New Mexico Geological Society Annual Spring Meeting — Abstracts

Gregory (1917) named the Todilto Limestone for outcrops in Todilto Park, New Mexico, and Darton (1928) and others subsequently extended its distribution across northern New Mexico. Two informal members of the Todilto have long been recognized: 1) a basal limestone member that is as much as 13m of laminated, kerogenic micrite; and 2) an upper gypsum member that is as much as 55m of laminar to massive gypsum-anhydrite.

Burbank (1930) named the Wanakah Member of the Morrison Formation near Ouray, Colorado for 20 m of shale, limestone and sandstone underlain by 18 m of gypsum and limestone that he termed the "Pony Express beds." It has long been clear that Burbank's pony Express beds = the Todilto Limestone of Gregory and that the remainder of Burbank's Wanakah Member = the unit long (but incorrectly) called Summerville Formation in northwestern New Mexico. The simplest nomenclatural solution thus is to abandon the term Pony Express, and restrict the name Wanakah to the upper, dominanatly clastic portion of Burbank's original Wanakah Member. This means that the Todilto Formation (with two unnamed members) in northwestern New Mexico and southwestern Colorado is overlain by the Wanakah Formation which underlies the Morrison Formation.

An alternative solution recently advocated by the U.S. Geological Survey is to reduce Todilto to member rank as the basal member of Wanakah Formation and term the upper, dominantly clastic portion of the Wanakah the Beclabito Member. This solution is less favorable because it: 1) unnecessarily introduces a new stratigraphic name (Beclabito); 2) reduces the two members of the Todilto to bed status even though they are mappable at 1:24,000 scale; and 3) runs contrary to longstanding and extensive published usage which has recognized the Todilto and Wanakah as separate formations.