Sedimentation rates in relation to sedimentary processes of the Yangtze Estuary, China

Abstract

Radioisotope analysis and Digital Elevation Model (DEM) method were combined to examine sedimentation rates and associated sedimentary processes in the Yangtze River Estuary. The major depocenter is validated at the delta front sedimentary facies above the normal wave base (NWB), where accumulation exceeds erosion. This alternated sedimentation does not accommodate Pb-210 and Cs-137 measurement, although sedimentation rates of less than 0.2–5.0 cm yr −1 were recorded in the fine-grained (silty) sediments, which were interbedded with coarse-grained (sandy) sediments. However, historical DEM data provide more detailed information on sedimentation in the delta front facies, where accumulation is dominant in the sandy shoals (1.73–8.30 cm yr −1) and delta front slope (5.22 cm yr −1) facies. The DEM data also show that erosion (1.61–7.32 cm yr −1) dominates in the northern estuarine distributaries, and accumulation (3.01–4.97 cm yr −1) prevails in the southern ones, primarily owing to the superimposed runoff and ebb tidal currents. Pb-210 and Cs-137 measurements reveal sedimentation rate from 2.0 cm yr −1 to 6.3–6.6 cm yr −1 in the delta front slope facies, which progressively decreases to <0.8 cm yr −1 in the prodelta facies and is unmeasurable in the delta-shelf transition zone. DEM analysis detects minor erosion in the delta front slope and prodelta facies, although accumulation predominates there. The present sedimentological database will be useful for estuarine environmental assessment after the Three-Gorges Dam is completed in 2009.