SOFT-SEDIMENT SLICKENSIDES IN THE STOCKTON FORMATION, STOCKTON, NEW JERSEY

A well-defined pattern of slickensides is observed in the Stockton Formation along Wickecheoke Creek in Stockton, New Jersey. Three lithologies are exposed along the stream bed: massive, well sorted gray arkose; coarse, micaceous red siltstone; and blocky, massive red mudstone. Three dominant joint sets are observed at the locality: N30°E, N60°W, and N80-90°W, however the mudstone units are relatively unjointed. Joints in layers above and below the mudstones generally end at the mudstone contact. The slickenside surfaces are confined to the clay-rich mudstone beds.

The slickensides are defined by irregular, shallow-dipping, concave-upward to planar polished surfaces that exhibit unsystematic orientations. Slickenlines on the surfaces indicate dip-slip motion. X-Ray Diffraction (XRD) analyses performed on the clay fraction of the mudstone beds that host the slickensides indicate that illite is the only clay mineral present. We interpret these surfaces as soft sediment deformation features. These features are preserved as discrete discontinuities within the beds, and appear to predate the formation of fractures and joint sets.

Gray and Nickelsen (1989) describe the formation of similar features in the Appalachian basin as the result of expansion in expansive clay soils and propose the term “pedogenic slickensides” to identify their origin. Guiraud and Séguret (1987) and Petit and Laville (1987) propose the term “hydroplastic slickensides” for comparable features that are generated by slumping and compaction of sediments in Spain and Morocco respectively.

Based on the concave morphology of the slickensides, the absence of swelling clays, and a dip-slip motion sense derived from slickenlines, we interpret a normal dip-slip motion along these surfaces. Irregularities consistent with normal motion are also present on the surfaces. This interpretation is consistent with hydroplastic features that are generated during slumping and compaction of sediment.