New Mexico Geological Society Annual Spring Meeting — Abstracts

Several different types of folds and fold-like structures, on different scales, affect the Todilto Limestone (Luciano Mesa Member of the Todilto Formation, Lucas et aL 1995), or are present within it, in the Grants uranium district. These include: regional large-scale folds affecting the Todilto and units above and below it; large-scale intraformational folds with mappable axes; mounds or pillow-like structures within the limestone, possibly stromatolites; and several types of small-scale intraformational folds. The latter include sharp folding of varve-like thin bedding; within-layer folds resembling those described elsewhere as "enterolithic"; and micro-folding of thin layers, including the "crinkly" bedding common in the middle and upper portions of the Luciano Mesa Member of the Todilto. The large-scale intraformational folds have clearly influenced the localization of uranium deposits in the Todilto, apparently by providing zones of permeability through which mineralizing solutions moved. Such intraformational folds are conspicuous only in or near known uranium deposits. Not all of the folds are mineralized, but all primary uranium deposits in the Todilto are associated with the folds. Many folds were exposed in mines that are now inaccessible; others are exposed on rim outcrops. Small, isolated uranium deposits distant from Grants (Laguna, Sanostee and Box Canyon) were also associated with folds of this type.

The origin of the ore-controlling folds has remained controversial since they were first recognized during initial development and mining of Todilto uranium deposits shortly after 1950. An early suggestion that the ore-controlling intraformational folds relate to Laramide tectonic activity seems unlikely, as isotopic dating of uraninite indicates that the deposits formed in Jurassic time shortly after the host rocks were deposited. Evidence of soft-sediment deformation is abundant, consistent with early formation.

One hypothesis is that the weight ofencroaching dune sands of the overlying Summerville Formation deformed soft lime muds of the TodiIto. Some folds resemble tepees; along with small-scale enterolithic folds, these could relate at least in part to forces of crystallization or hydration of calcite and gypsum. The extensive anhydrite/gypsum beds of the overlying Tonque Arroyo Member are absent where the limestone is mineralized with uranium. Mappable intraformational fold axes may be distributed more or less randomly, or may show a preferred orientation within a small area or single mine; this does not seem to uniquely favor any hypothesis.

Earthquakes occurring during sedimentation are increasingly recognized as a possible cause of soft-sediment structures, sometimes called "seismites," which can develop even on gentle slopes. Tectonic activity during Todilto deposition is possible; active folding appears to have influenced the geometry of mineralized sandstones of the younger Morrison Formation. Some Todilto folds resemble ones occurring in the Pleistocene Lisan Formation along the Dead Sea in Israel and Jordan, believed to be a sedimentological analog of the Todilto in a different tectonic setting. The Lisan folds apparently formed at least in part from seismogenic processes (Marco and Agnon, 1995).

The Todilto uranium deposits are the largest known deposits in limestone, and appear to reflect a rare combination of circumstances. Intraformational folds in the Todilto, many of which localize uranium deposits, also appear to also represent a somewhat unlikely combination of multiple origins; it is difficult to establish the relative importance of each process.