INTERPRETATION OF FLUID-FLOW PATHWAYS AND MINERALIZATION CONDITIONS OF BARITE CONCRETIONS IN THE LATE CRETACEOUS WAHWEAP FORMATION, SOUTHERN UTAH

The Late Cretaceous Wahweap Formation is well exposed along the East Kaibab monocline in Grand Staircase-Escalante National Monument, Utah. Two normal, syndepositional faults crosscut the Wahweap Formation separating it into southern, central, and northern blocks, each a few miles wide. The Wahweap Formation is informally divided into four units; the lower, middle, upper, and capping sandstone members. This study focuses on the distribution and provenance of authigenic barite concretions found within the fluvial litharenites of the upper member in the area of the central block.

Barite concretions, spherical in shape and 1-10 cm in diameter, are widely distributed throughout the central fault block; no apparent association was found between the locations of the concretions and the bounding faults. Stratigraphically, the characteristics of the concretions change. At the base of the upper member, barite concretions are surrounded by ferroan calcite macro-scale (1-7 m) concretions. The macro-concretions are within seismically distorted massive sandstone beds. Up-section, only smaller (1-3 cm) barite concretions are found. These smaller concretions do not have surrounding ferroan calcite concretions. At one location adjacent to the south fault, barite concretions are also found in the older Straight Cliffs Formation.

Petrographic and x-ray diffraction analyses indicate a paragenetic sequence of 1) poikilotopic barite cement, 2) calcite cement and 3) iron oxide precipitation. Fluid inclusion microthermetry indicates fluid salinity of >25 wt. % NaCl equiv., suggesting barite mineralization from high temperature and pressure sulfate-reducing brines. The syndepositional faults that crosscut the Wahweap Formation sole into the Jurassic Carmel Formation. The Carmel Formation contains evaporite deposits, and is the nearest possible source of sulfur for the barite forming solutions. These fluids migrated along the normal faults on the East Kaibab monocline and precipitated the authegenic barite in the Wahweap Formation.