THE NATURE OF CATACLASTIC DEFORMATION IN FAULTED POORLY LITHIFIED SEDIMENTS AND ITS STRUCTURAL AND HYDROLOGIC IMPLICATIONS

The Sand Hill fault zone (SHfz) is a large-displacement, high-angle, normal fault that cuts weak, poorly lithified sediments of the Albuquerque basin, New Mexico. Unlike faults in crystalline and lithified sedimentary rocks, the fault zone does not contain open fractures; the basic structural element is the deformation band. The fault core is a foliated clay veneer flanked by structurally and lithologically heterogeneous mixed zones, in turn flanked by little-deformed damage zones. These structures have a different impact on fluid flow than fractures. Permeability data from the SHfz and equivalent permeability calculations indicate that large-displacement faults in poorly lithified sediments may significantly impede horizontal groundwater flow and do not appreciably enhance vertical flow.

The internal structure and hydrologic properties of these faults are due in part to the deformation mechanisms operative during faulting. Specifically, much of the deformation in the SHfz is accommodated by particulate flow and localized cataclasis. The mode of grain fracture in the SHfz is controlled by relative grain strength. Transgranular fracturing of quartz is rarely observed - quartz typically deforms by flaking of grain edges, feldspar by transgranular fracture facilitated by easy cleavage, and lithic fragments by transgranular fracture or distributed microcracking. Particle size measurements indicate that progressive deformation produces a power-law, or fractal, particle size distribution characterized by high D values (3.49-3.74). This indicates a preponderance of fines over the constrained comminution model of cataclasis (D ~ 2.6) in crystalline and lithified sedimentary rocks. We interpret these results in terms of cataclastic deformation by controlled particulate flow under low confining pressure in a shear zone with non-rigid boundaries, in which extensive transgranular fracturing is not necessary for strain accumulation. Thus, the deformation mechanisms operative in poorly lithified sediment under low confining pressure result in a fault zone with a characteristic internal structure and hydrologic properties. We expect these results to be generally applicable to faults at shallow depths in poorly lithified sediments.