The Oldest Triassic Strata exposed in the 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—All Triassic rocks in the Petrified Forest National Park pertain to the Upper Triassic Chinle Group. We identify localized, topographically low exposures of siliciclastic redbeds and purple mudstones near Newspaper Rock as the Bluewater Creek Formation, the oldest unit exposed in the park. Mudstone and sandstone of the Blue Mesa Member of the Petrified Forest Formation are the oldest widely exposed strata in the park, and are overlain disconformably by sandstone and conglomerate of the Sonsela Member. The Sonsela Member is overlain conformably by mudstone and sandstone of the Painted Desert Member. In the extreme northern portion of the park, pedogenic limestones of the Owl Rock Formation conformably overlie the Painted Desert Member. Surface and subsurface stratigraphic evidence indicates that the Bluewater Creek Formation is the oldest unit exposed in the park—not the mottled strata, Shinarump Formation, or Moenkopi Formation, as claimed by some recent workers. Lithologic and tetrapod biostratigraphic evidence indicates that Chinle strata encompass a single major 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 (Norian) tetrapod fossils.

Introduction

Chinle Group deposits dominate the outcrops of the Petrified Forest National Park (PEFO), and the park contains several type sections of Chinle Group units. Geologic investigations of PEFO include Gregory (1917), Cooley (1957), Roadifer (1966), Stewart et al. (1972), Billingsley (1985a,b), Ash (1987a), Murry (1990), and Lucas (1993, 1995). 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. Recently, some workers (Dubiel et al., 1995; Hasiotis and Dubiel, 1995) have proposed that the oldest outcrops in PEFO pertain to the Moenkopi Formation, although descriptions in these works indicate that they actually mean "mottled strata," a name given to pedogenically reworked strata at the base of the Chinle Group by Stewart et al. (1972). This hypothesis is at odds with the available stratigraphic and paleontologic data. Here we use detailed surface stratigraphy, subsurface lithologic information, and tetrapod biostratigraphy to demonstrate that the oldest strata in PEFO are no lower stratigraphically than the top of the Bluewater Creek Formation and thus are of latest Carnian (Adamanian) age.

Surface Stratigraphy

The Petrified Forest National Park is well-known for its extensive badlands, which afford numerous opportunities to measure detailed stratigraphic sections. In the course of various studies, we have developed an extensive database of more than fifty measured sections. Of these, several are pertinent to discussion here. In particular, we have measured detailed sections near Newspaper Rock, from the Haystacks to Blue Mesa, and in the vicinity of the Rainbow Forest Museum.

The Blue Mesa Member has long been thought to be the stratigraphically lowest unit exposed in the park (Roadifer, 1966; Billingsley, 1985a,b; Ash, 1987; Lucas, 1993). However, we follow Cooley (1957) in identifying the Bluewater Creek Formation ("lower red member" of Cooley) as the lowest unit exposed in the park, based on our Newspaper Rock section and the type Blue Mesa Member section of Lucas (1993, 1995) (Figure 1). This effectively constrains the oldest beds in the park to a latest Carnian age (Lucas, 1993).


Figure 1—Correlated measured sections in the vicinity of Blue Mesa, the Teepees, and Newspaper Rock in PEFO. Note that the lowest exposures are of the uppermost Bluewater Creek Formation. Sections are located as follows: (A) SW1/4 SW1/4 SE1/4 sec. 21; (B) SW1/4 NW1/4 SW1/4 NE1/4 sec. 22; (C) SE1/4 NW1/4 SW1/4 sec. 23; (D) E1/2 SE1/4 SE1/4 SW1/4 sec. 23; (E) SE1/4 SE1/4 NW1/4 sec. 6; all in T18N, R24E.

Unit 3 of the Newspaper Rock section (Figure 1) is a bed of tuffaceous, highly micaceous sandstone 4.6 m thick (equivalent to unit 2 of Lucas [1993] type Blue Mesa Member type section) at the base of the Blue Mesa Member. This arenite/wacke interval is lithologically similar to the basal Blue Mesa Member in the vicinity of Fort Wingate, New Mexico, where approximately 5 m of ashy sandstone lie conformably on bentonitic mudstone "red beds" of the Bluewater Creek Formation (Lucas and Hayden, 1989; Heckert and Lucas, 1996; Lucas et al., 1997; Anderson et al., 1997). In both cases, this sandstone is muddy and/or ashy, micaceous, and poorly sorted. Near Fort Wingate, the basal Blue Mesa sandstones display more bedforms, particularly very low-angle trough crossbeds (see Chapter 4), but the bed in PEFO is similar to this in the vicinity of the Teepees, where it is approximately 12.9 m below the Newspaper Rock Bed.

Based on overall lithologic similarity to basal Blue Mesa Member outcrops elsewhere, we interpret this bed as the base of the Blue Mesa Member in PEFO. This lithology is unlike any found in units that underlie the Blue Mesa anywhere else in eastern Arizona or western New Mexico (Heckert, 1997). The pale purple and grayish red purple bentonitic mudstones below it belong to a different stratigraphic unit, either the Mesa Redondo Formation (Cooley, 1958, 1959) or the Bluewater Creek Formation (Lucas and Hayden, 1989). Lithologically, the red (grayish red purple and grayish red) mudstones below this sandstone suggest that the Blue Mesa Member is underlain here by the Bluewater Creek Formation, as the Mesa Redondo Formation is generally much sandier (Cooley, 1958; Stewart et al., 1972). Preliminary clay mineral analysis indicates that these strata also closely resemble the slightly bentonitic (proportionately less mixed layer smectite/illite) mudstones typical of the Bluewater Creek Formation and are unlike the highly bentonitic mudstones that typify the Blue Mesa Member in this outcrop belt (Heckert, 1997). These deposits more closely resemble the colors and lithologies of the Bluewater Creek Formation and represent, together with similar deposits immediately to the south and west of the park, the westernmost outcrops of this unit.

However, Dubiel et al. (1995) and Hasiotis and Dubiel (1995) identify some of these same strata as pertaining to the "Moenkopi Formation," by which they mean the "mottled strata." Dubiel et al. (1995) and Hasiotis and Dubiel (1995) however, represent an anomalous usage of this informal unit, that runs contrary to the description of either the "mottled strata" or the Moenkopi Formation as designated and utilized by Stewart et al. (1972) and used by most subsequent workers. The term "mottled strata" is used by most to refer to color-mottled paleosols and other deposits that represent isolated deposition and paleosol formation during the development of the pre-Chinle Tr-3 unconformity of Pipiringos and O'Sullivan (1978). As such, it is used exclusive of the term Moenkopi Formation, even though it is possible that some mottled strata may represent pedogenically modified Moenkopi Formation sediments.

In PEFO, the bioturbated mudstones near the base of the section are lithologically distinct from "mottled strata" on the rest of the Colorado Plateau. True "mottled strata" are generally limy to siliceous and form distinct porcellanite beds such as those at Fort Wingate and Bluewater, New Mexico (Stewart et al., 1972; Lucas and Hayden, 1989; Heckert and Lucas, 1996). Many outcrops of the mottled strata clearly represent pedogenically modified conglomerates, with numerous pebble and cobbles of quartzite and chert and occasional relict trough crossbedding. We have not observed these lithologies anywhere in the park. Rather, in the vicinity of the Teepees, and indeed, throughout exposures of the Blue Mesa Member in the southern portion of the park, bentonitic mudstones locally display a variety of reduction spots, color-mottling and vertical coloration changes long interpreted as representing paleosol development on floodplains associated with fluvial systems in the Blue Mesa Member (Cooley, 1959; Kraus and Middleton, 1987). However, this alteration is limited to fine-grained sediments of the Blue Mesa Member. Thus, we do see some paleosol development in the Blue Mesa Member, but no evidence of the extensive bioturbation, color mottling, and silicification characteristic of the "mottled strata" and the presence of tuffaceous sandstones and bentonitic mudstones throughout PEFO demonstrates that all of the strata contain abundant volcanic detritus, which is absent in the "mottled strata" (Stewart et al., 1972; Lucas and Hayden, 1989; Hasiotis and Dubiel, 1993) and uncommon in the upper Moenkopi Formation (McKee, 1954; Lucas and Hayden, 1989).

A cursory look at any large-scale geologic map of the region surrounding PEFO (e.g.,Wilson et al. 1960) indicates that the Moenkopi-Chinle contact must lie between PEFO and the town of Holbrook approximately 20 miles (32 km) to the west. Therefore, we investigated this issue by examining outcrops of the Chinle Group west of the southern entrance of the park. Here we observed several outcrops of lower Chinle Group strata and attempted to extrapolate our interpretations into subsurface of the southern PEFO, where there is additional well control (Harrell and Eckel, 1938).

Approximately 9.6 km WNW of the southern PEFO headquarters a series of low outcrops consists of (ascending) Moenkopi, "mottled strata," Shinarump, and Bluewater Creek Formations. Just to the ESE and slightly higher topographically are outcrops of the Blue Mesa Member of the Petrified Forest Formation. All beds are essentially flat-lying. Moenkopi strata at point A (Figure 2) consist of grayish red siltstones and sandstones. Overlying "mottled strata" are thin (1.5 m), limy to siliceous, pedogenically modified silt- and sandstones. Above the "mottled strata" are 1.6 m of well-indurated, trough-crossbedded, quartzose sandstones with pebble- to cobble-sized, siliceous conglomerate typical of the Shinarump Formation. At point B are greenish-gray and blue bentonitic mudstones typical of the lower Bluewater Creek Formation as described by Lucas and Hayden (1989) and Heckert and Lucas (1996). Outcrops between B and C are mostly covered, but at point C, bentonitic, pedoturbated mudstones and ash-rich silty sandstones typical of the Blue Mesa Member crop out, thus demonstrating the superposition of the Moenkopi, mottled strata, Shinarump, Bluewater Creek, and Petrified Forest Formations typical of these strata in western New Mexico (Lucas and Hayden, 1989; Heckert and Lucas, 1996).

Local subsurface information from the two wells discussed below indicates that perhaps 35 m of additional Chinle strata underlie the lowest mudstone unit exposed in the park before contact with the Shinarump Formation (Harrell and Eckel, 1939). Therefore, we consider the lowest grayish red and reddish purple mudstones exposed in PEFO to belong to the Bluewater Creek Formation.



Figure 2—Cross-section of Triassic strata near the southern edge of PEFO. The Petrified Forest well reported by Harrell and Eckel (1938) provides stratigraphic control at the east (right) edge of the cross section. Note that mottled strata and the Moenkopi Formation are at least 120 ft (35 m) below the surface of PEFO. All outcrop control points surface checked. Synclinal structure probably the result of dissolution collapse in underlying (Permian) rocks.

Subsurface Stratigraphy

No recent subsurface stratigraphic information is known from within the park, so we draw principally on water-well logs published by Harrell and Eckel (1939). Two wells drilled near the rail lines at Adamana west of PEFO indicate that, after drilling through approximately 15 m of alluvium and fine-grained Chinle sediments, sandstones and conglomerates pertaining to the Shinarump Formation were encountered (Harrell and Eckel, 1939, p. 100). Adamana is approximately 10-20 m lower in elevation than the base of our Newspaper Rock Bed type section. This yields a minimum of 25-30 m of Chinle strata below the Blue Mesa Member (Figure 3), a number more consistent with the Mesa Redondo Formation at its type locality (Cooley, 1958) than the Bluewater Creek Formation. However, the dominance of fine-grained lithologies in this region indicates that this may just be a relatively thin section of the Bluewater Creek Formation, which is normally 60 m thick (Lucas and Hayden, 1989; Heckert and Lucas, 1996). One of the wells may have encountered mottled strata ("blue shale") 10 m below the top of the Shinarump Formation.

A well drilled at the south park headquarters penetrates the Sonsela Member and approximately 90 m of fine-grained strata before the logger, park naturalist M.V. Walker, interpreted a Shinarump contact (Harrell and Eckel, 1939, p. 94). This figure and the fine-grained nature of the strata penetrated _ "shales" and "sandy shales" _ are consistent with a similar interpretation of a thick Blue Mesa Member with an underlying thin Bluewater Creek Formation (Figure 2). It is not clear on what grounds Walker identified the Shinarump. Perhaps he did so because at this level the well produced water, and the Shinarump is the first likely aquifer to be encountered (Harrell and Eckel, 1939). Therefore, we present these data much more tentatively than those of the Adamana wells.

Tetrapod Biochronology



Figure 3—Correlation of well information of Harrell and Eckel (1938) with the Newspaper Rock section. Note that mottled strata and Moenkopi Formation sediments are well below the surface of the PFNP.


Figure 4—Comparison of the stratigraphic interpretations of Dubiel et al. (1995) and this paper in a schematic stratigraphic column. Subsurface information follows that in Figures 2 and 3.

Two distinct vertebrate fossil assemblages have long been recognized in PEFO: a late Carnian assemblage and a younger, Norian assemblage (Gregory, 1957; Long and Padian, 1986; Murry and Long, 1989; Murry, 1990; Lucas, 1993, 1995; Lucas and Hunt, 1993; Hunt and Lucas, 1995; Long and Murry, 1995). These assemblages were collected from narrow stratigraphic intervals below and above the Sonsela Member. Other fossils from PEFO used for biochronology include palynomorphs, megafossil plants, and calcareous microfossils (Lucas, 1993).

Gregory (1957) differentiated two vertebrate fossil assemblages in PEFO but was unable to assign either to a stage within the Late Triassic. Murry and Long (1989), Lucas (1993, 1995), and Long and Murry (1995) noted that a Norian vertebrate assemblage is present above the Sonsela, whereas a Carnian assemblage has been found below. Palynology (Litwin et al., 1991), plant megafossils (Ash, 1980, 1987b), and calcareous microfossils (Kietzke, 1989; Lucas and Kietzke, 1993) indicate that the unconformity between the Sonsela and its correlatives and the underlying units approximates the Carnian-Norian boundary. Further biochronological studies by Lucas and Hunt (1993) and Hunt and Lucas (1995) assigned these faunas to the Adamanian and Revueltian land-vertebrate faunachrons, of latest Carnian and early- to mid-Norian age, respectively.

Conclusions

We use lithologic, stratigraphic, subsurface, and biochronologic information to demonstrate that there are no outcrops of the Moenkopi Formation, Shinarump Formation, or "mottled strata" in PEFO, contrary to the arguments made by some earlier workers. Figure 4 shows our interpretation of the available evidence, and how this interpretation differs from the stratigraphic interpretations advocated by Dubiel et al. (1995). Abundant surface, subsurface, and biochronologic evidence indicates that the stratigraphically lowest exposed strata in PEFO pertain to the top of the Bluewater Creek Formation, which crops out at the base of mudstone-dominated slopes in the vicinity of Newspaper Rock, the Teepees, and the Haystacks. Biochronologic evidence indicates that these strata are of latest Carnian age. Correlation from nearby outcrops to wells in and near PEFO demonstrates that at least 25-30 m of Chinle Group sediments underlie the park (Figure 4). Therefore, we reaffirm that the oldest strata in the park are latest Carnian.

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.

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