Geology and volcanic evolution in the southern part of the San Salvador Metropolitan Area

Abstract

We have carried out geological studies including mapping at the scale 1 : 50 000 in the southern part of the San Salvador Metropolitan Area to support urban planning and natural hazard mitigation. The study area extends over the Cordillera del Blsamo, marginal fault system and southern part of the Central Graben between the active San Salvador volcano and Ilopango caldera. It represents a segment in the Central American Volcanic Front. Volcanic rocks of the Late Miocene to recent age, classified as the Blsamo, Cuscatln and San Salvador formations, occur in the area. Remnants of two large basaltic andesite to andesite stratovolcanoes, Panchimalco and Jayaque, represent the Blsamo Formation. They show periclinal dips and facies zoning from lava flows and coarse epiclastic volcanic breccias of the proximal zone through epiclastic volcanic breccias/conglomerates of the medial zone to epiclastic volcanic conglomerates and sandstones of the distal zone. Their ages are 7.2-6.1 Ma and 2.6-1.5 Ma respectively. The Cuscatln Formation comprises the Jayaque and Santo Toms calderas, the andesitic-dacitic Ilopango and Jayaque ignimbrites (1.9-1.4 Ma) in the SW and SE parts of the area, the Ilopango andesitic volcano (1.5-0.8 Ma), the Loma Larga basaltic volcano (0.8-0.5 Ma), the Planes de Renderos caldera, the dacite-andesite San Jacinto extrusive domes and effusive cone (0.4-0.25 Ma), the San Jos tuff/scoria cone, the Ilopango caldera extrusive domes (0.25-0.05 Ma), the Antiguo Cuscatln scoria cone (0.2-0.08 Ma) and older tephra deposits of the Coatepeque and Ilopango calderas exposed along marginal faults of the Central Graben. The San Salvador Formation occurs as tephra cover along the crest of the Cordillera del Blsamo where it rests on laterites atop the Blsamo Formation and in the Central Graben. Tephra units belong to the Coatepeque caldera (Arce and Congo), San Salvador volcano (Apopa, G1 and G2) and Ilopango caldera (Tierra Blanca 1-4) spanning 70-1 ka. Tephra units are separated by palaeosols and aeolian dusty deposits.