FRACTURE AND GYPSUM DIAGENESIS IN THE TRIASSIC MOENKOPI FORMATION, SOUTHERN UTAH: ANALOG RELEVANCE TO MARS

The Triassic Moenkopi Formation is a fine-grained, tide-dominated marginal marine unit, widespread across southern Utah. Dominant lithologies are red mudstones, red siltstones, and orange sandstones. Thin green horizons associated with silty layers are interpreted as products of early diagenesis in response to water table fluctuations. Distinctive gypsum “stringer” veins ~1-5 cm thick commonly cross-cut upper Moenkopi lithologies. Thinner, densely spaced ~1-2 mm thick veins cross-cut green siltstones. Veins are fibrous to elongate blocky crystals of syntaxial satin spar or selenite. Most veins are at a low angle to bedding and commonly intersect at perpendicular to oblique angles, either terminating or bending to merge with the other intersecting vein.

Vein geometries reflect complex dilation of the mudrock via hydraulic fracturing, and multiple crack-and-seal events. Vein generation probably occurred during exhumation associated with the uplift of the Colorado Plateau because sub-horizontal veins indicate vertical dilation of the host rock. Many veins exhibit internal shear fabrics and en échelon arrays, indicating general horizontal northeast compression. Sr isotope geochemistry suggests that the gypsum in veins and host rock cement is re-precipitated from primary marginal marine gypsum beds. However, S isotopes in veins and the host mudrock tend to be lighter than S values of primary gypsum beds, suggesting another source of S during diagenesis. Despite the presence of abundant horizontal and vertical fractures in the upper ~10 m of the Moenkopi mudstones and siltstones, the lack of gypsum veins in this interval suggests possible diagenetic conditions that inhibited gypsum precipitation.

Hydrous calcium sulfate veins have been imaged by the NASA rover Curiosity in fine-grained rocks in the Yellowknife Bay formation at Gale Crater, Mars, with similar sizes, geometries and weathering characteristics as the terrestrial Moenkopi gypsum veins. Veins are most abundant in lower mudrocks and decrease in abundance in the overlying sandstones and conglomerates. Fracture networks on Mars may have been influenced by hydraulic fracturing. Gale Crater vein distribution is likely controlled by lithology, diagenetic conditions, and proximity to sulfate sources.