New Mexico Geological Society Annual Spring Meeting — Abstracts

Suspected miscorrelations between the Dockum Formation (Upper Triassic) of West Texas and strata of the Chinle Group (Upper Triassic) of eastern New Mexico have led Mesozoic stratigraphers to re-examine outcrop data. Utilizing both litho-and bio-stratigraphy these recent investigations have among other things established that the Trujillo Member of the Dockum correlates with the middle sandstone or "Cuervo Member" of the Chinle. The name Cuervo has subsequently been abandoned in favor of Trujillo which has precedence and which facilitates regional correlations. The long-standing, Texas engendered, correlation of the Trujillo sand body with the much older Santa Rosa Formation is not supported by the field evidence and should be discontinued.

Taking a cue from these newly established correlations, depositional environments of the entire Dockum and Chinle sections were reviewed and compared. The concept of a lacustrine depositional environment in the so-called Dockum basin, long held by Texas investigators, was found to lack supporting evidence. Outcrop data support a low gradient fluvial model with paleo flow generally to the north, northwest, and west. The extensive floodplains associated with the major stream systems received fine-grained, mud-dominated overbank deposits which now comprise the Garita Creek and Bull Canyon Formation in New Mexico and Tecovas and Cooper Members of the Dockum in Texas.

In this study subsurface data from the South Lovington pool in Lea County, New Mexico was examined. Records from this pool on file at the New Mexico Bureau of Mines reveal a complete Chinle section, 1510 ft thick, preserved in this area capped by Cretaceous rocks. A 20 ft thick conglomeratic sandstone at the base is correlated with the Santa Rosa Formation. The overlying section is mud-dominated, with a sandy interval 400 ft above the base which may correlate with the Trujillo. Sand-mudrock ratio for the post-Santa Rosa section is a low 0.08 to 1.0. This does not support the concept of a "high constructive lobate delta" prograding eastward into a Dockum basin, as depicted for the Lovington area in earlier accounts. Furthermore, stratigraphic continuity from West Texas, across the Tucumcari basin, and into central and western New Mexico is not consistent with the concept of a separate Dockum depositional basin, distinct from the Chinle basin.

While we do not protest the continued usage of Dockum as a stratigraphic name in Texas, we do lament its usage to suggest deposition in a separate and distinct basin as a primarily deltaic-lacustrine unit. Thus the point of nomenclature change is best adopted as the stateline
rather than some vaguely defined western margin of a Late Triassic basin that assumes the presence of a residual Pedernal uplift in central New Mexico.