Sedimentary facies of the tide-dominated paleo-Changjiang (Yangtze) estuary during the last transgression

A large estuary was formed by marine inundation of the paleo-Changjiang (Ya ngtze) incised valley during the transgression after the Last Glacial Maxim um. This paper presents the sedimentary facies and architecture of the estu ary fill, based on the analysis of three sediment cores (CM97, JS98, and HQ 98) obtained from the present Changjiang delta plain. Estuary fill deposits showing an upward-fining succession were grouped into five depositional facies: tidal river, distributary channel, muddy interti dal to subtidal flats, transgressive lag, and estuary front, based on sedim entary textures, lithology, and physical sedimentary structures. Sand-mud c ouplets are common in these deposits, indicating that tides played a signif icant role in producing these sedimentary structures and that the estuary t ype was tide-dominated. Some of the successive sand-mud couplets probably r ecorded neap-spring cycles as well as semidiurnal tidal cycles. The nature of the estuary was very different from other representative tide -dominated estuaries in sediment facies, its distribution, and sediment sou rce for estuarine fill. Unlike the other estuaries that receive sediments m ainly from the sea, the paleo-Changjiang estuarine fill deposits were suppl ied largely from the river. This difference would also have a great influen ce on the sedimentological and morphological component in the estuary. The sediment distribution of the estuary showed fining-seaward and estuary-mout h sand bodies fed by marine-source sand were absent. The architecture model of tide-dominated estuaries should be divided into two types by the degree of fluvial sediment supply. The paleo-Changjiang estuary shows a good exam ple for an estuary of large rivers. (C) 2001 Elsevier Science B.V. All righ ts reserved.