DIFFERENTIAL TECTONIC ACTIVITIES IN PALEOCENE-EOCENE AND ITS BEARING ON THE EVOLUTION OF SEDIMENTARY FILLINGS IN THE SOUTHWEST BOZHONG SAG

The evolution of sedimentary fillings in the southwest Bozhong Sag are not so clear up to date. This time, tectonic activities, sedimentary systems and evolution history of fillings in the faulted lake basins of this area are integrated and systematically studied by means of cores, well logs and 3D seismics. 3 tectonic activities were recognized in the Paleocene-Eocene deposits in the tectonic-inversion area of the southwest Bozhong Sag. As the results, three key sequence interfaces formed by uplifting and denudation were discovered, which controlled the sequence structure of the lake deposits in the Kongdian-Shahejie Formations. There are four types of sedimentary facies, i.e the fan delta, braided river delta, lacustrine and sublacustrine fan, of which the distribution patterns are controlled mainly by provenance, paleogeomorphology, faulting and other factors. In the early rifting stage of Kongdian Period, small lake basins occurred with adequate source supply, all the steep and gentle slope belts were predominated by fan-delta sediments. In the centers of lake basins, however, there laid down sublacustrine fan deposits. Lake basins gradually shrank later on. During the third stage of Shahejie Period, the subsiding of the basin was accelerated, and with the expansion of the basin area, the supply of the source was no longer sufficient, and the sublacustrine fans were mainly developed in the depressions of the lake. In the post- rifting subsidence period in the first and second stages of the Shahejie Period, the braided river deltaic deposits became widely distributed, and lake carbonate deposits might occur on some local geomorphic highs. The differential tectonic activities and sediments supply constrained the evolution of the sedimentary fillings of the region, and such a differentiation of sedimentary fillings may also occur in other tectonic areas of the Bohai Sea.