Laserfiche WebLink
<br />The stratigraphy and geologic structure .vi thin the Twentymile Park Synclinal <br />3a sin control the ground water flow within the permit and adjacent areas. <br />There are three regional bedrock aquifers within the permit and adjacent <br />areas. These aquifers are the Trout Creek sandstone, the Twentymile <br />sandstone, and the Wadge coal-overburden zone. All three aquifers exhibit <br />artesian heads in the center of the Twentymile Park Basin. The water quality <br />in all aquifers is moderately poor and is tolerable for use as a drinking <br />supply. <br />The folding and faulting (geologic structure) within the Twentymile Park <br />Synclinal Basin affects the direction of flow within the rock aquifers. In <br />the permit and adjacent areas, the ground water flow in the bedrock aquifers <br />is down the dip of the strata into the Twentymile Park Basin. Faults in the <br />area produce localized fracture zones within the rock strata. These zones of <br />increased hydraulic conductivity may increase the potential for vertical <br />movement of ground water within the rock strata. <br />The Twentymile sandstone is separated from the Wadge coal-overburden aquifer <br />by up to 700 feet of very low permeability shale within the permit and <br />adjacent areas. The underlying 700-foot shale and the overlying shales and <br />siltstones of the upper 'rilliams Fork Formation and the Lewis Shale Formations <br />confine the Twentymile sandstone aquifer, allowing artesian conditions to <br />develop. To the northeast, the Twentymile sandstone recharges the alluvial <br />aquifer and provides base flow to '':',e tributaries of Trout Creek down gradient <br />of the proposed permit area. Salts in the Twentymile sandstone waters are <br />predominantly sodium bicarbonate and result in TDS levels of 150 mg/1. This <br />aquifer is occasionally used for domestic needs. <br />The alluvia of Trout Creek and its tributaries are important sources of ground <br />water. The alluvial bodies are used as water supplies, provide sub-irrigation <br />to vegetation, and supply base flow to the streams. The alluvial bodies are <br />recharged by both the streams and the bedrock aquifers. <br />The climate of the permit area is classified as semi-arid, with significant <br />elevationally related variations in temperature and precipitation, Average <br />annual precipitation within the permit area would be somewhere between the <br />15.45 inch average for Hayden and the 24 inch average for Steamboat Springs, <br />with precipitation increasing with elevation. <br />Lithologic units in the permit area are upper Cretaceous in age and generally <br />lie at the bottom of the Williams Fork Formation. Rocks in the proposed mine <br />area lie stratigraphically between the younger Twentymile and older Trout <br />Creek Sandstones. The Wadge coal seams, part of the i4iddle Coal Group, will <br />be mined in the proposed area. The Lennox coal seam has been removed from <br />parts of the mine area by erosion, <br />Rock units and thicknesses from oldest to youngest are: <br />1. Trout Creek Sandstone (50-100') - fine grained, massive sandstone; <br />2. 'golf Creek Underburden (30') - predominately fine-grained sandstone and <br />siltstone with minor interbedded shales; <br />6 <br />