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I I I I I I I I I I I I I I I I I I I VII-l SECTION VI I MINERAL RESOURCES The Colorado Geological Survey (1974) published a comprehensive description of sand, gravel and quarry resources for Colorado Front Range counties. Special studies of sand and gravel deposits in Adams County were made by Turner (1974) covering the area from Henderson to Baseline Road at Brighton. The sand, gravel and aggregate deposits in the South Platte River floodplain are extensive. Gravel operations exist for several miles immediately down- stream from Chatfield Reservoir, in Englewood, Denver and most extensively in Adams County. State legislation and county regulations accommodate and protect the gravel resources for mining while at the same time specifying constraints on the extraction limits, reclamation of sites, water depletion and operational practices. General The South Platte River flowed in a valley cut into the landscape during Pleistocene or recent times. The valley floor is mostly reworked gravel overlain by a few feet of rich sandy silt. The valley floor is from 1/2 to 2 miles wide in the Denver area. The average width is approximately 3/4 mile. Upstream from Sixth Avenue in Denver, there are a few outcrops of Wisconsin Stage gravel fills in the valley floor. One is at the Overland Park Municipal Golf Course. These gravels are Chiefly granitic pebbles mostly less than 1 inch in di ameter, well-bedded and well sorted. The mineral resources have a direct effect upon many aspects of the master planning for the South Platte River corridor. Breached dikes adjacent to the river allow the river to flow into old gravel pit lakes, where sediment is collected. Upstream erosion of the river bed is aLcelerated when the river is deepened, or diverted into a pit. Entrapment of sediment causes downstream erosion. Long term river instability and bed and bank erosion results from unplanned diversion of the river into a gravel pit. Given enough time, unregulated gravel mining in Adams County would result in addi- tional erosion and/or headcutting moving southwards towards Denver. This would be first noticed by bridge piers being undermined and degradation below diversion dams. Other floodplain uses are constrained where commer- cially extractable gravel exists. Gravel pit lakes provide terrestrial and aquatic habitat. Gravel pits can provide recreational opportunities on a region-wide basis. The Denver formation underlies alluvium and granitic gravels. This forma- tion is deeply weathered but has compact conglomerate and siltstone and some clay shale of Upper Cretaceous and Paleocene age. The formation has lenticular bedding. The conglomerate commonly forms channel deposits in finer text ur ed beds. The Denver and underlying Arapahoe Formations consist of nearly flat-lying shale and sandstone. These two formations are similar in lithology and are sometimes not differentiated. All samples of rocks in the Denver Fonnation disintegrate in water after being allowed to dry in air. This section of the report provides background on the geology, type of min- eral deposits, and an evaluation of the extent of the deposits. An inven- tory of gravel operations and ownership is given on Figure VII-I. The alluvial deposits of the South Platte River are from 5 to 30 feet thick with a maximum in excess of 50 feet. The Denver Formation outcrops in the river bed at a number of locations through the study reach. GEOLOGY The geologic setting of the South Platte River study reach from Chatfield Reservoir to Baseline Road at Brighton has generally been described by Stevens (1983). Much of this description was based on work by Hunt (1954). The Broadway Terrace is the most prominent of the terraces in the Metropoli- tan Denver area. It forms the right valley wall of the South Platte River throughout the study reach. It also exists along the left side of the valley floor downstream from the confluence with Clear Creek. The terrace