Characteristics of Palynological Assemblage and Paleoclimate Analysis of the Miocene Formation in the Eastern Bohai Bay Basin

Abstract

Through the sporopollen and algal data of 13 wells selected throughout the area, more than 50 generas of plants are found. Angiosperms are abundant and the most common are juglandaceae; gymnosperms are mainly pinaceae, with the common being keteleeria, abies and hemlock; ferns are mostly aquatic ferns, especially magnastriatites; algae are all freshwater algae, including plicatidium, pediastrum and chaetosphaeridium. Detailed analysis of PL7-6-1 well in Bodong low uplift and PL14-3-1 well in Miaoxi Sag shows that the evolution of arborescent stratum in sporopollen composition can be roughly divided into two stages: Early Guantao Formation (sporopollen assemblage zone III, IV) – Coniferous dominated mixed forests, Late Guantao Formation (sporopollen assemblage zone I, II) – Deciduous forests contain many subtropical elements. The composition of under forest is single, mainly herbaceous, shrubs and terrestrial ferns are rare. In the herb, piceaepollenites are developed, the aquatic sporotrapoidites and graminidites are also common. Different plants adapt to different temperatures and humidity. Statistics show that the humid plants of the entire Guantao Formation in the work area are significantly more than the dryophytes, and the philotherm plants are significantly more than the thermophilic plants, which have typical characteristics of subtropical monsoon climate. With the passage of time, the characteristics of subtropical monsoon climate are more and more remarkable. The T/A, H/A, S/A and E-T parameters calculated from tall trees, herbs, tropical elements, ephedra sporopollen and aquatic plants can better describe the climate change among the sample points, which proves that the climate fluctuates frequently during Guantao Formation, and the overall trend of change is consistent with the previous statistical analysis. Generally speaking, the period of Guantao Formation belongs to subtropical monsoon climate, and has the characteristics of numerous lakes and marshes, the lake surface is wide and the water body is shallow.