Seepage-induced sedimentary structures and their significance in beach bars of the Zhoushan Archipelago, China

Sedimentary structures induced by water seepage are described from modern beach bars within the Zhoushan Archipelago in China, including two unique types, herein referred to as: 1) seepage tubes; 2) seepage beads. Their formation is related to water seepage in sandy sediments, where the seepage tubes are formed by the downward seepage of pore water in sandy sediment, while the seepage beads are formed by the vertical superposition of multiple beaded horizontal seepage layers. The results show that unique terrain and rapid water ingress and retreat related to tides control the formation of these sedimentary structures. The seepage-induced sedimentary structures can be divided into vertical vadose belts that develop seepage tubes, horizontal undercurrent belts with seepage beads, and water-table variation belts with both seepage tubes and seepage beads appearing alternately. These sedimentary structures are hitherto undescribed and contribute to an improved understanding of similar sedimentary structures in the geological record.