All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
528.
Hydrothermal alteration in the Bosumtwi impact structure: Evidence from 2M1-muscovite, alteration veins, and fracture fillings
Meteoritics and Planetary Science,
42
(4-5)
655 – 666
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ Drill-core samples from the Bosumtwi impact structure (1.07 Myr old and 10.5 km in diameter) in Ghana exhibit mineralogical evidence for post-impact hydrothermal alteration. Nine samples of drill core obtained through the 2004 International Continental Scientific Drilling Project (ICDP) were studied, including an uppermost fallback layer overlying impactite breccias, and partly deformed massive meta-graywacke bedrock. The petrographic study revealed alteration veins containing secondary sericitic muscovite (comparable to 2M1-muscovite) crosscutting original bedding in meta-graywacke and forming a matrix between clasts in impactite breccias. X-ray diffraction (XRD) shows that these impactite samples are rich in 2M1-muscovite, consistent with post-impact fluid deposition and alteration. Optical analysis indicates the presence of a pre-impact stratiform chlorite in meta-graywacke samples and a secondary alteration chlorite occurring in all samples. Secondary illite was detected in upper impactites of drill core LB-08A and samples containing accretionary lapilli. The lower temperature constraint for the hydrothermal event is given by 2M1-muscovite, secondary chlorite, and illite, all of which form at temperatures greater than 280 °C. An absence of recrystallization of quartz and feldspar indicates an upper temperature constraint below 900 °C. The presence of alteration materials associated with fractures and veins in the uppermost impactites of drill cores LB-07A and LB-08A indicates that a post-impact hydrothermal system was present in and adjacent to the central uplift portion of the Bosumtwi impact structure. A sample containing accretionary lapilli obtained from drill core LB-05A exhibits limited evidence that hydrothermal processes were more widespread within the impactites on the crater floor. © The Meteoritical Society, 2007. Printed in USA.
527.
Impact cratering - Fundamental process in geoscience and planetary science
Journal of Earth System Science,
116
(2)
81-98
2007
Abstract: ▾ Impact cratering is a geological process characterized by ultra-fast strain rates, which generates extreme shock pressure and shock temperature conditions on and just below planetary surfaces. Despite initial skepticism, this catastrophic process has now been widely accepted by geoscientists with respect to its importance in terrestrial - indeed, in planetary - evolution. About 175 impact structures have been discovered on Earth so far, and some more structures are considered to be of possible impact origin. One major extinction event, at the Cretaceous-Paleogene boundary, has been firmly linked with catastrophic impact, but whether other important extinction events in Earth history, including the so-called "Mother of All Mass Extinctions" at the Permian-Triassic boundary, were triggered by huge impact catastrophes is still hotly debated and a subject of ongoing research. There is a beneficial side to impact events as well, as some impact structures worldwide have been shown to contain significant (in some cases, world class) ore deposits, including the gold-uranium province of the Witwatersrand basin in South Africa, the enormous Ni and PGE deposits of the Sudbury structure in Canada, as well as important hydrocarbon resources, especially in North America. Impact cratering is not a process of the past, and it is mandatory to improve knowledge of the past-impact record on Earth to better constrain the probability of such events in the future. In addition, further improvement of our understanding of the physico-chemical and geological processes fundamental to the impact cratering process is required for reliable numerical modeling of the process, and also for the correlation of impact magnitude and environmental effects. Over the last few decades, impact cratering has steadily grown into an integrated discipline comprising most disciplines of the geosciences as well as planetary science, which has created positive spin-offs including the study of paleo-environments and paleo-climatology, or the important issue of life in extreme environments. And yet, in many parts of the world, the impact process is not yet part of the geoscience curriculum, and for this reason, it deserves to be actively promoted not only as a geoscientific discipline in its own right, but also as an important life-science discipline.
526.
Halogen systematics in the Mallik 5L-38 gas hydrate production research well, Northwest Territories, Canada: Implications for the origin of gas hydrates under terrestrial permafrost conditions
Applied Geochemistry,
22
(3)
656-675
2007
ISSN: 08832927Keywords:▾
Earth atmosphere; Gas hydrates; Natural gas wells; Seawater; Sedimentology, Gas hydrate production; Gas hydrate stability zone (GHSZ); Halogen systematics; Pore waters, Halogen compounds, bromine; coastal sediment; enrichment; gas hydrate; iodine; methane; mixing; organic matter; permafrost; porewater; seawater, Canada; Mackenzie Delta; North America; Northwest Territories
Abstract: ▾ The authors report here halogen concentrations in pore waters and sediments collected from the Mallik 5L-38 gas hydrate production research well, a permafrost location in the Mackenzie Delta, Northwest Territories, Canada. Iodine and Br are commonly enriched in waters associated with CH4, reflecting the close association between these halogens and source organic materials. Pore waters collected from the Mallik well show I enrichment, by one order of magnitude above that of seawater, particularly in sandy layers below the gas hydrate stability zone (GHSZ). Although Cl and Br concentrations increase with depth similar to the I profile, they remain below seawater values. The increase in I concentrations observed below the GHSZ suggests that I-rich fluids responsible for the accumulation of CH4 in gas hydrates are preferentially transported through the sandy permeable layers below the GHSZ. The Br and I concentrations and I/Br ratios in Mallik are considerably lower than those in marine gas hydrate locations, demonstrating a terrestrial nature for the organic materials responsible for the CH4 at the Mallik site. Halogen systematics in Mallik suggest that they are the result of mixing between seawater, freshwater and an I-rich source fluid. The comparison between I/Br ratios in pore waters and sediments speaks against the origin of the source fluids within the host formations of gas hydrates, a finding compatible with the results from a limited set of 129I/I ratios determined in pore waters, which gives a minimum age of 29 Ma for the source material, i.e. at the lower end of the age range of the host formations. The likely scenario for the gas hydrate formation in Mallik is the derivation of CH4 together with I from the terrestrial source materials in formations other than the host layers through sandy permeable layers into the present gas hydrate zones. © 2006 Elsevier Ltd. All rights reserved.
525.
Impactities as a random medium - Using variations in physical properties to assess heterogeneity within the Bosumtwi meteorite impact crater
Meteoritics and Planetary Science,
42
(4-5)
849 – 858
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ The recent drilling of the Bosumtwi impact structure, Ghana, has provided a unique opportunity to study the petrophysical properties of a young, well-preserved impact crater. The damage induced by impact results in extensive fracturing and mixing of target materials. We discuss here a means of using sonic velocity and density logs from two boreholes through the Bosumtwi crater fill and basement to estimate the degree of heterogeneity and fracturing within the impacted target, in order to understand the discrepancy between the large impedances derived from the log data and the nonreflective zone of impactites observed in seismic sections. Based on an analysis of the stochastic fluctuations in the log data, the Bosumtwi impactites are characterized by vertical scale lengths of 2-3 m. From the resolution of the seismic data over the crater, horizontal scale lengths are estimated at <12 m. The impactites therefore fall within the quasi-homogeneous scattering regime, i.e., seismic energy will propagate through the medium with little disruption. Scale lengths as small as these are observed in the fractured basement rocks of impact structures, whereas non-impact related crystalline environments are characterized by scale lengths an order of magnitude larger. Assuming that the high-frequency fluctuations observed in the log data are more sensitive to fracture distribution than petrology, this suggests that the small scale lengths observed within impact structures are characteristic of impact-induced damage, and could be used to estimate the extent of fracturing undergone by the rocks at any depth below an impact structure. © The Meteoritical Society, 2007. Printed in USA.
524.
Implication of seismic attenuation for gas hydrate resource characterization, Mallik, Mackenzie Delta, Canada
Journal of Geophysical Research: Solid Earth,
112
(10)
2007
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
gas hydrate; permafrost; seismic attenuation; seismic data, Canada; Mackenzie Delta; North America; Northwest Territories
Abstract: ▾ Wave attenuation is an important physical property of hydrate-bearing sediments that is rarely taken into account in site characterization with seismic data. We present a field example showing improved images of hydrate-bearing sediments on seismic data after compensation of attenuation effects. Compressional quality factors estimated from zero-offset Vertical Seismic Profiling data acquired at Mallik, Northwest Territories, Canada, demonstrate significant wave attenuation for hydrate-bearing sediments. These results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals. The application of an inverse Q-filter to compensate attenuation effects of permafrost and hydrate-bearing sediments improved the resolution of surface 3D seismic data and its correlation with log data, particularly for the shallowest gas hydrate interval. Compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone. Our results show that characterization of the Mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate-bearing strata and hence, the volume of hydrates in place. Copyright 2007 by the American Geophysical Union.
523.
Impact melting in sedimentary target rocks: An assessment
Special Paper of the Geological Society of America,
4371-18
2007
Abstract: ▾ Despite being present in the target sequence of ∼70% of the world's known impact structures, the response of sedimentary rocks to hypervelocity impact remains poorly understood. Of particular significance is the relative importance and role of impact melting versus decomposition in carbonate and sulfate lithologies. In this work, we review experimental evidence and phase equilibria and synthesize these data with observations from studies of naturally shocked rocks from several terrestrial impact sites. Shock experiments on carbonates and sulfates currently provide contrasting and ambiguous results. Studies of naturally shocked materials indicate that impact melting is much more common in sedimentary rocks than previously thought. This is in agreement with the phase relations for calcite. A summary of the criteria for the recognition of impact melts derived from sedimentary rocks is presented, and it is hoped that this will stimulate further studies of impact structures in sedimentary target rocks. This assessment leads us to conclude that impact melting is common during hypervelocity impact into both crystalline and sedimentary rocks. However, the products are texturally and chemically distinct, which has led to much confusion in the past, particularly in terms of the recognition of impact melts derived from sedimentary rocks. © 2008 The Geological Society of America. All rights reserved.
522.
Inorganic geochemistry of El'gygytgyn Lake sediments (northeastern Russia) as an indicator of paleoclimatic change for the last 250 kyr
Journal of Paleolimnology,
37
(1)
123-133
2007
ISSN: 09212728Keywords:▾
chemical alteration; climate change; geochemistry; inorganic compound; lacustrine deposit; paleoclimate; Quaternary; sediment chemistry, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ The inorganic geochemistry of sediments from El'gygytgyn Lake shift in phase with interpreted paleoclimatic fluctuations seen in the record over the past 250 ka. Warm periods, when the lake was seasonally ice free and fully mixed, are characterized by increased concentrations of SiO2, CaO, Na2O, K2O, and Rb, by decreased contents of TiO 2, Fe2O3, Al2O3, and MgO, and by a lower chemical index of alteration (CIA). Increased levels of SiO 2 reflect increases in limnic productivity whereas many of the other elements and the CIA likely reflect increased hydrological activity coincident with an increase in coarser sand and silt content and a decrease in clay mineral content. For cold/cooler periods when perennial lake ice cover lead to a stratifed water column and anoxic bottom waters, the opposite is generally observed suggesting a decrease in hydrological activity and an increase in post-depositional chemical alteration. Peaks in P2O3 and MnO, coincident with an increased abundance of vivianite, suggest possible linkages to the paleoproductivity of local fish fauna regardless of climate change across the region surrounding Lake El'gygytgyn. Strontium is high in concentration during warmer intervals and may also be linked to paleoproductivity. Enrichment of the post-Eemian portion of the sediment record in niobium, and yttrium appears independent of glacial-interglacial change; rather it may reflect a gradual shift in the geomorphology of the catchment, particularly the hydrology of large alluvial fans along the western side of the lake. In contrast to some lake records, changes in Zr concentration over time suggests only a weak, if any, increase in eolian sediment supply during colder periods. © 2006 Springer Science+Business Media B.V.
521.
Lake Van Drilling Project: A long continental record in eastern Turkey
Scientific Drilling
(4)
40-41
2007
ISSN: 18168957520.
Integrated 3-D model from gravity and petrophysical data at the Bosumtwi impact structure, Ghana
Meteoritics and Planetary Science,
42
(4-5)
859 – 866
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ The Bosumtwi impact structure of central Ghana was drilled in 2004 as part of the International Continental Scientific Drilling Program (ICDP). A vast amount of geoscience data is available from the pre-site surveys and the actual drilling phase. A 3-D gravity model was constructed and calibrated with the available data from the two ICDP boreholes, LB-07A and LB-08A. The 3-D gravity model results agree well with both the sediment thickness and size of the central uplift revealed by previously collected seismic data, and with the petrophysical data from the LB-08A and LB-07A core materials and the two borehole logs. Furthermore, the model exhibits lateral density variations across the structure and refines the results from previous 2.5-D modeling. An important new element of the 3-D model is that the thickness of the intervals comprising polymict lithic impact breccia and suevite, monomict lithic breccia and fractured basement is much smaller than that predicted by numerical modeling. © The Meteoritical Society, 2007. Printed in USA.
519.
Joint IODP-ICDP workshop examines challenges of fault zone drilling
Scientific Drilling
(1 SUPPL. 1)
5-16
2007
ISSN: 18168957518.
Joint IODP/ICDP scientific drilling of the Chicxulub impact crater
Scientific Drilling
(4)
42-44
2007
517.
Lake Baikal climatic record between 310 and 50 ky BP: Interplay between diatoms, watershed weathering and orbital forcing
Palaeogeography, Palaeoclimatology, Palaeoecology,
250
(1-4)
50 – 67
2007
ISSN: 00310182Keywords:▾
Eurasia; Lake Baikal; Russian Federation; Aulacoseira baicalensis; Aulacoseira skvortzowii; Bacillariophyta; Cyclotella; Cyclotella minuta; Stephanodiscus; climate change; diatom; interglacial; interstadial; lacustrine deposit; magnetic field; Northern Hemisphere; orbital forcing; proxy climate record; record; spectroscopy; watershed; weathering
Abstract: ▾ The environmental record from Lake Baikal, Russia, from 310 to 50 ky BP (MIS 9a to MIS 3) was interpreted using rock magnetic, UV-Vis spectral, mineralogical, and diatom analyses. The age model was based on a correlation of the diatom and chemical weathering records and the summer insolation curve at 55°N and checked against an age model based on the proxy of relative palaeointensity of the Earth's magnetic field. Peaks in chemical weathering within the watershed, inferred from maximum concentration of magnetic and coloured minerals and mica, the lowest mean Fe oxidation state in silicates and highs in expandable clay minerals correlated with the Northern Hemisphere summer insolation minima at 55°N. Reconstructed changes in weathering intensity are better correlated to insolation patterns than to global ice volume records. We propose a scheme of yet missing palaeoenvironmental interpretation of the diatom assemblage, including also some extinct species. Aulacoseira baicalensis and Aulacoseira skvortzowii were abundant in the early stages of lake flora recovery immediately after deglaciation and during MIS 7e and MIS 5e; periods of more pronounced continental climate and peak chemical weathering. Stephanodiscus formosus var. minor, Cyclotella minuta and Cyclotella ornata dominated in intervals of decreased seasonality and decreased humidity at the end of most interglacial/interstadial diatom zones. Stephanodiscus grandis, Stephanodiscus carconeiformis and Stephanodiscus formosus were ubiquitous between MIS 8 and MIS 5, an interval marked by high seasonality, i.e., large differences between winter and summer insolation, and low humidity revealed by a low hydrolysis of expandable clay minerals in the watershed. Diatom concentrations peaked in the climatic optima of MIS 7e and MIS 5e and in the short periods marked by shifts to warmer conditions in the upper sections of MIS 5: MIS 5c (103-99 ky BP), MIS 5b (90-88 ky BP), and MIS 5a (84-79 ky BP) in which increased humidity resulted in enhanced hydrolysis of clay minerals. No such short similar climatic optimums were found from MIS 9a to MIS 6. Sharp climate deteriorations recorded in the diatom and clay mineral records at 107, 94, and 87 ky BP, however, occurred within 1-2 ky of cold extremes in North Atlantic sea surface temperature emphasizing the strong teleconnections between the two localities. © 2007 Elsevier B.V. All rights reserved.
516.
Late Cenozoic climate changes in China's western interior: a review of research on Lake Qinghai and comparison with other records
Quaternary Science Reviews,
26
(17-18)
2281 – 2300
2007
ISSN: 02773791Keywords:▾
Asia; Atlantic Ocean; Atlantic Ocean (North); China; Eurasia; Far East; Qinghai-Xizang Plateau; Qionghai Lake; Sichuan; Tarim Basin; Xinjiang Uygur; Carbonates; Glaciers; Global warming; Precipitation (meteorology); Sea level; carbonate; Cenozoic; climate change; climate conditions; comparative study; global warming; historical record; Holocene; ice sheet; Last Glacial Maximum; marginal basin; Northern Hemisphere; regional climate; Lake Qinghai; Tibetan Plateau; Climate change
Abstract: ▾ We review Late Cenozoic climate and environment changes in the western interior of China with an emphasis on lacustrine records from Lake Qinghai. Widespread deposition of red clay in the marginal basins of the Tibetan Plateau indicates that the Asian monsoon system was initially established by ∼8 Ma, when the plateau reached a threshold altitude. Subsequent strengthening of the winter monsoon, along with the establishment of the Northern Hemisphere ice sheets, reflects a long-term trend of global cooling. The few cores from the Tibetan Plateau that reach back a million years suggest that they record the mid-Pleistocene transition from glacial cycles dominated by 41 ka cycles to those dominated by 100 ka cycles. During Terminations I and II, strengthening of the summer monsoon in China's interior was delayed compared with sea level and insolation records, and it did not reach the western Tibetan Plateau and the Tarim Basin. Lacustrine carbonate δ18O records reveal no climatic anomaly during MIS3, so that high terraces interpreted as evidence for extremely high lake levels during MIS3 remain an enigma. Following the Last Glacial Maximum (LSM), several lines of evidence from Lake Qinghai and elsewhere point to an initial warming of regional climate about 14 500 cal yr BP, which was followed by a brief cold reversal, possibly corresponding to the Younger Dryas event in the North Atlantic region. Maximum warming occurred about 10 000 cal yr BP, accompanied by increased monsoon precipitation in the eastern Tibetan Plateau. Superimposed on this general pattern are small-amplitude, centennial-scale oscillations during the Holocene. Warmer than present climate conditions terminated about 4000 cal yr BP. Progressive lowering of the water level in Lake Qinghai during the last half century is mainly a result of negative precipitation-evaporation balance within the context of global warming. © 2007.
515.
Rare earth element composition as evidence of the precursor material of Cretaceous-Tertiary boundary sediments at distal sections
Chemical Geology,
232
(1-2)
1-11
2006
Abstract: ▾ The Chicxulub impact event led to a worldwide deposition of impact materials originated from target rocks and the vaporized bolide. Relative contributions of both types of material to the K/T ejecta deposits vary with distance to the crater site. At distal sites (e.g., Agost and Caravaca in the SE of Spain) a major contribution of extraterrestrial material is indicated by different impact signatures, such as Os and Cr isotope composition, abundant microkrystites, platinum group elements and other siderophile elements that are typical of extraterrestrial components. Closer settings to the Chicxulub crater, for example the Blake Nose Plateau in the North American margin, display major continental crustal rock contributions in the ejecta layer. REE compositions provide additional evidence for terrestrial vs. extraterrestrial rock contributions. Previous research has not focused specifically on REE concentrations and corresponding C1- and NASC-normalized patterns. However, normalized REE patterns are already generating supplementary insights into the nature of the original material of the K/T boundary layer. Thus, Blake Nose ejecta C1-normalized patterns indicate a derivation from continental crustal target rocks. In more distal sections REE compositions point to a probable mafic precursor and confirm that extraterrestrial materials represent a major contribution the ejecta layer. © 2006 Elsevier B.V. All rights reserved.
514.
Refining the image of the San Andreas Fault near Parkfield, California using a finite difference travel time computation technique
Tectonophysics,
426
(1-2)
189-205
2006
ISSN: 00401951Keywords:▾
data inversion; fault zone; ray tracing; seismic tomography; seismic zone; travel time, California; North America; Parkfield; United States
Abstract: ▾ The Parkfield Area Seismic Observatory (PASO) was a dense, telemetered seismic array that operated for nearly 2 years in a 15 km aperture centered on the San Andreas Fault Observatory at Depth (SAFOD) drill site. The main objective of this deployment was to refine the locations of earthquakes that will serve as potential targets for SAFOD drilling and in the process develop a high (for passive seismological techniques) resolution image of the fault zone structure. A challenging aspect of the analysis of this data set was the known existence of large (20-25%) contrasts in seismic wavespeed across the San Andreas Fault. The resultant distortion of raypaths could challenge the applicability of approximate ray tracing techniques. In order to test the sensitivity of our hypocenter locations and tomographic image to the particular ray tracing and inversion technique employed, we compare an initial determination of locations and structure developed using a coarse grid and an approximate ray tracer [Thurber, C., Roecker, S., Roberts, K., Gold, M., Powell, M.L., and Rittger, K., 2003. Earthquake locations and three-dimensional fault zone structure along the creeping section of the San Andreas fault near Parkfield, CA: Preparing for SAFOD, Geophys. Res. Lett., 30 3, 10.1029/2002GL016004.] with one derived from a relatively fine grid and an application of a finite difference algorithm [Hole, J.A., and Zelt, B.C., 1995. 3-D finite-difference reflection traveltimes, Geophys. J. Int., 121, 2, 427-434.]. In both cases, we inverted arrival-time data from about 686 local earthquakes and 23 shots simultaneously for earthquake locations and three-dimensional Vp and Vp/Vs structure. Included are data from an active source seismic experiment around the SAFOD site as well as from a vertical array of geophones installed in the 2-km-deep SAFOD pilot hole, drilled in summer 2002. Our results show that the main features of the original analysis are robust: hypocenters are located beneath the trace of the fault in the vicinity of the drill site and the positions of major contrasts in wavespeed are largely the same. At the same time, we determine that shear wave speeds in the upper 2 km of the fault zone are significantly lower than previously estimated, and our estimate of the depth of the main part of the seismogenic zone decreases in places by ∼ 100 m. Tests using "virtual earthquakes" (borehole receiver gathers of picks for surface shots) indicate that our event locations near the borehole currently are accurate to about a few tens of meters horizontally and vertically. © 2006 Elsevier B.V. All rights reserved.
513.
SAFOD Penetrates the San Andreas Fault
Scientific Drilling,
2006
(2)
32-33
2006
ISSN: 1816-8957Abstract: ▾ SAFOD, the San Andreas Fault Observatory at Depth (Fig. 1), completed an important milestone in July 2005 by drilling through the San Andreas Fault at seismogenic depth. SAFOD is one of three major components of EarthScope, a U.S. National Science Foundation (NSF) initiative being conducted in collaboration with the U.S. Geological Survey (USGS). The International Continental Scientific DrillingProgram (ICDP) provides engineering and technical support for the project as well as online access to project data and information (https://www.icdp-online.org/sites/sanandreas/news/news1.html). In 2002, the ICDP, the NSF, and the USGS provided funding for a pilot hole project at the SAFOD site. Twenty scientifi c papers summarizing the results of the pilot hole project as well as pre-SAFOD site characterization studies were published in Geophysical Research Letters (Vol.31, Nos. 12 and 15, 2004).
512.
Scanned core image database and web-sharing of Chinese continental scientific drilling project
Diqiu Kexue - Zhongguo Dizhi Daxue Xuebao/Earth Science - Journal of China University of Geosciences,
31
(4)
575-577
2006
ISSN: 10002383Abstract: ▾ The only main hole of the Chinese Continental Scientific Drilling (CCSD) Project, the deepest scientific drilling project of all the ICDP projects, reached its depth of over 5000 m, from which more than 4400 m cores were collected. Hence the scanned core image database of CCSD project is also the largest of all the databases of ICDP projects. After various samplings, some cores have been completely damaged or disappeared. To make full use of the scanned core images, an MS ACCESS database was developed, on the basis of the original DIS system. More than 8000 images (about 100 gigabyte) have been efficiently and systematically managed and shared on the Internet with some user-friendly query program and interfaces using the ASP and VB techniques. This new information technology, proved by one year trial test, can not only store permanently and efficiently the original image data, but also help geologists to study efficiently the cores for their scientific purpose.
511.
Seismic characterization and continuity analysis of gas-hydrate horizons near Mallik research wells, Mackenzie Delta, Canada
Leading Edge,
25
(5)
599-604
2006
ISSN: 1070485X
Publisher: Society of Exploration Geophysicists
Keywords:▾
Gases; Hydration; Petroleum prospecting, Clastic sediments; Conventional hydrocarbons; Gas hydrate saturations; Hydrate accumulations; Permafrost region; Potential sources; Significant gas; Three interval, Gas hydrates, clastic sediment; gas hydrate; hydrocarbon exploration; natural gas; permafrost; porosity; supermarket, Arctic; Arctic Ocean; Beaufort Sea; Canada; Mackenzie Delta; Northwest Territories
Abstract: ▾ Gas-hydrate accumulations located onshore in Arctic permafrost regions are seen as a potential source of natural gas. Surprisingly, most of the gas hydrate found in the Mackenzie Delta and Beaufort Sea areas was indirectly discovered or inferred from conventional hydrocarbon exploration programs. One of these occurrences, the Mallik gas-hydrate field (Figure 1), has received particular attention over the last 10 years. Two internationally partnered research well programs have intersected three intervals of gas hydrates and have allowed successful extraction of subpermafrost core samples with significant gas hydrates. The gas-hydrate intervals are up to 40 m thick and have high gas-hydrate saturation, sometimes exceeding 80% of pore volume of unconsolidated clastic sediments with average porosities from 25-40%. At Mallik, the gas-hydrate intervals are located at depths of 900-1100 m and are localized on the crest of an anticline. © 2006 Society of Exploration Geophysicists.
510.
Seeking chemical clues to earthquakes
Chemical and Engineering News,
84
(4)
39-41
2006
ISSN: 00092347
Publisher: American Chemical Society
Keywords:▾
Flow of fluids; Pressure effects; Real time systems; Rocks; Tectonics, Chemical clue; Frictional change; San Andreas Fault; San Andreas Fault Observatory at Depth (SAFOD),, Earthquakes
509.
Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL)
Tectonophysics,
426
(1-2)
7 – 30
2006
ISSN: 00401951Keywords:▾
Gulf of Corinth; Ionian Sea; Mediterranean Sea; active fault; deformation; earthquake magnitude; geophysical survey; normal fault; seismic hazard; seismicity
Abstract: ▾ This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes. © 2006 Elsevier B.V. All rights reserved.
508.
Seismic evidence for asthenospheric updoming beneath the western Bohemian Massif, central Europe
Geophysical Research Letters,
33
(5)
2006
ISSN: 00948276Keywords:▾
Bohemian Massif; Central Europe; Eurasia; Europe; Carbon dioxide; Degassing; Earthquakes; Flow of fluids; Tectonics; asthenosphere; crustal thickness; earthquake swarm; fluid flow; imaging method; mantle process; Asthenospheric updoming; Carbon dioxide mantle-derived degassing; Earthquake swarm activity; Seismology
Abstract: ▾ Detailed images of the lithosphere beneath the western Bohemian Massif were obtained by analysis of more than 8500 P receiver functions. At the intersection of Regensburg-Leipzig-Rostock zone and Eger Rift, crustal thickness decreases to 26 km from approx. 31 km in the surroundings. The receiver functions display a positive phase at about 6 s delay time and a strong negative phase at 7 to 8 s, which coincides with an area of Moho updoming, CO2 mantle-derived degassing and earthquake swarm activity. These phases can be modeled by a velocity increase at 50 km and a velocity decrease at 65 km depth. The velocity decrease, observed over an area of 5300 km2, gives evidence for local asthenospheric updoming and/or a confined body of partial melt, which might be the cause for high CO2 mantle fluid flow and earthquake swarm activity in this recently nonvolcanic, intracontinental rift area. Copyright 2006 by the American Geophysical Union.
507.
Seismic reflections of gas hydrate from perturbational forward modeling
Geophysics,
71
(6)
F165-F171
2006
ISSN: 00168033
Publisher: Society of Exploration Geophysicists
Keywords:▾
Gas hydrates; Mineralogy; Organic compounds; Perturbation techniques; Petroleum reservoirs; Porosity; Reflection, Seismic amplitude; Seismic reflections, Seismology, amplitude; clay; elastic property; porosity; sand; seismic attenuation; seismic reflection; seismic velocity; seismology
Abstract: ▾ We perturb the elastic properties and attenuation in the Arctic Mallik methane-hydrate reservoir to produce a set of plausible seismic signatures away from the existing well. These perturbations are driven by the changes we impose on porosity, clay content, hydrate saturation, and geometry. The key is a data-guided, theoretical, rock-physics model that we adopt to link velocity and attenuation to porosity, mineralogy, and amount of hydrate. We find that the seismic amplitude is very sensitive to the hydrate saturation in the host sand and its porosity as well as the porosity of the overburden shale. However, changes to the amount of clay in the sand only weakly alter the amplitude. Attenuation, which may be substantial, must be taken into account during hydrate reservoir characterization because it lowers the amplitude to an extent that may affect the hydrate-volume prediction. The spatial structure of the reservoir affects the seismic reflection: A thinly-layered reservoir produces a noticeably different amplitude than a massive reservoir with the same hydrate volume. © 2006 Society of Exploration Geophysicists.
506.
Shock re-equilibration of fluid inclusions in crystalline basement rocks from the Ries crater, Germany
Meteoritics and Planetary Science,
41
(2)
247-262
2006
Abstract: ▾ This study examines the effects of shock metamorphism on fluid inclusions in crystalline basement target rocks from the Ries crater, Germany. The occurrence of two-phase fluid inclusions decreases from shock stage 0 to shock stage 1, while single-phase inclusions increase, likely as a result of re-equilibration. In shock stages 2 and 3, both two-phase and single-phase inclusions decrease with increasing shock stage, indicating that fluid inclusion vesicles are destroyed due to plastic deformation and phase changes in the host minerals. However, quartz clasts entrained in shock stage 4 melts contain both single-phase and two-phase inclusions, demonstrating the rapid quenching of the melt and the heterogeneous nature of impact deformation. Inclusions in naturally shocked polycrystalline samples survive at higher shock pressures than those in single crystal shock experiments. However, fluid inclusions in both experimental and natural samples follow a similar trend in re-equilibration at low to moderate shock pressures leading to destruction of inclusion vesicles in higher shock stages. This suggests that shock processing may lead to the destruction of fluid inclusions in many planetary materials and likely contributed to shock devolatilization of early planetesimals. © The Meteoritical Society, 2006.
505.
Post-Chicxulub depositional and diagenetic history of the northwestern Yucatan Peninsula, Mexico
Sedimentary Geology,
183
(1-2)
51-69
2006
Abstract: ▾ The Chicxulub Sedimentary Basin of the northwestern Yucatan Peninsula, Mexico, which was formed because of the largest identified Phanerozoic bolide impact on Earth, became a site of deposition of dominantly marine carbonate sediments during most of the Cenozoic Era. This is a study of the filling and diagenetic history of this basin and surrounding areas. The study makes use of lithologic, biostratigraphic, petrographic, and geochemical data obtained on core samples from boreholes drilled throughout the northwestern Yucatan Peninsula. The core sample data indicate that: 1) The Chicxulub Sedimentary Basin concentrated the deposition of pelagic and outer-platform sediments during the Paleocene and Eocene, and, in places, during the Early Oligocene, as well, and filled during the Middle Miocene, 2) deeper-water limestone also is present within the Paleocene and Lower Eocene of the proposed Santa Elena Depression, which is located immediately south of the Basin, 3) shallow-water deposits are relatively more abundant outside the Basin and Depression than inside, 4) the autigenic and allogenic silicates from the Paleogene formations are the most abundant inside the Depression, 5) sediment deposition and diagenesis within the Basin also were controlled by impact crater topography, 6) the abundance of the possible features of subaerial exposure increases upward and outward from the center of the Basin, and 7) the formation of replacive low-magnesium calcite and dolomite, dedolomitization, dissolution, and precipitation of vug-filling calcite and dolomite cement have been more common outside the Basin than inside. δ18O in whole-rock (excluding vug-filling) calcite from core samples ranges from -7.14‰ to + 0.85‰ PDB. δ13C varies from -6.92‰ to +3.30‰ PDB. Both stable isotopes correlate inversely with the abundance of subaerial exposure features indicating that freshwater diagenesis has been extensive especially outside and at the edge of the Chicxulub Sedimentary Basin. δ18O and δ13C in w hole-rock (excluding vug-filling) dolomite ranges from -5.54‰ to +0.87‰ PDB and -4.63‰ to +3.38‰ PDB, respectively. Most dolomite samples have negative δ18O and positive δ13C suggesting that replacive dolomitization involved the presence of a fluid dominated by freshwater and/or an anomalously high geothermal gradient. Most dolomite XRD-determined mole percent CaCO3 varies between 51 and 56. Replacive dolomite is larger, more euhedral, and less stoichiometric inside the Chicxulub Sedimentary Basin than outside. © 2005 Elsevier B.V. All rights reserved.
504.
Origin and significance of clay-coated fractures in mudrock fragments of the SAFOD borehole (Parkfield, California)
Geophysical Research Letters,
33
(16)
2006
ISSN: 00948276
Publisher: American Geophysical Union
Keywords:▾
Clay; Crystal growth; Rocks; Scanning electron microscopy; Textures; Thin films; Transmission electron microscopy; X ray diffraction analysis, Aqueous fluids; Drilling mud; Fault creep; Slickenfiber striations, Boreholes, bentonite; borehole; calcium; cation; clay; crystal structure; hydration; mudstone; potassium; rock property; sampling; San Andreas Fault; scanning electron microscopy; smectite; texture; transmission electron microscopy; X-ray diffraction
Abstract: ▾ The clay mineralogy and texture of rock fragments from the SAFOD borehole at 3067 m and 3436 m measured depth (MD) was investigated by electron microscopy (SEM, TEM) and X-ray-diffraction (XRD). The washed and ultrasonically cleaned samples show slickenfiber striations and thin films of Ca-K bearing smectite that are formed on polished fault surfaces, along freshly opened fractures and within adjacent mineralized veins. The cation composition and hydration behavior of these films differ from the Namontmorillonite of the fresh bentonite drilling mud, although there is more similarity with circulated mud recovered from 3479 m MD. We propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep, probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the San Andreas Fault. Copyright 2006 by the American Geophysical Union.