1. Field characteristics

Coastal dune rocks in China are aeolian sand cemented by calcium carbonate under subaerial conditions, distributing on tropical and subtropical coasts of South China. The northernmost location of the dune rock is found on the coast of Fujian Province and the southernmost in Shidao of Xisha Archipelago. Dune rocks are founded in the areas where coastal dunes are fully developed. However, only the top layers of the dunes closed to shorelines can be cemented into rocks, inland extent distance of which is no more than 500 m. Two types of dune rocks can be identified: (1) the rocks developing in patches on seaside slopes of coastal hills at elevations from 2 meters to more then 80 meters above seal level; (2) the sand being cemented on the top of coastal dunes and sand bars on coastal plains. The topset strata of the dun rocks are in form of flat deddings, laminae of which are parallel to each other with small dip angles. The foresets are planar cross-bedded strata on a large scale with high dip angles of 32°-40° generally pointing in one direction. The foresets are cut by the topsets at top or extend continuously up to the flat topsets.

2. Depositional characteristics of dune rocks

The coastal dun rocks are composed of find and medium, well or moderately sorted quartz grains and bio-detritus (mainly shell fragments). Contents of the bio-detritus are different from place to place, ranging from 10 to 40 percent. The rocks have very high percentage of porosity (>20%). Shape of the quartz grains in the dune rocks varys from sub-rounded to sub-angular with worn and rounded edges. However, some grains contained in samples taken from coastal hills have sharp edges, fresh and flat fractures without any traces made by abrasion. Grain surface is characterized by textures formed in aeolian environment such as dish-shaped depressions, meniscus depressions, upturned plates, pitted surfaces and worn particle shape. They also keep surface textures formed under water such as V-shaped depressions, triangular solution pits and conchoidal fractures. The features of the cements are the followings:

1. pore spaces filled with granular low-Mg calcite mosaic;
2. menicus cements between grains or gravitational cements beneath grains;
3. quartz grains or shell fragments cemented by columnar or microlitic calcite cements, and the size of the crystals increasing towards the centre of the pores where several generations of the crystals can be found;
4. the moulds of the pre-existing fragments or some times micrite envelops filled by columnar low-Mg calcite.

Contents of Sr and Mg are very low in the dun rocks.

3. Formation of dune rocks and their ages

Ages of the most coastal dune rocks in China is no more than 5000 years, with an exception of the dune rocks in Shidao of Xisha Archipelago, the age of which is 25000 B.P.. So that the coastal dune rocks in China formed especially during the period of late Holocene times (3000-1000B.P.), when the monsoon climate in China has been fully established. High temperature with distinct dry and wet seasons together with more than 10 percent biogenic fragments in coastal dunes provides a suitable environment for the coastal dunes to be cemented into dune rocks, sources of which were from beach sand and hill side detritus. Pore water between the sand in the dunes closed to shorelines contains certain amount of CaCO₃ resulting from soaking of wave sprays or leaching of biogenic fragments by rain. After evaporation taking place on the surface of the dunes, the pore water becomes over-saturated and then low-Mg calcite precipitate. The upper layers of the coastal dunes near shorelines are thus cemented into dune rocks.