Abstract

The Pacific coastal plain in Central America forms the subaerial part of an arc-trench gap that is terminated as much as 60 km inland from the Pacific shoreline by the steep slopes of a row of active Quaternary volcanoes. The Samalá fluvial-deltaic system, which heads inland from the volcanic arc, displays a combination of all of the Holocene features (frequently shifting braided channel, flanking elongate dendritically branched lakes, and a high-destructive wave-dominated arcuate delta) that characterize the coastal plain in southwestern Guatemala. Historical records indicate that the catastrophic eruption of Santa María Volcano in 1902 dramatically increased the amount of sediment supplied to the Samalá River. Within a few years, deposition of sand and gravel raised the river bed 10 to 15 m, blocking the drainage of the river's tributaries to produce flanking lakes and allowing the river to easily shift its channel. Frequently, the braided system was diverted laterally into a flanking lake, interrupting deposition of organic-rich muds and locally filling the lake with prograding deltaic deposits. Continued channel shifting and aggradation of sand and gravel on the lower 25 km of the coastal plain have contributed to the growth of an elongate alluvial fan that probably originated during the Pleistocene.

Contemporaneous with aggradation of the fluvial system, an elongate deltaic platform prograded about 7 km seaward, between 1902 and 1922, in response to the deposition of approximately 4 km3 of deltaic sediments. However, with waning sediment supply, the delta entered a destructive phase, and sands were redistributed laterally to prograding shoreface and beach environments, developing the present arcuate shoreline. The position of the arcuate shoreline has remained essentially unchanged since sometime before 1947; the volume of sediment derived from the active Santiaguito–Santa María volcanic complex and contributed to the shore zone by the Samalá River has been sufficient, since that time, to prevent further shoreline retreat. A new constructive phase of deltation would begin in the event of another great eruption.

Fluvial-deltaic sedimentation in arc-trench geologic settings, under conditions of torrential rainfall, differs significantly from fluvial-deltaic sedimentation in other geologic settings. This difference may be attributed to periodic catastrophic volcanic eruptions, which locally produce high-relief unvegetated slopes and allow extremely high local rates of erosion and subsequent fluvial-deltaic sedimentation.

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