The quaternary evolution of the Gordano Valley, North Somerset, UK

The Gordano Valley is a low-lying valley positioned in close proximity to the Severn Estuary, in southwest England. Although its late Quaternary and Holocene sedimentary archive is extensive, the spatial and temporal evolution of the Gordano Valley has received only limited attention from previous researchers.In this study, stratigraphic analysis of the valley archive, combined with pollen, diatom and particle size analysis are utilised in a detailed reconstruction of the evolution of the Gordano Valley. Two contrasting depositional environments are present, separated by a sedimentary ridge traversing the width of the valley. A tentative mechanism is presented for the development of the ridge in response to periglacial hillslope erosion and deposition during the Devensian glacial period (c. 115,000-11,500 Cal. yrs BP). In contrast, the chronology and development of the two depositional environments is clearer. Detailed multiproxy analysis of the two sedimentary archives reveals the development of an enclosed lake basin headward of the ridge during the late glacial period (c. 18,000-15,000 Cal. yrs BP). Subsequent climatic amelioration in response to the transition from glacial to interglacial conditions resulted in the terrestrialisation of the lake at the onset of the BØlling/AllerØd interstadial, c. 15,000 Cal. yrs BP. A complete hydroseral succession sequence is present in the headward region of the valley, where the terrestrialised lake sequence developed into the fen peatland that is present today. Seaward of the ridge, interbedded peat and marine sediments have developed since c. 7,200 Cal. yrs BP and reflect the typical deposits that characterise the Severn Estuary lowlands. Whilst an overall tend of sea-level rise occurred during the Holocene, periods of episodic marine inundation occurred in the Gordano Valley in response to variations in the rate of sea-level rise. This thesis also attempts for the first time in southwest England, to construct and apply a diatom-based sea-level transfer function to an interbedded coastal sequence in order to quantitatively infer changes in sea level since sedimentation began. Although the transfer function proved successful, autocompaction of the sedimentary archive restricted the reconstruction of sea level as age-altitude plots.The high-resolution lithostratigraphic analysis combined with the multiproxy analysis of the sedimentary archive has enhanced the understanding of the evolution of the Gordano Valley since the late glacial period. The terrestrialised lake sequence may also be the oldest of its kind in southwest England. Although autocompaction restricted a true reconstruction of sea level since sedimentation began, the initial success of the transfer function has indicated its applicability to a macrotidal environment such as the Severn Estuary