3D GEOLOGIC MODEL OF THE TRANSBOUNDARY SAN DIEGO-TIJUANA AREA, USA AND MEXICO

Prior geologic studies of the San Diego-Tijuana transboundary region focused on site investigations, characterization of rock formations, or earthquake hazards. No comprehensive, quantitative model characterizing the three-dimensional (3D) geology of the entire area has been developed. The lack of such a model limits understanding of large-scale geologic processes that span the international boundary area: such as development of ancient landforms, subsurface structures, and groundwater movement and availability in the transboundary Tijuana River basin deposits. To evaluate these regional processes, the United States Geological Survey (USGS) conducted a study to better understand the geologic structure of the San Diego transboundary area, including onshore and offshore provinces. A cornerstone of this study is the installation and analysis of 77 wells at 12 multiple-depth monitoring-well sites. Geologic information from these wells was combined with lithologic data from 81 oil exploration wells and municipal and private water wells, gravity and seismic interpretations, and paleontological interpretations. These data were analyzed in conjunction with geologic maps and digital elevation models to develop a 3D geologic model of the transboundary area, in particular of the San Diego embayment. Existing interpretations of regional surficial geology, faulting, and tectonic history provided the framework for this model, which was refined by independent evaluation of subsurface geology. Geologic formations were simplified into five sedimentary units (Quaternary, Plio-Pleistocene, Oligocene, Eocene and Cretaceous ages), and one basal crystalline unit (primarily Cretaceous and Jurassic). Complex fault systems are represented in the model by ten fault strands that maintain overall displacement. The 3D geologic model corroborates existing geologic concepts of the San Diego transboundary area and refines the extent of subsurface geology, which can be traced across the international border, both onshore and offshore. Users of this model can utilize this subsurface information to holistically evaluate geologic structures and regional hydrogeology.