New Mexico Geological Society Annual Spring Meeting — Abstracts

Magnetostratigraphic data for the Upper Triassic Chinle Group of New Mexico are interpreted to indicate that multiple, and often subtle, disconformities are common in the Chinle Group and, in conjunction with recent detrital zircon age information, that deposition of these strata may have been more sporadic than previously assumed. Comparison of magnetic polarity chronologies among the Four Corners area, northcentral New Mexico (Chama Basin) and eastern New Mexico (Mesa Redonda) implies a substantial disconformity between the Petrified Forest Formation and the uppermost strata preserved in the Chama Basin. We speculate that strata that would be time equivalent to the Late Norian Redonda Formation of eastern New Mexico and the Owl Rock and type Rock Point Formations of Arizona and Utah are not preserved in northern New Mexico. Instead, much younger strata sit above the Petrified Forest Formation and may be partially age equivalent to the Rhaetian-Hettangian Moenave Formation of Utah. A possible explanation of these stratigraphic relations is that a roughly north-south trending topographic high existed in central New Mexico and separated the Tucumcari area from western New Mexico, Arizona and Utah, in late Norian and perhaps early Rhaetian time. Such a paleohigh may have resulted in either erosion or nondeposition of Redonda-Rock Point equivalent strata in northern New Mexico. Baldwin and Muehlberger (1959) reported a striking angular unconformity between Upper Triassic redbeds and the Middle Jurassic Exeter (=Entrada) Sandstone in the Dry Cimarron Valley, NE New Mexico. Triassic redbeds are folded in open anticlines and synclines with trending fold axes. The overlying Jurassic Exeter Sandstone varies substantially in thickness (nonexistent to tens of meters) along the length of the Dry Cimarron Valley. We hypothesize that Exeter Sandstone deposits filled an erosional surface that mimicked the topography of deformed pre-upper Norian strata, resulting in a widespread unconformity, locally with well-developed angular relations. The presence of a disconformity in the Chama Basin together with evidence for deformation of Upper Triassic strata in northeastern New Mexico is interpreted to indicate that a paleotopographic high in central New Mexico resulted in substantial changes in depositional patterns during the late Norian to early Rhaetian in this area.