Reservoir characterization, paleogeomorphology, and genesis of the Mississippian Redwall Limestone paleokarst, Hualapai Indian Reservation, Grand Canyon area, Arizona, U.S.A.

by


Tony John Troutman, M.S. Geo. Sci. Sought

The University of Texas at Austin, 2004

SUPERVISOR: William Fisher


            The Mississippian Redwall Limestone paleokarst in the western Grand Canyon area provides an excellent outcrop analog for understanding paleokarst in three dimensions. Major controlling factors in the formation of this paleokarst include exposure time, tectonic and eustatic events, paleohydrology, and paleoclimate. The Redwall paleokarst shows examples of polyphase development, with earlier cave systems exerting strong influence on the formation of later phases of karstification. A close association of the paleokarst with the Surprise Canyon Formation paleochannels was found, suggesting these channels formed local base level and outlets for the paleocave drainage. Parts of the paleocave system show evidence of a vadose phase of speleogenesis, with evidence of relict flowstone and bedded fluvial sediments in the base of the collapse breccia. Parts of the breccia bodies show strong correlation to a fracture set that is found only in Mississippian strata, suggesting that fractures were a control on speleogenesis and paleocave system morphology. No breccia bodies were found that penetrated deeper than the chert beds of the Thunder Springs Member. These chert beds likely formed a paleohydrologic barrier that limited the depth of karstification. Very little evidence of paleosols or other clues to paleoclimate could be found, demonstrating that this may be one of the most difficult factors to utilize. There is evidence that the paleokarst together with the paleochannels of the Surprise Canyon Formation once formed a well-connected but geometrically complex aquifer or potential reservoir. The Redwall paleokarst presently has little porosity and is acting as a flow barrier, but it retains open fracture porosity in outer parts of the collapse beds surrounding the paleokarst breccia bodies. This open fracture porosity appears to be contributing to the formation of later phases of karstification. A hypothetical model of the paleocave distribution is developed using evidence of fracture control, base level control, and paleohydrology to constrain the style of paleocave morphology.