The type of datum gyratory structure, the hierarchical nature of datum gyration

Hierarchicality is one of the essential characteristics of geological phenomena and a universal law followed by geological theories (et al., 1993). Due to the cyclical nature of tectonic movements, sedimentation and climate changes in large and small sedimentary basins, the sediments in the basin are cyclic in time and space, segmented and periodic in the vertical direction, and zonal in the plane, and the cyclic nature of the genesis is hierarchical in energy, period and scale, so the genesis and distribution of the strata themselves are also hierarchical (Wang Zhenqi et al., 2002).

The hierarchical nature of sedimentary strata is the response to the hierarchical nature of the datum cycle. Datum gyration is a relative concept, and the research scale and affiliation of stratigraphic gyration according to different research objectives constitute the level of datum cyclus. The Lower Stone Box Formation in this area consists of one long-term cycle (LSC mid-cycle (MSC3 5) and 15 short-term cycles (SSC13 27) (see Chapter 3), which also includes three different levels.

The long-term datum gyration is a sequence entity bounded by the maximum lacustrine flood surface or the large regional scouring interface at the top and bottom, which can be identified in the ** profile. Short-term stratigraphic cyclic (genetic sequence) is a stratigraphic record preserved in a sedimentary environment with genetic connections from the side to the side during the datum cyclic change, and is a stratigraphic unit that conforms to the law of facies sequence or facies combination (Walther's law) (2000), mainly through facies sequence analysis. After identifying the short-term datum cycle, the medium-term cyclic datum cycle is formed by analyzing the longitudinal superposition pattern of successive genetic stratigraphic units and the change trend of the A S value to gradually merge the short-term cyclic cycle (Fig. 3-2).

The high-precision datum gyratory analysis and the establishment of the medium and short-term cyclic sequence stratigraphic framework have laid a foundation for the division of small layers, the comparison of sand bodies, the study of reservoir heterogeneity and the study of reservoir flow units.

According to the meaning of the sedimentary datum, that is, "the sedimentary datum is a potential energy surface that is undulating relative to the surface and is continuously slightly inclined downward towards the basin, and its position, movement direction and rise and fall amplitude are constantly changing with time, and it is basically the storm wave base plane parallel to the horizontal plane within the sea basin (lake basin), and the landward direction is a undulating curved surface" (Fig., and it is considered that it is controlled by comprehensive factors such as sea (lake) level, tectonic subsidence, sediment recharge, subsidence load compensation and sedimentary topography, In fact, it also includes the four major factors that control the formation and development of the sequence summarized by the classical sequence stratigraphy, and the results of the comprehensive action of these elements are reflected in the change of the datum, so the datum is the main basis for controlling the formation of the sequence and dividing and comparing it.

Figure Datum, accommodating space, and geomorphological morphology reflecting the balance between accommodating space and sediment supply.

Different from marine basins, in continental basins, basement subsidence, sediment** and climate play a more obvious role in sequence control, in which tectonic control is crucial. The continental basin has strong tectonic movement and strong segmentation. For example, in the Late Triassic Ordos Basin, the curtain tectonic activity of the boundary control basin formed a long-term stratigraphic cycle, and the secondary curtain activity generated during the period formed a secondary stratigraphic cycle, which led to the multi-level sub-cyclic process of the filling strata in the fault depression basin.

The main controlling factors of datum cyclic formation are different in different orders, but the larger-scale datum cyclic sequences in continental basins are mainly controlled by tectonic factors (such as i-order sequences), which is consistent with the sequence of classical sequence division. The higher the order of the tectonic datum cyclic sequence, the worse the degree of contrast between the stratigraphic cycles in the basin, and the more obvious the control effect of tectonic subsidence, climate and sediment. In addition to the tectonic movement and sediment recharge, the influence of the short-term datum cycle on the formation of stratigraphic cycles is gradually increasing, such as the rupture of rivers and the migration of delta polymorphs, so it can only be traced and compared in the sediments.