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Analysis of the clay mineralogy of 537 samples from Cenozoic sediments of the Peruvian forearc reveals distinctive geographic and stratigraphic patterns in the distribution of clay minerals. Post-Oligocene clay mineral assemblages show relatively constant illite-chlorite-kaolinite ratios with a variable expandable mineral component (smectite, vermiculite, and mixed-layer chlorite-vermiculite and/or chlorite-smectite). The distribution of Quater­ nary clay minerals is primarily controlled by provenance. In Quaternary assemblages from the Salaverry, Lima, and Trujillo basins, illite is more abundant than expandable minerals, and the expandable phase is primarily vermiculite, whereas in the Quaternary of the West Pisco Basin (Site 686), expandable minerals are more abundant than illite, and the expandable phase shows a substantial smectite component. This reflects low abundances of smectite in the source area between 4° and 14°S, which is a consequence of the Quaternary "volcanic gap" in this region. A change from smectiteto vermiculite-dominated assemblages during the Quaternary at Site 687 suggests relative uplift of the outer-shelf high at this time and isolation of the West Pisco Basin from the Lima Basin. The greater abundance of expandable phases seaward within the Quaternary is attributed to mixing of fluvial smectite-poor material with either smectite-rich eolian material or authigenic smectite, rather than differential settling. Slope sites exhibit a gradual increase in expandable minerals within the Quaternary. This may be explained in terms of a greater reworked shelf-derived component in the lower Quaternary. High-resolution studies of clay minerals at Sites 680 and 686 may provide some evidence of cyclicity. Most sites exhibit a sharp decrease in expandable minerals at or within the Quaternary or Pliocene. This is attributed to the reduction of volcanic activity associated with the progressive southward migration of the Nazca Ridge. At all sites, older sediments are characterized by smectite-rich assemblages, with smectite generally more abundant at depth. This is caused by the addition of allogenic and authigenic smectite from alteration of volcanic material, produced extensively during the Miocene and late Eocene.

Clay Mineralogy of Cenozoic Sediments from the Peruvian Continental Margin: Leg 112