Holocene coquina beachrock from Haishan Island, east coast of Guangdong Province, China

Coquina beachrock, with a thickness of 10 m and an exposed surface area covering 3.4 km2, is well developed along the southern coast of Haishan Island on the east coast of Guangdong Province, South China. Part of the deposit not treated in this study also extends under the surface of the sea. The coquina beachrock was deposited in a large embayment semi-enclosed by palaeocoastal hills and islands, but with good marine circulation. Holocene strata on Haishan Island demonstrate a mixed carbonate/siliciclastic regime in a series of fining- and deepening-upward shelly limestone successions. Mass accumulations of whole and fragmented mollusc shells are the most remarkable features expressed throughout the series. Coralline red algae are commonly found overgrowing the bivalve bioclasts. Other bioclasts include gastropods, coralline red algae, bryozoans, foraminifera, and scleractinian coral. Cement is well developed within the interstices formed by the bioclasts, showing two or three generations of development. On both a qualitative and quantitative basis, the consecutive coquina series document an overall shallowing-upward pattern of beachrock deposition, which was strongly influenced by marine transgression on Haishan Island. Dating by AMS 14C indicates an age in the Middle to early Late Holocene for these coquina beachrock deposits. Siliciclasts incorporated within the beachrock deposits mainly consist of quartz grains. Not fully hardened, the binding cement is high in calcium carbonate, including aragonite mud, fibrous aragonite and granular calcite. Such carbonate deposits dominated by mollusc bioclasts with quartz grains indicate a pattern of littoral sedimentation. The massive coquina beachrock on the southern coast of Haishan Island formed during Middle to early Late Holocene time, and represents a highstand of sea level from 4000 BP to 2800 BP with short-lived and rapid sea level fall.