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ABSTRACT: Sedimentation and Syndepositional Deformation in Modern Convergent Settings

Neil Lundberg

The interpretation of depositional environments in ancient fore-arc deposits is complicated by a general lack of diagnostic sedimentary structures and lithologic variety in mud-dominated slope sequences. Useful sedimentologic characteristics include faunal assemblages, especially of benthic foraminifers and trace fossils; carbonate dissolution patterns; and features indicative of a well-developed oxygen-minimum zone. In addition, sediments that accumulate in fore-arc environments are commonly deformed although still unlithified, and analysis of early formed deformational features provides additional genetic information on slope deposits.

The distribution of structural fabrics in Deep Sea Drilling Project cores from modern fore arcs serves to distinguish between contrasting deformational environments. Slope sediments from upper and middle-slope settings tend to be dominated by structures indicative of layer-parallel extension, including vein structure and spaced foliation, which likely form during gravity induced downslope movement of upper sediment layers. Crenulation folds are observed in laminated mudstones, and apparently represent compressional regimes at downslope margins of transported masses of slope sediment. Slope sediments from lower slope settings are dominated by structures indicative of compression and layer-parallel shear. These structures include kink bands, stratal disruption, cataclastic fabrics, and caly foliation, and are formed by horizontal shortening during offscraping of trench sediments at the toe of the trench slope. Thus, structural observations complement sedimentologic analyses in an integrated approach designed to interpret depositional and deformational environments represented in trench-slope sequences.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990