Boulder, Colo. - The April issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of newsworthy items. 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.
Geochemical investigation of a Neoproterozoic glacial unit: The Mineral Fork Formation in the Wasatch Range, Utah.
Grant M. Young, Department of Earth Sciences, University of Western Ontario, London, Ontario N6A 5B7, Canada. Pages 387-399.
The Earth has undergone considerable climatic changes long before the arrival of humans. One of the greatest of these changes was the period of extreme cold between about 750 million years ago and the beginning of the Cambrian Period at about 540 million years ago. This frigid time is evidenced by glacial deposits from all of the continents. A small part of this evidence is preserved in the rocks that make up part of the Wasatch Range just southeast of Salt Lake City in Utah. In this study, the chemical composition of these glacial deposits is documented and compared with those from a much more ancient glacial epoch that occurred about 2300 million years ago. The most significant difference is that the deposits of the younger glacial episode contain evidence that they incorporated much more sedimentary materials than the older ones, indicating that recycling of surficial materials was much more common in the younger deposits.
Dolomitization-induced aquifer heterogeneity: Evidence from the upper Floridan aquifer, southwest Florida.
Robert G. Maliva et al. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK. Pages 419-427.
A multidisciplinary study of a well field completed in the upper Floridan aquifer in Collier Country, southwest Florida, revealed that variation in performance among wells is related to the replacement of the native limestone by dolomite. Some dolomite-replaced intervals have very high hydraulic conductivities (water moves through the rock very well), which causes them to be preferred fluid-flow paths in the aquifer. The three-dimensional geometry of the dolomite bodies with high hydraulic conductivities is indeterminate. The high degree of heterogeneity and unpredictability that may occur in carbonate aquifers as a result of dolomitization and other diagenetic processes may greatly impact projects requiring the recovery of specific volumes of water, such as pump-and-treat remediation system and aquifer storage-and-recovery systems.
North American margin origin of Quesnel terrane strata in the southern Canadian Cordillera: Inferences from geochemical and Nd [neodymiam] isotopic characteristics of Triassic metasedimentary rocks.
Jennifer L.E. Unterschutz et al. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. Pages 462-475.
The geological history of ~220 million-year-old (Triassic) rocks that presently make up part of southeastern British Columbia (B.C.), Canada, is poorly understood. These rocks are remnants of an ancient volcanic arc that was active off the coast of North America during the Triassic. The location of where the volcanic arc originally formed is hotly debated. Some researchers propose that the arc developed more than 2000 km south of its present position and moved north with the passage of time. In contrast, other workers suggest the arc developed just offshore of its present location along the North American margin. To address uncertainty in the origin of this volcanic arc, this study examines the geochemical and radioisotope compositions of metamorphosed sedimentary rocks deposited east of, and associated with, these arc rocks. The compositions of these metamorphosed sedimentary rocks suggest derivation of sediment from two different nearby sources, a volcanic arc, and continental crust that is North American in origin. The new data, when combined with existing geologic evidence, indicates that this Triassic volcanic arc developed on, or near, the edge of the North American margin in southeastern B.C., and not 2000 km south.
Reactivation of prethrusting, synconvergence normal faults as ramps within the Ordovician Champlain-Taconic thrust system.
Nicholas W. Hayman, Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA, and W.S.F. Kidd, Department of Geological Sciences, State University of New York, Albany, New York 12222, USA. Pages 476-489.
This paper documents a significant example of a generally unrecognized style of faulting and basin formation in convergent margin thrust systems and, more locally, explains otherwise puzzling geologic relationships in the Champlain thrust system that remain following the results of previous studies. This article is based on and presents new bedrock geologic maps and corresponding cross sections of an area in the Champlain Valley of Vermont and adjacent New York Hudson River valley. The older rocks in the region were deposited on the passive margin of proto-North America (Laurentia) and were subsequently buried by deeper water sediments during convergence between Laurentia and a developing subduction zone in the Ordovician (circa 450 million years ago). As the crust descended into this subduction zone it flexed, creating an extending forebulge with resultant normal faults. With continuing subduction and convergence, thrust faults propagated into the passive margin sequence and transported the rocks about 100 kilometers from their original position. During this transport, the individual thrust faults utilized the earlier normal faults, which localized ramps that now offset the thrust faults laterally. The normal faults also permit explanation of otherwise puzzling large vertical offsets, in the “unexpected” sense of downwards in the propagation direction, of the basal Champlain thrust detachment fault.
Geochemical indicators of separate sources for eolian sands in the eastern Mojave Desert, California, and western Arizona.
James R. Zimbelman et al. Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560-0315, USA. Pages 490-496.
Stabilized transverse sand dunes cover much of the Cactus and La Posa Plains in western Arizona, near the town of Parker. The source of the sand in these dunes was not well delineated, and it had been hypothesized that they were possibly related to extensive sand deposits in the eastern Mojave Desert of California, on the western side of the nearby Colorado River. Laboratory analysis of the chemical make-up of sands from western Arizona, southeastern California, and along the Colorado River have revealed a very close association between the western Arizona and Colorado River sands, which are in marked contrast to the sands from southeastern California. The chemistry results are consistent with mineralogical studies that suggest the California sands are relatively immature, with a large component of the easily weathered mineral feldspar, while the Arizona and Colorado River sands are relatively mature, with a preponderance of quartz grains. Field relationships and the chemical data indicate that the sand in the Mojave Desert of southeastern California is derived mostly from mountains in the vicinity of the sand deposits, whereas the Colorado River and western Arizona sands have been transported significant distances from their potential sources.
To view abstracts for the GSA BULLETIN, go to http://www.gsajournals.org. To obtain a complimentary copy of any GSA BULLETIN article, contact Ann Cairns at acairns@geosociety.org. The Geological Society of America: http://www.geosociety.org