Late Quaternary evolution of the sub-aqueous Yangtze Delta, China: Sedimentation, stratigraphy, palynology, and deformation

Four sediment belts (A-D) of the Yangtze sub-aqueous delta are defined from the sea-bottom sediment distribution. Belt A, delta front fine sand and silt, with trough cross stratification; Belt B, prodelta silty clay and clayey silt, with abundant burrowing and rich in organic matter; Belt C, prodelta to continental shelf sand-silt-clay, with thin layers of sand and clay interbedded; and Belt D, late Pleistocene relict sands (medium to fine grained), mixed with shell fragments. The late Quaternary stratigraphy of the delta consists from bottom upward of: late Pleistocene terrigenous (fluvial and lacustrine) sand and mud; Holocene transgressive silt, prodelta clay and delta front fine sand and silt. In addition, four pollen-spore zones (a-d) derived from these sediment sequences reveal a general trend of climate warming from early to late Holocene. The Zone c, distributed primarily within the prodelta muds, provides evidence for a temperature maximum starting at ca. 7000 years B.P. (Atlantic period). This indicates that the delta initiation was essentially coeval with the climate optimum in early Holocene. Sub-bottom seismic surveys reveal three (I-III) acoustic facies. Facies I and II are characterized by seaward remobilization of water-saturated Holocene prodelta muds; facies III is composed of highly-contorted late Pleistocene strata and mud diapirs that erupted from deep-seated late Pleistocene sediments in front of the estuarine depocenter. Gravity subsidence due to Holocene sediment loading within the depocenter is the major factor in causing the deformation of underlying unconsolidated strata. Furthermore, the sub-aqueous Yangtze Delta proper is now obviously diverting to southeast owing to the long-term, tectonically induced southward shift of the river mouth, and to the marine currents associated with the Coriolis effect. We thus predict that the large Chongming island in river mouth area will have been coalesced with the northern Yangtze coast in the recent future. (C) 2000 Elsevier Science B.V.