As the product of the evolution of geological history, the stratigraphic sequence only records less than half of the earth's history, and the longer time is the period of discontinuity and erosion (Li Sitian, 1995). As a negative record, sequence interface can provide the following five aspects of information: ① physical interface of various identification features; (2) Records available for study, including special deposits completely different from overlying and underlying sequences, and residual records related to underlying sequences; ③ erosion discontinuity of physical records; ④ Discontinuity of time series; ⑤ An event or a series of events that happened. It can be seen that the study of sequence boundary is of great significance. Since the birth of sequence stratigraphy, the role of sequence boundary in basin analysis and oil and gas exploration has become more and more obvious, so it has attracted much attention. Krapez(1996,1997) preliminarily discussed the relationship between sequence and basin, Xu Xiaosong et al. (1996,1997) systematically expounded the genetic types of sequence interface and its relationship with basin-mountain transformation, Li Sitian et al. (1995) emphasized the significance of sequence interface in the study of sedimentary basin dynamics, and Qin Jianxiong et al. (2111) discussed the hierarchical types of sequence interface and its relationship with basin. With the study of geodynamics and sedimentary basin dynamics becoming hot spots, the research on sequence boundary will be more in-depth and extensive.

Xichang basin is located in the western margin of the Yangtze block in the Pan-Cathaysian block group and in the Tethyan tectonic domain (Xu Xiaosong et al., 1997). As a product of the combination of the western continental margin of the Yangtze block and the Tethys Ocean, the Xichang composite basin has different tectonic attributes in different geological periods, showing obvious compound characteristics of vertical superposition and horizontal recombination. During the evolution of about 611Ma from Late Sinian to Paleocene, Xichang composite basin experienced the reconstruction and superposition of various events, such as tectonic ups and downs, climate mutation, sea level change, biological extinction, thermal events, etc., and formed sequence boundaries of various origins. These interfaces are widely distributed, of different types, with remarkable characteristics, different erosion and discontinuous, and are ideal areas and horizons for studying sequence boundaries. This paper focuses on the types and characteristics of interfaces, analyzes the genetic attributes and mechanisms of interfaces, and then discusses the significance of interfaces in stratigraphic division, basin genesis and oil and gas exploration.

8.1.1 level types and characteristics

According to the interface fabric characteristics, extension range, erosion degree and discontinuity period, Xichang composite basin * * * identified six levels of sequences, namely super, super, I, II, III and IV (Table 7.3 and Figure 8.1).

8.1.1.1

This kind of interface is characterized by micro-angle unconformity, with wide distribution range, great erosion change and long interruption time (Table 7.3). There are qualitative differences between the overlying and underlying sequences in basin properties, structural attributes and dynamic mechanisms, which represent the product of global tectonic movement. The area includes the bottom boundary of Upper Sinian (T1), the bottom boundary of Baiguowan Formation of Upper Permian (T69) and the top boundary of Paleocene (T78). Among them, T1 and T78 are the products of Chengjiang Movement and Himalayan Movement, respectively, which indicate the beginning and final extinction of Xichang composite basin evolution.

8.1.1.2 super

area includes Silurian-Devonian boundary (T41), Devonian-Permian boundary (T54), Middle-Upper Permian boundary (T61) and Middle-Lower Jurassic boundary (T73) (Figure 8.1). Signs mainly include: ① regional parallel unconformity; ② Regional eluvial layer; ③ obvious stratum loss; ④ Long-term sedimentary discontinuity (Table 7.3). It is revealed that the regional tectonic movement is the product of the main controlling factors, such as interface T73, which is mainly manifested in the regional parallel unconformity between the Middle Jurassic and the Lower Jurassic (Figure 8.1), representing the first act of Yanshan Movement, which implies the transformation of Xichang composite basin from peripheral foreland basin to intracontinental depression basin.

fig. 8.1 Division of sequence boundary level in Xichang composite basin and its relationship with basin evolution

1— super grade; 2-super; Grade 3-I; Grade 4-II; Grade 5-III; 6-Ⅳ level

(Note: only the typical interface of Ⅳ level is marked, and all the interfaces of other levels have been marked)

8.1.1.3 Class I

identification marks are similar to those of super interfaces, but the interface distribution is small, the erosion degree is weak, the scale of residual layer is small, and the erosion discontinuity is relatively inconspicuous (Table 7.3), which represents the product of regional tectonic movement. For example, the interface T76 is mainly marked by the absence of the Datongchang Formation in the upper part of the Lower Cretaceous. At the same time, the interface is a biological extinction event surface, which marks the beginning of the radiation evolution of angiosperms and is the product of the third act of Yanshan tectonic movement. In addition, there are the Sinian-Cambrian boundary (T11) and the Cambrian-Ordovician boundary (T22) at Class I interface (Figure 8.1).

8.1.1.4 Ⅱ

marks include two types. One is the obvious sedimentary discontinuity within the basin (usually equivalent to 1 periods or several biological zones), such as interface T47, which is the interface between the upper and lower Devonian, that is, the movement surface of Xichang structure II. The identification features of this interface include: ① the Bianjinggou Formation at the top of the lower Devonian is generally missing; ② Hematite layer is developed at the top of Pojiao Formation of Lower Devonian. Second, the special stratigraphic interface with relatively inconspicuous sedimentary discontinuity-comprehensive event stratigraphic interface, such as the interface between Permian and Triassic (T63) (Figure 8.1), which is the comprehensive product of many events such as biological extinction, sea level decline, tuff settlement, sedimentary geochemical anomaly, magnetic pole reversal, etc. (Yang Zunyi et al., 1992).

8.1.1.5 Grade III

marks mainly include karst surface, dissolved surface, exposed surface, dilute diagenetic section, dissolved cavity and fracture zone, dolomitization, dissolved breccia, paleosol, etc. Typical Grade III interfaces in this area include T16, T26, T31, T62, T71, T75 and T77. Among them, T16 is the most typical grade III interface, which runs through the whole region. The evidences mainly include: ① The central and western regions are uneven, and karst dolomite is developed at the top of the underlying Longwangmiao Formation, which is overlapped by the thin layer of limestone mudstone in the overlying Douposi Formation; ② 1 ~ 1.1m eluvial phase on the top, thinning eastward and pinching out.

grade p>8.1.1.6 Ⅳ

the identification features of this kind of interface are not obvious. It can be divided into two situations: one is the interface related to submarine diagenesis, which is characterized by hard bottom, soft bottom or solid bottom (Qin Jianxiong et al., 2111), such as interface T11 in the lower part of Lower Cambrian and interface T58； in the bottom of Middle Permian Maokou Formation; The second is the transgressive transformation interface related to the early exposure (dissolution) surface. The specific signs are: ① exposure (dissolution) of the transgressive transformation surface; ② High-amplitude transgression sequence above the interface; ③ There is obvious "phase jump" or abrupt phase change in adjacent system tracts overlying and underlying the interface. This kind of interface is mainly found in Qixia Formation of Lower Permian, such as T55, T56 and T58 (Figure 8.1).

8.1.2 genetic analysis

on the basis of the above-mentioned interface identification characteristics, combined with the formation background, development mechanism and its relationship with tectonic activities, sea level changes, basin properties and evolution, the above-mentioned sequences of different levels of Xichang composite basin are summarized into the following six genetic types (Figure 8.2).

8.1.2.1 orogenic erosion

T69 is the typical representative, and the genetic signs include: ① low-angle unconformity, land scouring and cutting or false conformity; (2) The underlying layer is marine carbonate deposits from Upper Sinian to Middle Permian, and the overlying layer is continental low-water wedge-alluvial fan-fan delta-piedmont molasse formation caused by the injection of external sources, which reveals the great turning point of Xichang composite basin from passive continental margin to foreland basin from the end of Middle Permian to the beginning of Late Permian. The mechanism of this kind (Figure 8.2A) is mainly as follows: orogeny and uplift led to the continental margin changing into foreland process, craton formed foreland uplift zone, and the underlying sediments uplifted, which was not integrated with the overlying sequence at a low angle.

8.1.2.2 Uplift erosion

includes T41, T54, T61 and T73, especially T54, with the following signs: ① Carboniferous deposits are generally absent, which affects Upper Silurian in some areas; ② Fe-Al weathering eluvium widely distributed in the upper Yangtze region; (3) The underlying karst stratum or weathered and reformed stratum is distributed regionally; ④ There is no obvious change in dynamic properties between the underlying old basin and the overlying new basin. It shows that at the end of Devonian, due to the influence of tectonic movement in Guangxi, the strata were uplifted regionally and suffered from long-term weathering and erosion, which finally caused the sequence interface of uplift and erosion (Figure 8.2B). This kind of interface represents the extinction of the old basin and the rebirth of the new basin under the similar structural background.

8.1.2.3 land erosion

the land erosion interface is mainly caused by regional tectonic movement, which exposes the stratum to the surface and suffers relatively slight erosion, resulting in a certain degree of sedimentary discontinuity. The identification mark is similar to the uplift erosion interface, but the distribution range of the interface is limited, the erosion degree is weak, and the residual scale is small (Figure 8.2C). For example, T11, as a product of Tongwan Movement, led to the false integration of the Qianzhusi Formation on the Dengying Formation, and T22, as a product of Xichang Movement, covered the Hongshiya Formation and the underlying Erdaoshui Formation, lacking the new stage. This type of interface reveals the transition surface between secondary basins or basin structural stages.

fig. 8.2 sequence genesis diagram of Xichang composite basin

1— structural uplift; 2-Basin subsidence; 3-sea level rise; 4-sea level decline; 5— Pelvic floor deflection; 6-rebound of basin bottom; 7-material source supply; 8-transgression overtopping; 9-submarine diagenetic surface; 11— limestone. A-D in the figure represents the sequence of sequence units. Fig. A is a diagram of the formation of orogenic erosion T69: A-1 is a sedimentary model of Middle Triassic; A-2 is the flexural deformation of foreland basin; A-3 is the rebound of foreland basin bottom and the orogenic erosion of sediments. Fig. B is a diagram of the formation of T54 caused by uplift erosion: B-1 is a late Devonian sedimentary model; B-2 represents the process of regional tectonic uplift, which makes strata C and D above the water table; B-3 indicates weathering and denudation of C and D, and older strata A and B are exposed to the surface. Fig. C is a diagram showing the formation of land erosion T22: C-1 represents the late Cambrian sedimentary model; C-2 indicates regional tectonic uplift, which makes stratum D above the water table; C-3 indicates that unit D is weathered and denuded, and the older stratum C is exposed to the surface. Fig. D is a diagram showing the formation of exposure erosion T4: D-1 represents the late Sinian sedimentary model; D-2 represents the exposed surface above the slope break zone; D-3 indicates that the exposed erosion interface above the slope break zone is preserved. Fig. E is a diagram showing the formation of underwater discontinuous interface T58: E-1 represents the Middle Permian sedimentary model; E-2 means that the sea level rises rapidly for a long period, which leads to hunger deposition; E-3 indicates the development and preservation of submarine diagenesis. Fig. f is a diagram of the formation of transgressive upwelling; F-1 represents the late Ordovician sedimentary model; F-2 indicates the development of early exposed surface; F-3 indicates the rapid sea level overlaping to the land, which leads to the transgression transformation of the underlying shallow-water sediments and the overlaping of the deep-water sediments

8.1.2.4. The exposed dissolution

was formed during the long-term main sea level decline period, that is, the main filling period of the basin, and it is the product of the sediments exposed to the surface at the end of the third-level sea level decline and subjected to fresh water dissolution, diagenetic transformation or surface residual (Figure 8.2D). This kind of interface constitutes the main interface types in the area, including T16, T26, T31, T62, T71, T75 and T77, which reflect the suspension or pause in the evolution of the basin. The combination of exposed and dissolved interfaces is the product of gradual contraction of the basin.

8.1.2.5 transgressive suprainterface

transgressive suprainterface is formed in the long-period main sea level rise cycle, which is the product of the superposition of long-period sea level rise and short-term sea level change (Figure 8.2E). Usually, the upper super-interface combination is characterized by increasing water depth from bottom to top, and the water depth of the top interface in the combination is the largest; As far as singleness is concerned, it is characterized by short-term static sea level or short-term low-amplitude sea level decline between the overlying and underlying transgressive sequences. There are a large number of such interfaces in the Lower Cambrian, Lower Devonian and Lower Permian, such as T4, T12, T51, T55～T58, which represent the products of sudden extension of continental margin and rapid expansion of basin.

8.1.2.6 Underwater discontinuity

The underwater discontinuity interface is formed in the seabed environment with relatively deep water body and obvious lack of sedimentation, which is the result of superposition of long-period sea level rise cycle and short-period sea level rise and fall (Figure 8.2F). It is mainly characterized by stagnation of sedimentation or early diagenesis of seabed, with underwater hard bottom, solid bottom or soft bottom, which is often generated with transgressive suprainterface, reflecting the maximum extension and stable development period of the basin. This kind of sequence developed in the Lower Permian in this area.

8.1.3 Research significance

8.1.3.1 Solving stratigraphic boundary problem

1) Sinian-Cambrian boundary. The classic CAMBRIAN-Sinian boundary is located at the bottom of the second small shell fossil Paragloborilus-Siphogonuchites belt in the upper part of the third member of Dengying Formation (Maidiping Section), and now it has moved up to the top of Dengying Formation according to the theoretical system of sequence stratigraphy (Figure 8.3). The evidence includes: ① Tongwan tectonic movement surface: a parallel unconformity surface distributed in a region; (2) Lithologic transformation surface: the overlying layer is transgressive phosphorus-containing siliceous silty shale distributed in the region, and the underlying layer is tidal flat dolomite; ③ Mutation surface of biota: the crustacean assemblages above and other metazoan assemblages below; ④ Polar deflection event; ⑤ abrupt climate change surface: the upper part is gypsum-bearing strata with dry and hot climate, and the lower part is carbonate rocks with warm climate.

fig. 8.3 Cambrian-Sinian boundary based on sequence stratigraphy (Ganluo section)

2) Ordovician-Cambrian. The traditional Ordovician-CAMBRIAN interface is usually drawn at the bottom of the conodont Cordylodus proavus belt or Monocostodus sevierensis belt in the upper part of Erdaoshui Formation, and now it is set at the top of Erdaoshui Formation (that is, between Erdaoshui Formation and Hongshiya Formation). The signs mainly include: ① Xichang tectonic movement surface: the new factory steps equivalent to the lower part of Lower Ordovician are missing; ② Lithologic abrupt change surface: clastic rocks of Hongshiya Formation cover dolomite of Erdaoshui Formation (Figure 8.4); ③ biotransformation surface: above it is the radiation evolution and biological phase differentiation of invertebrates, and below it is the evolution period of crustaceans; ④ Climatic abrupt change surface: The upper layer is rich in hematite and siderite with hot and humid climate, and the lower layer is dry and hot climate combination.

3) Silurian-Ordovician. The traditional Silurian-Ordovician interface is located on the five-peak Dicellograptus szechuanensis belt and Dalmanitina belt in the lower part of Longmaxi Formation, and now it moves down to the bottom of Wufeng Formation. The main marks are as follows (Figure 8.5): ① Exposed erosion unconformity surface: below it is the fresh water cemented breccia interval at the top of Linxiang Formation; (2) Typical transgression event surface: black shale of Gulf facies of Wufeng Formation is superimposed on dolomite of Linxiang Formation; ③ The tectonic palaeogeography and sedimentary pattern of Wufeng period are consistent with Silurian, but obviously different from the former Wufeng period.

Figure 8.4 Ordovician-Cambrian Boundary Based on Sequence Stratigraphy (Puge Profile)

Figure 8.5 Silurian-Ordovician Boundary Based on Sequence Stratigraphy (Ganluo Profile)

8.1.3.2 Explain the nature and evolution of the basin

Different structural backgrounds and basin attributes have resulted in different genetic types of sequences, and different structural amplitudes and basin scales have formed different levels. As the product of sedimentary basin evolution, sequence boundary reflects the evolution of sedimentary crust