HYDROLOGICAL BEHAVIOR OF FAULTS NEAR SALT: FIELD AND ISOTOPIC CONSTRAINTS FROM EXAMPLES NEAR THE GYPSUM VALLEY SALT WALL, PARADOX BASIN, COLORADO

Rising salt bodies create unique deformation patterns including concentric and radial fracture patterns and faulting associated with salt tectonics. These fracture networks can influence the hydrological behavior of rocks near salt bodies by acting as barriers or conduits for the movement of fluids like sedimentary brine, meteoric water, hydrocarbons and various ore-forming fluids. Numerous studies use the chemistry of well fluids and pressure differences to document compartmentalization of hydrocarbon reservoirs near salt, but studying the field characteristics of brittle deformation near salt can help identify and rank the significance of variables associated with the compartmentalization of subsurface fluid by faults adjacent to salt. In the southwestern portion of the Paradox Basin, in southwestern Colorado, the southern end of the Gypsum Valley salt wall features a NW-trending counter-regional fault and two SW-trending radial faults. We combined field and laboratory analyses to investigate the paleohydrological behavior of these faults. Field observations suggest radial fractures formed first and are enhanced by the two radial faults, while concentric fractures formed later, around the southeast plunging nose of the salt wall. Fracture intensity reaches 23.2m/m² and generally decreases with distance from the radial faults suggesting that some of the fractures were induced by faulting. Microtextures in calcite veins suggest mineralization was primarily postkinematic, indicating that mineralizing fluids moved through the fracture network after the fractures formed. Barite is found in large (> 10 m³), irregular masses near the radial faults. Stable isotopes of carbon and oxygen in calcite show the presence of two paleofluids (average δ¹⁸O = -13.23 ‰ and -7.05 ‰ PDB; δ¹³C ranges from -5 to -8 ‰ PDB): meteoric water or sedimentary brines. Both paleofluid types are found along the radial faults, while one type is found along the counter-regional fault suggesting the faults in this area served as conduits to flow, but that different fluids may have moved along each type of fault. There is no partitioning of paleofluid types across the faults, indicating that they did not compartmentalize the regional, kilometer-scale fluid system