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The Chile Trench is a long, linear basin that concentrates clastic
terrigenous material at the foot of the Andean continental margin. In southern Chile, high rainfall and river runoff, combined with intense Pleistocene glacial activity, transport large volumes of detrital sediment to the offshore regions. Sediment bypassing the continental margin has produced a wedge-shaped deposit which swells to > 30 km (> 18.6 mi) in width and 2 km (6,560 ft) or more in thickness. Convergence of the Nazca plate with South America at the rate of 10 cm/yr indicates that these trench wedge deposits are no older than 300,000 years. Good piston core control over the wedge shows that very fine to medium-grained sand may locally constitute > 50% of the stratigraphic section.
Two distinct depositional environments exist in the south Chile trench. South of 41°S lat., it appears that unconfined turbidity sheet flows spread radially from the mouths of submarine canyons to deposit laterally extensive, rhythmic sequences across the entire trench wedge. Seismic reflectors in this region are flat and continuous. In seven piston cores, graded sand beds can be correlated across 30 km (18.6 mi) by stratigraphic position, mineralogical content, ash content, radiometric dates, and characteristic grain-size distributions (i.e., mean size, sorting, skewness); individual sheet flows can be identified and studied spatially. Skewness is consistently positive; the mode is perhaps the most descriptive grain-size parameter since it most closely reflects the carrying comp tency of the turbidity current at the time of deposition.
At 41°S lat., an axial channel (up to a few 100 m in depth) develops in the trench wedge, trending northward along the gravitational gradient. Sediments may be transported parallel to the margin, via the axial channel, hundreds of kilometers from their canyon mouth source. In this region, submarine fans develop at the mouths of submarine canyons, prograding across the trench wedge and displacing the axial channel seaward; the fan distributaries become confluent with the main axial channel. Tensional faulting in the subducting oceanic basement commonly displaces the overlying trench strata; these normal faults can influence the position of the axial and fan channels. Alternating periods of deposition (characterized by flat-floored channels, levees about 50 m [165 ft] high, and gra ed silt and sand facies) and periods of erosion and/or winnowing (characterized by V-shaped furrows; reflection hyperbolics; seismic reflectors truncated against erosional scarps; and massive, nongraded, laminated sand facies) combine to produce complex fan morphologies (such as inactive or remnant lobes, hanging valleys, and possibly braided channels) and sedimentary facies that change rapidly over space and through time.
Late Pleistocene to Holocene sedimentation rates in the Chile trench range from several centimeters to > 1 m/1,000 yr (> 3 ft/1,000 yr). Erosional hiatuses and a variety of depositional environments contribute to the wide range of sedimentation rates.
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