Terrestrial records of Early Cretaceous paleoclimate fluctuations in the Yin'e Basin, northern China: Evidence from sedimentology and palynomorphs in lacustrine sediments

Rapid changes in paleoclimate in the Early Cretaceous had profound impacts on the global ecosystem, which is recorded not only in marine sediments but also in coeval sediments deposited in continental lake systems. This study performed the lake sedimentary characteristics and paleontological data of the upper member of Bayingebi Formation (K1b2) of Lower Cretaceous in the Yin'e Basin (North China) and revealed the paleoclimate disturbances and the evolution of paleovegetation and paleoenvironment recorded in the continental lake. The sediments of K1b2 can be divided into 10 kinds of lithofacies according to grain size, sedimentary structure and overall lithofacies. The three lithofacies associations are nearshore subaqueous fan, semi-deep lake/deep lake and shallow lake respectively. Seventy-eight palynomorph genera and two hundred and thirty-one palynomorph species were identified in the K1b2 samples, which allowed distinguishing 3 palynozones. The sporopollen spectrum and the ecological characteristics of sporopollen parent plants indicate that there was abundant temperate coniferous forest vegetation during the deposition of Bayingebi Formation. The mixed phenomenon of a few cold-loving molecular spores and pollen such as spruce, cedar and fir with tropical and subtropical spores and pollen suggest vertical zoning of plants. It is inferred that the ground shape and height difference around Yin'e Basin in Early Cretaceous is large, and there may be paleogeomorphic features of high mountains, and the paleoclimate has vertical zoning. The K1b2 depositional period was mainly in the warm temperate-temperate paleoclimate background, and oil shale was mainly developed in the semi-humid warm temperate climate. The suitable paleotemperature and the sedimentary environment of semi-deep lake/deep lake provide rich organic matter sources and good preservation conditions for the formation of oil shale. The dropstone structures commonly found in the lower strata of the oil shale member of the Bayingebi Formation in the study area were deposited in a semi-humid temperate climate and were inferred to be ice-rafting deposits, which may be related to a brief climatic cooling event or seasonal freezing during the Early Cretaceous in northern China. The typical East Asian paleontological fossils (Jehol Biota) and lacustrine hydrocarbon source rocks developed in the Early Cretaceous in the study area were affected by the paleoclimate warming in the early Aptian, which was closely related to the increase of global CO2 concentration in this period.