Bulletin 147—Middle Jurassic Todilto Formation of northern New Mexico and southwestern Colorado: marine or nonmarine?

By D. W. Kirkland, R. E. Denison, and R. Evans, 1995, 37 pp., 3 tables, 32 figs.

Resolution of the controversy about the origin of this evaporitic limestone and gypsum unit is possible only if the position that the Todilto is exclusively marine or exclusively nonmarine is abandoned. The Todilto Formation was deposited in a coastal body of saline water (a salina) adjacent to the Sundance Sea. After an initial flooding by marine water, the salina was maintained by inflow of fresh water from streams, mostly intermittent, and by influx of sea water by seepage through or over-topping of physiographic barriers. The arid climate promoted evaporation, which led to precipitation of laminated carbonates and nodular gypsum. The mixed brine gave its precipitates a distinctive and indelible isotopic signature-the carbon and sulfur isotopic ratios could be interpreted as originating from marine waters, the strontium ratios as being influenced by nonmarine waters. Reconstruction of the Todilto paleoenvironment as a salina reconciles the apparent conflicts in interpretation of chemistry, as well as similar conflicts in interpretation of the paleontology and of the mineralogy and stratigraphy of the Todilto sediments.

The Todilto Formation can be seen in widely distributed, conspicuous outcrops in northern NM and southwestern CO. It consists of a Limestone Member typically 5–20 ft thick and a Gypsum Member typically 30–100 ft thick. The distribution of the latter is really more restricted, but where it is absent a Breccia Member may be present. The approximate deposition extents of the Todilto Limestone and Todilto Gypsum Members are shown. For more than 40 years there has been a controversy about the environment of deposition of the Todilto Formation: Was it marine or nonmarine? In this bulletin, the authors combine isotropic, paleontological, and stratigraphic information in an attempt to determine the environment of deposition of the Todilto Formation. Much of this information is new, including all the values for strontium isotopes, and many of the values for carbon and sulfur isotopes. Particularly the strontium isotopic data provides the authors with a new approach to resolving the question of the original depositional setting, an approach that can be used for other deposits of carbonates and evaporites for which the environment is unknown. Also new are data on the distribution of fossil insects and, in part, of fossil fish. Many of the individual pieces of evidence may be equivocal, but when viewed together they lead us to only one conclusion in the authors attempts to resolve this protracted controversy.