The Genetic Mechanism of the Sequence Stratigraphy of the Rift Lacustrine Basin in Jiyang Depression, East China

Abstract

Abstract Through the studies of sequence stratigraphy of early Tertiary in the east part of Jiyang depression, the characteristics of sequence evolution in continental basin of fault depression were analyzed theoretically, and a series of sequence stratigraphy patterns and study methods that are suitable to the continental fault depression were summed up. According to hydrogeologic characters, lacustrine basins can be divided into open and closed basin with different characters of relative lake level changes. The changes of the lake level are mainly controlled by tectonic up-down warping in open basin. But in closed basin the changes of the lake level controlled by climate changes. The formation and the characters of the continental rift sequence are mainly controlled by tectonic movements and climate changes. So the sequences of continental lacustrine basin can be divided into tectonicsequence and climatic sequence. Moreover, tectonic sequence is divided into simple, synsedimentary and multi-time fault depression sequence. Different kinds of sequence have different type of system tracts. Periodic changes of climate lead to lake-level changes in lacustrine, and then it controlled the formation of climatic sequence which are composed of lacustrine lowstand system tracts(LLST), lacustrine extension system tracts(LEST), lacustrine highstand system tracts(LHST) and lacustrine contraction system tracts (LCST). Introduction According to the principle of sequence stratigraphy, the stratal pattern of sequence and the formation of sequence boundary are determined by the ratio of the rate of sediments supply and the rate of the variation of accommodation space[1]. The rate of variation of accommodation space is controlled by the rate of rise and fall of sedimentary base level. In marine basin, the sedimentary base level is basically corresponded to sea level. In lacustrine basin, sedimentary base level is basically corresponded to the lake level. In the environment of stream, sedimentary base level is basically corresponded to the stream profile. The sediments above the sedimentary base level are unstable, and will be eroded at last. But if time of erosion is not long enough, the erosion will not reach the sedimentary base level[2]. The type of lacustrine basins Lacustrine basins can be divided into two types according to hydrology, that is open basin and closed basin[3]. Closed lacustrine basin. In a closed lacustrine basin, the amount of water flowing into the basin is less than the amount of water lost by evaporation and seepage, so the lake level is often lower than the elevation of the lowest outlet of the basin. In arid climate, lacustrine basin can often be developed into closed lacustrine basin (Fig.1). Open lacustrine basin. In an open lacustrine basin, the amount of water flowing into the basin is equal to or more than the amount of water lost by evaporation and seepage. So the lake level is constantly as high as the elevation of the lowest outlet of the basin, the superfluous water flows out from the outlet. In humid climate lacustrine basin can often be developed into open lacustrine basin (Fig.1).