FORMATION AND RHEOLOGICAL IMPLICATIONS OF CLAY-BEARING FAULT ROCKS OF THE MOAB FAULT SYSTEM, UTAH

Clay-bearing fault rocks are common along the Moab fault and therefore played a role in the rheological and hydrologic behavior of that fault system. There are three sources of clay-bearing fault rocks: 1) folding of clay-rich horizons into the fault zone, called shale smear; 2) mechanical incorporation of clay-rich protolith; 3) clay authigenesis. Previous studies of fault seal potential have focused only on incorporation of detrital clays. Here we focus on the role of clay authigenesis. Clay authigenesis is promoted by three mechanisms: 1) fluid flux and chemistry; 2) heating, either frictional or geothermal; 3) strain energy associated with fault slip. Authigenesis in clay gouge along the Moab fault is characterized by the formation of illite. Authigenesis in shale smear and altered protolith (regardless of original clay mineralogy) is characterized by mixed-layer illite-smectite (I-S), although detrital clays are dominant. This variation in clay mineralization with respect to location in the fault zone indicates that strain energy, and possibly frictional heating, plays a significant role in clay authigenesis. Alternatively, this variation in clay mineralogy could result from fluid flow confined to the ~1 m thick gouge layer. In either case, or by some combination of these processes, the evolution of the shale smear and clay gouge is clearly different. Direct dating of authigenic clays indicates that both types of clay mineralization were coeval with hydrocarbon migration at ~60 Ma. Bleaching of the clay gouge, located between sandstone in the hanging wall and smeared shale in the footwall, suggests reduction of the clay gouge due to contact with hydrocarbons. The spatial limitation of the reduction to clay gouge, and not the shale smear, indicates a difference in the sealing potential of those fault rocks. The spatial variability in clay mineralization may also indicate variation in deformation mechanisms within the fault zone. In addition changes in clay authigenesis due to changes in ambient conditions might cause a related evolution in the frictional and hydrologic properties of the fault system. These results demonstrate that deformation mechanisms and clay authigenesis must be considered when evaluating fault seal behavior.