Determination of slip sense along a fault in an unlithified sediment

It is usually difficult to measure slip sense along a fault in unlithified sediments due to the lack of mesoscopic indicators such as slickenlines. In this paper, scanning electron microscope (SEM) images of fault surfaces in mudstone at the sandstone–mudstone contacts are used for measuring the microscopic slickenlines formed by the scratching of sand grains. The lineations are subparallel to the presumed slip direction deduced from a conjugate fault set. Therefore, even in the case of no mesoscopic slickenlines, a microscopic scratching by a sand grain on the fault surface where sandstone and mudstone are juxtaposed by the fault is useful as an indicator of slip sense along a fault in sediments lithified subsequent to faulting.