Revisions to stratigraphic nomenclature of the Upper Triassic Chinle Group in New Mexico: New insights from geologic mapping, sedimentology, and magnetostratigraphic/paleomagnetic data

摘要

Upper Triassic strata, termed the Chinle Formation or Group, reveal a complex sedimentologic history, which has led to numerous modifications to the lithostratigraphic nomenclature for this unit. Recent revisions to Chinle nomenclature in New Mexico are based on local stratigraphic section inspection. The utility of some stratigraphic subdivisions, especially at formation rank, are problematic when mapping units away from type areas. New magnetostratigraphic records for the Chinle Group, as well as 1:24,000-scale mapping, stratigraphic section logs, and sedimentologic analyses support the following revisions to current nomenclature of the Chinle: (1) formally raising Chinle strata to group status in New Mexico; (2) abandoning the term Zuni Mountains Formation for lower Chinle strata in the Chama Basin; (3) including the lower member of the Salitral Formation in the Shinarump Formation; and (4) reassigning strata termed Rock Point Formation in the Chama Basin to the Moenave Formation. Sedimentologic and basin-analysis interpretations utilized in earlier studies of the Chinle should be resurrected. The revised stratigraphic nomenclature for the Chinle Group and sedimentologic analyses, both old and new, greatly expands our understanding of depositional changes in the Chinle Basin of northern New Mexico. The trunk stream for Chinle deposition in New Mexico coursed westward through the site of the modern Sierra Nacimiento of north-central New Mexico during Shinarump time (?late Carnian or early Norian), then moved northward to flow to the west in the area of the modern Chama Basin. In addition, north-central New Mexico was probably a topographic high during the time of Owl Rock and Rock Point deposition in the late Norian, and thus no uppermost Triassic (late Norian to early Rhaetian) strata were deposited in the area of the modern Chama Basin.