New Mexico Geological Society Annual Spring Meeting — Abstracts

The Upper Triassic Whitaker (= Ghost Ranch, = Coelophysis) quarry is known for its large accumulation of fossil bones of the theropod dinosaur Coelophysis. Developed in the Rock Point Formation (Chinle Group), the Whitaker quarry has received much attention that has yielded a deep understanding of the fauna of the quarry and its biostratigraphic importance. Indeed, both vertebrate (principally tetrapods) and invertebrate (conchostracans) biostratigraphy agree that the Whitaker quarry is Norian in age. Specimens of the phytosaur Redondasaurus have been recovered from the quarry and place it within the Apachean land-vertebrate faunachron and indicate a Norian or Rhaetian (latest Triassic) age for the assemblage. The presence of the conchostracan genus Shipingia from the quarry further constrains the age of the assemblage and allows correlation to the Sevatian (late Norian), based on global conchostracan biostratigraphy. Recently, workers have advocated a Rhaetian (latest Triassic) or even Hettangian (earliest Jurassic) age for the quarry based on magnetostratigraphy. The Hettangian age in particular runs counter to both the vertebrate and invertebrate biostratigraphy. In addition, they correlated the Rock Point (which they refer to as the “inferred” Rock Point) Formation to the Moenave-Wingate interval based on paleomagnetic pole similarities. However, this confounds chronostratigraphy with lithostratigraphy, and runs counter to basic stratigraphic principles, by assigning a lithosome to a stratigraphic unit based on its inferred age and not on its lithologic characteristics and stratigraphic position. The conclusion that the Rock Point Formation, and thus the Whitaker quarry, is Rhaetian or Hettangian in age based on magnetostratigraphy is not corroborated by the litho- or biostratigraphic data. Thus, three datasets, the vertebrate biostratigraphy, invertebrate biostratigraphy and regional lithostratigraphy, all indicate a Norian age for the Rock Point Formation and the Whitaker quarry.