2002-03-13_REVISION - M1979089 (2)

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Grand Junetioa Pipe Surficial Geology Investigation, 23 Road Proposed Gravel Pit, Grand Junction, Colorado February 26, 2002 Page SITE GEOLOGY The tract is located within the geologically active flood plain of the Colorado River. This active flood plain is composed of approximately 15 to 25 feet of unconsolidated soils which aze underlain by a very thick sequence of sedimentary rocks. Portions of the Dakota Formation and Basal Mancos Shale aze believed to be the `bedrock' beneath this site but, do not outcrop on the north side of the Colorado River in the Grand Junction area. The Dakota Formation and Mancos Shale is part of a thick sequence of sedimentary rocks which gently dip or slope toward the north northwest. This dip in this general area ranges from approximately 4 to 8 degrees from the horizontal. At the present ground surface, a sandy gravel and gravel and cobble alluvial deposit was encountered at the south end of the tract. In the northern portion, where there aze existing agricultural fields, the surface soils aze wmposed of a silty clayey sand, clayey silty sand and silty clan which maybe 5-10 feet thick. Numerous small Sand, Siit and Clayey deposits are present on the ground surface, as a result of overbank flooding and normal river depositional processes. This coarse grained gravel and cobble deposit is the Present Terrace Deposit of the Colorado River. This gravel and cobble deposit is relieved to be between 15 to 25 feet in thiclmess, based upon auger boring logs in this general area. This is the gravel deposit which is to be extracted and processed. ht general, the upper 3 to 5 feet ofthe gravel and cobble is somewhat sandier and may contain some thin silt, sand and clayey deposits. The maximum cobble size is no~u,a".; ~Z the range of 7 to 9 inches (square screened size) but the lower 4 to 6 feet of the deposit may wntain occasional cobbles up to 12 to l6 inches in diameter. In this particular area of the grand valley the bedrock is discordant and does not show a fine line between the Dakota Fom~ation or the Marcos Shale. Thus, the bedrock is somewhat of a combination of the two formations, however, most prominently by the Dakata Formation. The Dakota Formation is believed to be less than 100 feet thick in this particular area, based upon exposures of the geologic rock section in the river bluff on the south side of the Colorado River. The rock exposures in the river bluffto the south also include the Burro Canyon/Morrison Formations (undivided). The Dakota Formation th the Redlands Area of Grand Junction is generally described as a series of interbedded sdtstone and sandstone, with dazk gray to gray black thin bedded to laminated bedded carbonaceous shale, mudstone and siltstene. The Marcos Shale is described as a thin bedded, drab, light to dark gray marine shale, with thinly interbedded fine grain sandstone and siltstone layers. Some portions of the Marcos Shale are bentonitia, and therefore, are highly expansive. The majority of the shale, however, has only a low to moderate expansion potential. GROUND WATER The ground water elevations aze believed to be within 3 to 8 feet of the ground surface and are largely controlled by the elevation of the Colorado River . nd irrigation practie. > north of this site and north of Grand Junction. This ground water should be considered a permanent feature of the site. Dewatering of the open pit during mitring will be required. A deeper ground water is present in the underlyu:; :Ji:k 2~,.osits. This tower ground water is within several