Decollement zones (plate-boundary faults) separate offscraped and underthrust sequences in subduction zones. ODP Legs 156 and 110 penetrated the toe of the northern Barbados accretionary complex. Clay mineralogy and geochemistry (by XRD, SEM/EDS, and XRF) have added more insights into what controls the stratigraphic position of the Barbados decollement (a 32-m-thick zone). A sharp lithologic boundary occurs within the middle of the decollement zone between Miocene pelagic-hemipelagic clays (Unit II) and Oligocene fine-grained turbidites (Unit III). This zone also coincides with a pronounced mineralogic gradient from smectite-rich sediments at the top (>60 wt-% smectite in bulk) to illite-rich sediments at the base (<10 wt-% smectite). The smectite-rich interval retains higher porosity, abnormally high pore fluid pressure, and mechanical weakness. Mixed-layer I/S clays in Unit II are approximately 20 % less illitic that those in Unit III, which also lowers shear strength. Chemical and XRD data indicate that smectite in Unit II is montmorillonite, whereas smectite in Unit III is more Fe-rich and includes beidellite and nontronite. Relative clay-mineral abundances are partitioned by grain sizes, but smectite shows no significant variations in chemical composition between the <1 and <0.2 micro size fractions. We found no evidence to support an in situ diagenetic origin for these mineralogic changes. Instead, we attribute the variations to shifts in detrital provenances and sediment dispersal pathways from a continental source from South America (Unit III) to a mixed continental-volcanic source (Unit II).

West: -59.4500 East: -58.4207 North: 15.3224 South: 13.0000

West: NaN East: NaN North: NaN South: NaN

Expedition: 110

Expedition: 156

Provider: SEDIS Publication Catalogue

Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=1997-066334 (c.f. for more detailed metadata)

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