All ICDP Publications with Abstracts
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569.
An international and multidisciplinary drilling project into a young complex impact structure: The 2004 ICDP Bosumtwi Crater Drilling Project - An overview
Meteoritics and Planetary Science,
42
(4-5)
483 – 511
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ The Bosumtwi impact crater in Ghana, arguably the best-preserved complex young impact structure town on Earth, displays a pronounced rim and is almost completely filled by Lake Bosumtwi, a hydrologically closed basin. It is the source crater of the Ivory Coast tektites. The structure was excavated in 2.1-2.2 Gyr old metasediments and metavolcanics of the Birimian Supergroup. A drilling project was conceived that would combine two major scientific interests in this crater: 1) to obtain a complete paleoenvironmental record from the time of crater formation about one million years ago, at a near-equatorial location in Africa for which very few data are available so far, and 2) to obtain a complete record of impactites at the central uplift and in the crater moat, for ground truthing and comparison with other structures. Within the framework of an international and multidisciplinary drilling project led by the International Continental Scientific Drilling Program (ICDP), 16 drill cores were obtained from June to October 2004 at six locations within Lake Bosumtwi, which is 8.5 km in diameter. The 14 sediment cores are currently being investigated for paleoenvironmental indicators. The two impactite cores LB-07A and LB-08A were drilled into the deepest section of the annular moat (540 m) and the flank of the central uplift (450 m), respectively. They are the main subject of this special issue of Meteoritics & Planetary Science, which represents the first detailed presentations of results from the deep drilling into the Bosumtwi impactite sequence. Drilling progressed in both cases through the impact breccia layer into fractured bedrock. LB-07A comprises lithic (in the uppermost part) and suevitic impact breccias with appreciable amounts of impact melt fragments. The lithic clast content is dominated by graywacke, besides various metapelites, quartzite, and a carbonate target component. Shock deformation in the form of quartz grains with planar microdeformations is abundant. First chemical results indicate a number of suevite samples that are strongly enriched in siderophile elements and Mg, but the presence of a definite meteoritic component in these samples cannot be confirmed due to high indigenous values. Core LB-08A comprises suevitic breccia in the uppermost part, followed with depth by a thick sequence of graywacke-dominated metasediment with suevite and a few granitoid dike intercalations. It is assumed that the metasediment package represents bedrock intersected in the flank of the central uplift. Both 7A and 8A suevite intersections differ from suevites outside of the northern crater rim. Deep drilling results confirmed the gross structure of the crater as imaged by the pre-drilling seismic surveys. Borehole geophysical studies conducted in the two boreholes confirmed the low seismic velocities for the post-impact sediments (less than 1800 m/s) and the impactites (2600-3300 m/s). The impactites exhibit very high porosities (up to 30 vol%), which has important implications for mechanical rock stability. The statistical analysis of the velocities and densities reveals a seismically transparent impactite sequence (free of prominent internal reflections). Petrophysical core analyses provide no support for the presence of a homogeneous magnetic unit (= melt breccia) within the center of the structure. Borehole vector magnetic data point to a patchy distribution of highly magnetic rocks within the impactite sequence. The lack of a coherent melt sheet, or indeed of any significant amounts of melt rock in the crater fill, is in contrast to expectations from modeling and pre-drilling geophysics, and presents an interesting problem for comparative studies and requires re-evaluation of existing data from other terrestrial impact craters, as well as modeling parameters. © The Meteoritical Society, 2007. Printed in USA.
568.
Petrophysical and paleomagnetic data of drill cores from the Bosumtwi impact structure, Ghana
Meteoritics and Planetary Science,
42
(4-5)
829 – 838
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ Physical properties from rocks of the Bosumtwi impact structure, Ghana, Central Africa, are essential to understand the formation of the relatively young (1.07 Ma) and small (10.5 km) impact crater and to improve its geophysical modeling. Results of our petrophysical studies of deep drill cores LB-07A and LB-08A reveal distinct lithological patterns but no depth dependence. The most conspicuous difference between impactites and target lithologies are the lower bulk densities and significantly higher porosities of the suevite and lithic breccia units compared to meta-graywacke and metapelites of target lithologies. Magnetic susceptibility shows mostly paramagnetic values (200-500 × 10-6 SI) throughout the core, with an exception of a few metasediment samples, and correlates positively with natural remanent magnetization (NRM) and Q values. These data indicate that magnetic parameters are related to inhomogeneously distributed ferrimagnetic pyrrhotite. The paleomagnetic data reveals that the characteristic direction of NRM has shallow normal (in a few cases shallow reversed) polarity, which is in agreement with the Lower Jaramillo N-polarity chron direction, and is carried by ferrimagnetic pyrrhotite. However, our study has not revealed the expected high magnetization body required from previous magnetic modeling. Furthermore, the LB-07A and LB08-A drill cores did not show the predicted high content of melt in the rocks, requiring a new interpretation model for magnetic data. © The Meteoritical Society, 2007. Printed in USA.
567.
A helium isotope cross-section study through the San Andreas Fault at seismogenic depths
Geochemistry, Geophysics, Geosystems,
8
(1)
2007
ISSN: 15252027Abstract: ▾ We have analyzed noble gas isotopes in 19 mud gas samples from 116-3943 m borehole depth of the San Andreas Fault Observatory at Depth (SAFOD) Main Hole in the context of origin and spatial variability of fluids occurring at seismogenic depths. The measured 3He/4He ratios range between 0.40 and 1.02 Ra (Ra is the atmospheric 3He/4He ratio of 1.39 × 10-6), with 4He/20Ne values between 0.33 and 4.92, revealing a mixture of three components to the total helium: (1) atmospheric helium, (2) helium with a crustal signature, and (3) mantle-derived helium. The air-corrected 3He/4He ratios fall between 0.2 Ra and 0.9 Ra. Samples from the 2117-3196 m depth show a relatively constant helium isotope composition (0.35-0.46 Ra), indicating that ∼5% of the helium in this section the Pacific Plate is derived from the mantle. The contribution of mantle-derived helium increases slightly in the transition from the Pacific Plate to the North American Plate and reaches maximal values of ∼12% on the North American Plate (below ∼3500 m borehole depth). On the basis of our observations, we suggest that the San Andreas Fault plays a role for fluid flux from greater depths, but higher amounts of mantle-derived fluids rise up through other, more permeable faults, situated on the North American Plate of the San Andreas Fault Zone (SAFZ). Lateral fluid dispersion at shallow depths through permeable country rock of the North American Plate may explain the observed increase in 3He/ 4He ratios with increasing distance to the SAF. Copyright 2007 by the American Geophysical Union.
566.
A revised age model for core PG1351 from Lake El'gygytgyn, Chukotka, based on magnetic susceptibility variations tuned to northern hemisphere insolation variations
Journal of Paleolimnology,
37
(1)
65-76
2007
ISSN: 09212728Keywords:▾
anoxic conditions; climate variation; core analysis; lacustrine deposit; luminescence dating; magnetic susceptibility; Northern Hemisphere; opal; oxic conditions; total organic carbon, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ A combined analysis of magnetic susceptibility, total organic carbon (TOC), biogenic silica (opal), and TiO2 content of the 12.6 m long composite core PG1351 recovered from Lake El'gygytgyn, Chukotka Peninsula, indicate a clear response of the lacustrine sedimentary record to climate variations. The impact is not direct, but through variations in oxygenation of the bottom waters. Mixing of the water body is typical for warmer climates, whereas the development of a stratified water body associated with anoxic conditions at the lake floor appears during cold climates. Oxic conditions lead to a good magnetite preservation and thus to high magnetic susceptibilities, but also to a large-scale degradation of organic matter, as reflected by low TOC (total organic carbon) values. During anoxic conditions, magnetite is severely dissolved yielding very low susceptibility values, whereas organic matter is best preserved, reflected by high TOC values. Hence, in general, neither susceptibility reflects the lithogenic fraction, nor does TOC reflect bioproductivity in case of the studied El'gygytgyn sediments. Based on available infrared stimulated luminescence (IRSL) dating, the obtained susceptibility pattern of core PG1351 shows an obvious correlation to northern hemisphere insolation variations, with a dominating impact of the Earth's 18 and 23 kyr precessional cycles for the upper half of PG1351, that is, during the past 150 ka. Therefore, the whole susceptibility record, together with biogenic silica (as a proxy for bioproductivity), TOC (as an indicator for redox conditions), and TiO2 (as a proxy for lithogenic input), was systematically tuned to the northern hemisphere insolation yielding an age of about 250 ka for the base of the composite core. © 2006 Springer Science+Business Media B.V.
565.
Accurate relocation of İzmit earthquake (Mw = 7.4, 1999) aftershocks in Çιnarcιk basin using double difference method
Geophysical Research Letters,
34
(10)
2007
ISSN: 00948276Keywords:▾
Aftershocks; Double difference method; Double difference relocation algorithm; Seismic network; Seismicity, Algorithms; Correlation methods; Earthquake resistance; Mathematical models; Seismic response; Seismic waves; Waveform analysis, Earthquakes, aftershock; arrival time; Kocaeli earthquake 1999; seismicity; travel time; waveform analysis
Abstract: ▾ We relocated part of the aftershock activity in Çιnarcιk Basin and surrounding areas that are associated with the 1999 İzmit earthquake Mw 7.4. Double difference relocation algorithm is used to relocate the aftershocks. The data set was obtained from a temporary seismic network deployed 10 days after the main shock by cooperation between Boǧaziçi University Kandilli Observatory and Earthquake Research Institute, LGIT (Grenoble), and IPGP (Paris). For a better station coverage, additional data set was obtained from a network operated by TUBITAK Marmara Research Center. Differential travel times were calculated using both arrival time readings and waveform cross correlation method. We relocated 1145 of the aftershocks and interpreted the results with emphasis on the Yalova and Tuzla cluster located within the Marmara Sea. The results show better focused seismicity patterns for the Yalova cluster, providing clear evidence for the proposed models. We present a revised location of the 1963 Çιnarcιk earthquake which took place in area of Yalova cluster. Finally we propose that the activity of Tuzla cluster represents a parallel subsidiary fault of the Main Marmara Fault. Copyright 2007 by the American Geophysical Union.
564.
Amplitude loss of sonic waveform due to source coupling to the medium
Geophysical Research Letters,
34
(5)
2007
ISSN: 00948276Keywords:▾
Acoustic wave velocity; Boreholes; Gas hydrates; Sediments; Waveform analysis, Energy radiation; Sonic waveform, Petroleum prospecting, amplitude; pore space; seismic attenuation; sonic boom; wave velocity; waveform analysis, Canada; North America; Northwest Territories
Abstract: ▾ In contrast to hydrate-free sediments, sonic waveforms acquired in gas hydrate-bearing sediments indicate strong amplitude attenuation associated with a sonic velocity increase. The amplitude attenuation increase has been used to quantify pore-space hydrate content by attributing observed attenuation to the hydrate-bearing sediment's intrinsic attenuation. A second attenuation mechanism must be considered, however. Theoretically, energy radiation from sources inside fluid-filled boreholes strongly depends on the elastic parameters of materials surrounding the borehole. It is therefore plausible to interpret amplitude loss in terms of source coupling to the surrounding medium as well as to intrinsic attenuation. Analyses of sonic waveforms from the Mallik 5L-38 well, Northwest Territories, Canada, indicate a significant component of sonic waveform amplitude loss is due to source coupling. Accordingly, all sonic waveform amplitude analyses should include the effect of source coupling to accurately characterize a formation's intrinsic attenuation.
563.
An asteroid breakup 160 Myr ago as the probable source of the K/T impactor
Nature,
449
(7158)
48-53
2007
Abstract: ▾ The terrestrial and lunar cratering rate is often assumed to have been nearly constant over the past 3 Gyr. Different lines of evidence, however, suggest that the impact flux from kilometre-sized bodies increased by at least a factor of two over the long-term average during the past ∼100 Myr. Here we argue that this apparent surge was triggered by the catastrophic disruption of the parent body of the asteroid Baptistina, which we infer was a ∼170-km-diameter body (carbonaceous-chondrite-like) that broke up Myr ago in the inner main asteroid belt. Fragments produced by the collision were slowly delivered by dynamical processes to orbits where they could strike the terrestrial planets. We find that this asteroid shower is the most likely source (>90 per cent probability) of the Chicxulub impactor that produced the Cretaceous/Tertiary (K/T) mass extinction event 65 Myr ago. ©2007 Nature Publishing Group.
562.
Application of Coring with Formation Kept Techniques in SLCORE-I (in Chinese with English abstract);[保形取心技术及其在松科1井中的应用研究]
Drilling Engineering,
749-51
2007
561.
Chromium isotopic studies of terrestrial impact craters: Identification of meteoritic components at Bosumtwi, Clearwater East, Lappajärvi, and Rochechouart
Earth and Planetary Science Letters,
256
(3-4)
534 – 546
2007
ISSN: 0012821XKeywords:▾
Africa; Canada; Eurasia; Europe; Finland; France; Ghana; North America; Northern Europe; Scandinavia; Sub-Saharan Africa; West Africa; Western Europe; Chromium; Landforms; Meteorites; Rocks; Stratigraphy; Chondrites; Crater; Meteoritic component; chromium; crater; hydrothermal alteration; isotopic composition; meteorite; weathering; Geochemistry
Abstract: ▾ Chromium isotopic compositions and elemental abundances in impact melt rock and impact glass samples from four terrestrial impact craters were measured to verify the presence of an extraterrestrial component and to identify the meteorite type of the impactor. All meteorite classes have Cr isotopic signatures that are different from those of terrestrial rocks; thus, precise measurements of Cr isotopic abundances can unequivocally distinguish terrestrial from extraterrestrial materials. For all four studied craters - Bosumtwi (Ghana), Clearwater East (Canada), Lappajärvi (Finland), and Rochechouart (France) we found positive 53Cr excesses that eliminate carbonaceous chondrite projectiles (because those would show apparent negative excesses) and enstatite chondrites (because of the magnitude of the excess). In all four cases, ordinary chondrites have been identified as the best fit for the data; in the case of Lappajärvi interelement correlations together with the Cr isotope data make an H-chondrite the most likely projectile, whereas in the case of Clearwater East both L or H chondrites are possible. For Bosumtwi and Rochechouart the high indigenous contents of the siderophile elements, and disturbances of the elemental abundances by weathering and hydrothermal alteration, respectively, do not allow further constraints to be placed on the type of ordinary chondrite involved in the impact. © 2007 Elsevier B.V. All rights reserved.
560.
Application of oriented Coring Techniques in the SLCORE-I (in Chinese with English abstract);[定向取心技术在松科1井中的应用]
Drilling Engineering,
1069-71
2007
559.
Applying SAR-IRSL methodology for dating fine-grained sediments from Lake El'gygytgyn, north-eastern Siberia
Quaternary Geochronology,
2
(1-4)
187-194
2007
ISSN: 18711014Keywords:▾
age determination; bedrock; climate change; crater; fine grained sediment; geochronology; insolation; lacustrine deposit; luminescence dating, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation; Siberia
Abstract: ▾ Lake El'gygytgyn is situated in a 3.6 Ma old impact crater in north-eastern Siberia and probably represents one of the most complete archives of Arctic climate change. Investigated here is the potential of infra-red stimulated luminescence (IRSL) using the single-aliquot regenerative-dose (SAR) approach for dating sediments from this lake. Independent age control is available from a published age model of a parallel core that is based on tuning sediment proxies with regional insolation and the results of previous multiple aliquot IRSL dating. Although the site is located within volcanic bedrock, anomalous fading seems to have little effect on the calculated ages. The modelled water content for the entire time of burial is seen as the most prominent uncertainty at this particular site. Despite these potential error sources, SAR-IRSL ages are in acceptable agreement with the given timeframe and clearly point to the possibility to establish independent chronologies at this site up to at least 400,000 years. © 2006.
558.
Basic hydrology, limnology, and meteorology of modern Lake El'gygytgyn, Siberia
Journal of Paleolimnology,
37
(1)
17-35
2007
ISSN: 09212728Keywords:▾
crater lake; digital elevation model; hydrology; lacustrine deposit; lacustrine environment; lake dynamics; limnology; meteorology; sediment core, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ A survey of the modern physical setting of Lake El'gygytgyn, northeastern Siberia, is presented here to facilitate interpretation of a 250,000-year climate record derived from sediment cores from the lake bottom. The lake lies inside a meteorite impact crater that is approximately 18 km in diameter, with a total watershed area of 293 km2, 110 km2 of which is lake surface. The only surface water entering the lake comes from the approximately 50 streams draining from within the crater rim; a numbering system for these inlet streams is adopted to facilitate scientific discussion. We created a digital elevation model for the watershed and used it to create hypsometries, channel networks, and drainage area statistics for each of the inlet streams. Many of the streams enter shallow lagoons dammed by gravel berms at the lakeshore; these lagoons may play a significant role in the thermal and biological dynamics of the lake due to their higher water temperatures (>6°C). The lake itself is approximately 12 km wide and 175 m deep, with a volume of 14.1 km3. Water temperature within a column of water near the center of this oligotrophic, monomictic lake never exceeded 4°C over a 2.5 year record, though the shallow shelves (<10 m) surrounding the lake can reach 5°C in summer. Though thermally stratified in winter, the water appears completely mixed shortly after lake ice breakup in July. Mean annual air temperature measured about 200 m from the lake was -10.3°C in 2002, and an unshielded rain gage there recorded 70 mm of rain in summer of 2002. End of winter snow water equivalent on the lake was approximately 110 mm in May 2002. Analysis of NCEP reanalysis air temperatures (1948-2002) reveals that the 8 warmest years and 10 warmest winters have occurred since 1989, with the number of days below -30°C dropping from a pre-1989 mean of 35 to near 0 in recent years. The crater region is windy as well as cold, with hourly wind speeds exceeding 13.4 m s-1 (30 mph) typically at least once each month and 17.8 m s-1 (40 mph) in winter months, with only a few calm days per month; wind may also play an important role in controlling the modern shape of the lake. Numerous lines of evidence suggest that the physical hydrology and limnology of the lake has changed substantially over the past 3.6 million years, and some of the implications of these changes on paleoclimate reconstructions are discussed. © 2006 Springer Science+Business Media B.V.
557.
Characteristics of the lithology, fault-related rocks and fault zone structures in TCDP hole-A
Terrestrial, Atmospheric and Oceanic Sciences,
18
(2)
243-269
2007
ISSN: 10170839Keywords:▾
chemical property; fault zone; lithology; lithostratigraphy; mechanical property; Miocene; physical property; Pleistocene; Pliocene, Asia; Eurasia; Far East; Taiwan, Mollusca
Abstract: ▾ The main objective of the Taiwan Chelungpu-fault Drilling Project (TCDP) was to conduct an in-depth probe into a fault zone of recent major activity so as to gain a better understanding of and more insight into the physical, mechanical and chemical properties involved. By the end of 2004, with the completion of the drilling of Hole-A, cuttings from 0 to 431.34 m and cores from a 431.34- to 2003.26-m depth had been obtained. Stratigraphically, the Pliocene to Pleistocene Cholan Formation is found from the surface to a 1029-m depth and is predominantly composed of sandstone and sandstone-siltstone alternations with weak to intense bioturbation. The Pliocene Chinshui Formation is observed from a depth of 1029- to 1303-m and predominantly consists of siltstone with weak bioturbation. From 1303- to 1712-m down there is the late Miocene to early Pliocene Kueichulin Formation which is predominantly composed of massive sandstone with minor siltstone. Below 1712 m, the Formation again resembles the younger Cholan Formation with mollusca-rich, thick, layered shale and heavy bioturbated sandstone. Four types of fault-related rocks are identified in the cores. They are the fault breccia, gouges, foliated and non-foliated cataclasites and pseudotachylytes. At least six major fault zones are found in the cores: FZ1111, FZ1153, FZ1220, FZ1580, FZ1712, and FZ1812. Among these, FZ1111 most probably corresponds to the slip surface of the Chi-Chi earthquake, the Chelungpu fault, while FZ1712 very likely represents the Sanyi fault.
556.
Characterization of aftershock-fault plane orientations of the 1999 İzmit (Turkey) earthquake using high-resolution aftershock locations
Geophysical Research Letters,
34
(20)
2007
ISSN: 00948276Keywords:▾
Earthquake effects; Mathematical models; Seismic waves; Waveform analysis, Hypocenters; Seismically active regions; Turkey, Seismology, aftershock; earthquake event; earthquake hypocenter; fault plane; focal mechanism; seismic velocity, Eurasia; Izmit; Kocaeli [Turkey]; Turkey
Abstract: ▾ Joint inversion for hypocentral parameters and the velocity field is nowadays a state of the art tool to obtain high-resolution images of seismically active regions. In this study, we focus on the location accuracy of aftershocks of the 1999 Mw = 7.4 İzmit (NW Turkey) earthquake. We obtained a new velocity model for the region, and depicted its improvement on absolute locations in terms of uncertainty and misfit. Two well-developed aftershock clusters located in the Akyazi area and Karadere-Düzce region, were analyzed in detail based on a waveform cross-correlation approach that allowed improving the location accuracy by a factor of 6. Relocation results reveal that hypocenters form narrow planes of activity that can be correlated with focal mechanisms of the larger aftershocks as well as nearby clouds of activity with no internal structure down to the resolved scale of ∼300 m. Copyright 2007 by the American Geophysical Union.
555.
Characterization of the log lithology of cores LB-07A and LB-08A of the Bosumtwi impact structure by using the anisotropy of magnetic susceptibility
Meteoritics and Planetary Science,
42
(4-5)
839 – 847
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ Petrophysical data are commonly used for the discrimination of different lithologies, as the variation in mineralogy, texture, and porosity is accompanied by varying physical properties. A special field of investigation is the analysis of the directional dependence (anisotropy) of the petrophysical properties, which can provide further information on the characteristics of the lithologies, due to the fact that this parameter is different in the various rock-forming and rock-changing processes, e.g., deformation or sedimentation. To characterize the rocks in drill cores LB-07A and LB-08A, which were drilled into the deep crater moat and central uplift of the Bosumtwi impact structure, Ghana, samples were taken for the study of petrophysical properties. In the present work the magnetic properties of these samples were determined in the laboratory. The results are discussed in relation to the various lithologies represented by this sample suite. The shape and degree of magnetic anisotropy, in combination with the magnetic susceptibility, proved useful in distinguishing between the different lithologies present in the drill cores (polymict lithic breccia, suevite, shale component, and meta-graywacke). It was possible to correlate layers of high (shale component), intermediate (graywacke, polymict lithic breccia), and low (suevite) anisotropy degree with the lithostratigraphic sequences determined for cores LB-07A and LB-08A. The shape of the anisotropy showed that foliation is most dominant within the shale component, whereas lineation is more pronounced in the meta-graywacke and polymict lithic breccia. An overall increase of the anisotropy degree was observed from core LB-07A towards core LB-08A. Thus magnetic anisotropy data provide a useful contribution towards an improved petrophysical characterization of the lithostratigraphic sequences in drillcores from the Bosumtwi impact structure. © The Meteoritical Society, 2007. Printed in USA.
554.
Chicxulub impact predates K-T boundary: New evidence from Brazos, Texas
Earth and Planetary Science Letters,
255
(3-4)
339-356
2007
Abstract: ▾ Multidisciplinary studies, including stratigraphy, sedimentology, mineralogy and geochemistry, of the new core Mullinax-1 and outcrops along the Brazos River and Cottonmouth Creek, Falls County, Texas, reveal the complex history of the Chicxulub impact, the event deposit and the K-T boundary event. The K-T boundary, as identified by the negative δ13C shift, first occurrence of Danian planktic foraminifera and palynomorphs occurs 80 cm above the event deposit in core Mullinax-1. The underlying 80 cm interval was deposited in a shallow low oxygen environment during the latest Maastrichtian, as indicated by high stress microfossil assemblages, small shells and burrows infilled with framboidal pyrite. The underlying event deposit, commonly interpreted as K-T impact tsunami, consists of a basal conglomerate with clasts containing Chicxulub impact spherules, repeated upward fining units of spherule-rich sands, followed by hummocky cross-bedded and laminated sands, which are burrowed by Thalassinoides, Planolites and Ophiomorpha and truncated by erosion. This suggests a series of temporally separated storm events with re-colonization of the ocean floor by invertebrates between storms, rather than a series of waning tsunami-generated waves. The lithified clasts with impact spherules at the base of the event deposit provide strong evidence that the Chicxulub impact ejecta layer predates the event deposit, but was eroded and re-deposited during the latest Maastrichtian sea level lowstand. The original Chicxulub ejecta layer was discovered in a 3 cm thick yellow clay layer interbedded in undisturbed late Maastrichtian clay- and mudstones 40 cm below the base of the event deposit and near the base of planktic foraminiferal zone CF1, which spans the last 300 kyr of the Maastrichtian. The yellow clay consists of cheto smectite derived from alteration of impact glass, as indicated by rare altered glass spherules with similar chemical compositions as reworked spherules from the event deposit and Chicxulub impact spherules from NE Mexico and Haiti. The Brazos sections thus provide strong evidence that the Chicxulub impact predates the K-T boundary by about 300 kyr, consistent with earlier observations in NE Mexico and the Chicxulub crater core Yaxcopoil-1. © 2007 Elsevier B.V. All rights reserved.
553.
Fluid infiltration associated with seismic faulting: Examining chemical and mineralogical compositions of fault rocks from the active Chelungpu fault
Tectonophysics,
443
(3-4)
243-254
2007
ISSN: 00401951Keywords:▾
active fault; chemical composition; Chi-Chi earthquake 1999; core analysis; fault zone; mineralogy, Asia; Eurasia; Far East; Taiwan
Abstract: ▾ In order to understand the fault zone architecture and mechanisms that caused the Chi-Chi earthquake, the Chelungpu drilling project was conducted during April 2000 through a collaborative project between Japan and Taiwan. In this study, chemical and mineralogical variations within the overall Chelungpu fault zone, including variations between less damaged host rocks, damaged zones, and fault cores caused by the Chi-Chi earthquake were examined. Slopes of TiO2 immobile isocons were consistently > 1 for analyses comparing host rocks with rocks from damaged zones or with gouges from fault cores, indicating that volume loss occurred in damaged zones and the fault cores. These results strongly imply that pervasive fluid infiltration occurred within the fault zone. Volume loss within the damaged zone and fault core is interpreted to result from a two-stage process involving: (i) coseismic mechanical wearing and/or dissolution in the fault core, and (ii) fluid infiltration within the fault zone during postseismic and interseismic periods along cracks caused by seismic failure. Semi-quantitative XRD analysis indicates that the kaolinite content consistently increases from the less damaged host rocks to the damaged zone and gouges in each fault core. Mineralogic changes indicate that pervasive acidic fluid infiltration occurred within the fault zones and reacted with the feldspars or muscovite to form kaolinite. Enrichment of kaolinite and illite found in the fault zones of southern drilling site could play some role on the slipping behavior of the southern part of the Chelungpu fault. Greater volume loss in the fault core may have resulted from moderate permeability, combined with the very fine grain nature of pulverized material in the fault core, which enhanced chemical reactions including transformation of feldspars and muscovite to clay minerals. The study results indicate that pervasive fluid infiltration occurred and changed the mineralogical and chemical architecture of fault zones caused by the cyclic earthquakes. © 2007 Elsevier B.V. All rights reserved.
552.
Broadside interferometric and reverse-time imaging of the San Andreas Fault at depth
SEG Technical Program Expanded Abstracts,
26
(1)
2175-2179
2007
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
Abstract: ▾ The San Andreas Fault Observatory at Depth provides the most comprehensive set of data on the structure and dynamics of the San Andreas fault. We use two independent experiments recorded by the seismometer arrays of the SAFOD Pilot and Main Holes to resolve the localized structure of the San Andreas fault zone and of an intermediate fault zone at depth. From Pilot Hole recordings of the drilling noise coming from the Main Hole, we reconstruct the waves that propagate between the Pilot Hole sensors and use them to image the fault zone structure. The use of correlated drilling noise leads to a high-resolution image of a major transform fault zone. Another independent image is generated from the detonation of a surface explosive charge recorded at a large 178-sensor array placed in the Main Hole. The images reveal the San Andreas fault as well as an active blind fault zone that could potentially rupture. This is confirmed by two independent methods. The structure and the activity of the imaged faults is of critical importance in understanding the current stress state and activity of the San Andreas fault system. © 2007 Society of Exploration Geophysicists.
551.
Geochemistry of impactites and basement lithologies from ICDP borehole LB-07A, Bosumtwi impact structure, Ghana
Meteoritics and Planetary Science,
42
(4-5)
667 – 688
2007
ISSN: 10869379
Publisher: University of Arkansas
Abstract: ▾ In 2004, a drilling project by the International Continental Scientific Drilling Program (ICDP) at the Bosumtwi impact crater, Ghana (1.07 Myr old and 10.5 km in diameter), obtained drill core LB-07A, which sampled impactites and underlying metasediments in the crater moat surrounding the small central uplift of the structure. The LB-07A core consists of three sequences: 82.29 m of an upper impactite sequence of alternating polymict lithic and suevitic impact breccias overlying 54.88 m of so-called lower impactite of monomict impact breccia with several suevite intercalations, and 74.53 m of meta-graywacke and altered shale of the basement, also containing a number of suevite intercalations. Major- and trace-element characteristics of all three sequences have been determined to investigate breccia formation and the role of the respective basement lithologies therein. Compositions of polymict impact breccias of the crater fill revealed by core LB-07A are compared with the compositions of the Ivory Coast tektites and the fallout suevites. The impactites of the LB-07A borehole appear well homogenized with respect to the silicate component, and little change in the ranges of many major- and trace-element differences is seen along the length of the borehole (except for Fe2O3, MgO, and CaO contents). Much scatter is observed for a number of elements, and in many cases this increases with depth. It is proposed that any variability in composition is likely the function of clast population differences (i.e., also of relatively small sample sizes). No systematic compositional difference between polymict lithic and suevitic impact breccias is evident. An indication of carbonate enrichment due to hydrothermal alteration is observed in samples from all lithologies. The impactites of the borehole generally show intermediate compositions to previously defined target rocks. The fallout suevites have comparable major element abundances, except for relatively lower MgO contents. The Ivory Coast tektites are generally similar in composition to the LB-07A suevites, but broader ranges in MgO and CaO contents are observed for the LB-07A suevites. © The Meteoritical Society, 2007. Printed in USA.
550.
Nondestructive continuous physical property measurements of core samples recovered from hole B, Taiwan Chelungpu-Fault Drilling Project
Journal of Geophysical Research: Solid Earth,
112
(7)
2007
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
active fault; Chi-Chi earthquake 1999; fault zone; faulting; magnetic susceptibility, Asia; Eurasia; Far East; Taiwan
Abstract: ▾ The Taiwan Chelungpu-Fault Drilling Project was undertaken in 2002 to investigate the faulting mechanism of the 1999 Mw 7.6 Taiwan Chi-Chi earthquake. Hole B penetrated the Chelungpu fault, and core samples were recovered from between 948.42- and 1352.60-m depth. Three major zones, designated FZB1136 (fault zone at 1136-m depth in hole B), FZB1194, and FZB1243, were recognized in the core samples as active fault zones within the Chelungpu fault. Nondestructive continuous physical property measurements, conducted on all core samples, revealed that the three major fault zones were characterized by low gamma ray attenuation (GRA) densities and high magnetic susceptibilities. Extensive fracturing and cracks within the fault zones and/or loss of atoms with high atomic number, but not a measurement artifact, might have caused the low GRA densities, whereas the high magnetic susceptibility values might have resulted from the formation of magnetic minerals from paramagnetic minerals by frictional heating. Minor fault zones were characterized by low GRA densities and no change in magnetic susceptibility, and the latter may indicate that these minor zones experienced relatively low frictional heating. Magnetic susceptibility in a fault zone may be key to the determination that frictional heating occurred during an earthquake on the fault. Copyright 2007 by the American Geophysical Union.
549.
Luminescence geochronology for sediments from Lake El'gygytgyn, northeast Siberia, Russia: Constraining the timing of paleoenvironmental events for the past 200 ka
Journal of Paleolimnology,
37
(1)
77-88
2007
ISSN: 09212728Keywords:▾
crater lake; eolian deposit; geochronology; lacustrine deposit; luminescence dating; paleoenvironment; sediment core, Chukchi; Eurasia; Lake El'gygytgyn; Russian Federation
Abstract: ▾ This study focused on the luminescence dating of sediments from Lake El'gygytgyn, a meteorite impact crater 100 km north of the Arctic Circle in northeast Siberia, formed 3.58 Ma ago. The sediment is principally eolian deposited in to a lake with nearly permanently ice. The fine-grained polymineral and quartz extracts taken from nine distinct levels from the upper 12.3 m of sediment core PG1351 were dated by infrared stimulated (IRSL) and green stimulated luminescence (GSL) using multiple aliquot additive dose procedures. The veracity of these ages is evaluated by comparing to an age model for the core derived from magnetic excursions and from correlation of variations of the magnetic susceptibility record to similar magnitude variations in δ 18O in the Greenland Ice core record. The IRSL ages from the upper 9 m of core correspond well with the independent age control for the past ca. 200 ka. However, sediments deeper in the core at 12.3 m with an inferred age of ca. 250 ka age yield a saturated IRSL response and therefore a non-finite OSL age. The youngest sediment dated from 0.70 m depth yielded the IRSL age of ca. 11.5 ka, older than the corresponding age of 9.3-8.8 ka, indicating a discrepancy in dating the youngest sediments in the upper 1 m of core. This study confirms the utility of IRSL by the multiple aliquot additive dose method to date sediments <200 ka old from eastern Siberia. © 2006 Springer Science+Business Media B.V.
548.
Magnetic hysteresis parameters and Day plot analysis to characterize diagenetic alteration in gas hydrate-bearing sediments
Journal of Geophysical Research: Solid Earth,
112
(6)
2007
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
chemical alteration; detrital deposit; diagenesis; formation mechanism; gas hydrate; greigite; hysteresis; isotherm; magnetic field; magnetic property; magnetite; porosity; remanent magnetization; viscosity, Canada; Mackenzie Delta; North America; Northwest Territories
Abstract: ▾ The J meter coercivity spectrometer is a machine capable of rapid and simple measurement of magnetic hysteresis, isothermal remanence acquisition and magnetic viscosity of rocks and sediments. The J meter was used to study a suite of samples collected from strata in the gas hydrate-bearing JAPEX/JNOC/GSC Mallik 5L-38 well (69.5°N, 134.6°W) in the Mackenzie Delta of the northwestern Canadian Arctic. The Day plot of magnetic hysteresis ratios for these samples is exotic in that the points do not plot along a hyperbola as is usually observed. Rather, they plot as a scatter which is shown to contour into vertical slices using coercivity field (HC) or saturation magnetization (JS), and horizontal slices using the relative quantity of superparamagnetism (JSPM/JS). Optical microscopy reveals that the magnetic minerals are detrital magnetite and authigenic greigite. Greigite is dominant in sands which in situ had >70% gas hydrate saturation and in silts in which gas hydrate growth was blocked by insufficient porosity. We infer that the silts were the accumulation sites for solutes which had been excluded from the pore waters in neighboring coarser-grained sediments during the course of gas hydrate formation. Consequently, we conclude that magnetic properties are related to gas hydrate-related processes, and as such, may have potential as a method of remote sensing for gas hydrate deposits. Copyright 2007 by the American Geophysical Union.
547.
Measuring attenuation and velocity dispersion using vibrator sweeps
SEG Technical Program Expanded Abstracts,
26
(1)
3115-3119
2007
ISSN: 10523812
Publisher: Society of Exploration Geophysicists
Keywords:▾
Boreholes; Crystalline rocks; Dispersion (waves); Gas hydrates; Petroleum prospecting; Rocks; Seismology; Velocity; Vibrators, Heterogeneous rocks; Seismic frequencies; Seismic processing; Velocity dispersion; Waveform distortions, Seismic prospecting
ISBN:
9781604238976
Abstract: ▾ Heterogeneity of rocks, such as porosity, fractures, and fluids, causes attenuation and velocity dispersion of seismic waves, and induces waveform distortion. This distortion, once detected, offers an insight into the heterogeneous rock properties. In order to detect small velocity dispersion in the exploration seismic frequency band, a new seismic processing method has been developed for uncorrelated vibrator data. This method has been applied to the uncorrelated vibrator VSP data from Mallik gas hydrate research well, MacArthur River uranium mine area, and Outokumpu crystalline rock borehole. Different trends of Q and velocity dispersion have been detected in the above areas, which is a result of the heterogeneities in the rock volume surrounding the boreholes. © 2007 Society of Exploration Geophysicists.
546.
Mesoscopic structural observations of cores from the Chelungpu Fault System, Taiwan Chelungpu-fault Drilling Project Hole-A, Taiwan
Terrestrial, Atmospheric and Oceanic Sciences,
18
(2)
359-377
2007
ISSN: 10170839Keywords:▾
borehole; breccia; core analysis; deformation mechanism; drilling; fault gouge; fault zone; fracture zone, Asia; Eurasia; Far East; Taiwan
Abstract: ▾ Structural characteristics of fault rocks distributed within major fault zones provide basic information in understanding the physical aspects of faulting. Mesoscopic structural observations of the drilled cores from Taiwan Chelungpu-fault Drilling Project Hole-A are reported in this article to describe and reveal the distribution of fault rocks within the Chelungpu Fault System. The Chelungpu Fault System in Hole-A was encountered at a depth of between 1050 - 1250 m where deformation structures increased. Three major fault zone structures were found at approximate depths of 1111, 1153, and 1221 m. The presence of wide fault rock regions were mostly concentrated in these 3 fault zones. The fault zone at 1111 m mainly consists of a nearly brecciated fracture zone and a clayey fault gouge zone of about 1.05 m in thickness. Fault rocks from the fault zone at 1153 m are characterized by the presence of sand grains in the matrix content, consisting of a 1.1-m thick fault breccia zone and a 0.35-m thick fault gouge zone. The fault zone at 1221 m consists of fault breccia and fault gouge of 1.15 m in total thickness. These are relatively harder and darker in color than the previous 2 fault zones. Each of the 3 fault zones contains a few layers of dark colored rocks of approximately 5 - 80 mm in thickness within the fault breccia and fault gouge zones. These dark colored rocks were found distinctively within the fault rocks. However, there relation to the process of faulting is not clearly understood and shall be discussed in detail with the aid of microscopic observations.
545.
Microfacies of Deep-water Deposits and Forming Models of the Chinese Continental Scientific Drilling-SKII
Acta Geologica Sinica - English Edition,
81
(6)
1026-1032
2007
Keywords:▾
Cretaceous, deep-water deposit, model of microfacies, Songliao basin, SKII
Abstract: ▾ Abstract: Extensive transgression of lake water occurred during the Cretaceous Qingshankou Stage and the Nengjiang Stage in the Songliao basin, forming widespread deep-water deposits. Eleven types of microfacies of deep-water deposits have been recognized in the continuous core rocks from the SKII, including mudstone of still water, marlite, dolostone, oil shale, volcanic ashes, turbidite, slump sediment, tempestite, seismite, ostracoda limestone and sparry carbonate, which are divided into two types: microfacies generated due to gradually changing environments (I) and microfacies generated due to geological events (II). Type I is composed of some special fine grain sediments such as marlite, dolomite stone and oil shale as well as mudstone and Type II is composed of some sediments related to geological events, such as volcanic ashes, turbiditie, slump sediment, tempestite, seismite, ostracoda limestone. The formation of sparry carbonate may be controlled by factors related to both environments and events. Generally, mudstone sediments of still water can be regarded as background sediments, and the rest sediments are all event sediments, which have unique forming models, which may reflect controlling effects of climatics and tectonics.