排序方式： 共有14条查询结果，搜索用时 15 毫秒
1.
KROON NORRIS KLAUS IAN T. ALEXANDER LEON PAUL BARDOT CHARLES E. BARKER JEAN-PIERRE BELLIER CHARLES D. BLOME LEON J. CLARKE JOCHEN ERBACHER KRISTINA L. FAUL MARY ANNE HOLMES BRIAN T. HUBER MIRIAM E. KATZ KENNETH G. MACLEOD SANDRA MARCA FRANCISA C. MARTINEZ-RUIZ ISAO MITA MUTSUMI NAKAI JAMES G. OGG DOROTHY K. PAK THOMAS K. PLETSCH JEAN M. SELF-TRIAL NICHOLAS J. SHACKLETON JAN SMIT WILLIAM USSLERIII DAVID K. WATKINS JOEN WIDMARK & PAUL A. WILSON 《Geology Today》1998,14(6):222-226
2.
Major and trace element chemical analyses of the Plio‐Pleistocene Bardin Bluffs Formation, on the margin of a major ice‐stream of the East Antarctic Ice Sheet, yield an anomalous chemically altered sediment composition. The Bardin Bluffs Formation of the Pagodroma Group is one of the key deposits on the Antarctic continent recording glaciomarine sedimentation under open marine fjord conditions as recently as the Plio‐Pleistocene. In modern fjords occupied by outlet glaciers of ice sheets, the composition of fine‐grained terrigenous sediments approaches that of unweathered rock types exposed upstream. In the Bardin Bluffs Formation, average abundances of stable elements (Ti, Al, Zr) approach average upper crustal compositions and the element ratios are consistent with sediments with a cratonic source, implying glacial dispersal from a large shield area through the Lambert Glacier drainage system. Interestingly, the chemical index of alteration (CIA) of these sediments has values similar to those of average shales formed under conditions of chemical weathering. The sediments are particularly depleted in silicate Ca, which has been observed elsewhere in glacial muds sourced from pre‐glacial saprolites. The anomalous chemistry of the Bardin Bluffs Formation can be explained by a sequence of events, involving chemical weathering prior to glacial expansion and erosion. The presence of a remnant 1·5 m deep late Neogene weathering profile at the base of the Bardin Bluffs sequence corroborates this conclusion. Supply of large quantities of chemically weathered materials to Antarctic marginal basins requires at least partial deglaciation of the continent and was previously regarded as uncharacteristic for late Neogene Antarctica. 相似文献
3.
The snowball Earth hypothesis describes episodes of Neoproterozoic global glaciations, when ice sheets reached sea‐level, the ocean froze to great depth and biota were decimated, accompanied by a complete shutdown of the hydrological cycle. Recent studies of sedimentary successions and Earth systems modelling, however, have brought the hypothesis under considerable debate. The Squantum ‘Tillite’ (Boston Basin, USA), is one of the best constrained snowball Earth successions with respect to age and palaeogeography, and it is suitable to test the hypothesis for the Gaskiers glaciation. The approach used here was to assess the palaeoenvironmental conditions at the type locality of the Squantum Member through an analysis of sedimentary facies and weathering regime (chemical index of alteration). The stratigraphic succession with a total thickness of ca 330 m documents both glacial and non‐glacial depositional environments with a cool‐temperate glacial to temperate non‐glacial climate weathering regime. The base of the succession is composed of thin diamictites and mudstones that carry evidence of sedimentation from floating glacial ice, interbedded with inner shelf sandstones and mudstones. Thicker diamictites interbedded with thin sandstones mark the onset of gravity flow activity, followed by graded sandstones documenting channellized mass gravity flow events. An upward decrease in terrigenous supply is evident, culminating in deep‐water mudstones with a non‐glacial chemical weathering signal. Renewed terrigenous supply and iceberg sedimentation is evident at the top of the succession, beyond which exposure is lost. The glacially influenced sedimentary facies at Squantum Head are more consistent with meltwater dominated alpine glaciation or small local ice caps. The chemical index of alteration values of 61 to 75 for the non‐volcanic rocks requires significant exposure of land surfaces to allow chemical weathering. Therefore, extreme snowball Earth conditions with a complete shutdown of the hydrological cycle do not seem to apply to the Gaskiers glaciation. 相似文献
4.
McLAREN SANDRA; SANDIFORD MIKE; POWELL ROGER; NEUMANN NARELLE; WOODHEAD JON 《Journal of Petrology》2006,47(12):2281-2302
The effect of radiogenic heat production within the crust onthermal processes such as crustal anatexis is generally disregardedas bulk geochemical models suggest that crustal heat generationrates are too low to effect significant heating. However, theMount Painter Province in northern South Australia is characterizedby a total crustal contribution to surface heat flow of morethan twice the global average. The province is composed dominantlyof Proterozoic granites and granite gneisses with an area averageheat production of 16·1 µW/m3; individual lithologieshave heat production >60 µW/m3. These Proterozoic rocksare intruded by the British Empire Granite, a younger intrusivewhose origin has remained enigmatic. Isotope geochemistry suggestscrustal sources for the melt and it has a crystallization ageof 440–450 Ma, which places the setting >750 km inboardof the nearest active plate boundary zone at this time. Phaseequilibria calculations suggest that temperatures of at least720–750°C are required to produce the granite butthe intensity of crustal thickening during Palaeozoic deformation(12%) cannot account for these conditions. Here we describea model for the generation of the British Empire Granite inwhich the primary thermal perturbation for mid-crustal anatexiswas provided by the burial of the high heat-producing MountPainter basement rocks beneath the known thickness of Neoproterozoiccover sediments. The high heat-producing rocks at Mount Painterimply that the natural range and variability of crustal heatproduction is much greater than previously believed, with importantconsequences for our understanding of temperature-dependentcrustal processes including the exploitation of geothermal energyresources. KEY WORDS: geothermal energy; low-pressure anatexis; thermal conductivity; thermal regime 相似文献
5.
Climate and landscape during Heinrich Event 3 in south‐western Europe: the small‐vertebrate association from Galls Carboners cave (Mont‐ral,Tarragona, north‐eastern Iberia) 
下载免费PDF全文
下载免费PDF全文 6.
7.
8.
9.
SANDRA PASSCHIER CEES LABAN CHRIS S. MESDAG KENNETH F. RIJSDIJK 《Boreas: An International Journal of Quaternary Research》2010,39(3):633-647
Passchier, S., Laban, C., Mesdag, C.S. & Rijsdijk, K.F. 2010: Subglacial bed conditions during Late Pleistocene glaciations and their impact on ice dynamics in the southern North Sea. Boreas, Vol. 39, pp. 633–647. 10.1111/j.1502‐3885.2009.00138.x. ISSN 0300‐9483. Changes in subglacial bed conditions through multiple glaciations and their effect on ice dynamics are addressed through an analysis of glacigenic sequences in the Upper Pleistocene stratigraphy of the southern North Sea basin. During Elsterian (MIS 12) ice growth, till deposition was subdued when ice became stagnant over a permeable substrate of fluvial sediments, and meltwater infiltrated into the bed. Headward erosion during glacial retreat produced a dense network of glacial valleys up to several hundreds of metres deep. A Saalian (MIS 6) glacial advance phase resulted in the deposition of a sheet of stiff sandy tills and terminal moraines. Meltwater was at least partially evacuated through the till layer, resulting in the development of a rigid bed. During the later part of the Saalian glaciation, ice‐stream inception can be related to the development of a glacial lake to the north and west of the study area. The presence of meltwater channels incised into the floors of glacial troughs is indicative of high subglacial water pressures, which may have played a role in the onset of ice streaming. We speculate that streaming ice flow in the later part of the Saalian glaciation caused the relatively early deglaciation, as recorded in the Amsterdam Terminal borehole. These results suggest that changing subglacial bed conditions through glacial cycles could have a strong impact on ice dynamics and require consideration in ice‐sheet reconstructions. 相似文献
10.
Mesoproterozoic metaplutonic rocks in northern Cape Breton Island,Nova Scotia, occur in a tectonic inlier within the Appalachianorogen. Although they have been multiply metamorphosed and variablydeformed, the petrology and geochemistry of these rocks provideinsight into the tectonomagmatic evolution of easternmost Laurentia.Anorthosite, syenite, and granitoid plutons (1100–980Ma) intruded the Sailor Brook and Polletts Cove River gneisses.New Nd isotopic data are presented from a biotite-rich partof the Sailor Brook gneiss ( 相似文献