All ICDP Publications with Abstracts
From parent-sysfolder "Publications" + 2 folder-levels deep
944.
Impact of lake level change on deep-water renewal and oxic conditions in deep saline Lake Van, Turkey
Water Resources Research,
46
(11)
2010
ISSN: 00431397Keywords:▾
Conductivity temperature depths; Deep-water renewals; Deepwater; Eastern Turkey; Freshwater inflow; Hydrological regime; Lake level fluctuations; Lake levels; Long lasting; Mixing conditions; Oxic conditions; Saline lake; Transient tracers; Winter cooling, Biochemical oxygen demand; Dissolved oxygen; Light transmission; Neon; Oxygenation; Oxygenators; Pumps; Saline water; Sulfur; Sulfur hexafluoride, Lakes, deep water; dissolved oxygen; freshwater input; hydrological regime; inflow; lake level; oxic conditions; oxygenation; saline lake; salinity; surface water; tracer, Lake Van; Turkey
Abstract: ▾ Changes in the hydrological regime of the saline closed basin Lake Van, a large, deep lake in eastern Turkey, resulted in a lake level increase by about 2 m between 1988 and 1995, followed by a 1.5 m decrease until 2003 and a relatively constant lake level thereafter. Based on measurements of transient tracers (sulfur hexafluoride, CFC-12, 3H, 3He, 4He, Ne), dissolved oxygen, light transmission, conductivity- temperature-depth profiles, and thermistor data, we investigate the implications associated with lake level fluctuations for deep-water renewal and oxygenation. Our data suggest that deep-water renewal was significantly reduced in Lake Van between 1990 and 2005. This change in mixing conditions resulted in the formation of a more than 100 m thick anoxic deep-water body below 325 m depth. Apparently, the freshwater inflows responsible for the lake level rise between 1988 and 1995 decreased the salinity of the surface water sufficiently that the generation of density plumes during winter cooling was substantially reduced compared to that in the years before the lake level rise. Significant renewal and oxygenation of the deep water did not occur until at least 2005, although by 2003 the lake level was back to almost the same level as in 1988. This study suggests that short-term changes in the hydrological regime, resulting in lake level changes of a couple of meters, can lead to significant and long-lasting changes in deep-water renewal and oxic conditions in deep saline lakes. Copyright © 2010 by the American Geophysical Union.
943.
Geoelectrical Investigations in the Cheb Basin/W-Bohemia: An Approach to Evaluate the Near-Surface Conductivity Structure
Studia Geophysica et Geodaetica,
54
(3)
443 – 463
2010
ISSN: 00393169Keywords:▾
Bohemia; Cheb Basin; Czech Republic; Karlovarsky; active fault; electrical conductivity; electrical resistivity; fault zone; geodynamics; geoelectric field; mantle source; numerical model; tomography; upper crust; vertical electrical sounding
Abstract: ▾ The Cheb Basin, located in the western Eger (Ohře) Rift, is part of the European Cenozoic Rift system. Although presently non-volcanic, it is the most active area within the European Rift with signs of recent geodynamic activity like emanations of mantle derived CO2, and the repeated occurrence of swarm earthquakes, which are common features in active volcanic regions. It is assumed that the fluids, uprising in permeable channels, play a key role for the genesis of these earthquake swarms. An image of the distribution of the electrical conductivity (resp. resistivity) in the upper crust can give information about the fluid distribution since the electrical patterns reflect pathways of fluids and fluid properties like ionic content. This was the motivation to start both a regional-scale direct current (DC) geoelectrical test covering the Cheb Basin area and several local-scale near surface investigations inside the basin at the seismically active faults Počátky-Plesná Zone (PPZ) and Mariánské Lázně Fault Zone (MLF) near Nový Kostel. It was the research idea to test electrical tomography's ability to detect faults and tectonic deformation in a complex geological environment and to identify characteristic features of these faults. The more methodically justified regional field test mainly aimed at clarifying the opportunity to trace DC electrical signals in an area with known high industrial noise in a sufficient quality. A field set-up with a range of 15-20 km is necessary for an investigation depth of 4-5 km in case of DC geoelectrics. A new developed inversion strategy for sparse electrical data sets allows for a first (coarse) model of resistivity distribution. The near surface investigations in prominent fault zones of the Cheb Basin give more detailed information about the structure of fault zones, divide the sedimentary units in different resistivity zones and detect vertical displacements in the quaternary formations. © 2010 Institute of Geophysics of the ASCR, v.v.i.
942.
Evolution of ancient Lake Ohrid: A tectonic perspective
Biogeosciences,
7
(10)
3377 – 3386
2010
ISSN: 17264189Keywords:▾
Albania; Lake Ohrid; Macedonia [Southern Europe]; data set; hydrothermal activity; lake evolution; landscape; magmatism; metamorphism; Neogene; neotectonics; observational method; Paleozoic; seismic anisotropy; seismic hazard; stratigraphy; uplift
Abstract: ▾ Lake Ohrid Basin is a graben structure situated in the Dinarides at the border of the Former Yugoslavian Republic of Macedonia (FYROM) and Albania. It hosts one of the oldest lakes in Europe and is characterized by a basin and range-like geological setting together with the halfgraben basins of Korca, Erseka and Debar. The basin is surrounded by Paleozoic metamorphics in the northeast and north and Mesozoic ultramafic, carbonatic and magmatic rocks in the east, northwest, west and south. Paleocene to Pliocene units are present in the southwest. With the basin development, Neogene sediments from Pliocene to recent deposited in the lows. There are three major deformation phases: (A) NWĝ€"SE shortening from Late Cretaceous to Miocene; (B) uplift and diminishing compression during Messinian ĝ€" Pliocene; (C) vertical uplift and (N)Eĝ€"(S)W extension from Pliocene to recent led to the basin formation. Neotectonic activity of the study area concentrates on Nĝ€"S trending normal faults that bound the Ohrid Basin eastwards and westwards. Seismic activity with moderate to strong events is documented during the last 2000 yrs; the seismic hazard level is among the highest in Albania and Macedonia. Activity of the youngest faults is evidenced by earthquake data and field observations. Morphotectonic features like fault scarps, a stepped series of active normal faults, deformed paleosols, a wind gap and fault-related hydrothermal activity are preserved around Lake Ohrid and allow delineating the tectonic history. It is shown that the Lake Ohrid Basin can be characterized as a seismogenic landscape. This paper presents a tectonic history of the Lake Ohrid Basin and describes tectonic features that are preserved in the recent landscape. The analysis of morphotectonic features is used to derive the deformation history. The stratigraphy of the area is summarized and concentrates on the main units. © 2010 Author(s).
941.
Hydrothermal alteration in the Reykjanes geothermal system: Insights from Iceland deep drilling program well RN-17
Journal of Volcanology and Geothermal Research,
189
(1-2)
172-190
2010
ISSN: 03770273Keywords:▾
Albitization; Amphibole compositions; Bulk compositions; Clinopyroxenes; Deep drilling; Fe content; Ferric iron; Fluid composition; Fluid pressures; Geothermal fluids; Geothermal systems; Greenschist; Host rocks; Hyaloclastites; Hydrothermal alterations; Hydrothermal system; Icelandics; Icelands; Intrusive rocks; Loss on ignition; Meteoric waters; Pleistocene; Prehnite; Pressure and temperature; Protoliths; Sea floor; Seafloor hydrothermal systems; Sedimentary sequence, Basalt; Boiling point; Crystalline rocks; Feldspar; Garnets; Geothermal fields; Geothermal prospecting; Glacial geology; Hydrostatic pressure; Mineralogy; Minerals; Seawater; Sedimentary rocks; Silicate minerals; Textures; Trace elements; Underwater mineral resources; Well drilling, Igneous rocks, amphibole; basalt; breccia; diabase; geothermal system; host rock; hyaloclastite; hydrothermal alteration; hydrothermal system; mid-ocean ridge basalt; mineralization; Pleistocene; seafloor; solid solution; tuff; water-rock interaction, Iceland; Reykjanes Peninsula, Aves
Abstract: ▾ The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems in terms of host rock type and low water/rock alteration. The similarities make the Reykjanes system a useful proxy for seafloor vents. At some time during the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The basaltic host rock includes hyaloclastite, breccia, tuff, extrusive basalt, diabase, as well as a marine sedimentary sequence. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite, epidote and garnet, occurs at depths as shallow as 350 m; these minerals are commonly found in Icelandic geothermal systems at temperatures above 250 °C (Bird and Spieler, 2004). This requires hydrostatic pressures that exceed the present-day depth to boiling point curve, and therefore must record alteration at higher fluid pressures, perhaps as a result of Pleistocene glaciation. Major, minor, and trace element profiles of the cuttings indicate transitional MORB to OIB composition with limited metasomatic shifts in easily mobilized elements. Changes in MgO, K2O and loss on ignition indicate that metasomatism is strongly correlated with protolith properties. The textures of alteration minerals reveal alteration style to be strongly dependent on protolith as well. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusive rocks display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. Hydrothermal anorthite (An92-An98) occurs in veins in the most altered basalt cuttings and is significantly more calcic than igneous feldspar (An48-An79). Amphibole compositions change from actinolite to hornblende at depth. Hydrothermal clinopyroxene, which occurs in veins, has greater variation in Fe content and is systematically more calcic than igneous pyroxene and also lacks uralitic textures. Solid solutions of prehnite, epidote, and garnet indicate evolving equilibria with respect to aluminum and ferric iron. © 2009 Elsevier B.V. All rights reserved.
940.
Extreme coring
Geodrilling International
(162)
18-22
2010
ISSN: 09693769Abstract: ▾ The science program of the Iceland Deep Drilling Project (IDDP) requested that as much coring as possible should be done in the transition zone to supercritical and inside the supercritical zone in the depth interval 2,400-4,500m. The coring system selected is of conventional design, non-wireline with a 184.15mm OD and capable of collection of a 101.6mm diameter core using a 215.9mm OD core bit. The effective cooling is attributed to a top drive being employed that allows circulation while tripping in or out, except for the very short time when a new drill pipe connection is being made. The core barrels were made by Rok-Max Drilling Tools Ltd and the core bits were made by GeoGem Ltd, both UK companies with a good track record in making specialist coring equipment. The cored section consisted of a hyaloclastite breccia, thoroughly altered to greenschist facies mineralogy.
939.
Fault zone structure at depth from differential dispersion of seismic guided waves: Evidence for a deep waveguide on the San Andreas Fault
Geophysical Journal International,
182
(1)
343-354
2010
ISSN: 0956540XKeywords:▾
arrival time; fault zone; finite difference method; image analysis; San Andreas Fault; seismic data; seismic velocity; seismic wave; wave dispersion; wave propagation
Abstract: ▾ Seismic guided wave dispersion can be used to image fault-zone structure at seismogenic depth. A two-station differential group velocity technique previously used for surface waves was adapted to solve for local fault-zone structure between two stations. This method was extended to solve for fault-zone structure between two earthquakes using differential group arrival times at a single station. The method was tested with finite-difference synthetic data for an inhomogeneous fault, as well as with a pair of shallow earthquakes recorded in the San Andreas Fault Observatory at Depth (SAFOD) borehole station. Results from a pair of deep earthquakes recorded in the SAFOD borehole station indicate that the low-velocity waveguide of the San Andreas Fault extends to >10 km depth. The waveguide at 10-12 km depth is 120-190 m wide and the velocity contrast is >20 per cent, similar to the values in the shallow subsurface. Multiple earthquakes and receivers could be used to map fault zone structure at seismogenic depth as a function of depth and strike. © 2010 The Authors Journal compilation © 2010 RAS.
938.
Fourier transform infrared spectroscopy, a new method for rapid determination of total organic and inorganic carbon and biogenic silica concentration in lake sediments
Journal of Paleolimnology,
43
(2)
247-259
2010
ISSN: 09212728Keywords:▾
biogenic deposit; biogeochemistry; concentration (composition); correlation; FTIR spectroscopy; inorganic carbon; lacustrine deposit; paleolimnology; quantitative analysis; sediment chemistry; silica; total organic carbon, Greece; Macedonia [Greece]; Siberia; Sweden
Abstract: ▾ We demonstrate the use of Fourier transform infrared spectroscopy (FTIRS) to make quantitative measures of total organic carbon (TOC), total inorganic carbon (TIC) and biogenic silica (BSi)concentrations in sediment. FTIRS is a fast and cost-effective technique and only small sediment samples are needed (0.01 g). Statistically significant models were developed using sediment samples from northern Sweden and were applied to sediment records from Sweden, northeast Siberia and Macedonia. The correlation between FTIRS-inferred values and amounts of biogeochemical constituents assessed conventionally varied between r = 0.84-0.99 for TOC, r = 0.85-0.99 for TIC, and r = 0.68-0.94 for BSi. Because FTIR spectra contain information on a large number of both inorganic and organic components, there is great potential for FTIRS to become an important tool in paleolimnology. © Springer Science+Business Media B.V. 2009.
937.
Erratum: Constraints on the stress state of the San Andreas Fault with analysis based on core and cuttings from San Andreas Fault Observatory at Depth (SAFOD) drilling phases 1 and 2 (Journal of Geophysical Research B: Solid Earth)
Journal of Geophysical Research: Solid Earth,
115
(3)
2010
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
936.
Geochemistry of basement rocks and impact breccias from the central uplift of the Bosumtwi crater, Ghana - Comparison of proximal and distal impactites
Special Paper of the Geological Society of America,
465443 – 469
2010
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Ashanti; Ghana; Lake Bosumtwi; Chemical analysis; Core drilling; Fallout; Infill drilling; Magnesia; Rocks; Sodium compounds; Trace elements; Bulk chemical composition; Chemical compositions; Continental scientific drillings; Different proportions; Drilling projects; Impact structures; In compositions; Major and trace elements; basement rock; breccia; chemical composition; crater; geochemistry; impact structure; impactite; suevite; trace element; Meteor impacts
Abstract: ▾ Crater-fill impact breccia and basement rock samples from the 1.07 Ma Bosumtwi impact structure (Ghana) were recovered for the first time in 2004 during an International Continental Scientific Drilling Program (ICDP)-sponsored drilling project. Here, we present detailed results of major-and trace-element analyses of 119 samples from drill core LB-08A, together with the chemical compositions of melt particles from suevite. The meta-graywacke and phyllite/slate crater basement rocks can be easily distinguished from each other on the basis of their bulk chemical compositions. A comparison of the chemical compositions of crater-fill and fallout suevites, as well as between proximal and distal impactites, reveals that LB-08A suevites have higher MgO, CaO, and Na 2 O contents than fallout suevites and, similarly, that the CaO and Na 2 O contents are higher by a factor of approximately two in LB-08A suevites than in Ivory Coast tektites. Noticeable differences occur in Cr, Co, and Ni contents between the different impactites; higher abundances are observed for these elements in distal impactites. The observed differences in composition in the various impactites mainly reflect mixing of different proportions of the original target lithologies, as can be seen in the differences in the clast populations between crater-fill and fallout suevites. However, the original impactite compositions may have also been modifi ed by postimpact alteration, particularly in the proximal impactites. Melt particles in suevite show signifi cant differences in major-element compositions between the different samples investigated, but also within a given sample, indicating that they represent melts derived from different lithologies.© 2010 The Geological Society of America. All rights reserved.
935.
Geology: A trip to dinosaur time
Nature,
467
(7312)
150-151
2010
ISSN: 00280836DOI:10.1038/467150a
Keywords:▾
environmental temperature; geological time; geology; greenhouse; note; paleoclimatology; Paleogene; priority journal; science; sediment; volcano, Dinosauria
934.
Geophysical properties within the San Andreas Fault Zone at the San Andreas Fault Observatory at Depth and their relationships to rock properties and fault zone structure
Journal of Geophysical Research: Solid Earth,
115
(12)
2010
ISSN: 21699313
Publisher: Blackwell Publishing Ltd
Keywords:▾
borehole logging; data inversion; deformation; fault zone; foliation; geophysical method; porosity; rock property; San Andreas Fault; sandstone; shale; siltstone
Abstract: ▾ We examine the relationships between borehole geophysical data and physical properties of fault-related rocks within the San Andreas Fault as determined from data from the San Andreas Fault Observatory at Depth borehole. Geophysical logs, cuttings data, and drilling data from the region 3-to 4-km measured depth of the borehole encompass the active part of the San Andreas Fault. The fault zone lies in a sequence of deformed sandstones, siltstone, shale, serpentinite-bearing block-in-matrix rocks, and sheared phyllitic siltstone. The borehole geophysical logs reveal the presence of a low-velocity zone from 3190 to 3410 m measured depth with Vp and Vs values 10-30% lower than the surrounding rocks and a 1-2 m thick zone of active shearing at 3301-3303 m measured depth. Seven low-velocity excursions with increased porosity, decreased density, and mud-gas kick signatures are present in the fault zone. Geologic data on grain-scale deformation and alteration are compared to borehole data and reveal weak correlations and inverse relationships to the geophysical data. In places, Vp and Vs increase with grain-scale deformation and alteration and decrease with porosity in the fault zone. The low-velocity zone is associated with a significant lithologic and structural transition to low-velocity rocks, dominated by phyllosilicates and penetratively foliated, sheared rocks. The zone of active shearing and the regions of low sonic velocity appear to be associated with clay-rich rocks that exhibit fine-scale foliation and higher porosities that may be a consequence of the fault-related shearing of foliated and fine-grained sedimentary rocks. Copyright 2010 by the American Geophysical Union.
933.
High-precision orientation of three-component magnetic downhole logs
Scientific Drilling
(9)
37 – 40
2010
ISSN: 18163459932.
Hydrogeology of the south-eastern Yucatan Peninsula: New insights from water level measurements, geochemistry, geophysics and remote sensing
Journal of Hydrology,
389
(1-2)
1-17
2010
Abstract: ▾ The Yucatan Peninsula is one of the world's largest karstic aquifer systems. It is the sole freshwater source for human users and ecosystems. The region hosts internationally important groundwater-dependent ecosystems (GDEs) in the 5280km2 Sian Ka'an Biosphere Reserve. The GDEs are threatened by increasing groundwater abstractions and risks of pollution. Hydrogeological exploration work is needed as basis for sound groundwater management. A multidisciplinary approach was used to study this data-scarce region. Geochemical data and phreatic surface measurements showed distinct hydrogeological units in the groundwater catchment of Sian Ka'an. The hilly southwestern areas had a low hydraulic permeability, likely caused by a geology containing gypsum, whereas the transition zone and flat areas in the east and north had a high permeability. In the latter areas, the fresh groundwater could be described by a Dupuit-Ghyben-Herzberg lens. Geophysical borehole logging and time-domain electromagnetic soundings identified a shallow, low-resistive and high-gamma-radiation layer present throughout the hilly area and transition zone. Its thickness was 3-8m, apparent conductivity was 200-800mS/m and natural gamma-radiation about 80 counts pr. second. The layer is proposed to be ejecta from the Chicxulub impact (Cretaceous/Paleogene boundary). Spatial estimates of recharge were calculated from MODIS imagery using the 'triangle method'. Average recharge constituted 17% of mean annual precipitation in the study area. Recharge was greatest in the hilly area and towards Valladolid. Near the coast, average actual evapotranspiration exceeded annual precipitation. The multidisciplinary approach used in this study is applicable to other catchment-scale studies. © 2010 Elsevier B.V.
931.
Hydrological and solute budgets of Lake Qinghai, the largest lake on the Tibetan Plateau
Quaternary International,
218
(1-2)
151 – 156
2010
ISSN: 10406182Keywords:▾
China; Qinghai; Qinghai Lake; carbon cycle; carbonate; paleohydrology; residence time; solute; water budget; water chemistry; water level; weathering; wet deposition
Abstract: ▾ Water level and chemistry of Lake Qinghai are sensitive to climate changes and are important for paleoclimatic implications. An accurate understanding of hydrological and chemical budgets is crucial for quantifying geochemical proxies and carbon cycle. Published results of water budget are firstly reviewed in this paper. Chemical budget and residence time of major dissolved constituents in the lake are estimated using reliable water budget and newly obtained data for seasonal water chemistry. The results indicate that carbonate weathering is the most important riverine process, resulting in dominance of Ca2+ and DIC for river waters and groundwater. Groundwater contribution to major dissolved constituents is relatively small (4.2 ± 0.5%). Wet atmospheric deposition contributes annually 7.4-44.0% soluble flux to the lake, resulting from eolian dust throughout the seasons. Estimates of chemical budget further suggest that (1) the Buha-type water dominates the chemical components of the lake water, (2) Na+, Cl-, Mg2+, and K+ in lake water are enriched owing to their conservative behaviors, and (3) precipitation of authigenic carbonates (low-Mg calcite, aragonite, and dolomite) transits quickly dissolved Ca2+ into the bottom sediments of the lake, resulting in very low Ca2+ in the lake water. Therefore, authigenic carbonates in the sediments hold potential information on the relative contribution of different solute inputs to the lake and the lake chemistry in the past. © 2009 Elsevier Ltd and INQUA.
930.
Paleontological interpretations of crater processes and infilling of synimpact sediments from the Chesapeake Bay impact structure
Special Paper of the Geological Society of America,
458633-654
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Gravitational effects; Infill drilling; Mica; Stars, Biostratigraphic analysis; Calcareous nannofossil; Calcareous nannofossils; Chesapeake bay impact structures; Continental scientific drillings; Microfossil assemblages; U.s. geological surveys; Unconsolidated sediment, Sediments, biostratigraphy; breccia; crater; diamicton; dinoflagellate cyst; fossil assemblage; infill; microfossil; micropaleontology; Paleogene; palynomorph; pollen; research program; sedimentation; turbulent flow, Chesapeake Bay; United States, Dinophyceae
Abstract: ▾ Biostratigraphic analysis of sedimentary breccias and diamictons in the Chesapeake Bay impact structure provides information regarding the timing and processes of late-stage gravitational crater collapse and ocean resurge. Studies of calcareous nannofossil and palynomorph assemblages in the International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville A and B cores show the mixed-age, mixed-preservation microfossil assemblages that are typical of deposits from the upper part of the Chesapeake Bay impact structure. Sparse, poorly preserved, possibly thermally altered pollen is present within a gravelly sand interval below the granite slab at 1392 m in Eyreville core B, an interval that is otherwise barren of calcareous nannofossils and dinocysts. Gravitational collapse of watersaturated sediments from the transient crater wall resulted in the deposition of sediment clasts primarily derived from the nonmarine Cretaceous Potomac Formation. Collapse occurred before the arrival of resurge. Low pollen Thermal Alteration Index (TAI) values suggest that these sediments were not thermally altered by contact with the melt sheet. The arrival of resurge sedimentation is identified based on the presence of diamicton zones and stringers rich in glauconite and marine microfossils at 866.7 m. This horizon can be traced across the crater and can be used to identify gravitational collapse versus ocean-resurge sedimentation. Glauconitic quartz sand diamicton dominates the sediments above 618.2 m. Calcareous nannofossil and dinoflagellate data from this interval suggest that the earliest arriving resurge from the west contained little or no Cretaceous marine input, but later resurge pulses mined Cretaceous sediments east of the Watkins core in the annular trough. Additionally, the increased distance traveled by resurge to the central crater in turbulent flow conditions resulted in the disaggregation of Paleogene unconsolidated sediments. As a result, intact Paleogene clasts in Eyreville cores are rare, but clasts of semilithified Potomac Formation silts and clays are common. © 2009 The Geological Society of America.
929.
Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville B core, Chesapeake Bay impact structure, USA
Special Paper of the Geological Society of America,
458255-275
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Biotite; Buildings; Calcite; Feldspar; Granite; Quartz; Sedimentary rocks; Silicate minerals, Amphibolite facies; Biotite granite; Chesapeake bay impact structures; Crystalline basement; Metamorphic grade; Metasedimentary rocks; Mineral chemistry; Silicate rocks, Mica, amphibolite; crater; geochemistry; granite; impact structure; impactite; metasedimentary rock; petrography; schist, Chesapeake Bay; United States
Abstract: ▾ The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ± fibrolite ± garnet ± tourmaline ± pyrite ± rutile ± pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite- K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase- quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ± biotite ± garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ± muscovite ± pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ± epidote ± amphibole. The lower basement-derived section and both megablocks exhibit similar middleto upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafi c source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites. © 2009 The Geological Society of America.
928.
Petrographic and geochemical comparisons between the lower crystalline basement-derived section and the granite megablock and amphibolite megablock of the Eyreville B core, Chesapeake Bay impact structure, USA
Special Paper of the Geological Society of America,
458255-275
2009
Abstract: ▾ The Eyreville B core from the Chesapeake Bay impact structure, Virginia, USA, contains a lower basement-derived section (1551.19 m to 1766.32 m deep) and two megablocks of dominantly (1) amphibolite (1376.38 m to 1389.35 m deep) and (2) granite (1095.74 m to 1371.11 m deep), which are separated by an impactite succession. Metasedimentary rocks (muscovite-quartz-plagioclase-biotite-graphite ± fibrolite ± garnet ± tourmaline ± pyrite ± rutile ± pyrrhotite mica schist, hornblende-plagioclase-epidote-biotite- K-feldspar-quartz-titanite-calcite amphibolite, and vesuvianite-plagioclase- quartz-epidote calc-silicate rock) are dominant in the upper part of the lower basement-derived section, and they are intruded by pegmatitic to coarse-grained granite (K-feldspar-plagioclase-quartz-muscovite ± biotite ± garnet) that increases in volume proportion downward. The granite megablock contains both gneissic and weakly or nonfoliated biotite granite varieties (K-feldspar-quartz-plagioclase-biotite ± muscovite ± pyrite), with small schist xenoliths consisting of biotite-plagioclase-quartz ± epidote ± amphibole. The lower basement-derived section and both megablocks exhibit similar middleto upper-amphibolite-facies metamorphic grades that suggest they might represent parts of a single terrane. However, the mica schists in the lower basement-derived sequence and in the megablock xenoliths show differences in both mineralogy and whole-rock chemistry that suggest a more mafi c source for the xenoliths. Similarly, the mineralogy of the amphibolite in the lower basement-derived section and its association with calc-silicate rock suggest a sedimentary protolith, whereas the bulk-rock and mineral chemistry of the megablock amphibolite indicate an igneous protolith. The lower basement-derived granite also shows bulk chemical and mineralogical differences from the megablock gneissic and biotite granites. © 2009 The Geological Society of America.
927.
Petrographic and shock metamorphic studies of the impact breccia section (1397-1551 m depth) of the Eyreville drill core, Chesapeake Bay impact structure, USA
Special Paper of the Geological Society of America,
458317-348
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Binary alloys; Clay alteration; Crystalline materials; Crystalline rocks; Drills; Feldspar; Igneous rocks; Infill drilling; Metamorphic rocks; Mica; Quartz; Sedimentary rocks; Sedimentology; Silicate minerals; Silicon alloys, Chesapeake bay impact structures; Continental scientific drillings; Crystalline basement; Derived materials; Hydrothermal alterations; Planar deformation; Polycrystalline quartz; U.s. geological surveys, Core drilling, breccia; crater; drilling; hydrothermal alteration; impact structure; impactite; petrography; shock metamorphism; suevite, Chesapeake Bay; United States
Abstract: ▾ The moat of the 85-km-diameter and 35.3-Ma-old Chesapeake Bay impact structure (USA) was drilled at Eyreville Farm in 2005-2006 as part of an International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) drilling project. The Eyreville drilling penetrated postimpact sediments and impactites, as well as crystalline basement-derived material, to a total depth of 1766 m. We present petrographic observations on 43 samples of suevite, impact melt rock, polymict lithic impact breccia, cataclastic gneiss, and clasts in suevite, from the impact breccia section from 1397 to 1551 m depth in the Eyreville B drill core. Suevite samples have a fine-grained clastic matrix and contain a variety of mineral and rock clasts, including sedimentary, metamorphic, and igneous lithologies. Six subunits (U1-U6, from top to bottom) are distinguished in the impact breccia section based on abundance of different clasts, melt particles, and matrix; the boundaries between the subunits are generally gradational. Sedimentary clasts are dominant in most subunits (especially in U1, but also in U3, U4, and U6). There are two melt-rich subunits (U1 and U3), and there are two melt-poor subunits with predominantly crystalline clasts (U2 and U5). The lower part (subunits U5 and U6), which has large blocks of cataclastic gneiss and rare melt particles, probably represents ground-surge material. Subunit U1 possibly represents fallback material, since it contains shard-like melt particles that were solidified before incorporation into the breccia. The melt-poor, crystalline clast-rich subunit U2 could have been formed by slumping of material, probably from the central uplift or from the margin of the transient crater. Melt particles are most abundant near the top of the impact breccia section (above 1409 m) and around 1450 m, where the suevite grades into impact melt rock. Five different types of melt particles have been recognized: (1) clear colorless to brownish glass; (2) melt altered to fine-grained phyllosilicate minerals; (3) recrystallized silica melt; (4) melt with microlites; and (5) dark-brown melt. Proportions of matrix and melt in the suevite are highly variable (̃2-67 vol% and 1-67 vol%, respectively; the remainder consists of lithic clasts). Quartz grains in suevite commonly show planar fractures (PFs) and/or planar deformation features (PDFs; 1 or 2 sets, rarely more); some PDFs are decorated. On average, ̃16 rel% of quartz grains in suevite samples are shocked (i.e., show PFs and/or PDFs). Sedimentary clasts (e.g., graywacke or sandstone) and polycrystalline quartz clasts have relatively higher proportions of shocked quartz grains, whereas quartz grains in schist and gneiss clasts rarely show shock effects. Rare feldspar grains with PDFs and mica with kink banding were observed. Ballen quartz was noted in melt-rich samples. Evidence of hydrothermal alteration, namely, the presence of smectite and secondary carbonate veins, was found especially in the lower parts of the impact breccia section. © 2009 The Geological Society of America.
926.
Paleoenvironmental recovery from the Chesapeake Bay bolide impact: The benthic foraminiferal record
Special Paper of the Geological Society of America,
458747-773
2009
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.
925.
Petrographic observations on the Exmore breccia, ICDP-USGS drilling at Eyreville, Chesapeake Bay impact structure, USA
Special Paper of the Geological Society of America,
458655-698
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Crystalline materials; Crystalline rocks; Deposition; Drills; Feldspar; Infill drilling; Mica; Rocks; Sedimentology; Sediments; Stratigraphy, Bioclastic carbonates; Chesapeake bay impact structures; Continental scientific drillings; Crystalline target rocks; Derived materials; Microscopic scale; Strong heterogeneities; U.s. geological surveys, Core drilling, bedform; breccia; clast; crater; deposition; heterogeneity; impactite; particle size; petrography; research program; sedimentary petrology; shock metamorphism, Chesapeake Bay; United States; Virginia
Abstract: ▾ The International Continental Scientific Drilling Program (ICDP)-U.S. Geological Survey (USGS) Eyreville A and B drill cores sampled crater fill in the region of the crater moat, ̃9 km to the NE of the center of the Chesapeake Bay impact structure, Virginia, USA. They provide a 953 m section (444-1397 m depth) of sedimentary clast breccia and intercalated sedimentary and crystalline megablocks knownas Exmore beds, deposited on top of the impactite sequence between 1397 and 1551 m depth. We petrographically investigated the sandy-clayey groundmass-dominated breccia, which resembles a diamictite ("Exmore breccia"), and which, in its lower parts, carries sedimentary and crystalline blocks. The entire breccia interval is characterizedby the presence of glauconite and bioclastic carbonate, which distinguishes the Exmore breccia from other sandy facies above and below in the stratigraphy. The sediment-clast breccia exhibits strong heterogeneity from sample to sample with respect to groundmass nature, e.g., clay versus sand content, as well as clast content, in general, and shocked clast content, in particular. There is a consistently signifi cantly larger macroscopic sedimentary to crystalline clast content. On the microscopic scale, the intersample sediment to crystalline clast ratios are quite variable. A very small component of shocked material, in the form of shock-deformed quartz, and to an even lesser degree feldspar, and somewhat more abundant but still relatively scarce shardshaped,altered melt particles, is present throughout the section. However, between ̃458 and 469 m, and between 514 and 527 m depths, the abundance of such melt particlesis notably enhanced. These sections are also chemically distinct and relatively more mafic than the other parts of the Exmore breccia. It appears that from the time of deposition of the 527 m material, calming of the ocean occurred over the crater area as a result of abatement of resurge activity, so that ejecta from the plume abovethe crater could accumulate within the crater area to a larger degree. Deposition ofejecta fallout from the collapsing ejecta plume was terminated by the time of deposition of the 458 m material. This raises questions about the positioning of the exact upper contact of Exmore breccia to post-Exmore sediment (Chickahominy Formation), which is currently placed at 444 m depth and which possibly should be revisedto 458 m depth. Based on a signifi cant record of granite-derived material with shocked minerals, the shocked debris component seems to be largely derived from crystalline target rocks. This provides further evidence that the basement-derived material of the basal section of the Eyreville drill cores, which is essentially unshocked, is likely of an allochthonous nature and that the drilling did not intersect the actual crater floor. 76°W. © 2009 Geological Society of America.
924.
Petroleum surface oil seeps from a Palaeoproterozoic petrified giant oilfield
Terra Nova,
21
(2)
119--126
2009
923.
Polar organic compounds in pore waters of the Chesapeake Bay impact structure, Eyreville core hole: Character of the dissolved organic carbon and comparison with drilling fluids
Special Paper of the Geological Society of America,
458891-903
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Boreholes; Dissolution; Electrospray ionization; Infill drilling; Ions; Mass spectrometry; Molecular weight; Mud logging; Organic carbon; Stream flow; Water, Chesapeake bay impact structures; Dissolved organic carbon; Electrospray ionization mass spectrometry; Inorganic salts; Low molecular weight; Polar organic compounds; Sample collection; Weight-average molecular weight, Drilling fluids, analytical method; detection method; dissolved organic carbon; drilling fluid; fractionation; impact structure; ionization; mass spectrometry; organic compound; porewater, Chesapeake Bay; United States; Virginia
Abstract: ▾ Pore waters from the Chesapeake Bay impact structure cores recovered at Eyreville Farm, Northampton County, Virginia, were analyzed to characterize the dissolved organic carbon. After squeezing or centrifuging, a small volume of pore water, 100 μL, was taken for analysis by electrospray ionization-mass spectrometry. Porewater samples were analyzed directly without filtration or fractionation, in positive and negative mode, for polar organic compounds. Spectra in both modes were dominated by low-molecular-weight ions. Negative mode had clusters of ions differing by -60 daltons, possibly due to increasing concentrations of inorganic salts. The numberaverage molecular weight and weight-average molecular weight values for the pore waters from the Chesapeake Bay impact structure are higher than those reported for other aquatic sources of natural dissolved organic carbon as determined by electrospray ionization-mass spectrometry. In order to address the question of whether drilling mud fluids may have contaminated the pore waters during sample collection, spectra from the pore waters were compared to spectra from drilling mud fluids. Ions indicative of drilling mud fluids were not found in spectra from the pore waters, indicating there was no detectable contamination, and highlighting the usefulness of this analytical technique for detecting potential contamination during sample collection. © 2009 The Geological Society of America.
922.
Petrology of impact melt rocks from the Chesapeake Bay crater, USA
Special Paper of the Geological Society of America,
458377-396
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Feldspar; Heat transfer; Infill drilling; Petrology; Silica; Silicate minerals; Structural geology; Zeolites; Zircon, Chemical compositions; Chesapeake Bay; Chesapeake bay impact structures; Conductive heat transfer; Crystalline basement; Emplacement conditions; Mixing calculations; Titanomagnetites, Core drilling, crater; crystallization; impact structure; marine sediment; petrogenesis; shock metamorphism, Chesapeake Bay; United States
Abstract: ▾ The Eyreville B drill core in the inner annular moat of the 85-km-diameter Chesapeake Bay impact structure recovered the first coherent impact melt volumes from within the crater as two bodies, 1 and 5.5 m thick. This study focuses on the petrogenesis of these well-preserved rocks. Mixing calculations reveal that the chemical composition of these melts can be modeled as a hybrid of ̃40% sedimentary target and ̃60% crystalline basement component. The melt rocks contain abundant lithic and mineral clasts that display all stages of shock metamorphism. Zircon clasts record the cooling of the melt from temperatures above 1700 °C to below 1200 °C within the first minutes after formation. Glassy melt with a peraluminous, rhyolitic composition that contains ̃5 wt% water is preserved. This melt records a crystallization sequence of aluminum-rich orthopyroxene and hercynitic spinel, followed by plagioclase, titanomagnetite and cordierite, and late sanidine. Spherulitic aluminosilicate-SiO 2 -cordierite aggregates that are comparable to buchites at temperatures below ̃1465 °C complement this assemblage. Lack of hyaloclastic fragmentation suggests dry emplacement conditions. Complete cooling by conductive heat transfer took ̃7 weeks and ̃4 years for the 1-m- and the 5.5-m-thick melt bodies, respectively. Alteration stages below ̃100 °C produced smectite, phillipsite, chalcedony, and a rare zeolite phase that is tentatively identified as terranovaite. © 2009 The Geological Society of America.
921.
Petrology of impact melt rocks from the Chesapeake Bay crater, USA
Special Paper of the Geological Society of America,
458377-396
2009
Abstract: ▾ The Eyreville B drill core in the inner annular moat of the 85-km-diameter Chesapeake Bay impact structure recovered the first coherent impact melt volumes from within the crater as two bodies, 1 and 5.5 m thick. This study focuses on the petrogenesis of these well-preserved rocks. Mixing calculations reveal that the chemical composition of these melts can be modeled as a hybrid of ̃40% sedimentary target and ̃60% crystalline basement component. The melt rocks contain abundant lithic and mineral clasts that display all stages of shock metamorphism. Zircon clasts record the cooling of the melt from temperatures above 1700 °C to below 1200 °C within the first minutes after formation. Glassy melt with a peraluminous, rhyolitic composition that contains ̃5 wt% water is preserved. This melt records a crystallization sequence of aluminum-rich orthopyroxene and hercynitic spinel, followed by plagioclase, titanomagnetite and cordierite, and late sanidine. Spherulitic aluminosilicate-SiO 2 -cordierite aggregates that are comparable to buchites at temperatures below ̃1465 °C complement this assemblage. Lack of hyaloclastic fragmentation suggests dry emplacement conditions. Complete cooling by conductive heat transfer took ̃7 weeks and ̃4 years for the 1-m- and the 5.5-m-thick melt bodies, respectively. Alteration stages below ̃100 °C produced smectite, phillipsite, chalcedony, and a rare zeolite phase that is tentatively identified as terranovaite. © 2009 The Geological Society of America.
920.
Physical property data from the ICDP-USGS Eyreville cores A and B, Chesapeake Bay impact structure, Virginia, USA, acquired using a multisensor core logger
Special Paper of the Geological Society of America,
458165-179
2009
ISSN: 00721077
Publisher: Geological Society of America
Keywords:▾
Boreholes; Infill drilling; Magnetic susceptibility; Seismic waves; Thermal logging; Wave propagation, Chesapeake bay impact structures; Continental scientific drillings; Measured properties; P-wave velocity; Spatial variations; Temperature log; Thermal history; U.s. geological surveys, Structural properties, crater; deformation; drilling; geophysical method; impact structure; magnetic susceptibility; P-wave; physical property, Chesapeake Bay; United States
Abstract: ▾ The International Continental Scientific Drilling Program (ICDP) and the U.S. Geological Survey (USGS) drilled three core holes to a composite depth of 1766 m within the moat of the Chesapeake Bay impact structure. Core recovery rates from the drilling were high (̃90%), but problems with core hole collapse limited the geophysical downhole logging to natural-gamma and temperature logs. To supplement the downhole logs, ̃5% of the Chesapeake Bay impact structure cores was processed through the USGS GeoTek multisensor core logger (MSCL) located in Menlo Park, California. The measured physical properties included core thickness (cm), density (g cm -3 ), P-wave velocity (m s -1 ), P-wave amplitude (%), magnetic susceptibility (cgs), and resistivity (ohm-m). Fractional porosity was a secondary calculated property. The MSCL data-sampling interval for all core sections was 1 cm longitudinally. Photos of each MSCL sampled core section were imbedded with the physical property data for direct comparison. These data have been used in seismic, geologic, thermal history, magnetic, and gravity models of the Chesapeake Bay impact structure. Each physical property curve has a unique signature when viewed over the full depth of the Chesapeake Bay impact structure core holes. Variations in the measured properties reflect differences in pre-impact target-rock lithologies and spatial variations in impact-related deformation during late-stage crater collapse and ocean resurge. © 2009 The Geological Society of America.