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Clay mineralogy and diagenesis of sediments from deformation zones in the Barbados accretionary wedge (Deep Sea Drilling Project Leg 78A)

Jane Schoonmaker
Jane Schoonmaker
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January 01, 1986

Drilling during DSDP Leg 78A in the Barbados accretionary complex provided an opportunity to study the clay mineralogy and chemistry of sediments in relation to development of structural fabrics in the sediments and location of the samples relative to major structural features in the wedge. The clay mineral suite, comprised of smectite, illite, kaolinite, and chlorite, was generally similar in all samples analyzed from both accretionary wedge sites (541 and 542) and from the Atlantic reference site (543). Differences in the mineralogy of the smectite component were noted, however, for several samples from the accretionary wedge Site 541. In most of the sediments analyzed, the smectite is a trioctahedral, mixed-layer phase. Sediments near a reverse fault and from the basal decollement zone from Site 541, however, contain a trioctahedral smectite with no mixed-layering, probably an Fe-rich saponite. The sediments containing saponite all have well-developed structural fabrics including scaly foliation, fracturing, and strata! disruption. This association, however, is not ubiquitous, and thus it is concluded that the occurrence of saponite is not directly related to development of structural fabrics. It is likely, rather, that the reverse fault and the basal décollement zone, by acting as conduits for upward migration of fluids released at depth in the wedge, provided unique chemical environments in which formation of saponite was favored. Evaluation of the chemical data indicates that the saponite probably formed by alteration of pre-existing mixed-layer smectite.

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Figures & Tables


GSA Memoirs

Structural Fabrics in Deep Sea Drilling Project Cores From Forearcs

J. Casey Moore
J. Casey Moore
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Geological Society of America
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Publication date:
January 01, 1986




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