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1.
TIMOTHY G. FISHER 《Boreas: An International Journal of Quaternary Research》2004,33(4):349-358
Boulders resting on meltwater-sculpted and striated-granite bedrock near the head of the southern outlet spillway of glacial Lake Agassiz are used to generate paleodischarge calculations. The rounded nature of many boulders suggests fluvial transport and a corestone origin. The distribution of boulders in clusters and linear trains records the interaction of clasts during transport and deposition. The geomorphology of the spillway with streamlined erosional remnant hills is characteristic of other large flood spillways. Using the Manning equation and a variety of empirical equations to determine paleovelocity, preferred discharges between 0.364 and 0.102 Sv are calculated. These discharges agree well with flood discharges using modeling methodologies, and most likely represent ephemeral and catastrophic flood events linked to either episodic incision at the outlet or the result of rapid inputs of meltwater to Lake Agassiz. 相似文献
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The 1984 PROTEA expedition, leg 5, to the central SouthwestIndian Ridge recovered basaltic lavas from fracture zones andridge segments between 25?E and 48?E. In terms of petrographyand major element variations the samples are unremarkable forocean ridge basalts and range from aphyric to highly plagioclasephyric and from primitive (mg-number = 70) to moderately evolved(mg-number = 40) in composition. Multiply saturated (i.e., olivine,plagioclase, and clinopyroxene) basalts are common within thisregion. There is no systematic difference in compositional characteristicsbetween basalts dredged from fracture zone walls and those dredgedfrom ridge segments, and fractional crystallization has playedan important role in controlling the overall range in lava compositionin both tectonic environments. Incompatible element abundance ratios in the basalts are morenotable and distinguish between geochemically depleted (N-type)MORB with high Zr/Nb (1668) and Y/Nb (4?723) ratios and low(La/Sm)m, ratios (0-?76–1?00), and geochemically enriched(E-type) MORB with low Zr/Nb (3?4–15?8) and Y/Nb (0?5–8?8)and high (La/Sm). ratios (1?07–3?8). N-type MORB appearsto be absent in the immediate vicinity of Marion Island, butoccurs further along the ridge to the northeast and southwest.Geochemically enriched MORB occurs at scattered localities alongthe ridge but is particularly abundant along the section ofthe ridge closest to the Marion hotspot. In detail, two distinct varieties of E-type MORB can be recognized.The one type has incompatible element and isotopic ratios similarto, although slightly less enriched than, those characteristicof the Marion hotspot (Zr/Nb=5?8–8?6; Y/Nb=0?5–0?8;Ba/Nb=5?1–9?0). The second type can be distinguished byhaving high Ba/Nb ratios (9–22), unlike any lavas directlyassociated with the Marion hotspot, but similar to those characteristicof DUPAL ocean island basalts (OIB). A single sample from thisgroup for which there are isotopic data indicates derivationfrom an isotopically anomalous source region. A model is proposed whereby the sub-oceanic mantle below thisportion of the southwest Indian Ocean has experienced at leasttwo distinct enrichment events. The one is associated with theupwelling of the Marion mantle plume (geochemically characterizedby having low Ba/Nb ratios and normal OIB isotopic ratios).The other is associated with upwelling from a DUPAL source (characterizedby having high Ba/Nb ratio and unusual isotopic ratios) whichhas been proposed to exist beneath this portion of the southwestIndian Ocean (Hart, 1984). On the basis of Ba/Nb and Nb/U ratios,recycled oceanic lithosphere is favoured as a source for theMarion hotspot, while recycled oceanic lithosphere plus ancientpelagic sediment appears to be the most likely source for theDUPAL anomaly and the DUPAL E-type MORB in this region. 相似文献
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Propagating effects of database generalization on the viewshed 总被引：2，自引：0，他引：2
PETER FISHER 《Transactions in GIS》1996,1(2):69-81
Few studies have systematically examined the effects of different possible generalization methods on the products of GIS operations. In this paper the effects of generalizing a Digital Elevation Model (DEM) on the area which is determined to be visible (the viewshed) is examined. Among the many different methods of generalization which are possible, a limited set of operators is examined here. First, they all fit geometrically from one resolution to another, and they can be grouped into two types: regular sampling and statistical summaries. In the latter class four different operations are used: for any cell at the target resolution, elevations are determined, from the arithmetic mean, the maximum, the minimum, and the maximum difference from the mean of the cells within the kernel at the original scale. Changes in resolution of 0.5, 0.33, 0.25, and 0.2 of the original study area are studied at 100 viewing points in each of two study areas. At the original resolution a 120 × 120 pixel area was examined, giving generalizations to 60 × 60, 40 × 40, 30 × 30, and 24 × 24 pixels. The viewsheds determined over these different resolution DEMs are compared with a number of possible viewsheds derived by generalization of the viewshed over the original DEM. Of those tested, the maximum deviation from the mean within kernel emerges as the most reliable estimate of the pattern and area of the viewshed at all resolutions. The importance of this conclusion is that different generalization operators yield more or less faithful versions of the ungeneralized product (the viewshed), and it seems indicative that a similar variety in stability of the product with generalization operator will result in many, if not all, complex products of spatial analysis. 相似文献
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Consider a lamina of ore of thickness 2t whose electrical resistivity p2 is much smaller than the resistivity p1 of the surrounding host rock. The induced polarization response of such an ore body is investigated under the assumption that it arises from the variation of p2 with the frequency of measurement. Let p2l and p2h be the resistivities of the ore-body for the low and high frequencies of measurement and L a length of the order of the distance between the transmitting electrodes. A theory is developed under the assumptions that each of the quantities t/L, p2l/p1, p2h/p1, Lp2l/2tp1, and Lp2h/tp1 is small. The main conclusion is that the frequency effect parameter P is given approximately by P=cL(p2l ? p2h)/2tp1, where the constant c is independent of t, p2l, p2h, and p1. Thus for a family of similar ore bodies having differing values of t, P will be the larger the smaller t. Detailed results are given for a semi-infinite submerged dipping dyke and the two dimensional Wenner array. 相似文献
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JOHN A. FISHER CARMEN B. E. KRAPF SIMON C. LANG GARY J. NICHOLS TOBIAS H. D. PAYENBERG 《Sedimentology》2008,55(6):1915-1930
Douglas Creek terminal splay, sited on the western shoreline of Lake Eyre North, central Australia, covers a surface area of approximately 4 km2 with a down‐system length of 2·5 km from the distributary channels terminus to the splay fringe. Two distributary channels feed two sediment lobes which have amalgamated to form the terminal splay. Three primary facies associations have been identified sub‐dividing the creek terminus into distributary channel, proximal and distal splay sections. Proximal splay sediments are characterized by erosionally based, relatively thick (> 100 mm), stacked sheets of coarse to medium sand which commonly display trough and planar cross‐bedding, whereas the distal splay is characterized by thin (generally < 50 mm) massive beds of very fine sand, silt and clay. The change in splay sedimentology is interpreted as reflecting the transition from bedload‐dominated deposition to suspended load‐dominated deposition from decelerating sheetfloods as they spread out from the channel onto the dry lake bed. A proximal to distal splay transition zone is also noted where deposits of both facies associations interfinger laterally and vertically. In scale, geometry and facies associations, the Douglas Creek terminal splay is very different to the often cited Neales terminal splay complex located 70 km to the north. It is suggested that these architectural differences reflect variations in discharge volume, input sediment distribution and the degree of vegetation cover. Understanding the variation in terminal splay architecture has very significant implications for the modelling of analogous subsurface petroleum systems, which at present relies on few modern‐day analogues. 相似文献
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Antidune and chute and pool structures in the base surge deposits of the Laacher See area, Germany 总被引：5，自引：0，他引：5
Grey tuffs of late Pleistocene age form broad fans radiating from the Laacher See basin. They were derived from phreatomagmatic outbursts, and transported in turbulent pyroclastic flows, in contrast with the underlying white pumice tuffs of air fall origin. Flow origin of the grey tuffs is inferred from the well-bedded plane parallel to cross-bedded tephra characteristic of base surge deposits, and a variety of other sedimentary structures, as well as grain size distributions. We recognize a tentative sequence of five main kinds of dune structures or cross-bedded strata. With some reservations these may be compared with the high flow-regime alluvial bedforms produced experimentally in flumes. Most of the cross-bedded structures in the Laacher See deposits resemble antidunes, with steep stoss sides and very low-dipping lee sides. Upcurrent migration of antidune crests is dominant close to the source, but changes to downcurrent migration at greater distances, presumably because of decay in flow energy. The most spectacular cross-bedding is somewhat similar to chute and pool structures formed under experimental condition in alluvial flumes, but not recognized in ancient sedimentary rocks. We suggest that these structures of the Laacher See tuffs formed during deposition from phreatic pyroclastic flows of very high flow energy and high sediment concentration. The antidunes apparently formed at lesser flow velocity than chute and pool structures, although interpretation of velocity conditions by examination of the deposits is difficult because of other factors such as the cohesiveness of wet material erupted by explosive phreatic volcanic activity. The large wave lengths of the dune-like structures, however, suggest unusually high velocities. The Laacher See magmas were of phonolitic to tephritic composition, and may have erupted with greater explosive energy and in greater volume than comparable basaltic eruptions. 相似文献
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Pyrite replacement of mollusc shells from the Lower Oxford Clay (Jurassic) of England 总被引：1，自引：0，他引：1
IAN ST JOHN FISHER 《Sedimentology》1986,33(4):575-585
Replacement of originally aragonite mollusc shells by pyrite commonly occurs in the Lower Oxford Clay. Petrographic studies show the shells to have constituted complex microenvironments in the sediment. A range of replacement textures is found showing a variable amount of solution of the original aragonite. Three distinct textures were found in crushed pyrite-replaced ammonite shells from heavily pyritized concretions. (1) A texture reflecting the original shell structure due to the replacement of the organic shell-matrix by pyrite. (2) An ovoid texture seen at several stages of replacement reflecting processes occurring at discrete centres of sulphate reduction. (3) Euhedral crystals lining cracks and fractures in the shell. Three types of replacement are found in small gastropods and bivalves from shell bed, some of which may relate to those seen in the ammonites. (1) Replacement of organic shell-matrix by pyrite preserving good shell-microstructure. (2) Replacement showing outwardly good preservation of morphological features but inwardly only the gross structure, such as growth lines, is preserved. (3) Replacement of the shell in a matrix of euhedral pyrite leaving only lines of carbonate inclusions marking the margins of the shell. The replacement textures and types appear to be dependent on the initial structure of the shell and the access of iron and sulphate into the shell. Early stages of replacement appear to proceed by pyrite formation within the organic matrix of the shell, with little or no solution of the carbonate, this produces textures which faithfully mimic the original shell microstructure. It is thought that the lack of carbonate solution is due to a limited availability of iron, brought about by the less intensively reducing nature of the sediment. Later stages of replacement are promoted by the cracking and fracturing of the shell and are, generally, not as faithful to the original shell structure. This is due to the greater availability of iron as the sediment becomes more reducing with burial. 相似文献
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Distributed hydrological models are becoming increasingly complex with respect to spatial phenomena, and with the widespread availability of spatial data from GIS, this trend is likely to increase. In all such models the spatial arrangement of phenomena, such as soil properties and land-use categories is fundamental, and so the arrangement should have an influence on the model output. Testing for this influence we term spatial sensitivity analysis. Here, we report on the spatial sensitivity of two widely used models, AgNPS (agricultural non-point source pollution model) and ANSWERS (areal nonpoint source watershed environment response simulation). The input spatial data were subjected to spatially random mixing to varying degrees, such that the organized landscape became disorganized. The chemical discharge from AgNPS, and the sediment and water discharge from ANSWERS, are examined. In both cases most outputs exhibited little or no sensitivity to the spatial distribution of most input data. Only infiltration-related inputs produced large variations, but these changes were not in the sense that might have been predicted. Although the analytical methods used require further refinement, there must now be some doubt as to the validity of the models, and whether they repay their computational complexity. Furthermore, it is felt that spatial sensitivity analysis should become a fundamental part of the verification of all such models. © 1997 by John Wiley & Sons, Ltd. 相似文献
10.
TIMOTHY G. FISHER LAWRENCE D. TAYLOR HARRY M. JOL 《Boreas: An International Journal of Quaternary Research》2003,32(2):328-336
Hummocky terrain composed of boulder gravel and a wavy contact between stratified till and sand are described and explained as products of subglacial meltwater activity beneath the Saginaw Lobe of the Laurentide Ice Sheet in south-central Michigan. Exposures and geophysical investigations of hummocky terrain in a tunnel channel reveal that hummocks (˜100m diameter) are glaciofluvial bedforms with a supraglacial melt-out till or till flow veneer. The hummocky terrain is interpreted as a subglacial glaciofluvial landscape rather than one of stagnant ice processes commonly assumed for hummocky landscapes. Sandy bedforms at another site are in-phase with a wavy contact at the base of a stratified till exposed for 50m along the margin of a tunnel channel. The 0.4m thick stratified till is overlain by up to 5m of compact, pebble-rich, sandy subglacial melt-out till. The contact between the till and sand has a wave form with a 0.5m amplitude and 3-5m wavelength. Bedding within the stratified till, sandy bedforms and melt-out till are mostly in-phase with each other. Clasts from the overlying stratified till penetrate and deform the underlying sand recording recoupling of the ice to its bed. Ice ripples cut into the base of river ice have a similar morphology and are considered analogs for cavities cut into the base of the glacier and subsequently filled with sand. Subglacial meltwater activity was not coeval at each study site, indicating that subglacial meltwater played important roles in the evolution of the subglacial environment beneath the Saginaw Lobe at different times. 相似文献