Holocene transgressions and regressions on the Essex coast outer Thames estuary

Abstract

Four main environments are described from tropical estuaries of the Freetown Peninsula, Sierra Leone: A) sand bats, B) channels, C) intertidal flats and D) mangrove swamps. The sand bars are predominantly well-sorted medium sands, with dunes as the main bed form. The channel sediments vary in grain size and bed form up the estuaries, but generally contain lag deposits (mostly of shell debris and laterite pebbles) coarser than the adjacent intertidal sediments. The intertidal flats are mostly muddy sands, commonly with scour pits and current lineation. The sedimentary structures are obliterated by infaunal bivalves and burrowing crustaceans. The mangroves, developed peripherally around the estuaries, are important in trapping and binding the finer grades of sediment. Sierra Leone has an extreme two-season climate, considerably affecting the estuarine sediments. During the dry season, a period of accretion, much sediment (mainly bed load) is taken into the estuaries from offshore. Crustaceans and bivalves increase in numbers and occupy a larger area of the intertidal flats. During the wet season, mud and plant debris are brought down by the rivers and some bed load is moved down or out of the estuaries. With rising sea level, the estuarine deposits are prograding landward, over fluviatile sediments and soils (laterite in this case), producing a coarsening upward sequence from rootlet beds through bioturbated muddy sands to well-sorted cross-bedded medium sands.