The morphology and formation of a binary sand ridge in the Beibu Gulf, northwestern South China Sea

The overall morphological characteristics and shallow architecture of a tidal sand ridge were examined for the first time in the Beibu Gulf, northwestern South China Sea. The linear sand ridge is characterized by a unique binary feature in terms of its shape, constituents, architecture, and behavior. In the northern part, the crestline has rotated anticlockwise to the principal tidal currents, the sediment is coarse, and large active sand waves occur on the surface. In the southern part, the crestline is parallel to the principal tidal currents, the sediment is fine, and no large bedforms have developed on the crest. The sand ridge has shrunk in the northern part and elongated at its southern end in recent decades, leading to a southward or southeasterly movement. The upper strata of the northern part mainly comprise inclined bedding, whereas parallel bedding prevails in the southern part. The bedding implies historically dominant transport directions in different sand ridge parts and suggests that flow modification occurred during ridge evolution. The inclined bedding in the lower strata of the northern part dips to the south or southeast, indicating the existence of unidirectionally moving sand waves. This is unusual for a growing sand ridge where convergent sand transport should have already occurred. The present sand ridge is interpreted as a source from the joining of a sand wave assemblage and an adjoining nascent sand ridge, which has experienced stages of connection, accretion, and migration. The transformation of a sand wave assemblage to the northern part is attributed to the hydraulic change from a unidirectional residual flow regime over sand waves to a bidirectional residual flow regime. This flow regime change was not related to sea level changes but benefited from a new sand body that was produced by sand waves merging into the original nascent ridge.