Nonmarine Sedimentation in an Active Fore Arc Basin
Richard K. Vessell, David K. Davies, 1981. "Nonmarine Sedimentation in an Active Fore Arc Basin", Recent and Ancient Nonmarine Depositional Environments: Models for Exploration, Frank G. Ethridge, Romeo M. Flores
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The modern fore arc basin of Guatemala receives non-marine sediments as a result of deposition of airfall ash, glowing avalanches (nuées ardentes), debris flows (lahars), and fluvial sediments. The lateral and vertical distribution of these deposits allows ready subdivision into four facies, including 1) the volcanic core facies, 2) proximal volcan- iclastic facies, 3) medial volcaniclastic facies, and 4) distal volcaniclastic facies.
Recent sediments of the study area have been deposited during the past 20,000 to 30,000 years, in a series of similar and repeated cycles. Each cycle consists of four phases: 1) The Inter-Eruption Phase (Phase 1) which is characterized by low rates of sediment deposition, incision of meandering streams, and delta reworking. This phase has a duration of 80 to 125 years. 2) The Eruptive Phase, (Phase 2) dominated by the ejection of airfall ash and glowing avalanches. This phase generally lasts less than one year. 3) The Fan Building Phase (Phase 3) dominated by debris flows and the deposition of coarse grained fluvial sediment. This phase lasts for some two years after an eruption. 4) The Braiding Phase, (Phase 4) characterized by the introduction of large volumes of sediment into the stream systems, resulting in the transformation of the incised meandering channels to rapidly aggrading, braided channels, and rapid deltaic progradation. This phase lasts for some 20 to 30 years following an eruption.
Phases 3 and 4 are triggered only by major eruptions which produce more than 6 × 107m3 ejecta. Smaller eruptions do not significantly affect the sedimentation system. Non-marine volcaniclastic sedimentation therefore proceeds as a series of relatively short-lived pulses (20 to 30 years duration) separated by longer periods (80 to 125 years duration) of comparatively minor depositional activity. The failure of the sedimentary system to respond in a similar fashion to all eruptions, indicates the existence of a geomorphic threshold, controlled entirely by the amount of ejecta produced during an eruption. In areas, dominated by fluvial activity (generally the distal volcaniclastic facies) the geologic record consists of superimposed or interdigitating deposits from braided and meandering streams. The change from braiding to meandering is not a function of change in slope or rainfall. Rather it is a response to the amount of sediment introduced into the fluvial systems.