Late Mesozoic transgressions of eastern Heilongjiang and their significance in tectonics, and coal and oil accumulation in northeast China

The spatial and temporal distribution of Late Mesozoic marine and brackish-water sediments in eastern Heilongjiang records three main transgressive interludes: The late Middle Callovian–Valanginian, Barremian–Aptian and Albian marine invasion phases. The Callovian–Valanginian transgressions were limited to the northeastern corner of eastern Heilongjiang, but the others intermittently extended northwest-, southwest- and westwards along the Yunshan shallow embayment, which opened into and deepened towards the northwestern Palaeo-Pacific. They not only flooded almost all of the subsiding basins surrounding the embayment, but during peak transgressions also widely flooded areas outside eastern Heilongjiang, such as the Yanji, Beipiao–Fuxin and Songliao basins along the Tan–Lu fault system of northeast China. All the transgressions came from the northwestern Palaeo-Pacific Ocean, and probably the Arctic Sea as well. In addition to the eustatic sea level changes, the late Early Cretaceous transgressions of northeast China were closely related to the tectonic movements within the Tan–Lu fault system and the circum-Paleo-Pacific, which were very pronounced and went hand in hand with a strong volcanic activity and ongoing subsidence. During the late Early Cretaceous, the long-term existence of a marine embayment in eastern Heilongjiang, successive marine transgressions, and flooding along the Tan–Lu fault system in northeast China ensured a humid climate across all of northeast China. These transgression and climatic conditions produced a number of extensive and long-lasting swamps and marsh lands in both paralic and limnic environments. Luxuriant plant and thriving animal growth led to the accumulation of abundant organic matter in the deposits. As a result, a number of late Early Cretaceous coal basins and oil fields formed in eastern Heilongjiang, the Yanji basin of eastern Jilin, Beipiao–Fuxin basin of western Liaoning, and the Songliao basin, northeast China.