A Conceptual Model for Hypogene Speleogenesis in Grand Canyon, Arizona

Abstract

Although Grand Canyon hosts exceptionally well‐formed vadose caves and exemplary paleokarst, it also contains hypogene caves that provide important information on the canyon’s age and evolution. These caves exhibit speleogenetic materials, passage morphologies, and locations near the top of the Mississippian Redwall Limestone that represent major hypogene dissolution phases. Cave origin resulted from CO2 and H2S in solutions that upwelled from depth and mixed with Redwall–Muav aquifer water. Lack of abundant speleogenetic gypsum suggests that CO2 was the primary solutional agent, while upwelling H2S likely played only a minor role. Each phase of hypogene speleogenesis in our model encompasses the following sub‐events, from deep to shallow: (1) dissolution of cave passages 500 ± 250 m below the water table or potentiometric surface, sometimes with Fe‐ and Mn‐oxide by-products; (2) deposition of calcite spar linings (~50–100 m below the water table); (3) deposition of calcite mammillary coatings (1–20 m below the water table); (4) deposition of calcite folia at the water table; and (5) deposition of gypsum rinds a few meters above the water table. Controls on the amount of cave dissolution and speleogenetic by-products probably include regional water table fluctuations during the Miocene and Pliocene, in combination with magmatic/tectonic pulses that pumped CO2 and H2S from below. The complete cycle of Grand Canyon hypogene speleogenesis includes a largely dissolutional phase under confined conditions and a later (mostly by-product) phase taking place under unconfined conditions. The process terminates when the water table descends below the cave.