Architecture and evolution of the tide-dominated Changjiang (Yangtze) River delta, China

The Changjiang (Yangtze) River, China, has formed a large tide-dominated delta on the western coast of the East China Sea. This paper presents the architecture and Holocene evolution of the delta, based on analyses of three borehole samples taken from the present delta plain. The deltaic sediments are 25-30 m thick and composed largely of subaqueous deposits, which can be divided into three facies: prodelta, delta front and delta plain, in ascending order. Tide-influenced sedimentary structures such as sand-mud couplets and bi-directional crosslaminations characterize the delta plain and delta front deposits. An upward-coarsening facies succession from prodelta to delta front is overlain by an upward-fining facies succession from the uppermost delta front to the delta plain. The deposits at the boundary between delta plain and delta front are composed of well-sorted, coarse deposits because it is in the zone of highest energy. The depth from the top of the intertidal facies of the delta plain to this boundary is almost the same, 9-11 m, among the three cores. Sediment accumulation rates are highest in the delta front facies (ca. 10m/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 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 rise of sea level from 8 to 6 kyr BP, and the progradational system developed after sea level reached or approached its present position. The evolution of distributary channels and related river-mouth sand bars was an important process of delta migration or progradation.