UPLIFTED MARINE TERRACES IN THE BAHÍA DE LORETO REGION, BAJA CALIFORNIA SUR, MEXICO

Uplifted marine terraces are geomorphic indicators of past sea-level highstands and can provide insights into the vertical tectonic history of a given region. Uplifted marine terraces are present near Loreto, Baja California Sur, Mexico along the mainland coast, as well as on at least three islands within the Bay of Loreto: Isla Coronados, Isla del Carmen, and Isla Monserrate. Previous workers identified terraces in some of these areas and completed local mapping and measurement, but more precise field methods are required to accurately constrain terrace elevations and terrace ages. Here, we expand on previous work by providing regional mapping of terraces throughout the Bahía de Loreto region, including differential GPS elevation measurements, limited dating, and assessment of Late Quaternary uplift history of the Bahía de Loreto region. We measured the inner edges of the terraces using a Trimble GeoExplorer 2008 GPS receiver and post-processed the data to the permanent GPS reference station in La Paz, Baja California Sur, Mexico, reducing the vertical error of terrace elevations to ±0.3m to ±2.4m. We collected coral samples from the T1 terrace at Bahía Marquer, Isla del Carmen for U-series analysis, yielding an average age of 131.11 ±0.84 ka. This U-series age suggests that this terrace, and perhaps others at similar elevations, were formed during the MIS 5e highstand. We used measured terrace elevations to infer ages of the terraces that have not been radiometrically age-dated, based on constant calculated uplift rates since MIS 5e ranging from ~0.05 m/ka to ~0.16 m/ka. We found that, assuming constant uplift rates, it is possible that the terraces in this region could date back to as early as MIS 17 (~690 ka), earlier than ages that have previously been proposed.