Rhythmic deposition on mudflats in the mesotidal Changjiang Estuary, China

The mesotidal Changjiang Estuary is characterized by high suspended sediment concentration, broad mudflats, no barrier islands, and a semi-diurnal tidal regime controlled by the tropical period. Study mudflats are located at the south Hank of the Changjiang Delta, facing the East China Sea. Thickness and number of tidal couplets deposited on the mudflats were measured daily and fortnightly over three neap-spring cycles. Changes in mudflat elevation were monitored from May (pre-typhoon season) to August (mid-typhoon season). Daily observation of sedimentary features shows that short-term deposition on the mudflats during calm weather is controlled mainly by tides. Neap-spring cyclicity can be extracted from the variation of sand-mud couplet thickness. High suspended sediment concentration allows deposition of two couplets per semidiurnal tide. However, number of couplets preserved per tidal cycle is reduced because of reworking and erosion by tidal currents and waves. Over a fortnightly period, tidal cyclicities are obscured, with ca. 80% of the couplets reworked. On a seasonal scale, sand-dominated layers reflect deposition during storms and mud-dominated layers reflect deposition from tides during calm weather. Two orders of sand-mud lamina alternations were examined in the tidal-flat succession. The first-order alternations are in the range of centimeters to decimeters (consisting of 16-26 couplets) and related to storm-calm weather variation; the second-order, millimeter-thick alternations are related to tidal rhythm. Deposition rates decrease exponentially as time scales increase. The open-coast tidal-flat succession is truncated with numerous depositional hiatuses.