SEDIMENTOLOGICAL, MINERALOGICAL, AND GEOCHEMICAL CHARACTERIZATION OF MUDROCK IN THE UPPER BURRO CANYON FORMATION (LOWER CRETACEOUS), “RATTLESNAKE RIDGE,” WESTERN COLORADO

The Burro Canyon Formation forms a nearly continuous outcrop (~80 km) along the Gunnison and Colorado rivers between Delta and Loma, CO. The lower two-thirds (15-40 m thick) is a fluvial, sandstone-mudrock complex, whereas the upper third (9-20 m) is a green mudrock interval (GMI) that has minor fine-grained sandstone and carbonate nodules and beds. The GMI was deposited in lacustrine, floodplain, and mudflat settings and is probably equivalent to the Ruby Ranch Member of the Cedar Mountain Formation in eastern Utah. The GMI is disconformably overlain by the Naturita (Dakota) Formation (Upper Cretaceous).

The GMI has two main lithofacies: 1) soft, fissile mudrock (SFM), and 2) brittle, porcellanitic mudrock (BPM). The SFM facies is found across the outcrop belt; however, the BPM is more restricted. The most complete exposure of both facies is between the Escalante and Rattlesnake Canyons along a divide informally called Rattlesnake Ridge (39.764º; -108.242º). Here, the SFM facies is 7.9 m thick and the BPM facies is 3.7 m thick. A transition zone (0.9 m thick) occurs between the two facies. Field work at Rattlesnake Ridge included spectral radiometric analysis (SRA) along a measured section and collection of nine samples.

The outcrop SRA data through the SFM interval (N=26) show slight upward increases in the total-count (TC), equivalent potassium (eK), uranium (eU), and thorium (eTh) values. The TC, eK, and eU values noticeably increase in the SFM-BPM transition zone (N=3), then gradually decrease upward through the BPM (N=12), especially for eK. The eTh values do not show a vertical trend through the BPM. XRD analyses indicate that illite is the dominant clay (49-93%) in the SFM samples (N=6), followed by minor kaolinite and smectite (<30%). The BPM samples (N=3) show an upward decrease in illite (62 to 9%) and a noticeable increase in kaolinite (38 to 91%). Smectite is absent in the BPM. Major-element analyses by EDXRF show relatively uniform concentrations of Mg, Si, Al, and K through the SFM interval (N=6); however, the BPM (N=3) displays a progressive decrease in Mg and K, coupled with a strong increase in Al. The results suggest that the BPM is the result of the alteration of illite to kaolinite. The timing and circumstances for the alteration are uncertain.