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1.
Abstract The transition from impact to post‐impact rocks in the Yaxcopoil‐1 (Yax‐1) core is marked by a 2 cm‐thick clay layer characterized by dissolution features. The clay overlies a 9 cm‐thick hardground, overlying a 66 cm‐thick crossbedded unit, consisting of dolomite sandstone alternating with thin micro‐conglomerate layers with litho‐ and bioclasts and the altered remains of impact glass, now smectite. The micro‐conglomerates mark erosion surfaces. Microprobe and backscatter SEM analysis of the dolomite rhombs show an early diagenetic, complex‐zoned, idiomorphic overgrowth, with Mn‐rich zones, possibly formed by hot fluids related to cooling melt sheet in the crater. The pore spaces are filled with several generations of coelestite, barite, K‐feldpar, and sparry calcite. XRF core scanning analysis detected high Mn values in the crossbedded sediments but no anomalous enrichment of the siderophile elements Cr, Co, Fe, and Ni in the clay layer. Shocked quartz occurs in the crossbedded unit but is absent in the clay layer. The basal Paleocene marls are strongly dissolved and do not contain a basal Paleocene fauna. The presence of a hardground, the lack of siderophile elements, shocked quartz, or Ni‐rich spinels in the clay layer, and the absence of basal Paleocene biozones P0 and Pa all suggest that the top of the ejecta sequence and a significant part of the lower Paleocene is missing. Due to the high energy sedimentation infill, a hiatus at the top of the impactite is not unexpected, but there is nothing in the biostratigraphy, geochemistry, and petrology of the Yax‐1 core that can be used to argue against the synchroneity of the end‐Cretaceous mass‐extinctions and the Chicxulub crater. 相似文献
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TUCCILLO M. E.; MEZGER K.; ESSENE E. J.; VAN DER PLUIJM B. A. 《Journal of Petrology》1992,33(6):1225-1259
Textural evidence, thermobarometry, and geochronology were usedto constrain the pressure-temperature-time (PTt)history of the southern portion of the Britt domain in the CentralGneiss Belt, Ontario Grenville Province. Typical metapeliticassemblages are quartz+plagioclase+ biotite + garnet + kyanite alkali feldspar sillimanite rutile ilmenite staurolite gahnite muscovite. Metatonalitic assemblages have quartz+ plagioclase + garnet biotite + hornblende + rutile + ilmenite.Metagabbroic rocks contain plagioclase + garnet + clinopyroxene+ biotite + ilmenite hornblende rutile quartz. Notabletextural features include overgrowths of sillimanite on kyaniteand of spinel on staurolite. The spinel overgrowths can be modeledby the breakdown of staurolite via the reaction Fe-staurolite= hercynite +kyanite + quartz + H2O. The decomposition of stauroliteto her-cynite has a steep dP/dT slope and constrains the lateprograde path of a staurolite metapelite. GarnetAl2SiO5plagioclasequartz(GASP) barometry applied to metapelitic garnets that preservecalcium zoning reveals a pressure decrease from 11 to 6 kbat an assumed temperature of 700 C. Garnetplagioclaseilmeniterutilequartzand garnetclinopyroxeneplagioclasequartzbarometry is in good agreement with pressures obtained withthe GASP barometer. Geochronologic data from garnet, allanite,and monazite in metapelitic rocks give ages that fall into twogroups, 14 Ga and 1.1 Ga, suggesting the presence ofat least two metamorphic events in the area. It is most reasonableto assign the 1.4 Ga age to the high-pressure data and the 1.1Ga age to the lower-pressure data. Collectively the PTtdata indicate a complex and protracted history rather than asingle cycle of burial and uplift for this part of the GrenvilleProvince. 相似文献
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At Delft Geotechnics the technique of ground-penetrating radar is in use for the detection of buried objects such as pipes. To enable us to give our ‘measurements in the field’ a more quantitative interpretation than can be deduced from these alone, a series of experiments has been started under well-defined conditions. A cylindrical vessel containing water simulates wet soil. Mounted horizontally above the water surface is a pulsed triangular half-wave dipole which is used as a transmitting antenna (TA). It has a carrier-frequency of about 160 MHz and a pulse repetition-frequency of about 50 kHz. A movable receiving dipole (‘probe’) in the water measures the transverse, mutually orthogonal Eφ,- and Eθ-components of the pulses as a function of probe-position (r, θ, φ) and of the height h of the TA above the water surface. When these pulses are Fourier-transformed, the transverse electric fields Eφ and Eθ at 200 MHz are obtained. The resulting field patterns are compared with computational results on the basis of the theory of the continuous wave, infinitesimal electric dipole (‘point dipole’). It can be concluded that:
- 1 Far-field conditions have not fully developed at a depth of about 2.50 m, the largest value of the radius r at which field patterns were measured, although it represents a distance of about 15 wavelengths.
- 2 The attenuation constant of the tapwater used, as deduced from E-field measurements for θ= 0, 2.50 m < r < 2.75 m, is slightly less than the value measured using a network analyser and air line combination, in agreement with (1).
- 3 E φ field patterns calculated using the value of the conductivity σ corresponding to the former value of the attenuation constant agree reasonably well with the measured patterns for r≤ 2.50 m and for θ < 20° at all antenna heights considered. Calculated Eφ patterns do not agree so well with the measured patterns when h is close to zero. With increasing height the agreement inproves.
- 4 In accordance with the theory of the point-dipole, the angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases.
- 5 The normalized underwater pulse-spectra shift to lower frequencies with increasing r. This can be explained since the attenuation constant of the water rises with rising frequency.
5.
Origin of Geochemical Variability by Arc-Continent Collision in the Biru Area, Southern Sulawesi (Indonesia) 总被引:1,自引:0,他引:1
Analyses of igneous rocks from the Eocene calc-alkaline andMiocene potassic volcanic arc in southwest Sulawesi indicatethat magmas became more heterogeneous in their trace elementand PbSrNd isotopic signature following the collisionof the Buton microcontinent with the arc at 相似文献
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渤海湾老狼坨子海岸带^14C,^137Cs、^210Pb测年与现代沉积速率的加速趋势 总被引:10,自引:5,他引:10
通过对渤海湾西岸L剖面中同一关键层的碳酸钙和原生腹足类壳债权的^14C测年对比研究,确定了晚全新世的2800cal BP的层位及沉积速率0.045cm/a;8个样柱的^137Cs和^210Pbex强度、蓄积量揭示了距今约120年来堤后盐沼的平均沉积速率约为0.35cm/a,而面向开放海湾的潮坪上部则达到约2-3cm/a。研究区近1个世纪以来沉积速率的加速趋势,是渤海湾西岸泥质海岸带的特征之一。 相似文献
7.
STÉPHANE POCHAT JEAN VAN DEN DRIESSCHE VALÉRIE MOUTON FRANÇOIS GUILLOCHEAU 《Sedimentology》2005,52(4):809-825
Lacustrine environments are an excellent indicator of continental palaeoclimate. In particular, the sedimentary record of waves in lakes may be used to constrain atmospheric palaeocirculation. Wave ripples have been identified in a Permian lacustrine basin (the Salagou Formation, 260–250 Ma, Lodève Basin) located in the southern French Massif Central, part of the western European Hercynian mountain chain. Wave ripple patterns are interpreted with regards to hydrodynamics and water palaeodepth. It is shown that, in the case of the Salagou Formation, wave ripple orientations were controlled by the direction of the prevailing palaeowind. The Late Permian wind blew from between north and 20° east of north, possibly over several millions of years and certainly throughout the period of deposition of about 2000 m of strata in the Lodève Basin. Permian lacustrine sedimentation is widespread and well preserved on the Earth's surface and so wave ripple data may help constrain numerical modelling of the Earth's past climates, especially with regards to Permian times outside of desert regions. 相似文献
8.
DUGGEN S.; HOERNLE K.; VAN DEN BOGAARD P.; GARBE-SCHONBERG D. 《Journal of Petrology》2005,46(6):1155-1201
Post-collisional magmatism in the southern Iberian and northwesternAfrican continental margins contains important clues for theunderstanding of a possible causal connection between movementsin the Earth's upper mantle, the uplift of continental lithosphereand the origin of circum-Mediterranean igneous activity. Systematicgeochemical and geochronological studies (major and trace element,SrNdPb-isotope analysis and laser 40Ar/39Ar-agedating) on igneous rocks provide constraints for understandingthe post-collisional history of the southern Iberian and northwesternAfrican continental margins. Two groups of magmatic rocks canbe distinguished: (1) an Upper Miocene to Lower Pliocene (8·24·8Ma), SiK-rich group including high-K (calc-alkaline)and shoshonitic series rocks; (2) an Upper Miocene to Pleistocene(6·30·65 Ma), Si-poor, Na-rich group includingbasanites and alkali basalts to hawaiites and tephrites. Maficsamples from the SiK-rich group generally show geochemicalaffinities with volcanic rocks from active subduction zones(e.g. IzuBonin and Aeolian island arcs), whereas maficsamples from the Si-poor, Na-rich group are geochemically similarto lavas found in intraplate volcanic settings derived fromsub-lithospheric mantle sources (e.g. Canary Islands). The transitionfrom Si-rich (subduction-related) to Si-poor (intraplate-type)magmatism between 6·3 Ma (first alkali basalt) and 4·8Ma (latest shoshonite) can be observed both on a regional scaleand in individual volcanic systems. SiK-rich and Si-poorigneous rocks from the continental margins of southern Iberiaand northwestern Africa are, respectively, proposed to havebeen derived from metasomatized subcontinental lithosphere andsub-lithospheric mantle that was contaminated with plume material.A three-dimensional geodynamic model for the westernmost Mediterraneanis presented in which subduction of oceanic lithosphere is inferredto have caused continental-edge delamination of subcontinentallithosphere associated with upwelling of plume-contaminatedsub-lithospheric mantle and lithospheric uplift. This processmay operate worldwide in areas where subduction-related andintraplate-type magmatism are spatially and temporally associated. KEY WORDS: post-collisional magmatism; Mediterranean-style back-arc basins; subduction; delamination; uplift of marine gateways 相似文献
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