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Models for Volcaniclastic Sedimentation

Stephen Self, John G. McPherson

Volcaniclastic deposits, including pyroclastic and effusive volcanics and epiclastic sediments, constitute an important component of both continental and oceanic magmatic arc and rift settings. Volcanologic and sedimentologic analysis of the lithofacies and lithofacies assemblages in volcaniclastic sequences not only better defines the evolution of the volcanic system itself, but has direct relevance to the development and infill of associated sedimentary basins. Models for volcaniclastic sedimentation must integrate emplacement processes and evolution of pyroclastics and lavas with the sedimentology of terrigenous clastics with which they are closely associated. Examination of multicompositional, voluminous, primary and secondary volcaniclastic deposits in and around a l rge continental volcanic center with a considerable life span (many millions of years) provides examples of most variants in volcaniclastic sequences. These include deposits typical of intracaldera (vent) and proximal, medial, and distal facies. The type of volcaniclastic sequence produced depends on the proximality and volcanic activity (type and intensity) of the source, and the principal tectonic and depositional setting of the receiving basin. Pyroclastics and lavas are volumetrically dominant in the proximal facies; medial facies are characterized by a high percentage of epiclastic sediments containing significant proportions of interstratified pyroclastic deposits. Although the ratio of pyroclastic to epiclastic constituents generally decreases away from the volcanic center, primar pyroclastic deposits may be voluminous in distal areas due to the energy-efficient transport mechanism of pyroclastic flows and the widespread dispersal of some ash falls. Welded ignimbrites are an important diagnostic rock type in this association, and are prominent in the ancient record due to their high preservation potential. The temporal and spatial relationships among lavas, fall deposits, ignimbrites, block-and-ash pyroclastic flow deposits, and epiclastics are important for interrelating the various volcaniclastic facies and establishing event stratigraphy.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.