Surface processes, deposits, and development of star dunes: Dumont dune field, California

Abstract

The Dumont dune field in the northern Mojave Desert of California consists of a core of well-developed star dunes flanked by smaller barchan, crescentic-ridge, and linear dunes. Many of the small dunes on the margin of the dune field develop star-dune–like features, such as a central peak and multiple, radiating arms. These incipient star dunes develop in the complex wind regime at Dumont by the merging and modifying of small bed-forms during the seasonal changes in wind directions, but they typically lose their star-dune–like features a few weeks later. The larger, mature star dunes maintain their overall integrity throughout the seasons. Mapping of surface airflow and sedimentary processes on mature and incipient star dunes show that the arm trends correspond to directions of the primary winds over the seasons. For any given time and wind direction, however, each arm behaves as an active transverse, oblique, or longitudinal element. Secondary airflow over star dunes is complex, and the direction of the wind at the surface is markedly different over parts of the dune at a single time and at a high variance with the primary wind direction. Most of the depositional surface of star dunes is covered by wind ripples, with slipfaces commonly confined to the upper portions of the dunes. The strength, direction, and duration of the seasonal primary winds, the size of the dune and its arms, and the secondary airflow all play critical roles in the maintenance and growth of star dunes. A multidirectional primary wind favors star-dune formation, but the persistence of winds from any one direction for a period of time favors some arms over others and, hence, loss of the star-dune features. A “minimum survival size” is necessary for incipient star dunes to maintain their star-dune morphologies from season to season. Secondary airflow favors the maintenance of the arms as well as the accumulation of sand on the bedform, thus allowing for growth. The nature of star-dune surface processes (plus the internal structure revealed in shallow trenches) suggests that the most common deposits of star dunes are low- to moderate-angle wind-ripple laminae arranged in packages or sets separated by gently dipping truncational surfaces. This may explain the rarity with which star-dune deposits are identified in the rock record.