ABSTRACT

Exposures of an Oligocene small-volume dome field in central Arizona, USA, provide an excellent example of the interaction of volcanism and sedimentation in a proximal volcanic setting. Volcanism in the field began with eruption of a shoshonite cinder cone and lava flows, which were emplaced onto braided-stream gravels. Stream sediments aggraded to the top of the lava flows before eruption of an amphibole-trachyte dome and its derivative pyroclastic flows. Deposition from the pyroclastic flows was punctuated by fluvial sedimentation, which took the form of both sheetflow reworking and debris-flow and hyperconcentrated-flow deposition. The lack of pronounced channel formation may indicate that inter-eruptive periods were brief.

Two principles of volcaniclastic sedimentation are well documented in this study: First, active aggradation may preserve proximal deposits that are normally eroded quickly from a volcano. This aggradation is seen in the interstratification of braided-stream gravels derived from outside the area of volcanism with dome-derived primary pyroclastic deposits. The growing dome apparently did not form a large topographic feature, and thus did not overwhelm the preservation caused by regional aggradation. The landscape response to volcanism was highly complex, and the response of the stream system to short-term volcanic processes merely accentuated the effects occurring due to longer-term tectonic subsidence.

Second, because sediments in the proximal areas of the dome were preserved, the interaction of the regional dispersal system with the dome complex is clear. Pyroclastic deposits were in some cases localized in channels. Reworked pyroclastic deposits have structures indicative of sheetflow or mass flow, but are entirely pyroclastic in composition. Deposits strongly influenced by the regional system are also mass flows or sheetflows, but they contain appreciable concentrations of non-volcanogenic material, and were deposited by debris-choked braided streams.