Stratigraphy and Paleontology of the Lower Chinle Group (Adamanian: latest Carnian) in the Vicinity of St. Johns, Arizona

ASU Author/Contributor (non-ASU co-authors, if there are any, appear on document)
Andrew B. Heckert Ph.D., Professor (Creator)
Appalachian State University (ASU )
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Abstract: Topographically low exposures of Upper Triassic Chinle Group strata near St. Johns, Arizona, overlie the Middle Triassic Moenkopi Formation and can be assigned to four formation-rank units. These are, in ascending order, the Zuni Mountains (= “mottled strata”), Shinarump, Bluewater Creek (= “Mesa Redondo”), and Petrifi ed Forest formations. Petrifi ed Forest Formation strata in the vicinity of St. Johns pertain to three members, the basal Blue Mesa, medial Sonsela, and upper Painted Desert members. There are two principal unconformities in this section, the Tr-3 unconformity at the base of the Chinle and the Tr-4 unconformity at the top of the Blue Mesa Member. The Sonsela Member of the Petrifi ed Forest Member disconformably overlies the Blue Mesa Member throughout this region. However, although the Sonsela consists of basal extrabasinal conglomerates overlain by coarse-grained sandstones, it does not always form a persistent bench or ledge, and often must be identifi ed by lithology alone, not merely by topographic expression. In the northern and easternmost Blue Hills northeast of St. Johns, the Painted Desert Member of the Petrifi ed Forest Formation crops out as badlands of reddish-brown, bentonitic mudstone.

Additional Information

Heckert, A.B., and Lucas, S.G. (2003), Stratigraphy and paleontology of the lower Chinle Group (Adamanian: latest Carnian) in the vicinity of St. Johns, Arizona. New Mexico Geological Society Guidebook 54, 281-288. Archived in NCDOCKS with permission of the editor. Version of record available at:
Language: English
Date: 2003

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