Geometry and preservation process of tidal sand bar deposits of Middle Pleistocene macrotidal tide-dominated delta succession, southern Vietnam

Abstract

Three-dimensional geometry and preservation processes of Middle Pleistocene tidal sand bar deposits were studied in the wall of a pit where macrotidal tide-dominated delta succession in the Ba Mieu Formation, southern Vietnam, is exposed. The delta succession at this site was divided into units A, B, and C in ascending order. Units A and B are interpreted as upward-fining tidal sand bars that migrated seaward. Unit C consists of upward-fining tidal flat deposits those onlap units A and B. As a consequence of successively weakened tidal current during the progradation of the delta and tidal flat, the tidal sand bar deposits were buried under stacked tidal flat deposits. The lower part of the tidal sand bar deposits were partially reworked by tidal currents while the upper part was not modified. After the reworked sands were deposited as mixed flat deposits on the tidal sand bars, muddy tidal flat spread and covered the tidal sand bars. As a result, the upward-convex geometry of the tidal sand bars was preserved in he tidal flat deposits.