Straigraphic Redivision And Tectonic Evolution In The Middle Sag Of The Wuliyasitai Depression, Erlian Basin

The Erlian Basin is a large Mesozoic sedimentary basin in northeast China. The stratigraphic subdivision of the subsurface strata in the basin has long been based on paleontological data, lacking accurate isotopic dating evidences. This situation has brought a lot of obstacles in studying the evolution history of the basin, and making stratigraphic correlation among the basin, its neighboring basins and the Greater Xingan mountain belt.The present study selects the Upper Jurassic-Lower Cretaceous strata in the middle sag of the Wuliyasitai depression, northeastern Erlian Basin as the targets. The LA-ICP-MS zircon U-Pb dating of seven samples of volcanic and clastic rocks is carried out. The dating results show that the conglomerate sample Tai8 which was smapled from previously-defined Jurassic stata is younger than 121 ± 6 Ma, and the conglomerate sample TC2 which was thought to have been deposited in the earliest Cretaceous is younger than 124 ± 2 Ma. The 206Pb/238U ages of five volcanic rock samples Tai6, Tai28, Tai14, Tai18, and Tai26 are 436.6 ± 9.7Ma,152.0 ± 4.4 Ma, 120.7 ± 1.4 Ma,124 ± 13 Ma and 151.0 ± 2.9 Ma, respectively. However, the last three samples of Tai14, Tai18, Tai26 were thought to have been deposited during the Jurassic, Jurassic, and Paleozoic, respectively.Based on detrital zircon U-Pb dating results of the first member of Tenggeer Formation in Well Tai8 and TC2, isotopic geochronologic data of outcrop sections in the neighboring areas of the Wuliyasitai depression, and sedimentary characteristics of the depression, the present study analyzes possible sediment source areas of the depression. It is suggested that ancient highlands around the depression are probably the main source areas of the Lower Cretaceous strata.Based on the theory of sequence stratigraphy, using seismic and drilling data, the present study analyzes sequence stratigraphic characteristics for the the Upper Jurassic-Lower Cretaceous strata of the Wuliyasitai depression. We interpret and trace twenty-one seismic sections in the depression, and indentify three unconformities (T11, T6, T3). Using the isochronous stratigraphic framework established by seismic and drilling data, combining with zircon U-Pb dating results, we re-divide the Upper Jurassic-lower Cretaceous strata.Based on the new knowledge of the Upper Jurassic-lower Cretaceous strata, the present study analyzes the evolution history of the middle sag of the Wuliyasitai depression. The upper Jurassic strata were deposited before 150 Ma. In the latest Jurassic-earliest Cretaceous, the depression experienced a long period of uplift and denudation. From 130 Ma, the depression underwent an intense rifting period. In the early Late Cretaceous, the extensional environment in the depression came to a sudden end, and strong folding and uplifting occurred. Based on the new stratigraphic subdivision, the study discusses the influence Mongolia-Okhotsk Ocean closure during the late Jurassic-Early Cretaceous and the collision between the Okhotomorsk block and East Asia during the early Late Cretaceous to the evolution of the Wuliyasitai depression, Erlian Basin.