The Evolution Mirrored By The Cretaceous Sedimentary Sequences In The Nanxiong-Danxia Basin And Its Response To The Tectonic Transition At The SCS’s Area

Nanxiong and Danxia basins are located on the continental basement of thenorthern margin of South China Sea. They, together with South China Sea basins,experience geological processes in close contact with each other in the same tectonicsystem. Most of formation and activities time just had happened in the South ChinaSea tectonic transition process. Accordingly, they could be the "land-base monitors"of South China Sea evolution to a certain extent.The characteristics of sedimentary sequences, clastic geochemical, clasticcomponents and heavy mineral combination in Nanxiong and Danxia basins havebeen discussed and researched. The main components of the basin floor is molasseaccumulation, which is represented by cuttings conglomerate, greywacke, feldsparsandstone facies. The components mostly show active continental marginal arcfeatures, while the changes of sedimentary association of the upper basin shows therift environment，which marked by dual structure. Through tectonic environment ofNanxiong and Danxia basins analysis, there exists conversion node of tectonicconditions in about96Ma. It is a sign of extrusion environment of active continentalmarginal arc conversion to rifting environment in the subduction of Pre-South ChinaSea conditions. And it may not be related to conversion of subduction of Pre-SouthChina Sea to the rift of South China Sea, that is, conversion of active continentalmargin to passive continental margin.Through the study of sedimentary sequence and source, the tectonicenvironment of Nanxiong and Danxia basins had converted from foreland basin ofposterior-arc to rift basin in time and activity center migrated from Danxia basin toNanxiong basin in space. The rift sedimentary source material is similar to passivecontinental margin. Because of the complexity of volcano activity participation andpre rift stage tectonic background, the discrimination and analysis of single detritaland chemical composition is limitation in rift stage. Therefore, the tectonicenvironment of rift basin should be studied by composite approach as a whole.