Evolution of a regionally extensive evaporite removal paleokarst complex : Mississippian Madison Group, Wyoming
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Paleokarst systems owe their complex geometries to the interaction between the karst aquifers and the host rock being dissolved. The majority of paleokarst research to date has considered dissolution of carbonate strata (James and Choquette 1987), but rapid and extensive dissolution of interstratified evaporites can be an important if largely undocumented style of paleokarst that may play an important role in near-surface environmental settings as well as providing a unique style of reservoir heterogeneity in the subsurface (Sando 1967, 1974, 1988; Smith et al. 2004). This study is designed to answer the question, “How do we recognize evaporite paleokarst as distinct from standard meteoric carbonate paleokarst?” using spectacular, laterally continuous exposures in the upper Madison Formation within Bighorn Canyon, Wyoming. Key characteristics of the Madison intrastratal evaporite karst complex were documented and contrasted with the top-Madison surficial karst system resulting in a suite of data that includes detailed section measuring, facies mapping using high resolution photo panels and ground based LiDAR for control. Hand samples, thin sections and x-ray diffraction analysis also contributed to this study. High resolution mapping of key surfaces, karst facies and petrophysical properties were used to develop a stepwise evolutionary model of the evaporite removal paleokarst complex. The interplay between surficial karstification, solution enhanced fractures, subsurface intrastratal evaporite dissolution, collapse and infill, were considered in constructing this model. Similar to standard meteoric paleokarst systems, the Madison evaporite paleokarst has been divided into 7 distinct karst “facies” including laminated cave floor fill, roof collapse chaotic breccias, and suprastratal dissolution complexes. Features proposed to be unique to evaporite paleokarst that will aid in future studies are (1) presence of relic gypsum breccia clasts within cave-fill facies, (2) the near absence of cave pillars or roof touch down within the chaotic breccia zones, indicating removal of a laterally extensive soluble stratum, (4) a striking absence of sub-cave floor breccias or fractures, (5) a distinct breccia matrix consisting of primarily autochthonous detrital dolomite with a minor component of allochthonous detrital clays from the overlying Amsden, suggesting that the bulk of the breccia matrix is locally sourced insoluble residue from evaporite dissolution, and finally (6) close facies associations of the depositional sequence suggesting that evaporites were a likely part of the original stratigraphic record in the Madison. These criteria are considered to be a solid starting point for an evaporite paleokarst model and should assist in the recognition of similar paleokarst breccias in the ancient rock record.