DATES AND RATES IN ANCIENT LAKES - AR-40-AR-39 EVIDENCE FOR AN EARLY CRETACEOUS AGE FOR THE JEHOL GROUP, NORTHEAST CHINA

The correlation of freshwater sediments in small, fault-bound basins i n Liaoning Province, northeast China, known as the Jehol (or Rehe) Gro up, has been a subject of debate for many years, with biochronological estimates ranging from Late Jurassic to the Cretaceous periods. We ha ve applied the laser Ar-40-Ar-39 technique to volcanic intercalations and lacustrine sediments from the Yixian Formation at the base of the Jehol Group. Minerals and whole-rock chips from the upper parts of the Yixian Formation give concordant ages with a mean of 121.1 +/- 0.2 Ma (1 sigma). Ages for samples near the base of the Yixian Formation giv e 121.4 +/- 0.6 and 122.9 +/- 0.3 Ma, and appear to be synchronous or only slightly older than the top of the formation. Integrated ages of 122 Ma for glaucony from the lacustrine sediments lying stratigraphica lly between the upper and lower parts of the Yixian are in very good a greement with the absolute age framework provided by the volcanic unit s. Such disseminated facies of this clay show promise for directly dat ing lacustrine sediments. All of the above Ar-40-Ar-39 dates provide a n absolute calibration of the Yixian Formation, and show that the whol e formation was deposited entirely within Early Cretaceous time over a n interval of no more than 2-3 Ma.