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Three hundred samples of Cenozoic sediments from Sites 671 and 672 were analyzed by X-ray diffraction for their clay mineralogy. In addition, clay particles from selected samples were submitted to microprobe analysis. Throughout the Eocene-Miocene section, the smectite abundance exceeds that of all other minerals. From the lower Pliocene upward, a noticeable increase of illite and chlorite abundance occurs. The presence of thrust faults does not seem to correlate with the composition of the clay species. Quantitative differences in clay mineralogy are mainly due to paleoenvironmental conditions. Prior to the Pliocene, clay mineral assemblages were influenced by climatic and tectonic conditions existing on the adjacent islands and South America. Since the Miocene, the acceleration of the worldwide climatic cooling and the northwestward migration of currents along the South American coast increased the supply of illite and chlorite. Tectonic rejuvenations in the northern part of South America (late Miocene-early Pliocene; late Pliocene-early Pleistocene) were marked by illite and chlorite input and were superimposed on other paleoenvironmental controls.

Clay Mineralogy of ODP Sites 671 and 672, Barbados Ridge Accretionary Complex and Atlantic Abyssal Plain: Paleoenvironmental and Diagenetic Implications