THE LAST GASP OF THE EARLY MISSISSIPPIAN SEAWAY; SEQUENCE STRATIGRAPHY OF HORSESHOE MESA MEMBER OF THE REDWALL LIMESTONE, GRAND CANYON, AZ

The Redwall Limestone of northern Arizona was deposited on the early Mississippian carbonate ramp complex which extended across Montana, Utah, Colorado, New Mexico, and western Texas. Regionally these strata are important oil and gas reservoirs as well as aquifers. The upper portion of this formation, the Osagean to early Meramecian Horseshoe Mesa Member, exhibits well developed parsequences and sequences which have received little recent study. This member was investigated in the Grand Canyon at three locations; South Canyon (near Vasey’s Paradise), the type locality, Horseshoe Mesa, and along North Kaibab Trail. Thickness measured at 29.5, 23.5, and 7 meters respectively, with variation in thickness due to post depositional dissolution and erosion of the top strata. The Horseshoe Mesa Member contains five marine and two non-marine facies; oolitic grainstones, peloidal grainstones, crinoidal grainstones, peloidal packstones, microbial boundstones, sandstone, and non-marine shale. Within these shoreline facies, flooding surfaces define the 6-7 parasequences of this unit, and 2 subaerial exposures define a minimum of 3 sequences. The shallow water facies and thin parasequences suggest that deposition occurred on the up-dip portion of the shelf with limited accommodation space, which ultimately resulted in the retreat of marine conditions. The petrographic investigation of diagenesis of these deposits is on-going, but early results confirm the influence of compaction through stylolites, moldic dissolution, and calcite cements. The peritidal sediments include partially dolomitized microbial laminites, and clay and silt-rich siliciclastic intervals creating permeability barriers between layers. These compartmentalized layers reacted differently to both karst development and later burial diagenesis. The late Mississippian period of sub-aerial exposure and dissolution created passageways for karst-filling clays, silts and sandstones to infiltrate the top ten meters of the unit. The exceptional exposures of the Horse Shoe Mesa Member near South Canyon reveal a clear sequence stratigraphic history of up-dip facies and an intriguing set of potential feedbacks between facies-controlled compartmentalization, Mississippian karst development, and subsequent diagenesis.