THE ABANDONED DELTAS OF THE LUANHE RIVER AND THE BARRIER-LAGOON SEDIMENTARY SYSTEMS
Li Congxian Chen Gang Wang Li ( Institute of Marine Geology, Tongji University, Shanghai )
The distributions of the recent barrier-lagoon coasts are widespread in the world.Offshore barriers and lagoons are interdependent and interact on each other, thus forming unified sedimentary systems.The barrier-lagoon sedimentary systems in the coastal zone from Luanhe river mouth to Caofeidian, Hebei Province, are of different types and develop at different stages, which provides favourable conditions for the study of the coastal sediments of this kind. The sedimentary characteristics of the main sedimentary units in barrier-lagoon sedimentary systems are shown by the analyses of grain size of the sediments, heavy minerals,surface microtextures of quartz sands and microfossils. In addition, based upon the analyses of the sedimentary characteristics, coastal geomorphology, marine shallow profile and ore hole data in this region, a lot of vestiges of the abandoned deltas of the Luanhe river arc found, and thus it is proved that the barrier-lagoon systems are formed and evolved by the reworking of the abandoned deltas of the Luanhe river. Closed lagoon, semiclosed lagoon and gulf-lagoon are of three different types of lagoon and represent different stages of its development .During the transgression, the developmental trend of lagoon was closed lagoon-semiclosed la-goon-gulf-lagoon ( and eventually ) -the shallow sea;during the regression, the developmental series of the lagoon was gulf-lagoon-semiclosed lagoon -closed lagoon ( and at last ) -the land. Nevertheless, transgression and regression of one region might result from the world wide eustatic movement, the regional eusta-tic movement and the increase, or the decrease, or even the absence of material source.The barrier-lagoon sedimentary systems in the coastal zone from Luanhe river mouth to Caofeidian developed at the time when the Luanhe river changed its course, material source decreased or lacked.ancient delta was abandoned and eroded so that transgression happened partially in the area. Therefore, the barrier-lagoon sedimentary systems in this area essentially followed the developmental pattern of the system during the transgression. Historical records and radiocarbon dating indicate that the Luanhe channel migrated in a proper order from west to east, and the ancient deltas were abandoned successively. The earlier the time the deltas became abandoned from Luanhe river mouth to Caofeidian,the longer the time the delta was eroded,and the worse the closed degree of the lagoon was.Therefore, the closed degree of the lagoon reflects the abandoned time-sequence of the Luanhe river delta.