Stratigraphic Distribution and Age of Petrified Wood in Petrified Forest National Park, Arizona


Andrew B. Heckert1 and Spencer G. Lucas2

1Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131
2New Mexico Museum of Natural History and Science, 1801 Mountain Road NW, Albuquerque, NM 87104


Abstract—We present detailed stratigraphic and sedimentologic evidence that supports the conclusion that two stratigraphic units in the Upper Triassic Chinle Group contain almost all of the petrified wood in the Petrified Forest National Park. Although incidental deposits of petrified wood occur throughout the stratigraphic section, the Sonsela Member of the Petrified Forest Formation hosts almost all of the brightly colored, large trunks and logs for which the park is known. The Black Forest Bed in the Painted Desert Member of the Petrified Forest Formation contains another large deposit of petrified logs. Incidental petrified wood deposits occur primarily in sandstone beds within the Painted Desert Member, although some are also known in the Blue Mesa Member. Available biochronological evidence, including tetrapods, megafossil plants, pollen, and calcareous microfossils, indicates that both the Sonsela Member and the Black Forest Bed are of early- to mid-Norian (220-215 Ma) age.

Introduction


Figure 1—Map of the Petrified Forest National Park. Arrows show locations of measured sections used in Figure 3. Rose diagrams show paleocurrent measurements from logs in the Sonsela Member outside park boundaries, indicating a predominantly southwest-northeast channel trend of Sonsela Member sediments.

Figure 2—Schematic stratigraphic section showing the Chinle Group stratigraphy of the Petrified Forest National Park and the stratigraphic distribution of major petrified wood accumulations in the park.

All the petrified wood in the Petrified Forest National Park (PEFO) occurs in deposits of the nonmarine Upper Triassic Chinle Group. Geologic investigations of the Chinle in PEFO include Gregory (1917), Cooley (1957), Roadifer (1966), Stewart et al. (1972), Billingsley (1985a,b), Ash (1987a), Murry (1990), Ash (1992), and Lucas (1993, 1995). These studies, combined with our own, have resulted in an extensive database of detailed measured sections throughout PEFO and vicinity (Figure 1). In PEFO, the following previously named Chinle Group units are exposed (ascending order): the Bluewater Creek Formation, the Blue Mesa, Sonsela, and Painted Desert Members of the Petrified Forest Formation, and the Owl Rock Formation (Figure 2). The bulk of the petrified wood in PEFO, including the spectacular deposits ("forests") of large, highly colorful, trunks, occurs in the Sonsela Member of the Petrified Forest Formation. The gray-black logs of the Black Forest occur in the Black Forest Bed of the Painted Desert Member. Isolated logs also occur in bench-forming sandstones of the Painted Desert Member, and in deposits of the Blue Mesa Member.

Published studies on the systematics and stratigraphic occurrence of petrified wood in PEFO include Blake (1856) Goeppert (1858), Knowlton (1889), Jeffrey (1910), Daugherty (1934, 1941), and Gould (1971). Almost all of the petrified wood in PEFO was transported prior to deposition, as evidenced by a lack of limbs, bark, or roots preserved with most specimens. A few in situ stumps have been recorded, predominantly in the Blue Mesa Member or in the Black Forest Bed (Ash and Creber, 1992). In general, the spectacularly colored logs in the Sonsela Member are typically referred to the genus Araucarioxylon, and most are probably representatives of the species A. arizonicum (Daugherty, 1941). The Black Forest logs, so named because of their dark gray to black color, also include A. arizonicum and most of the area's specimens of Woodworthia and Schilderia (Ash, 1992).

Historically, most workers have agreed that the bulk of the petrified wood in PEFO is concentrated in a few horizons. Some, including Cooley (1957, 1959), Billingsley (1985a,b), and Ash (1987a, 1992), argued that three highly fossiliferous horizons were present, the informal "Rainbow Sandstone," Sonsela Member, and the Black Forest Bed. Others, including Roadifer (1966) and Deacon (1990) considered the "Rainbow Sandstone" and the Sonsela Member homotaxial, and thus thought that only two principal horizons were present, namely the Sonsela Member and the Black Forest Bed. Here we present stratigraphic and paleocurrent information that supports the conclusions of Roadifer (1966) and Deacon (1990) and demonstrates that most petrified wood deposits in PEFO are in either the Sonsela Member or the Black Forest Bed (Figure 2).

Sonsela Member forests.—The vast majority of the fossil wood in PEFO occurs in trough crossbedded sandstones and conglomerates of the Sonsela Member of the Petrified Forest Formation. Almost all of the famous localities in the southern portion of the park, including the spectacular "forests" of trunks preserved in Giant Logs, Rainbow Forest, Long Logs, Crystal Forest, Jasper Forest, Agate Bridge, and Blue Mesa, are in the Sonsela Member, as are several "forests" on adjacent private and state lands. The Sonsela Member typically consists of gray, grayish brown, and grayish green sandstones and conglomerates. Conglomerate clasts include pebble- to cobble-sized chert and quartzite clasts and rip-ups of underlying Blue Mesa Member mudstones, some of which approach boulder size. Deacon (1990) thoroughly investigated the sedimentology of the Sonsela Member and concluded that it represented a low sinuosity fluvial system consisting of northerly to northeasterly draining braided channels. Lucas (1993) demonstrated that the Sonsela disconformably overlies an erosional surface on the Blue Mesa Member that represents his Tr-4 unconformity, and Heckert and Lucas (1996) examined the details of this relationship in PEFO and vicinity.

In the past, numerous workers, including Cooley (1957, 1959), Billingsley (1985a,b), and Ash (1987a), have referred to an informal unit called the "Rainbow Sandstone" and considered it the sandstone unit that contained the "forests" at Giant Logs, Rainbow Forest, and Long Logs. However, we agree with Roadifer (1966) and Deacon (1990) that this interval actually represents the same stratigraphic interval as the Sonsela Member. Recently, one of us (ABH) conducted extensive stratigraphic work on the private lands adjoining PEFO and determined that all the trough-crossbedded, wood-bearing, extrabasinal conglomerate and conglomeratic sandstone is at a single horizon (Figure 3). Paleocurrent data, both our own and those of Deacon (1990) corroborate this stratigraphic hypothesis, and show an overall channel trend toward approximately N30E. This channel can be traced from the extreme southwestern portion of the park into the subsurface, re-emerging at the Crystal Forest, where it is readily traced to Jasper Forest, Agate Bridge, and Blue Mesa (Figure 1). Araucarioxylon is by far the most common genus of tree preserved in these forests (Ash and Creber, 1992).


Figure 3—Correlated measured sections showing the stratigraphic distribution of the major "forests" of petrified wood in the southern portion of the park. Location of measured sections given in Figure 1. Sections credited to original author as appropriate, sections not credited are our own.

Painted Desert Member.—The Painted Desert Member contains much less petrified wood than the underlying Sonsela Member. Scattered petrified wood occurs in the bench-forming sandstones in the Painted Desert, usually referred to as "Flattops Sandstones" in the southern portion of the park and as "Painted Desert Sandstones" in the northern portion (e.g., Billingsley, 1985a,b). Most logs are found in Jim Camp Wash, and are typically less colorful and smaller than those in the Sonsela Member. None of the major forests in PEFO are composed of this wood, and most of these deposits appear to represent more typical, isolated wood deposition in fluvial systems. Because of its scattered and fragmentary nature, this wood is not well-studied, but Araucarioxylon dominates the identifiable specimens (Daugherty, 1941; Ash and Creber, 1992).

Black Forest Bed.—Ash (1992) named the Black Forest Bed for the extensive deposits of limestone-pebble conglomerate and reworked tuff in the Painted Desert Member in the northern portion of the park. These deposits occur approximately 60-65 m above the top of the Sonsela and range from 0 to 12.6 m in thickness in the park (Ash, 1992; Lucas, 1993). Ash (1992) described the Black Forest Bed as a basal, well-indurated calcrete pebble conglomerate overlain by reworked, andesitic tuff with thin interbeds of mudstone and siltstone. The petrified logs here are gray-black and most abundant in the Black Forest itself, roughly 2.5 km (1.5 miles) north of the Painted Desert overlooks, although numerous logs derived from the Black Forest Bed can be found as float on the slopes below those overlooks. The Black Forest Bed wood, like the Sonsela, includes abundant specimens of Araucarioxylon, but also includes rarer Woodworthia and Schilderia (Ash, 1992). Most of the wood is concentrated in the upper, tuffaceous portion of the unit, which is the highest stratigraphic occurrence of petrified wood within the park.

Biostratigraphy, biochronology, and numerical ages.—The superposition of the petrified forests clearly demonstrates that the oldest extensive wood deposits are those in the Sonsela Member, with scattered wood deposition occurring in the various Painted Desert Member sandstone beds until another major depositional event resulted in the preservation of the fossil logs in the Black Forest Bed. Unfortunately, the petrified logs
are not themselves age diagnostic, as is well demonstrated by the universal occurrence of Araucarioxylon in petrified wood deposits throughout the park. Extensive work on tetrapod vertebrates (summarized in Hunt and Lucas, 1995), megafossil plants (Ash, 1980, 1987b), pollen (Litwin et al., 1991), and calcareous microfossils (Lucas and Kietzke, 1995) indicates that both the Sonsela and the Painted Desert Members are early-mid Norian in age. Lucas (1997) summarized this biostratigraphic evidence and noted that the available biochronologic evidence indicated that the absolute age for these strata probably ranges between 215 and 220 Ma.

Two numerical ages have been reported from PEFO, both from the Black Forest Bed. Ash (1992) suggested that a K-Ar age on a biotite of 239±9 Ma is the age of a Middle Triassic tuff that was subsequently reworked during Late Triassic time to form the Black Forest bed. Riggs et al. (1994a) reported a U-PB age on zircons from the Black Forest Bed of 207±2 Ma and suggested that this is the syndepositional age of the unit. Riggs et al. (1994b) went further to conclude that the Black Forest Bed represents the fluvial deposition of a Plinian ash fall that disrupted stream flow, causing avulsion and crevasse splay formation.

This sedimentological interpretation seems unlikely because: (1) the tuff of the Black Forest Bed is extremely localized and only known from one location outside of PEFO (Ash, 1992), which would not be the case in a Plinian ash fall; and (2) geometry and sedimentary structures of the Black Forest Bed differ little from those of other Painted Desert Member channel deposits, which suggests an ash fall was not needed to produce this type of facies architecture. Furthermore, an age of 207 Ma for the early Norian Black Forest Bed seems unlikely, unless all previous calibration of the Late Triassic timescale is incorrect (Lucas, 1994, 1997). Therefore, neither of the published numerical ages of the Black Forest Bed is a reliable syndepositional age, and abundant biostratigraphic evidence constrains deposition of both the Sonsela Member and the Black Forest Bed to the early- to mid-Norian.

Conclusions

Two horizons, the Sonsela Member and the Black Forest Bed, contain the vast majority of the petrified wood in PEFO. Sonsela Member deposits are the oldest widespread deposits in the park and include the spectacular logs associated with Giant Logs, Rainbow Forest, Long Logs, Crystal Forest, Jasper Forest, Agate Bridge, and Blue Mesa in the southern portion of the park. The most extensive deposits in the Painted Desert Member are those in the Black Forest Bed in the northern portion of the park, although scattered petrified wood also occurs in other sandstone beds within the Painted Desert Member. Available stratigraphic and sedimentologic evidence suggests that deposits termed the "Rainbow Sandstone" by some authors are actually basal Sonsela Member deposits. These are the oldest major deposits of petrified wood in PEFO, and are early Norian in age. The Black Forest Bed is also well-constrained biochronologically to the early-mid Norian, in spite of isotopic data that suggest other ages. Acknowledgments

The Petrified Forest Museum Association generously supported this research. Additional support was provided by the New Mexico Museum of Natural History. Numerous personnel have facilitated field work involved with this project, including P. Bircheff, C. Davis, P. Huber, A. Hunt, K. Kietzke, P. Reser and park employees L. Bolich, D. Humphries, and M. Schmitt. Personnel of PEFO, especially Carl Bowman, Mark DePoy, David Dewitt, Pat Quinn, and Vince Santucci facilitated logistics of our study. We are also indebted to the New Mexico and Arizona Land Company, especially J.D. Sphar, for allowing access to lands adjacent to the park and allowing us to publish data collected there.

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