Development of stratigraphically controlled, eolianmodified unconsolidated gravel surfaces and yardang fields in the wind-eroded Hami Basin, northwestern China

Here, we present a stratigraphic and geomorphologic study of the northern part of the hyperarid and internally drained Hami Basin, located south of the Tian Mountains in northwestern China. 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 in the past. The northern Hami Basin exposes at least 430 m of subhorizontal (dipping locally < 1° to the south) Neogene strata. The strata consist of interbedded paleosol, siltstone, sandstone, and massive and cross-bedded matrix- and clast-supported pebble conglomerate associated with alluvial-fan to playa deposition. 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) subhorizontal planar surfaces of unconsolidated gravel. These gravel surfaces are topographically tiered and coincide spatially with prominent conglomerate beds within the wind-eroded Neogene stratigraphy. Conversely, the yardang fields developed in finer-grained and presumably more easily eroded intervals of the stratigraphy between the conglomerate beds. Unconsolidated gravels are also exposed within interyardang corridors, on yardang flanks and tops, in Pleistocene-Holocene ephemeral stream channels, and atop fluvial terraces. There is a scarcity of unconsolidated sediment composed of sand- to smaller-sized particles, which implies the basin is sandstarved and/or that near-surface winds are efficient in removing sand and fine-grained sediment from the basin. We 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. The distribution of conglomerates, which are more abundant in the lower part of the wind-eroded section, would be consistent with initially higher wind erosion rates and a decreasing trend through time as gravel surfaces coalesced, armored the landscape, and suppressed wind deflation. We estimate a minimum vertically integrated average of 178 m of erosion based on the projection of wind-eroded strata within the field area. We infer that the abundance of dark-colored (low-albedo) unconsolidated gravels in the Hami Basin has contributed to its anomalously high land skin temperatures and frequent extreme near-surface wind events.