海南岛全新世海滩岩

在海南岛及邻近小岛沿岸,往往可以发现现代潮间带海滩上的砂砾石沉积物已被胶结成岩的现象。同样,全新世中、晚期形成的各种类型的滨海相沉积物,也经常程度不同地胶结起来了。

HOLOCENE BEACHROCKS AT HAINAN ISLAND

Two types of beachroeks with different ages and lithology occur at the coast of Hainan Island. They consist of sands and gravels which were deposited and cemented in the intertidal and spray zones during the Recent and the Middle-Late Holocene respectively. Most of the Middle-Late Holocene beachroeks are subaerially exposed.In this paper, the geology, geomorphology and petrology of these beachroeks are described, while their origin and geological significance are also discussed.The cements of these beachroeks are of three types: muddy and acicular ara-gonite, columnar high Mg-calcite and granular calcite mosaic. The aragonite and high Mg-calcite making up the skeletons within the Middle-Late Holocene beachroeks have mostly been transformed into ealcite or low Mg-calcite, but the same minerals in the Recent beachroeks have not been changed. This change (neomorphism) is due to the effect of meteoric fresh-water on the exposed beachroeks. The term "marine-freshwater diagenetic limestone" or "marine-epidiagenetic limestone" is proposed for these carbonate sediments which were deposited in marine environment and lithi-fied by the action of meteoric water.The study of beachroeks indicates that the annual average temperature during the deposition of Middle-Late Holocene beachroeks was 2-4℃ higher than that of the present, and the sealevel then was 3-5 m higher than that of to-day. Since then the northeast coast of Hainan Island has been considerably uplifted, wheseas its southwest corner has slightly subsided.