Bahia Adair and Vicinity, Sonora: Modern Siliciclastic-Dominated Arid Macrotidal Coastline

Brian E. Lock, Stephen M. Sinitiere, Lester J. Williams

The northwestern Sonoran coastline, in the vicinity of Bahia Adair, combines several important geologic features. The arid landward environments are dominated by the dunes of the Gran Desierto and the surrounding alluvial fans and ephemeral streams. The Colorado River, whose delta lies to the northwest, has been an important source of sediment until very recently. The high tidal energy of the region has profoundly influenced the distribution and geometries of coastal and shallow-marine sand bodies, and the active tectonic setting has also played a role. The Cerro Prieto splay of the San Andreas fault system has been responsible for local uplift and downwarp and resulting transgression and regression.

The intertidal and supratidal zones are dominated by sand and constitute a sand-body type that has been seldom considered by petroleum explorationists or other students of ancient sand bodies, and the associated evaporites are rather different from those described from the superficially analogous Persian Gulf sabkhas.

The intertidal zone of the inner bay is most readily divided into lower, middle, and upper subzones on the basis of vegetation; halophytic plants, particularly Salicornia, are dominant in the middle part of the intertidal range and are associated with flats adjacent to a complex pattern of tidal creeks with point bars, overbank and crevasse splays, and megarippled channels. The lower intertidal subzone is an area of large, shifting sandbars, again commonly with megaripples, and the upper intertidal areas consist of essentially featureless sand flats which are submerged by sheet floods at the highest tides. The surrounding desert has large areas of active dunes, but close to the shoreline much of the sand is partially stabilized by a relatively rich xerophytic flora. Evaporites are of elatively minor importance in the high intertidal to supratidal flats, probably because of the relatively porous, sandy nature of the sediments; thin, ephemeral halite crusts and sporadic intrastratal gypsum occur throughout the area. Of rather greater interest are the numerous salinas and pans that occur scattered through the coastal desert. Some of these pans include thick salt deposits (at Salina Grande), and others have accumulations of trona, schairerite, and gaylussite. The source of these materials is complex, including marine flooding and seepage of seawater in come cases, sea spray concentrated by rainwater runoff, and ions introduced by barely potable springs. The latter are significant; series of spring mounds are located along shear zones and provide small but verdant oases. he trona-bearing pans are clearly fed by continental ground waters, probably rising toward the surface over a deeper, landward penetrating wedge of denser marine ground waters. Saltwater encroachment has been recorded in municipal wells at Puerto Penasco, on the eastern side of the bay.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.