Revised Upper Triassic stratigraphy of the Petrified Forest National Park, Arizona, U.S.A.

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: During the last 40 years, a variety of informal names have been proposed for Upper Triassic stratigraphic units in the Petrified Forest National Park (PFNP) and vicinity. We both simplify and formalize this stratigraphy. All Triassic exposures in the PFNP pertain to various units of the Chinle Group. We identify localized, topographically low exposures of siliciclastic redbeds and purple mudstones as the Bluewater Creek Formation, the oldest unit exposed in the park. The oldest widely exposed strata in the park belong to the overlying Blue Mesa Member of the Petrified Forest Formation. Within the Blue Mesa Member, we recognize one formal, sandstone-dominated unit of bed-level rank, the Newspaper Rock Bed. Sandstones and conglomerates of the Sonsela Member of the Petrified Forest Formation rest disconformably on Blue Mesa Member mudstones on an erosional surface (Tr-4 unconformity) that approximates the Carnian-Norian boundary. There is no separate "Rainbow Sandstone;" instead, beds previously assigned to this unit pertain to the basal Sonsela Member. The Sonsela Member is thus composed of three bed-level units in the park, units we designate as Rainbow Forest (basal sandstone and conglomerate), Jim Camp Wash (middle slope-forming, finer-grained interval) and Agate Bridge (upper sandstone and conglomerate) beds. The Sonsela Member is conformably overlain by the Painted Desert Member. In the southern portion of the park, the Painted Desert Member includes three sandstone-dominated units we formalize as Flattops beds 2-4 (ascending). North of 1-40, bed-level units cropping out in the Painted Desert include the Lithodendron Wash Bed (formalized here) and the Black Forest Bed. In the extreme northern portion of the park, pedogenic limestones of the Owl Rock Formation conformably overlie the Painted Desert Member. Locally, the Neogene Bidahochi Formation, including some interbedded lava flows, overlies the Painted Desert Member or the Owl Rock Formation. The youngest lithostratigraphic units exposed in the park are a variety of unconsolidated Quaternary colluvial, fluvial and eolian deposits. Lithologic and tetrapod biostratigraphic evidence indicates that Chinle strata encompass a single significant unconformity at the base of the Sonsela Member, with strata below the Sonsela bearing tetrapod fossils of Adamanian (latest Carnian) age and the Sonsela and overlying units containing Revueltian (early-mid-Norian) tetrapod fossils.

Additional Information

Heckert, A.B., and Lucas, S.G. (2002) Revised Upper Triassic stratigraphy of the Petrified Forest National Park, Arizona, U.S.A. New Mexico Museum of Natural History and Science Bulletin 21, p. 1-36. (ISSN 1524-4156) Archived in NC DOCKS with permission of the editor. The version of record is available at:
Language: English
Date: 2002

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