Differential compaction within alluvial sediments results from the inherent juxtaposition of sand, si It, clay and peat on the floodplain. Differential compaction is primarily dependent on (a) the nature of overbank mudstone compaction, and (b) the relative timing of channel and adjacent overbank sediment deposition. Results from the analysis of modern overbank sediments indicate that the near-surface porosity of alluvial overbank sediment Is controlled by the complex interaction of grain shape, grain size, clay content, groundwater fluctuations, evaporation and transpiration, suggesting that the subsequent compaction of these sediments would be an extremely heterogeneous process, with n'o single controlling parameter. Porosity-depth curves derived from modern sediment analysis indicate that a significant amount of compaction occurs during the first few metres of burial. Examination of samples suggest that ·porosity loss may be due to the expulsion of water from the sediment pores, and the rearrangement of grains from an unstable packing arrangement to a more stable one. Early differential compaction will affect floodplain topography, and therefore directly influence the subsequent pattern of facies distribution. However, as significant amounts of compaction have occurred during very early burial, later stages of compaction will have a less marked effect on alluvial stratigraphy than has been previously suggested. In particular, simulation models such as Bridge & Leeder (1979) which suggest that channel "packing" increases with depth may not be entirely correct. If, within an alluvial sequence, there is no compactional deformation of features such as alluvial channel wings, it is unlikely that channel packing will have increased with depth. Studies of the Middle Jurassic alluvial sediments of North Yorkshire and the Cretaceous of the Isle of Wight, illustrate that the effects of differential compaction can be observed on several scale~: (a) channel belt, (b) individual channel or bedform, and (c) microscopic
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:536314 |
| Date | January 1991 |
| Creators | Anderson, Stephen |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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