GOING WITH THE FLOW; THE SURPRISING PREFERENTIAL KARST DEVELOPMENT IN DOLOMITES OF THE REDWALL LIMESTONE, GRAND CANYON, AZ

The Mississippian Redwall Limestone is an important constituent of the regional karstic R-aquifer, which is the sole source of drinking water in Grand Canyon National Park and a major source of water in the region. Although faults and fractures have been recognized as focal points for localizing karst development in the aquifer (Huntoon, 1974, 1996, 2000b; Hill and Polyak, 2010; Jones et al.2017), the cause of the high concentration of caves in the Mooney Falls Member of the Redwall Limestone is not well understood. We propose an explanation for stratigraphic localization of karst networks and relate this to preferential dissolution of dolomitized facies in the Mooney Falls Member. This member has over 20 dolomite-rich dolowackestone beds with moldic and intracrystalline porosity ranging from 0-35%, with an average of 18% (Dohm et al., 2017). We posit that the localization of caves in the Mooney Falls Member of the Redwall Limestone can be attributed to the preferential dissolution of these highly porous dolomite beds. Photographic analyses of karst exposed within the canyon demonstrate that cave width and location are primarily controlled by faults and fractures, and secondarily controlled by the location of dolomite bands. These porous dolomites continue to be a pathway for groundwater, with springs of a wide range of flow discharging from dolomite bands (e.g. Vasey’s Paradise Spring).

Preferential dissolution of dolomitic facies over calcite is counterintuitive because dolomite has a lower solubility product constant (Ksp) than that of calcite. Our data suggests that this is controlled by increased porosity and permeability of the dolowackestone facies, allowing for increased fluid flow, saturated water mobilization, and hence, dissolution. Permeability analyses are needed to further evaluate this hypothesis. These findings are significant for understanding both the characteristics and behavior of the R-aquifer, and more broadly, the interaction of dolomitization and karst genesis.