The Late Cretaceous Sedimentary-tectonic Evolution Of Yongfeng-chongren Basin In Jiangxi Province

Jiangxi Yongfeng-Chongren Basin is a continental rift sedimentary basin formedunder the south china extensional geotectonic environment during the Late Mesozoicperiod, has developed a hugely thick red coarse clastic rock formation. As being located inthe edge of Fuzhou Basin and Taihe Basin, it has an obvious sedimentary differentiationand is a favorable area of studying basin sedimentary–tectonic evolution. At present,pepole only have very little research in the basin and are short of understanding for thedepositional environment and the geological evolution process of the basin.On the basis of participating in the1:5million regional survey work, I mainlyexpanded field observations and laboratory analysis researches on the Upper CretaceousHekou Formation, Tangbian Formation and Lianhe Formation in Jiangxi Yongfeng-Chongren Basin. Through the researches of lithologic features, sedimentary structures,grain size distributional characteristics and paleontological features, I analyzed thesedimentary facies of the Upper Cretaceous in this basin detailedly, and explored thedepositional environment of each group.From the bottom up, it substantially consists of a coarse-fine-coarse approximatelysymmetrical sedimentary cycles formed by the fluvial-alluvial fan, braided river-river,lake-braided river, fluvial-alluvial fan, the Upper Cretaceous strata in Yongfeng-Chongren basin reflects an alluvial fan, braided river and coastal shallow lacustrinedepositional environment on the whole. According to the development of sedimentarycharacteristics and the depositional environment, I have divided three sedimentary-tectonic evolution processes including the Hekou Period, Tangbian Period and LianhePeriod, each period shows its own unique features.In the Hekou Period, because of the intensely faulted tensile action, both sides of theland area were subjected to strong erosion. At the edge of the basin, it rapidly developedseveral accumulations of alluvial fan and braided river which were close to the source. Themain lithology contain polymictic conglomerate, sandy conglomerate, medium–fineconglomerates and pebbly fine sandstone. There are a lot of sedimentary structuresinvolving parallel bedding, large-scale trough or tabular cross-bedding, normal gradedbedding. Furthermore, both the sieve plot structure, imbricated structure and the scoursurface structure are very common. In the late depositional period of Hekou, tectonicmovement became gently, the retrograde superposition phenomenon could clearly be seenon the top of alluvial fan. The faulted depression pulled apart weakly during the Tangbian Period. In therelatively stable tectonic background, the basin formed a frequently stacked profilestructure of river and coastal shallow lacustrine as the cyclic rise and fall of base-level.Tangbian Group is characterized by the development of large set of argillaceous siltstoneformation, contains both sandy conglomerate, the gravelly coarse sandstone and pebblyfine sandstone strata yet. It can be generally found that there are several sedimentarystructures in the strata, mainly are parallel bedding, small-scale trough or tabularcross-bedding and normal graded bedding. It also grew peeling lineation and imbricatedstructure. The segmentation cracks and calcareous concretions were formed in the aridclimate. In this period, paleontology were prosperous, but their types were monotonous,sometimes there was an individual species with a higher abundance.The tectonic movement became frequently and actively in this basin during theLianhe Period. The basin-controlled fracture violently actived and the northwest side of thebasin intensely uplift, which resulted in Lianhe Formation only developing in thenorthwest of the basin and expressing as either braided river or alluvial fan facies.However, it mainly presents as an alluvial fan depositional environment, and thephenomenon of several alluvial fan multiple depositional superimposed can been commonseen. The Lianhe Formation is made up of conglomerate, sandy conglomerate, pebblymedium-fine sandstone and striped siltstones, forms parallel bedding, troughcross-bedding, normal graded bedding and large-scale scour surface structure.Overall, the formation and evolution of Yongfeng-Chongren basin was significantlycontrolled by tectogenesis, climate and provenance, but tectogenesis is the maincontrolling factor. Tectonic movement impacted most directly on the distribution of thestrata and combination of sedimentary facies in this area, and dominated the wholeevolutional processes of the basin from formed by stretching to extruded closed.