## 6 Sequence Evolution Model  
The geometry and lithology of the sequence stratigraphic units are controlled by four basic factors: tectonic subsidence, global sea level rise and fall, sediment supply rate, and climate change [25; 26]. Among them, tectonic subsidence provides the accommodation space for sediment deposition. Secondly, global sea level change controls the distribution pattern of strata and lithology. Lastly, the sediment supply rate, largely influenced by climate, controls the type and quantity of sediment, filling process, and ancient water depth in the basin [27]. The formation of sequence strata is mainly controlled by accommodation space (tectonic movement, global sea level rise and fall) and sediment supply. The establishment of the third-order sequence isochronous framework is mainly limited by sequence boundary (unconformity or disconformity) and flooding surface [28]. According to the relationship between the rate of sea level change and basin subsidence rate at the slope break zone of the sedimentary shoreline and the sequence boundary types, the marine sedimentary sequences can be divided into two types of third-order sequences [28; 29], type I and type II. Type I sequence is composed of low-stand (LST), transgressive (TST), and highstand (HST) system tracts, and the bottom boundary of the sequence is a type I unconformity interface and its corresponding integration surface. Type II sequence is composed of a shelf margin system tract (SMST) and a transgressive (TST) and high-position (HST) system tract, and the bottom boundary of the sequence is the type II unconformity interface and its corresponding integration surface. The type-I sequence boundary is a regional unconformity interface, which is produced when the global sea level drops faster than the basin subsidence speed at the slope break of the sedimentary shoreline. Its upper and lower strata lithology, sedimentary facies, and strata occurrence can change greatly, and it has land exposure signs and deep valleys formed by river rejuvenation. Type II sequence boundary is formed when the global sea level decline rate is lower than the basin subsidence rate at the slope break zone of the sedimentary shoreline, so there is no relative sea level decline at this position, and, thus, there is a lack of regional erosion caused by river rejuvenation. The changes in lithology, sedimentary facies, and stratigraphic occurrence of the upper and lower strata are not as dramatic as those in Type I, but there is obvious accretion of overlying strata or the offshore shift towards the basin above the boundary. Because the overall slope of the study area is extremely slow, the sequence boundary lacks obvious river rejuvenation and deep valley, and the lower system tract lacks basin floor fan and slope fan, so the third-order sequence in the study area consists of only a transgressive system tract (TST) and high-position system tract (HST) (Figure 13). The sequence boundary of this sequence development model is formed when the global sea level decline rate is lower than the basin subsidence rate at the slope break zone of the sedimentary shoreline, so there is no relative sea level decline at this position, and, thus, there is a lack of regional erosion caused by river rejuvenation. The changes in lithology, sedimentary facies, and stratigraphic occurrence of the upper and lower strata  
Figure 13: Sequence development model diagram. are not drastic, but there is obvious accretion of overlying strata or the shift towards the basin on the coast above the boundary. Therefore, tectonic subsidence controls the shape and provenance direction of the study basin and controls the boundary (unconformity) and development of the third-order sequence at the same time as the global third-order relative sea level change. The fourth-order relative sea level change simultaneously controls the boundary of the fourth-order sequence, its sedimentary environment, and sediment types.