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41.
An international iron-formation reference sample is warranted as a basis for geochemical investigations of Precambrian iron-formations. A better knowledge of the geochemistry of iron formations will hopefully elucidate the long standing discussion on the genesis of iron-formations. The reference sample can furthermore be used in studies of deep-sea metalliferous deposits. 相似文献
42.
PETER H. STAUFFER 《Sedimentology》1966,7(3):261-263
Criticisms by VAN DER PLAS (1962) of thin-section point counting as a method of size analysis appear to be based on confusion between number percent and volume percent. Points are samples of volume, and point counts yield volume percents. This is as much true of size classes as of other types of constituents, though distortions are caused in the distribution by the sectioning effect. The “ribbon” method advocated by Van der Plas counts individuals rather than points and hence yields number percents. The two types of statistic are fundamentally different. 相似文献
43.
A six-channel wide-band digital system for magnetotelluric measurements is described The system is designed to operate in the frequency band 10-.0001 Hz. The recording is made on a 7 track tape in an IBM compatible 12 bit format. The amplitude and phase responses of the system and sample records are presented. 相似文献
44.
45.
DARREL A. SWIFT DAVID C. W. SANDERSON PETER W. NIENOW ROBERT G. BINGHAM IAN C. COCHRANE 《Boreas: An International Journal of Quaternary Research》2011,40(3):446-458
Swift, D. A., Sanderson, D. C. W., Nienow, P. W., Bingham, R. G. & Cochrane, I. C. 2010: Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland – a consequence of resetting at the glacier bed? Boreas, Vol. 40, pp. 446–458. 10.1111/j.1502‐3885.2010.00196.x. ISSN 0300‐9483. Luminescence has the potential to elucidate glacial geomorphic processes because primary glacial sediment sources and transport pathways are associated with contrasting degrees of exposure to light. Most notably, sediment entrained from extraglacial sources should be at least partially reset, whereas sediment produced by glacial erosion of subglacial bedrock should retain substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of this assumption at Haut Glacier d'Arolla, Switzerland using sediment sampled extraglacially and from the glacier bed. Contrary to our expectations, the subglacial samples exhibited natural signals that were substantially lower than those of other sample groups, and further (albeit limited) analyses have indicated no obvious differences in sample‐group luminescence characteristics or behaviour that could account for this observation. For glaciological reasons, we can eliminate the possibilities that the subglacial sediment has been extraglacially reset or exposed in situ to heat or light. We therefore advocate investigation of possible resetting processes related to subglacial crushing and grinding, and speculate that such processes, if more generally present, may enable the dating of subglacially deposited tills using luminescence‐based techniques. 相似文献
46.
47.
The use of Very High Frequency measurements applied to the detection of the variation in dielectric constant as a precursor to earthquakes is examined. It is concluded that such a technique is feasible and that the installation of source and sensor in separate deep boreholes provides a suitable environment for this type of study. The boreholes could also be used for a considerable range of other types of instrumentation. 相似文献
48.
49.
The extent of a large mountain icefield that existed in the western Grampians during the Loch Lomond stade has been mapped. The main types of evidence used in establishing the limits comprise moraines, thick drift, fluvioglacial landforms, erratics, ice-smoothed bedrock, striae, friction cracks and relict periglacial forms. Trimlines on 198 spurs, and various forms of glacial and periglacial evidence on 73 cols and in c . 200 cirques, enable the upper limits and morphology of the icefield to be reconstructed. Abundant striae and friction cracks in many areas enable ice-flow directions and the surface form of the icefield to be inferred in some detail. The icefield had ice-shed altitudes of c . 750 m, a maximum width of 80 km and an area over 2,000 km2 . At least 60 nunataks stood above the icefield, and on its western side outlet glaciers reached the sea and flowed for as much as 28 km along major tidal water lochs. 相似文献
50.
ANDREW J. MITCHELL DAVID ULIČNÝ GARY J. HAMPSON PETER A. ALLISON GERARD J. GORMAN MATTHEW D. PIGGOTT MARTIN R. WELLS CHRISTOPHER C. PAIN 《Sedimentology》2010,57(2):359-388
Lower to Middle Turonian deposits within the Bohemian Cretaceous Basin (Central Europe) consist of coarse‐grained deltaic sandstones passing distally into fine‐grained offshore sediments. Dune‐scale cross‐beds superimposed on delta‐front clinoforms indicate a vigorous basinal palaeocirculation capable of transporting coarse‐grained sand across the entire depth range of the clinoforms (ca 35 m). Bi‐directional, alongshore‐oriented, trough cross‐set axes, silt drapes and reactivation surfaces indicate tidal activity. However, the Bohemian Cretaceous Basin at this time was over a thousand kilometres from the shelf break and separated from the open ocean by a series of small islands. The presence of tidally‐influenced deposits in a setting where co‐oscillating tides are likely to have been damped down by seabed friction and blocked by emergent land masses is problematic. The Imperial College Ocean Model, a fully hydrodynamic, unstructured mesh finite element model, is used to test the hypothesis that tidal circulation in this isolated region was capable of generating the observed grain‐size distributions, bedform types and palaeocurrent orientations. The model is first validated for the prediction of bed shear stress magnitudes and sediment transport pathways against the present‐day North European shelf seas that surround the British Isles. The model predicts a microtidal to mesotidal regime for the Bohemian Cretaceous Basin across a range of sensitivity tests with elevated tidal ranges in local embayments. Funnelling associated with straits increases tidal current velocities, generating bed shear stresses that were capable of forming the sedimentary structures observed in the field. The model also predicts instantaneous bi‐directional currents with orientations comparable with those measured in the field. Overall, the Imperial College Ocean Model predicts a vigorous tide‐driven palaeocirculation within the Bohemian Cretaceous Basin that would indisputably have influenced sediment dispersal and facies distributions. Palaeocurrent vectors and sediment transport pathways however vary markedly in the different sensitivity tests. Accurate modelling of these parameters, in this instance, requires greater palaeogeographic certainty than can be extracted from the available rock record. 相似文献