December media highlights - GSA BULLETIN

December 10, 2001

Boulder, Colo. - The December issue of the GSA BULLETIN includes research on a range of newsworthy topics including a probable volcano eruption in Peru, estuaries being key to helping prevent erosion, and plate movement in central California.

Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the GSA BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.

Chemical sedimentology and paleoenvironmental history of Lake Olduvai, a Pliocene lake in northern Tanzania. R.L. Hay, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, and T.K. Kyser, Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6, Canada. Pages 1505 - 1521.

The Olduvai basin of northern Tanzania was inhabited by at least two types of hominids in Pliocene Pleistocene time, and their remains and associated archaeological materials have been found in sedimentary deposits bordering a former saline lake that is here termed Lake Olduvai. Lacustrine deposits of Lake Olduvai provide a sedimentary record of about 200,000 years, from ~1.9-1.7 million years ago.

The main goal of this study is the paleoenvironmental history of the lake as determined from evidence of water depth, areal extent, and water chemistry. Lacustrine minerals indicate that the lake fluctuated widely in salinity and was most saline late in its history. Evidence suggests an overall trend toward a hotter and drier climate that was punctuated by wetter- and drier-than-average episodes that may have lasted as much as 1,000 years or more.

Net ebb sediment transport in a rock-bound, mesotidal estuary during spring-freshet conditions: Kennebec River estuary, Maine. Michael S. Fenster, et al. Environmental Studies Program, Randolph-Macon College, Ashland, Virginia 23005, USA. Pages 1522-1531.

A recent study by the Heinz Center for Science, Economics and the Environment concluded that erosion will claim one out of four houses along U.S. beaches, at a cost to coastal property owners of $530 million per year during the next 60 years.

An understanding of the causes and mitigation of beach erosion is facilitated by studies dealing with sand supply and sand transport. Current research and models suggest that estuaries act as sediment "sinks" by temporarily or permanently storing sand and removing it from the coastal sediment budget.

This research documents a mechanism by which estuaries serve as net sand sources for beaches in high-latitude locations such as Maine. Three boat teams concurrently collected current velocity, water salinity, and water temperature data the 25 km longitudinal axis of the Kennebec River estuary for 13 hours. The data were collected during a time of high tidal energy and high river discharge (a spring snowmelt flood) to test the hypothesis that, unlike lower-latitude Coastal Plain estuaries, spring floods supplant the tidal energy and deliver sediment to the coast.

The hydrographic data were compared to maps of the estuary bottom produced by side-scan sonar and data from other seasons to reveal that spring floods and seaward-directed tidal flow did move sediment to beaches adjacent to the mouth of the estuary during spring months.

Geochronology of late Pleistocene to Holocene speleothems from central Texas: Implications for regional paleoclimate. MaryLynn Musgrove, et a. Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA. Pages 1532-1543.

This article presents a detailed chronology of speleothems (secondary mineral deposits formed in a cave) that were created during the last 71,000 years in three central Texas caves. The agreement of uranium-thorium and uranium-protactinium ages strongly suggests that the ages are accurate and represent some of the first uranium-protactinium ages determined on these types of deposits.

Variations in the growth rates of the speleothems are indicative of changes in effective moisture supplying the growth of the speleothems and reflect the response of the hydrologic system to regional and global climate variability. Consistent shifts in speleothem growth rates correspond in part with global glacial-interglacial climatic shifts.

Sedimentary record and tectonic implications of Mesozoic rifting in southeast Mongolia. S.A. Graham et al. School of Earth Sciences, Stanford University, Stanford, California 94305, USA. Pages 1560-1579.

The remote Gobi Desert region of southeastern Mongolia contains a geologic record of widespread structural extension during late Mesozoic time, as well as a significant portion of Mongolia's petroleum reserves. Understanding this record is important for reconstructing the tectonic evolution of eastern Asia and for predicting undiscovered petroleum resources.

Extensional faulting and volcanism markedly affected southeastern Mongolia for at least 35 and perhaps as much as 60 million years during the Late Jurassic and Early Cretaceous periods, creating a landscape of closed basins dominated by lakes and fringing deltas, rivers, and alluvial fans. The extensional deformation likely reflects the resolution of stresses transmitted to the interior of the Asian continent from tectonic activity along continental margins to the north, east, and south.

Present tectonic motion across the Coast Ranges and San Andreas fault system in central California. Donald F. Argus. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA, and Richard G. Gordon, Department of Earth Science, Rice University, Houston, Texas 77005, USA. Pages 1580- 1592. Very accurate space-based surveying techniques, including the Global Positioning System (GPS), are used to determine the motion across the San Andreas fault system.

To obtain accurate estimates of this motion we do not use the motion of the Pacific plate relative to the North American plate. Instead, we use motion relative to the Sierran microplate, which itself moves relative to the North American plate at about 12 mm/yr. The motion between the Pacific plate and the Sierran plate is about 39 mm/yr. Slip along the San Andreas fault and related faults takes up much of this motion either steadily by creep or dramatically during earthquakes. The motion is nearly, but nowhere exactly, parallel to the San Andreas fault and related faults. In most places there is a small and variable component of convergence (at rates up to 3 mm/yr) where the Sierran and Pacific plates are in collision. This results in crustal thickening manifested as the coastal ranges, including the Diablo Range.

The amount of convergence required to build the mountains indicates that the present episode of plate motion has persisted for about 10 million years. Along a segment just north of San Francisco, however, the Pacific and Sierran plates are slowly pulling apart, opening a hole manifested as a topographic low. It is at this low that rivers originating in the Sierra Nevada mountains drain through the coastal mountains on their way to passage under the Golden Gate Bridge and into the Pacific Ocean.

Geology of El Misti volcano near the city of Arequipa, Peru. Jean-Claude Thouret et al,,CNRS Laboratoire Magmas et Volcans, and Institut de Recherche pour le Développement, Université Blaise-Pascal, 5 rue Kessler, 63038 Clermont-Ferrand cedex, France. Pages 1593-1610.

Approximately 750,000 people live at risk in the city of Arequipa whoe center lies 17 km from the summit (5820 m asl.) of the active El Misti volcano. The composite edifice comprises a stratovolcano termed Misti 1 (about 833,000 to 112,000 years old) and three stratocones, termed Misti 2 to 4 (younger than 112,000 years). Seven eruptive periods have successively built up Misti 2 to 4. Repeated episodes of growth and destruction of domes have triggered avalanches and pyroclastic flows.

They alternated with explosive events, which emplaced ignimbrites, probably related to the formation of two calderas, and pyroclastic flows, surges, and tephra falls. On average, pumice falls occurred every 2000 to 4000 years, and ashfall occurred every 500 to 1500 years. The extent and volume of the small event during the 1400's and of the eruption ca. 2050 yr ago indicate that future Misti's eruptions, even moderate in magnitude, will entail considerable hazards to the densely populated area of Arequipa.
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To view abstracts for the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, go to http://www.gsajournals.org To obtain a complimentary copy of any BULLETIN article, contact Ann Cairns at acairns@geosociety.org. The Geological Society of America: http://www.geosociety.org

Geological Society of America

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