Sediment slump structures: a review of diagnostic criteria and application to an example from Newfoundland

Abstract

In the study of the history of deformed sedimentary rocks it is important that the cause of the deformation and the timing of deformation relative to metamorphic and tectonic events be assessed. There is a continuous gradation between the deformation of freshly deposited sediments by gravitational forces and the deformation of well-lithified sediments by tectonic forces, so determining the degree to which gravity, tectonism and lithification influenced deformation can be very difficult. The characteristics that may be considered in determining the origin of deformational structures in sedimentary rocks can be divided into five categories: Ductile deformation structures (1) can provide definitive criteria for recognizing post-lithification deformation, but not pre-lithification deformation. This also holds for (2), brittle deformation structures and décollements. Overprinting of sediment reworking or remobilization structures (3) are the best criteria for recognizing pre-lithification structures. Fabrics (4) can be very useful but are as yet poorly understood, and there are misconceptions in the literature about what sort of fabrics are or are not found in unlithified sediments. Spatial relationships (5) may also tell us a great deal, but are most useful when taken in conjunction with other types of evidence. Complex fold patterns in sandstones on Farmer Head, north-central Newfoundland, provide a case study for criteria that may be used to determine the degree of lithification during deformation. Although these folds have been interpreted as slump folds by earlier workers, fabrics and spatial relationships point to a tectonic post-lithification origin for the structures, a conclusion which has significant implications for the interpretation of the regional geology.