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2.04.6 <br />2.04.6 Geology Description. <br />(1) No response required. <br />(2)(a) Not applicable. <br />(3)(a) The North Fork Valley of the Gunnison River is located near the western margin of <br />the Colorado portion of the central Rocky Mountain system. The permit and <br />adjacent areas are situated on the southern flank of the Piceance Creek structural <br />and sedimentary basin. The area is bounded by Larimide structural and <br />physiographic features on the following sides: West Elk and Elk Mountains on the <br />east; Gunnison Uplift on the south; Uncompahgre Uplift on the west - southwest; and <br />Grand Mesa - Piceance Basin on the north (Collins, 1976). <br />Generally, Paleozoic strata comprise the Elk Mountains on the East; <br />Mesozoic and Cenozoic strata area exposed within the Uncompahgre Uplift, <br />pre- Cambrian within the Gunnison Uplift, and Mesozoic Strata comprise the <br />immediate North Fork Valley. The area is further modified by Tertiary <br />intrusive porphyritic rocks, which are predominantly laccolithic in structure <br />(West Elk Mountains, Hail, 1972) and Miocene basalt flows capping Grand <br />Mesa (Junge, 1978). <br />Stratiaraphv; The stratigraphy of Cretaceous coals has been addressed by <br />numerous authors since the early 1900's. During the close of the Jurassic <br />System, the present Rocky Mountain area was inundated by the Cretaceous <br />Ephric Seaway, which extended from Canada to Mexico (Weimer, 1976). <br />The basal transgressive rock unit deposited at this interval was the Dakota <br />sandstone (lower Cretaceous). As the basin developed, 2,000 - 5,000 ft. (or <br />greater) of marine sediments were deposited and are preserved as the <br />Mancos shale. <br />The Mesaverde Formation of Holmes, (1877) has been extended throughout <br />western Colorado, eastern Utah, and southwestern Wyoming, and represents <br />a dominantly regressive sequence which formed in response to larger detrital <br />input from the Cordilleran region to the west, than contemporaneous basin <br />- subsidence -(- Weimer, - 1976). - -T -he Rollins Sandstone - Member (Lee, 1912) of <br />the Mesaverde Formation represents the basal regression of the Cretaceous <br />Seaway, although Collins (1976) identified seven cycles of marine - nonmarine <br />PR -12 2.04-14- 02/10 <br />APQRov6D <br />l! /16 /ja <br />