TIMING OF SILICIFICATION OF THE MIDDLE PROTEROZOIC MESCAL PALEOKARST AND THE TRANSITION FROM THE APACHE GROUP TO THE TROY QUARTZITE IN CENTRAL ARIZONA

Uplift and subaerial exposure of the Mescal Limestone (dolomite) during the Middle Proterozoic led to a regional karsting event that hosts evidence of the earliest known microbial land life. North of the Salt River in the Sierra Ancha widespread karsting in the Mescal involved pervasive dissolution of dolomite and collapse and deflation of the formation as a whole. The result was thinning of the Mescal in places by up to one half the original thickness. Karsting in the Mescal was characterized by narrow, bedding-parallel caves, collapse pits, localized collapse breccias, and minor clastic dikes. Comparison of relatively unkarsted sections of the Mescal at Roosevelt Dam indicates that the original stratigraphy of the early diagenetic cherts is preserved in the karsted sections. Abundant fine-grained hematite and terra rossa residuum fills cracks and cavities on all scales within the karst. Wholesale replacement of the remaining dolomite by silica occurred in a northeast-striking belt across the Sierra Ancha. Petrographic examination of fabrics within this 'secondary' silica indicates replacement occurred prior to burial. Widespread replacement of the remaining dolomite by granular microcrystalline quartz and close-packed chalcedony spherules was followed by crystallization of cavity-lining botryoidal chalcedony and void-filling megaquartz. Caverns filled with siltstone and sandstone derived from the overlying argillite and Troy Quartzite attest to the presence of caverns after silicification of the karst. Weathered, hematite-rich basalt overlying the Mescal was the most likely source of silica in the silicified paleokarst. Thinly laminated argillite interbedded with the basalt is also locally strongly silicified and probably represents weathered products of the basalt washed into low-lying areas. The extent of near-surface silicification and weathering suggests a subhumid to subarid environment. Eolian sands and braided stream deposits of the overlying Troy Quartzite herald a change to arid conditions. Abundant microfossils and possible strange and unique megafossils entombed in the silica of the paleokarst provide a rare look at a possible cave- and/or near-surface land-based, Proterozoic biota.