New Mexico Geological Society Annual Spring Meeting — Abstracts

The Todilto limestone unit of the Wanakah Formation is 1-30 ft (0.3-9.1 m) thick in the Grants district and records changes in depositional environments from a restricted embayment, with an ephemeral connection to the Curtis-Summerville sea, to a completely enclosed and shrinking body of gypsiferous water. The salina was 300 miles (483 km) eastwest, 250 miles (402 km) north-south and fringed by an extensive limestone-gypsum sabkha. Arenaceous lime mudstones to arenaceous-ooid-ostracode-microbial mat-wackestones record a
shoaling upward sequence, deposited in an arid, subtidal to supratidal, sabkha environment in alternating brackish to hypersaline marine waters. Dolomite is absent in the study area. Petrographic studies show that the calcite lime mudstones were subjected to extensive neomorphism and were derived primarily from an aragonite mud precursor. The aragonite to calcite diagenetic history is evident in the poorly preserved ooids.

The salina waters did not support a normal marine invertebrate fauna or flora. Fragments of the marine calcareous algae Dasyclads, belonging to the Tribe Salpingoporelleae, indicate short periods of near normal sea water resulting from marine transgression into the Todilto embayment. Ostracodes are abundant in the sabkha facies and lived in ephemeral-gypsiferous-ponds.

The salinity of the marine waters was influenced by addition of seasonal water from streams and rainfall and by periods of drought and intermittent connections to the Curtis-Summerville sea. The overlying gypsum unit, 0-110 ft (0-33.5 m) thick, found to the east of the study area, was deposited in the center of the basin during the final salina phase. Lacustrine alkaline evaporites, such as trona and shortite, are unknown in the Todilto Limestone.

Megapolygon mounds and tepee box folds, up to 10 ft (3 m) high and 45 ft (14 m) wide and of early diagenetic origin, are found in the supratidal facies, associated with calcite pseudomorphs of gypsum. The tepee sutures and cracks on the sabkha crust may have acted as sites of saline ground water outflow. The tops of the tepees are not eroded but are overlapped and buried by thick-bedded microbial mats of ostracode-supratidal-lime-mudstone and caliche. Commercial orebodies of uranium are found in the Todilto Limestone Member in the tepee box folds. Isotopic ages of 155-150 Ma from uraninite indicate that the uranium mineralization is syngenetic with the carbonate deposits.