Late Quaternary incised-valley fill of the Yangtze delta (China): its stratigraphic framework and evolution

A Late Quaternary stratigraphic framework of the Yangtze delta, China, has been established based on the analysis and correlation of over 600 cores. Thirty cores were drilled and analyzed during this study, and the rest were collected from numerous publications. The Late Quaternary stratigraphic framework of the Yangtze delta consists of incised-valley fill and two interfluvial sequences. The incised valley roughly coincides with the modern Yangtze delta. From bottom to top, the incisedvalley fill sequence is typically composed of channel, floodplain–estuary, estuary–shallow marine, and modern deltaic facies. The interfluvial sequences occur along the two flanks of the incised-valley fill. On the southern flank, seaward of the maximumtransgression line, the interfluvial sequence is typically composed of marsh–nearshore, shallow marine, and tidal flat facies with predominantly muddy deposits. On the northern flank, the interfluvial sequence is typically composed of barrier– lagoon, tidal sand ridge, and littoral– tidal flat facies with predominantly sandy deposits. The interfluvial sequence landward of the maximum-transgression line on both flanks is composed mainly of lacustrine muddy deposits. The post-glacial fining-upward incised-valley fill sequence was formed by retrogressive aggradation in the lower reach of the Yangtze River, induced by sealevel rise. The upstream extent of the retrogressive aggradation exceeds the reach of flood tidal currents, which explains the lack of marine fossil and tidal sedimentary structures in the channel sand deposits. A huge estuary was formed in the present Yangtze delta area when the post-glacial transgression reached maximum at about 7500 years BP. The shallow marine mud was deposited broadly in the estuary and the maximum flooding surface, which separates the underlying transgressive sequence from overlying regressive sequence, located within the muddy stratum. An erosional surface between the channel sand unit and deltaic sand unit exists at the apical area of the Yangtze delta. The modern Yangtze delta developed after the post-glacial transgression maximum was reached. The delta development was dominated by the abundant fluvial sediment supply. Six subdeltas with a southward migrating trend have developed. D 2002 Elsevier Science B.V. All rights reserved.