Carbonate Diagenesis and Secondary Mineralization of Cavernous Porosity in Amazing Maze Cave, Pecos County, Texas

Amazing Maze Cave (AMC), in Pecos County, Texas is a rectilinear maze cave that displays many characteristics conducive to transverse hypogene speleogenetic origin. The cave resides in Lower Cretaceous Fort Terrett limestone of Fredericksburg Group carbonates that have undergone varying degrees of dolomitization and dedolomitization. Morphological features within the cave support a primary hypogene phase associated with confined, mixed convection speleogenesis.;Several mineral species are found in the cave that indicate hydrogen sulfide (H2S) hypogenic dissolution processes, such as secondary gypsum, carnotite, alunite, natroalunite, illite, jarosite, sulfur, and others. Also, the cave contains other mineral species (diagenetic microcline and gibbsite) that have no other documented occurrences as a cave mineral, and some mineral species that are documented, but not in the United States or western hemisphere. Some are conducive to sulfur acidic processes, while others are not, yet are conducive mineralogically as later epigenic overprinting. Most of these minerals are spatially limited, but common throughout the cave are massive secondary gypsum forms filling passage floors near intersections and dominating the spatial volume of many larger rooms. Secondary minerals and morphological features in the cave support a sulfuric-acid transverse hypogene model for speleogenetic origin with H2S likely originating from San Andres hydrocarbon reservoirs proximal to the study area. This study utilized Ground Penetrating Radar (GPR) and detailed digital mapping for morphologic characterization of gypsum massives. Application of GPR in confined spaces like those of AMC is not known to have ever been attempted before due to perceived drawbacks from excessive reflections from walls and ceilings. However, this study achieved useable data given certain limitations. Also utilized are X-Ray Powder Diffraction (XRPDor XRD), Mass Spectroscopy, and Scanning Electron Microscopy (SEM) for qualitative analyses and identification of the mineral species present in samples collected. Furthermore, stratigraphic interpretations and facies delineations of the cave forming horizons were completed to aid in development of speleogenetic models.;These studies provide a better insight for the fluid chemical and fluid kinetics responsible for the development of AMC which may also translate elsewhere for development characteristics of cavernous porosity and permeability in carbonate environments decoupled from surficial influences. The speleogenetic model proposed for AMC, based on data analyses, indicates a primary phase of dissolution associated with H2S enriched ascending fluids being oxidized through mixing with more shallow, confined oxic solutions. The mixing of these fluids, driven by forced convection towards the Pecos River and free convection associated with density gradients produced cavernous porosity as well as the precipitation of large volumes of secondary gypsum and rare sulfuric acid minerals, including alunite, jarosite, sulfur and others. As surface denudation breached confinement, late speleogenetic phases shifted from hypogene to epigene processes, resulting in vadose and phreatic overprinting as well as the precipitation of epigene minerals, including calcite, aragonite, carnotite and several exotic mineral species. The speleogenetic model for AMC reflects a complex, continuously evolving system in response to the changing climatic conditions of the Trans-Pecos region of Texas that is reminiscent of processes documented in the caves of the Guadalupe Mountains.