The Late Mesozoic Sequence Stratigraphic Framework And Palaeogeographic Evolution In Sanjiang Area

The Late Mesozoic basins in Sanjiang area include Sanjiang basin, Boli basin, Jixi basin, Hulin basin, Tangyuan fault basin, Hegang basin and Ning'an basin. This thesis emphasizes on Sanjiang basin, Boli basin, Jixi basin and Qihulinhe depression of Hulin basin. Though nowdays each of these basins is isolated for the tectonic destroying consumingly,it has similar vertical sedimentary filled sequence in Lower Cretaceous and it suggests that the basins should be have the same evolutionary process. The article establishes the sequence stratigraphic framework and analyzes the distribution of depositional system and palaeogeographic evolution in second-sequence framework based on the analysis of sequence stratigraphy and sedimentology. The author considers the Sanjiang basins as a unite of Lower Cretaceous and studies the sequence for the first time. It has an important significance to further prospect for oil & nature gas.By utilizing the core,outcrop,logging data and seismic materials, according to lithology changes,lacustrine flooding surface,cessation of deposition,logging data saltation,shape of logging curve,truncation and onlap in seismic profiles,the thesis recognizes all levels sequence boundaries and identifies 2 megasequences,5 supersequences and 20 sequences. Among them, Upper Jurassic and Cretaceous are recognized respectively as a megasequence, 1 supersequence and 3 sequences are distinguished in Upper Jurassic;3 supersequences and 12 sequences are distinguished in Early Cretaceous; 1 supersequence and 5 sequences are distinguished in Upper Cretaceous.According to the unconformity developed by tectonic movements,the similar cycles of sedimentation with marine event and volcanic event,the second order sequences of Late Mesozoic in Sanjiang area can be contrasted completely. The second order sequences(supersequences) are controlled by tectonic movements. Supersequence A corresponds to the extrusion stage of Upper Jurassic; Supersequence B corresponds to the fault stage; Supersequence C corresponds to the integer depression stage; Supersequence D corresponds to the petty fault again and Supersequence E corresponds to the shrinked stage of basin. Among them, Supersequence C develops all over the area and the sequencesⅥ～ⅩⅢ(Chengzihe Formation and Muleng Formation) can be contrasted by differences of tectonic movements ,extension of marine bed,number of coal beds, palaeontologica evidences and the sequences evolution in-phase indicates that Sanjiang area is one uniform depression inshore basin.Sequence is the result of base level changes controlled by several factors during the sedimentary period.From Chengzihe Formation (Qihulin Formation+Yunshan Formation) to Muling Formation (Zhushan Formation),the numbers of sequence in each basin are different for the influences of tectonic,sea level change, sedimentary supply,climate and palaeogeorelif. When Chengzihe Formation (Qihulin Formation+Yunshan Formation)deposited, the quantities of sequence in Hulin basin and Boli basin were the most and it had 5 sequences,including sequenceⅥ,Ⅶ,Ⅷ,Ⅸ,Ⅹ,but it only developed sequenceⅧ,Ⅸ,Ⅹin Jixi basin and Suibin depression of Sanjiang basin. When Muling Formation (Zhushan Formation) deposited,each basin in target area all developed sequenceⅪ,Ⅻ,ⅩⅢand the sedimentary center was in Boli basin.Because of different denudation ,The rudimental sequenceⅪ,Ⅻ,ⅩⅢare reserved intactly in Suibin depression,but in Hulin basin they are reservered badly nowdays. SequenceⅥ—ⅩⅢdeveloped gradually from east to west and from south to north. With the rise of base level, when sequence X , XI , XII developed,the lacustrine level ascended to tiptop and sedimentary centre was in Boli basin,so the quantity of sequence in it is the most.According to the integrative analysis of the Sanjing area utilizing cores and field sections,there are 6 kinds of sedimentary facies including alluvial fan facies, fluvial facies,(fan)delta, lacustrine, lacustrine bottom fan facies, marine facies and 16 kinds of subfacies and 31 kinds of microfacies in target area. Using well log and seismic materials to analyze seismic facies in second order sequences framework,based on transforming relation between seismic facies and sedimentary facies,the thesis indicates the sedimentary evolution characteristics in target area. During the Supersequence A sedimentary periods of Suibin+Dongrong Formation, it was rudimental bay environment in Suibin Depression and it developed offshore , shallow marine facies and marine-continental transitional facies. At the late Supersequence A,seawater retreated from the study area;During the Supersequence B sedimentary periods of Didao (Peide) Formation, the basins were isolated fault basins, which mainly developed alluvial fan-(fan)delta-lacustrine sedimentary systems with volcanic coarse clastic rock;During the Supersequence C sedimentary periods of Chengzihe Formation (Qihulin Formation+Yunshan Formation) and Muling Formation (Zhushan formation), the basins were uniform depression basins, which developed (fan)delta-shallow lake-semi-lake- lacustrine bottom fan sedimentary systems,and the sedimentary extension was most widely in Muleng Formation. Meanwhile, it existed marine events. The direction of marine transgressions were from the east of Qihulinhe depression in Hulin basin and north of Suibin depression; During the Supersequence D sedimentary periods of Dongshan Formation, the basins developed (fan)delta front-lacustrine sedimentary systems with quantities of volcanic coarse clastic rock.The uniform basin was destroyed and the rudimental Dongshan Formation distributes locally nowdays for the strongly uplift after Dongshan Formation deposited. During the Supersequence E sedimentary periods of Late Cretaceous, the contour of the basins looks like nowdays basin contour ,the depth of the water was shallow. It mainly developed alluvial fan -(fan)delta -lacustrine sedimentary systems .