ASTRONOMICAL FORCING OF MIDDLE PERMIAN CHERT SEQUENCE IN LOWER YANGTZE AREA, SOUTH CHINA

Astronomical forcing is believed to be a fundamental driver of climate change over geological history. Oceanic bedded cherts of Mesozoic age recently have been demonstrated to have astronomical origins. Here we report on older, Permian chert sequences with similar cyclic features. In the Lower Yangtze area (Chaohu City), South China, we have found a Middle Permian radiolarian chert sequence characterized by rhythmic chert-mudstone couplets that formed in anoxic conditions on a limited continental shelf. This sequence occupies the principal part of the Gufeng Formation, of early Wordian to middle Capitanian age. Recent U-Pb geochronologic data from zircons in volcanic tuff layers below the first chert layer provide a constraining age of 272+5.5 Ma. Through the comparison of four Gufeng Formation sections outcropping in the Lower Yangtze area, a continuous, high-resolution Middle Permian chert sequence has been established. We present an analysis of the thickness variations of this chert-mudstone sequence to show that its key signal is indicative of astronomical forcing. We use the objective modeling to show that the signal is composed cycles that are consistent with combined obliquity and precession forcing. The results allow a reconstruction of an approximately 2.5 million year long chronology for the lower 9 m section of the sequence.