Speleogenesis of caves in a cretaceous shale: Bighorn Basin, Wyoming

摘要

Blind valleys, aligned dolines, and openings leading to 50 to 70 m long, linear caves, developed entirely within the lowerCretaceous Cody Shale, are found along the west flank of a 150 m high and 7 km long ridge on the eastern side of theBighorn Basin in north-central Wyoming. Precipitation events on a swelling soil allow water and oxygen to reach theshale a few meters below the surface and to react with pyrite in the shale. Microbially-assisted oxidation of the pyrite,possibly by Acidithiobacillus ferrooxidans, produces sulfuric acid that reacts with calcite in the shale, resulting in gypsumfracture fillings, observed as 2 to 4 cm thick beds on cave walls and sub-mm-diameter deposits within shale beds.Evidence for pyrite oxidation is provided by the presence of a ferric oxyhydroxide (goethite) as a by-product, visibleas brown fillings on cave walls, confirmed by XRD. Stable sulfur isotope analysis, using gypsum samples taken fromfracture fillings in the caves, was conducted with negative values for 34S obtained (versus positive values for marinesulfate), providing additional evidence for pyrite as the source of sulfur in the gypsum. This was confirmed by SEMimaging of shale samples. These samples showed gypsum inclusions in the shale, biofilm-coated, framboidal pyritepseudomorphs, and iron oxyhydroxide residue remaining on the framboidal surfaces, evidence for pyrite oxidation.The 2× molar-volume increase, resulting from calcite re-crystallization to gypsum and subsequent growth of gypsumcrystals, leads to fracturing and separation of individual shale beds, reducing the structural integrity of the shale. Disassociatedshale beds along passage walls and rubble slopes of decomposed shale beneath the walls are evidence ofshale decomposition. Subsequent dissolution of gypsum by meteoric water moving through beds and fractures in theshale results in the creation of small, localized voids. When wetted, the shale decomposes into micron-scale particlesthat are removed by episodic water flowing downslope. Sapping occurs at the places where sediment-laden wateremerges, creating openings that progress headward. As material is removed on a grain-by-grain basis by corrasion,the small voids coalesce into more integrated spaces, ultimately permitting human entry.