HOLOCENE EVOLUTION OF THE TIDE-DOMINATED CHANGJIANG (YANGTZE) DELTA

Deltas are classified into three end members, fluvial-, wave-, and tide-dominated, based on the relative intensities of these influences. Interestingly, most of the largest rivers today, in terms of sediment load, have their mouths along tide-dominated coasts and are forming tide-dominated or tide-influenced deltas. Here, we present the sedimentary facies, architecture, and Holocene evolution of the tide-dominated Changjiang (Yangtze) delta, China. The deltaic sediments are 25 to 30 m thick and composed largely of subaqueous sediments, which can be divided into three facies: prodelta, delta front, and delta plain, in ascending order. Tide-influenced sedimentary structures, such as bi-directional cross-laminations and thinly interlaminated to thinly interbedded sand and mud, characterize the delta plain and delta front sediments. The deltaic sequence from the prodelta sediments to delta front sediments showed an upward-coarsening succession, overlain by an upward-fining succession from the uppermost delta front sediments to the delta plain sediments. The sediments at the boundary between delta front and subaqueous delta plain are composed of well-sorted, coarse sediments with parallel laminations because it is in the zone of highest energy. Radiocarbon ages show that sediment accumulation rates are highest in the delta front facies (ca. 10 m/kyr maximum) and decrease toward both delta plain and prodelta facies (ca. 1 m/kyr). The evolution of the Changjiang delta, which began in a drowned-bay approximately 8 kyr BP, can be divided into two stages by its stacking pattern: aggradational and progradational. The aggradational system developed during the slow sea-level rise from 8 to 6 kyr BP, and the progradational system developed after sea level reached or approached its present position or a little higher. The results of this study will be useful for understanding the other large, tide-dominated delta systems, such as the Amazon, Ganges-Brahmaputra, and Fly.