On the formation of sand ramps: A case study from the Mojave Desert

Sand ramps are dune-scale sedimentary accumulations found at mountain fronts and consist of a combination of aeolian sands and the deposits of other geomorphological processes associated with hillslope and fluvial activity. Their complexity and their construction by wind, water and mass movement means that sand ramps potentially hold a very rich store of palaeoenvironmental information. However, before this potential can be realised a full understanding of their formation is necessary. This paper aims to provide a better understanding of the principal factors influencing the development of sand ramps. It reviews the stratigraphic, chronometric and sedimentological evidence relating to the past development of sand ramps, focussing particularly on Soldier Mountain sand ramp in the Mojave Desert, as well as using observations of the modern movement of slope material to elucidate the formation of stone horizons within sand ramps. Findings show that sand ramps cannot easily be interpreted in terms of a simple model of fluctuating palaeoenvironmental phases from aeolian dominated to soil/fluvial dominated episodes. They accumulate quickly (perhaps in <5 ka), probably in a single phase before becoming relict. Based on the evidence from Soldier Mountain, they appear strongly controlled by a 'window of opportunity' when sediment supply is plentiful and cease to develop when this sediment supply diminishes and/or the accommodation space is filled up. Contemporary observations of stone movement both on rock and sandy sloping surfaces in the Mojave region indicate movement rates in the order of 0.6 and 11 mm yr(-1), which is insufficiently fast to explain how stone horizons could have been moved across and been incorporated into sand ramps on multiple occasions. Stone horizons found within the aeolian sediments lack evidence for soil development and are interpreted as very short-term events in which small streams moved and splayed discontinuous stone horizons across the sand ramp surface before aeolian deposition resumed. Surface stone horizons may form by creep from mountain slope sources across sand ramps but require enhanced speed compared to measured rates of runoff creep. We propose the mechanism of fluvio-aeolian creep. Our study suggests that current models of alternating aeolian and colluvial deposition within sand ramps, their palaeoenvironmental significance and indeed how sand ramps are distinguished from other dune forms require amendment. (c) 2012 Elsevier B.V. All rights reserved.