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LARAMIE FORMATION <br />The thick shale sequences of the Laramie Formation forms the aquitard or hydraulic <br />barrier that separates the Arapahoe aquifer from the underlying Laramie -Fox Hills aquifer. <br />The formation consists of approximately 400 feet of clay shales with minor interbeds of <br />sandstone and siltstone. Coal beds are common within the formation and have been mined <br />commercially along the eastern margin of the basin. Minor amounts of water are present <br />within the upper sandstones, but are rarely developed for water supplies. Water in the <br />sandstone units is of generally poor quality due to the presence of sulfides and biogenic gas. <br />The basal portion of the Laramie Formation contains two relatively thick sandstone <br />units (the A and B sandstones) which are hydraulically isolated from the upper part of the <br />formation. These sandstone units, combined with the underlying Fox Hills Sandstone, <br />compose the hydrogeologic unit of the Laramie -Fox Hills aquifer. <br />LARAMIE-FOX HILLS AQUIFER <br />The lowermost of the four Denver Basin aquifers is the Laramie -Fox Hills aquifer which <br />underlies approximately 6,700 square miles and marks the areal extent of the basin for <br />economic groundwater development. The aquifer is made up of both the marine sandstones <br />of the Fox Hills and the overlying fluvial sandstones in the lower Laramie Formation. The <br />Laramie -Fox Hills aquifer is generally between 250 and 300 feet thick, and includes about 150 <br />to 200 feet of fine-grained and medium -grained sandstone. <br />Wells yields of 350 gallons per minute are common. The aquifer is extensively utilized <br />throughout the basin for commercial development and less so for domestic due to the deeper <br />well depths over most of the basin. Both the Laramie -Fox Hills and Arapahoe aquifers are <br />generally under artesian pressure at the present time. <br />PIERRE SHALE <br />The Pierre Shale underlies the Fox Hills Formation and is composed of up to 4,500 feet <br />of marine shale with minor sandstone/ siltstone lenses. in general, the intermittent <br />sandstone /siltstone intervals in the upper portion of the formation contain sufficient <br />groundwater for domestic uses or for the watering of livestock. This formation defines the <br />lower boundary for development of groundwater for commercial purposes in the Denver <br />Basin. <br />