Stratigraphy, Sedimentology, and Paleoenvironment of Lushi Basin (central China) in Early to Middle Eocene

The Lushi Basin in the eastern Qinling Mountains has yielded a large number of mammalian fossils and is an important locality of the Eocene. Recently, we have conducted sedimentology analysis, paleoenvironment reconstruction and chronology analysis based on a multi-disciplinery method on a ~1000 m thick section in Lushi Basin. Age model based on a combination of detrital apatite fission track ages, zircon U-Pb ages, authigenic carbonate U-Pb ages and paleomagnetostratigraphy shows that this sedimentary sequnece deposited in early to middle Eocene. Based on facies analysis, the Eocene strata in the studied site can be divided into 18 lithofacies and 6 facies associations. Zhangjiacun Fm and Dayu Fm mainly consist of conglomerates, sandstones and siltstones, abundant carbonate-rich paleosol and a low-quality coal lense are also observed. These two formations are interpreted as deposited in alluvial fan, gravel-bed channel plain, floodplain with or without ephemeral channels. Lushi Fm mainly consist of palustrine to lacustrine carbonate and siltstones, interpreted as siliciclastic or carbonate dominated palustrine deposits and shallow carbonate lake deposits. The abundant pedogenic/palustrine carbonate indicate a semi-arid to sub-humid climate, probably with marked seasonality. A comparison of early to middle Eocene environment reconstructions between Lushi Basin and other basins arounding has shown that fluvial-lacustrine sediments with chemical deposition dominated by carbonate is common in this region, with minor basins have thin gypsum layers. Thus, the semi-arid to sub-humid climate may have regional significance. Vegetation reconstructed by floral fossils or pollen assemblages in surrounding basins show woodland or forest, which is consistent with the mammalian fossil assemblage being dominated by forest and wetland fauna in the Eocene Lushi basin. To conclude, Lushi Basin may exhibit a landscape with active surface runoff, lakes, wetlands and forest under semi-arid to sub-humid climate in early to middle Eocene.