Modern Depositional Systems and Their Evolutional Process in the Paipu Coral Reef Area, Hainan Island, China

Quan Songqing Wang Guozhong Lu Bingquan Jiang Panliang (Department of Marine Geology. Tongji University. Shanghai. 200092. China) Paipu Coral Reef Area, about 100 km2 in area, located at the Northwest Part of Hainan Island in the South China Sea , is consist of recent coral fringing reefs, coral barrier reef and their internal water body. Because of the obstacle of the barrier reef in the seaward side and rich supplement of terrigenous elastics from island, in this area exist two kinds of sedimentary environments - the fresh water carbonate sedimentary environment and the muddy water carbonate sedimentary environment. Therefore two sedimentary systems -the fresh water carbonate sedimentary system and muddy water carbonate sedimentary system are formed relatively. In these sedimentary environments accumulate three kinds of sediments; the reefoid sediments, consisted of fresh water coral reef facies and muddy water coral reef facies, the terrigenous sediments and the mixed sediments, deposited in the muddy water environments. In this paper the sedimentary characteristics of all kinds of sediments are described in detail. The framework of muddy water coral reefs is built up by mound - like and massive corals with large and high knob of coralline skeletons and thick tentacles. The low limit of coral growing zone is 3. 4m under the sea level; absent of spurs and grooves systems and lower coverage of growing corals are the other characteristics of muddy water coral reefs. The evolutional process of sedimentary systems is as follows: in the early Holocene epoch a single tel-rigenous clastic sedimentary system existed there, in the time of middle Holocene transgression, the climate was becoming warmer, the early fringing coral reefs and Dachau Barries Coral reef formed, and in the late Holocene epoch, as the barrier coral reef was going into senile stage, the obstacle role of barrier reef was be- coming strong, two kinds of sedimentary systems - fresh water sedimentary system and muddy water sedimentary system formed finally. Authors of this article considered that the percentage of terrigenous elastics and coral debris can be used as the index of the reefoid sedimentary facies and terrigenous sedimentary facies. but the amount of the shell debris can't be used as the inedx of the reefoid or terrigenous sedimentary facies.