|2 June 2014
GSA Release No. 14-38
Figure 4. Section of ichthyosaur-bearing sediment unit at Tyndall ice field: http://dx.doi.org/10.1130/B30964.1.
New Ichthyosaur Graveyard Found
Boulder, Colo., USA – In a new study published in the Geological Society of America Bulletin, geoscientists Wolfgang Stinnesbeck of the University of Heidelberg and colleagues document the discovery of forty-six ophthalmosaurid ichthyosaurs (marine reptiles). These specimens were discovered in the vicinity of the Tyndall Glacier in the Torres del Paine National Park of southern Chile. Among them are numerous articulated and virtually complete skeletons of adults, pregnant females, and juveniles.
Preservation is excellent and occasionally includes soft tissue and embryos. The skeletons are associated with ammonites, belemnites, inoceramid bivalves, and fishes as well as numerous plant remains. The enormous concentration of ichthyosaurs is unique for Chile and South America and places the Tyndall locality among the prime fossil Lagerstätten for Early Cretaceous marine reptiles worldwide.
Four different species have been identified. Both concentration and diversity of ichthyosaurs are unique for South America and place the Tyndall locality among the prime fossil Lagerstätten marine reptiles worldwide. The deposit is Early Cretaceous in age (about 146 million years ago) and forms part of a deep water sequence located in the Rocas Verdes Basin, a straight separating Antarctica and South America from Late Jurassic to late Early Cretaceous times.
The Tyndall ichthyosaurs were gregarious and likely hunted in packs in a submarine canyon near the east coast of this sea. Their potential prey, belemnites and small fishes, were abundant due to plankton blooms caused by cold water upwelling. Occasionally, high energy turbiditic mudflows sucked down everything in their reach, including ichthyosaurs. Inside the suspension flows, the air-breathing reptiles lost orientation and finally drowned. They were instantly buried in the abyss at the bottom of the canyon.
A Lower Cretaceous ichthyosaur graveyard in deep marine slope channel deposits at Torres del Paine National Park, Southern Chile
W. Stinnesbeck et al., Institut für Geowissenschaften, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 234-236, 69221 Heidelberg, Germany. Published online 22 May 2014; http://dx.doi.org/10.1130/B30964.1.
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(U-Th)/He geochronology and chemical compositions of diagenetic cement, concretions, and fracture-filling oxide minerals in Mesozoic sandstones of the Colorado Plateau
P.W. Reiners et al., Department of Geosciences, Gould-Simpson Building, 1040 E. 4th Street, University of Arizona, Tucson, Arizona 85721, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/B30983.1.
Water dissolving, and water removing--under the rocks and stones, there is water flowing underground. But is it the same as it ever was? In fact, the geologic record of secondary minerals left behind by ancient fluid flow through bedrock has the potential to record changes through time in the tectonics, landscape, and climate that influence groundwater and hydrocarbon flow. By analyzing the geochronologic ages and compositions of iron and manganese oxide cement, concretions, and fracture-fill in sandstones on the Colorado Plateau, Reiners et al. have shown that some of these diagenetic minerals formed as early as 25 million years ago, but continued to react with later groundwater until much more recently. Many concretions and other secondary oxides record ages suggesting fluid-rock reactions between about 2-3 million years ago, a time of both major climate and geomorphic changes in the region, and some fracture fill material suggests an episode of formation at 300 thousand years ago, possibly associated with a wave of rapid incision. Taken together these results show that diagenetic oxide minerals can record the timing of ancient fluid flow events, and could be used to understand past episodes of groundwater flow on Earth and other planets.
Postglacial denudation of western Tibetan Plateau margin outpaced by long-term exhumation
H. Munack et al., Institute of Earth and Environmental Sciences, Universität Potsdam, 14476 Potsdam, Germany. Published online 22 May 2014; http://dx.doi.org/B30979.1.
The Indus River, one of Asia’s premier rivers, links the western Tibetan Plateau and the Nanga Parbat region of Kashmir. These two areas juxtapose some of the lowest and highest topographic relief and commensurate denudation rates in the high-elevated regions of Himalaya and Tibet, respectively. Yet it remains unclear how rapidly rivers incise into the seemingly long-lived Tibetan Plateau margin by removing material and migrating headwards into the arid plateau interior. We explore the regional denudation pattern across this western margin of the Roof of the World by combining independent radiometric dating, mineralogical analysis of river sands, and computational landscape analysis. Our data reveal a systematic decrease of landscape downwearing towards the Tibetan Plateau. Yet we find that the highest denudation rates occur >100 km below the most prominent knickpoint on the Indus River, where it drops off the Tibetan Plateau. From this observation we infer that any erosion attacking the interior of the plateau has mostly concerned reaches well below the plateau margin. Moreover, reported rock exhumation rates averaging over millions of years consistently exceed our millennial-scale estimates of postglacial denudation rates. To counterbalance this apparent mismatch, denudation rates must have been higher in the Quaternary during glacial-interglacial intervals.
Ontogeny of point bars on a river in a cold semi-arid climate
J.A. Moody and R.H. Meade, U.S. Geological Survey, 3215 Marine St., Suite E-127, Boulder, Colorado 80303, USA and U.S. Geological Survey, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA. Published online 22 May 2014; http://dx.doi.org/10.1130/B30992.1.
Point bars are distinctive features of meandering rivers, appear to be persistent, but are transient in nature. Each has a history that includes birth, evolution, abandonment, and burial. We documented the ontogeny of new point bars that formed in cutoffs during the extreme flood of 1978 on Powder River in southeastern Montana. Ontogeny includes the evolution stage of the point bar but also the formation stage of the point-bar platform--the foundation of the point bar itself. Point-bar platforms were built by a random process of deposition and erosion during multiple floods each year. The number and magnitude of these floods characterizes the hydro-climatic regime. The random process gradually changed the original concave channel shape to a convex shape upon which the point bar could form and evolve. Convex surfaces reduce the likelihood that sediment would be eroded. However, erosion was found to be a significant process in determining the ultimate shape of the point bars. The characteristics of the point bars on Powder River were compared with those described on other rivers with different hydro-climatic regimes. We found that the sediment composition and the form of point-bar features depended on the nature of the hydro-climatic regime for each river.
A universal power-law scaling exponent for fracture apertures in sandstones
J.N. Hooker et al., Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, Oxfordshire OX1 3AN, UK. Published online 22 May 2014; http://dx.doi.org/10.1130/B30945.1.
From the abstract: A high-resolution data set of kinematic aperture (opening displacement) of opening-mode fractures, from large (up to 2 m long) quartz-cemented sandstone samples, shows that microfractures are ubiquitous and that most natural-fracture sets are better fit by power-law size distributions than by exponential, normal, or log-normal distributions. The data set includes 3822 fractures within 68 scanlines from eight formations on three continents. Kinematic apertures were measured along scanlines using scanning electron microscope–based cathodoluminescence (SEM-CL) and, for field data, using a hand lens. Microtextural evidence from SEM-CL shows that power law–distributed fractures typically have crack-seal texture and are composed of opening increments having a narrow (characteristic) aperture size range. In contrast, rare non-power-law–distributed fracture populations lack crack-seal texture. Power-law exponents, as measured in one dimension, have values of -0.8 plus or minus 0.1. Most variation among fracture sets results from power-law coefficients, which constitute a scale-invariant measure of fracture intensity. We show how observed scaling patterns can be used to improve estimations of large-fracture spacing in cases where fracture sampling is limited, as by the width of cores. The low (less than 1) value of the power-law scaling exponent reflects but a gentle increase in fracture frequency with decreasing size, such that microfracture abundances in core are commonly too low for statistically robust sampling. On the other hand, the consistency of the scaling exponent among fracture sets within various tectonic settings is such that the exponent can be assumed, facilitating large-fracture spacing estimations. The assumption of the scaling exponent should be supported by the presence of crack-seal texture within fractures.