Litho-, Bio-, and Magnetostratigraphy and Paleoenvironment of Tunggur Formation (middle Miocene) in Central Inner Mongolia, China

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The Tunggur Formation is presently exposed along the edge of the Tunggur Tableland. Being mostly flat-lying and with a maximum thickness of less than 80 m (most sections are less than 40 m thick), exposures along the northern, western, and southern edges of the platform all produce vertebrate fossils. The American Museum Wolf Camp Quarry and Platybelodon Quarry produced most of the early collections. To the east, the Tableland gradually blends into the landscape, and its distribution becomes less well delineated. At the extreme eastern end, the Tunggur Formation is probably overlain by the late Miocene Baogeda Ula Formation. The Tunggur Formation has its stratotype along a small exposure at Mandelin Chabu in the northwest edge of the Tableland. The Tunggur Formation in the Tableland is divided into two informal sedimentary units, the upper and lower beds. The stratotype section, Wolf Camp, and Platybelodon Quarry, along with most localities in the northern escarpment, belong to the upper beds, which are characterized by cross-bedded sandstones, variegated mudstones, and occasional marls. The entire southern Tairum Nor badland and lower part of the Aletexire section belong to the lower beds, which are characterized by more uniform red mudstones interrupted by a channel sandstone. The upper beds produce the Tunggur Fauna, which includes most of what is traditionally known as the Platybelodon Fauna. The lower beds contain a more recently named Tairum Nor Fauna, which has the latest occurrence of Tachyoryctoides and three musteloid carnivorans: Leptarctus neimenguensis, a new species of Sthenictis, and a new form possibly related to Aelurocyon, with all three being well represented in the North American Miocene. Paleomagnetic study of two key sections at Moergen small mammal locality and at Tairum Nor locality suggests a correlation in the magnetozones C5Ar. 3r through part of C5r.3r, with an age range of 11.8-13 Ma. Systematic prospecting in known and previously unexplored areas has resulted in an expansion of the faunal representations in the region. By integrating the faunal data with magnetostratigraphy, we provide the first chronological tie point in the Chinese middle Miocene mammalian stratigraphy. Paleoenvironmental studies of the Tunggur Formation indicate a mosaic of grassland and mixed conifer-broadleaf woodland. Shallow channels were abundant, and floodplains had undergone different degrees of soil formation. Large mammal communities contain many low-crowned browsers and a few high-crowned grazers, but small mammals are dominated by grassland-adapted forms.

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