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

Abstract

Late 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.

Sedimentation history of the Palaeoproterozoic Dhanjori Formation, Singhbhum, eastern India

“…For example, the sudden influx of voluminous volcanic and volcaniclastic material into a restricted basin can create resistant barriers and may have pronounced effect on the sediment dispersal patterns and depositional sequences (Kuenzi et al, 1979;Mueller et al, 1994). Although volcanic, pyroclastic, and sedimentological aspects of terrestrial depositional systems have been discussed by a number of authors (Smith, 1987(Smith, , 1988Smith and Swanson, 1987;Mueller et al, 1994;Altermann, 1996;Mueller and Corcoran, 2001;Mueller and Mortensen, 2002), the influence of allocyclic controls such as tectonism and volcanism on basin formation, sediment dispersal patterns and facies architecture have not been adequately addressed (cf. Sato and Amano, 1991;Corcoran et al, 1999;Kataoka and Nakajo, 2002).…”

A medial to distal volcaniclastic record of an andesite stratovolcano: detailed stratigraphy of the ring-plain succession of south-west Taranaki, New Zealand

“…Neall 1975;Smith 1987Smith , 1988Cronin and Neall 1997;Lecointre et al 1998;Belousov et al 1999;Davidson and De Silva 2000). Deposits recording these events can often be dated by applying the radiocarbon method to intercalated soil/peat layers and ripped-up fragments of wood, as well as by identifying interbedded tephras of known age.…”

Build‐up and depositional dynamics of an arc front volcaniclastic complex: the Miocene Tepoztlán Formation (Transmexican Volcanic Belt, Central Mexico)

“…The occurrence of prismatically fractured clasts in debris-flow deposits suggests that the debris flows were generated by mixing of water with recently emplaced debris from block-and-ash flows (e.g. Smith, 1986Smith, , 1988.…”

“…In non-marine sedimentary basins, deposition of this facies association is mostly concentrated on fluvial system deposits and overbank alluvium (Smith, 1988;Hackett & Houghton, 1989;Riggs & Busby-Spera, 1990). Primary volcanic products are generally restricted to fine-grained fall deposits and far travelled ignimbrites comprising minor contributions to the stratigraphy (Smith & Landis, 1995).…”

“…Accumulation is thickest near the volcanic cone and fine and deposits thin distally, giving rise to proximal-distal facies patterns (Vessell & Davies, 1981;Smith, 1988Smith, , 1991.…”