Compaction bands: a structural analog for anti-mode I cracks in aeolian sandstone

Abstract

We present evidence for the existence of tabular zones of localized deformation in aeolian sandstone, that accommodate pure compaction. In this sense they are analogs for anticracks or closing mode I fractures such as pressure solution surfaces or stylolites. The so called “compaction bands” are exposed in outcrops of the Jurassic Navajo Sandstone in the Kaibab monocline, Utah. They are characterized by lack of shear offset across their plane, volume loss, micro fracturing and very little grain crushing or comminution. Based on their geometry, two kinds of compaction bands are distinguished: the first kind is 0.5-1.5 cm thick and fairly straight over lengths of about 5–10 m. The second kind is 0.1-0.5 cm thick over lengths up to 2 m, and is conspicuously crooked with wavelengths of 1–5 cm and amplitudes of a few mm to a few cm. Compaction bands preferentially developed in the compressive quadrant at the tips of small faults or “deformation band faults” which suggests, together with the direction of shear along the deformation band faults, that compaction bands form perpendicular to the largest compressive stresses induced by motion along the deformation band faults. Also, the compaction bands typically occur in sedimentary layers with large grain sizes (0.3-0.8 mm) and high porosity (20–25%) whereas the deformation band faults occur in the layers with smaller grain sizes (0.05-0.25 mm) and lower (< 20%) porosity.