Modern Analogues and the Evaluation of a Pleistocene Sedimentary Sequence

Abstract

Recent sands from the Saunton Sands-Braunton Burrows beach-dune complex bordering Bideford Bay provide modern analogues for interpreting the depositional history of Pleistocene raised littoral sediments on Saunton Down and Baggy Point. Use of size-frequency statistics determined by moments and the analysis of size-curve components illustrate the similarity between the Recent dune sands from Braunton Burrows and the Pleistocene deposits on Saunton Down. A linear discriminant function derived for the classification of modern eolian and wavelaid sands is used as a predictive model to identify the environments of deposition of the Pleistocene sands. Of the twenty-three sand samples from Saunton Down, twenty-one are clearly classified as eolian using size-frequency statistics. Three sand samples from the basal section on Baggy Point are classified as wave-laid while a sample from higher in the section is eolian. Graphical dissection of the size-frequency curves supports the inference of the sizefrequency statistics. The Saunton Down sands can be analysed into two phi-normal components in the majority of cases and three components at most. This is similar to the Recent backshore and dune sands where a fine suspension population, a dominant saltation population and only occasionally a coarse rolling population are evident. In contrast, the lower layers at Pencil Rock on Baggy Point illustrate the existence of a double saltation population (associated with swash-backwash currents) and a coarse bed rolling or sliding component. The upper layers are typically eolian in form. Electron-microscopic analysis of the surface textures of quartz reveal similar patterns in both Recent and Pleistocene sands. Particularly common on wave-laid sands from both Saunton Sands and the lower layers on Baggy Point are V-shaped impact fractures generally less than 0-75 ,um in size. They are not found on either the Recent or Pleistocene dune sands of Saunton Down. Instead, conchoidal fractures arranged in concentric series together with smooth blocky structures characterize the wind-blown material. The tentative conclusion to be drawn is that the Saunton Down sands are most probably eolian in origin throughout and form part of a Pleistocene backshore-dune sequence backing a foreshore, with a strand-line running close to the Pencil Rock sands on Baggy Point and sea level higher than at present. A MAJOR field in geomorphological research is the chronological analysis of the evolution of the landscape and the study of the associated changes in the spatial pattern of geomorphological environments. Often evidence of such changes is scarce or absent but, frequently, past environments are represented by the presence of superficial sedimentary deposits. This has led to the use of techniques of sedimentological analysis in investigating previous depositional environments. Complete understanding of past geomorphological environments does, however, necessitate the use of modern systems as analogues for ancient ones and the adoption of the principle of uniformitarianism1. This paper examines certain techniques of sedimentological analysis which have proved to be of value in differentiating Recent sediments and compares the results with those obtained from a study of a Pleistocene sedimentary sequence. An attempt is made to provide methods which are useful in analysing coastal sands, and the paper presents the initial results of a larger study of the sedimentology of the Pleistocene littoral sands in several sites in south-west England. I45 This content downloaded from 157.55.39.144 on Wed, 11 May 2016 05:16:45 UTC All use subject to http://about.jstor.org/terms