New 40Ar/39Ar dating results from the Shanwang Basin, eastern China: Constraints on the age of the Shanwang Formation and associated biota

The fluvio-lacustrine sequence of the Shanwang Basin, eastern China, preserves a rich and important terrestrial fossil fauna and flora; the exceptional preservation of these fossils reveals the dynamics of ancient mammalian ecosystems and plant biology. However, the timing of this sedimentary sequence has been the subject of debate for decades. Here we contribute to this debate by presenting the detailed results of 40Ar/39Ar analysis of the basalts above, below, and within the Shanwang Formation. These dates place stringent constraints on the age of Shanwang Formation and associated biota. 40Ar/39Ar ages obtained from basalts of the Niushan and Yaoshan Formations, which underlie and overlie the Shanwang Formation, are 21.0 +/- 2.5 Ma (2 sigma, full external error) and 17.3 +/- 1.5 Ma (2 sigma, full external error), respectively. The 40Ar/39Ar age of the basalt in the Shanwang Formation is 17-18 Ma. Given the age constraints of the basalts of the Yaoshan and Shanwang Formations, the age of the Shanwang biota is estimated to be ca. 17 Ma, late Burdigalian of the Early Miocene, indicating that the deposition of this fauna coincided with the onset of the mid-Miocene Climatic Optimum. The results provide new age constraints on the Shanwang mammal fauna, and independently support interpretations that this fauna can be assigned to chronozone MN4, and correlated with middle Orleanian of the European Land Mammal Age, and to late Hemingfordian of the North American Land Mammal Age. Biological diversity of the Shanwang Formation could reflect the global-scale mid-Miocene Climatic Optimum. (C) 2011 Elsevier B.V. All rights reserved.