An integrated stratigraphic record from the Paleocene of the Chijiang Basin, Jiangxi Province (China): Implications for mammalian turnover and Asian block rotations

Abstract

New paleomagnetic and isotopic results from a ∼ 1000 meter thick Paleocene stratigraphic section in the Chijiang Basin of China's Jiangxi Province provide chronostratigraphic constraints on the Shanghuan–Nongshanian Asian Land Mammal Age boundary and allow for a more accurate determination of an early Paleogene paleomagnetic pole for the South China Block. Paleomagnetic analysis of 121 sites (326 samples) reveals that the Shanghuan–Nongshanian boundary lies close to a normal-to-reverse polarity change. Stable carbon isotope analysis of dispersed organic matter and paleosol carbonates indicate a secular increase of ∼ 1.5‰ superimposed on higher frequency variations. Correlation of this magnetochemostratigraphic pattern to the global timescale suggests that the polarity reversal near the Shanghuan–Nongshanian boundary likely represents the Chron C27n–C26r transition. The Torrejonian–Tiffanian North American Land Mammal Age boundary is closely correlated to this same polarity transition, indicating nearly synchronous global mammalian turnover at this time. Because both Land Mammal Age boundaries are thought to record turnover of mostly endemic taxa, it is unlikely that the synchroneity of faunal change is due to intercontinental dispersal as documented for other early Paleogene faunal transitions (e.g. Paleocene–Eocene boundary). Instead, these mammalian faunal transitions may represent independent ecological or evolutionary responses to environmental changes that have been interpreted from marine records at this time (e.g. “Top Chron 27n Event”). These new paleomagnetic results from the Chijiang Basin are also used to augment other published data and to calculate a Paleocene paleomagnetic pole for the South China Block. The new pole shows no significant vertical axis rotation compared to the Paleocene reference pole for Eurasia indicating that much of the clockwise rotation that has been documented for the South China Block from Cretaceous deposits cannot be the result of extrusion tectonics associated with the early Paleogene India–Asia collision. Observed Cretaceous rotations of the South China Block may be the result of late Cretaceous–early Paleocene rifting in the backarc of the Kula–Pacific subduction zone.