ORIGIN OF THE MIDDLE JURASSIC TODILTO SALINA, NORTHERN NEW MEXICO AND SOUTHWESTERN COLORADO

The Jurassic Todilto Formation of northern New Mexico and southwestern Colorado is a distinctive lithostratigraphic unit consisting mostly of carbonates and evaporites in a thick section otherwise dominated by siliciclastic eolianites. It crops out, and is present in the subsurface, over an area of about 100,000 km². It overlies the Entrada Sandstone with minor disconformity and is overlain disconformably to conformably by the Summerville Formation. The Todilto includes two members: the lower Luciano Mesa Member (predominantly limestone) and the upper Tonque Arroyo Member (predominantly gypsum). Regional stratigraphic relationships indicate that the Todilto is homotaxial with the Middle Jurassic marine Curtis Formation of Utah. The origin and nature of the Todilto waterbody has remained uncertain. Some earlier studies suggested that the Todilto was deposited in a marine embayment of the Curtis sea. However, no direct stratigraphic continuity of Todilto strata and Curtis marine strata is known to exist. It seems more likely that the Todilto waterbody began to form during transgression of the Curtis sea, or, alternatively, the Sundance sea, but became separated from these seaways by coastal ergs and a regional decrease in base level. Dune topography below the Todilto at the top of the Entrada is known in the subsurface, but is not prominent on the outcrop. The initial flooding of marine waters was apparently sufficiently energetic to have eroded and deflated much of the dune topography, forming a nearly planar surface, and a thin (<5 cm) zone in the basal limestone containing sand grains reworked from the Entrada sandstone. However, there is no evidence of churning and deformation of the lowermost Todilto in a catastrophic flood. After being separated from the seas, the Todilto basin became a paralic salina, maintained by freshwater stream runoff, seepage through the erg, and possible short-lived overtopping of the erg. Increased aridity eventually produced a smaller evaporitic basin in which gypsum precipitated.