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SEAN J. FITZSIMONS 《Boreas: An International Journal of Quaternary Research》1989,18(2):127-134
Pebble counts of the lithology of glacial sediments in the King Valley show that the content of distantly derived erratics of many sections decreases upwards in near surface sediments. Two factors that contribute to this lithological stratification are dilution of the erratic content of surface sediments by locally derived rocks and lithological stratification of debris within the Pleistocene King Glacier. The common diluting mechanism appears to have been slope detritus derived from the valley sides and small hills that crop out on the valley floor. Lithological stratification of debris in the King Glacier resulted from the altitude of the equilibrium line of the King Glacier relative to the position and altitude of the rock source areas and the thermal regime at the ice-bed interface. The Jurassic dolerite and Permian sediments that crop out above the equilibrium line altitude were transported in subglacial and englacial positions. In contrast, below the equilibrium line sediments that accumulated and were transported in a supraglacial position contained no erratic lithologies. When deposited, the supraglacial sediments formed a siliceous, non-erratic cover over sediments that were transported in subglacial and englacial positions. The model of the mode of sediment transport in the King Valley may have application to areas of alpine glaciation where the distribution of some rock types is restricted to areas above the equilibrium lines of glaciers. 相似文献
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On the transition between 2D and 3D dunes 总被引：3，自引：0，他引：3
Sediment transport in sand-bedded alluvial channels is strongly conditioned by bedforms, the planimetric morphology of which can be either two- or three-dimensional. Experiments were undertaken to examine the processes that transform the bed configuration from two-dimensional (2D) dunes to three-dimensional (3D) dunes. A narrowly graded, 500 μm size sand was subjected to a 0·15 m deep, non-varying mean flow ranging from 0·30 to 0·55 m sec−1 in a 1 m wide flume. Changes in the planimetric configuration of the bed were monitored using a high-resolution video camera that produced a series of 10 sec time-lapsed digital images. Image analysis was used to define a critical value of the non-dimensional span (sinuosity) of the bedform crestlines that divides 2D forms from 3D forms. Significant variation in the non-dimensional span is observed that cannot be linked to properties of the flow or bedforms and thus appears random. Images also reveal that, once 2D bedforms are established, minor, transient excesses or deficiencies of sand are passed from one bedform to another. The bedform field appears capable of absorbing a small number of such defects but, as the number grows with time, the resulting morphological perturbations produce a transition in bed state to 3D forms that continue to evolve, but are pattern-stable. The 3D pattern is maintained by the constant rearrangement of crestlines through lobe extension and starving downstream bedforms of sediment, which leads to bifurcation. The experiments demonstrate that 2D bedforms are not stable in this calibre sand and call into question the reliability of bedform phase diagrams that use crestline shape as a discriminator. 相似文献
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SIWAN M. DAVIES WIM Z. HOEK SJOERD J. P. BOHNCKE J. JOHN LOWE SEAN PYNE O''DONNELL CHRIS S. M. TURNEY 《Boreas: An International Journal of Quaternary Research》2005,34(2):123-135
Three distal tephra layers or cryptotephras have been detected within a sedimentary sequence from the Netherlands that spans the last glacial-interglacial transition. Geochemical analyses identify one as the Vedde Ash, which represents the southernmost discovery of this mid-Younger Dryas tephra so far. This tephra was found as a distinct horizon in three different cores sampled within the basin. The remaining two tephras have not been geochemically 'fingerprinted', partly due to low concentrations and uneven distributions of shards within the sequences sampled. Nevertheless, there is the potential for tracing these tephra layers throughout the Netherlands and into other parts of continental Europe. Accordingly, the possibilities for precise correlation of Dutch palaeoenvironmental records with other continental, marine and ice-core records from the North Atlantic region are highlighted. 相似文献