Abstract

The Todilto Limestone was deposited in or near a large lake that at times became restricted and evaporated to dryness. The formation has two members: a lower limestone and an upper gypsum member. The limestone member has been divided informally into three zones: the lower “platy” zone, a middle “crinkly” zone, and an upper “recrystallized” zone. The platy zone is interpreted to have been deposited below wave base under anoxic conditions. The crinkly zone has thin stromatolitic laminations and may form algal domes. The upper recrystallized zone appears in part to be a collapsed breccia caused by the removal of interbedded gypsum. Uranium ore is restricted primarily to the “crinkly” and recrystallized zones. These two zones may have been formed in a sabkha-like environment.

A. R. Renfro has proposed a sabkha origin for some stratiform copper deposits. The same conditions that cause copper to precipitate would also cause uranium to precipitate. Groundwater bearing U⁺⁶ could also be drawn upward by evaporative pumping through the decaying algal-mat zone where the uranium would be reduced to U⁺⁴ and precipitated. Carbonate materials lithify early destroying permeability so that uranium emplacement must occur before lithification. Radioisotope dates on uraninite in the Todilto Limestone indicate ore emplacement shortly after deposition. Uranium-bearing groundwater moved basinward in the underlying Entrada Sandstone and was drawn upward through the stromatolitic zones along the southwest margins of Lake Todilto and uranium was precipitated.