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The geochemistry of the major components plus Ba and Sr are interpreted in the context of data on lithology and mineralogy and of the available geological information. The role of biogenic, clastic, authigenic, and hydrothermal constituents in the history of post-Middle Jurassic sedimentation is quantitatively estimated. The results of factor analysis of the chemical components and peculiarities of the distribution of average accumulation rates are considered. Five main stages in the geochemical history of sedimentation are distinguished: 1) Late-Middle Jurassic to Early Cretaceous, 160.0-106.0 Ma: Sedimentation in a shallow-water basin. 2) Middle/early Albian to middle Maestrichtian, 106.0(?)-66.4 Ma: Sedimentation in an extending and deepening open oceanic basin. 3) Late Maestrichtian to late-middle Eocene, 66.4-57.0(7) Ma: An erosional hiatus of almost global extent. 4) Late/middle Eocene to early Miocene, 57.0-15.0 Ma: Widespread development of erosional hiatuses with accumulation of residual products and intense sedimentation of biogenic CaCO3 and SiO2. 5) Neogene to Quaternary, 15.0 Ma to present: Opening of the Drake Passage at the Paleogene/Neogene boundary, followed by deepening of the basin, northward invasion of Antarctic water masses rich in nutrient components, and a depression of the CCD. In the Quaternary the Polar Front retreated somewhat, followed by decreased rates of biogenic sedimentation and increased accumulation of glacial products. INTRODUCTION The main objectives of DSDP Leg 71 were to study the evolution of sedimentation and paleoceanographical environments in the southwestern Atlantic during the post-Middle Jurassic (Ludwig et al., 1980) (see Fig. 1). The geochemical study of the major components and of the trace elements Ba and Sr in the context of mineralogical, lithological, and geological data permit a quantitative estimation of the effects of biogenic, clastic, authigenic, and hydrothermal components on sedimentation and lithogenesis, and of their variations in time and space. MATERIALS AND METHODS This work is based on the results of lithologic-geochemical studies of core samples from DSDP Leg 71, received from V. A. Krasheninnikov; the work was carried out at the Geological Institute of the U.S.S.R. Academy of Sciences. The lithologic-mineral composition of the sediments is described in other chapters (e.g., Varentsov et al., this volume). All chemically analyzed samples were also studied as thin sections under a microscope. The mineral varieties were studied by X-ray diffraction using slides prepared either from a natural sediment or its size fractions, particularly the fraction < 0.001 mm. Chemical components were identified at the laboratories of the Institute of Geochemistry of the Siberian Branch of the U.S.S.R. Academy of Sciences, Irkutsk, under the direction of Dr. V. P. Afonin, by means of X-ray fluorescence spectroscopy, using international reference standards. In addition, control identifications were made by wet chemistry methods. Analytical data were processed by computer (EC-1022) in the laboratory of mathematical methods of the Geological Institute of the U.S.S.R. Academy of Sciences (D. A. Kazimirov) using the factor Ludwig, W. J., Krasheninnikov, V. A., et al., Init. Repts. DSDP, 71: Washington (U.S. Govt. Printing Office). analysis program (R-, Q-mode; Davis, 1973; Herman, 1967). The specific features of paragenetic assemblages of the factor clusters determined by factor analysis of oceanic sediments have been considered by Lehvn and Stakes, 1979; Varentsov, 1980; and Varentsov et al., 1981. The identification of the assemblages was based on grouping the components with significant factor loadings of one sign. For a better representation of relationships between the components of an assemblage, each chemical component is characterized by the factor loading value, given in parentheses, for the factor with which it is most closely connected. The better expressed values of factor loadings were obtained after transformation of the data by the rotation method (Davis, 1973). Calculation of the average rates of accumulation for chemical components are considered elsewhere (Varentsov et al., 1981). Sediment densities were derived from measurements made aboard ship (see site chapters, this volume). The Mesozoic and Cenozoic geochronology is that adopted by the shipboard scientists (Berggren, 1973; Geological Society of London, 1964; Hardenbol and Berggren, 1978; Larson and Hilde, 1975; Van Eysinga, 1975). PARAGENETIC ASSEMBLAGES OF COMPONENTS (Table 1) Assemblage IA( +) Na2O(0.23)-MgO(0.14)-Al2O3(0.94)-SiO2(0.87)K2O(0.95)-TiO2(0.94)-Fe2O3(0.89) The components of this factor group compared with the real mineral composition of sediments shows that this assemblage is predominantly represented by mixedlayer clay components: mica/smectite. (More detailed data on the distribution of these clay minerals are given in Varentsov et al., this volume.) Distribution of this assemblage vertically and laterally through the sequence (Table 2) distinctly indicates its dual nature: 1) Relatively high factor scores (exceeding 0.5) occur in geochronological intervals with increased accumula-

Geochemical History of Post-Middle Jurassic Sedimentation in the Southwestern Atlantic, Deep Sea Drilling Project Leg 71; Ba, Sr, and Major Components