2022-05-10_GENERAL DOCUMENTS - M1993041

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Dowe Flats Quarry Soils and Geology Report 3.1 Regional Geologic Setting 3.L I Regional Stratigraphy A maximum of approximately 12,700 feet of sediments are found in the vicinity of Dowe Flats, but only those formations beneath the lower Pierre Shale were present at Dowe Flats. In general, the geologic section is composed of alternating sandstone and shale layers with some limestones. Beginning at the bottom of the section and working upward in both depth and time, the individual units display a variety of depositional origins. Mineral utilization near Dowe Flats has included production of cement from the Fort Hays limestone in a pit south of Dowe Flats and quarrying of Lyons sandstone west of the site for building blocks and silica (from sandstone) used by CEMEX, Inc. in the cement making process. Existing and former gravel pits are present along St. Wain Creek just south of Dowe Flats from Lyons to Longmont. Farther to the east are several oil fields that produce from sandstones and limestones at depth. Coal mining occurred in the early part of the century from the Laramie Formation but is presently inactive in the area. 3.1.2 Regional Geologic Structure The Dowe Flats area is in the Foothills Belt, a 5- to 10-mile-wide transition zone between the Front Range physiographic province to the west and the Denver Basin physiographic province to the east. The sedimentary beds adjacent to the Precambrian mountain front are steeply dipping and occasionally overturned. The dip progressively decreases in the younger formations as they outcrop at greater distances from the mountain front. Between Lyons and the northern Colorado border a series of northwest-trending high-angle bedrock faults offset the sedimentary beds. Draping of sediments over these faults produces a series of en-echelon folds and faults. Taken together, these intermediate-scale structural deformations, regional deformations produced by the Front Range and the Denver Basin, and more localized small-scale folding and faulting, result in a generally complex geologic setting throughout the Foothills Belt. 3.2 Dowe Flats Site Geology 3.2.1 Local Stratigraphy The important formations at Dowe Flats are all Upper Cretaceous in age and are limited to the middle of the regional stratigraphic column. They include, from oldest to youngest, the Dakota Group, the Benton Formation, the Niobrara Formation, and the Pierre Shale. The general stratigraphic descriptions in this section are summarized from three graduate theses that contained detailed lithologic descriptions compiled during field mapping and section measurement activities (Quam 1932, Hunter 1947, and Masters 1957). These documents provide the best available specific lithologic information on Dowe Flats. The most important stratigraphic units in Dowe Flats are, from oldest to youngest: the Dakota Group, containing the Lytle and South Platte formations; the Benton Formation; the Niobrara Formation, containing the Fort Hays and Smoky Hill members; and the Pierre Shale. The uppermost unit at Dowe Flats is the Niobrara Formation, everything above it has been eroded away over geologic time. 4 May 2, 2022