Abstract One of the most distinctive Jurassic lithostratigraphic units in the American Southwest is the Todilto Formation of northern New Mexico and southwestern Colorado (Figure 1). This relatively thin (>75 m) unit is mostly carbonates and evaporates in a thick section otherwise dominated by siliciclastic eolianites (Figures 2, 3). The To dilto Formation is extremely significant economically as a source rock for petroleum (Vincelette and Chittum, 1981)and uranium (Chenoweth, 1985); it also provides all the gypsum mined in New Mexico (Weber and Kottlowski,1959).Some earlier workers regarded the Todilto as having been deposited in a marine embayment of the Middle Jurassic Curtis seaway (e.g., Harshbarger etal., 1957; Ridgley and Goldhaber, 1983), but morerecent studies of stratigraphy, paleontology, and geochemistry indicate that any marine connection tothe Todilto Basin was short-lived or intermittent(Lucas et al., 1985; Kirkland et al., 1995). Todilto deposition took place in a paralic salina culminated by a gypsiferous evaporitic lake.