2010-02-24_REVISION - C1996083 (2)

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2.04.6 2.04.6 Geology Description. (1) No response required. (2)(a) Not applicable. (3)(a) The North Fork Valley of the Gunnison River is located near the western margin of the Colorado portion of the central Rocky Mountain system. The permit and adjacent areas are situated on the southern flank of the Piceance Creek structural and sedimentary basin. The area is bounded by Larimide structural and physiographic features on the following sides: West Elk and Elk Mountains on the east; Gunnison Uplift on the south; Uncompahgre Uplift on the west - southwest; and Grand Mesa - Piceance Basin on the north (Collins, 1976). Generally, Paleozoic strata comprise the Elk Mountains on the East; Mesozoic and Cenozoic strata area exposed within the Uncompahgre Uplift, pre- Cambrian within the Gunnison Uplift, and Mesozoic Strata comprise the immediate North Fork Valley. The area is further modified by Tertiary intrusive porphyritic rocks, which are predominantly laccolithic in structure (West Elk Mountains, Hail, 1972) and Miocene basalt flows capping Grand Mesa (Junge, 1978). Stratigraphy; The stratigraphy of Cretaceous coals has been addressed by numerous authors since the early 1900's. During the close of the Jurassic System, the present Rocky Mountain area was inundated by the Cretaceous Ephric Seaway, which extended from Canada to Mexico (Weimer, 1976). The basal transgressive rock unit deposited at this interval was the Dakota sandstone (lower Cretaceous). As the basin developed, 2,000 - 5,000 ft. (or greater) of marine sediments were deposited and are preserved as the Mancos shale. The Mesaverde Formation of Holmes, (1877) has been extended throughout western Colorado, eastern Utah, and southwestern Wyoming, and represents a dominantly regressive sequence which formed in response to larger detrital input from the Cordilleran region to the west, than contemporaneous basin subsidence (Weimer, 1976). The Rollins Sandstone Member (Lee, 1912) of the Mesaverde Formation represents the basal regression of the Cretaceous Seaway, although Collins (1976) identified PR-12 2.04-14- 02/10