Geologic and stratigraphic setting of the Coelophysis Quarry, Ghost Ranch, northern New Mexico: a reassessment

Uses paleomagnetic data to argue that the Ghost Ranch Coelophysis quarry is about 223 million years old.

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Although the Coelophysis quarry at Ghost Ranch in northern New Mexico has long been considered to lie within the informally designated "upper siltstone" member of the Chinle Formation, no systematic, detailed work has unequivocally located the quarry within a regional stratigraphic framework. Detailed bed by bed mapping throughout the Ranch shows that the quarry site actually lies 3-4 meters below the base of the "upper siltstone" member, within a lithogically persistent orange muddy siltstone bed. There is no apparent unconformity between the top of this bed and the base of the "upper siltstone". The orange bed is similar to the uppermost bed within the Petrified Forest Member that is found at the type section of the Owl Rock and Rock Point Members on the south side of Monument Valley in northern Arizona, as well as throughout the Colorado Plateau region. If this long-distance physical correlation is correct, then the Coelophysis quarry at Ghost Ranch clearly lies within the Petrified Forest Member. Analysis of facies variations within the "upper siltstone" indicates that this unit may contain the condensed eastern extension of both the Owl Rock and the Rock Point Members. At the Ranch, these are represented by pedogenically modified overbank deposits (?Owl Rock) which are discontinuously overlain, and erosionally truncated, by medium-coarse grained channel sandstones (?Rock Point).

During the late-Quaternary, incision by Arroyo del Yeso (the main tributary of the Rio Chama at Ghost Ranch) undercut the cliff-and-slope topography of the "upper siltstone" member above the quarry, progressively exposing the easily eroded badland-forming Petrified Forest Member below. However, before the thin remnant of Chinle bedrock was eroded below the level of the quarry, an aggradational episode (graded to the main of Rio Chama outwash terrace of the last glacial maximum) partially buried the narrow fin of bedrock. This was all subsequently covered by a landslide that originated from adjacent Kitchen Mesa. The net result was fortuitous preservation of the quarry-bearing rocks by terrace deposits on the valley side and by landslide debris above.