Abstract

The depositional history of the Eocene-Oligocene Burwash strike-slip basin is characterized by a transition from non-volcanic clastic sedimentation of the Amphitheatre Formation to deposition of lavas and volcaniclastic rocks of the overlying lower Wrangell volcanic sequence. The lowermost Wrangell volcanic deposits within the Burwash basin consist mostly of 110 m of trachybasalt, basaltic trachyandesite, and trachyte lava flows which directly overlie about 440 m of stream-dominated alluvial fan, fan-delta, and lacustrine deposits of the Amphitheatre Formation. Overlying the lava flows are about 50 m of interbedded volcaniclastic fluvial deposits, pumice-rich, andesitic pyroclastic-fallout deposits, and lithic-rich trachyandesitic pyroclastic-flow deposits. Although volcanism is known to occur within strike-slip basins, the role of volcanism in strike-slip basin evolution and the effects of volcanism on sedimentation in strike-slip basins are largely unknown. The depositional history of the sedimentary rocks (Amphitheatre Formation) and the volcanic rocks (Wrangell volcanic sequence) in the Burwash basin can be divided into four progressive stages: 1) progradational stream-dominated alluvial fan deposition; 2) lacustrine deposition; 3) renewed basin margin faulting, proximal alluvial fan deposition and initial volcanic vent formation; and 4) intrabasinal volcanism resulting in lava flow emplacement, pyroclastic eruptions, and fluvial volcaniclastic sedimentation. This study focuses on the complex interplay between volcanism and fluvial sedimentation that can be demonstrated during stages 3 and 4, and on the overall reorganization of depositional systems in this strike-slip basin during the transition from non-volcanic clastic sedimentation to lava flows and volcaniclastic sedimentation. Paleocurrent data collected from the Burwash basin document major reorganization of paleodrainage during the clastic to volcaniclastic transition. Prior to volcanism, fluvial systems flowed westward toward and along the basin axis, whereas after the onset of volcanism fluvial systems drained northeastward, outward from the basin center. In addition, conglomerate compositions in the basin change at the clastic to volcaniclastic transition from extrabasinally-derived basement clasts eroded from uplifted source areas along the strike-slip basin margin to intrabasinally-derived volcanic clasts. Fluvial depositional style was also strongly influenced by contemporaneous explosive volcanism during the late stage of Burwash basin evolution. Fluvial aggradation, which occurred in the form of sand-dominated hyperconcentrated flood-flows and braided streams, was contemporaneous with episodes of pyroclastic activity. These syneruption fluvial deposits are sheet-form and contain abundant juvenile detritus (vitric grains and pumice), euhedral to subhedral and angular crystal fragments of plagioclase and hornblende, and a small percentage of basaltic lithic fragments. Fluvial degradation, marked by the incision of steep valleys, occurred during periods between eruptions. The inter-eruption fluvial deposits are characterized by lenticular beds of clast-supported conglomerate and trough-crossbedded sandstone and are composed predominantly of basaltic lithic grains with small amounts of crystal and vitric fragments. Subsequent eruptions generated pyroclastic flows which ponded in the fluvial valleys. These pyroclastic-flow deposits exhibit crystal-rich bases, lithic-rich lapilli-sized middle zones with out-sized lithic blocks and gas-escape structures, upper pumice-rich zones, and interbedded stratified surge deposits. Our results show that the onset of volcanism in the Burwash basin produced a major reorganization of fluvial drainage systems and sediment source areas. In addition, changes in fluvial depositional style were closely linked to alternating periods of explosive volcanic activity and quiescence during the late stages of strike-slip basin evolution. The excellent exposures in the Burwash basin provide an apt opportunity to examine the role of volcanism in the depositional history of an intracontinental strike-slip basin.

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