Detailed Fault Zone Architecture Mapped from Inside a Tunnel Near Salina, Utah

Anne H. Covault¹, Russell K. Davies², and Janok Bhattacharya^3
1 University of Texas at Dallas and Rock Deformation Research USA, Inc, Richardson, TX
² Rock Deformation Research, USA Inc, McKinney, TX
³ University of Houston, Houston, TX

A well-exposed fault zone (FZ) in a tunnel on the western flank of the San Rafael Swell, developed in Late Cretaceous Blackhawk and Castlegate fluvial sediments, provides an exceptional opportunity to evaluate the evolution of fault zone architectures. The fault has a throw of ~8m and a ~6m wide damage zone of numerous small throw faults. Most sands and some shales are initially incorporated into the FZ by block faulting into discrete lenses of meter and centimeter scale separated by discrete fault planes. The deformation evolution can be related to the clay content of the lithologies being incorporated into the fault. A sand bed with a clay content of < 2%, for example, shows some smearing but is extensively faulted. Shales exhibit the most smearing. The most continuous smear has a clay content of 21%, and forms part of a £2cm thick clay smear visible along the 9m length of the main slip plane. Source bed thicknesses variations into the FZ arise from a combination of local faulting and ductile flow. For example, over a distance of < 2m, the thickness of one sand bed decreases dramatically from ~1m to 2.5cm next to the main fault slip plane. Deformation bands are also pervasive in the sandstones in the FZ. These observations are consistent with an initial corrugated fault plane that is "smoothed" during offset, preserving fault lenses and smearing the section along the resulting fault plane, which have important implications for similar structures in the subsurface for reservoir compartmentalization.