Paleoclimatic and paleogeographic significance of the early Santonian ice-rafted dropstones in the Songliao Basin, NE China

Abstract

Two ice-rafted dropstones were discovered in the Yaojia Formation (early Santonian) of wells Ao14 and F247 in the Songliao Basin, northeastern China. The dropstone from well Ao14 was further studied using computed tomography (CT) scanning, sporopollen, carbon and oxygen isotope, and zircon U–Pb isotope analyses. The results show that the dropstone is composed of quartz monzonite and has an age of 178.0 ± 3.3 Ma. Based on the bedrock age analysis around the Songliao Basin, it is suggested that the dropstone in well Ao14 may have originated from the Zhangguangcai Range. The palynoflora assemblage indicates a mixed survival of cold and warm palynomorphs. The carbon and oxygen isotope compositions of the synsedimentary calcareous clasts surrounding the dropstone exhibit obvious negative biases, suggesting that glacier meltwater flowed into the paleo-lake, probably due to the significant topographic height difference between the Songliao Basin and adjacent areas during the early Santonian. Additionally, it can be inferred that coastal mountains formed in the Zhangguangcai Range area, which is located southeast of the Songliao Basin, and the dropstones derived from this region. During the cold season, the dropstones may be transported to the edge of Songliao Lake by avalanche and mixed with carbonate debris. As the seasons changed, the lake ice broke into pieces and carrying the dropstones and carbonate debris into the deep lake area. The present research provides direct evidence of a brief global cold event and the presence of high mountains in the southeastern Songliao Basin during the early Santonian in the Cretaceous greenhouse world.