The Sequence Stratigraphy Of Tertiary In Tangyuan Fault Depression Of Yishu Graben

Yishu Graben is one of the northern branches of Tancheng-Lujiang fault zone which crosses throughout the northest of China. Tangyuan fault depression is the northen sub-tectonic unit of Yishu Graben. It is a dustpan-like fault depression, that is faulting in the east, overlapping in the west and is controlled by eastern border fault F1. The main sedimentary deposit of Tangyuan fault depression is Tertiary stratigraphy. In Tangyuan fault depression faults develop very much and its structural framework is characterized by the zonation from east to west and the segmentation from south to north. It can be divided into four sub-tectonic units from east to west which are the eastern hollow zone, the center bossy zone, the western hollow zone and the western zone of slope. From south to north it is made up of five big blocks which are relatively single parts and obviously different from each other. These blocks are portioned further into fifteen secondary building units and some third class tectonic units. Every secondary hollow possesses different geological characteristics and evolutionary history on account of the different features of the basement, the different shape of the basement and the different fault activity. The nine-class sequence unit (Van Wagoner, et al) was adopted to the study of the Tertiary sequence of the Tangyuan fault depression. The terminology system of four-division scheme within a third order sequence (Liu Zhaojun, etc.) is adopted to divide the third order sequence. Based on the detailed information and the author's abundant researching experiences in different types of continental basin, the author used seismic analysis, field observation, logging and core as primary means in sequence division. Besides those the theories of paleontology, geochemistry, mineralogy, event sedimentology are used as the accessorial study means. The key of the study of the sequence stratigraphy is the classification of diversified sequences and the sequence features study. The study of the third order sequence classification is very important and it is the base of sequence study in the region of interest. The marks of parasequence and parasequence group are mainly recognized by well core, outcrop and logging; the marks of sequence are mainly recognized by logging, seismic, paleontology, geochemistry, mineralogy; The marks of supersequence and megasequence are mainly recognized by physical geography, stratigraphy and paleontology, areal geology; and related research methods. The sequences characteristics study mainly includes two sides of content: one is the characteristics of the sequence interface, the other is the inner structural characteristics of sequences, among them, interface characteristics is the main mark to distinguish the sequence types. By analyzing a lot of materials of well core, outcrop, logging, seismic ang other geological data, testing data, the author summarizes the sequence interface characteristics and its recognition marks of 5 order sequences from megasequence to parasequence of Tangyuan fault lacustrine basin, and mainly marks eight main seismic .reflection boundaries, that is T0,T1,T2,T3,T4,T14,T5,Tg. According to the characters of its reflected inpedance , the author makes clear the geologic signification of every boundary : T0,T5,Tg are the megasequence bounderies, T1 is the supersequence sets boundary, T2,T3,T4 are the supersquence boundaries, T14 is the sequence boundary. Although the grade of sequences is different, their boundaries are all different order unconformity surfaces, which have the similar recognition marks. Of these recognition marks, the marks of seismic facies, logging facies and lithologic facies are the most reliable and operationable, which are also the main marks to divide and distinguish the Tertairy sequence units of Tangyuan fault lacustrine basin. paleontology, clay minerals, heavy mineral assemblages and geochemistry index can be the direct recognition marks to divide the megasequence and supersequence boundaries, and can be the supplymentary marks of sequence, parasequence and systems tract boundaries. In this thesis, terminology of four-division scheme within a third order sequence is used to study sequence stratigraphy of Tertiary lacustrine basin of Tangyuan fault depression, and the author distinguishes and divides the sequence stratigraphic units of Tertiary lacustrine basin of Tangyuan fault depression, which generally consists of 1 megasequence, 2 supersequence sets, 5 supersequences and 13 sequences. Among them, sequence VIII-IX is equal to Wuyun formation, sequence X-XI is equal to Xin'ancun formation, sequence XII-XIII is equal to Dalianhe formation, sequence XIV-XV is equal to Baoquanling formation, sequence XIX-XX is equal to upper Tertiary. On the base of summarizing the characteristics of every sequence stratigraphic interface and their recognition marks, using various kinds of subject research methods and utilizing intergrative results of seismic well,core,outcrop,palaeontologic and geochemistry data, the author carefully analyzes and studies the inner composition characteristics of sequence stratigraphic units from sequences to parasequences (three order sequences) in research region. According to lithology and original sedimentary structure of well,core and outcrop, this thesis interprets 15 kinds of lithologic and structural facies, makes up 14 kinds of basic stratigraphic units to explain the vertical innercharacteristics of parasequences. According to the parasequence stacking patterns,it can be divided into progradational,retrogrational and aggradational parasequence sets, the grain size,silt ratio,well logs and depth of parasequence of different types have distinctions in characteristics of vertical change. By studying the formation mechanism,type characteristics and recognition methods of Tertiary sequence boundaries of Tangyuan fault lacustrine basin,combining the inner structural characteristics of sequences, the thesis sets up the sequence stratigraphic framework of research region. Looking far and wide the regional tectonic evolutionary history of research region, according to analyzing fault filling and heaping characteristics ,tectonic feature of the basement and sedimentary cover structure, Tertiary is made up of 1 megasequence, which can be divided into fault depression supersequence sets and post-fault depression supersequence sets, equaling to the develop period of Tangyuan fault depression graben and post-period graden, two different tectonic evolutionary stages. Fault period is the main evolutionary stage of Tangyuan fault depression, fault depression evolution can be divided into four stages, that is intense fault period(T5-T4),continue fault period(T4-T3),transform of fault to depression period(T3-T2),fault shrinking period(T2-T1), every period all forms corresponding supersequences, each other equals to sequence VIII-XI, sequence XI-XIII, sequence XIV-XVI, sequence XVII-XVIII. post-fault depression sets can be divided into 1 supersequence, including sequence XIX and sequence XX. The factors, such as tectonic movements,climates,rate of sedimentary supply and so on, control the development of Tertiary sequences of Tangyuan fault lacustrine basin. By analyzing the cause of sequence interface,classification of every sequence stratigraphy and their developed characteristics, the results go step further to confirm that tectonic movements,climates and rate of sedimentary supply achieve to control the development of sequence stratigraphy of fault depression. Structure,rate of sedimentary supply and climates have a great significance of the formation of sequence stratigraphy ,tectonic movements control the formation type of basin, that is the development of megasequences. Tectonic screen and climate cycle control the development of supersequence sets, fault movements in the inner basin and climate factors control the development of sequences and their interfaces. The relationship between the rate of sedimentary supply and the changeable rate of the accommodated room decides the stacking relationship of parasequence sets or high-frequency sequences. Thanks to the same tectonic screen has the similar tectonic movements,climate background and characteristics of sedimentary supply ,which decides that every third order sequence in supersequence sets formed by the VI...