Abstract

Surface sediment samples from 86 locations on the Peru-Chile continental margin and the Nazca plate have been analyzed for bulk chemistry and texture to evaluate the factors influencing sediment formation on continental slope and adjacent abyssal plain depositional environments. Sand-sized calcareous microfossils are abundant above the carbonate compensation depth (CCD), but their removal by dissolution leaves fine-grained deposits in the deeper basins. Terrigenous silts are found seaward of the Peru-Chile Trench, especially south of 7°S, and may be transported there by wind, mid-water turbid layers, or bottom waters flowing north through the deep ocean basins. Sediments become finer grained away from shore as coarse terrigenous particles settle out. The more humid climate of northern Peru produces finer fluvial sediments there than to the south. This textural change also appears in adjacent marine sediments. In the Peru Basin, bottom nepheloid layers are found where clay-sized terrigenous material dominates the sediments. On the adjacent Galapagos Rise, however, nepheloid layers are absent, and eolian-derived sedimentary components in the <5 μ size range may exceed those transported to the area by currents.

R-mode factor analysis has outlined sediment constituents from the geochemical data. Terrigenous components dominate the slowly accumulating clay-sized sediments of the Peru and Chile Basins, and the coarser, more rapidly forming margin deposits south of 14°S. Biogenic deposits form at intermediate rates on topographic highs on the Nazca plate. Very slow hydrogenous sedimentation occurs in the seaward portions of the deep basins. Sediments rich in organic components are concentrated in rapidly forming deposits along the margin north of 14°S, beneath centers of strong coastal upwelling.

Several distinct sediment accumulations have been mapped on the continental shelf and upper slope. An upper-slope deposit is anomalously fine grained and organic rich; it lies between 10.5°S and 13.6°S, in the region of most intense upwelling. Preservation of this body is enhanced by the inclusion of fine inorganic material in biogenic fecal pellets, and by the impingement of the shallow-water oxygen minimum layer on the slope at the same level.

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