EFFECTS OF STRUCTURAL AND DEPOSITIONAL HETEROGENEITIES ON FLUID FLOW IN AN EXHUMED SANDSTONE AQUIFER: THE AZTEC SANDSTONE AT THE VALLEY OF FIRE, NEVADA

Paleo-fluid flow conditions are reconstructed for an exhumed faulted and fractured sandstone aquifer, the Jurassic Aztec Sandstone at Valley of Fire, Nevada. This reconstruction is based on detailed mapping of multicolored alteration patterns that resulted from syndepositional reddening of the eolian sandstone and repeated episodes of dissolution, mobilization, and re-precipitation of iron oxide and hydroxides. A first stage of bleaching and local redeposition of hematite is attributed to upward migration of reducing basinal brine during and subsequent to Late Cretaceous Sevier thrusting and foreland deposition of clastic sediments. A second stage of bleaching and iron remobilization, precipitating predominantly goethite and minor iron sulfates, occurred during and after Miocene strike-slip faulting associated with Basin and Range tectonics. This second stage is explained by mixing of reducing sulfide-rich basinal brine with meteoric water entering the aquifer.

The distribution of diagenetic alteration patterns indicates that regional fluid migration pathways were controlled by stratigraphic contacts and by thrust faults. Outcrop-scale focusing of flow was controlled by structural heterogeneities such as joints, joint-based faults, and deformation bands as well as the sedimentary architecture. Slight shearing increases the efficiency of flow along joint zones resulting in haloes of enhanced wallrock alteration. Diagenetic alteration zones are deflected adjacent to joint-based faults with large offset, consistent with their dual role as aquitards for cross-fault flow and preferred pathways for fault-parallel flow. Fluid flow is similarly impeded across deformation bands resulting in alteration gradients adjacent to these structures. The complex interaction of structural heterogeneities with alteration is consistent with their measured hydraulic properties demonstrating the significance of structural heterogeneities for focused fluid flow in a porous sandstone aquifer.