PRELIMINARY GEOCHRONOLOGY OF UPLIFTED MARINE AND FLUVIAL TERRACES IN THE SANTA ROSALÍA BASIN, CENTRAL BAJA CALIFORNIA, MEXICO

The Santa Rosalía basin in Mexico’s central Baja California Peninsula records sedimentation during Gulf of California rifting and subsequent tectonic uplift of the basin. Gravel and calcareous fossiliferous conglomerate and sandstone at the top of the sedimentary sequence have been included in the Pleistocene Santa Rosalía Formation. Distinct marine terraces within this formation, at 10 m, 25–30 m, 100 m, and ~200 m (above sea level [asl]) are identified near the town of Santa Rosalía. The height of these terraces increases towards the NW and decreases towards the SE. Four distinct fluvial terraces with top elevations ranging from 2 m to 50 m above the modern stream level were observed in the Santa Agueda, Boleo, Montado, and Providencia arroyos. Soil profiles in the higher elevation terraces were observed to be deeper and more developed than the soil profiles in the lower elevation terraces.

To constrain abandonment and stabilization of the observed marine and fluvial terraces, we dated them using a combination of cosmogenic exposure dating using ³⁶Cl and luminescence dating of feldspars. Preliminary results indicate that the oldest and highest marine terrace (at >200 m asl) contains boulders having exposure ages between 480 ± 64 ka and 720 ± 150 ka (³⁶Cl exposure age). The youngest uplifted fluvial terrace (2 m above the modern stream) is dated at 15 ± 3 ka (post IR–IRSL on feldspar). These results indicate that the basin stopped subsiding and began uplifting ca. 800 ka. This is consistent with local geomorphological evidence for uplift such as stream capture and the presence of stream knickpoints.