1. periodicity and coupling of sea level change and tectonic subsidence
Global sea level fluctuation and tectonic activity are twin brothers, which exist in the global crustal evolution, but each has its own laws.
The rise and fall of sea level may be uneven for a long time. In a tectonic cycle, the rising wing of sea level starts from the turning end, from slowly rising to rapidly rising, then slowly rising to stagnation, and then to the descending stage. After the superposition of short-period and long-period sea level rise and fall, a coupled sea level rise and fall law is formed, which controls the sedimentation, sedimentation mode and superposition relationship of sediments. The sediment and stratigraphic records actually seen are the sea level change effect after superposition (see figure 1-6).
The general law of global tectonic activity is the cracking and polymerization of plates, which correspondingly leads to the rise and fall of global sea level. The effective accommodation space is in the cycle of first-order structure and sea level change, which is the formation and extinction of the basin. If understood from the perspective of tectonic subsidence, it is the changing process of tectonic subsidence rate from high to low.
Like the sea level change cycle, the tectonic cycle also has structural orders. However, due to the inhomogeneity of the crust-mantle thermal column, the tectonic activities in different regions are very different. Obviously, the secondary tectonic activity is the result of superposition under the overall tectonic background, which is much more complicated than the superposition effect of long and short periods of sea level. It can be seen that there is a complex causal relationship between tectonic subsidence and sea level change and sediments and stratigraphic records, which constitutes the framework of sequence stratigraphy.
The basic assumption of sequence stratigraphy is that the cycle of apparent sea level change rate leads to the change of new space for accommodating sediments. The change of accommodation space does not depend on sediment injection, but the potential sediment filling thickness is a function of accommodation space, and accommodation space is a response to sea level change. No matter what the tectonic framework of the basin is, the apparent sea level at the edge of all basins has the characteristics of different change rates. The basement activities at the edge of the basin vary greatly, from uplift to rapid subsidence. The total tectonic subsidence along the dip direction of the basin margin has three trends: (1) the subsidence rate increases towards the sea; (2) The settlement rate increases towards the land; (3) The settlement rate along the inclined section is constant.
The first situation is more common in cratons, passive continental margins and continental margins along marginal active zones. The second situation is most common in foreland basins and semi-graben basins adjacent to orogenic belts, and the craton side of foreland basin looks like a passive edge from the trend of tectonic subsidence. The third situation is in a basin where regional tectonic activity is stagnant.
2. Calibration of tectonic sea level fluctuation domain
The prerequisite for calibrating the tectonic sea level fluctuation zone under the background of passive continental margin and foreland basin is that given the global sea level change and apparent sea level change rate, the change of accommodation space can be calibrated according to the three tectonic subsidence trends introduced above. Posamentier et al. (1993)[2 1] expounded the first change trend of the basin margin, and put forward the tectonic-sea level fluctuation zone (figure 1-8) in order to predict the stratigraphic composition. Area B is characterized by low settlement rate. In a certain sea level period, the global sea level decline rate exceeds the settlement rate, resulting in the relative sea level decline effect (Posamentier and Allen, 1993) (Figure 1-8). In the whole global sea level cycle, the subsidence rate in area A is always greater than the global sea level decline rate, which shows obvious sea level rise. According to the comprehensive analysis of global sea level and tectonic subsidence, this zoning is very useful to predict the sequence stratigraphic composition of the basin margin. In a given sea level cycle, the position of the coastline in the maximum ocean direction determines whether a type I sequence or a type II sequence is formed. If the most seaward position of the coastline is in area B, type I sequence develops; If the maximum seaward position of coastline is in Area A, then type II sequence will develop (Posamentier and Allen, 1993a) (Figure 1-9).
Figure 1-8 Illustration of the characteristics of tectonic subsidence profile of foreland basin (a) and passive margin (b) with active margin (according to Posamentier and Allen, 1993).
In the foreland basin, due to the thrust of thrust block, the tectonic subsidence in the land direction increased obviously (A). On the contrary, due to the cooling of the crust in the land direction, the tectonic subsidence increases in the ocean direction (b).
Figure 1-9 Sequence Stratigraphic Composition Diagram of Area A or Area B when the coastline changes (revised by Posamentier)
Shoreline A is located in Area A, and the sequence is progradation → accretion → retrogression, with no sedimentary crossing, unconformity and low water level deposition (A); Shoreline B is located in Area B, and the sequence includes low water level shoreline, sediment crossing area and possible river valley cutting and filling deposits (B).
Lst-low water level system tract; TST-transgressive system tract; SMST-shelf margin system tract; HST-Advanced System Domain
3. Sedimentary input and basin paleotopography
Above, the importance of shoreline position in the sea level domain of these two structures is analyzed, and sediment supply is an important parameter to determine stratigraphic composition (Schlager, 1992, 1993) [23].
With the change of global sea level and total tectonic subsidence, the boundary position between Area A and Area B has changed greatly, and the sequence stratigraphic composition will also change (Figure 1-9). As for deep-water sedimentation, sediment supply plays an important role. When the sediment supply is sufficient, the sedimentary center of the passive edge will move to the shelf slope break zone. Therefore, when the relative sea level begins to decline, shelf margin and deep water deposition will occur in the sedimentary center. Another important parameter that restricts the formation of sequence stratigraphy is the paleogeomorphology of the basin margin. Under the background of gentle slope edge, there is no deep-water turbidite fan in low-water system tract. When the edge of the coastline retreats to the slope break zone of the continental slope, the low-water system tract has deep-water turbidite system of valley cutting and filling deposits, basins or slopes on the shelf.
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