QUATERNARY TERRACE DEVELOPMENT IN THE NORTHERN GULF EXTENSIONAL PROVINCE OF BAJA: INSIGHTS FROM COMBINED BURIAL AND EXPOSURE AGES

The Sierra El Mayor - Sierra Cucapa (SEM-SC) mountain range in northern Baja separates the Colorado River delta from the sub-sea level Laguna Salada Basin and is important in terms of understanding the structural transition from the northern Gulf Extensional Province to the San Andreas system. One of the principal Quaternary features of the SEM-SC is a prominent pediment surface that extends basinward and forms a 10-15 m high escarpment. Along the eastern side of the Sierra El Major and facing the Colorado River delta, the pediment is terminated by a series of stepped strath terraces. The straths are cut into fine-grained fluvial-deltaic deposits from the Colorado River delta system and are capped by ~1 m thick gravels. Optically stimulated luminescence (OSL) ages from interbedded coarse sands in the capping gravels range from ~20 to 28 ka. One additional sample from the underlying fluvial-deltaic deposits has an OSL age of 40 ka, indicating Colorado River delta deposition during the interglacial OIS-3. Capping gravel depth-profile and surface sample 10Be cosmogenic radionuclide (CRN) ages range from 42 to 117 ka (mean=60 ka) and are considerably older than the OSL ages. Depth profiles lack a systematic decrease in 10Be concentration with depth suggesting considerable inheritance. CRN ages from active drainages and scree slopes range from 26 to 44 ka (mean=38 ka), which we interpret as inheritance for the sources of the capping gravels. We compute an average age of ~22 ka (CRN surface age minus inheritance) for exposure of the capping gravels on the pediment and straths, which is in close agreement with the OSL burial ages. The pediment and strath surfaces formed 20-28 ka 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 than at present. Computed uplift rates for the eastern side of the SEM range from 0.4 to 2 mm/yr depending on whether the 40 ka delta deposits graded to a base level similar to the present delta level or if they graded to a base level that was -60 m during OIS-3. Sequential strath incision may have been punctuated by tectonic uplift.