## 4 Sequence Stratigraphic Analysis of Volcanic Rock-Evaporite-Clastic Rock Symbiotic Zone
### Saline Lake Sequence
#### 4.3.2 Saline Lake Sequence Stratigraphic Superposition Patterns  
The saline lake sequence mainly developed in the center of the southern lake basin in the Laizhou Bay Sag, which was formed mainly due to the paleoclimate influence. During the period from the lower sub-member of the fourth member of the Shahejie Formation to the upper sub-member, the climate changed from drought to semi-drought, forming two sets of different stratigraphic superposition patterns.  
A sulfate lake superposition pattern of progradation-accretion saline lake salt rock-saline lake mud mixed-sedimentation developed in the lower sub-member of the fourth member of the Shahejie Formation. This superposition pattern mainly developed in the center of the northern depression. In the early stage of the LST, the sedimentation of saline lake mud, salt rock, and sandy beach bar was discovered, showing weak progradation overall. Affected by the dry climate, the lake level dropped, the water body became shallow, and the accretion was strong. Dark gray mudstone and thick gypsum rock were superposed in multiple periods. Thick mudstone developed in the lake TST. During the period of HST, large sets of gypsum developed. The sedimentary lithology was gypsum-mudstone-gypsum rock. The sedimentary microfacies of gypsum salt beach-sulfate lake mud-gypsum salt beach reflected the change of the water body from shallow to deep and then to shallow, as well as the change of the LST-lake TST-HST (Figure 13a).  
Figure 12: Identification of the salt lake rock–electrostrata–sequence interface (B-1-1): (**a**) SBS4 drilling sequence interface identification mark; (**b**) SBS3– drilling sequence interface identification mark.  
Figure 13: Salt lake sequence stratigraphic overlay pattern: (**a**) the paste salt lake sequence stacking style; (**b**) the carbonate lake sequence stacking style.  
A programation-retrogression type carbonate rock-clastic rock mixed-sedimentary type superposition pattern developed in the upper sub-member of the fourth member of the Shahejie Formation, which formed under the sedimentary environment of carbonate lake in the center of semiarid lake basin. In the early stage of the LST, lake mud and an underwater distributary channel developed and showed progradation characteristics; in the lake TST, the water body became shallower with the lake level descending; mudstone and carbonate rocks began to develop in the late stage of the LST; and sandstone, mudstone, and carbonate rocks were deposited alternately. During the period of the lake TST, the lithology changed from the limestone of the underwater distributary channel to mudstone, and the sediments in the underwater distributary channel turned into lake mud, which reflected the rising water body. During the HST, the lake level dropped again. Carbonate beach and thick limestone sedimentation developed, changing from percarbonate beach bar-underwater distributary channel progradation to carbonate beach bar-lake mud retrogradation, and later to lake mud-carbonate beach bar progradation. The lithological changes of limestone-mudstone-limestone under the shallow lake-carbonate lake mud-carbonate beach bar stratigraphic stacking and superposition pattern reflected the shallowing of the lake level and the change of the LST-lake TST-HST (Figure 13b).