LATE CENOZOIC WIND EROSION IN THE HAMI BASIN, NORTHWESTERN CHINA

The hyper-arid and internally-drained Hami Basin, south of the Tian Mountains in northwestern China, is part of a network of arid wind-deflated basins in central Asia. The Hami Basin exposes wind-eroded Neogene sedimentary bedrock and was potentially an important source of atmospheric dust transported by westerly winds and deposited in the northern Pacific Ocean during latest Cenozoic time. At least 430 m of sub-horizontal Neogene strata are exposed in the Hami Basin and consist of interbedded paleosol, siltstone, sandstone, and pebble conglomerate associated with alluvial fan to playa deposition. These have been extensively wind eroded presumably starting during Pliocene time. The Neogene strata are sculpted into widespread (>2000 km²) but spatially discontinuous fields of megayardangs, separated and locally overlain by spatially-extensive (1×10⁴–10⁶ m²) sub-horizontal planar surfaces of unconsolidated gravel. These gravel surfaces are topographically layered and coincide spatially with prominent conglomerate beds within the wind-eroded Neogene stratigraphy. The finer-grained, and presumably more easily wind eroded intervals, between the topographically layered gravel surfaces and conglomerates are sculpted into the yardang fields. The clasts of the unconsolidated gravels are generally dark in color and show variable degrees of ventifaction. There is a scarcity of unconsolidated sediment composed of sand- to smaller-sized particles, which implies the basin is sand-starved and that near surface winds are efficient in reducing and/or removing sand and finer-grained sediment from the basin. These sedimentary, stratigraphic, and geomorphic observations suggest that the rate of eolian dominated downcutting through the heterogeneous lithologies within the Neogene strata was modulated, in part, by the distribution of conglomerate beds and the coalescences of gravels on the surface. The distribution of conglomerates, more abundant in the lower part of the wind-eroded section, would be consistent with initially higher wind erosion rates with a decreasing trend through time as gravel surfaces coalesced, armored the landscape, and suppressed subsequent wind deflation. The abundance of dark-colored gravels in the Hami Basin has contributed to its anomalously high land skin temperatures.