First-order and super bounding surfaces in eolian sequences—Bounding surfaces revisited

Abstract

Extensive bounding surfaces in eolian sequences form with bedform migration (first-order bounding surfaces) and hiatuses in erg development (super bounding surfaces). First-order bounding surfaces represent the floors of interdune areas between simple or compound-complex dunes (draas), and form by ordinary migration of bedforms and interdune areas within a single erg. In contrast, super surfaces represent the truncation of ergs, or portions of ergs, and are a measure of how ergs respond to changing conditions and external events. Ergs are dynamic systems whose initiation, location, evolution and termination are a function of climate, sea level, basin configuration, sediment supply, and sandflow patterns; the latter are in turn determined by regional and local winds, pressure gradients and topography. As the overall conditions for erg development in an area change from favorable to unfavorable, ergs respond relatively rapidly or with a very long lag time. This response is evidenced by three broad categories of super surfaces. These categories are: (1) surfaces originated by regional termination of ergs because of climatic causes; (2) surfaces formed by erg contraction because of changes in sea level or tectonic setting; and (3) surfaces formed by the migration of ergs. Super surfaces can represent deflation of ergs and erg deposits to the water table, to an armored lag, to a point where a protective mantle of vegetation occurs, or to a point where renewed deposition of any sort begins. Features associated with super surfaces include root structures, paleosols, evaporites, polygonal fractures, preferential cementation, lag surfaces, zibars and granule ripples, and the surface may be planar, irregular, or show degraded dune topography. Super bounding surfaces allow the recognition of genetic units within thick and laterally extensive eolian sequences, and represent tools for correlating within (and possibly between) eolian bodies. The potential exists for relating the cause of super-surface development to specific geological events. Work thus far has recognized super surfaces largely in Pennsylvanian-Permian eolian units that are coastal and associated with glacial cycles. This in part may reflect ease in recognition, and identification of less obvious super surfaces in thick, inland erg sequences may ultimately show that these sequences also consist of diachronous, unrelated erg deposits.