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101.
John Cushing Marla Daily Elmer Noble V. Louise Roth Adrian Wenner 《Quaternary Research》1984,21(3):376-384
Mammoth remains on Santa Cruz Island, one of the four Northern Channel Islands of California, are very sparse, in marked contrast to those reported from Santa Rosa and San Miguel Islands of the same island group. A probable major reason for this scarcity is that Quaternary deposits are greatly restricted on Santa Cruz Island. It is proposed, contrary to popular opinion, that fossils found on Santa Cruz Island were derived from animals which died on the island, and were not transported there by humans. Reasons for this conclusion are that the size and geological context of the fossils are similar to those of the largest mammoth fossils of Santa Rosa Island, and that, in spite of extensive investigations by many persons, mammoth remains have not been found in middens, either on the islands or on the adjacent mainland. 相似文献
102.
Colin R. Nash 《Ore Geology Reviews》1986,1(2-4)
Structural analysis of remotely sensed data provides a method of assessing the tectonic significance of regional metallogenic lineaments in the New England Orogen of southeastern Queensland. Photogeological analysis of Landsat imagery and small-scale aerial photography reveals a pattern of WNW—NNW-oriented structures, which were apparently generated in response to Mesozoic crustal extension and reactivated during Early Tertiary block faulting. These structures tend to overprint arcuate late Palaeozoic to early Mesozoic trends and batholith belts, and exert a control over Middle to Late Triassic rifting and epizonal plutonism. The distribution of epigenetic base and precious metal deposits in the Rockhampton—Maryborough area is locally but not regionally related to identifiable structural lineaments. 相似文献
103.
A self-contained, field-portable recirculating seawater flume was designed and constructed in order to measure in situ the erodibility of cohesive estuarine muds. The flume can be easily deployed by one person and is designed for subaerial use only. Bed shear stresses up to 0.6 Nm−2 can be generated by the flume. Rates of sediment erosion are assessed in terms of the mass of particulate material eroded with time. Flow calibrations yield a drag coefficient (CD) of 4.5 × 10−3 which enables single mid-depth velocity measurements to be related directly to the imposed bed stress. Water motions within the flume are complex, but secondary (radial) circulations are not considered sufficiently competent to dominate sediment erosion. Stratification effects due to high levels of suspended sediment ( 1.5 gl−1) are negligible. However, the drag-reducing properties of the sediment suspension are uncertain. 相似文献
104.
Adrian C. Armstrong 《地球表面变化过程与地形》1982,7(3):283-284
The constant base boundary condition used by Kirkby to derive characteristic form solutions to the continuity equation are unlikely in practice. Constant form solutions relying on constant basal erosion are however more probable. 相似文献
105.
106.
Five samples from historical lava flows on Mt. Etna, which had previously been used in a palaeointensity study, were examined using a combination of rock magnetic and microscopic techniques to elucidate the causes of failure of palaeointensity determination. The samples were characterised using a combination of low-temperature susceptibility (LT-), Bitter pattern imaging and scanning electron microscope (SEM). High-temperature susceptibility curves and hysteresis loops had been previously measured by Calvo et al. (2001). Of the five samples only one gave an accurate palaeointensity. This sample was deuterically oxidised and consisted of large exsolved ore grains. It was only possible to distinguish this sample either optically or magnetically from a similar sample by the LT- warming curves; the unsuccessful sample displayed alteration in the LT- warming curves, which was not readily observed in the high temperature susceptibility curves. It is proposed that the measurement of LT- curves before and after heating could be a more sensitive method of determining suitability for palaeointensity determination than previous rock magnetic pre-selection techniques. 相似文献
107.
108.
109.
Colin M. Sayers 《Geophysical Prospecting》2007,55(5):699-705
Large changes in seismic reflection amplitude have been observed around injectors, and result from the decrease in elastic‐wave velocity due to the increase in pore pressure in the reservoir. In contrast, the velocity change resulting from the decrease in pore pressure in depleting reservoirs is observed to be smaller in magnitude. Elastic‐wave velocities in sandstones vary with stress due to the presence of stress‐sensitive grain boundaries within the rock. Grain‐boundary stiffness increases non‐linearly with increasing compressive stress, due to increased contact between opposing faces of the boundary. This results in a change in velocity due to a decrease in pore pressure that is smaller than the change in velocity caused by an increase in pore pressure, in agreement with time‐lapse seismic observations. The decrease in porosity resulting from depletion is not fully recovered upon re‐pressurization, and this leads to an additional steepening of the velocity vs. effective stress curve for injection relative to depletion. This difference is enhanced by any breakage of cement or weakening of grain contacts that may occur during depletion and by the reopening or formation of fractures or joints and dilation of grain boundaries that may occur during injection. 相似文献
110.
Christophe COLIN 《中国科学D辑(英文版)》2007,50(11):1674-1684
High-resolution clay mineral records combined with oxygen isotopic stratigraphy over the past 450 ka during late Quaternary from Core MD05-2901 off Middle Vietnam in the western South China Sea are re-ported to reconstruct a history of East Asian monsoon evolution. Variations in Illite, chlorite, and kaolinite contents indicate a strong glacial-interglacial cyclicity, while changes in smectite content present a higher frequency cyclicity. The provenance analysis indicates a mixture of individual clay minerals from various sources surrounding the South China Sea. Smectite derived mainly from the Sunda shelf and its major source area of the Indonesian islands. Illite and chlorite originated mainly from the Mekong and Red rivers. Kaolinite was provided mainly by the Pearl River. Spectral analysis of the kaolin-ite/(illite chlorite) ratio displays a strong eccentricity period of 100 ka, implying the ice sheet-forced win-ter monsoon evolution; whereas higher frequency changes in the smectite content show an ice sheet-forced obliquity period of 41 ka, and precession periods of 23 and 19 ka and a semi-precession period of 13 ka as well, implying the tropical-forced summer monsoon evolution. The winter monsoon evolution is generally in coherence with the glacial-interglacial cyclicity, with intensified winter monsoon winds during glacials and weakened winter monsoon winds during interglacials; whereas the summer monsoon evolution provides an almost linear response to the summer insolation of low latitude in the Northern Hemisphere, with strengthened summer monsoon during higher insolation and weakened summer monsoon during lower insolation. The result suggests that the high-latitude ice sheet and low-latitude tropical factor could drive the late Quaternary evolution of East Asian winter and summer monsoons, respectively, implying their diplex and self-contained forcing mechanism. 相似文献