HOLIER THAN THOU? DOLOMITES IN PARADISE: MATRIX POROSITY DEVELOPMENT IN THE REDWALL LIMESTONE, GRAND CANYON

Sequence stratigraphic and diagenetic analyses of the iconic Redwall Limestone in Grand Canyon National Park (GCNP) demonstrate that facies-controlled matrix porosity may be an important control on aquifer storage and behavior, in addition to the karst and fracture porosity described by previous studies. The regional Redwall-Muav Aquifer (R-Aquifer) is the source of Roaring Spring, the sole source of water for the Trans-Canyon Pipeline and the South Rim. While this study focuses on the Mooney Falls Member, the upper three members of the Redwall Limestone (Thunder Springs, Mooney Falls and Horseshoe Mesa Members) were studied near South Canyon and along the North Kaibab trail. At South Canyon, a measured section (95m) of the complete Mooney Falls Member contained five facies, ranging from highly porous dolomitized mudstone and wackestone to low porosity, syntaxial calcite cemented crinoidal and ooilitic grainstone. These facies occur within seven parasequences which thicken upwards into grainier, shallower-water lithologies. A partial section of this member measured on the North Kaibab Trail showed similar facies, and sequence development; 1-2m thick dolomitized marker beds in both sections occur within 1.5 m of the same stratigraphic position despite the 40km between sections, indicating a very low relief ramp setting. Our sequence stratigraphic model interprets a shift from deeper water lithologies in the Thunder Springs Member (dolomitized mudstone and wackestone) to higher energy lithologies (crinoidal and ooilitic packstone to grainstone) in the Mooney Falls Member. This succession is capped by the supratidal lithologies of the Horseshoe Mesa Member in a prograding system in a lagoonal to high energy near-shore environment. In the Leadville/Redwall Limestone Northeast of GCNP, similar facies to those in the Thunder Springs and lower Mooney Falls Members in GCNP exhibit porosities of 35%. This suggests portions of the R-Aquifer hosted in the Redwall Limestone may have significant water storage in more porous dolomitic layers of the aquifer. Importantly, these dolomitic strata are also the localized sites of preferential dissolution in karst and the source of seeps adjacent to the famous Vasey’s Paradise spring which supports seep ecosystem communities.