THREE DIMENSIONAL FAULT ZONE GEOMETRY AND THE PRESENCE OF SEALING CHARACTERISTICS IN INTERBEDDED SAND/SHALE UNITS OF THE MESA VERDE FORMATION, SALINA CANYON TUNNEL FAULT

Detailed mapping in an abandoned railroad tunnel near Salina, Utah provides an exquisite three-dimensional illustration of the architecture produced by normal faults in interbedded clastic sediments and the sealing potential of such faults. Fault zone architecture consists of an inner core zone, which marks the primary displacement surface, an inner band of fault slivers that have been transported and deformed within the fault zone, and an outer damage zone of less intense deformation in both the hanging and footwalls. The style of deformation within the fault zone is both ductile and brittle. The core zone is characterized by alternating bands of clay and sand fault gouge that contain well-developed ductile shear fabrics offset by small normal faults. Slivers of footwall sediments (that include clay gouge source beds) have been transported down the fault and extend into fault-bounded dominoes by synthetic listric normal faults. Hanging wall deformation in the damage zone is characterized by spaced minor faults and calcite filled joints dominated by two sets of secondary shears. These faults have an orientation of 11° and 78° to the fault plane and are consistent with Riedel and secondary Riedel shears. Each of these secondary shears appear as subsidiary normal faults to the main orientation of movement. Pronounced displacement in siltbeds of the hanging wall showed this pattern extending 30-40 cm from the core zone. A thin veneer of clay gouge in the core zone is derived from a shale source bed. The total thickness of the source, along with the amount of displacement, affects both the thickness and amount dragged into the core zone. A 14 cm thick shale bed in the footwall produced a smear extending 119 cm below the top of the source. No smear was observed above the top of the source. In addition, similar lithologies along strike produced smears of similar fashion that also terminates above the source bed. Across the width of the tunnel, it is difficult to correlate zones of clay gouge and fault slivers, suggesting there is significant variability along strike in the sealing capacity of the fault. Correlating these zones is fundamental in determining if normal faults compromise the capacity of the fault zone to seal.