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

The correlation of freshwater sediments in small, fault-bound basins in Liaoning Province, northeast China, known as the Jehol (or Rehe) Group, has been a subject of debate for many years, with biochronological estimates ranging from Late Jurassic to the Cretaceous periods. We have applied the laser 40Ar-39Ar 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 give 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 stratigraphically between the upper and lower parts of the Yixian are in very good agreement with the absolute age framework provided by the volcanic units. Such disseminated facies of this clay show promise for directly dating lacustrine sediments. All of the above 40Ar-39Ar dates provide an absolute calibration of the Yixian Formation, and show that the whole formation was deposited entirely within Early Cretaceous time over an interval of no more than 2-3 Ma.