Dispersal of Volcanic Sediment in Central Aleutian Forearc and Trench

M. B. Underwood, A. J. Stevenson

Late Quaternary patterns of marine sediment dispersal are somewhat unusual in the central Aleutian forearc. The upper trench slope is incised by numerous small submarine canyons, all of which debouch along the midslope Aleutian Terrace. In effect, these closely spaced point sources act as a single line source for turbidite sedimentation. Evidently, deposition in forearc basins (e.g., Atka basin) involves either sheet flow or shallow braided channels; there is no evidence of channelization on available seismic reflection records. The depositional geometry is perhaps best described as a turbidite ramp.

No throughgoing submarine canyons cross the central Aleutian forearc. Consequently, confined turbidity currents cannot be responsible for transverse delivery of arc-derived sands to the trench floor. However, the central trench wedge is thick (up to nearly 4 km or 13,000 ft), and trench sands are comprised almost exclusively of volcanic rock fragments, glass shards, and plagioclase. Apparently, arc-derived turbidity currents were able to traverse the forearc terrace as unconfined flows and maintain enough flow strength to billow over the ridges that mark the trench-slope break. Small notches in the bounding ridges may have served as preferred basin exit points, but the flows remained unconfined as they accelerated down the lower trench slope.

The uppermost 10 m (32 ft) of trench fill is actually dominated by mud. Whether the described style of sediment dispersal is unique to post-glacial time remains unknown, but it may be favored by muddy sediment textures.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.