HIGH-PRECISION U-PB GEOCHRONOLOGIC CONSTRAINTS ON THE LATE CRETACEOUS LACUSTRINE SUCCESSIONS OF THE SONGLIAO BASIN, NORTHEAST CHINA

The ca. 2500 meters-deep drill core recovered from the predominantly lacustrine deposits of the Songliao Basin in East Asia provides an extraordinary record of Late Cretaceous terrestrial paleoenvironments during a crucial period of climate perturbations and environmental change in Earth history. This near-continuous early Turonian to Paleogene stratigraphic record exhibits cyclic sedimentation that has been attributed to the Milankovic forcing of climate and has been used to construct an astronomically calibrated time scale for the basin. Numerous volcanic ash (bentonite) layers intercalated with the lacustrine strata provide a unique opportunity for high-resolution radioisotopic age calibration of the cyclostratigraphy, as well as temporal correlation to the global Cretaceous. We present high-precision U-Pb zircon geochronology by the ID-TIMS method for bentonite beds from the Qingshankou and Nenjiang Formations (Turonian – Campanian) of the Songliao succession, with significant enhancement in precision and accuracy over the previously published geochronology. The results allow the Songliao cyclostratigraphy to be calibrated at a cycle level and its astronomical time scale to be directly tested. The geochronology from the Nenjiang Formation also improves present age constraints on the termination of the Cretaceous superchron of normal geomagnetic polarity (C34n).

Reliable correlations of the Upper Cretaceous successions, especially those between terrestrial and marine realms, are necessary for a better understanding of the Late Cretaceous climate and its driving mechanisms. The new high-resolution chronostratigraphy of the Songliao Basin facilitates unequivocal correlation to the Upper Cretaceous of the Western Interior Basin of North America; one that has been hindered by contrasting biostratigraphies and by the Cretaceous Normal Superchron. The geochronology of the basal Qingshankou Formation, in particular, places precise temporal constraints on the East Asian lake anoxic events and thus elucidates their possible links to the global sea level changes recorded in shallow marine depositional settings.