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Jan/Feb GSA Bulletin media highlights

Geological Society of America

Boulder, Colo. -- Beginning in January 2004, the GEOLOGICAL SOCIETY OF AMERICA BULLETIN will be published 6 times per year. The January/February issue includes a number of potentially newsworthy items. Of particular interest is discovery of a large impact structure in western Wisconsin.

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Analysis of modern and Pleistocene hydrologic exchange between Saginaw Bay (Lake Huron) and the Saginaw Lowlands area
John R. Hoaglund III, The Pennsylvania State University, Earth and Mineral Sciences Environment Institute, 2217 Earth-Engineering Sciences, University Park, Pennsylvania 16802-6813, USA; et al. Pages 3-15.
Keywords: Saginaw Bay, Port Huron, glacier, Michigan Basin, chloride, groundwater.

A model simulating present ground-water flow conditions between the Michigan Basin and Saginaw Bay shows that although there is a strong potential for flow from the basin to the bay, groundwater discharge through the Saginaw Bay floor is small and depends upon the structural extent of fine grained rocks of the Michigan Formation. Another model of the Port Huron glacial ice and subglacial meltwater recharge shows that during the Pleistocene, groundwater flow was reversed from the ice sheet to proglacial Lake Saginaw. The models constrain the origin of saline, isotopically light groundwater, and porewater in the Saginaw Bay and Lowlands areas. The area of reversed groundwater flow is largely commensurate with the distribution of the isotopically light groundwater. Mixing scenarios, constrained by chloride and δ18O measurements, demonstrate that mixing between a brine and subglacial meltwater, followed by the mixing of the resultant saline water with modern freshwater, could have produced the saline, isotopically light groundwater and porewater chemistry currently observed in the Saginaw Lowlands Area. Porewaters of Saginaw Bay could have resulted from the mixing of freshwater with either the resultant saline water or directly with brine, and thus whether or not the samples reflect mixing with subglacial meltwater is unclear. The introduction of subglacial meltwater to the Saginaw Bay floor may have been retarded by the Michigan Formation. The combination of models and mixing scenarios indicates that bedrock structure is a major influence on both the present and Pleistocene flow system.

The geology of Damavand volcano, Alborz Mountains, northern Iran, p. 16-29
Jon Davidson, Department of Earth and Space Sciences, University of California, Los Angeles, California 90095-1567, USA, and Department of Earth Sciences, University of Durham, Durham, DH1 3LE, UK; et al. Pages 16-29.
Keywords: Alborz Mountains, Damavand volcano, argon-argon, geochronology, (U-Th)/He, volcanic centers, volcaniclastic rocks.

Damavand Volcano, at 5,670 m above sea level, is the highest peak in the Middle East. This study presents the first compilation of age determinations and modern geochemical data for this little-known volcano. We provide a general overview of the geological setting and volcanic history of the volcano, along with a summary of the geochemistry and mineralogy of the volcanic rocks. Volcanic activity began nearly 2 million years ago with the development of a precursor volcano, now largely eroded. The current cone has been active for about the past 500,000 years; the most recent eruption occurred within the past 10,000 years. Many of the volcanic deposits are rapidly eroded and transported away from the high slopes of the volcano. Some of this material is deposited in the adjacent river valleys where it provides a partial record of the volcano's activity, although most travels northward to the Caspian Sea. The compositions of the erupted material are quite homogeneous and consistent with melting of locally upwelling mantle material, perhaps in response to tectonic activity in the Alborz Mountains.

Glacial stratigraphy and paleomagnetism of late Cenozoic deposits of the north-central United States, p. 30-41
M. Roy, Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA; et al. Pages 30-41.
Keywords: glacial stratigraphy, paleomagnetism, Laurentide ice sheet, glaciation, midcontinent.

The glacial stratigraphy of the north-central United States has the particularity of recording the onset of late Cenozoic glaciations in the Northern Hemisphere, and understanding the chronological setting of the midcontinent sedimentary sequences is thus important for research in Quaternary geology and paleoclimatology. This paper provides new data on the composition of glacial deposits within the context of an improved chronology based on paleomagnetic measurements of glacial and nonglacial deposits and the presence of three volcanic ashes dating back about two million years. This study identified periods of normal and reversed magnetic polarity in the sedimentary sequences, and within this context, the documented till petrographic and mineralogic changes suggest an increase with time in the surface area of unweathered crystalline bedrock.

Evolution of a strike-slip fault network, Valley of Fire, southern Nevada
Eric A. Flodin and Atilla Aydin, Department of Geological and Environmental Sciences, Room 118, Building 320, Stanford University, Stanford, California 94305-2115, USA. Pages 42-59.
Keywords: strike-slip faults, joints, rotation, brittle deformation, Basin and Range, Aztec Sandstone.

In the Valley of Fire State Park of southern Nevada, the Jurassic Aztec sandstone is deformed by two predominately strike-slip fault sets with opposite slip sense. At a regional scale, most of the right-lateral faults terminate against the larger offset left-lateral faults and are found localized between en echelon and parallel left-lateral faults and at the ends of the larger left-lateral faults. At a local scale, right- and left-lateral faults of smaller size show mutually abutting relationships. Furthermore, splay fractures related to fault slip are observed sharing the same orientation and abutting relationships as members of the left- and right-lateral fault sets. A conceptual model for the evolution of the strike-slip fault network in the Valley of Fire is proposed whereby the fault network forms via progressive splay fracturing and their subsequent shearing.

Detrital-zircon fission-track ages for the "Hoh Formation" Implications for late Cenozoic evolution of the Cascadia subduction wedge
Richard J. Stewart, Department of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA; and Mark T. Brandon, Department of Geology and Geophysics, Kline Geology Laboratory, Yale University, P.O. Box 208109, New Haven, Connecticut 06520-8109, USA. Pages 60-75.
Keywords: Cascadia subduction wedge, Olympic Mountains, Coastal OSC, Hoh Formation, zircon fission-track dating.

Rocks of the "Hoh Formation", exposed along the Pacific Coast of the Olympic Peninsula, Washington, were accreted to the North American Plate about 16 to 24 million years ago. By analyzing the record of spontaneous decay of the 238U isotope in natural crystals of zircon, the authors demonstrate that sandstones in the "Hoh Formation" were derived from contemporaneous sources that include volcanoes in the adjacent Cascade Range. This sediment was deposited on submarine fans or in slope basins adjacent to the Cascadia subduction zone. Comparison of the decay record of 238U with independent age estimates from fossils indicates these sediments were added to North America by the process of accretion as soon as they were deposited. Following accretion, these rocks moved eastward about 140 kilometers from their original position adjacent to an oceanic trench and are now exposed both along the coast and in the high peaks of the interior of the Olympic Mountains.

Complex patterns of fluid flow during wollastonite formation in calcareous sandstones at Laurel Mountain, Mount Morrison Pendant, California
Jade Star Lackey and John W. Valley, Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 76-93.
Keywords: contact metamorphism, wollastonite, fluid flow, Sierra Nevada, stable isotopes, oxygen isotopes, carbon isotopes.

Understanding the patterns of fluid flow away from crystallizing magma chambers provides insight into heat and material transport processes in Earth's crust. This study describes the hydrothermal interaction of the Laurel Mountain pluton with its surrounding metamorphic wallrocks 89 million years ago in the Eastern Sierra Nevada, California. Our findings show that upon leaving the crystallizing granodiorite, igneous fluids caused metamorphism and extensive wollastonite (CaSiO3) mineralization in surrounding sandstone wallrocks. This mineralization is accompanied by prominent bleaching to white, evident in Sevahah Cliff above Convict Lake. The abundance and mapped distribution of wollastonite in the sandstones indicates that the magmatic fluids were water-rich and flowed in a dominantly vertical orientation, which is confirmed by carbon isotope ratios measured on calcite. Oxygen isotope analyses of co-existing wollastonite and quartz indicate isotopic disequilibrium between these two minerals in many rocks and emphasize that there was variable isotopic exchange between the magmatic fluids and the wallrocks they infiltrated. The production and escape of CO2 by metamorphic reactions such as this is an important natural source of greenhouse gases.

Holocene landslides and a 3500-year record of Pacific Northwest earthquakes from sediments in Lake Washington
Robert E. Karlin, Department of Geological Sciences, University of Nevada, Reno, Nevada 89557, USA; et al. Pages 94-108.
Keywords: paleoseismology, seismotectonics, Holocene, landslide, Seattle, Washington, Lake Washington.

Lake Washington, just east of Seattle, contains numerous landslides and a 3500-year sediment record of turbidites (seismites), probably associated with strong ground motion due to large earthquakes in the Pacific Northwest. Seismic reflection profiling, sidescan swath imagery, and sediment coring were used to define deformation of Holocene lake sediments and the distribution, geometry, age, and causes of observed submarine landslides. Numerous large block slides, sediment slumps, and debris flows occur throughout the lake, obscuring the subsurface structure of Seattle fault strands under the lake. Buried landslides suggest that submarine slope failures and mass wasting occurred several times in the Holocene. Most bays along the lake margin are the headwalls of large submarine slides. Submerged forests show evidence of deep-seated block failures that have exposed glacial sediments and Tertiary rocks. Sediments in 36 gravity cores from Lake Washington preserve a record of episodic turbidite deposition. Characteristic patterns in magnetic susceptibility profiles can be traced throughout the lake to identify turbidite source areas. The areal extent and magnetic signatures of the sediments suggest multiple sources for a given event, which is consistent with triggering of local landslides due to large (mb>7) earthquakes on the Seattle fault and/or large to great (mb>8) temblors occurring elsewhere in Cascadia. Radiocarbon dating and correlation of the down-core magnetic profiles indicate that strong ground shaking may have occurred up to seven times in the last 3500 years. The Lake Washington sediment record is consistent with the timing of other paleo-earthquake proxies in the Pacific Northwest.

Anatomy and evolution of a pull-apart basin, Stellarton, Nova Scotia
John W.F. Waldron, Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada. Pages 109-127.
Keywords: strike-slip faults, pull-apart basins, Canadian Appalachians, Maritimes, coal.

Pull-apart basins are rapidly subsiding small areas of Earth's crust that are located on major fault lines, such as the San Andreas, where two plates are moving horizontally past each other. A pull-apart basin forms where a small irregularity exists in the fault that causes a gap to open up as the plates move. Fault-bounded blocks typically subside into such basins, and thick piles of sediment accumulate in the subsiding area.

During Pennsylvanian time, about 310 million years ago, a major plate-bounding fault, similar to the modern San Andreas, ran east-west through what is now Nova Scotia in Atlantic Canada. A thick succession of sand, mud, and peat, now sandstone, shale, and coal, accumulated in lakes and deltas in a pull-apart basin, the Stellarton basin. Extensive coal mining and exploration in the last two centuries have provided detailed information on the three-dimensional form of these sediments below the surface. By examining the thickness of sediment accumulated in different parts of the basin, it is possible to determine that different parts of the basin subsided at different rates. In addition, soon after the sediments were deposited, they were affected by faults and folds generated by shearing motion along the bounding faults. This distorted the coal seams and other sedimentary layers. The geometry of these distorted shapes can be reconstructed from archived plans of the underground coal mines that formerly operated in the basin. In this way, the Stellarton basin provides information necessary for the detailed understanding of the formation and deformation of pull-apart basins.

Fault and fault-rock characteristics associated with Cenozoic extension and core-complex evolution in the Catalina-Rincon region, southeastern Arizona
George H. Davis, et al., Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 128-141.
Keywords: core complex, crustal extension, detachment fault, fault rock, normal fault.

Stretching of the continental crust in the desert southwest over the past 25 million years has involved successive phases of deformation marked by different styles of extensional faulting. Close attention to the progressive evolution of superimposed faults and related structures preserved in the rock record reveals the separate effects of different phases of extension over time. Analysis shows that the classic metamorphic core complex exposed in the Catalina and Rincon Mountains near Tucson, where originally midcrustal rocks were translated toward the surface along a gently inclined mid-Tertiary detachment fault, has assumed its modern configuration only during the past few million years as a result of subsequent steeper normal faulting and associated flexural deformation. The influence of post-detachment events on the present morphology of the core complex was not fully appreciated before the current study was undertaken.

U-Pb geochronology and geochemistry of the McCoy Mountains Formation, southeastern California: A Cretaceous retroarc foreland basin
Andrew P. Barth, Department of Geology, Indiana University-Purdue University, Indianapolis, Indiana 46202, USA; et al. Pages 142-153.
Keywords: California, stratigraphy, tectonics, geochronology, geochemistry.

The McCoy and Palen Mountains of southeastern California consist of sedimentary rocks (known as the McCoy Mountains Formation) whose ages are not well known. Knowing the ages of these rocks is important for improving our understanding of the environmental evolution of this region, as well as little-known older faults in this area. Results of this study indicate that the sedimentary rocks are about 116 to 84 million years old. However, an unexpected result of our study shows that most granitic rocks in this region are actually younger than the McCoy Mountains Formation. This result suggests that the source of the sand and gravel that comprise most of the McCoy Mountains Formation must still be sought in ancient mountains, now mostly worn away, that once existed north and west of this region. Understanding how these ancient mountains were formed and subsequently eroded away will ultimately provide a much better picture of the environmental evolution of this part of California.

An orphaned basement block: The Arequipa-Antofalla Basement of the central Andean margin of South America
Staci L. Loewy, et al., Department of Geological Sciences, The University of Texas at Austin, Austin, Texas 78712, USA. Pages 171-187.
Keywords: U/Pb, whole-rock lead, Sm/Nd, Arequipa-Antofalla Basement, Amazonia, isotopes, absolute age.

The Arequipa-Antofalla Basement (AAB) comprises a series of exposures of ca. 1-2 billion year old rocks within the young central (younger than 200 million year old) Andes Mountains of South America. These ancient rocks of the AAB provide a key to interpreting the pre-Andean history of South America. New U/Pb geochronology and whole-rock Pb and Nd isotope geochemistry from the AAB refine the tectonic and geologic history. Based on this refined characterization of the AAB and comparison with that of adjacent South American rocks, we conclude: (1) the isolated exposures of the AAB are related and comprise a single basement block, (2) the AAB was not derived from South America, but from another continent, and 3) the AAB became part of South America ca. 1.0 billion year ago when it collided with the pre-Andean margin.

Structure and evolution of the central Gulf of Mexico continental margin and coastal plain, southeast United States
D.L. Harry and J. Londono, Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, Alabama 35487-0338, USA. Pages 188-199.
Keywords: Gulf of Mexico, North America, rifting, Ouachita orogeny, gravity profiles.

Analyses of variations in Earth's gravity field, combined with seismic data and rocks recovered in petroleum wells, reveal new details of the complicated geologic history buried beneath the central Gulf of Mexico coastal plain and continental shelf. The data reveal an ancient continental margin that formed more than 500 million years ago. This buried continental margin is very different from the modern Gulf of Mexico margin, which formed during breakup of the Pangea supercontinent as Earth's tectonic plates moved apart. Instead, the ancient margin bears many similarities to the northwest Australian continental margin, which formed as tectonic plates moved laterally past one another. The Ouachita mountain belt formed in this region roughly 300 million years ago, when an Island arc collided with North America during the Late Paleozoic Era. The culminating geologic event in this region was the breakup of the Pangean supercontinent during the the Triassic Period, roughly 225 million years ago, which led to formation of the modern Gulf of Mexico. Remarkably, the new analysis shows that the ancient continental margin and Ouachita mountain systems are preserved in a nearly pristine state beneath the modern Gulf of Mexico coastal plain in Mississippi. As such, they provide a remarkable picture of the geologic history of the region and offer new opportunities to understand the processes of continental breakup, the formation of mountain belts, and the formation of coal, natural gas, and oil deposits in the region. The remarkably good preservation of the ancient tectonic systems provides an excellent opportunity for geologists to examine fundamental issues concerning the breakup of continents, formation of mountain belts, and development of energy resources in sedimentary basins that impact studies in similar environments across the globe.

The Rock Elm meteorite impact structure, Wisconsin: Geology and shock-metamorphic effects in quartz, p. 200-218
Bevan M. French, Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560, USA; et al. Pages 200-218.
Keywords: Rock Elm (Wisconsin), meteorite impact craters, shock metamorphism, quartz deformation, quartz cleavage, planar deformation features (PDFs).

Unique microscopic deformation features in tiny mineral grains have led three scientists to the discovery of a large and ancient meteorite impact structure in western Wisconsin. As reported in the Bulletin of the Geological Society of America, the recognition of the Rock Elm, Wisconsin, structure by Dr. Bevan M. French of the Smithsonian Institution in Washington, D.C., Prof. William S. Cordua of the University of Wisconsin at River Falls, and Dr. Jeff B. Plescia of the U.S. Geological Survey at Flagstaff, Arizona, was based on the discovery of microscopic fractures and other deformation features in grains of quartz in rocks from the center of the structure, features identical to those observed in many established impact craters. Their geological studies show that the Rock Elm structure, 6.5 kilometers (4 miles) in diameter, was formed by the impact of a comet or asteroid about 200 meters across, traveling at possibly 30 km/sec (67,500 mph). The impact event released more than 1000 megatons of explosive energy, instantly lifted the central part of the structure more than 500 meters (1650 feet), and sent intense shock waves through the target rocks, crushing and deforming their minerals. The structure, with an age estimate of 400-450 million years, is now so deeply eroded that the scientists were fortunate to find the shock-deformed rocks still preserved. The Rock Elm structure is too small to have caused a major extinction of life as did the large dinosaur-killing impact event 65 million years ago, but the local geological effects could have been important, and the structure provides an example of new deformation features in quartz that might be used to identify other impact structures in the future.

Evolution of marine terraces and sea level in the last interglacial, Cave Hill, Barbados
R.C. Speed (deceased) and H. Cheng, Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55812, USA. Pages 219-232.
Keywords: marine terraces, last interglacial, sea level history, reefal facies, past highstand levels.

Examination in Barbados of an uplifted, coral-bearing marine terrace of last interglacial age (about 120,000 years old) gives clues to the timing and processes of sea level rise and fall during episodes of warmth in Earth's major climatic cycles. Sea level behavior in last interglacial is perhaps the best predictor for that of the current interglacial as affected by natural cyclic effects. We found that sea level rose from before 136,000 to 128,000 years ago at as much as 3.5 m/1000 years to reach a nearly constant high stand for about 7000 to 9000 years. Sea level then fell slightly, a few meters, until 116,000 years ago. It then fell precipitously more than 37 m by 115,000 years ago. Given that the present-day interglacial sea level has been constant for 6000 years, it will remain as now for another 1000 to 3000 years before falling as a result of cooling toward the next ice age.

The 26 May 1982 breakout flows derived from failure of a volcanic dam at El Chichón, Chiapas, Mexico
J.L. Macías, Instituto de Geofísica, Universidad Nacional Autónoma de México, Coyoacán 04510, México, D.F., México; et al. Pages 233-246.
Keywords: volcanic dam, breakout, debris flow, Chichón, Chiapas, Mexico.

As a consequence of the cataclysmic eruptions of March-April 1982, the terrain surrounding El Chichón volcano, Mexico, was devastated, and local rivers and streams became dammed by volcanic material. This situation caused the ponding of incoming water from the rivers and rainfall and the formation of a hot lake. The dam of volcanic material failed May 26, forming a hot flood that traveled about 35 km downstream, killing one person and scalded three others.


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