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 Bálsamo, 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 Bálsamo, Cuscatlán and San Salvador formations, occur in the area. Remnants of two large basaltic andesite to andesite stratovolcanoes, Panchimalco and Jayaque, represent the Bálsamo 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 Cuscatlán Formation comprises the Jayaque and Santo Tomás 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 José tuff/scoria cone, the Ilopango caldera extrusive domes (0.25-0.05 Ma), the Antiguo Cuscatlán 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 Bálsamo where it rests on laterites atop the Bálsamo 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.Las amenazas naturales afectan al territorio de El Salvador en toda su extensión de manera constante. Se ha llevado a cabo un mapeo geológico en la parte Sur del Área Metropolitana de San Salvador (AMSS), asimismo se han evaluado las amenazas naturales potenciales que pueden afectar a la zona. El área de estudio se extiende sobre la Cordillera del Bálsa-mo, el sistema de fallas marginales y en la parte Sur del Graben Central entre los volcanes activos de San Salvador y la Caldera de Ilopango; representando un segmento del frente volcánico de Centro América. Las rocas volcánicas del Mioceno tardío hasta de edad reciente que pertenecen a las Formaciones Bálsamo, Cuscatlán y San Salvador conforman la geología del área. Los remanentes de dos extensos estratovolcanes basálticos-andesíticos hasta andesíticos, Panchimalco y Jayaque, representan la Formación Bálsamo. Estos presentan un buzamiento periclinal y zonas con facies que van desde flujos de lava y brechas epiclásticas volcánicas gruesas de ...

Problemática y conocimiento actual de las tefras tierra blanca joven en el Área Metropolitana de San Salvador, el Salvador

“…Las rocas volcánicas que conforman la superficie del AMSS (figuras 2 y 3) están, en su mayoría, incluidas en la formación San Salvador, producto de los volcanes pertenecientes al frente volcánico de América Central (Plioceno-Cuaternario) (Lexa et al, 2011). Hernández (2008) describe la estratigrafía del AMSS y sus características; los estratos más importantes son: TB4, TB3, TB2 (entre ellos existen paleosuelos) y Tierra Blanca Joven (TBJ), que son productos eruptivos de la Caldera de Ilopango.…”

Unsaturated Volcanic Tephra and Its Effect for Soil Movement in El Salvador

“…Beside alteration minerals there is present plagioclase and quartz and minor amount of illite that may represent admixture from altered rocks in the volcanic explosion center (Communication of J. Lexa, 2011). …”

“…Important areas of weathering in the MASS belong to the laterites [1] associated to the rocks of the Balsamo formation, especially atop rocks of the older Panchimalco stratovolcano which are covered by the younger tephras.…”

“…The location of the tuffs was controlled depending on the direction of the winds, the erosion processes and explosive force during the eruption. The basement of the area is composed [1] by the products of calderas and volcanoes of the Cuscatlán formation and remnants of andesite stratovolcanoes of the Balsamo formation.…”

“…The MASS stratigraphy consists mainly of the San Salvador formation, product [1,2] of the late Pliocene-Quaternary Central American Volcanic Front volcanoes. The more important strata are: the tephras TB4, TB3, TB2 (between them exist paleosoils) and Tierra Blanca Joven (TBJ) which are eruptive products of the Ilopango Caldera; also strata from the San Salvador stratovolcano like G1, G2 including different lava flows and ash falls are included.…”