A NEW EARLY TO LATE MIOCENE FOSSILIFEROUS REGION IN CENTRAL NEI MONGOL:LITHOSTRATIGRAPHY AND BIOSTRATIGRAPHY IN AOERBAN STRATA

Vertebrate fossil localities of Early Miocene age are rare in north China in general and in Nei Mongol in particular.Here we report a recently discovered,richly fossiliferous Early Miocene through Late Miocene sequence in central Nei Mongol.The new Aoerban strata from Sonid Zuoqi represent one of the longest exposures in the region,spanning up to 50 m in total thickness,and produce fossils along much of the section with distinct lithologies,resulting in an easily correlated in situ fossil collection with excellent stratigraphic documentation. Three lithologic units are recognized.We formally name the Aoerban Formation at the lower part of the section,which contains three discrete members of fine-grained sediments altered by paleosols:1) Lower Red Mudstone Member,2) Middle Green Mudstone Member,and 3) Upper Red Mudstone Member.Resting disconformably above the Aoerban Formation is the Balunhalagen bed,which is characterized by basal conglomerates followed by orange-colored mudstones and siltstones.Cutting into the Balunhalagen bed is the Bilutu bed,which is another unit of channel gravel sediments and overbank deposits. Paleontologically four discrete faunal units are clearly recognizable,mostly based on small mammals obtained from five wash sites.We distinguish two Early Miocene faunas.The Lower Aoerban Fauna and Upper Aoerban Fauna,produced from the Lower Red Mudstone and Upper Red Mudstone Members, respectively,contain characteristic Early Miocene elements of north China and/or Europe.The Balunhalagen Fauna,derived from the Balunhalagen bed,is characteristic of the Middle to Late Miocene transition.The Bilutu Fauna from the capping Bilutu bed features a mixed composition of Middle and Late Miocene taxa.Both faunal and sedimentological evidence suggest that some of the earlier faunal elements have been reworked into the later Bilutu deposits;thus the Bilutu Fauna is mostly likely of Late Miocene age.