All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
919.
Physical rock properties of the Eyreville core, Chesapeake Bay impact structure
Special Paper of the Geological Society of America,
458137-163
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Boreholes; Core drilling; Granite; Infill drilling; Lithology; Porosity; Structural geology; Thermodynamic properties, Chesapeake bay impact structures; Geophysical models; Good correlations; Impact structures; Laboratory measurements; Petrophysical interpretation; Small variations; Structural arrangement, Structural properties, breccia; bulk density; impact structure; physical property; porosity; rock property; sedimentary structure, Chesapeake Bay; United States
Abstract: ▾ The physical properties of rocks in drill core from impact structures can be used to distinguish individual nonimpact and impact-generated lithologies, and to investigate the effect of the impact process on the target rocks. Here, we present the results of laboratory measurements of porosity, density, velocity, and thermal properties on the densely sampled cores from the Eyreville borehole in the Chesapeake Bay impact structure, USA. With increasing depth, the lithologies encountered (and porosities) are: postimpact sediments (40%-60%), Exmore breccia and sedimentary blocks (27%-44%), a large megablock of granitoids (<1%), suevite and polymict lithic impact breccia (1%-25%), and schist, granite, and pegmatite of the basementderived section (1%-13%). The low bulk densities and thermal properties of the postimpact sediments show a good correlation with the high porosity values. The physical properties within the Exmore bed sequence overall display relatively small variation but are heterogeneous on the core sample scale. Physical properties along the impact-breccia sequence are highly variable on all scales, and they are interpreted to be controlled by the structural arrangement of particles as well as by the highly variable mineral and clast compositions of the samples. The physical properties of the rocks of the lowermost basement-derived section are also heterogeneous and are interpreted as having been influenced by both lithology and overprinting as a result of the impact process. These results are important for further lithological and petrophysical interpretation and for calibrating future geophysical models of the Chesapeake Bay impact structure. © 2009 The Geological Society of America.
918.
Physical rock properties of the Eyreville core, Chesapeake Bay impact structure
Special Paper of the Geological Society of America,
458137-163
2009
Abstract: ▾ The physical properties of rocks in drill core from impact structures can be used to distinguish individual nonimpact and impact-generated lithologies, and to investigate the effect of the impact process on the target rocks. Here, we present the results of laboratory measurements of porosity, density, velocity, and thermal properties on the densely sampled cores from the Eyreville borehole in the Chesapeake Bay impact structure, USA. With increasing depth, the lithologies encountered (and porosities) are: postimpact sediments (40%-60%), Exmore breccia and sedimentary blocks (27%-44%), a large megablock of granitoids (<1%), suevite and polymict lithic impact breccia (1%-25%), and schist, granite, and pegmatite of the basementderived section (1%-13%). The low bulk densities and thermal properties of the postimpact sediments show a good correlation with the high porosity values. The physical properties within the Exmore bed sequence overall display relatively small variation but are heterogeneous on the core sample scale. Physical properties along the impact-breccia sequence are highly variable on all scales, and they are interpreted to be controlled by the structural arrangement of particles as well as by the highly variable mineral and clast compositions of the samples. The physical properties of the rocks of the lowermost basement-derived section are also heterogeneous and are interpreted as having been influenced by both lithology and overprinting as a result of the impact process. These results are important for further lithological and petrophysical interpretation and for calibrating future geophysical models of the Chesapeake Bay impact structure. © 2009 The Geological Society of America.
917.
Pore-water chemistry from the ICDP-USGS core hole in the Chesapeake Bay impact structure-Implications for paleohydrology, microbial habitat, and water resources
Special Paper of the Geological Society of America,
458867-890
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Chemical analysis; Chlorine compounds; Dissolution; Groundwater; Groundwater flow; Hydrochemistry; Infill drilling; Organic carbon; Rock drills; Salt water intrusion; Sulfur compounds, Chesapeake bay impact structures; Dissolved organic carbon; Electron acceptor; Groundwater system; Microbial activities; Molecular diffusion; Pore-water chemistry; Stable isotopes of water, Saline water, cation; dissolved organic carbon; geological survey; groundwater flow; impact structure; microbial activity; microbial community; paleohydrology; porewater; saline intrusion; sediment core; stable isotope; sulfate; transport process; water chemistry; water resource, Chesapeake Bay; United States; Virginia
Abstract: ▾ We investigated the groundwater system of the Chesapeake Bay impact structure by analyzing the pore-water chemistry in cores taken from a 1766-m-deep drill hole 10 km north of Cape Charles, Virginia. Pore water was extracted using high-speed centrifuges from over 100 cores sampled from a 1300 m section of the drill hole. The pore-water samples were analyzed for major cations and anions, stable isotopes of water and sulfate, dissolved and total carbon, and bioavailable iron. The results reveal a broad transition between freshwater and saline water from 100 to 500 m depth in the postimpact sediment section, and an underlying synimpact section that is almost entirely filled with brine. The presence of brine in the lowermost postimpact section and the trend in dissolved chloride with depth suggest a transport process dominated by molecular diffusion and slow, compaction-driven, upward flow. Major ion results indicate residual effects of diagenesis from heating, and a pre-impact origin for the brine. High levels of dissolved organic carbon (6-95 mg/L) and the distribution of electron acceptors indicate an environment that may be favorable for microbial activity throughout the drilled section. The concentration and extent of the brine is much greater than had previously been observed, suggesting that its occurrence may be common in the inner crater. However, groundwater-flow conditions in the structure may reduce the saltwater-intrusion hazard associated with the brine. © 2009 The Geological Society of America.
916.
Postimpact alteration of sedimentary breccias in the ICDP-USGS Eyreville A and B cores with comparison to the Cape Charles core, Chesapeake Bay impact structure, Virginia, USA
Special Paper of the Geological Society of America,
458699-721
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Boreholes; Calcite; Dissolution; Feldspar; Flow of fluids; Kaolinite; Mica; Pyrites; Quartz; Rocks; Scanning electron microscopy; Sedimentology; Silicate minerals; Temperature; X ray powder diffraction; Zeolites, Chesapeake bay impact structures; Feldspar dissolution; Hydrothermal circulation; Hydrothermal mineral assemblages; Low temperature alteration; Low temperatures; Maximum temperature; Sedimentary substrates, Clay alteration, albitization; breccia; dissolution; feldspar; hydrothermal activity; hydrothermal alteration; hydrothermal system; impact structure; kaolinite; mineralogy; muscovite; petrography; quartz; recrystallization; research program; sedimentary petrology; smectite; vermiculite, Chesapeake Bay; United States, Calluna vulgaris; Micas
Abstract: ▾ In this study, we extend the knowledge of postimpact alteration processes through an investigation of mineralogy and petrology of 24 samples from the Exmore Formation and sedimentary megablock intervals in the Eyreville borehole within the Chesapeake Bay impact structure and comparisons to similar studies of cored intervals of the Cape Charles borehole. The bulk mineralogical studies reveal quartz, feldspars (microcline and albite), muscovite, smectite-vermiculite clays, and kaolinite with variable quantities of pyrite, zeolites, calcite, and chlorite. X-ray diffraction analysis of the clay (<2 μm) fraction of samples indicates that the clays are dominated by expandable clays with lesser quantities of illite, kaolinite, glauconite, and mixed- layered clays. The expandable clays include smectite, vermiculite, and smectite-vermiculite intergrade varieties; illite interlayering is minimal (generally, <10% illite layers). Thin section and scanning electron microscope petrography in the Exmore breccia show evidence for extensive authigenic expandable clay in the matrix and dispersed pyrite lepispheres and fine calcite rhombs. Grain alteration includes feldspar dissolution and albitization, glauconite recrystallization, and dissolution and expandable-clay replacement of micas. Taken together, the results indicate that low-temperature alteration (maximum temperatures 60-80 °C) is prevalent in the sedimentary clast-rich intervals in the Eyreville cores, and the maximum effects are observed between 600 and 970 m depth. In comparison, the Exmore Formation from the Cape Charles borehole, 8 km to the southwest and overlying the central peak of the inner crater, shows more advanced authigenesis with Fe-rich chlorite, common quartz overgrowths, and mixed-layered illite-smectite clay with as much as 20% interlayered illite. A low-temperature hydrothermal mineral assemblage is documented in suevite and crystalline-clast breccia at depths of 725-820 m in the Cape Charles borehole. The fine-grained clastic target material and contained seawater are argued to have limited initial target melting and initial crater-floor temperatures in the Chesapeake Bay impact structure to an even greater degree than that of other marine craters targeted in consolidated sedimentary substrates. Subsequent hydrothermal circulation was confined to the central uplift and neighboring fractured zones, whereas alteration in the overlying sedimentary breccias involved conductive heat flow, reaction with hypersaline pore fluids, and minor fluid flow into more porous, permeable sedimentary blocks adjacent to the central uplift. © 2009 The Geological Society of America.
915.
Palaeoproterozoic petrified oil field (Shunga Event)
Paleontological Journal,
43
(8)
972--979
2009
914.
Postimpact alteration of sedimentary breccias in the ICDP-USGS Eyreville A and B cores with comparison to the Cape Charles core, Chesapeake Bay impact structure, Virginia, USA
Special Paper of the Geological Society of America,
458699-721
2009
Abstract: ▾ In this study, we extend the knowledge of postimpact alteration processes through an investigation of mineralogy and petrology of 24 samples from the Exmore Formation and sedimentary megablock intervals in the Eyreville borehole within the Chesapeake Bay impact structure and comparisons to similar studies of cored intervals of the Cape Charles borehole. The bulk mineralogical studies reveal quartz, feldspars (microcline and albite), muscovite, smectite-vermiculite clays, and kaolinite with variable quantities of pyrite, zeolites, calcite, and chlorite. X-ray diffraction analysis of the clay (<2 μm) fraction of samples indicates that the clays are dominated by expandable clays with lesser quantities of illite, kaolinite, glauconite, and mixed- layered clays. The expandable clays include smectite, vermiculite, and smectite-vermiculite intergrade varieties; illite interlayering is minimal (generally, <10% illite layers). Thin section and scanning electron microscope petrography in the Exmore breccia show evidence for extensive authigenic expandable clay in the matrix and dispersed pyrite lepispheres and fine calcite rhombs. Grain alteration includes feldspar dissolution and albitization, glauconite recrystallization, and dissolution and expandable-clay replacement of micas. Taken together, the results indicate that low-temperature alteration (maximum temperatures 60-80 °C) is prevalent in the sedimentary clast-rich intervals in the Eyreville cores, and the maximum effects are observed between 600 and 970 m depth. In comparison, the Exmore Formation from the Cape Charles borehole, 8 km to the southwest and overlying the central peak of the inner crater, shows more advanced authigenesis with Fe-rich chlorite, common quartz overgrowths, and mixed-layered illite-smectite clay with as much as 20% interlayered illite. A low-temperature hydrothermal mineral assemblage is documented in suevite and crystalline-clast breccia at depths of 725-820 m in the Cape Charles borehole. The fine-grained clastic target material and contained seawater are argued to have limited initial target melting and initial crater-floor temperatures in the Chesapeake Bay impact structure to an even greater degree than that of other marine craters targeted in consolidated sedimentary substrates. Subsequent hydrothermal circulation was confined to the central uplift and neighboring fractured zones, whereas alteration in the overlying sedimentary breccias involved conductive heat flow, reaction with hypersaline pore fluids, and minor fluid flow into more porous, permeable sedimentary blocks adjacent to the central uplift. © 2009 The Geological Society of America.
913.
Paleoenvironmental recovery from the Chesapeake Bay bolide impact: The benthic foraminiferal record
Special Paper of the Geological Society of America,
458747-773
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
C (programming language); Deposition; Infill drilling; Offshore oil well production; Silt, Agglutinated foraminifera; Benthic foraminifera; Foraminiferal assemblages; Impact craters; Microfossil assemblages; Offshore targets; Paleoenvironments; U.s. geological surveys, Boreholes, accumulation; benthic foraminifera; bolide; crater; deep drilling; deep-sea sediment; deposition; depositional environment; Eocene; fossil assemblage; geological survey; impact structure; paleoenvironment; turbidite, Chesapeake Bay; United States; Virginia, Foraminifera
Abstract: ▾ The late Eocene Chesapeake Bay bolide impact transformed its offshore target site from an outer neritic, midshelf seafl oor into a bathyal crater basin. To obtain a depositional record from one of the deepest parts of this basin, the U.S. Geological Survey (USGS) and the International Continental Scientifi c Drilling Program (ICDP) drilled a 1.76-km-deep core hole near Eyreville, Virginia. The Eyreville core and eight previously cored boreholes contain a rarely obtainable record of marine deposition and microfossil assemblages that characterize the transition from synimpact to postimpact paleoenvironments inside and near a submarine impact crater. I used depositional style and benthic foraminiferal assemblages to recognize a four-step transitional succession, with emphasis on the Eyreville core. Step 1 is represented by small-scale, silt-rich turbidites, devoid of indigenous microfossils, which lie directly above the crater-fi lling Exmore breccia. Step 2 is represented by very thin, parallel, silt and clay laminae, which accumulated on a relatively tranquil and stagnant seafl oor. This stagnation created a dead zone, which excluded seafl oor biota, and it lasted ~3-5 ka. Step 3 is an interval of marine clay deposition, accompanied by a burst of microfaunal activity, as a species-rich pioneer community of benthic foraminifera repopulated the impact site. The presence of a diagnostic suite of agglutinated foraminifera during step 3 indicates that paleoenvironmental stress related to the impact lasted from ~9 ka to 400 ka at different locations inside the crater. During step 4, the agglutinated assemblage disappeared, and an equilibrium foraminiferal community developed that contained nearly 100% calcareous species. In contrast to intracrater localities, core sites outside and near the crater rim show neither evidence of the agglutinated assemblage, nor other indications of long-term biotic disruption from the bolide impact. © 2009 The Geological Society of America.
912.
Late quaternary mass movement events in lake el'gygytgyn, north-eastern Siberia
Sedimentology,
56
(7)
2155-2174
2009
ISSN: 00370746Keywords:▾
Basal erosion; Debris flows; Depositional models; Eastern Siberia; Flow path; Impact craters; Lake areas; Late Quaternary; Mass movement; Pelagic sediments; Pliocene; Quaternary climate; Radiocarbon dates; Sediment core; Sedimentary records; Siberian Arctic; Stokes' law; Temporal distribution; Turbidite; Unconsolidated sediment, Anoxic sediments; Climate change; Debris; Deposits; Erosion; Petroleum geology; Sediment transport; Sedimentation; Sedimentology, Lakes, climate variation; debris flow; deposition velocity; echo sounding; erosion rate; formation mechanism; lacustrine deposit; mass movement; paleoclimate; Pliocene-Pleistocene boundary; Quaternary; sediment core; sediment transport; Stokes formula; turbidite; unconsolidated medium, Chukchi; Elgygytgyn Lake; Russian Federation
Abstract: ▾ Lake El'gygytgyn is situated in a 3·6 Myr old impact crater in North-eastern Siberia. Its sedimentary record probably represents the most complete archive of Pliocene and Quaternary climate change in the terrestrial Arctic. In order to investigate the influence of gravitational sediment transport on the pelagic sediment record in the lake centre, two sediment cores were recovered from the lower western lake slope. The cores penetrate a sub-recent mass movement deposit that was identified by 3·5 kHz echo sounding. In the proximal part of this deposit, deformed sediments reflect an initial debris flow characterized by limited sediment mixture. Above and in front of the debrite, a wide massive densite indicates a second stage with a liquefied dense flow. The mass movement event led to basal erosion of ca 1 m thick unconsolidated sediments along parts of its flow path. The event produced a suspension cloud, whose deposition led to the formation of a turbidite. The occurrence of the turbidite throughout the lake and the limited erosion at its base mainly suggest deposition by 'pelagic rain' following Stokes' Law. Very similar radiocarbon dates obtained in the sediments directly beneath and above the turbidite in the central lake confirm this interpretation. When applying the depositional model for the Late Quaternary sediment record of Lake El'gygytgyn, the recovered turbidites allow reconstruction of the frequency and temporal distribution of large mass movement events at the lake slopes. In total, 28 turbidites and related deposits were identified in two, 12·9 and 16·6 m long, sediment cores from the central lake area covering approximately 300 kyr. © 2009 The Authors. Journal compilation © 2009 International Association of Sedimentologists.
911.
Palaeolimnological evidence for environmental change over the past millennium from Lake Qinghai sediments: A review and future research prospective
Quaternary International,
194
(1-2)
134 – 147
2009
ISSN: 10406182Keywords:▾
Asia; China; Eurasia; Far East; Qinghai; Qinghai Lake; climate change; Holocene; lacustrine deposit; paleoenvironment; paleolimnology; proxy climate record
Abstract: ▾ Lake Qinghai, on the NE Tibetan Plateau, is China's largest natural lake that lies at a triple junction of major climatic influences, making it sensitive to global climate change. As such, the sediments of Lake Qinghai have been the focus of numerous palaeoenvironmental studies spanning a range of timescales. However, as a result of uncertain age controls, uncertainties over interpretation of the proxies, the relative dearth of proxy calibration and lack of understanding of the modern lake system a coherent picture of climate over the NE Tibetan Plateau has yet to emerge from Lake Qinghai's sediment record. We review the state of knowledge for this important site, focusing on the last millennium. A comparison of the major proxy records show significant variability with a general pattern of change over the last 1000 years, notably those linked to the onset of the Little Ice Age, but due to poor chronological constraints a detailed picture of climate change cannot be established. Further, some of the proxy records produced from Lake Qinghai's sediments are open to alternative explanations. This compounds the sediment record as a palaeoenvironmental archive. To fully realise the potential of Lake Qinghai, future research must concentrate on defining a reliable old carbon effect for the lake, calibrating proxy records with climatic processes and understanding spatial variability of proxy records within this large lake. © 2008 Elsevier Ltd and INQUA.
910.
Megablocks and melt pockets in the Chesapeake Bay impact structure constrained by magnetic field measurements and properties of the Eyreville and Cape Charles cores
Special Paper of the Geological Society of America,
458195-208
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Buildings; Electromagnetic field effects; Granite; Magnetic materials; Magnetic susceptibility; Magnetization; Refraction; Seismology, Chesapeake bay impact structures; Magnetic anomalies; Magnetic field data; Remanent magnetization; Seismic reflections; Seismic refraction data; Short wavelengths; Structural feature, Structural properties, crater; gravity anomaly; magnetic anomaly; magnetic field; magnetic property; magnetic susceptibility; remanent magnetization; seismic reflection; seismic refraction, Chesapeake Bay; United States
Abstract: ▾ We use magnetic susceptibility and remanent magnetization measurements of the Eyreville and Cape Charles cores in combination with new and previously collected magnetic field data in order to constrain structural features within the inner basin of the Chesapeake Bay impact structure. The Eyreville core shows the first evidence of several-hundred-meter-thick basement-derived megablocks that have been transported possibly kilometers from their pre-impact location. The magnetic anomaly map of the structure exhibits numerous short-wavelength (<2 km) variations that indicate the presence of magnetic sources within the crater fill. With core magnetic properties and seismic reflection and refraction results as constraints, forward models of the magnetic field show that these sources may represent basementderived megablocks that are a few hundred meters thick or melt bodies that are a few dozen meters thick. Larger-scale magnetic field properties suggest that these bodies overlie deeper, pre-impact basement contacts between materials with different magnetic properties such as gneiss and schist or gneiss and granite. The distribution of the short-wavelength magnetic anomalies in combination with observations of small-scale (1-2 mGal) gravity field variations suggest that basement-derived megablocks are preferentially distributed on the eastern side of the inner crater, not far from the Eyreville core, at depths of around 1-2 km. A scenario where additional basement-derived blocks between 2 and 3 km depth are distributed throughout the inner basin-and are composed of more magnetic materials, such as granite and schist, toward the east over a large-scale magnetic anomaly high and less magnetic materials, such as gneiss, toward the west where the magnetic anomaly is lower-provides a good model fi t to the observed magnetic anomalies in a manner that is consistent with both gravity and seismic-refraction data. © 2009 The Geological Society of America.
909.
Postimpact heat conduction and compaction-driven fluid flow in the Chesapeake Bay impact structure based on downhole vitrinite reflectance data, ICDP-USGS Eyreville deep core holes and Cape Charles test holes
Special Paper of the Geological Society of America,
458905-930
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Boreholes; Compaction; Heat conduction; Infill drilling; Reflection; Rocks; Saline water; Sediments; Software testing, Chesapeake bay impact structures; Conductive heat; Continental scientific drillings; Metamorphic events; Reflectance data; U.s. geological surveys; Vertical fluid flow; Vitrinite reflectance, Flow of fluids, black shale; breccia; crater; fluid flow; geological survey; heat flow; impact structure; Ocean Drilling Program; temperature effect; thermal conductivity; thermal regime; uplift; vitrinite reflectance, Chesapeake Bay; United States, Calluna vulgaris
Abstract: ▾ Vitrinite reflectance data from the International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville deep cores in the centralcrater moat of the Chesapeake Bay impact structure and the Cape Charles test holes on the central uplift show patterns of postimpact maximum-temperature distribution that result from a combination of conductive and advective heat flow. Within the crater-fill sediment-clast breccia sequence at Eyreville, an isoreflectance (-0.44% Ro) section (525-1096 m depth) is higher than modeled background coastal-plain maturity and shows a pattern typical of advective fluid flow. Below an intervening granite slab, a short interval of sediment-clast breccia (1371-1397 m) shows a sharp increase in reflectance (0.47%-0.91% Ro) caused by conductive heat from the underlying suevite (1397-1474 m). Refl ectance data in the uppermost suevite range from 1.2% to 2.1% Ro. However, heat conduction alone is not sufficient to affect the temperature of sediments more than 100 m above the suevite. Thermal modeling of the Eyreville suevite as a 390 °C cooling sill-like hot rock layer supplemented by compaction- driven vertical fluid flow (0.046 m/a) of cooling suevitic fluids and deeper basement brines (120 °C) upward through the sediment breccias closely reproduces the measured reflectance data. This scenario would also replace any marine water trapped in the crater fill with more saline brine, similar to that currently in the crater, and it would produce temperatures sufficient to kill microbes in sediment breccias within 450 m above the synimsuevite. A similar downhole maturity pattern is present in the sediment-clast breccia over the central uplift. High-reflectance (5%-9%) black shale and siltstone clasts in the suevite and sediment-clast breccia record a pre-impact (Paleozoic?) metamorphic event. Previously published maturity data in the annular trough indicate no thermal effect there from impact-related processes. © 2009 The Geological Society of America.
908.
Interpretation and mathematical modeling of temporal changes of temperature observed in borehole Yaxcopoil-1 within the Chicxulub impact structure, Mexico
Journal of Hydrology,
372
(1-4)
9-16
2009
Abstract: ▾ Geothermal research of the Chicxulub impact structure on the Yucatan Peninsula, Mexico, included repeated temperature logs following 0.3-0.8, 15, 24, 34 and 50 months after shut-in of drilling operations at the 1.5 km deep Yaxcopoil-1 borehole. A gradual distortion of the linear temperature profile by a cold wave propagating downward from 145 m to 317 m was detected within the observational period of 49 months (March 2002-April 2006). The amplitude of the cold wave was increasing with depth and time in the range of 0.8-1.6 °C. As an explanation of this unusual phenomenon, the hypothesis of downward migration of a large volume of drilling mud, reported lost during drilling within the overlying and cooler highly porous and permeable karstic rocks, has been proposed. The thermal effects of the migrating fluid have been evaluated by solving numerically the heat conduction-convection equation in appropriate geothermal models. The best coincidence between the observed data and the simulations was yielded by the model of the drilling mud migrating as a large body. Parameters of this model are constrained by the measured temperature logs relatively tightly: (i) the vertical extent of the downward migrating fluid body is about 5-10 m, possibly increasing within the observational period of 49 months by a factor of 2; (ii) the horizontal extent of the body must be at least 15-20 m, i.e. by order(s) of magnitude larger than the diameter of the borehole; (iii) the average speed of the migration is about 5 metres per month and (iv) the fluid must migrate through a highly porous rock (80% porosity or more). This high porosity, which is necessary for the model to fit the observed data, and the observed relatively stable velocity of the migration indicate the existence of a well-developed system of interconnected cavities down to more than 300 m about 150 m more than the deepest cave system known in Yucatan yet. © 2009 Elsevier B.V. All rights reserved.
907.
Joint inversion for Vp, Vs, and Vp/Vs at SAFOD, Parkfield, California
Geochemistry, Geophysics, Geosystems,
10
(11)
2009
ISSN: 15252027Abstract: ▾ We refined the three-dimensional (3-D) Vp, Vs and Vp/Vs models around the San Andreas Fault Observatory at Depth (SAFOD) site using a new double-difference (DD) seismic tomography code (tomoDDPS) that simultaneously solves for earthquake locations and all three velocity models using both absolute and differential P, S, and S-P times. This new method is able to provide a more robust Vp/Vs model than that from the original DD tomography code (tomoDD), obtained simply by dividing Vp by Vs. For the new inversion, waveform cross-correlation times for earthquakes from 2001 to 2002 were also used, in addition to arrival times from earthquakes and explosions in the region. The Vp values extracted from the model along the SAFOD trajectory match well with the borehole log data, providing in situ confirmation of our results. Similar to previous tomographic studies, the 3-D structure around Parkfield is dominated by the velocity contrast across the San Andreas Fault (SAF). In both the Vp and Vs models, there is a clear low-velocity zone as deep as 7 km along the SAF trace, compatible with the findings from fault zone guided waves. There is a high Vp/Vs anomaly zone on the southwest side of the SAF trace that is about 1-2 km wide and extends as deep as 4 km, which is interpreted to be due to fluids and fractures in the package of sedimentary rocks abutting the Salinian basement rock to the southwest. The relocated earthquakes align beneath the northeast edge of this high Vp/Vs zone. We carried out a 2-D correlation analysis for an existing resistivity model and the corresponding profiles through our model, yielding a classification that distinguishes several major lithologies. © 2009 by the American Geophysical Union.
906.
Lake El'gygytgyn's emerging IPY record of Pliocene to recent Arctic change.
PAGES (Past Global Changes) News,
17
(1)
19-21
2009
905.
Lake Van, Eastern Anatolia, hydrochemistry and history
Aquatic Geochemistry,
15
(1-2)
195-222
2009
ISSN: 13806165Keywords:▾
desiccation; dissolution; Holocene; hydrochemistry; hydrogeochemistry; lake level; paleolimnology; porewater; water column; water depth, Eurasia; Lake Van; Turkey, Bacillariophyta
Abstract: ▾ Saline, 450-m-deep Lake Van (Eastern Anatolia, Turkey) is, with 576 km3, the third largest closed lake on Earth and its largest soda lake. In 1989 and 1990, we investigated the hydrochemistry of the lake's water column and of the tributary rivers. We also cored the Postglacial sediment column at various water depths. The sediment is varved throughout, allowing precise dating back to ca. 15 ka BP. Furthermore, lake terrace sediments provided a 606-year-long floating chronology of the Glacial high-stand of the lake dating to 21 cal. ka BP. The sediments were investigated for their general mineralogical composition, important geochemical parameters, and pore water chemistry as well. These data allow reconstructing the history of the lake level that has seen several regressions and transgressions since the high-stand at the end of the Last Glacial Maximum. Today, the lake is very alkaline, highly supersaturated with Ca-carbonate and has a salt content of about 22 g kg-1. In summer, the warmer epilimnion is diluted with river water and forms a stable surface layer. Depth of winter mixing differs from year to year but during time of investigation the lake was oxygenated down to its bottom. In general, the lake is characterized by an Na-CO3-Cl-(SO4)-chemistry that evolved from the continuous loss of calcium as carbonate and magnesium in the form of Mg-silica-rich mineral phases. The Mg cycle is closely related to that of silica which in turn is governed by the production and dissolution of diatoms as the dominant phytoplankton species in Lake Van. In addition to Ca and Mg, a mass balance approach based on the recent lake chemistry and river influx suggests a fractional loss of potassium, sodium, sulfur, and carbon in comparison to chloride in the compositional history of Lake Van. Within the last 3 ka, minor lake level changes seem to control the frequency of deep water renewal, the depth of stratification, and the redox state of the hypolimnion. Former major regressions are marked by Mg-carbonate occurrences in the otherwise Ca-carbonate dominated sediment record. Pore water data suggest that, subsequent to the major regression culminating at 10.7 ka BP, a brine layer formed in the deep basin that existed for about 7 ka. Final overturn of the lake, triggered by the last major regression starting at about 3.5 ka BP, may partly account for the relative depletion in sulfur and carbon due to rapid loss of accumulated gases. An even stronger desiccation phase is proposed for the time span between about 20 and 15 ka BP following the LGM, during which major salts could have been lost by precipitation of Na-carbonates and Na-sulfates. © The Author(s) 2008.
904.
Mantle deformation beneath the Chicxulub impact crater
Earth and Planetary Science Letters,
284
(1-2)
249-257
2009
Abstract: ▾ The surface expression of impact craters is well-known from visual images of the Moon, Venus, and other planets and planetary bodies, but constraints on deep structure of these craters is largely limited to interpretations of gravity data. Although the gravity models are non-unique, they do suggest that large impact craters are associated with structure at the base of the crust. We use seismic data to map Moho (crust-mantle interface) topography beneath the Chicxulub crater, the youngest and best preserved of the three largest known terrestrial impact craters. The Moho is upwarped by ~ 1.5-2 km near the center of the Chicxulub crater, and depressed by ~ 0.5-1.0 km at a distance of ~ 30-55 km from the crater center. A comparison with numerical modeling results reveal that immediately following impact a transient crater reached a maximum depth of at least 30 km prior to collapse, and that subsequent collapse of the transient crater uplifted target material from deep below the crater floor. These results demonstrate that deformation from large terrestrial impacts can extend to the base of the continental crust. A similar Moho topography is also modeled for some large lunar and Martian craters, which suggests that mantle deformation may play a prominent role in large crater formation. © 2009 Elsevier B.V. All rights reserved.
903.
Measuring velocity dispersion and attenuation in the exploration seismic frequency band
Geophysics,
74
(2)
WA113-WA122
2009
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
Keywords:▾
Data handling; Dispersion (waves); Gas hydrates; Petroleum prospecting; Petrophysics; Seismic waves; Seismology; Velocity; Vibrators, Frequency dependent; Instantaneous phase; Petrophysical properties; Seismic data processing; Seismic frequencies; Spectral decomposition methods; Velocity dispersion; Vertical seismic profiles, Seismic prospecting, accuracy assessment; conference proceeding; data processing; efficiency measurement; frequency analysis; frequency dependence; heterogeneity; observational method; seismic attenuation; seismic data; seismic velocity; seismic wave; wave dispersion
Abstract: ▾ No perfectly elastic medium exists in the earth. In an anelastic medium, seismic waves are distorted by attenuation and velocity dispersion. Velocity dispersion depends on the petrophysical properties of reservoir rocks, such as porosity, fractures, fluid mobility, and the scale of heterogeneities. However, velocity dispersion usually is neglected in seismic data processing partly because of the insufficiency of observations in the exploration seismic frequency band (∼5 through 200 Hz). The feasibility of determining velocity dispersion in this band is investigated. Four methods are used in measuring velocity dispersion from uncorrelated vibrator vertical seismic profile (VSP) data: the moving window crosscorrelation (MWCC) method, instantaneous phase method, time-frequency spectral decomposition method, and cross-spectrum method. The MWCC method is a new method that is satisfactorily robust, accurate, and efficient in measuring the frequency-dependent traveltime in uncorrelated vibrator records. The MWCC method is applied to the uncorrelated vibrator VSP data acquired in the Mallik gas hydrate research well. For the first time, continuous velocity dispersion is observed in the exploration seismic frequency band using uncorrelated vibrator VSP data. The observed velocity dispersion is fitted to a straight line with respect to log frequency to calculate Q. This provides an alternative method for Q measurement. © 2009 Society of Exploration Geophysicists. All rights reserved.
902.
Integrated sequence stratigraphy of the postimpact sediments from the Eyreville core holes, Chesapeake Bay impact structure inner basin
Special Paper of the Geological Society of America,
458775-810
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Lithology, Chesapeake bay impact structures; Chronostratigraphy; Measurements of; Middle Pleistocene; Paleoenvironments; Pleistocene strata; Sequence boundary; Sequence stratigraphy, Stratigraphy, biostratigraphy; chronostratigraphy; deposition; Eocene; impact structure; lithology; Miocene; paleoenvironment; Pleistocene; Pliocene; sequence boundary; sequence stratigraphy; strontium isotope, Chesapeake Bay; Delaware; New Jersey; United States, Foraminifera
Abstract: ▾ The Eyreville core holes provide the first continuously cored record of postimpact sequences from within the deepest part of the central Chesapeake Bay impact crater. We analyzed the upper Eocene to Pliocene postimpact sediments from the Eyreville A and C core holes for lithology (semiquantitative measurements of grain size and composition), sequence stratigraphy, and chronostratigraphy. Age is based primarily on Sr isotope stratigraphy supplemented by biostratigraphy (dinocysts, nannofossils, and planktonic foraminifers); age resolution is approximately ±0.5 Ma for early Miocene sequences and approximately ±1.0 Ma for younger and older sequences. Eocene-lower Miocene sequences are subtle, upper middle to lower upper Miocene sequences are more clearly distinguished, and upper Miocene- Pliocene sequences display a distinct facies pattern within sequences. We recognize two upper Eocene, two Oligocene, nine Miocene, three Pliocene, and one Pleistocene sequence and correlate them with those in New Jersey and Delaware. The upper Eocene through Pleistocene strata at Eyreville record changes from: (1) rapidly deposited, extremely fi ne-grained Eocene strata that probably represent two sequences deposited in a deep (>200 m) basin; to (2) highly dissected Oligocene (two very thin sequences) to lower Miocene (three thin sequences) with a long hiatus; to (3) a thick, rapidly deposited (43-73 m/Ma), very fi ne-grained, biosiliceous middle Miocene (16.5-14 Ma) section divided into three sequences (V5-V3) deposited in middle neritic paleoenvironments; to (4) a 4.5-Ma-long hiatus (12.8-8.3 Ma); to (5) sandy, shelly upper Miocene to Pliocene strata (8.3-2.0 Ma) divided into six sequences deposited in shelf and shoreface environments; and, last, to (6) a sandy middle Pleistocene paralic sequence (~400 ka). The Eyreville cores thus record the fi lling of a deep impact-generated basin where the timing of sequence boundaries is heavily infl uenced by eustasy. © 2009 The Geological Society of America.
901.
Palaeoenvironmental significance of carbon- and oxygen-isotope stratigraphy of marine Triassic-Jurassic boundary sections in SW Britain
Journal of the Geological Society,
166
(3)
431 – 445
2009
ISSN: 00167649Keywords:▾
England; Eurasia; Europe; Glamorgan; Saint Audrie's Bay; Somerset; United Kingdom; Wales; Western Europe; Bivalvia; Ostreidae; carbon cycle; carbon dioxide; carbon isotope; chemical composition; Jurassic; oxygen isotope; paleoenvironment; paleotemperature; seawater; stratigraphic boundary; stratigraphic correlation; Triassic
Abstract: ▾ Carbon-isotope stratigraphy is a useful tool for stratigraphic correlation, especially for strata deposited during major perturbations of the carbon cycle that affected the marine, terrestrial and atmospheric reservoirs. For the Triassic-Jurassic boundary, effectively defined by a first-order mass extinction, major fluctuations in carbon-isotope values have been well documented, but these datasets have generally been derived from bulk-rock samples. Hence, the extent to which features of the isotopic curve reflect diagenetic alteration or changing proportions of constituent materials is unconstrained. Here, carbon- and oxygen-isotope data are presented from well-preserved oyster shells (Liostrea) comprising low-magnesium calcite, a mineral species relatively resistant to diagenetic alteration. Samples were obtained from Lavernock Point, Glamorgan, Wales, a coastal section close to a candidate stratotype for the base of the Jurassic at St Audrie's Bay, Somerset, England. The carbon-isotope signature from St Audrie's Bay, previously defined on the basis of analysis of bulk organic matter, is confirmed by our new data. Major features are (1) the upper part of an 'initial' negative isotope excursion in the lowest part of the section, followed by (2) a pronounced positive excursion, and (3) an extended 'main' negative isotope excursion in the highest part of the section. The data confirm that the carbon-isotope stratigraphy previously documented from bulk organic matter in SW England records the chemical composition of the contemporaneous seawater. Bulk carbonates sampled over the same interval near Lyme Regis, England, show similar trends to those from oyster calcite in the lower part of the study section, but there are more 13C-depleted values up-section. These lower values probably result from an admixture of primary and diagenetic carbonate. Palaeotemperatures calculated from oxygen-isotope values from Lavernock Point oyster shells are relatively cool at the beginning of the positive carbon-isotope excursion, and increased by up to 10 8C during the main negative carbon-isotope excursion. The new results are compatible with the view that positive carbon-isotope excursions correspond to times of low atmospheric carbon dioxide content, whereas negative carbon-isotope excursions correspond to times of high atmospheric carbon dioxide content, as is also found to be the case during the Early Jurassic (Toarcian) Oceanic Anoxic Event. The Mg/Ca and Sr/Ca ratios and 18O of investigated Liostrea hisingeri show no correlation,supporting data from modern bivalves that indicate that incorporation of Mg and Sr is controlled mainly by factors other than temperature. © 2009 Geological Society of London.
900.
Microbial abundance in the deep subsurface of the Chesapeake Bay impact crater: Relationship to lithology and impact processes
Special Paper of the Geological Society of America,
458941-950
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Biospherics; Lithology; Submarine geology, Chesapeake Bay; Chesapeake bay impact structures; Impact craters; Impact process; Marine sediments; Microbial abundances; Subsurface biosphere; Surface ecosystems, Sediments, abundance; asteroid; biosphere; comet; crater; Eocene; hydraulic conductivity; impact structure; lithology; marine sediment; microbial activity; microbial community; microbiology; Miocene; schist; suevite, Chesapeake Bay; United States
Abstract: ▾ Asteroid and comet impact events are known to cause profound disruption to surface ecosystems. The aseptic collection of samples throughout a 1.76-km-deep set of cores recovered from the deep subsurface of the Chesapeake Bay impact structure has allowed the study of the subsurface biosphere in a region disrupted by an impactor. Microbiological enumerations suggest the presence of three major microbiological zones. The upper zone (127-867 m) is characterized by a logarithmic decline in microbial abundance from the surface through the postimpact section of Miocene to Upper Eocene marine sediments and across the transition into the upper layers of the impact tsunami resurge sediments and sediment megablocks. In the middle zone (867-1397 m) microbial abundances are below detection. This zone is predominantly quartz sand, primarily composed of boulders and blocks, and it may have been mostly sterilized by the thermal pulse delivered during impact. No samples were collected from the large granite block (1096-1371 m). The lowest zone (below 1397 m) of increasing microbial abundance coincides with a region of heavily impact-fractured, hydraulically conductive suevite and fractured schist. These zones correspond to lithologies infl uenced by impact processes. Our results yield insights into the infl uence of impacts on the deep subsurface biosphere. © 2009 The Geological Society of America.
899.
Microseismic activity and fluid fault interactions: Some results from the Corinth Rift Laboratory (CRL), Greece
Geophysical Journal International,
178
(1)
561 – 580
2009
ISSN: 1365246XKeywords:▾
Eurasia; Europe; Greece; Gulf of Corinth; Ionian Sea; Mediterranean Sea; Southern Europe; backarc basin; earthquake swarm; fault zone; fluid-structure interaction; permeability; porosity; rheology; seismicity; stress field
Abstract: ▾ The Gulf of Corinth, in western-central Greece, is one of the fastest continental rifts in the world. In its western section near the city of Aigion, the previous work has outlined the existence of a shallow dipping seismogenic zone between 5 and 12 km. This seismic activity has been monitored with a network of 12 three-component stations for the period 2000-2007. Three, few months long, seismic swarms have been observed. They mobilize a complex structural fault system that associates both shallow dipping elements and subvertical structures with very different azimuths, some of which extend to depths greater than that of the shallow dipping zone. The swarm activity associates intensely active, short crises (a few days) with more quiescent periods. The long-term growth velocity of the seismically activated domains is compatible with a fluid diffusion process. Its characteristics are discussed in the context of the results from the 1000 m deep AIG10 well that intersects the Aigion Fault at 760 m. The vertical growth directions of the seismically activated volumes outline two different sources for the fluid and imply non-steady pressure conditions within the seismic domain. The diffusivity along the cataclastic zone of the faults is in the order of 1 m2 s-1, while faults act as hydraulic barrier in the direction perpendicular to their strike. If the vertical direction is a principal stress component, the high pore pressure values that must be reached to induce slip on the shallowly dipping planes can result only from transitory dynamic conditions. It is argued that the shallow dipping active seismic zone is only local and does not correspond to a 100 km scale decollement zone. We propose to associate the localization process with deep fluid fluxes that have progressively modified the local stress field and may be the cause for the quiescence of the West Heliki Fault presently observed. © 2009 The Authors, Journal compilation © 2009 RAS.
898.
Microseismicity at the North Anatolian Fault in the Sea of Marmara offshore Istanbul, NW Turkey
Journal of Geophysical Research: Solid Earth,
114
(9)
2009
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
azimuth; depocenter; earthquake hypocenter; earthquake mechanism; earthquake rupture; fault geometry; fault zone; focal mechanism; observational method; P-wave; seismic tomography; seismicity; seismograph; spatial distribution; strike-slip fault; tectonic setting; travel time; waveform analysis, Anatolia; Eurasia; Istanbul [Turkey]; Sea of Marmara; Turkey
Abstract: ▾ The North Anatolian Fault Zone (NAFZ) below the Sea of Marmara forms a "seismic gap" where a major earthquake is expected to occur in the near future. This segment of the fault lies between the 1912 Ganos and 1999 İzmit ruptures and is the only NAFZ segment that has not ruptured since 1766. To monitor the microseismic activity at the main fault branch offshore of Istanbul below the Çinarcik Basin, a permanent seismic array (PIRES) was installed on the two outermost Prince Islands, Yassiada and Sivriada, at a few kilometers distance to the fault. In addition, a temporary network of ocean bottom seismometers was deployed throughout the Çinarcik Basin. Slowness vectors are determined combining waveform cross correlation and P wave polarization. We jointly invert azimuth and traveltime observations for hypocenter determination and apply a bootstrap resampling technique to quantify the location precision. We observe seismicity rates of 20 events per month for M < 2.5 along the basin. The spatial distribution of hypocenters suggests that the two major fault branches bounding the depocenter below the Çinarcik Basin merge to one single master fault below ∼17 km depth. On the basis of a cross-correlation technique we group closely spaced earthquakes and determine composite focal mechanisms implementing recordings of surrounding permanent land stations. Fault plane solutions have a predominant right-lateral strike-slip mechanism, indicating that normal faulting along this part of the NAFZ plays a minor role. Toward the west we observe increasing components of thrust faulting. This supports the model of NW trending, dextral strike-slip motion along the northern and main branch of the NAFZ below the eastern Sea of Marmara. Copyright 2009 by the American Geophysical Union.
897.
Modeling on gas hydrate formation conditions in the Qinghai-Tibet plateau permafrost
Acta Geophysica Sinica,
52
(1)
157-168
2009
ISSN: 00015733
Publisher: Science Press
Abstract: ▾ Based on field-investigated gas geochemistry and predecessors data such as the permafrost ground temperature, thermal gradients within/below the frozen layer, the modeling on gas hydrate formation conditions is conducted in the Qinghai-Tibet plateau permafrost. The modeled results show that the permafrost characteristics generally meet gas hydrate formation conditions in the study area. Gas composition, temperature-related permafrost parameters (e. g. permafrost thickness or its ground temperature and thermal gradients within / below the frozen layer) are the most important factors affecting gas hydrate occurrences, whose variance may cause the heterogeneity of gas hydrate occurrences in the study area. The most probable gas hydrate is the kind of hybrid of methane and weight hydrocarbon gases (ethane and propane). In the predicted gas hydrate locations, the upper gas hydrate occurrence depth may be around several ten to more than one hundred meters and the lower depth may range from several hundred meters to about one thousand meters and the thickness may reach several hundred meters. Compared with Canadian Mallik permafrost, the Qinghai-Tibet plateau permafrost has similar thermal gradients within / below the frozen layer and gas composition, except for relatively thinner permafrost, still suggesting great gas hydrate potentials.
896.
Modeling the formation of the K-Pg boundary layer
Icarus,
201
(2)
768-780
2009
Abstract: ▾ In this paper we investigate the formation of the Cretaceous-Paleogene (K-Pg) boundary layer through numerical modeling. The K-Pg layer is widely agreed to be composed of meteoritic material and target rock from the Chicxulub impact site, that has been ejected around the globe and mixed with local material during final deposition. The observed composition and thickness of the K-Pg boundary layer changes with azimuth and distance from the impact site. We have run a suite of numerical simulations to investigate whether we can replicate the observational data, with a focus on the distal K-Pg layer and the impact glasses at proximal sites such as Beloc, Haiti. Previous models of the K-Pg ejecta have assumed an initial velocity distribution and tracked the ejecta to its final destination. Here, we attempt to model the entire process, from impact to the arrival of the ejecta around the globe. Our models replicate the observed ejecta thickness at proximal sites, and the modeled ejecta is composed of sediments and silicate basement rocks, in agreement with observational data. Models that use a 45° impact angle are able to replicate the total ejecta and iridium volume at distal sites, and the majority of the ejecta is composed of meteorite and target sediments. Sub-vertical impacts generate too little iridium, and oblique impacts of ≤30 degrees generate too much. However, in contrast to observations, models that involve ballistic transport of ejecta lead to ejecta thickness decreasing with increasing distance, and are unable to transport shocked minerals (quartz and zircon) from the Chicxulub basement rocks around the globe. We suggest that much of the K-Pg ejecta is transported non-ballistically, and that the most plausible mechanism is through re-distribution from a hot, expanding atmosphere. The results are important for future investigations of the environmental effects of the Chicxulub impact. © 2009 Elsevier Inc. All rights reserved.
895.
Mollusc biodiversity and endemism in the potential ancient Lake Trichonis, Greece
Malacologia,
51
(2)
357 – 375
2009
ISSN: 00762997Keywords:▾
Bivalvia; Gastropoda; Lysimachia; Mollusca
Abstract: ▾ Ancient lakes are hotspots of biodiversity, often harboring a large number of endemic species that make them prime model systems for evolutionary biologists. Besides such well-recognized ancient or long-lived lakes as Baikal, Biwa, Ohrid, and Tanganyika, there are other potentially old and biodiverse lakes in the world with poorly specified ages and under-studied faunas. We here report on the mollusc fauna of one such lake, Lake Trichonis in continental Greece. This graben lake is situated in a highly tectonized area, characterized by karst features and probably of middle to late Pliocene origin. Lake Trichonis is deep, oligotrophic, and rich in such specific habitat types as macrophyte meadows, rocky shores and sublacustrine spring systems. Moreover, it is a hotspot of freshwater biodiversity in Greece, particularly in molluscs. After reviewing newly collected material and the published mollusc records, we found that at least 33 mollusc species occur in Lake Trichonis, with 24 gastropod and 9 bivalve species currently being recognized. This is 24% of the total freshwater mollusc diversity of Greece; 21% of the gastropods (five species) are endemic to Lake Trichonis. If the whole Trichonis Basin is considered, which also includes neighboring Lake Lysimachia, eight species (33%) of the total fauna appear to be endemic. Taking lake surface areas into account, the index of gastropod endemism of 0.442 (log Nendemic species/log Asurface area) for the Lake Trichonis Basin resembles on a world-wide scale values known for Lake Baikal, Russia, and Lake Biwa, Japan, and is only exceeded by Lake Ohrid, Macedonia/Albania, and ancient lakes of Sulawesi, Indonesia. Despite the limited knowledge about the lake's evolutionary history, the suggested age of origin, the palaeogeographical characteristics, and the potential timing of phylogenetic events reviewed here support the presumed status of Lake Trichonis as an ancient lake. From a conservational standpoint, more research, management and conservation efforts are necessary because ancient lakes are among the most vulnerable and threatened ecosystems on earth. Effects of human-induced environmental change are already noticeable in Lake Trichonis. Recognition of Lake Trichonis as a unique system with an unusually high biodiversity may help promoting conservation efforts.