Many sequences of geosynclinal strata are composed of pyroclastic ejecta and detritus eroded from contemporaneous volcanic accumulations. Their lithologic successions are governed largely by secular variations in the type and intensity of the contributory volcanism and their facies patterns, by the spatial distribution of eruptive centers. The linkage between volcanism and sedimentation can be as direct as, for example, when showers of airborne ash rain into the final site of deposition, or the linkage can be as indirect as the subtle influences caused by changes in submarine topography with the construction of volcanic piles within a basin of deposition. To reconstruct the depositional histories of the clastic volcanic sequences, the role of volcanism in controlling envi onments of deposition, determining the dispersal agents of sediment, and creating transitory sources of sediment must be correctly evaluated.

The following genetic assemblages of clastic volcanic strata can be recognized in Paleozoic and Mesozoic marine sequences of western North America.

1. Volcanic center assemblages include (a) pillow lavas, pillow breccias, and palagonitic tuffs from submarine eruptions; and (b) lavas, flow-breccias, and pyroclastic breccias from subaerial eruptions.

2. Volcanic shelf assemblages were deposited on (a) narrow shelves flanking volcanic piles and (b) broad platforms of tectonic origin. The rocks of the assemblages are composed of vitroclastic ash and abraded grains in varying proportions, and are commonly fossiliferous.

3. Volcanic slope assemblages were deposited on subaerial or submarine slopes forming the sides of (a) elevated tracts built up by volcanic outpourings and (b) tectonic ridges capped by volcanos. The sedimentary structures of the assemblages include erosive scour, sole markings, cross-bedding, and graded bedding.

4. Volcanic basinal assemblages were deposited in (a) low tracts lying between volcanic piles and (b) tectonic depressions lying within the dispersal radius of volcanic sources. The rocks of the assemblages include vitroclastic tuffs deposited by showers of airborne ash and rocks formed by the setting of suspended sediment.

These assemblages have counterparts in other regions where similar rocks are exposed. Modern analogues are present in Recent sediments of the Caribbean and lndonesian regions.

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