Ground Water Movement through the Highly-Fractured Crystalline Core of a Large-Scale, Foreland Uplift, Royal Gorge Arch, South-Central Colorado
Fifth Advisor: Dr. Christopher Schmidt
Sixth Advisor: Dr. Michael Stoline
Abstract
The objectives of this study were twofold: (1) to examine the well-exposed, basement core of the Royal Gorge arch as a large-scale, field test of ground water flow through a fractured, crystalline, aquifer system; and (2) to devise a method to more accurately determine the direction of maximum aperture of a fractured aquifer through structural and hydrogeologic analysis.
The recognition of a preferred direction of maximum aperture in a fractured aquifer remains an important hydrogeologic consideration because flow through a fracture is proportional to the cube o f the aperture width. Previous studies of hydraulic conductivity directions using fracture orientation techniques have met with varying degrees of success as many of the systems were complicated by multiple deformations, making the maximum stress direction, and corresponding maximum aperture direction, difficult to decipher.
Fracture orientations, fracture spacing, and other indicators of brittle deformation were collected at 565 outcrops within the basement core of Royal Gorge arch, and in the overlying sedimentary units. Over 500 foliation and bedding orientations were analyzed via stereographic projection, dividing the uplift into 10 homogeneous, structural domains. Arrays of slickensides were examined in each domain, resulting in determination of the stress field which caused the last episode of brittle deformation. The orientation of extension fracturing, and correspondingmaximum hydraulic conductivity, was then defined in each domain. Fifty water samples were collected and analyzed for stable isotopic signature, hydrogeochemical parameters, and ground water quality. Water well, spring, stream, isotopic, and hydrogeochemical data were used to define the water table surface within the aquifer and to define flow directions.
Results indicate that fracture orientation, fracture spacing, and the direction of maximum stress depend strongly on pre-existing basement fabric orientation and lithology. Ground water flow lines generally trend S 45-70 W, nearly identical to the direction of maximum stress interpreted for Laramide time. This direction also parallels the elevation gradient of the arch. All isotopic, hydrogeochemical, and potentiometric data are consistant with the interpretation that ground water is preferentially migrating along fractures parallel to the last operative direction of maximum stress.