MICROSTRUCTURE OF DEFORMATION BANDS IN POROUS SANDSTONES AT ARCHES NATIONAL-PARK, UTAH

At Arches National Park it is possible to distinguish three kinds of deformation bands on the basis of their distinctive microstructure: (1) deformation bands with little or no cataclasis: (2) deformation bands with cataclasis, and (3) deformation bands with clay smearing. The micromechanics of deformation band development consist of initial dilatancy followed by grain crushing and compaction. This process may be developed to different stages according to the interplay of porosity, confining pressure, clay content and amount of strain. Low porosities and low confining pressures promote the formation of dilatant bands with no cataclasis. High porosities and high confining pressures promote compaction and cataclasis. Two generations of deformation bands were documented. The older generation has little or no cataclasis and formed in relatively undisturbed sandstone probably under conditions of low confining pressure. The younger generation exhibits cataclasis, appears to be localized in proximity to major faults and seems to have developed under conditions of high confining pressure. The temporal sequence of deformation band development can be related to the regional geology of the area; where the first generation probably formed during growth of the salt anticline, and the second generation during its collapse.