New Mexico Geological Society Annual Spring Meeting — Abstracts

During the past 25 years, the U. S. Geological Survey has reinterpreted Middle and Late Jurassic stratigraphy and sedimentation on the Colorado Plateau by: (1) using and creating redundant and parochial stratigraphic names; (2) purportedly tracing marker beds, which, cannot, however, be replicated in the field; (3) postulating regional unconformities for which no stratigraphic or geochronologic evidence exists; and (4) creating sedimentologic models to which stratigraphic units are retrofitted regardless of actual stratigraphic relationships.

Examples of these practices include: (1) coining the redundant name Horse Mesa for the unit long called Bluff and using and extending the preoccupied name Wanakah in place of Summerville; (2) claiming that a marker bed in the "Wanakah Formation" in SW Colorado can be traced into the Curtis Formation of SE Utah, although the "marker bed" cannot be traced across gaps in the outcrop belt and, at different outcrops, is of differing thickness, lithology and stratigraphic position; (3) positing a regional J-5 unconformity for which no evidence exists;
(4) creating a sedimentological model whereby sandstones of the Salt Wash Member of the Morrison Formation must have fed a lake, and then creating the lake by co-opting upper Summerville strata and renaming them the Tidwell Member of the Morrison Formation, even though these strata are nowhere laterally equivalent to the Salt Wash. Particularly significant here is the correlation of Wanakah and Curtis, because this suggests the Todilto Formation of northern New Mexico is older than the Curtis (contrary to all available data, as summarized by Harshbarger et al., 1957, Imlay, 1980, Kocurek & Dott, 1983, Blakey et al., 1988 and Ridgley, 1989, among others) and that the New Mexico Entrada is older than the Utah Entrada.

We conclude that the recent U.S. Geological Survey interpretations of Middle and Late Jurassic stratigraphy on the Colorado Plateau have totally and erroneously confused a relatively stratightforward stratigraphy well understood by earlier workers. We prefer to base a stratigraphy of these rocks on sound stratigraphic principles and practices that eschew the use of redundant nomenclature, are based on demonstrable field relationships, recognize rock-stratigraphic units by their lithology and mappability not their age, avoid correlation based on imaginary unconformities and develop sound regional rockstratigraphy as the basis of sedimentological interpretation, not the reverse.