Seguence Stratigraphic Frame of Jurassic in East Fukang Slope, Junggar Basin

Key words: 
• Jurassic /
• seguence boundary /
• systems tract /
• seguence stratigraphic model /
• control factor

Abstract: This paper is concerned about the study of seguence stratigraphy of Jurasssic in East Fukang Slope, Junggar Basin. East Fukang Slope is located in Northeast Junggar basin, a dipping southwest sloping zone. The area is about 5 000 km². Because Jurassic was rarely deformed by the tectonic movement, there are few structural traps with obvious structural closure in this area. Therefore, it is necessary to seek lithological or stratigraphic traps guided by seguence stratigraphic theory. Jurassic in East Fukang Slope consists of Lower Jurassic, including Badaowan Formation（J₁ b）and Sangonghe Formation （J₁ s）, Middle Jurassic, Xishanyao Formation（J₂ x）and Toutunhe Formation（J₂t）, and Upper Jurassic, Oigu Formation （J₃ q）and Kalazha Formation（J₃ k）. Combining with core data, well logging data, seismic data and analysis data in lab, Jurassic could be divided into six seguences, that is：Seguence I（SI）（J₁ b¹）, SI（I J₁ b²）, SII（I J₁ s—J₂ x¹）, SIV（J₂ x²）, SV（J₂ t）and SV（I J₃ g—J₃ k）from bottom to top. Moreover, according to synthetic seismic data, the divisions of core and well logging seguence are identical with seismic seguence. The distinguished marks of seguence boundary include：surface of unconformity, the mutation surfaces of fossil, lithology, well logging curve and sedimentary facies, basal conglomerate and surface of scour, coal formation , etc. The definition of the first flooding surface（FFS）and the maximum surface（MFS）is the significant step for the division of systems tract. In the period of Jurassic, due to the steep slope from Baijiahai uplift to Changji sag and the gentle slope from Cainan region to Kelameili Mountain, a slope break formed in the southeast of Baijiahai uplift. Therefore, the FFS can be determined, resulting in the definition of lowstand systems tract（LST）, transgrassive systems tract（TST）and highstand system trac（t HST）. In the study area, identified marks of systems tract boundary include the followings：the first steady and continuous seismic axis across the slope break can indicate the FFS;coal formation or danby？can indicate FFS or MFS;the change of lapping style of subseguences and the change of the colour of sedimentary rocks are available as well. LST is mainly composed of prograding subseguence, TST of aggradating and retrograding subseguences, and HST of aggradating and prograding subseguences. Combining with the paleo - topography feature, the distribution of significant coal formation and the position of FFS and MFS, the seguence stratigraphic model or frame has been established in East Fukang Slope. This model shows that the seguence stratigraphic style of Jurassic apparently was influenced by cycle of relative change of lake level, resulting in the onlaps on the steep side of lake basin regularly moving. The principal factors of seguence stratigraphic style of Jurassic include tectonic subsidence, the supplying rate of sediment, climate and the change of base level. Tectonization was the direct one, by which the unconformity boundary, the most apparent identified mark, was formed. Tectonic subsidence controlled the generation and consumption of accommodation. The changes of sediment supplying rate and subsidence rate could influence the lapping styles of subseguence sets and systems tract. In addition, the change of base level was one of the most significant factors which resulted in the change of accommodation and relative lake level.