GEOMORPHOLOGY OF TERRACES DEVELOPED ON THE EASTERN FLANK OF THE SIERRA EL MAYOR, NORTHERN BAJA CALIFORNIA, MEXICO

A series of pediment and strath terrace surfaces are located along the eastern margin of the Sierra El Mayor (SEM) in northern Baja California.  They are located at the distal extents of the Colorado River delta, near the area where the delta converges with the Sea of Cortez.  The surfaces are located only on the eastern base of the SEM.  The SEM is bound to the west by active faults, which are believed to be the southern extent of the Elsinore-Laguna Salada fault system.  The terraces consist of an older pediment surface and four 1 to 3 m high terrace straths that were carved into a wedge of interbedded clays, silts, sands, and gravels deposited at the base of the eastern SEM.  These deposits formed by the intermingling of coarser alluvial deposits derived from the SEM and finer deltaic flood-plain deposits from the Colorado River.  The straths are capped by a ~1 m thick veneer of coarse, subangular to angular gravel.  All the surfaces exhibit desert pavement devopment, but it is most evolved on the highest pediment surface and less evolved on the lower terrace straths.  The surfaces dip NNW and are punctuated by incised channels that form four erosional ridges.  Some surfaces extend up to 100 m along the inside of the drainages as thin paired straths formed by dissection of surfaces.  The pediment and strath surfaces dip locally 0.4 to 8.0°, but generally are between l and 2°.  The total vertical relief from the top pediment surface to the active flood plain ranges from 13 to 20 m, depending on distance from the range front.  Desert pavement development and new optically-stimulated luminescence ages suggest a late Pleistocene age for the development of the pediment and strath surfaces.  The terraces are interpreted to have formed by the interaction of the uplift of the SEM and base level lowering during late Pleistocene sea level low stands when the Colorado River was probably entrenched east of the SEM.