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News Release August 24, 2001
GSA Release No. 01-36
Contact: Ann Cairns
Director–Communications and Marketing
acairns@geosociety.org, (303) 357-1056
FOR
IMMEDIATE
RELEASE

September Media Highlights:
The Geological Society of America Bulletin

BOULDER, Colo. - The Geological Society of America has just released the September issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, and this is the first media advisory that provides synopses of the BULLETIN articles. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.

The reptile Macroleter: First vertebrate evidence for correlation of Upper Permian continental strata of North America and Russia
Robert R. Reisz, Department of Zoology, Erindale Campus, University of Toronto, Mississauga, Ontario L5L 1C6, Canada, and Michel Laurin (Institut für Paläontologie, Naturhistorisches Forschungsinstitut, Museum für Naturkunde, Zentralinstitut der Humboldt-Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany. Pages 1229-1233.
We report on the startling discovery that the terrestrial reptile Macroleter, previously known only from the Late Permian (approximately 250 million years ago) of Russia, occurs in the continental deposits of Oklahoma. This discovery indicates for the first time that the sediments exposed in northern Oklahoma (Chickasha Formation) preserve terrestrial vertebrates of Late Permian age, bridging the gap between the richly fossiliferous sediments of North America (Early Permian) and Russia (Late Permian). This discovery also provides clear evidence for terrestrial vertebrate interchange between North America and Russia at this time.

Paleogeographic development of the east Laurentian margin: Constraints from U-Pb dating of detrital zircons in the Newfoundland Appalachians
Peter A. Cawood and Alexander A. Nemchein, Tectonics Special Research Centre, School of Applied Geology, Curtin University, GPO Box U1987, Perth 6845, Western Australia, Australia. Pages 1234-1246.
Sediments which accumulated along the ancient continental margin of eastern North America some 500 million years ago contain detritus that covers some three billion years of Earth history, with the oldest grains up to 3.6 billion years old. The material was derived from ancient mountains in the interior of North America and transported up to 2000 km to its present depositional site.

Compaction-corrected paleomagnetic paleolatitudes for Late Cretaceous rudists along the Cretaceous California margin: Evidence for less than 1500 km of post–Late Cretaceous offset for Baja British Columbia
Kenneth P. Kodama, Department of Earth and Environmental Sciences, Lehigh University, 31 Williams Drive, Bethlehem, Pennsylvania 18015-3188, USA, and Peter D. Ward, Department of Geological Sciences, University of Washington, Seattle, Washington 18195, USA. Pages 1171-1178.
Baja BC is the hypothesis, based on paleomagnetic data, that portions of British Columbia were at the same latitude as Baja California in the Cretaceous. In conflict with this interpretation is that observation that Baja BC rocks in British Columbia do not contain any rudists, yet coastal California rocks that were at the same paleomagnetic paleolatitudes as Baja BC do contain rudists (ancient bivalve Mollusks indicative of tropical marine water). The paleomagnetic inclination of rocks is used to calculate the rocks' paleolatitude. If burial compaction has caused the inclination to rotate closer to the horizontal, an inaccurate record of paleolatitude results. Paleomagnetic inclinations corrected for burial compaction were used to determine the paleolatitudinal extent of rudists. Rudists lived no further north than latitude 40° during the Cretaceous. This constrains Baja BC to be no further south than 40°N, and reduces the amount of offset for Baja BC from 3000 km to less than 1500 km, in agreement with most geologic observations.

Glacial incursion on a Neoproterozoic carbonate platform in the Kimberley region, Australia
Maree Corkeron et al. Tectonics Special Research Centre, Department of Geology and Geophysics, University of Western Australia, Crawley, Western Australia 6009, Australia. Pages 1121-1132.
In the Kimberley region of northwestern Australia, an 80-m-thick package of sedimentary rocks, the Egan Formation, records an unusual sequence of climatic events from the terminal Precambrian (about 550 million years ago). The change from carbonate rocks deposited in warm waters, to glacially derived sediments, followed by a return to carbonates typical of warm water environments such as patch reefs (stromatolitic), shoals, and lagoons indicates severe oscillations in climatic conditions. While the paradoxical carbonate/glacial rock association is commonly observed in the terminal Precambrian record worldwide, rarely are the depositional environments of the carbonate deposits so clearly preserved. Additionally, reassessment of the Egan Formation's correlation with other late Neoproterozoic glacial deposits suggests a third glaciation in the Australian, and possibly global rock record, younger than the widespread and widely recognised Marinoan glacial event.

Uranium isotopic evidence for groundwater chemical evolution and flow patterns in the eastern Snake River Plain aquifer, Idaho
Robert C. Roback et al. Environmental Division, M.S. J514, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. Pages 1133-1141.
This paper describes the use of 234U/238U isotope ratios to examine groundwater chemical evolution and aquifer flow patterns in the Snake River Plain aquifer, near the Idaho National Engineering and Environmental Laboratory, a major Department of Energy facility in Idaho. The paper shows that groundwater from different geographic regions has distinct uranium isotopic ratios. These distinct ratios provide a unique signature to track the movement of these waters and chemical changes that these waters experience along their path. The authors show that flow in the aquifer is quite heterogeneous, with areas of faster flow and parts of the aquifer that are apparently more stagnant. They also point out that flow directions may differ considerably from those expected based on widely spaced physical measurements of the water table. The paper should help workers better understand flow in the Snake River Plain aquifer as well as in fractured-rock aquifers worldwide.

West African proximity of the Avalon terrane in the latest Precambrian
Allen K. McNamara, et al. Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA. Pages 1161-1170.
Considerable debate surrounds the Earth's late Precambrian (about 550 million years ago) paleogeography, specifically whether the position of the Avalon terrane was adjacent to Amazonia (Avalon and Amazonia were ancient plates). New results from Upper Precambrian rocks in the Avalon terrane challenge this position. We performed a paleomagnetic study on rocks collected in Newfoundland, and results indicate that the Avalon terrane was located at relatively shallow latitudes in the latest Precambrian. Within the constraints of available plate tectonic reconstructions, our results imply that the position of Avalon was more likely located off the northern margin of West Africa rather than a position adjacent to Amazonia. This also casts doubt on tools such as zircon ages as a paleogeographic indicator.

Construction of a pluton: Evidence from an exposed cross section of the Searchlight pluton, Eldorado Mountains, Nevada
Carolyn A. Bachl et al. Department of Geology, Vanderbilt University, Nashville, Tennessee 37235, USA. Pages 1213-1228.
Miocene (15 million years ago) faulting south of Las Vegas, Nevada, exposed domino-like tilted blocks of crust that initially extended downward to depths of as much as 15 km. This faulting was a result of rapid rifting, which was accompanied by voluminous volcanism. The Searchlight pluton represents a top-to-bottom cross section of a 10-km-thick, solidified magma chamber that has been revealed by this crustal tilting. This unusual exposure permits evaluation of the solidification history of the magmas and the processes that affected them. The chamber, which was situated at depths of 3 to 13 km, was filled mostly by granitic magma. Basaltic magma also injected and interacted locally with the dominant granitic melt. The record of pluton (or magma chamber) history primarily reflects final solidification, which proceeded both by downward growth of a network of crystals that grew in response to heat loss from the roof and by accumulation of a thick pile of crystals that settled to the floor of the chamber.

To view abstracts for the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, go to www.gsajournals.org. To obtain a complimentary copy of any BULLETIN article, contact Ann Cairns.

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