Relict sheetflood bed forms on late Quaternary alluvial-fan surfaces in the southwestern United States

Late Pleistocene and early Holocene alluvial-fan surfaces in southeastern California are locally mantled by narrow, widely spaced, transverse-to-slope bands of fine gravel and coarse sand. These features are interpreted to be relict bed forms of high-magnitude sheetfloods that inundated abandoned alluvial-fan surfaces. The majority of these sheetflood events apparently occurred in the latest Pleistocene to middle Holocene and produced meso–bed forms with wavelengths of 2–6 m. These flood events mobilized sediment with mean grain sizes of 2–8 mm on inactive fan surfaces under calculated velocities of 30–60 cm/s and may be responsible for increased dissection of upper piedmonts and widespread alluvial-fan deposition in the lower piedmont regions. An older sheetflood event produced macro-bed forms with wavelengths of 20–80 m on a pedogenic CaCO₃-cemented piedmont in southwestern Arizona. Both scales of bed forms display characteristics of confined-flow fluvial bed forms which form under diverse flow regimes that cause either megaripples or transverse ribs.