Abstract

Observations of dune migration, wind velocity (measured near the dune surface on relatively short time scales and near the dune field over a period of several years), and dune surface processes (that is, wind-ripple migration, grainfall, avalanching, and superposed dune migration) were made on a complex-crescentic dune in the Algodones dune field, California, to document the link between eolian dunes, their deposits, and the wind regimes in which they form. The observations suggest that typically there is not a simple link between the orientation of eolian cross-strata and the primary, or regional, paleowind direction. Leeward of dunes with near-angle-of-repose lee faces, an alongslope-directed secondary airflow develops in response to primary winds that are oblique to the dune. This secondary airflow drives the migration of lee-side superposed dunes and wind ripples. The deposits of these superposed bedforms may significantly differ in dip direction from any one component of the primary wind regime, or the vector sum of all of its components. A record of the short-term fluctuations in primary wind direction is recorded by the interlamination of eolian stratification types, primarily grainflow and wind-ripple deposits.

Insights gained from this study of modern dunes, their deposits, and wind regimes were applied to two well-documented ancient eolian deposits, the Jurassic Page and Navajo Sandstones of the southwestern United States. Both units contain alternating wind-ripple and grainflow deposits. These cyclic stratification patterns are interpreted to have resulted from a seasonally varying wind regime with a component or components of flow oblique to the dune, responsible for wind-ripple deposition, and another component of flow perpendicular to the dune, responsible for deposition by grainflow.

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