Sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt: Ellensburg Formation (late Miocene), central Washington

Abstract

ABSTRACTLate Miocene volcaniclastics of the Ellensburg Formation (Washington, northwestern USA) are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a Cascade Range source area. Two facies associations are recognized. The first is composed of laterally restricted, well‐sorted, polymictic conglomerates representing a gravel‐bedload regime during inter‐eruption periods. The second comprises laterally extensive sheets, bounded by deeply‐developed paleosols, composed of monomictic pebbly, pumiceous dacitic sandstones with intercalated debris‐flow and hyperconcentrated flood‐flow deposits. These sheets aggraded in response to eruptive events that are sometimes recorded by air‐fall tephras at the bases of sequences. Debris‐flow and hyperconcentrated flood‐flow deposits occur as far as 120 km from source, but are uncommon beyond 50 km. Hyperconcentrated flood‐flow deposits initially increase in abundance away from source as debris‐flow deposits diminish, suggesting formation of hyperconcentrated flood flows by dilution of debris flows. Sandy facies form broad sheets dominated by scour‐fill bedding, in proximal and medial settings, and grade to narrower, trough cross‐bedded sheets in distal settings, suggesting moderation of flood discharges with distance. Base‐level changes associated with episodic sediment influx caused incision‐aggradation cycles in mainstream settings, and episodic impoundment of tributaries to form lakes or rapidly aggrading, poorly drained floodplains. Although volcanism was the primary control on depositional style, concurrent development of the Yakima fold belt produced a structure‐consequent drainage pattern that determined sediment dispersal, and basin subsidence permitted preservation of both syn‐eruption and inter‐eruption facies. Detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics, but dominates deposits of small tributary streams flowing toward, rather than from, the Cascades.