THE LITHOSTRATIGRAPHY AND BIOSTRATIGRAPHY OF A HOLOCENE COASTAL SEDIMENT SEQUENCE IN MARAZION MARSH, WEST CORNWALL, UK WITH REFERENCE TO RELATIVE SEA-LEVEL MOVEMENTS

A Holocene sedimentary sequence at Marazion Marsh, Mount's Bay, west C ornwall, U.K. is examined with reference to coastal evolution and rela tive sea-level change. A sample core analyzed in this paper shows the base of the sedimentary sequence rests on weathered bedrock material. Basal organic-rich deposits are overlain by minerogenic materials domi nated by sand. A sharp contact separates these two sedimentary units. Changing coastal conditions are reconstructed using lithostratigraphic , biostratigraphic and chronometric data from this sequence. The lower organic-rich unit accumulated between ca. 5420+/-60 and 4380+/-55 yrs B.P. Substantial changes in the marsh environment, involving vegetati on succession from initial herb domination to mesophytic woodland and eventually to fen-carr/reed-marsh development, is indicated by pollen records from this unit. Diatom data show increased salinity within the basal sequence as arboreal pollen values decline and fen-carr/reed-ma rsh becomes established. Radiocarbon dates from this and other cores f rom west Cornwall are used to construct a local time/altitude plot of relative sea level (RSL). It is concluded that the sediment sequence a t Marazion Marsh appears to have accumulated in an embayed environment . The sequence was both indirectly and directly affected by changes in relative sea-level position and the influence of protective/transgres sive coastal morpho-sedimentary structures and associated processes of sedimentation during the mid-Holocene period.