COMPACTION BANDS - A STRUCTURAL ANALOG FOR ANTI-MODE-I CRACKS IN AEOLIAN SANDSTONE

We present evidence for the existence of tabular zones of localized de formation in aeolian sandstone, that accommodate pure compaction. In t his sense they are analogs for anticracks or closing mode I fractures such as pressure solution surfaces or stylolites. The so called ''comp action bands'' are exposed in outcrops of the Jurassic Navajo Sandston e in the Kaibab monocline, Utah. They are characterized by lack of she ar offset across their plane, volume loss, micro fracturing and very l ittle grain crushing or comminution. Based on their geometry, two kind s 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 kin d is 0.1-0.5 cm thick over lengths up to 2 m, and is conspicuously cro oked with wavelengths of 1-5 cm and amplitudes of a few mm to a few cm . Compaction bands preferentially developed in the compressive quadran t at the tips of small faults or ''deformation band faults'' which sug gests, together with the direction of shear along the deformation band faults, that compaction bands form perpendicular to the largest compr essive stresses induced by motion along the deformation band faults. A lso, the compaction bands typically occur in sedimentary layers with l arge grain sizes (0.3-0.8 mm) and high porosity (20-25%) whereas the d eformation band faults occur in the layers with smaller grain sizes (0 .05-0.25 mm) and lower(< 20%) porosity.