Holocene dune-sourced alluvial fans in the Nebraska Sand Hills

The large, stabilized dunes of the Nebraska Sand Hills are in a phase of degradation. The deposition of small-scale alluvial fans composed of well-sorted, fine- to medium-grained sand occurs when sand is transported via gullies on the lee side of large barchanoid-ridge dunes during infrequent, intense summer rain storms (>5 cm/h). The hydraulic conductivity of the dune sand itself is too high to allow Hortonian overland flow, but organically bound mats and crusts on the surface of the dunes help to impede infiltration. Secondary eolian bed forms at the crests of the dunes focus shallow groundwater and overland flow that erodes gullies. Internally, alluvial fans are dominated by laminated sands interpreted as sheetflood deposits. Depositional couplets of structureless (bottom) and laminated (top) sands may form from a combination of hyperconcentrated flow and sheetflood. Thickness of deposits varies with storm intensity and sediment supply in the gullies. The alluvial fans are Holocene in age, and gully initiation may have begun at the onset of a wetter climate when vegetation was less dense than at present. Water-laid sandstones associated with large-scale eolian cross-strata have been recognized in Precambrian, Devonian, and Cretaceous sequences. The alluvial fans from the Sand Hills may provide a new modern analogue for interpreting these ancient sandstones.