Climbing Zibars of the Algodones

Abstract

Zibars are coarse-grained, low-relief, slipfaceless, eolian bedforms that occur on sand sheets and within interdune corridors of many eolian sand seas. Zibar surface features consist exclusively of wind ripples and local shadow and shrub-coppice dunes associated with vegetation. A study of zibars located on the southwestern margin of the Algodones dune field, California, shows that zibars, and their associated interzibar areas, are migrating features oriented transverse to oblique to the resultant sand-transport direction. Zibars leave deposits formed by deposition on the zibar lee face. The zibar shape modifies the near-surface airflow to produce a subtle grain-size change across the zibar/interzibar surface. Flow expansion (not separation) leeward of the zibar crest causes the lee-face deposits (those preserved upon bedform migration) to have the finest grain size on the zibar/interzibar surface. Sediment coarser than that deposited on the zibar lee face is transported more slowly and less frequently, and thus becomes concentrated on the zibar stoss slope and, ultimately, becomes interzibar deposits. Migration of zibars and interzibar areas can produce two characteristic depositional sequences depending upon the angle of climb. For low angles of climb, a sequence of stacked amalgamated interzibar deposits is preserved; zibar deposits are absent. Amalgamated interzibar deposits consist of low-angle (usually less than 5°) wind-ripple laminae of medium to coarse sand. Given higher angles of climb, deposits of both the entire interzibar and zibar lower lee face are preserved. Low-angle interzibar deposits of medium to coarse sand are overlain tangentially by finer grained (fine to medium sand) wind-ripple laminae (dipping less than 15°) of the zibar lee face. Lee-face wind-ripple laminae are sharply truncated and overlain by the succeeding set of interzibar wind-ripple deposits. This zibar/interzibar sequence is distinctive and allows for the detailed interpretation of some ancient sand-sheet deposits as left by migrating zibars. A possible example of ancient zibar deposits is present in the Entrada Sandstone (Jurassic) of southwestern Colorado.