Clay mineralogy of the Peru continental margin and adjacent Nazca plate: Implications for provenance, sea level changes, and continental accretion

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Abstract
The clay mineralogy and organic carbon contents of 52 surface and 53 subsurface samples were determined for Quaternary sediments of the Peru continental margin and adjacent Nazca plate. By using Q-mode factor analysis, three factors (oceanic, continental A, and continental B) can explain 99% of the variation in clay mineral composition and organic carbon content in surface sediment. Northeast Nazca plate surface sediment is characterized by an oceanic factor that is dominated by smectite with subordinate illite. Continental margin surface sediment is characterized by the following clay mineral assemblages: (1) continental factor A, which consists of smectite-chlorite mixed-layer clays and the detrital clay minerals illite, kaolinite, and chloride, or (2) continental factor B, which is composed of illite with subordinate kaolinite and chlorite. Upper continental margin (<2,000 m) surface sediment is dominated by either of these continental assemblages, while lower continental margin sediment is characterized by the oceanic or continental-A assemblage. All subsurface data were compared to the surface factor model by constructing a pseudo-factor matrix to examine the temporal changes in clay mineral assemblages. In the late Quaternary (≤0.44 m.y.b.p.), the boundary between surface sediment with a dominant oceanic factor and sediment with a continental factor (A or B) lies on the continental slope. Earlier in the Quaternary (0.44 to 2.0 m.y.b.p.), this boundary was about 100 km seaward of its present position, a shift that we attribute to regression of the shoreline as a result of glaciation. By comparing the subsurface factor loadings to the surface factor loadings, displaced sediment can be recognized. Four cores taken on the seaward side of the present Peru Trench axis contained terrigenous turbidites that have been uplifted since deposition. Two cores on the lower continental slope opposite the Nazca Ridge contained pelagic sediment (clay and calcareous ooze) accreted from the Nazca plate. However, most of the 28 cores recovered from the landward wall of the trench were terrigenous turbidites and hemipelagic muds, not accreted Nazca plate sediment.

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