Diagenetic Controls on Groundwater Sapping and Valley Formation, Colorado Plateau, Revealed by Optical and Electron Microscopy

Investigations into processes of valley formation on the Colorado Plateau have confirmed the important role of sapping in the Navajo Sandstone. The sapping process produces drainage systems that differ uniquely from fluvially eroded networks in their valley morphology, network pattern, spatial evolution, and degree of structural, lithologic, and stratigraphic control. The Navajo Sandstone is a highly transmissive aquifer. Sapping results from groundwater emergence above a permeability boundary formed by the underlying Kayenta Formation. This discharge undercuts cliff faces, and causes massive slab failures and vertical cliff recession. The principal agent for the physical weakening of the Navajo Sandstone at a site of seepage appears to be the mechanical separation of sand grains by the deposition of calcite from saturated waters. The control of porosity and permeability by textural and mineralogical features in the Navajo Sandstone and Kayenta Formation was studied using reflected light and cathodoluminescence examination of epoxy-impregnated, polished sections, and a scanning electron microscope study of unimpregnated, unpolished rock chips. The most significant diagenetic factor in porosity reduction in the Kayenta Formation is the abundance of clay filling the pores. Other factors incude the development of quartz overgrowths and extensive carbonate deposition.