CENOZOIC EVOLUTION OF THE NORTHERN ORDOS PLATEAU, NORTH CHINA

The northern Ordos Plateau, bordered on three sides by the Yellow River, is situated in a transition zone from desert to loess between northwestern and northeastern China. Knowledge of the geologic history of this neotectonically active and climatically sensitive area is key for recognition of environmental changes there. In the mid-Eocene the Hetao and Yinchuan basins formed in the north and west Ordos areas, respectively. By the Oligocene, deposits in the Yinchuan Basin expanded and overlapped eastward onto the north Ordos peneplain due to west warping of the Ordos block. Until the late Miocene an approximately north-south broad valley in east Ordos was filled with fluvial gravels, subsequently covered by red clays and interbedded in the lower parts with basalt lavas. These deposits extend westward into the inner Ordos probably indicating east warping of the north Ordos block. During the late Pliocene to early Quaternary broad valleys were eroded westward to the inner north Ordos. In some places these valleys have been eroded into the gravel-red clay deposits. Lakes and Quaternary lacustrine deposits, often underlain by fluvial pebbles, are scattered across the north-central Ordos Plateau. Quaternary river valley systems, especially those trending north or south, developed asymmetrically, with gently sloping eastern and steeply sloping western sides. The highest fluvial terraces of the Yellow River along the Jinshan Gorge in eastern Ordos are about 100-150 m higher than the correlative terraces along the Shizuishan Gorge in western Ordos. These height differences indicate that north Ordos was warped westward since the late Pliocene. As a result, some broad valleys were blocked and lakes formed. However, as the Yellow River and its tributaries in eastern Ordos eroded headwards in the late early Pleistocene some of the lakes on the plateau were drained and river valleys formed again.