Autocompaction in Holocene coastal back-barrier sediments from south Devon, southwest England, UK

The present-day elevation of superficial horizons situated above a competent basal stratum is likely to be lower than the original height of deposition. This is because sediments such as minerogenic fines and peat undergo a post-depositional reduction in volume as a result of the weight of overlying sediments, the downward movement being due to the cumulative compression of all the sediment below the level in question. This “autocompaction” can affect the palaeoenvironmental interpretation of lithofacies from which a vertical reference is required, e.g., when quantifying the height of a sea-level index point. Geotechnical theory was used to apply a correction to Holocene coastal back-barrier sediments from North Sands and Blackpool Sands in south Devon (UK) and so to return marker horizons to a level approximating their original height of deposition. In this model the total downward movement is calculated by notionally dividing the underlying soil into a number of thin (∼ 0.1 to 0.2 m) layers and calculating the individual compression in each one. The results were then summed to give the total compression. This approach can underestimate the full extent of autocompaction notably due to uncertainties arising from the behaviour of organic-rich facies and from inadequate knowledge of groundwater history. The results must be considered semiquantitative and are usually minimum estimates. Results of vertical corrections from coastal sedimentary units in south Devon range from < 0.1 m in fine-grained sediments situated above basal facies to > 1 m at contacts between minerogenic sediments and peat, increasing to > 2 m in more organic facies.