spring-water origins and pathways in the Cuatrociénegas Basin are revealed by linking structure and geochemistry via regionally extensive fault networks. Basement involved faults with complex reactivation histories are important in northeastern Mexico tectonics and affect hydrogeologic systems. The importance of faults as conduits for northeastern Mexico volcanism is recognized, but connections between faulting and the hydrogeologic system have not been extensively investigated. This research tests the hypothesis that Cuatrociénegas Basin springs are divided into two general classes based upon discharge properties: (1) regional carbonate aquifer discharge (mesogenic) mixed with contributions from deeply sourced (endogenic) fluids containing 3He and CO2 from the mantle that ascend along basement-involved faults; and (2) carbonate aquifer discharge mixed with locally recharged (epigenic) mountain precipitation. This study demonstrates the presence of mantle derived 3He and deeply sourced CO2 that ascend along basement-penetrating faults and mix with Cupido aquifer groundwater before discharging in Cuatrociénegas Basin springs.
Orbitally paced shifts in the particle size of Antarctic continental shelf sediments in response to ice dynamics during the Miocene climatic optimum
S. Passchier et al., Dept. of Earth and Environmental Studies, Montclair State University, 252 Mallory Hall, 1 Normal Avenue, Montclair, New Jersey 07043, USA. Posted online 13 Dec. 2012; http://dx.doi.org/10.1130/GES00840.1.
Drillholes within sediment archives on the Antarctic continental margin shed light on changes in ice cover during past warm periods. By analyzing the changes in the seafloor sediment composition in an ANDRILL core from the Ross Sea continental shelf, S. Passchier and colleagues investigate the interactions between the ice sheet and the ocean. They discuss how, over time, ice growth and decay control the available wave energy recorded in the grain size of the seafloor sediment and conclude that although melt at the top of the ice sheet may have been limited over the past 18 million years, warm ocean currents may have melted a large proportion of the ice that is in contact with the ocean during past warm periods.
(U-Th)/He thermochronologic constraints on the evolution of the northern Rio Grande Rift, Gore Range, Colorado, and implications for rift propagation models
Rachel L. Landman and Rebecca M. Flowers, Dept. of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA. Posted online 17 Dec. 2012; http://dx.doi.org/10.1130/GES00826.1.
The Rio Grande rift system is a zone of intracontinental extension that tapers northward into the center of the southern Rocky Mountains. Near its northern end, the rift is located in a region that contains some of the highest peaks in the Rockies. However, relationships between the rifting process and development of the Rocky Mountains are not well understood. The notion persists that the Rio Grande rift propagated northward in late Cenozoic time, with this propagation proposed as a possible cause of late Cenozoic uplift of the Rocky Mountains. This study by Rachel Landman and Rebecca Flowers of the University of Colorado Boulder uses low-temperature thermochronology to constrain the uplift and exhumation history of the Gore Range, a rift-flank uplift at the northern end of the rift in central Colorado. Their results show that the mid-Tertiary and younger history of the Gore Range area is similar to histories inferred along the rest of the rift to the south, suggesting that the onset and evolution of the Rio Grande rift were roughly synchronous along its length. This conclusion demonstrates that the idea of a northward propagating rift is a misconception.
Detrital zircon age distributions as a discriminator of tectonic versus fluvial transport: An example from the Death Valley, USA, extended terrane
Nathan A. Niemi, Dept. of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. Posted online 17 Dec. 2012; http://dx.doi.org/10.1130/GES00820.1.
The Basin and Range Province of the western United States is perhaps the premier example of a continental extensional orogen on Earth today. Nonetheless, the amount of extension that has occurred across the Basin and Range, and the mechanisms that accommodate it, remain strongly debated. This is particularly true in the Death Valley region, where up to 400% crustal extension has been proposed in the last ~15 million years. In part, this debate hinges on the interpretation of fluvial sediments located on the eastern side of Death Valley, which contain unique clasts derived from a source ~80 km to the WNW on the western side of Death Valley, with one interpretation positing that most of the transport of the clasts from source to sink was accomplished by tectonic processes, and another that the transport is primarily due to sedimentologic processes. In this paper, Nathan A. Niemi describes a new method to quantitatively assess the transport distance of fluvial sediments using the dilution of distinct detrital zircon U-Pb age populations. Detrital zircon U-Pb age spectra from sedimentary rocks on the east of Death Valley contain a Jurassic age peak that is similar in age and magnitude to unique plutonic source rocks in western Death Valley, supporting an interpretation of large-magnitude extension across Death Valley. The proposed methodology is applicable for discriminating tectonic versus sedimentary transport in any orogenic system in which a unique zircon source population can be identified.
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