Abstract

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 reprecipitation of iron oxide and hydroxide. A first stage of bleaching and local redeposition of hematite is attributed to upward migration of reducing basinal fluid 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 Miocene strike-slip faulting associated with Basin and Range tectonics. This second stage is explained by mixing of reducing sulfide-rich basinal fluid with meteoric water entering the aquifer. The distribution of alteration patterns indicates that regional-scale fluid migration pathways were controlled by stratigraphic contacts and by thrust faults, whereas the 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. 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.

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