Boulder, Colo., USA – Six new Gesophere articles, posted online on 14 Aug. 2013, offer insight into a variety of geologic problems, from the minute to the massive. Authors investigate inclusion and porosity patterns in a 23-carat carbonado diamond; sea-level change offshore of New Jersey (USA); a new age for Sierra Nevada faulting; a reconstruction of the dimensions and shape of the Great Basin over the past 500 million years; and deep-water perspectives on Hawaiian volcano growth.
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Sequence boundaries are impedance contrasts: Core-seismic-log integration of Oligocene-Miocene sequences, New Jersey shallow shelf
Kenneth G. Miller et al., Dept. of Earth & Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA. Themed issue: "Results of IODP Exp313: The History and Impact of Sea-level Change Offshore New Jersey." Posted online 14 Aug. 2013, http://dx.doi.org/10.1130/GES00858.1.
Integrated Ocean Drilling Program (IODP) Expedition 313 has continuously cored uppermost Eocene to Miocene sequences on the New Jersey shallow shelf (Sites M27, M28, and M29). Previously, 15 Miocene (about 13 to 23 million years ago) seismic sequence boundaries were recognized on several generations of multichannel seismic profiles using criteria of onlap, downlap, erosional truncation, and toplap. In this study, Kenneth G. Miller and colleagues independently recognize sequence boundaries in the cores and logs based on an integrated study of core surfaces, lithostratigraphy, and process sedimentology (grain size, mineralogy, facies, and paleoenvironments), facies successions, stacking patterns, benthic foraminiferal water depths, downhole logs, core gamma logs, and chronostratigraphic ages. Excellent recovery allows core-seismic integration that confirms the hypothesis that unconformities are a primary source of impedance contrasts. The core-seismic-log correlations presented by Miller and colleagues predict that key seismic surfaces observed in other subsurface investigations without core and/or well logs are stratal surfaces with sequence stratigraphic significance.
Initiation of Sierra Nevada range front–Walker Lane faulting ca. 12 Ma in the Ancestral Cascades arc
Cathy J. Busby et al., Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA. Themed issue: "Origin and Evolution of the Sierra Nevada and Walker Lane." Posted online 14 Aug. 2013, http://dx.doi.org/10.1130/GES00927.1.
This paper by Cathy J. Busby and colleagues gives evidence for initiation of Sierra Nevada faulting at around 12 million years ago, much older than previously assumed estimates (about 4 million years ago). It also shows that the transtensional plate margin on the eastern side of the Sierra Nevada micro plate was born by about 12 million years ago.
Phanerozoic palinspastic reconstructions of Great Basin geotectonics (Nevada-Utah, USA)
William R. Dickinson, Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA. Posted online 14 Aug. 2013, http://dx.doi.org/10.1130/GES00888.1.
The Great Basin of Nevada and western Utah is one of the most geologically complicated regions of the American Cordillera. Crustal extension over the past 20 million years has rearranged all of its geology on a massive scale by stretching its limits by nearly 100% over wide areas. Before the extension to form the modern Basin and Range Province, there was comparable crustal contraction along the Sevier thrust belt during the interval from 125 to 75 million years ago. This paper by William R. Dickinson and colleagues undertakes to reconstruct the dimensions and shape of the Great Basin over the past 500 million years with the effects of crustal contraction and extension removed. Without the reconstructions, the geometry of evolving tectonic patterns cannot be appraised accurately because many have been distorted since they were formed.
New textural evidence on the origin of carbonado diamond: An example of 3-D petrography using X-ray computed tomography
Richard A. Ketcham, High-Resolution X-ray Computed Tomography Facility, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA; and Christian Koeberl, Dept. of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, and Natural History Museum, Burgring 7, A-1010 Vienna, Austria. Posted online 14 Aug. 2013, http://dx.doi.org/10.1130/GES00908.1.
Three-dimensional textural observations of inclusion and porosity patterns in a 23-carat carbonado diamond using high-resolution X-ray computed tomography reveal new information bearing on the nature and origin of this enigmatic material. A prominent patinaed surface is texturally linked to a banding and grading of inclusions and pore space beneath, extending several millimeters into the specimen. In situ observation demonstrates that almost all inclusions are polymineralic and show replacement textures, corroborating previous work indicating that the pore network is fully three-dimensionally (3-D) connected, and that virtually all macro-inclusions are secondary. Large metal inclusions are only found immediately adjacent to the margin of the specimen, and are thus also likely to be secondary or even tertiary. These and other observations outlined in this study by Richard Ketcham and Christian Koeberl support recent work suggesting that carbonado crystallized from a carbon-supersaturated fluid and suggest that the second stage may correspond with the creation of the pore alignment fabric. They further postulate that, although the present-day macro-inclusions are certainly secondary, the bulk material may not be, and may instead be broken-down remains of the original included phase(s). While further verification is needed, a model built around this hypothesis may provide the simplest explanation to many of the unusual features of carbonado.
Modeling volcano growth on the Island of Hawaii: Deep-water perspectives
Peter W. Lipman and Andrew T. Calvert, U.S. Geological Survey, Menlo Park, California 94025, USA. Posted online 14 Aug. 2013, http://dx.doi.org/10.1130/GES00935.1.
Recent ocean-bottom geophysical surveys, dredging, and dives, which complement surface data and scientific drilling at the Island of Hawaii, document that evolutionary stages during volcano growth are more diverse than previously described. Based on combining available composition, isotopic age, and geologically constrained volume data for each of the component volcanoes, this overview by Peter Lipman and Andrew Calvert provides the first integrated models for overall growth of any Hawaiian island. In contrast to prior morphologic models for volcano evolution (preshield, shield, postshield), growth can be tracked by age and volume (magma supply), defining waxing alkalic, sustained tholeiitic, and waning alkalic stages. Lipman and Calvert compare three alternative models are compared: (1) near-constant volcano propagation; (2) near-equal volcano durations; (3) high peak-tholeiite magma supply. These models define inconsistencies with prior geodynamic models, indicate that composite growth at Hawaii peaked around 800-400 thousand years ago, and demonstrate a lower current rate. Recent age determinations for Kilauea and Kohala define a volcano propagation rate of 8.6 cm/yr that yields plausible inception ages for other volcanoes of the Kea trend. In contrast, a similar propagation rate for the less-constrained Loa trend would require inception of Loihi Seamount in the future and ages that become implausibly large for the older volcanoes. An alternative rate of 10.6 cm/yr for Loa-trend volcanoes is reasonably consistent with ages and volcano spacing, but younger Loa volcanoes are offset from the Kea trend in age-distance plots. Variable magma flux at the Island of Hawaii, and longer-term growth of the Hawaiian chain as discrete islands rather than a continuous ridge, may record pulsed magma flow in the hotspot/plume source.
Refinement of late-Early and Middle Miocene diatom biostratigraphy for the East Coast of the United States
John A. Barron et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Themed Issue: "Results of IODP Exp313: The History and Impact of Sea-level Change Offshore New Jersey." Posted online 14 Aug. 2013, http://dx.doi.org/10.1130/GES00864.1.
Integrated Ocean Drilling Program (IODP) Expedition 313 continuously cored Lower to Middle Miocene sequences at three continental shelf sites off New Jersey, USA. The most seaward of these, Site M29, contains a well-preserved Early and Middle Miocene succession of planktonic diatoms that have been independently correlated with the geomagnetic polarity time scale derived in studies from the equatorial and North Pacific. Shallow water diatoms (species of Delphineis, Rhaphoneis, and Sceptroneis) dominate in onshore sequences in Maryland and Virginia, forming the basis for the East Coast Diatom Zones (ECDZ). Integrated study of both planktonic and shallow water diatoms in Hole M29A as well as in onshore sequences in Maryland (the Baltimore Gas and Electric Company well) and Delaware (the Ocean Drilling Program Bethany Beach corehole) allows the refinement of ECDZ zones into a high-resolution biochronology that can be successfully applied in both onshore and offshore regions of the U.S. East Coast. Strontium isotope stratigraphy supports the diatom biochronology, although for much of the Middle Miocene it suggests ages that are on average 0.4 million years older. The ECDZ zonal definitions are updated to include evolutionary events of Delphineis species, and regional occurrences of important planktonic diatom marker taxa are included. Updated taxonomy, reference to published figures, and photographic images are provided that will aid in the application of this diatom biostratigraphy.
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