Abstract

A tephrochronological study that began as an active-fault mapping program in Managua, Nicaragua, following the 1972 earthquake has yielded information with broader geologic relevance. Seven principal, widespread Quaternary air-fall deposits (the “Managua sequence”) have been defined; the deposits are basaltic or rhyodacitic, span probably < 35,000 yr, and appear to represent the largest explosive eruptions in central Nicaragua during this period. They were erupted from vents at the present sites of Masaya Caldera, Apoyo Caldera, Jiloa Caldera, and Apoyeque Caldera, all within 35 km of Managua. In volume, they range from <0.5 to >11 km3 (dense-rock equivalent) and are classified as plinian or phreatoplinian, regardless of chemical composition. Two of the units were radiocarbon dated at 6,600 yr B.P. and 21,000 yr B.P. as a result of this study; ages of the other layers are not as well constrained but are estimated on the basis of soil development and position in the section.

Correlation and source assignment were particularly difficult because of incomplete exposure and similarity of physical and chemical features, but they were accomplished with a combination of detailed measurement and sampling at Managua, reconnaissance mapping and sampling at potential vent areas, and application of petrographic, mineralogic, and chemical correlation methods. The close similarity among the rhyodacite pumices posed the greatest difficulty for correlation; cluster analysis applied to multielement XRF chemical analyses of pure-glass separates gave the best results.

The clarification of the Managua sequence has revealed the local geologic history of Managua, has characterized the average largest recent Nicaraguan eruptions, and has defined time-stratigraphic markers used for archaeologic and engineering purposes. Additionally, it has provided a regional framework within which to interpret and correlate more detailed studies of individual Nicaraguan volcanoes.

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