Three-dimensional geometry and fabric of shear deformation-bands in unconsolidated Pleistocene sediments

Abstract

We present an outcrop-based study on the 3D geometry and fabric of shear deformation-bands that developed in Middle Pleistocene, unconsolidated, glaciolacustrine delta sands in northern Germany. With a three dimensional model of a 3.38m³ outcrop we demonstrate deformation band geometries, analysis of band thickness and along-strike juxtaposition maps of beds against the deformation bands. The parallel thicknesses of the shear deformation-bands ranges from 0 to 8cm, with a mean of 1.5cm. Analysis of along-strike band displacement of intersecting shear deformation-bands proves the true displacement vector is close to dip-slip movement. The shear deformation-bands compensated a large lateral extension of ca. 10–20%. Though the deformation is unevenly distributed, this is a very high amount of deformation and this has wide implications when upscaled to the whole outcrop or locality. Grain size within the shear deformation-bands is identical to the grain-size distribution of the parent sediment. Conventional Fry analysis of grains in the shear deformation-bands shows strain ratios of 1.4. This, however, is an artifact of bulk grain-shape, since a normalized Fry plot shows strain close to unity. We observe a two-stage calcite overgrowth on the grains of the deformation band (radial sparite followed by micrite). We thus propose grain-boundary sliding (possibly aided by calcite-rich fluids) to be the dominant deformation mechanism. The deformation bands are very likely the product of young basement tectonics and may be related to movements in the crest of a salt anticline. In the study area, the observation of young basement tectonics is a novelty.