DETRITAL ZIRCON GEOCHRONOLOGY OF QUATERNARY SEDIMENTS IN THE SANTA ROSALIA REGION, BAJA CALIFORNIA SUR, MEXICO

The Santa Rosalía Basin (SRB), in Baja California Sur, formed in the Gulf of California region as a rift-related basin during the Late Miocene. Quaternary sediments in this region form a series of terraces that were formed by fluvial erosion and deposition, as well as erosional cutback along the coastal shoreline. Detrital zircon analysis was conducted to reveal the provenance of the sediments that form the terraces and to constrain their origin.

Detrital zircons were collected from two sample sites. The first sample (AB) is interpreted to be fluvial in origin and then uplifted, while the second sample (EM) is interpreted to have formed through coastal processes. U-Pb geochronologic analysis of detrital zircon was conducted by LA-ICPMS at the University of Arizona Laserchron Center. A total of 98 zircons were analyzed from Sample EM and 90 zircons were analyzed from Sample AB. For Sample EM, a 10 Ma and 14 Ma age peak represent early rift volcanic activity that initiated in the late Miocene. An age peak of 21 Ma comes from the volcanic activity pre-rift, when the plate boundary was a subduction zone. Long shore coastal transport is indicated by the 94 Ma age peak, which is sourced from the Peninsular Ranges Batholith to the north. Proterozoic and Late Archean age peaks could also be a result of longshore transport or could be recycled zircon.

The youngest age peak in sample AB is 0.5 Ma. This age indicates rift related volcanic activity that initiated in the Pleistocene Epoch. This specific age is sourced from the north Aguajito Caldera, that was active during that time. There is also another age peak at 0.84 Ma that is also sourced from Pleistocene Epoch volcanic activity. The next two age peaks of 21.5 Ma and 93.7 Ma are similar in age to sample EM. These age peaks must have the same sources, pre-rift volcanic activity for the 21-21.5 Ma age, and longshore transport of the Peninsular Ranges Batholith for the 93.4-94 Ma age. Two older Proterozoic aged peaks must have also been deposited by the same method as sample EM; longshore transport or recycled zircon. These dates show how the two terraces in this study were formed, and what type of transportation was present. This research will give insight into not just the depositional events in the study area, but also the tectonic geomorphology of the area and how it ties into the overall geologic history of the SRB.