Analysis Of Sedimentary Characteristics And Tectonic Environment Of The Late Jurassic-early Cretaceous Mashiping Basin

On the basis of survey and exploration to the outcrops of the Upper Jurassic-Lower Cretaceous Mashiping Basin, investigation was made to Mashiping Basin on sedimentary characteristics and tectonic environment combining the way of section analysis, cathodoluminescence, grain-size analysis, sporo-pollen analysis, organic carbon analysis and so on.The results show that sedimentary cover is composed of Upper Jurassic Huangtuling and Nanzhao Formation, Lower Cretaceous Mashiping Formation in Mashiping Basin. The Huangtuling Formation is new discovery and naming stratum, and in conformable contact with overlying Upper Jurassic Nanzhao Formation. It is 361.2m and slump accumulation body with rich slump structure composed of the dark grey schist conglomerate, polymictic conglomerate, turbidite, payn grey gravel siltstone. On the basis of discovery of sporopollen fossils including Acanthotriletes, Leiotriletes, Granulatisporites, Lophotriletes, Spinozonotriletes, disaccate pollen and sedimentary characteristics and tectonic environment, the Huangtuling Formatio is temporarily fixed for Late Jurassic. Discovery of the new stratum has the vital significance to tectonic evolution of Mashiping Basin and Qinling-Dabie orogenic belt.The sedimentary facies of Mashiping Basin is successively nearshore subaqueous fan, fan delta near mountains, fan delta near fans and braided river delta combining the way of sedimentary sequence, sedimentary structure, grain-size analysis. The Huangtuling Formation is mainly deep lake debris flow and turbidite deposit at the top of nearshore subaqueous fan. The lower member of Nanzhao Formation is mainly front fan delta fan of delta near mountains and turbidite. Its upper and middle member is mainly fan delta front of fan delta near mountains. The lower part of Mashiping Formation is mainly fan delta plain and fan delta front about fan delta near fans. The upper member of Mashiping Formation is mainly braided river delta.The basin-controlling faults of south side controled the sedimentary evolution of Mashiping Basin combining the way of sedimentary facies analysis, structural background analysis and provenance analysis. The stratum are slump accumulation body composed of underling system, lubrication layer, sliding surface, sliding system, zone of outer edge pushing in Shibangou cross-section.Based on the above analysis, the Late Jurassic-early Cretaceous Mashiping Basin is a typical haft-graben fault basin with a fault as its south boundary and a slope to the north controled by Waxuezi-Xiaoluogou Fault. The evolution of basin contains three stages: subsidence quickly(Huangtuling Formation), subsidence smoothly(Nanzhao Formation and lower member of Mashiping Formation) and basement uplift(upper member of Mashiping Formation). The formation and evolution process show basement dipping toward south, slope decreased gradually in gradient, lake water shoaling, and tectonic activity changed from strong to weak.