Vegetation and climate during the primary formation of the Songliao Basin, NE China

Abstract

The International Continental Scientific Drilling Project (ICDP) in the Songliao Basin that has produced a continuous core of the Lower Cretaceous in 4134.81 m thickness, i.e. the SK-II borehole, provides an exceptional archive for studying tectonic and palaeoenvironmental history of the basin under the Cretaceous greenhouse climate. The Shahezi Formation that consists of the main part of this core, represents the primary basin fills at the syn-rift stage of the Songliao Basin and thus marks its commencement and is crucial for studying the timing and mechanism of this basin's early evolution. The precise age of the Shahezi Formation, however, remains controversial and little is known about the palaeovegetation and palaeoenvironment during its deposition. In this study, plenty of pollen and spores are extracted from core samples near the base of the Shahezi Formation. They are the oldest fossils so-far identified from the SK-Ⅱ borehole. The high abundance and diversity of Cicatricosisporites, in association with some other fossils as Pilosisporites, indicate a clear early–middle Aptian age for the base of the Shahezi Formation, i.e. the primary formation of the Songliao Basin. The vegetation was overall featured by abundant ferns, common cycads and some bryophytes in the ground storey, alongside conifer trees or shrubs of Pinaceae, Taxodiaceae, Araucariaceae and Cheirolepidiaceae in the upper canopy, with increases of Cheirolepidiaceae and cycads over time. A generally warm and humid tropical to subtropical climate with a trend to be slightly drier was developed, which might be a response of terrestrial ecosystem to the Oceanic Anoxic Event 1a.