Tectonic Characteristics And Evolution In Nantun Formation Of Lower Creteaous, Ta'nan Depression

Ta'nan Depression is one of the secondary tectonic units of Tamu Chaga basin. It's a double-faulted structure and the Lower Cretaceous strata from bottom to top are: Tongbomiao, Nantun, Damuguai River and Yimin Formation. The studied formation Nantun Formation. Based on well seismic, Nantun Formation is divided into two thrid-order and four cycle sequence. In this study, the author studies four cycle based on the tectonic characteristics and evolution of Nantun Formation.Through the detailed interpretation of seismic profiles, it is interpreted that Nantun faults evolved with sedimentary process. It can be seen from the seismic profiles that there are three major faults which divide into eastern, central, western-three parts. It is fewer faults in the southern part. The northern fault significantly increased, and it comes from south to North fault portfolio diversification. Combinations of seismic profiles in Nantun Formation are: "Y-shaped" fracture patterns; the"Ren-shaped"fracture patterns; the combination of graben-type fault combination; the horst fault combination and ladder fault combination. The most common type of fracture is the combination of "Y shaped"; Secondly, horst-type and ladder-type fracture patterns are also a large portion. The"Ren-shaped"and"pazi-character"types in the basin are rarely indentified. Judging from the regional distribution, the northern section is obviously more complex in structure of assemblages, and there are more structural styles in the north. The Basin holds the more simple styles in fault assemblages and fewer types of fracture combinations from north to south. Structural frameworks from studied works show the plane arrangement of the three sub-basins from east to west are trending with faults. Major faults within the depressions are described and graded, then the stylesof fault on plane are summed up. Fault patterns of the main fork-like faults, comb fault, step-like fault, fault brush, character-like faults and an echelon faults are alike. Judging from the regional distribution, it is shown that the comb and the ladder-like fault developed in the basin by south of the main faults, character-shaped and en echelon faults did in the basin on the northwestern main faults, brush faults are developed in western basin.By the calculation of the extension and drawing geological section, it is indicated that the basin has been tensional deposited in Nantun. At the same time, stretching factors can be seen in different sections from the bottom of the stretch coefficient different trend, suggesting that depressional tension at different positions vary on the degree of imbalance.Tectonic subsidence analysis shows that different sub-sag basins'subsidence profiles are different as follows: Nantun deposited first, with the southeastern basin of the tectonic subsidence the largest rate in the early deposition of Nantun Formation; in south-central and southeastern tectonic subsidence, the rate is relatively large. Deposition at the end of the Nantun Period, Western secondary depression and the tectonic subsidence rate of secondary depression was higher. By balanced cross section, it is indicated that in Nantun deposition period, the basin subsidence center changed from southeast to northwest.