||August 26, 2004
GSA Release No. 04-23
September/October Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. – The September-October issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics include Utah’s ancient Lake Bonneville, California’s San Gabriel Basin, and Argentina’s Ischigualasto Provincial Park, home of some of the earliest dinosaur-fossil-bearing rocks from the Late Triassic.
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- Tethyan magnetostratigraphy from Pizzo Mondello (Sicily) and correlation to the Late Triassic Newark astrochronological polarity time scale
Giovanni Muttoni, Department of Earth Sciences, University of Milan, Milan, Italy, et al. Pages 1043-1058.
- Keywords: magnetostratigraphy, biostratigraphy, Late Triassic, Tethys, Sicily, Newark astrochronological polarity time scale.
- The magnetostratigraphy, chemostratigraphy, and biostratigraphy of a 430-m-thick sequence of Upper Triassic marine limestones outcropping at Pizzo Mondello, Sicily, are presented. The Pizzo Mondello section was correlated magnetostratigraphically to the continental Newark astrochronological time scale (APTS) for development of an integrated Late Triassic time scale. The biostratigraphic Carnian-Norian boundary, located at Pizzo Mondello 12.5 m above a positive shift of d13C, corresponds to Newark magnetozone E7 at about 228-227 million years of age, implying a long Norian with a duration of about 20 million years and a Rhaetian of about 6 million years' duration. Based on its good exposure, accessibility, stratigraphic thickness and continuity, and multiple chronostratigraphic correlation possibilities, we propose Pizzo Mondello as the global stratigraphic section and point (GSSP) for the base of the Norian.
- Significance of porosity for stratigraphy and textural composition in subsurface, coarse fluvial deposits; Boise Hydrogeophysical Research Site
Warren Barrash and Edward C. Reboulet, Center for Geophysical Investigation of the Shallow Subsurface, and Department of Geosciences, Boise State University, Boise, Idaho 83725, USA. Pages 1059-1073.
- Keywords: fluvial sediments, grain-size analysis, porosity, statistical analysis, stratigraphy, subsurface geology.
- Identification of facies or sedimentary features from subsurface samples of coarse unconsolidated gravel and sand deposits is difficult because of inherent problems with sample recovery from augering or drilling due to mixing and grinding of grains, but also from coring due to truncation and breakage of large grains. Then, with a given sample, recognition of different sedimentary environments represented by the sediments is difficult because the lateral scale of sedimentary features is considerably greater than that of a core barrel. This paper contributes to the understanding of these deposits in the subsurface by using borehole data (porosity and grain-size distribution [GSD] data) to answer two basic questions: (1) can subsurface coarse river deposits be subdivided into units based on porosity and/or GSD data, and (2) if so, how do porosity and GSDs vary within and between the units? Lacking direct observation of identifying features in core samples (as can occur at an outcrop or quarry), relevant questions are examined with statistical tests. First we examine porosity data, next GSD data, and then porosity and GSD data together. Results taken as a whole are consistent with the recognition of two types of sedimentary units in the 20-m-thick coarse gravel and sand deposits at a research wellfield, the Boise Hydrogeophysical Research Site. Also, the sedimentary interpretation of these two types of units is constrained by their porosity and GSD characteristics.
- Significance of paleosurfaces in interpreting the paleogeographic and stratigraphic evolution of the late Paleozoic Paspébiac graben, a recently identified basin in the southern Gaspé Peninsula of Quebec, Canada
Pierre Jutras, Department of Geology, Saint Mary's University, Halifax, NS, B3H 3C3, Canada, and Gilbert Prichonnet, GEOTERAP, Département des Sciences de la Terre et de l'Atmosphére, Université du Québec à Montréal, Montréal, QC, H3C 3P8, Canada. Pages 1074-1086.
- Keywords: exhumed paleosurfaces, Saint-Jules Formation, upper Paleozoic stratigraphy, structural geomorphology, paleogeographic reconstruction, Paspébiac graben.
- This paper provides evidence for Carboniferous transpressive deformation in a region that was thought to have only experienced mild extension during that time. It casts doubt on the assumed Devonian age of the youngest strike-slip displacements within the rest of the Quebec Appalachians, away from Carboniferous rock exposures.
- Mid-Paleozoic initiation of the northern Cordilleran marginal backarc basin: Geologic, geochemical, and neodymium isotope evidence from the oldest mafic magmatic rocks in the Yukon-Tanana terrane, Finlayson Lake district, southeast Yukon, Canada
Stephen J. Piercey, Mineral Deposit Research Unit, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada, et al. Pages 1087-1106.
- Keywords: backarc basin, Yukon-Tanana terrane, Nd isotopes, Cordilleran tectonics, pericratonic, crustal growth.
- This paper contains a combined field- and laboratory-based study that examines ancient undersea volcanic rocks along the western edge of the Ancient Pacific continental margin of North America 365-360 million years ago. This period of time was a very active time along the entire Ancient Pacific margin of North America from Alaska to the southwest United States with numerous volcanoes, sedimentary basins, and fault systems occurring. This paper illustrates that one landmass, the Yukon-Tanana Terrane, began as a volcanic "arc" chain on the edge of the North American continent. At 360 million years ago, this "arc" fragment separated from the North American continent due to plate tectonic processes to form an intervening ocean, the Slide Mountain Ocean. This tectonic scenario and geometric relationships recorded by the rocks of this study is very similar to the recent (approximately 30 million years to present) plate tectonic evolution of the Japan (e.g., Yukon-Tanana Terrane), Japan Sea (e.g., Slide Mountain Ocean), and Sino-Korean mainland (e.g., North American margin).
- The 87Sr/86Sr ratios of lacustrine carbonates and lake-level history of the Bonneville paleolake system
William S. Hart, The Desert Laboratory and Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, et al. Pages 1107-1119.
- Keywords: Bonneville, Pleistocene, carbonates, strontium, lake.
- In our work we have developed a geochemical method for reconstruction the depth of ancient lake Bonneville (that once covered present day central and western Utah) using the carbonate deposits laid down around the lake's shoreline. We use strontium isotopes in the carbonates to make this lake depth reconstruction.
- Paleo-fluid flow and deformation in the Aztec Sandstone at the Valley of Fire, Nevada — Evidence for the coupling of hydrogeologic, diagenetic, and tectonic processes
Peter Eichhubl, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA, et al. Pages 1120-1136.
- Keywords: fluid flow, sandstone, hematite, diagenesis, deformation, thrust.
- Multicolored sandstone formations characterize the landscape of the United States' southwest. We studied the origin of these colorations at Valley of Fire State Park on the shores of Lake Mead in southern Nevada, interpreting these colorations as a geologic record of groundwater flow across these sandstone formations. The red, orange, yellow, and purple hues are caused by small amounts of iron oxides and hydroxides that precipitated by groundwater as it moved across the porous and permeable sandstone throughout 150-200 million years of its geologic evolution. This geologic evolution, starting with deposition in Jurassic times, included two periods of geologic unrest and mountain building. These periods of geologic unrest were accompanied by the formation of numerous faults and fractures that dissect the sandstone. Adjacent to faults and fractures, the sandstone coloration is often observed to change intensity, hue, or the orientation and thickness of color bands. These local changes of coloration adjacent to faults and fractures can be attributed to disturbances in the groundwater flow due to the higher or lower permeability of faults and fractures relative to the surrounding sandstone. Based on the interaction among faults, fractures, and sandstone coloration, we can distinguish two distinct stages of fluid flow that relate to the two periods of geologic unrest. In both cases, reducing water originating from deeper parts of the sedimentary sequence likely moved upward and bleached the sandstone along its way by dissolving the iron oxides. Mixing with oxidizing groundwater resulted in partial reprecipitation of the iron oxides. The complex patterns of layers and bands of different colors thus provide a geologic record of a complex history of groundwater flow, mixing of waters of different chemical composition, and repeated stages of dissolution and reprecipitation of iron oxides. Exhumation of the sandstone in geologically recent times exposed these alteration patterns at Earth's surface, providing insight for the geologist into groundwater movement in the subsurface and a stunning display of colors to the visitors of Valley of Fire State Park.
- The Yavapai-Mazatzal boundary: A long-lived tectonic element in the lithosphere of southwestern North America
M.B. Magnani, Department of Earth Science, Rice University, Houston, Texas, USA, et al. Pages 1137-1142.
- Keywords: inherited features, low angle boundary, transition zone, deep seismic profile, bivergent orogen.
- The growth of the southwest United States was accomplished by the amalgamation of juvenile lithosphere to the North American continent between 1.8 and 1.6 billion years ago. The accretion established a NE-SW fabric that has had a persistent influence in the tectonic and magmatic evolution of the southern Rocky Mountain lithosphere, according to geologic and geophysical investigations. This paper presents 170 km of new seismic data from north-central New Mexico, crossing the enigmatic Jemez Lineament, a NE-SW alignment of Tertiary volcanic centers. The data show that this magmatic feature coincides with one of the Proterozoic accretionary boundaries between the Yavapai and the Mazatzal provinces. The doubly vergent suture imaged at depth beneath the Jemez Lineament is characterized by low angle (15-20 degree), oppositely dipping features and a crustal scale duplex structure. High amplitude reflections cross cut the accretionary structures and are interpreted as mafic intrusions, likely associated with the magmatic activity along the Jemez Lineament. The authors suggest that the Proterozoic assembly structure acts as a broad crustal and possibly lithospheric zone of weakness that enables hot, young mantle to penetrate the Proterozoic lithosphere, suggesting that the ancient fabric still plays an important role in the tectonic deformation of the southwestern United States.
- Tectonics of the San Gabriel Basin and surroundings, southern California
Robert S. Yeats, Department of Geosciences, Oregon State University, Corvallis, Oregon 97331, USA. Pages 1158-1182.
- Keywords: tectonics, urban geology, earthquake hazards, well data, strike-slip faults, blind reverse faults.
- This paper marks the first detailed discussion of the San Gabriel Valley, a heavily populated suburb of metropolitan Los Angeles, California, using information from oil and gas exploratory wells as well as surface geology. The subsurface contains information about the Canton fault, an ancestor of the San Andreas fault system that ruptured the crust 15-20 million years ago. In 1987-1991, the valley was shaken by five moderate size earthquakes on active faults at the margins of the valley; however, no active faults have been found within the valley itself except for a low-angle fault that may be related to the Puente Hills blind thrust.
- Lithogeochemical and Sm-Nd and U-Pb isotope data from the Silurian-Lower Devonian Arisaig Group clastic rocks, Avalon terrane, Nova Scotia: A record of terrane accretion in the Appalachian-Caledonide orogen
J. Brendan Murphy, Department of Earth Sciences, St. Francis Xavier University, Antigonish, Nova Scotia B2G 2W5, Canada, et al. Pages 1183-1201.
- Keywords: Arisaig Group, LA-ICPMS, detrital zircons, Avalon terrane, accretion, Appalachian orogen.
- The collision between microcontinents and continental margins is a key mechanism of mountain building and continental growth. Such collisions were important in the development of the Appalachian orogen of eastern North America. Determining the timing and geometry of these collisional events is fundamental to the understanding of this orogen. This paper provides geochemical and isotopic data for sedimentary rocks of the Early Silurian-Early Devonian Arisaig Group of mainland Nova Scotia and assesses their provenance and regional tectonic setting. Deposition of the group spans much of the critical time interval during which accretion of several terranes, including Avalonia, occurred along the Laurentia-Baltica margin. The geochemical and isotopic characteristics of its strata clearly record the signal of regional tectonic events. The data suggest that these strata were primarily derived from Baltica but with increasing input from Laurentia by the time of deposition of the upper (early Devonian) strata. We interpret the Arisaig Group to have been deposited adjacent to the trailing edge of the Avalonia microcontinent during collisional events in the Appalachian orogen.
- Early Cretaceous stratigraphy, depositional environments, sandstone provenance, and tectonic setting of central Tibet, western China
Kai-Jun Zhang, Guangzhou Institute of Geochemistry, Laboratory of Marginal Sea Geology, Chinese Academy of Sciences, Guangzhou 510640, China and State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing University, Nanjing 210093, China, et al. Pages 1202-1222.
- Keywords: Tibet, Gangdese magmatic arc, Bangong-Nujiang suture, Lhasa block, Qiangtang block, Early Cretaceous, stratigraphy, paleogeography, sandstone compositions, bimodal volcanism, back-arc rifting.
- Could the collision between the Qiangtang and Lhasa continental blocks in western China have given rise to the elevation of the Tibetan plateau about 100 million years ago? Chinese geologists Kai-Jun Zhang and his colleagues say no. They found that, though coarse continental clastic sedimentation prevailed from 135 to 120 million years B.C., an extensive marine transgression occurred in southern Tibet that led to deposition of thick, marine clastic-carbonate sediments up to 5 km from 120 to 95 million years B.C. A detailed study of Lower Cretaceous strata of the Asa basin (31°00'-32°40', 84°30'-87°05') in central Tibet showed that the sedimentation was evidently controlled by coeval volcanism since 120 million years B.C. The sandstones deposited earlier contain abundant chert grains and some ultrabasic lithic fragments and radiolarian fragments and were obviously related to the orogen created by the Qiangtang-Lhasa blocks. However, the sandstones formed later contain abundant volcanic grains that seem to have a bimodal basaltic and rhyolitic composition. Bimodal basaltic and rhyolitic volcanic rocks and syndepositional normal faulting are found in places within the sedimentary sequences. Both bimodal volcanic rocks and normal faulting are key indicators of a rift; therefore, a significant tectonic transition from earlier contraction to later backarc rifting may have occurred in response to a rollback of the Tethyan oceanic slab. The collision between the Qiangtang and Lhasa blocks could have been completed at the earliest Cretaceous, and during Middle Cretaceous time, the entire southern Eurasian margin could have been characterized by a series of backarc rift basins (including those in Kohistan, Pakistan, and Ladakh, India), much like the present western Pacific margin. Therefore, the collision could not be the mechanism to create the elevation of Tibet.
- Ephemeral stream response to growing folds
Sarah A. Pearce, San Francisco Estuary Institute, 7770 Pardee Lane, 2nd Floor, Oakland, California 94621, USA, et al. Pages 1223-1239.
- Keywords: California, San Bernardino Mountains, ephemeral streams, channel geometry, alluvial fans, active tectonics.
- This paper documents the response of ephemeral alluvial streams to active tectonics. Active deformation of the north flank of the San Bernardino Mountains, southern California, is propagating northward across the piedmont as a series of growing folds. Ephemeral channels that encounter these folds display a distinct range of channel pattern morphologies. The response of ephemeral channels differs from that of perennial channels mainly because of differences in sediment transport processes. These documented channel pattern responses can be used to help identify unrecognized locations of growing folds and to better understand tectonic deformation. This study also allows investigation of transverse drainage development, where both antecedent and consequent channels are observed traversing the folds.
- Entrainment of debris in rock avalanches: An analysis of a long run-out mechanism
Oldrich Hungr, Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada, and S.G. Evans, Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. Pages 1240-1252.
- Keywords: rock avalanche, debris avalanche, dynamic analysis, runout, entrainment, British Columbia.
- Many large rock avalanches entrain large quantities of soil material from the path as they move from the slope. Two recent examples for British Columbia are described. Analysis indicates that this mechanism of entrainment of saturated debris is capable of modifying, often increasing, the mobility of rock avalanches and the distance of their travel.
- Provenance of Neoproterozoic and lower Paleozoic siliciclastic rocks of the central Ross orogen, Antarctica: Detrital record of rift-, passive-, and active-margin sedimentation
John W. Goodge, Department of Geological Sciences, University of Minnesota, Duluth, Minnesota 55812, USA, et al. Pages 1253-1279.
- Keywords: Ross orogen, Antarctica, siliciclastic rocks, U-Pb, detrital zircons.
- Detrital sedimentary rocks containing dateable minerals can be used to constrain the depositional age of the original sediment and its provenance; that is, what materials were eroded from a source terrain and transported to the site of deposition. Detrital mineral geochronology can therefore be used to place constraints on the age of deposition, as well as the characteristics of the source area. Stratigraphic evidence and detrital zircon geochronology show that clastic rocks in the Transantarctic Mountains record the tectonic transformation from a Neoproterozoic rift-margin setting to a passive margin and ultimately to an active early Paleozoic orogenic setting along the paleo-Pacific margin of East Antarctica. New U-Pb detrital zircon ages show that relatively older deposits of a passive margin record distal input from the adjacent cratonic shield, whereas younger deposits signify a fundamental shift to an active, subducting plate boundary in which the major source of sediment is from a nearby magmatic arc terrain. When integrated with stratigraphic relationships, the detrital zircon age patterns can be explained by a tectonic model involving Neoproterozoic rifting and development of a passive-margin platform, followed by rapid transition in the late Early Cambrian (circa 515 million years ago) to an active continental-margin arc and forearc setting. This transition is related to the breakup of the Rodinia supercontinent and the subsequent formation of Gondwanaland. Significant populations (up to 22%) of approximately 1.4 billion year old zircons in these Neoproterozoic and Lower Cambrian sandstones suggest a unique source of Mesoproterozoic igneous material in the East Antarctic craton; comparison with a unique igneous province of this age in North America suggests paleogeographic linkage between East Antarctica and Laurentia prior to circa 1.0 billion years ago, supporting a SWEAT model of Rodinia.
- Mineralogical and geochemical evolution of a basalt-hosted fossil soil (Late Triassic, Ischigualasto Formation, northwest Argentina): Potential for paleoenvironmental reconstruction
Neil J. Tabor, Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275-0395, USA, et al. Pages 1280-1293.
- Keywords: paleosols, clay mineralogy, oxygen and hydrogen isotopes of pedogenic minerals, paleoclimate, mineral weathering.
- Field studies combined with mineralogical and chemical analysis of an ancient soil, or paleosol, were used to better understand the environmental and climatic conditions in the area of Ischigualasto Provincial Park, northwestern Argentina, approximately 228 million years ago, during the Late Triassic time period. The rocks in this area preserve some of the earliest examples of dinosaurs; therefore, understanding the environmental and climatic conditions at this time and place in Earth's history may also help us to better understand the "life and times" of ancient creatures. The mineralogical and chemical analyses of this ancient soil indicate that it formed as a result of weathering of a lava flow that issued from volcanic vents about 228 million years before the present. Initially, weathering of the basalt flow took place in the presence of hot waters, or under hydrothermal conditions, with temperatures ranging from about 100 °C to 50 °C. Subsequent to this pulse of hydrothermal weathering, the lava flow cooled to ambient earth surface conditions and soil formation occurred. Characteristics of this soil indicate that climate was seasonal, with rainfall patterns that may have been similar to monsoons observed today in southeast Asia and India. In addition, the minerals that formed during soil development suggest that mean annual temperatures in this region were about 6 °C during Late Triassic time. These reconstructed Late Triassic conditions are in stark contrast to the modern arid and warm (about 17-20 °C) climate in this part of Argentina.
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