The analysis of dimensional grain orientation in sediments has traditionally been carried out either to determine the paleocurrent direction or to detect the mechanism of penecontemporaneous deformation. In this report, grain orientation in cross-sets and grain fabric in penecontemporaneous structures are summarized from the past studies.
Three types of grain orientation are recognized from cross-strata. 1) The planar and steep cross-bedding of micro-delta origin that have angular contact with the basin floor display a current-normal preferred orientation. 2) The cross-lamina characterized by concave of gentle inclination, together with a tangential basal contact, shows, parallelism between flow direction and the preferred grain orientation. Such parallelism of grain fabric is ascribed to the intergranular encounters while the grains are in suspension. 3) Grain fabric in the planar cross-set that is formed by lateral accretion shows an obliquely downslope preferred orientation. The skewed mode fabric is produced by the resultant force of gravity pull that orients the grains to the dip direction and the lateral traction current of the channel flow which tends to imbricate the grains to the upstream dip.
Two types of soft sedimentary deformations, fluidized and liquefied deformations, are discussed in this report. Fluidized deformation is produced by turbulent flow and all primary structures are destroyed within a sediment. Liquefied deformation is caused by laminar flow and continuity of primary structures is likely to be preserved. Because grain separations are commonly involved in turbulent flow, there may be no significant grain alignment in fluidized sedimentary structures. In contrast, sand-sized grain separations are generally small during liquefaction. Hence a significant preferred orientation is expected in these structures. However, silt- or clay-sized sediments are freely rotated between sand-sized grains even in the liquefied condition. Thus, fine-grained materials may not form preferred grain orientation by liquefaction.