|24 August 2011
GSA Release No.11-53
Director - GSA Communications & Marketing
GSA BULLETIN Highlights: New research posted 19 August 2011
Boulder, CO, USA - Research topics in the latest GSA BULLETIN posting include: the study of paleo-hurricane records from South Carolina marshlands; geochronology of the Chinle Formation that provides new insights into early dinosaur evolution; water tracks in Antarctica; analysis of magma ascent in large-scale volcanic systems; deep-seated, non-eruption produced volcano collapse and the role of weak bedrock foundation; the transportation of suevite; and a refined timeline of deposition to the famous fossil-rich deposits in the Cibao basin.
Keywords: Folly Island, South Carolina, paleotempestology, Chinle Formation, Colorado Plateau, Petrified Forest National Park, Ischigualasto Formation, Argentina, McMurdo Dry Valleys, Antarctica, water tracks, sheet swarms, South Victoria Land, Pouakai Volcano, Ferrar Large Igneous Province, Maitahi Formation, Cibao Valley, Cibao fossils, Dominican Republic, Ries Crater, Germany
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Spatio-lateral continuity of hurricane deposits in back-barrier marshes
Scott P. Hippensteel, Dept. of Geography and Earth Sciences, University of North Carolina, 9201 University City Blvd., Charlotte, North Carolina 28223, USA; doi: 10.1130/B30261.1.
Growing populations and recent hurricane activity along the Atlantic and Gulf coastlines have made more accurate and extensive records of paleostorm frequency and hurricane return periods imperative. Proxy records collected from marshes offer the potential to extend this record by several thousand years, thus providing improved statistical constraints for hurricane prediction. Scott P. Hippensteel of the University of North Carolina documents how storm records recovered from the marshes behind Folly Island, South Carolina, USA, are misleading with respect to hurricane frequency. Different storm records were recovered from cores taken only meters apart and different paleotempestology proxies (sediment and foraminifers) provided different storm records from the same cores. Overall, the spatio-lateral continuity of the deposits based on sedimentary correlation was poor and biostratigraphic correlation was nearly impossible. This indicates that only the largest, most robust, sand-rich storm layers survive in the Folly Island marsh strata and these layers were probably buried quickly and escaped extensive bioturbation.
High-precision U-Pb zircon geochronology of the Late Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA): Temporal constraints on the early evolution of dinosaurs
Jahandar Ramezani et al., Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA; doi: 10.1130/B30433.1.
The Upper Triassic Chinle Formation of the Colorado Plateau in southwestern United States preserves an important record of vertebrate evolution and environmental change, but its geologic interpretation is hampered by problematic regional correlations and a lack of reliable age data. Sedimentary rocks of volcanic origin were collected from the Chinle Formation exposed in the Petrified Forest National Park, Arizona, USA, within the context of 31 detailed measured stratigraphic sections, and were analyzed by the U-Pb ID-TIMS method. High-precision age results from nine tuffaceous beds bracket nearly 300 meters of strata that were deposited between 225 million years ago and 208 million years ago (or younger), spanning approximately the entire Norian Stage of the Late Triassic. The new geochronology of the Chinle Formation provides new insights into its intermittent depositional history and underscores the potential pitfalls of correlation of fluvial rock units over long distances. This new data presented by J. Ramezani of MIT and colleagues permit a significant time overlap between the lower Chinle sequence and the Ischigualasto Formation of northwestern Argentina, from which some of the earliest dinosaurs and their near relatives have been discovered. Near contemporaneity and faunal similarities of the trans-American deposits imply that early dinosaur evolution occurred rapidly across the Americas.
Water tracks and permafrost in Taylor Valley, Antarctica: Extensive and shallow groundwater connectivity in a cold desert ecosystem
Joseph S. Levy et al., Dept. of Geology, Portland State University, Portland, Oregon 97210, USA; doi: 10.1130/B30436.1.
Water tracks are a new class of Antarctic landform that pipe shallow groundwater through permafrost, routing moisture across an otherwise hyperarid and barren polar desert. Because water tracks do not flow overland like streams, water tracks constitute a cryptic or "shadow" hydrological system in the McMurdo Dry Valleys of Antarctica. Water tracks in Antarctica represent a new addition to the Antarctic hydrological paradigm, adding a groundwater component to a hydrological system that was previously thought to be dominated by overland flow through seasonal streams. Joseph S. Levy of Portland State University and colleagues note that the only evidence of water-track presence is linear zones of enhanced soil moisture (visible as dark streaks at the permafrost surface -- similar to slope streaks on Mars) and intense soil geochemical signatures. Like water tracks in the Arctic, water tracks in Antarctica are major determiners of where soil moisture, salts, and nutrients end up on permafrost hillslopes. Water in the water tracks acts as a thermal buffer, dramatically altering the seasonal freezing and thawing of the permafrost. Because water tracks in Antarctica drain from the high mountain peaks down to closed basin lakes on the valley floors, water tracks provide hydrological and ecological connectivity in the cold, polar desert.
Interconnected sills and inclined sheet intrusions control shallow magma transport in the Ferrar large igneous province, Antarctica
James D. Muirhead et al., School of Environment, University of Auckland, Private Bag 92019, Auckland, New Zealand; doi: 10.1130/B30455.1.
Immediately prior to the breakup of Gondwana, approximately 183 million years ago, 2.5 x 105 km cubed of magma intruded along the margin of the east Antarctic craton. Remnants of this voluminous magmatic activity are spectacularly exposed as intrusive sheets and sills of the Ferrar Large Igneous Province, Antarctica. James D. Muirhead of the University of Auckland and colleagues examine sheet swarms in South Victoria Land in order to address two fundamental questions: (1) what mechanism controls magma ascent in large-scale volcanic systems, and (2) what was the nature of the magmatic-tectonic regime prevailing in Antarctica and parts of Gondwana during the Jurassic? The predominance of regionally extensive sills throughout the Ferrar province demonstrates that magma travelled laterally by intruding bedding planes and uplifting overlying strata. Buckling of strata at the edges of these sills resulted in the formation of shallowly to moderately dipping inclined sheet intrusions, which acted as sub-vertical magma pathways. The final expression of the Ferrar plumbing system is of interconnected sills and inclined sheets, which fed explosive volcanism in various parts of South Victoria Land and, by inference, flood-basalt lavas. Muirhead and colleagues conclude that the predominance of these sill-fed inclined sheets, rather than steeply dipping dikes, indicates that the upper 4 km of the crust were not affected by a rift system during the Jurassic.
Subedifice collapse of an andesitic stratovolcano: The Maitahi Formation, Taranaki Peninsula, New Zealand
David R. Gaylord et al., School of Earth and Environmental Sciences, Washington State University, Webster Physical Sciences Building, Pullman, Washington 99164-2812, USA; doi: 10.1130/B30141.1.
David R. Gaylord of Washington State University and colleagues reconstruct the depositional history of the Maitahi Formation, an approximately quarter-million-year old debris-avalanche deposit that resulted from the collapse of Pouakai Volcano, an extinct stratovolcano located on the Taranaki peninsula of New Zealand. Blanketing an area greater than 350 km squared and totaling greater than 7.5 square kilometers in volume, the Maitahi debris avalanche traveled over 20 km from its source at speeds that likely exceeded 100 km per hour. The Maitahi debris avalanche deposited essentially intact, meters- to tens-of-meters-- wide pieces (megaclasts) of the original volcano as well as megaclasts derived from relatively weak mudstone-rich bedrock strata underlying the volcano. The incorporation of abundant bedrock megaclasts in the Maitahi Formation implies a deep-seated collapse origin that distinguishes it from all other Taranaki peninsula volcanic debris-avalanche deposits. Most if not all of the more than a dozen other Taranaki debris avalanches were generated by relatively shallow collapses of the still-active Egmont Volcano. Deep-seated, non-eruption produced, and probably earthquake-triggered volcano collapses, such as the one that produced the Maitahi Formation, speak to the role that weak bedrock foundations can play in promoting such cataclysmic events in similar, and often inhabited, geologic settings worldwide.
Lateral transport of suevite inferred from 3-D shape-fabric analysis: Evidence from the Ries impact crater, Germany
Cornelia Meyer et al., Museum of Natural History Berlin, Leibniz Institute at the Humboldt University Berlin, Invalidenstrasse 43, 10115 Berlin, Germany; doi: 10.1130/B30393.1.
Today, most impact scientists believe that suevite was formed in a raining-down process out of an ejecta cloud which is formed during a meteorite impact. Such clouds are imagined to be similar to a mushroom cloud after an atomic blast. Some scientists suggest that the suevite was transported horizontally. In order to differentiate between these two ideas, Cornelia Meyer of the Museum of Natural History in Berlin and colleagues analyzed suevite samples from the Ries crater, Germany, using 3-D measurements. Their observations indicate that it is more likely that the suevite material was transported by a lateral flow-like process known for pyroclastic flows from volcanic eruptions.
Late Neogene chronology and sequence stratigraphy of mixed carbonate-siliciclastic deposits of the Cibao Basin, Dominican Republic
Donald F. McNeill et al., Division of Marine Geology and Geophysics, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33149, USA; doi: 10.1130/B30391.1.
This study by Donald F. McNeill of the University of Miami and colleagues of the rocks in the northern valley of the Dominican Republic follows in a long line of research that has examined the evolutionary history of marine life ranging from about 6 million years ago to 3 million years ago. The Cibao Valley is a classic fossil locality because the shallow marine species living during that time are beautifully preserved, encased in beds of mud that have sealed them from major alteration. Prior to being uplifted and exposed, the Cibao Valley was at one time an east-west seaway rich in molluscs, coral reefs, and many other forms of marine life. Paleontologists have been coming to this valley for more than 100 years searching for fossils and examining them through time. The first scientific studies occurred in the 1850s with the study of fossil marine molluscs. Carlotta Maury, one of the few female paleontologists of the early 20th century, carried out the first comprehensive study of fossil molluscs and other invertebrates. This current study by McNeill and colleagues provides a refined timeline of deposition to the famous fossil-rich deposits in the Cibao basin. This refined age dating and better understanding of the conditions under which the fossils were deposited provides paleontologists with a better history of when species originated and when they became extinct. Geologists can now compare the differences between today's marine species and those of the past to better evaluate the stability of coral reef ecosystems, and the nature of originations and extinctions since the time of the Cibao fossils.
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