Volcaniclastic facies depend ultimately upon magma composition, which governs eruptive rates, types of particles, manner of emplacement, total volume and therefore type of volcano. In subduction settings, andesite to dacite suite magmas construct high-standing stratovolcanoes with large volumes and great heights, and therefore large reservoirs of sediment (Hackett and Houghton, 1989). They erode rapidly, providing large volumes of reworked pyroclastic and hydroclastic particles together with epiclastic volcanic debris that are deposited into surrounding basins. Large calderas, commonly built in extensional back-arc regions, are as voluminous as stratovolcanoes, but they are low-standing volcanoes. Very large craters of calderas are initially closed sedimentary basins in which lacustrine sediments and slump blocks from crater walls are deposited. Differences between volcanoes require that different facies aspects be considered in order to reconstruct volcanic areas. These facies aspects are (1) distance-related facies, (2) the type of source volcano and (3) whether vents were single, multiple, central or flank.
The presence of vitric fragments (shards, pumice) within sedimentary sequences indicates a pyroclastic or hydroclastic origin. Moreover, glass is metastable and readily alters to clays and zeolites, and therefore does not appear as an epiclastic fragment.
The proximal facies may include the source volcano (Vessel and Davies, 1981), but where the source is not exposed, proximal facies rocks can be defined by type of transport such as lava flows (short travel distance), lahars, and fallout layers (most far-travelled) and, in the case of reworked pyroclastics or volcanic epiclastic materials, on their coarsest and thickest parts (Smith, 1988a,b). Pyroclastic facies may be divided into different subfacies, such as lahar or pyroclastic flow and pyroclastic surge subfacies (mechanisms of transport), lacustrine, submarine fan or alluvial sub-facies (environment of deposition), etc. These criteria are the foundation for defining larger-scale facies environments such as source volcanoes and their surroundings (Hackett and Houghton, 1989; Fisher and Schmincke, 1984).
One example of a multilayered deposit from a single event comes from the 18 May 1980, 8:32 a.m. eruption of Mount Saint Helens, Washington (USA). Five different layered units were formed from a single blast erupted laterally from the north side of the volcano. The layers include a ground layer containing a poor mixture of material from the original ground surface with some juvenile lithics from the eruption, overlain by blast surge deposits, capped by an accretionary lapilli fallout layer (Fisher, 1990).
The concept of tephra event unit has been developed which includes proximal to distal facies, all produced in a geologically very brief time interval (Schmincke and Bogaard, 1991).
The growth rate of andesitic stratovolcanoes, with consequent influence upon depositional environments, is geologically extremely rapid --- on the order of a few hundred to a few thousand years. These large constructional landforms are composed of great volumes of easily remobilized fragmental material. Their growth is therefore reflected almost instantly in the sedimentary record of the surrounding region (Kuenzi et al., 1979; Vessel and Davies, 1981) by direct deposition from airborne tephra, by deposition of ground-hugging pyroclastic flows, or from eruption-related debris avalanches, lahars and fluvial materials. Rapid construction of a volcano results in an increase in rate of erosion as slopes steepen and local climates are altered. Large volcanoes create climatic barriers, where rainfall and consequent erosion can be dramatically high on the windward side.
Andesitic volcanism produces three conditions necessary to lahar development -- steep slopes, relatively high rainfall and abundant loose fragmental material. With high enough elevations glaciers may also form, including their consequent abundant outwash. Rapid growth of volcanoes can result in oversteepened unstable slopes leading to collapse of sections of the mountain and development of debris avalanches (Glicken, 1986; Siebert et al., 1987).
Rapid growth rates of volcanoes profoundly influence the progressive facies changes associated with an entire volcano system. For example, later products from a stratovolcano at its maximum height and volume can be carried farther than its earlier products when it was smaller. In subaqueous environments, as a volcano grows from deep through shallow water to subaerial environments, explosivity increases which leads to greater production of particles and their more efficient dispersal (Staudigel and Schmincke, 1984; Schmincke, 1982)). Thus, coarsening upward, progradational sequences in adjacent marine basins, as demonstrated by Busby-Spera (1988a), result from actively growing stratovolcanoes. Incision occurs during inactive periods with reworked primary pyroclastic and epiclastic volcanic debris being carried away by fluvial systems leaving little or no record of sedimentation near the source (Smith, 1987a; Smith et al., 1988b). Fining upward sequences develop in sedimentary basins as a volcano lowers by erosion, with products being dominantly of epiclastic and reworked pyroclastic origin as shown by the Great Valley sequence of California (USA) (Ingersoll, 1978).
Scott (1988) extends the concept of flow (laminar-turbulent) transformation to include changes in transport agents whereby lahars are transformed to lahar-runout flows (hyperconcentrated flows), or hot pyroclastic flows are transformed to lahars. Weirich (1989) demonstrates that subaqueous debris flows transform to turbidity currents by hydraulic jumps. Thus, the concept of transformations links the general volcaniclastic facies (multiple event facies) to flow processes (single event facies) within a space-time framework from source (the volcano) to final deposition in marine or non-marine basins (proximal to distal facies). Because of erosion, however, the lateral facies changes are commonly truncated. For example, in volcanic arc environments, the proximal source is commonly missing, with only distal facies (turbidite, submarine fan) and intermediate facies (fluvial, lahar to delta and shelf with or without submarine lahars) being present (Smith, 1988b).
As shown by the 1980 Mount St. Helens eruptions, one facies lineage, linked by flow transformations, is as follows (Scott, 1988): eruption of pyroclastic surge or flow > lahar > hyperconcentrated flood flow > normal fluvial transport (in the Columbia River). Another lineage is fallout ash from vertical eruption plumes > initial large-scale debris avalanches > stop-gap storage of sediment on submarine shelves or slopes > submarine landslides > subaqueous lahars > turbidity currents.
Thus, on present-day stratovolcanoes, major volcaniclastic facies associations can be divided on the basis of distance and geographic location -- cone-forming sequences surrounded by voluminous ring plains corresponding to proximal and medial facies as presented above. Distal facies are far-travelled ash blankets that may be physiographically separated from the other deposits of the source volcano (Fisher and Schmincke, 1984).
Facies analysis of volcaniclastic aprons surrounding oceanic islands has led to the definition of several overlapping stages in the evolution of oceanic islands: 1) deep water stage, (2) shallow water--shield stage, (3) mature island stage and (4) regressive erosional stage, each with different clastic processes giving rise to characteristic clast types and mixtures (Schmincke, 1987, 1988).
The above described sedimentary environments also occur in marine backarc regions, and therefore are difficult to separate only on the basis of volcaniclastic lithology or type of transporting agent. Tectonic associations (extensional structures, grabens), chemical affinities (more alkaline in back arc) and rock associations (interbedding of cratonic sediments) may also be necessary to determine tectonic environment. In the North and South American Cordilleran environment, fallout tephra is much more common in backarc regions than in forarc environments, whereas, in the Lesser Antilles, fallout tephra is far more common in the forearc region (Sigurdsson and Carey, 1981; Sigurdsson et al., 1980).
There are several types of extensional environments, generally in back arc regions, both marine and nonmarine, and in intra-arc regions (Busby-Spera, 1988a,b; Smith et al., 1987). Chemical affinities of extensional volcanics are commonly alkaline. Although stratovolcanoes may grow within extensional environments to provide volcaniclastic debris, felsic ignimbrite deposits are characteristic products associated with caldera formation. Basaltic scoria cones (and maar volcanoes in water-rich environments) may be abundant. Extensional environments generally include graben structures that act as sediment traps. In lowland graben environments, basaltic volcanism is likely to give rise to abundant hydroclastics, and volcanic land forms can be partly covered and preserved within the sedimentary fill, which is likely to be epiclastic volcanics depending upon the intensity of volcanism. In addition to chemical evidence and rock type mentioned above, preservation of the volcano edifice signifies an extensional tectonic environment different from that of the convergent island arc stratovolcano that stands high and erodes away. Features of and evidence for arc graben depressions are briefly reviewed by Busby-Spera (1988b) for the early Mesozoic of the southwest Cordilleran United States. Important evidence is the great thickening of the depositional bodies, and the interbedding of quartz sandstone from the craton trapped in the graben and interbedded with pyroclastic and epiclastic materials, including ignimbrite. A modern arc graben analogue occurs in Central America (Burkart and Self, 1985). Smith et al. (1988) describe a late Miocene graben from the central Oregon High Cascades filled with volcaniclastics of the Cascade arc.
Volcaniclastic rocks of the arc are characteristically calc-alkaline andesite or basaltic andesite. Rhyolitic to dacitic ignimbrites also occur in arcs but are more abundant and widespread in intra-arc grabens, backarc regions and continental extensional tectonic zones.
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