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1 The Marine Geoscience Research Group, Department of Earth Sciences, Cardiff University, PO Box 914, Cardiff CF1 3YE, UK
2 Départment des Sciences de la Terre, Université Blaise Pascal, 5 Rue Kessler, 63038 Clermont-Ferrand, France
A variety of grain fabrics are commonly observed in coarse-grained turbidites that form potential reservoir rocks. Such fabrics may impart an anisotropy to permeability; however, their origin is poorly understood. Observations of analogous volcanic and laboratory deposits are used here to infer the origin and character of clast alignments. Experimental gravity currents were produced by releasing suspensions of silt-grade silicon carbide (10, 15 or 200y volume) with minor volumes of larger, lower-density perspex clasts into a tank of water. Fabric was measured in the experimental deposits and in the Quaternary Upper Laacher See Tephra, Germany. The experimental and pyroclastic deposits both had a clast fabric with grain long-axes predominantly transverse, but not necessarily perpendicular, to the mean flow direction. The perspex clasts in the laboratory currents were initially transported in suspension, and marks on the sediment surface indicated that these clasts rolled along the bed immediately prior to final deposition. The perspex clasts were buried at differing levels and at a range of distances from source in the laboratory deposits, having been deposited at different stages in the flow history. In the volcanic deposits the same fabric is persistent vertically throughout the thickness of the bed at each site, suggesting that progressive aggradation provided a surface on which the clasts rolled prior to deposition. It is proposed here that similar fabrics in high-density turbidites are analogous to those observed in these experimental and pyroclastic deposits, and therefore they may be predicted from palaeocurrent interpretations, or conversely used to assess palaeocurrent directions.
