GEOMORPHOLOGY AND RELATIVE UPLIFT RATES OF QUATERNARY MARINE TERRACES OF SAN MARCOS ISLAND, BAJA CALIFORNIA SUR, MEXICO

The coastlines of San Marcos Island, located off the east coast of the central Baja California Peninsula, Mexico, exhibit an array of Quaternary marine terraces that were identified and mapped during this study. The elevations of these terrace surfaces were determined using Real Time Kinematic (RTK) GPS elevation data collected at study areas on the western and eastern coastlines, then mapped across the island using QGIS. Terraces were found at elevations of about 10 meters along the coastline (T1), as well as a second terrace (T2) at 40 meters in some localities, characterized by marine deposits including shell fragments and carbonate growth, the latter primarily on T1. Through comparison of elevation data, the amount of relative uplift occurred between the formation of terraces T2 and T1 is approximately 30m. Relative uplift between T1 and the present erosional surface is approximately 10m. The terrace surfaces tend to have higher elevations in the west than in the east of the island. This elevation trend could be explained by a possible east-west tilting of the island. The T2 and remnants of a higher T3 surface were located only on the western coastline, at approximately 40m and 200m respectively, suggesting that the eastern shore is more heavily eroded in comparison to the western shore. With future measurement of the amount of tilt and orientation of faults near the coastline, a structural analysis of the overall tilt of the island could be determined. Absolute rates of terrace uplift will also be determined through dating of terrace deposits. This study is the first to analyze in detail the geomorphology of these marine terraces on San Marcos island, setting a precedent for future geological mapping to be completed.