## 5 Discussion  
### Response of Structural Events to Paleoclimate Evolution  
5.1.1. Ages of the Hajiiazhuang Section  
(1) Zone I  
Zone I shows a predominance of gymnosperm pollen, with a scarcity of fern spores. Among the gymnosperms, _Classopolis_ dominates, accounting for an average percentage of 78.60%. _Classopolis_ flourished during the Upper Jurassic (with contents ranging from 60% to 80%), declined rapidly in the Lower Cretaceous, and became more and more abundant in the late Lower Cretaceous [2, 32, 38, 39, 40] (usually in the Aptian). The high abundance of _Classopolis_ is also characteristic of the pathological assemblages of the Liupanshan Group [19, 41, 42]. _Jiahepollis_, a representative Lower Cretaceous pollen from Northern China, occurred in the Berriasian-Albian period and was most prominent during the Hauterivian-Aptian time [29, 40, 43]. Additionally, Zone I contains typical Early Cretaceous spores, such as _Cicatriciosisporites_. _Cicatriciosisporites_ are often regarded as markers that distinguish between the Jurassic and Cretaceous [44, 45]. Furthermore, based on fossil assemblages of spores, pollen, ostracods, and algae in the Liupanshan Group of the Haiyuan Depression in the Liupanshan Basin, the Heshangpu and Liwaxia Formations possess Hauterivian-Barremian (Lower Cretaceous) ages [2]. Therefore, the sedimentation of Zone I may not have preceded the Barremian and was not later than the Albian. (2) Zone II  
Zone II is characterized by the dominance of gymnosperm pollen, followed by fern spores. Compared with Zone I, _Classopolis_ is significantly less abundant in Zone II (although still at a generally high level), whereas the abundances of _Araucariacites_, Bisaccates, and _Jiahepollis_ are significantly more common. _Jiahepollis_ was widespread in the Berriasian-Albian. A small amount of _Erliampollis_ (spanning Berriasian-Aptian) [46] appears in this zone. The palynological assemblage in the Madongshan Group in the Sikouzi section is similar to that of Zone III, where _Classopolis_ remains predominant but with reduced abundance. In contrast, the contents of Bisaccates and _Jiahepollis_ increase, while fern spores are scarce. The palynological evidence from the Sikouzi section points to the Aptian to Albian geological ages of the Madongshan Group [33]. In Zone II, characteristic Cretaceous fossils such as _Appendicisporites_ and _Cicatriciossi-porites_ also occur. _Appendicisporites_ first appeared in the Berriasian (Lower Cretaceous) and gradually declined during the early Upper Cretaceous [47]. Additionally, some typical Lower Cretaceous spores, such as _Epidriites_ and _Toroisporis_, were found in Zone II. Therefore, the sedimentation of Zone II probably occurred during the Aptian. (3) Zone III  
Zone III is characterized by the predominance of gymnosperm pollen, followed by fern spores. The angiosperm pollen _Magnolipollis_ is distributed throughout Zone III, although in small amounts; its presence distinguishes Zone III from the previous two. _Magnolipollis_ may have existed in minor quantities since the Upper Jurassic; however, it became widespread from the Aptian-Albian onwards [29; 48]. _Jiahoepollis_ became increasingly abundant in the Berriasian-Albian and peaked in the Hauterivian-Aptian. A small number of continuously distributed _Erlianpollis_ appeared in Zone III, and their distribution age was from Berriasian-Aptian. Additionally, characteristic Early Cretaceous spores, such as _Appendiisporites_, _Cicatricisporites_, and _Toroisporis_, were present. Therefore, the sedimentation of Zone III likely happened in the Aptian-Albian. From a lithological point of view, the Haijiazhuang section mainly comprises gray, dark gray mudstone, and black shale, which accords with the lithological characteristics of the Madongshan Formation. [PERSON] et al. conducted paleomagnetic surveys on the sedimentary strata of the Huoshizhai Section in the Liupanshan Basin and found magnetost stratigraphic ages of 118.3\(\sim\)109.6 Ma (Aptian-Albian) for the Madongshan Formation [16] (Figure 6).