Flandrian sea level changes and vegetational history of the lower Thames estuary

Abstract

A biostratigraphic study, investigating the interleaved Flandrian biogenic and inorganic deposits of the lower Thames Estuary, has been carried out between central London and the Isle of Grain. The vegetational and environmental history, showing the relation of the biogenic deposits to former sea level has been deduced from pollen, diatom and other microfossil studies. Radiocarbon dating has been used to establish an objective chronology. From this evidence the height of relative sea level movements, seen in marine transgression and regression surfaces, have been determined. These are plotted against time to show the rate of relative sea level change and subsidence trends for the Thames Estuary and southern England. Diatom studies show the early importance of marine and brackish water influences, at the beginning of Flandrian sedimentation in the Thames. Pollen and macrofossil analyses demonstrate the strong local effects of the saltmarsh and fen environment upon the vegetational history. The rise of Alms pollen is seen to occur before 8100 years b.p ., probably reflecting local physiographic conditions and valley flooding consequent upon the rising sea level. Elements of the regional vegetation development are recorded however, with the Ulmus and Tilia pollen declines shown. Five regression phases (Tilbury I-V), represented by the biogenic deposits and four marine transgressions (Thames I-IV), together with the possible existence of a fifth (Thames V), are recognized. The relative sea level for mean high water mark of spring tides (m.h.w.s.t.) is shown to rise at about 8500 b.p . from —26.5 m o.d . to above present Ordnance Datum (Newlyn) by about 1750 years b.p . Relative sea level curves for the Thames during Flandrian times correlate well with the form and rate of relative sea level changes shown for northwest Europe. Plotting these graphs against each other has allowed subsidence trends to be shown. Within the Thames, possible differential downwarping of approximately 1.5 m has been identified between Crossness and Tilbury for the Flandrian. The regional trends of west to east and north to south downwarping are supported. The amount of subsidence for southeast England, formerly given as 6.1 m since 6500 b.p ., is not confirmed. The figure for the Thames area relative to the Bristol Channel lies closer to 2-3 m since 7000 b.p., although rates of downwarping vary with the type of environment studied, making generalizations tenuous. Sea level only shows relative subsidence trends and is not as yet seen to provide an accurate fixed datum from which one can give precise figures for land subsidence.