2012-10-01_REVISION - M1977310 (10)

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JD -6 Mine Environmental Protection Plan 30 9. GROUNDWATER DATA 9.1 Geologic Setting 9.1.1 Regional Geology of the Paradox Basin The Paradox Basin' is located on the western margin of the Colorado Plateau Physiographic Province. Regional stratigraphy includes marine and terrestrial sedimentary rocks deposited in response to multiple stages of seaway transgression- regression and orogenic uplift during Late Paleozoic to Quaternary Periods (Table 19). Regionally, these sedimentary rocks include shale, salt, gypsum, claystone, siltstone, and thick sequences of sandstone which, for the most part, are flat lying except for areas deformed by local faulting and folding caused by the uplift of the Colorado Plateau. Many folds and faults occur in the area, with the most prominent being the Uncompahgre Plateau with a fold axis estimated at one hundred miles long in the northeast section of the region (Kleinfelder, 2009). Well- developed anticlines with intrusive gypsum and salt cores underlie Paradox Valley, Sinbad Valley, Gypsum Valley and other valleys (Cater, 1954). The present northwest- trending valleys are formed by the collapse of the anticline cores due to rapid dissolution of salt during the Tertiary uplift of the Colorado Plateau (Strauss, 1982). As a result of this collapse, these "salt anticline valleys" also developed systematic normal faults parallel to the axis of the anticline (Cater 1954 and 1955). These faults are downdropped towards the center of the anticlines where there are still remnants of the salt core. In the Paradox Valley, older Paleozoic rocks crop out on the valley floor and younger Triassic and Jurassic age rocks are exposed in the valley walls (Figure 6, Figure 7). The order of the geologic units correlates to the order of geologic succession of the Colorado Plateau with the exception of missing members (including the Jurassic Navajo Sandstone). The unconformable contacts with missing Mesozoic units observed elsewhere in the Colorado Plateau may be evidence for regional uplift. The missing units could result from either lack of deposition or erosion due to uplift. The uppermost geologic units are Holocene eolian (wind- blown) and sheet flood deposits, which are widely distributed on the valley floors, along the benches, and on top of the mesas (Cater, 1954). Although these younger units may be hosts for near - surface hydrologic activity, the alluvium has been shown to be dry at the Pinon Ridge site (Section 9.2) and the nearby JD -8 site (Section 9.2). 4 The "Paradox Basin" is defined as the part of the Colorado Plateau physiographic province that is underlain by Pennsylvanian evaporites (i.e., the Paradox Member of the Hermosa Formation). The "Paradox Valley" is a local structural /geomorphological feature (i.e., an eroded anticline) within the Paradox Basin. 4148A.120927 Whetstone Associates