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52.04 APPLICATION FOR PERMIT FOR SIIRFACE OR IINDERGROUND MINING ACTIVITIES -- <br />MINIMUM REQUIREMENTS FOR INFORMATION ON ENVIRONMENTAL RESOURCES. <br /> <br />2.04.6 Geology Description. <br />(1) Not applicable. <br />(2) (a) The North Fork Valley of the Gunnison River is located <br />near the western margin of the Colorado portion of the <br />central Rocky Mountain system. Colorado Westmoreland's <br />Orchard Valley Mine is situated on the southern flank <br />of the Piceance Creek structural and sedimentary basin. <br />The area is bounded by Larimide structural and physio- <br />graphic features on the following sides: West Elk and <br />Elk Mountains on the east; Gunnison Uplift on the <br />south; Uncompahgre Uplift on the west-southwest; and <br />Grand Mesa-Piceance Basin on the north (Collins, <br />1976). <br />• <br />Generally, Paleozoic strata comprise the Elk Mountains <br />on the East; Mesozoic and Cenozoic strata area exposed <br />within the Uncompahgre Uplift, pre-Cambrian within the <br />Gunnison Uplift, and Mesozoic strata comprise the imme- <br />diate North Fork Valley. The area is further modified <br />by Tertiary intrusive porphyritic rocks, which are pre- <br />dominantly lac colithic in structure (West Elk Moun- <br />tains, Hail, 1972), and Miocene basalt flows capping <br />Grand Mesa (Junge, 1978). <br />Stratigraphy: The stratigraphy of Cretaceous coals has <br />been addressed by numerous authors since the early <br />1900's. During the close of the Jurassic System, the <br />present Rocky Mountain area was inundated by the Creta- <br />ceous Ephric Seaway, which ex [ended from Canada to Mex- <br />ico (Weimer, 1976). The basal transgressive rock unit <br />deposited at this interval was the Dakota sandstone <br />(lower Cretaceous). As the basin developed, 2,000 - <br />3,000 ft. (or greater) of marine sediments were depos- <br />ited and are preserved as the Mancos shale. <br />The Mesaverde Formation of Holmes, (1877) has been ex- <br />tended throughout western Colorado, eastern Utah, and <br />southwestern Wyoming, and represents a dominantly re- <br />gressive sequence which formed in response to larger <br />detrital input from the Co rdilleran region to the west, <br />than contemperaneous basin subsidence (Weimer, 1976). <br />e Rollins Sandstone Member (Lee, 1912) of the Mesa- <br />verde Formation represents the basal regression of the <br />Cretaceous Seaway, although Collins (1976) identified <br />. Revised 08(01(84 <br />Corrected 10/12/86 <br />Revised 10/01/87 <br />15 <br />