TERRACE AGE AND DEVELOPMENT ON THE EASTERN SIERRA EL MAYOR, NORTHERN BAJA: COMBINED INFLUENCE OF TECTONICS, SEA LEVEL CHANGE, AND RIVER INCISION

The eastern Sierra El Mayor (SEM) faces the Colorado River delta and is directly adjacent to the modern river meander system.  Whereas active faults are present along most of the west side of the SEM, no active faults have been mapped at the surface on the east side where the erosional and depositional effects of the Colorado River have sculpted the range front.  A series of at least four stepped strath terraces and an upper pediment surface are present along much of the eastern range flank.  The strath and pediment surfaces all dip 1-2° away from the range, and the differences in height between each terrace typically are 1 to 3 m.  The strath terraces are cut into fine-grained fluvial-deltaic deposits and are capped by gravel deposits (~1 m thick). The fluvial-deltaic deposits probably formed when the Colorado River delta graded to the 125 ka Sangamonian high sea level stand at 7 m above present sea level (Carter, 1977).  Given that these deposits are now ~20 m above the present floodplain, it is likely that ~13 m of uplift has occurred in this region since their deposition. The capping pediment and terrace gravels were dated using optically stimulated luminescence (OSL) and ¹⁰Be cosmogenic radionuclide (CRN) depth-profile methods.  The OSL ages range from ~20 to 28 ka, whereas the CRN ages scatter from 45 to 128 ka.  We interpret the CRN ages to reflect inheritance due to exposure prior to final deposition and the OSL ages to represent the time of deposition of the capping gravels.  Therefore, the formation and capping of the pediment and strath surfaces occurred 20-28 ka at a time when sea level was falling during the last glacial cycle.  At this time, the Colorado River may have been entrenched immediately east of the SEM causing local base level to be 10’s of meters lower.  Sequential strath incision is interpreted to have occurred during this base-level lowering, but may have been punctuated by tectonic uplift.  Since the time of high sea level deltaic deposition 125 ka, the deltaic deposits have been uplifted with average surface uplift rates of ~0.1 mm/yr.  However, this rate may be higher if some of the high-stand fluvial-deltaic deposits were removed by erosion prior to pediment formation and/or if the pediment and strath surfaces graded to an entrenched Colorado River located just east of the SEM at 20-28 ka.