Coarse grained, late-Devensian, fluvial deposits underlie river terraces along the lower River Severn and in tributary valleys. Borehole evidence indicates that individual terrace features may be correlated downstream, supporting previous stratigraphic interpretations. Examinations of the terrace sediments were carried out at 10 major sections enabling comparisons to be made between 'paraglacial' and 'periglacial' deposits. Interpretations of the depositional environments are based upon detailed descriptions of the sediments and considerations of genetic relationships between lithofacies types, determined through Markov chain and architectural element analyses. Five depositional sub-environments may be identified; channel zone units, overbank deposits, unitary channel forms, slope and alluvial fan deposits. The occurrence of the latter two sub-environments depends on the pre-existing valley topography, whilst the characteristics of the channel-zone and overbank deposits vary in relation to the local aggradation rate, discharge regime, proximal to distal controls on the sediment size distribution and channel slope. The fluvial sequences were predominantly deposited by low sinuosity gravel bed rivers, often with braided planforms. However, the Worcester Terrace aggraded partly under sandy braided conditions. Channel zone lithofacies in a modern braided river are shown to develop through the formation and agglomeration of complex lateral and medial bar forms. These comprise bar-platform units representing primary, in-channel (unit-bar), sedimentation, and supra-platform sediments which are superimposed on, and modify, the bar-platform deposition. Mean annual flood discharges in the basin, and possible error limits, are estimated using palaeohydraulic and morphometric techniques. Palaeodischarges in the Severn and Avon are shown to have been similar, with maxima occurring during deglaciation. Avon discharges may have been up to 25 times present values, although in the lower Severn mean annual floods may only have reached 4 times present. The development and preservation of Pleistocene terrace deposits is considered within a model emphasizing differences apparent in the sedimentary sequences. A similar model explains the development of channel zone depositional features, and the origins of gravel stratification types are discussed in relation to observations of terrace and modern sediments.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:674157 |
| Date | January 1986 |
| Creators | Dawson, Martin |
| Publisher | University of Leicester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/2381/34480 |
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