REVISED UPPER TRIASSIC STRATIGRAPHY PETRIFIED FOREST NATIONAL PARK
Spencer G. Lucas
New Mexico Museum of Natural History and Science
1801 Mountain Road N.W.
Albuquerque, New Mexico 87104
Upper Triassic strata exposed in the Petrified Forest National Park belong to the Petrified Forest and Owl Rock Formations of the Chinle Group. In the park, the Petrified Forest Formation consists of the (ascending order) Blue Mesa, Sonsela, and Painted Desert Members. Fossil vertebrates and other biochronologically significant fossils indicate that Chinle Group strata in the park are of late Carnian-middle Norian age, about 222-212 million years old.
The Petrified Forest National Park (PEFO) encompasses 93,492 acres located in Navajo and Apache Counties, northeastern Arizona (Figure 1). Most of this acreage is spectacular badlands developed in Upper Triassic nonmarine strata. These rocks have long been referred to as Chinle Formation with various informal and formal subdivisions (Cooley, 1957; Roadifer, 1966; Espegren, 1985; Billingsley, 1985). Recent revisions of Upper Triassic stratigraphic nomenclature throughout the western United States affect the nomenclature of Upper Triassic strata at PEFO (Lucas, 1993a, b). Here, I summarize these changes by reviewing the Upper Triassic stratigraphy and chronology of PEFO.
Lucas (1993a, b) assigned all Upper Triassic nonmarine strata in the western United States (Wyoming, Idaho, Utah, Colorado, Nevada, Arizona, New Mexico, Oklahoma and Texas) to the Chinle Group to emphasize the lithostratigraphic integrity of these strata, which were deposited in a single, vast depositional basin. Upper Triassic strata exposed at PEFO belong to two formations of the Chinle Group, the Petrified Forest and Owl Rock Formations (Figure 1).
PETRIFIED FOREST FORMATION
Most of the Chinle Group strata exposed at PEFO belong to the Petrified Forest Formation (Figure 1). Three members are present (ascending order): Blue Mesa, Sonsela and Painted Desert.
Blue Mesa Member
Lucas (1993b) coined the name Blue Mesa Member for strata previously termed "lower Petrified Forest Member". The type section is in the Tepees-Blue Mesa area of the southern part of PEFO. Blue Mesa strata are the oldest rocks exposed at PEFO and are as much as 83 m thick here, but their base is not exposed in the park. Mudstone dominates the Blue Mesa Member and is bentonitic, mostly not calcareous and displays variegated hues of purple, blue, gray and red. Interbedded sandstones (about 20-30% of the Blue Mesa Member) are mostly trough-crossbedded micaceous (biotite-rich) litharenites. The Blue Mesa Member is best exposed in the southern portion of PEFO from the Tepees south. Exposures in the northern portion of the park are limited to the Alexander Wash-Wildhorse Wash area.
In Petrified Forest National Park, informal stratigraphic nomenclature exists for three locally persistent sandstone beds in the Blue Mesa Member (Figure 1):
1. Newspaper sandstone - Billingsley (1985) used the term Newspaper sandstone (it was the newspaper rock sandstone in Daugherty, 1941) to refer to the lowest ledge-forming sandstone in the Blue Mesa Member at PEFO. The Newspaper sandstone is very localized; it is only present along the mesa exposed primarily in secs. 9, 15 and 16, T19N, R24E in the southern part of PEFO. It is as much as 10.7 m thick and has 4.5 to 6.1 m of erosional relief at its base. This light brown, medium- to coarse-grained sandstone is ripple laminated and trough-crossbedded.
2. Rainbow sandstone - Cooley (1957) introduced the term rainbow forest sandstone for the stratigraphically lowest persistent sandstone bed in the southern part of PEFO; it is stratigraphically between the newspaper sandstone and the Sonsela Member. At Rainbow Forest, the stratigraphically lowest fossil-log field in the park occurs in the rainbow sandstone. All outcrops of the rainbow sandstone are in the Giant Logs-Agate House area at the southern end of PEFO. About 6 m thick, the rainbow sandstone is generally trough-crossbedded, locally conglomeratic and lithologically similar to the Sonsela Member.
3. Brown sandstone - Billingsley (1985) applied the name brown sandstone to the lowest mappable sandstone in the northern part of PEFO. It forms a persistent ledge along the banks of Lithodendron Wash between I-40 and the park boundary where it is a trough-crossbedded, pale orange to yellowish-brown, micaceous litharenite with a basal intraformational conglomerate of mudstone and calcrete pebbles.
The Sonsela Member is a cliff- and bench-forming sandstone and conglomerate throughout the southern part of PEFO that is only very locally exposed in the Alexander Wash-Wildhorse Wash area of the northern part of the park. It defends the mesa tops at Jasper Forest, Agate Bridge, Blue Mesa and Crystal Forest. The Sonsela Member is as much as 20 m thick and is mostly through-crossbedded litharenite and chert-pebble conglomerate (Deacon, 1990).
In the northern part of PEFO, there are local pinchouts of the Sonsela Member (Billingsley, 1985). This is particularly evident in the SE 1/4, NE 1/4 sec. 11, T19N, R23E along the eastern flank of Wildhorse Wash where a thin Sonsela sandstone ledge pinches out laterally into muddy sandstone and mudstone that lithologically resemble beds of the Blue Mesa Member.
The base of the Sonsela Member marks a major change in lithology from underlying Blue Mesa Member mudstones and siltstones to Sonsela conglomerates and sandstones. This contact shows clear evidence of channeling and erosion, marking it as an unconformity.
Painted Desert Member
The Painted Desert Member of Lucas (1993b) overlies the Sonsela Member conformably. The northern portion of PEFO, the "Painted Desert", is the type area of this unit and is badlands mostly developed in the Painted Desert Member. At PEFO, the Painted Desert Member is as much as 147 m thick, and 80-90% of its thickness is reddish-brown, calcareous, bentonitic mudstone. Several informally named sandstone beds are present in the lower part of the Painted Desert Member, as is a formally named bed of reworked tuff. The informally named sandstones are the Flattops and Painted Desert sandstones of Roadifer (1966) and Billingsley (1985).
The Flattops sandstones are exposed in the southern portion of PEFO, principally on the low buttes called the Flattops in secs. 29-32, T17N, R24E. These sandstones are four, locally persistent ledges separated by mudstone and siltstone slopes. They are trough-crossbedded, laminated and ripple laminated, micaceous litharenites and contain basal, intraformational conglomerates.
The Painted Desert sandstones are exposed only in the northern portion of the park, along and near Lithodendron Wash. Three sandstones have been identified and numbered (in ascending order) 1 through 3. Painted Desert Sandstone 1 is very localized, only cropping out along Lithodendron Wash near Zuni Well in sec. 7, T19N, R24E. Painted Desert sandstones 2 and 3 are more persistent, forming prominent benches throughout the Lithodendron Wash area. The Painted Desert sandstones are lithologically very similar to the Flattops sandstones, and the two sandstone complexes are broadly correlative (Roadifer, 1966).
The Black Forest Bed of Ash (1992) (formerly Black Forest tuff: Billingsley, 1985) is a prominent, white, ledge-forming bed of very tuffaceous sandstone and conglomerate as much as 10.6 m thick. Its base is typically brown-weathering limestone-pebble conglomerate overlain by white-weathering pink to purple reworked tuff. The "Black Forest" is a log and stump field - mostly of Araucarioxylon arizonicum - in the tuff. Ash (1992) reported a K-Ar age of 239 +9 Ma from biotite in the upper tuffaceous part of the Black Forest Bed. This is a Middle Triassic age by any timescale and underscores the fact obvious from outcrop examination that the volcanic detritus in the Black Forest Bed has been reworked from an older source. Painted Desert Member strata above the Black Forest Bed are dominantly reddish-brown mudstones and siltstones with some lenticular sandstones.
OWL ROCK FORMATION
The Owl Rock Formation conformably overlies the Painted Desert Member of the Petrified Forest Formation at Chinde Mesa and Pilot Rock along the northern edge of PEFO. Lucas (1993b) and Dubiel (1993) described Owl Rock strata here, which are at least 30 m thick and mostly consist of pale red litharenite and orange-pink silty mudstone and siltstone with color mottled ledge-forming beds of pisolitic limestone. Owl Rock strata are the youngest Triassic rocks exposed in the park, being overlain unconformably by upper Cenozoic rocks.
Fossil vertebrates of two land-vertebrate faunachrons are present at PEFO: Adamanian vertebrates from the upper part of the Blue Mesa Member and Revueltian vertebrates from the lower part of the Painted Desert Member (Lucas and Hunt, 1993). These vertebrates and other biostratigraphically significant fossils (especially palynomorphs) indicate a late Carnian age for the Blue Mesa Member and an early-middle Norian age for the lower part of the Painted Desert Member (Murry and Long, 1989; Murry, 1990). The Carnian-Norian boundary is thus at or near the base of the Sonsela Member. Evidence from outside of the park indicates the Owl Rock Formation is no younger than middle Norian. Thus, by the Harland et al. (1990) timescale, the Upper Triassic strata at PEFO were deposited about 212 to 222 million years ago.
The Petrified Forest Museum Association, National Geographic Society and New Mexico Museum of Natural History and Science supported this research. Vince Santucci helped with logistics, and Phil Bircheff, Phil Huber, Adrian Hunt, Ken Kietzke and Pete Reser assisted in the field. Orin Anderson and Adrian Hunt reviewed an earlier version of this article.
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Billingsley, G.H., 1985. General stratigraphy of the Petrified Forest National Park, Arizona: Museum of Northern Arizona Bulletin, v. 54, p. 3-8.
Cooley, M.E., 1957. Geology of the Chinle Formation in the upper Little Colorado drainage area, Arizona and New Mexico [M.S. Thesis]: Tucson, University of Arizona, 317 p.
Daugherty, L.H., 1941. The Upper Triassic flora of Arizona: Carnegie Institution of Washington Publication, no. 526, 108 p.
Deacon, M.W., 1990. Depositional analysis of the Sonsela Sandstone Bed, Chinle Formation, northeast Arizona and northwest New Mexico [M.S. Thesis]: Flagstaff, Northern Arizona University, 127 p.
Dubiel, R.F., 1993. Depositional setting of the Owl Rock Member of the Upper Triassic Chinle Formation, Petrified Forest National Park and vicinity, Arizona: New Mexico Museum of Natural History and Science, Bulletin 3, p. 117-121.
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Lucas, S.G., 1993b. The Chinle Group: revised stratigraphy and chronology of Upper Triassic nonmarine strata in the western United States: Museum of Northern Arizona Bulletin, v. 59, p. 27-50.
Lucas, S.G. and Hunt, A.P., 1993. Tetrapod biochronology of the Chinle Group (Upper Triassic), western United States: New Mexico Museum of Natural History and Science, Bulletin 3, p. 327-329.
Murry, P.A., 1990. Stratigraphy of the Upper Triassic Petrified Forest Member (Chinle Formation) in Petrified Forest National Park, Arizona, USA: Journal of Geology, v. 98, p.780-789.
Murry, P.A. and Long, R.A., 1989. Geology and paleontology of the Chinle Formation, Petrified Forest National Park and vicinity, Arizona and a discussion of vertebrate fossils of the southwestern Upper Triassic; in Lucas, S.G. and Hunt, A.P., eds., Dawn of the age of dinosaurs in the American Southwest: Albuquerque, New Mexico Museum of Natural History, p. 29-64.
Roadifer, J.E., 1966. Stratigraphy of the Petrified Forest National Park, Arizona [Ph.D. Dissertation]: Tucson, University of Arizona, 152 p.
Figure 1. Index map (left) of Petrified Forest National Park and generalized stratigraphy of the northern portion of the park (upper right) and the southern portion of the park (lower right).
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