Structural Characteristics, Deformation Sequences And The Relationship Between Structure & Sediment Of Urxun Sag, Hailar Basin

The structure and sediment are two important scientific questions in studying the formation and evolution of a basin. By summarizing and analyzing the predecessors'production, the paper find that there are two important ways in studying the relationship between sediment and structure from abroad to home at present: the one is to depict structural characteristics of geometry and kinematics just by geometry characteristics of syntectonic sediments through interpreting seismic reflection profiles; the other is to discuss the basin's uplift and/or subsidence just by studying what the basin's synsedimentary material composition is, how they assembled and distributed at three-dimensional. However both of them can not accurately illuminate the synsedimentary fill and deformation history of a basin, so we must integrate these two ways with each other in the studying of basin's structure and sediment, which is not only an available way to understand the basin's structural and sedimentary characteristics correctly, but also is a key of studying the coupling of basin and range.At present, there are many arguments about tectonic features and deposition of Urxun sag, but Urxun sag developed a series of integrated and thick strata. There are many high quality three-dimensional seismic reflection profiles and the well's data in Urxun sag, so it's the best natural laboratory for studying basin's structure and the relationship between structure & sedimentation.The structural geometry and kinematics characteristics, and sediments fill are three most important research subjects of the paper. At first, on the basis of analyzing and summarizing the predecessors'information, the paper finds out the limitation and deficiency/void in studying of Urxun sag. Secondly, combined with the date of the high quality well and other sedimentary information, the paper detailedly and accurately interprets many high quality three-dimensional seismic profiles including main lines and traces, on the basis of which the paper analyses the Urxun sag's main structural styles and the different structural characteristics on different orientations adopting present popular theory or production about structural geology, such as fault related fold and balanced section techniques. Thirdly, the paper illuminates the syntectonic kinematics characteristic and analyses some main seismic profiles'structural deformation processes on the base of balanced section restoration techniques, and summarized the deformation sequences of Urxun sag. After that, the paper constructs the fault's geometry and kinematics models on different tectonic stress field and discusses how they control the deformation of synsediments. At the end, the paper, combination with thepresent production of sequence stratigraphy about Urxun sag, discusses the relationship between structure and deposition, considering three aspects such as tectonic sequence, sedimentary systems (faces), and the migration of subsidence or sedimentary center.According to the scheme referred above, the paper discussed structural characteristics, deformation sequences and the relationship between structure & sediment in detail, and we obtained presently cognitions(conclusions) as follows:1. Urxun sag in Hailaer basin is a large unsymmetrical extensional- contractional sag with strike-slip characteristics during Mesozoic-Cenozoic era, located in Inner Mongolia-Daxing'anling orogenic belt.2. The Urxun sag's structural style is more complicated. The sag does not only develops the listric normal fault and rollover anticline or some extensional fault related fold at the regional extensional setting, but also develops other structural styles in different tectonic settings, such as thrust fault and fault related fold associated with contraction setting, flower structural, associated with strike-slip setting (palm tree structure and tulip structure), and reversion structural style associate with the change of regional stress field.3. the structural system includes three trends of listric normal faults (SN, NE and NEE) and fault related folds, which are SN-trending Wuxi faults zone (F1, F2) and Wuzhong faults zone (F3), NE-trending Huangqimiao faults zone (F0-1, F0-2) and other NE-trending faults (F0-3, F0-4) near Wuxi faults zone, and NEE-trending Surenuo'er faults zone. These listric normal faults generated at NW-SE trend stretching stress field with lots of bimodal volcanics erupting intensely during Xing'anling Group, meanwhile NE-trending structure have pure extensional features, and the SN and NEE trending's are extensio-shear features. They underwent many tectonic movements and displayed different structural types at different times. During Xing'anling Group-Nantun Formation sedimentation, they were dominating normal faults which controlled the syntectonic sediments, but from Damoguaihe formation to the first stage of Yimin Formation sedimentation, they reversed, or became thrust faults and fault related fold. At the middle stage of Yimin formation sedimentation, most early faults were steady, but some of NE and NEE trend faults revived again showing extensional structural characteristics. And on the hanging-wall of F1, a series of new SN-trending faults called Wuxi-Fault F2 and Wuzhong-Fault F3 separately, formed and became the dominating structure which controlled the basin-fill. At the end of Yimin Formation sedimentation, almost all faults reversed, the sag was uplift and eroded. After that the tectonic movements were so weak that just a few of fault cutoff the bottom of Qingyuangang Formation.4. Besides the intensive pressing at the end of Yinmin Formation sedimentation that has been proved by predecessors in many articles, the paper revealed that there has been another intensive pressing from the telophase of Nantun Formation to the first stage of Yinmin Formation sedimentation by detailed geological and geophysical researches. The paper divided the sequence of deformation of Urxun sag into five stages: (1) the intensive stretching of NW-NE orientation during the Xing'anling Group-Nantun Formation sedimentation; (2) the pressing of NW-NE trend during the Damoguaihe Formation-first stage of Yimin Formation sedimentation; (3) the intensive tenso-shearing of near EW orientation at the middle stage of Yimin Formation sedimentation; (4) the intensive compresso-shearing of near EW trend at the end of Yimin Formation sedimentation; (5) the feeble stretching of near EW trend (or depression stage) from Qingyuangang Formation sedimentation to now.5. The geometry and kinematics characteristics of faults on extensional and compressional settings in Urxun sag controlled the way of syntectonic sedimentary development and deformation. The paper concludes that Urxun sag's formation and deformation were controlled by simple-shear mechanics regime. The paper uses growth fault related fold theory to construction the geometry and kinematics models for the structure and its deformation which have the feature of kink-band migration. However, it is important to adopt the newest structural theory, that was presented by John Suppe (2004) called shear fault related fold, to construction the geometry and kinematics models for these including both kink-band migration and limb rotation features.6. The characteristics of dominating faults geometry and kinematics are dominating factors for syntectonic sediments during the sag development since the deposition always goes with the deformation of Urxun sag. The study area develops three types of tectonic sequence interfaces:â‘ the erosional/truncational unconformity caused by the intense uplift of Urxun sag (T5, T22, T04),â‘¡the toplap unconformity (T21, T20) formed when tectonic activity is weak or stable,â‘¢and the special interface (T5, T22, T1, T04-2, T04) caused by the change of regional stress field. The lower Cretaceous strata of Urxun sag should be divided into 4 tectonic sequences. When studying sedimentary system/faces, the paper finds that the types, distribution, and evolution of the sedimentary systems are severely controlled by the main faults at different deformation stages. In Urxun sag, the sedimentary centre nearly is subsidence centre, and its migration is controlled by the structural geometry and kinematics characteristics.