2023-02-17_HYDROLOGY - M2007044 - Laserfiche WebLink

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Whirlwind Mine Groundwater Characterization Report structures associated with upwelling of Pennsylvanian age salt deposits to the south of the study area. The Whirlwind Mine lies in the Canyon Lands Section of the Plateau. Basement rocks of the Plateau consist of mostly Proterozoic metamorphics and igneous intrusions, the closest exposures are seen in Unaweep Canyon approximately eight miles to the northeast of the study area. The La Sal Mountains, 12 miles to the west, are composed of laccolithic intrusives. The Plateau is recognized as an area of relative stability through the Paleozoic and Mesozoic Eras with some minor uplifts and gentle folding resulting in the fairly flat-lying sedimentary rocks observed today. About 330 million years ago, during the Carboniferous Period, the Uncompahgre Uplift occurred, forming the Uncompahgre Plateau to the east. A long period of erosion followed, resulting in nearly flat plain of Proterozoic and some Triassic-age rocks. This "Great Unconformity" surface forms the base for the long period of sedimentation that followed during the late Paleozoic Era (Permian-age) and Mesozoic Era. Locally, the geologic setting at the Whirlwind Mine involves the overlying strata of the Burro Canyon Formation, the Brushy Basin Member of the Morrison Formation, where the mine portal is collared, and the underlying Salt Wash Member of the Morrison Formation that hosts the ore zone. The Burro Canyon forms abrupt cliffs or large ledges a few hundred feet to the west of the portal. The eroded top of the Burro Canyon forms a relatively flat area called Beaver Mesa which extends several miles west across the state line into Utah, where another escarpment defines the mesa's western edge. The Brushy Basin forms a gentle to modest slope below the Burro Canyon that is characterized by rock fall rubble and slumps. This area forms a gentle bench in the topography and has been mapped as Quaternary slump and landslide material by Doelling (2001). This unconsolidated material varies in thickness, but is only a few feet to ten feet thick in the area of the mine operations. Site observations show that this area is characterized by hummocky and slump terrain that show well vegetated slope failures and small flat slump block features. A few hundred feet to the east and northeast, the underlying Salt Wash Member forms a more abrupt slope, and exhibits scattered sandstone ledges interspaced by mudstones and shales, where exposed. At the Packrat Mine, thick ledges of Salt Wash sandstone form the collar of the main portal, and a similar sandstone ledge is present at the emanation point of PR Spring, located approximately 20 ft down section from the Packrat Mine. Figure 3 is a geologic map of the Whirlwind Mine and vicinity. Figure 4 illustrates a stratigraphic column of the geologic formations in the area. Western Water& Land, Inc. 6