|27 August 2008
GSA Release No. 08-45
Director - GSA Communications & Marketing
September-October GSA Bulletin Media Highlights
This issue examines the diverse characteristics of Earth’s lithosphere, including the western Idaho shear zone, marine to continental transition in the Himalaya, Chicxulub impact deposits in Texas and New Jersey, and hillslope evolution in the Oregon Coast Range. Also covered: catastrophic flooding in Japan five thousand years ago; vast lakes on the ancient Tibetan Plateau; and dating ammonoids, pollen, and spores in Arctic Norway to understand the tempo of geological time on a global scale.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in articles published. Contact Christa Stratton for additional information or other assistance.
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Carbonate accretionary lapilli in distal deposits of the Chicxulub impact event
Thomas E. Yancey and Renald N. Guillemette, Dept. of Geology and Geophysics, Texas A&M University, College Station, Texas 77843-3115, USA. Pages 1105-1118.
The petrography and chemical composition of carbonate accretionary particles of Chicxulub impact origin are described from Cretaceous-Paleogene boundary deposits at Brazos River, Texas, and Bass River borehole, New Jersey. The particles consist of lapilli and lapilli fragments ranging in size from 0.05 to 0.3 cm; they are white, have an accretionary fabric at several scales, and are composed of micrometer-sized microspar of low-Mg calcite with an elevated sulfur content. The internal aggregate microfabric indicates that they formed by accretion of small solid particles, suggesting an origin from carbonate crystals generated within the vapor plume from the Chicxulub impact. Carbonate accretionary lapilli occur with altered glass spherules in Cretaceous-Paleogene boundary deposits at sites in Texas, USA, and northern Mexico and in the spherule layer in New Jersey, USA, indicating that a large amount of particulate carbonate was present within the impact plume.
Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho
Scott Giorgis et al., Dept. of Geological Sciences, State University of New York-Geneseo, 1 College Circle, Geneseo, New York 14454, USA. Pages 1119-1133.
This paper provides information about the timing of activity on an ancient, major fault zone in western Idaho, USA. The western edge of the Idaho batholith contains a plate-scale fault zone known as the western Idaho shear zone. The age data indicate that this shear zone was active in the Late Cretaceous from ca. 105 to 90 Ma. Shortly after movement on the fault zone ceased, upward movement brought the fault zone much closer to the surface between 85 and 70 Ma. The Baja British Columbia hypothesis suggests that pieces of present-day British Columbia may have collided with North America as far south as Baja California, then moved north along strike-slip fault zones. Data presented have two implications with respect to the Baja British Columbia hypothesis: first, the western Idaho shear zone can only be used to transport slices of British Columbia northward between 105 and 90 Ma; second, the timing of deformation on the western Idaho shear zone matches that of the collision recorded on the continental fragments in British Columbia. This suggests that movement on the western Idaho shear zone may record both the collision of fragments of British Columbia with North America and some of their northward transportation.
Constraining the travels of a "suspect" terrane: Paleomagnetism and geobarometry of two Early Cretaceous igneous complexes in the Peninsular Ranges Batholith, California
D.T.A. Symons et al., Dept. of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada. Pages 1159-1170.
The western zone of the Peninsular Ranges Batholith of northern Baja California--the Yuma terrane--has consistently yielded stable remanent magnetization directions that are shallower in inclination and rotated clockwise to the expected North American directions. Thus, excluding the Neogene opening of the Gulf of California, the Yuma terrane is suspected of originating more than 1000 km south of its present location either along the Central American coastline or offshore in the Pacific Ocean. Using aluminum-in-hornblende geobarometric and paleomagnetic results from the Cretaceous Alpine and Ramona complexes and from earlier studies along with other geological and geophysical evidence, Symons et al. propose that the Yuma terrane is composed of a mosaic of fault blocks that have undergone ENE-side-up tilting of about 10 plus-or-minus three degrees. The tilting is in response to shallow subduction of the Farallon plate beneath the North America plate and brings the measured and expected remanence directions into agreement, meaning that the Yuma terrane is not "suspect" but rather that it originated along Mexico's northwestern coastline.
Marine to continental transition in Himalayan foreland
M.K. Bera et al., Dept. of Geology and Geophysics, Indian Institute of Technology, Kharagpur 721302, India. Pages 1214-1232.
Mapping, sedimentological studies, and sequence stratigraphic analysis in parts of the Himalayan peripheral foreland basin of northwest India suggest that deposition of basinal turbidites, derived from both the orogenic and ramp sides, took place due to progressive uplift of the basin margin, causing forced regression. The forced regressive wedge of shoreface white sandstone, thus deposited throughout the foreland basin across the regressive surface of marine erosion, cannot be included in the alluvial sediments of Dagshai Formation. Contrary to earlier inferences, the unconformity between the Subathu and Dagshai Formations is found to occur at the top of the white sandstone marked by caliche development or erosion by Dagshai channel sand interpreted as a Type 1 sequence boundary. The reworked fossils in calciturbidite units suggest that the upper limit of the Subathu Formation must be significantly younger than ca. 44 Ma, and the proposition of a synchronous orogen-scale unconformity of a greater than 10 m.y. duration and early exhumation of Himalayan rocks should be reassessed. The duration of unconformity between Subathu and Dagshai Formations is interpreted to be less than or equal to 3 m.y. The sea-level fall and shoaling of Subathu Sea that was already set in by forced regression received tectonic enhancement only at the beginning of the Dagshai Formation, which resulted in a total turnaround from a marine to a continental alluvial system.
Breakout flood from an ignimbrite-dammed valley after the 5 ka Numazawako eruption,
Kyoko S. Kataoka et al., Research Center for Natural Hazards and Disaster Recovery, Niigata University, Ikarashi 2-cho 8050, Nishi-ku, Niigata 950-2181, Japan. Pages 1233-1247.
This paper centers on the breakout flood event that occurred in the aftermath of the 5 ka Numazawako ignimbrite eruption. A catastrophic release from the ignimbrite-dammed lake (more than 1.6 cubic kilometers of water) resulted in a large-scale flood that traveled 150 km downstream, with a peak discharge of 30,000 to 50,000 cubic meters per second. The flood caused inundation and burial of Neolithic occupation sites even at distal locations from the volcano.
How well can hillslope evolution models "explain" topography? Simulating soil transport and production with high-resolution topographic data
Joshua J. Roering, Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272, USA. Pages 1248-1262. NSF funding received.
Hillslopes contribute sediment to river systems, spawn landslides that endanger human life and affect aquatic habitat, and support grazing and agricultural practices. The form of most hillslopes is convex, such that as one walks down from a hillcrest, slope angles get progressively steeper. This ubiquitous morphologic pattern has inspired simple equations describing soil erosion and thus enabled dozens of landscape simulations to explore how Earth's surface responds to changes in tectonic processes, climate fluctuations, and ecological change. Such studies seldom compare their results to actual landscape features. In this paper, Roering explores how well state-of-the-art hillslope evolution models can "explain" meter-scale morphologic patterns (as documented via high-resolution topography derived from airborne laser altimetry) for a sequence of hillslopes in the Oregon Coast Range. The results indicate that current models can account for a significant fraction of topographic variability at the study site and further suggest that biological processes are the predominant means by which soil is produced and transported across the landscape. As such, long-term ecological changes (via natural or anthropogenic processes) should impart a topographic signature on Earth's surface.
Lack of inhibiting effect of oil emplacement on quartz cementation: Evidence from Cambrian reservoir sandstones, Paleozoic Baltic Basin
Nicolaas Molenaar et al., Institute of Environment and Resources, Technical University of Denmark, Bygningstorvet, Building 115, DK-2800 Kongens Lyngby, Denmark,and Dept. of Geology and Mineralogy, Vilnius University, K.M. Ciurlionio 21/27, LT-03101 Vilnius, Lithuania. Pages 1280-1295.
Molenaar et al. have used Cambrian fine-grained quartzarenites in the Baltic Basin to verify the supposed effects of oil emplacement. Modeling of the stratigraphic and organic maturation data into a burial history in combination with petrographic analyses and the establishment of a paragenetic succession of diagenetic features showed that quartz cementation and oil migration and the filling of the reservoirs must have occurred largely simultaneous. A large database of petrophysical properties of dry structures and oil fields, derived from extensive well logging and core analyses, has been used to compare the porosity distribution. Results indicate that both porosity and permeability decrease with increasing temperature for sandstones from dry structures and oil fields alike. In addition, plotting the porosity versus the distance to the oil-water contact in oil fields shows no particular or consistent trend. From this the authors conclude that oil emplacement has no effect on quartz cementation and thus it does not lead to the preservation of primary porosity. They also see indications that silica supply for quartz cementation is derived from internal sources, independent of large-scale fluid flow. The observed heterogeneity is due to local lithological factors influencing the sourcing and precipitation of quartz cement.
Biomagnetostratigraphy of the Vikinghøgda Formation, Svalbard (Arctic Norway), and the geomagnetic polarity timescale for the Lower Triassic
Mark W. Hounslow et al., Center for Environmental Magnetism and Palaeomagnetism (CEMP), Geography Dept., Lancaster University, Bailrigg, Lancaster LA1 4YB, UK. Pages 1305-1325.
Switches in the polarity of Earth's magnetic field provide one of the key means by which the tempo of geological time can be correlated from place to place, with high resolution. This study on the Svalbard (Arctic Norway) Lower Triassic demonstrates the combination of dating based on ammonoids, pollen, and spores with changes in the magnetic field polarity to better understand how these dating tie points were linked. In combination with data from other locations on the planet, this paper provides a key for calibrating geological events in time on a global scale.
Vast early Miocene lakes of the central Tibetan Plateau
Wu Zhenhan et al., Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China. Pages 1326-1337.
Vast lake complexes are found to have covered the central Tibetan Plateau, as it was rising, for several million years between ca. 23.5 Ma and ca. 13.5 Ma in the early Miocene. The lakes outlined most of the uplands seen today on the plateau and were surrounded by a forest dominated by spruce and fir. The forest grew in a cool climate at a high elevation much different from the treeless, cold, 4500-m-high plateau of today. Prior to the time the lakes formed, the climate was warm and dry and supported a vegetation of mostly broad-leaved trees, especially birch and oak, and herbs, along with some evergreens. The cooling of the local climate at a time of stable, warm global conditions in the early Miocene indicates that uplift of the plateau continued. The lake deposits remain little deformed, except locally, and have only recently begun to be eroded by the headwaters of the Yangtze and other rivers.
Tibetan uplift intensified the 400 k.y. signal in paleoclimate records at 4 Ma
Junsheng Nie et al., Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, Rhode Island 02882, USA; Institute of Tibetan Plateau Research, Chinese Academy of Science, P.O. Box 2871, Beijing 100085, China; and Key Laboratory of Western China’s Environment System, Ministry of Education, Lanzhou University, Lanzhou, Gansu 730000, China. Pages 1338-1344.
Sedimentation rates and grain sizes in a variety of settings, including active and inactive mountain belts, have been reported to increase dramatically during the interval of 4-2 Ma. The global increase in sedimentation rates and grain sizes has been previously explained as the result of increased climate variability, rather than tectonism. If increased climate variability caused the increased erosion rates, what then caused the increased climate variability? In this paper, Nie et al. show that the increased climate variability starting at 4 Ma may have been caused by the Pliocene uplift of the Tibetan Plateau. Therefore, it may be tectonism (i.e., the Tibetan uplift) that ultimately increased global sedimentation rates and sediment grain sizes during the interval of 4-2 Ma.
Review abstracts for these articles at http://www.gsajournals.org/perlserv/?request=index-html&issn=0016-7606.