Small scale syn-sedimentary faults in the Upper Jurassic ‘Boulder Beds’

Abstract

Synopsis Syn-sedimentary faults are common in the Kimmeridge ‘Boulder Beds’ in Sutherland, NE Scotland. Various styles are recognised: (1) closely spaced normal faults, generated by earthquake shock waves or listric faulting in the basal region of sediment gravity glides; (2) isolated normal faults with soft-sediment dykes along fault planes, possibly due to hydraulic fracturing that relieved high pore fluid pressures; (3) isolated conjugate normal faults with the acute bisectrix oriented perpendicular to bedding, probably formed by sediment overburden pressures; (4) decollement surfaces along which sliding or slumping has occurred, and (5) isolated reverse faults or thrusts; (4) and (5) possibly resulting from shear stresses generated as a result of sedimentation on a relatively steep basin slope. Sediment draping is visible above most of the fault surfaces, indicating that failure occurred in the superficial sediment cover and affected packets of beds up to a few metres thick. Stereograms of poles to the fault planes indicate a NNE–SSW girdle, with a concentration of poles showing fault planes dipping at about 45° to the ESE: the fault plane orientations corroborate earlier interpretations of a southeasterly dipping basin slope.