Coupling controls of neotectonism and paleoclimate on the Quaternary sediments of the Yangtze (Changjiang) coast.

Attempt has been made to discuss the coupling relation of neotectonism and paleoclimate that have played a critical role in controlling the Quaternary sediment distribution and sedimentary facies evolution on the Yangtze coast On the basis of petrological analysis, up to six sedimentary cyclicities have been identified in the Quaternary sediment boreholes on the time scale of 0.4-0.5 Ma. The lower sedimentary cyclicities (Pliocene to Early Pleistocene) are characterized largely by thicker gravelly coarse sands, topped by thinner fine silt and silty clay. Gravels are very angular and clayey gravels prevail. Sediments are usually of poor sorting, rich in log fragments and have no microfossils. Bedload as denoted by C-M plot occurs throughout the section. This evidence represents the alluvial fan sedimentation which took place as the basin began to subside. The middle sedimentary cyclicities (often including Early to Middle Pleistocene) consist of basal gravelly sands and clayey silt at the middle and upper sediment sections. Sorting becomes relatively better, and foraminifer appears sporadically. C-M plot reveals the mixture of sediment transport media via bedload, saltation and suspension, representing sedimentation of braided river system at the early stage and subsequently, the sedimentation of meandering river pattern. The upper sedimentary cyclicities (including Late Pleistocene to Holocene) comprise basal gravelly sands (exclusive of the Holocene sediment) and thick fine sand and silty clay on the upper section. Sorting becomes fine and foraminifer appears throughout the sediment section. C-M plot shows that saltation and suspension serve as the main sediment transport media in the sediment section and bedload transport weakens. These evidence the sedimentation of meandering river pattern near coast during Late Pleistocene and Holocene time, when marine transgression invaded into the paleoriver valleys, which often forms drowned-valley facies and shallow marine facies. Of note is the Recent delta facies developed on the top of the cyclicities, and characterized by a large proportion of fine sand and silty clay, and various sedimentary beddings. These phenomena have proved enhanced climate function on sedimentary facies evolution on the basis of previous alluvial fan system, largely affected by tectonic subsidence setting. [ABSTRACT FROM AUTHOR]