A GEOSPATIAL ANALYSIS OF SPELEOGENESIS: USING PASSAGE MORPHOLOGY AND RESOURCE INVENTORY DATA TO QUANTIFY CONTROLS ON CAVE FORMATION

The Shinumo cave system is a series of network maze caves in the Mississippian Redwall Limestone of the Grand Canyon, Arizona. Hundreds of feet above the canyon floor, the modern Shinumo caves are hydrologically inactive but give insights into the evolution of the ancient R aquifer. These caves may indicate hypogene speleogenesis in the Grand Canyon, providing insight into modern interactions between epigenic and hypogenic groundwater dynamics.

Hypogene speleogenesis produces unique morphologies and mineral assemblages that can be analyzed and classified using spatial and statistical methods. We developed a geodatabase of cave-survey and resource-inventory data for Double Bopper Cave, the longest cave in the system with more than 40 miles of passage. Linear regressions were performed to determine if there is a significant relationship between variations in passage size and either the proximity of major faults or mineralogy distributions.

Twenty-three percent of the variability in passage size is explained by spatial relationships with major faults and a suspected hypogene conduit. Passage size decreases with distance from the Leandras Fault and conduit; however, passage size increases with distance from the Double Bopper Fault. No relationship was observed for the Noble Fault. Gypsum flowers and iron crust are found in smaller passages, whereas stalactites, stalagmites, and calcite spar are found in larger passages.

Faults have been shown to be both pathways and barriers for fluid flow, an important control on speleogenesis. Our results suggest that the Leandras Fault and conduit play a role in sourcing the fluids that created the Shinumo caves. The Double Bopper Fault may have been a fluid barrier, based on its inverse relationship with passage size. Calcite deposits are found near entrances, major faults, and larger passages; sulfate and iron-based speleothems are found farthest from entrances and in smaller passages, indicating a spatial pattern of elemental precipitation along the route of fluid movement. Our results suggest that fluids were oversaturated and aggressive near the Leandras Fault, decreasing with distance toward the Double Bopper Fault, with potential mixing of fluid sources. Further geochemical analysis is needed to determine approximate epigene and hypogene fluid proportions.