GENERAL30229 - Laserfiche WebLink

Home Browse Search

CHAPTERTHREE Affected Environment ' Verde Group. The stratigraphic sequence of interest to the Yankee Gulch Project is the Tertiary- age rocks of the Pazachute Creek Member of the Green River Formation (Figure 3.1-2). ' Quaternary Unconsolidated Sediments Unconsolidated Quatemary sediments consist of Holocene alluvium and Pleistocene terrace ' deposits. The Quatemary alluvium is found in the river valleys and along dry washes. Pleistocene terrace deposits of glacial origin occur as accumulations of remnant older stream deposits on terraces high above the present-day valley floors. ' Alluvial floodplain deposits are mostly gray, brown, and buff=colored silt, sand, and gravel. Alluvial fan deposits consist of angular sandstone and mazlstone boulders and pebbles mixed with silts and sands derived from nearby hilly terrain. The alluvium, which may reach up to 140 feet thick in the Piceance Creek Basin, is generally less than 50 feet thick at the Piceance Site. Terrace deposits contain smoothly rounded pebbles and boulders of chert, limestone, ' quartzite, azkose, and sandstone derived from Paleozoic and Mesozoic rocks east of the Piceance Creek Basin. The thickness of these deposits varies from 0 to 25 feet (Duncan 1976a and 1976b). ' Uinta Formation ' The Tertiary Uinta Formation outcrops throughout most of the Piceance Creek Basin and is situated below the unconsolidated Quaternary sediments. The Uinta Formation consists of sandstones with interlayered sequences of siltstones and mazly siltstones. Marlstone is more abundant in the lower portion of the formation. In the central part of the Piceance Creek Basin, the Uinta Formation generally exceeds 1,000 feet in thickness; it is 674 feet thick at monitoring well 20-1 (USGS C1), an existing discovery hole drilled at the Piceance Site in 1965. The ' sandstones are massive, usually devoid of visible stratification, and generally fine- to medium- grained. Occasional coarse-grained and pebbly sandstone layers 1- to 2-feet thick aze also present. The interbedded Finer-grained rocks (i.e., siltstone, sandy siltstone, and mazly siltstone) tend to be tabular, and stratification is not very distinct. The lower contact of the Uinta Formation with the Green River Formation is marked by an abrupt transition from gray siltstone to dark brown, moderately rich oil shale. Below this contact, siltstone and other detrital rocks are raze. ' Green River Formation The Tertiary Green River Formation in the Piceance Creek Basin is divided into four members: ' the Pazachute Creek (upper member), Garden Gulch (intermediate member), Douglas Creek (lowest member), and Anvil Points (lateral correlative of the Douglas Creek and Garden Gulch Members, and part of the lower Parachute Creek Member). At the top of the Parachute Creek ' Member, tongues of the Green River Formation are interfingered with the lower part of the Uinta Formation. The Green River Formation rests conformably on top of the Wasatch Formation. Toneues of the Green River Formation Five major tongues of the Green River Formation composed of maz[stone, silty mazlstone, and lean oil shale aze interstratified with the sandstones ' Geology 3-3