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

At Arches National Park it is possible to distinguish three kinds of d eformation 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 mi cromechanics of deformation band development consist of initial dilata ncy followed by grain crushing and compaction. This process may be dev eloped to different stages according to the interplay of porosity, con fining 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 co mpaction and cataclasis. Two generations of deformation bands were doc umented. The older generation has little or no cataclasis and formed i n relatively undisturbed sandstone probably under conditions of low co nfining pressure. The younger generation exhibits cataclasis, appears to be localized in proximity to major faults and seems to have develop ed 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.