40Ar/39Ar age constraints on Cretaceous fossil-bearing formations near the China–North Korea border

Abundant Mesozoic terrestrial plant, fish and invertebrate fossils have been discovered from the Lishugou Formation and the underlying Xiaolin Formation in the eastern Liaoning province of NE China since the 1970s. These discoveries have great potential for substantially increasing our knowledge on the evolution of paleoecosystems and paleoenvironments in East Asia. However, previous geological and geochronological studies for these fossil-bearing formations are rare because the fossil outcrops are near the border between China and North Korea. Here we present robust 40Ar/39Ar ages of 99.9 ± 0.4 Ma for one sample from the Lishugou Formation and 109.1 ± 0.2 Ma and 122.4 ± 0.2 Ma for two samples from the upper and lower parts of the Xiaolin Formation from Dandong City, Liaoning, China. Our new age data indicate that the Lishugou and Xiaolin Formations were deposited contemporaneously with the Jiufotang Formation from the classic Jehol outcrops in western Liaoning. Therefore, our results provide an accurate age calibration for the fossils discovered from the Liaodong Peninsula and suggest that the hypothesis of the Jehol migration should be reappraised.