New Mexico Geological Society Annual Spring Meeting — Abstracts

Fossil fishes from the Todilto Formation in New Mexico are Caturus dartoni, Hulettia americana and Todiltia schoewei. The distribution of these fishes in the Sundance Formation of Wyoming and Montana suggests the Todilto Formation in New Mexico is latest Bathonian or early-middle Callovian in age. This indicates that Todilto deposition took place between 165 and 170 Ma.

U²³⁸-Pb²⁰⁶ dates of uraninite from the Todilto near Grants, New Mexico reported by Berglof indicate an age of 150-155 Ma. This suggests that uranium mineralization in the Todilto took place at least 10 million years after deposition. Models of syndepositional emplacement of uranium in the Todilto (sabkha model of Rawson, limestone-reef model of Perry) thus can be discounted. The idea that uranium mineralization in the Todilto occurred shortly after deposition, during diagenesis, also is difficult to accept. Indeed, Todilto uraninite is about the same age as the Jackpile Member of the Morrison Formation based on biostratigraphic data (youngest Morrison dinosaurs are Tithonian) and Rb-Sr dates on barren-rock montmorillonite from the Jackpile reported by Brookins. This suggests that Todilto uraninite was secondarily derived from the Morrison Formation long after Todilto deposition ceased.