New Mexico Geological Society Annual Spring Meeting — Abstracts

Middle Triassic strata of the Moenkopi Formation and Upper Triassic strata of the Chinle Group are readily correlated from the Colorado Plateau eastward across the northern Rio Grande rift to the southern High Plains of eastern New Mexico. Lithostratigraphic and biostratigraphic data thus provide the basis for a detailed correlation of Triassic strata from Fort Wingate on the west to Lamy on the east. This correlation demonstrates the continuity of Middle and Late Triassic deposition across northern New Mexico and that:

1. There is substantial (tens of meters of) stratigraphic relief across three pervasive unconformities: Tr-1, Tr-3 and J-2. These are tectonosequence boundaries that correspond to significant tectonic reorganizations of the Triassic-Jurassic depositional basins. Thus, the Tr-1 erosional surface represents the hiatus from Permian to Middle Triassic time (at least 30 million years) and is overlain by Moenkopi fluvial deposits. Tr-3 is the unconformity between early Middle Triassic (Moenkopi) and early Late Triassic (Chinle) rocks (about a 10 million year hiatus), and this surface is overlain by initial fluvial deposits of the Chinle Group. J-2 is the erosional surface between latest Triassic and Middle Jurassic strata (about a 45 million year hiatus) overlain by Entrada eolianites.

2. There is much less regional stratigraphic relief on the Tr-4 unconformity. This is consistent with the fact that the Tr-4 unconformity is an intrabasinal unconformity that represents a relatively short temporal hiatus close to the Carnian-Norian boundary

3. The nature of the Tr-5 (= J-O) unconformity is difficult to evaluate using these data because in most of the sections the youngest Triassic strata pre-date the Tr-5 unconformity. Nonetheless, the varying thicknesses of the Painted Desert Member of the Petrified Forest Formation and its correlatives may in part be due to erosion beneath Tr-5 the unconformity.