Abstract

To assess provenance variations during Late Cretaceous–Tertiary uplift of the Colorado Front Range, detailed petrographic analysis was performed on core samples from the Kiowa Cored Well, Elbert County, Colorado. Forty-two samples from five stratigraphic intervals were used in the analysis. Samples range in depth from 91.5 to 2,242 ft (27.9 to 683.4 m).

Results indicate that minor but significant amounts of low-grade, metasedimentary debris occur in the Fox Hills Sandstone and Pierre Shale samples and require a relatively distal, westerly source. Subsequent to a strong volcanic pulse during deposition of the Upper Cretaceous Laramie Formation and lowermost D1 sequence, a gradual unroofing sequence is recorded in the overlying Upper Cretaceous–middle Paleocene D1 synorogenic deposits. The volcanics encountered in the Laramie and lowermost D1 sequence are primarily silicic types.

An influx of chert detritus to sandstones of the lower D1 sequence is interpreted to reflect unroofing of post-Lyons Sandstone, chertrich, Mesozoic sandstone. This was followed by overlapping depositional pulses of feldspathic and then chert- and dolomiterich sandstone. The first of these is interpreted to have coincided with unroofing of the arkosic Fountain Formation and the second with the removal of the underlying lower Paleozoic carbonates and clastics. The shallowest occurrence of components having a definite sedimentary origin is at a depth of 976.2 ft (297.5 m) in the upper D1 sequence. The shallowest sample containing quartz that exhibits significant rounding, indicating derivation from sedimentary sources, occurs at a depth of 1,061 ft (323.4 m).

Abundances of feldspathic components exhibit a dramatic increase in the upper D1 sequence and continue into the lower Eocene D2 sequence sandstones, where they comprise the bulk of the framework minerals. A second major pulse of volcanic debris occurs in the uppermost D1 sequence. The D2 sequence was derived almost exclusively from plutonic materials, with no evidence indicating input from high-grade metamorphics (feldspathic gneiss and amphibolite) that tend to dominate the Front Range foothills north of the Castle Rock area.

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