1. CLAY MINERALOGY AND CLAY GEOCHEMISTRY IN THE VICINITY OF THE DÉCOLLEMENT ZONE, NORTHERN BARBADOS RIDGE 1

Fine-grained sediments recovered during Leg 156 of the Ocean Drilling Program contain assemblages of clay minerals that change considerably with depth. Key structural and stratigraphic features within the toe of the northern Barbados accretionary prism include the basal decollement and a sharp boundary between lithologic Units II and III; this lithologic boundary occurs within the decollement zone. We analyzed three size fractions (4 -8 µm, 1-4 µm, and <1 µm) by X-ray diffraction and obtained chemical data from the <1-µm size fractions by scanning electron microscopy coupled with energy dispersive spectroscopy. Relative proportions of expandable clay minerals increase significantly, but erratically, near the top of the decollement zone. Proportions of discrete smectite and smectite-rich illite/smectite (I/S) mixed-layer clays decrease steadily down the decollement zone. The base of the decollement zone is characterized by unusually high concentrations of illite. These changes in clay mineralogy influence the shear strength of claystones at the deformation front and help control the position of the decollement . Relative abundances of illite within all lithofacies units increase in coarser size fractions, and this trend is consistent wit h a detrital source. Kaolinite does not display any systematic partitioning within Unit II, but its proportions increase in the fin er size fractions within Unit III. The smectite content of Unit II increases with decreasing grain size; Unit III shows the opposi te trend between particle size and % smectite. Bentonite-like samples probably originated through in situ alteration of disseminated volcanic ash, but we believe that the pyroclastic material was mixed into a background sediment that already contained abundant detrital smectite. Illitic mixed-layer clays within the underthrust domain (Unit III) also appear to be detrital in or igin. Correlations among both % smectite and % kaolinite and most major-element oxides are statistically significant, but correlations for illite are not. Expandable clays vary considerably in their chemical make-up; the most common type appears to be a Mg-poor, K-rich, Fe-Al-beidellite. We rely upon actualistic provenance connections for late Quaternary clays to guide our interpretations of Miocene and Oligocene sediment dispersal. If these analogues are valid, then the proto-Orinoco and protoAmazon Rivers were the most important detrital sources for the older hemipelagic and turbidite deposits of Barbados Ridge.