JOURNAL OF CAVE AND KARST STUDIES / ARTICLES / VOLUME 85

 

Rock Pigeon Cave, Colorado: Development and Mineralogy

Douglas M. Medville

Publication Date: 2023/12/01

Publication Volume: Journal of Cave and Karst Science 85 #

DOI Link: https://doi.org/10.4311/2023ES0104

Article Link: https://caves.org/wp-content/uploads/Publications/JCKS/v85/85_3-4_102.pdf

Publication Keywords: Mancos Shale, Vadose Caves, Pyrite Oxidation, Gypsum Formation, Sulfate Minerals

Article Download

ABSTRACT:

The late Cretaceous Mancos Shale in western Colorado contains several vadose caves developed entirely within the shale. The largest of these, Rock Pigeon Cave, is over 300 m in length and contains a seasonal stream. The cave, and others like it, is hypothesized to develop as a result of the oxidation of disseminated pyrite in the shale by descending meteoric water, with resulting sulfate ions reacting with the carbonate component of the shale to produce gypsum. The gypsum pries apart the shale, increasing its secondary porosity and allows flowing water to remove shale particles via corrasion. As these particles are removed, they are transported down-gradient to an outlet, allowing continuous openings (cave passages) to develop. Extensive sulfate mineralization is observed within the cave and takes several forms: soft globular deposits on rocks at water level, a white to tan crust on shale surfaces, dry crusts on the passage floor following evaporation of pools, and needle and hair-like extrusions on passage walls. Powder X-ray diffraction (XRD) indicates that these deposits are a mixture of thenardite and blodite, with lesser amounts of gypsum, hexahydrite, and konyaite. Na+, Mg2+, and SO4^2- ions in the entering stream are the source of the sulfate minerals at stream level and on the wetted slopes above. These come out of solution as the cave stream and pools evaporate, with sulfates precipitating as saturation is reached. Fibrous, needle-like sulfates on walls above the wetted zone are a result of crystallization by evaporation: fluids containing sulfate ions are extruded and evaporate at the rock/air interface. Other minerals, e.g., deposits containing goethite and jarosite, are seen on passage walls as reaction products from oxidation of pyrite in the shale.

SIMPLE LANGUAGE SUMMARY:

The Mancos Shale in Colorado has caves that were not created by water erosion, which is how most caves are formed, but through a chemical process. These caves, like Rock Pigeon Cave, formed because of a reaction between rainwater and pyrite, a mineral in the shale. This reaction creates gypsum, a different mineral that breaks the shale apart and makes room for water to flow and carry away pieces of shale. This process slowly forms the cave. Inside the cave, there are various sulfate minerals, which are made from substances in the water that come from the shale. These minerals form different shapes and colors on the cave walls and floors, especially when the water evaporates. Some of these minerals look like soft globules, crusts, or needle-like structures. Other minerals in the cave are formed when the pyrite in the shale reacts with oxygen in the air. In short, the caves in the Mancos Shale are special because they form through a mix of chemical reactions and erosion, and they have unique mineral formations inside.

REFERENCES:

 

Cabestrero, O., P. del Buey, and M. Esther Sanz-Montrero. 2018. "Biosedimentary and geochemical constraints on the precipitation of mineral crusts in shallow sulphate lakes." Sedimentary Geology, vol. 366, pp. 32-46.
Cline, A.J., C. Spears, F. Mehaffey, et al. 1967. "Soil Survey Delta-Montrose Area Colorado." U.S. Department of Agriculture Soil Conservation Service, 73 p.
De Waele, J., C. Carbone, L. Sanna, M. Vattano, E. Galli, F. Sauro, and P. Forti. 2017. "Secondary Minerals from Salt Caves in the Atacama Desert (Chile): a hyperarid and hypersaline environment with potential analogies to the Martian subsurface." International Journal of Speleology, vol. 46, pp. 51-66.
del Buey, P., M. Esther Sanz-Montero, O. Braissant, O. Cabestrero, and P.T. Visscher. 2021. "The role of microbial extracellular polymeric substances on formation of sulfate minerals and fibrous Mg-clays." Chemical Geology, vol. 581, pp. 1-16. [https://doi.org/10.1016/j.chemgeo.2021.120403]
Elliott, J. G., J.R. Herring, G.P. Ingersoll, J. Kosovich, and J. Fahy. 2007. "Rainfall-Runoff and Erosion Data from the Mancos Shale Formation in the Gunnison Gorge National Conservation Area, Southwestern Colorado 2003-2006." U.S. Geological Survey Open-File Report 2007-1002G, 68 p.
Evangelou, V. P., L. D. Whittig, and K. K. Tanji. 1984. "Dissolved Mineral Salts Derived from Mancos Shale." Journal of Environmental Quality, vol.13, issue 1, pp. 146-150.
Gu, X., D. Rempe, W. Dietrich, A. Joshua West, T-C Lin, L. Jin, and S. Brantley. 2020. "Chemical reactions, porosity, and microfracturing in shale during weathering: The effect of erosion rate." Geochimica et Cosmochimica Acta, vol. 269, pp. 63-100.
Hansen, W.R. 1971. "Geologic Map of the Black Canyon of the Gunnison River and Vicinity, Western Colorado." U.S. Geological Survey Miscellaneous Geologic Investigations Map I-584, 2 sheets, scale 1:31680.
Hill, C., and P. Forti. 1997. "Cave Minerals of the World," 2nd Edition. Huntsville, Alabama: National Speleological Society, 463 p.
Kellogg, K. W., Hansen, K., Tucker, and D. Paco Van Sistine. 2004. "Geologic Map Gunnison Gorge National Conservation Area, Delta and Montrose Counties, Colorado." U.S. Geological Survey Scientific Investigations Map 2825, scale 1:45 000, 1 sheet.
Miller, R.O., R. Gavlak, and D. Horneck. 2013. "Soil, Plant, and Water Reference Methods for the Western Region." Western Coordinating Committee on Nutrient Management WERA-103.
Medville, D. 2018. "Speleogenesis of Caves in a Cretaceous Shale: Bighorn Basin, Wyoming." Journal of Cave and Karst Studies, vol. 80, pp. 66-80.
Mees, F., C. Castaneda, J. Herrero, and E. Van Ranst. 2011. "Bloedite sedimentation in a seasonally dry saline lake (Salada Mediana, Spain)." Sedimentary Geology, vol. 238, issues 1-2, pp. 106-115.
Merkler, D., N. McMillan, and B. Buck. 2006. "Salt Mineralogy of Las Vegas Wash, Nevada: Morphology and Subsurface Evaporation." Soil Science Society of America Journal, vol. 70, pp. 1639-1651.
Noe D.C., J. L. White, and M. Nelson, 2015. Geologic Map of the North Delta quadrangle, Delta County, Colorado: Colorado Geological Survey Open File Report 15-09, scale 1: 24 000, 1 sheet, 9 p. text.
Noe D.C., M.L. Morgan, P.R. Hanson, and S. M. Keller, 2007. Geologic Map of the Montrose East Quadrangle, Montrose County, Colorado: Colorado Geological Survey Open-File Report 07-02, scale 1: 24 000, 2 sheets, 101p.
Nordstrom D.K., 1982. Aqueous Pyrite Oxidation and the Consequent Formation of Secondary Iron Minerals. Chapter 3 in Kittrick A., D.S. Fanning, and L.R. Hossner eds. Acid Sulfate Weathering, v. 10, Soil Science Society of America, p. 56-77.
Onac B. P., W. B. White, and I. Viehmann, 2001. Leonite [K2Mg(SO4)2∙4H2O], konyaite [Na2Mg(SO4)2∙5H2O], and syngenite [K2Ca(SO4)2∙H2O] from Tausoare Cave, Rodnei Mts., Romania. Mineralogical Magazine, February 2001, v. 65(1), p. 103-109.
Onac B. P., and P. Forti, 2011. Minerogenetic mechanisms occurring in the cave environment: an overview: International Journal of Speleology, v. 40, p. 79-98.
Palmer A., 2007. Cave Geology: Dayton, Ohio, Cave Books, 454p.
Parkhurst D.L., and C.A.J. Appelo, 1999. User’s Guide to PHREEQC (version 2); A Computer Program for Speciation, Batch-Reaction, One-Dimensional Transport, and Inverse Geochemical Calculations: U.S. Geological Survey Water Resources Investigation Report 99-4259, 312p.
Penner E., W.J. Eden, and P.E. Grattan-Bellew, 1972. Expansion of Pyritic Shales: Canadian Building Digest, Division of Building Research, National Research Council Canada, 6p.
Sonmez I., 2010. Actual Bloedite mineral Precipitation in a Seasonal Lake in Cankiri-Corum Basin: Bulletin of the Mineral Research and Exploration, v. 140, p. 35-53.
Sonnenfeld P., 2003. Evaporites in R. A. Meyers ed., Encyclopedia of Physical Science and Technology (third edition): San Diego, California, Academic Press, p. 653-671.
Tuttle M.L., J. Fahy, J. Elliott, R. Grauch, and L. Stillings, 2014. Contaminants from Cretaceous black shale I. Natural weathering processes controlling contaminant cycling on Mancos Shale, southwestern United States, with emphasis on salinity and selenium: Applied Geochemistry, v. 46. p. 57-71. [https://doi.org/10.1016/j.apgeochem.2013.12.010]
U.S. Department of Agriculture, 2018. Final Watershed Plan and Environmental Assessment for the Lower Gunnison Project: Figure 3.2-1.
U.S. Department of Energy, 2011. Natural Contamination from the Mancos Shale: Report ESL-RPT-2011-1, 78 p.
van Doesburg J., L. Vergouwen van der Plas, 1982. Konyaite Na2Mg(SO4)2∙5H2O, a new mineral from the Great Konya Basin, Turkey: American Mineralogist, v. 67, p. 1035-1038.
White J.L., R. MacLean, and C.J. Carroll, 2014. Geologic Map of the Whitewater Quadrangle, Mesa County, Colorado, Colorado Geological Survey Open File Report 14-09, scale 1:24 000, 2 sheets, 42 p text.
White J.L., and C. Greenman, 2008. Collapsible Soils in Colorado: Denver, Colorado, Colorado Geologic Survey Publication EG-14, 108 p.
Whittig L. D., A. E. Deyo, and K. K. Tanji, 1982. Evaporite mineral species in Mancos Shale and Salt Efflorescence, Upper Colorado River Basin: Soil Science of America Journal, v. 46, issue 3, p. 645-651.