OCCURRENCES OF ZAMITES POWELLII IN
OLDEST NORIAN STRATA IN
PETRIFIED FOREST NATIONAL PARK, ARIZONA
1
2
ALISA S. HERRICK1, DAVID E. FASTOVSKY1, AND
GREGORY D. HOKE2
Department of Geosciences, University of Rhode Island, 8 Ranger Road, Kingston, 02881
Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology,
77 Massachusetts Avenue, Building 54-1116, Cambridge, 02139
____________________
ABSTRACT—Zamites powellii is one of the most common fossil leaves found in Carnian sediments of
the Upper Triassic Chinle Formation and Dockum Group. It is rarely found in Norian sediments. Here,
however, we report a Norian occurrence in Petrified Forest National Park, Arizona. Z. powellii was found
in channel-fill deposits up to 5 m above the Sonsela Sandstone, generally considered to be the
Carnian/Norian boundary in the Park. The geology of this locality highlights the reasons why plant
assemblages are rarely found in Norian deposits: preservational biases limit the abundance of the fossils
and a lack of stratigraphic control across the Carnian/Norian boundary makes age identification uncertain.
____________________
INTRODUCTION
amites powellii provides one of the most common fossil leaves, if not the most
common fossil leaf, found in the lower part (Carnian) of the Upper Triassic Chinle
Formation and Dockum Group in the southwestern United States (Daugherty, 1941;
Ash, 1967, 1972a, 1974, 1975, 1978). The fossil is found in the Eoginkigites and
Dinophyton floral zones (Litwin et al., 1991) which, in Petrified Forest National Park
(PEFO), Arizona, occur in and below the Sonsela Sandstone (Ash, 1967, 1980). The
Sonsela Sandstone is a widely recognized marker bed throughout the Park, and is
believed to approximate the Carnian/Norian boundary (Litwin, 1986; Litwin et. al.,
1991). With few exceptions, Z. powellii only occurs below it (Ash, 1975). The absence of
Z. powellii in Norian sediments may be attributed to preservational biases that have
limited the abundance of the fossils and/or a lack of stratigraphic control across the
Carnian/Norian boundary resulting in localities with an uncertain age affiliation. It is
these few exceptional localities that are discussed here.
Z
KNOWN LOCALITIES
Carnian localities of Z. powellii in the western United States are known from the
lower part of the Chinle Formation in western New Mexico, Arizona, Utah and the
Dockum Group of eastern New Mexico, western Texas and western Oklahoma
(Daugherty, 1941; Ash, 1975, 1988). In the Carnian sediments of the Chinle Formation
fossils have come from the Sonsela and Poleo Sandstones, and the Lower Petrified
Forest, Monitor Butte, Mesa Redondo, and Shinarump Members (Ash, 1972b, 1974,
1980). In the Carnian sediments of the Dockum Group, fossils are known from the Santa
Rosa Sandstone (referred to as the Santa Rosa Formation, Ash, 1972a, 1988), as well as
the Trujillo and Tecovas Formations (Ash, 1980).
A few possible Norian localities of Z. powellii have been documented by Ash
(1970, 1972b, 1975). One locality is reported to be in the Upper Petrified Forest Member
(Norian) of the Chinle Formation in the southern end of PEFO at a locality called
"Walker's Stump" (Ash, 1970; Ash and Hevly, 1974; Walker, 1974). Two other
potentially Norian localities, both in the Dockum Group, were described by Ash (1975).
Near Santa Rosa, New Mexico, Z. powellii is in the base of the Chinle Shale Formation,
and at Boys Ranch in Texas, it is found in the Trujillo Sandstone (Ash, 1972a, 1975).
Z. powellii AT GATESY'S PLUNGE IN PEFO
Z. powellii (identified by S. Ash, personal commun., August 1998; Fig. 1) was
found during the summer of 1998 in the Gatesy's Plunge area (near Jasper Forest) in
PEFO (Fig. 2) in sediments that lie up to 5 m above the Sonsela Sandstone (Fig. 3).
Because the Sonsela Sandstone is considered to be the marker bed for the Carnian/Norian
transition in PEFO, the locality may be placed confidently within the Norian.
Figure 1---Photograph of Z. powellii from Gatesy's Plunge, PEFO. Penny is 1.8 cm.
Within PEFO, the Sonsela Sandstone is a highly recognizable and traceable unit
averaging between 10 and 12 m thick and is characterized as light gray-to-tan, fine- to
medium-grained, moderately sorted, subangular-to-subrounded, quartz arenite and
conglomerate with horizontal bedding, and tablular and trough cross-bedding (Elzea,
1983; Billingsley, 1985; Ash, 1987; Murry and Long, 1989). Typically, the sandstone
forms cliffs, but it is known to thin out and intertongue with sediments above and below
it (Billingsley, 1985; Murry and Long, 1989). This is the case with the Z. powellii
locality. The Sonsela Sandstone can be seen in its "typical" form as a prominent cliff,
capping mesas in the Jasper Forest area of PEFO. It can be physically traced from these
cliffs to a more modest sand body at the base of hills in the Gatesy's Plunge area. In the
region where stratigraphic section 4 was measured (Figs. 3 and 4) the Sonsela Sandstone
is a fine to very-fine grained reddish lithic arenite as described by Dott (1964). This is
one of the more inconspicuous manifestations of this sandstone in the Park.
Figure 2—PEFO indicating Gatesy's Plunge. Inset: Arizona. Modified from Billingsley (1985).
The plant material was found in an approximately 4 m thick unit of gray, silty
mudstone and siltstone interlayered with fine- to medium-grained, moderately to poorly
sorted lithic wacke (Fig. 3; stratigraphic section 1). Most of the carbonaceous material
found was disseminated fragments, but whole leaf impressions of Z. powellii were also
obtained (Fig. 1). The plant-bearing stratum can be traced laterally about 0.75 km, (see
stratigraphic section 2; Fig. 3), but no whole leaf impressions were found elsewhere in it,
only carbonaceous fragments and dark stained (presumably from carbon) rock.
The plant locality is in channel-fill sediments at the base of a +15 m deep
paleochannel (Herrick, in prep.). Preliminary work done on reconstructing the
paleoenvironment of the Gatesy's Plunge area indicates that deposition took place in a
meandering fluvial system (Herrick, in prep.). Taphonomic settings like those at the Z.
powellii locality are reported to favor plant preservation in the Chinle Formation (Demko
et al., 1998).
DISCUSSION AND CONCLUSIONS
"Carnian" plant fossils have been found in younger sediments (Daugherty, 1941;
Ash, 1972b, Ash, 1992) and one might expect in the future to find Z. powellii among
them. However, there are relatively few localities of leaf compressions reported above
Carnian sediments, and the age of these is only questionably referred to the Norian. These
two issues will be briefly examined.
Though the leaf assemblages that are found in the Norian are very similar to those
in the Carnian, they are considerably more rare (Ash, 1972b). This may be attributed to
preservational biases. Plant assemblages are concentrated in lower Chinle members
where they were preserved within incised valleys that were cut into the underlying
Triassic rock (Demko et al., 1998). In contrast, the upper Chinle members were deposited
across the fluvial plain that formed once the incised valleys were filled in (Demko et al.,
1998). Accordingly, as Ash (1972b) has noted, the apparent difference between the
abundance of plant assemblages in Carnian and Norian sediments may not be real. This
may simply be an artifact of different depositional systems and associated preservation,
as has been shown to occur elsewhere on the Colorado Plateau (Demko, et al., 1998). It
follows that if we look in Norian valley-fill and channel-fill sequences, we may indeed
find more floral assemblages.
Figure 3----Stratigraphic sections of Gatesy's Plunge showing lithostratigraphic correlation between the
Sonsela Sandstone (Section 4) and the Z. powellii locality (Section 1). Horizontal spacing not to scale.
A lack of stratigraphic control across the Carnian/Norian boundary has resulted in
uncertainty in assigning a Norian age to some localities. Two Z. powellii localities, the
Chinle Shale Formation locality near Santa Rosa New Mexico, and the Trujillo
Sandstone locality at Boy's Ranch in Texas, are questionably referred to the Norian (Ash
1972a, 1975).
Recent stratigraphic work does not shed light on the Chinle Shale Formation
locality. We have not found further reference to a "Chinle Shale Formation" of the
Dockum other than by Ash (1972a, 1975). Reeside et al. (1957) and Chatterjee (1986)
cite an unnamed shale member of the "Chinle" (quotations their's) Formation of the
Dockum Group. Lucas and Hunt (1989) recognized and renamed a lower (Carnian) and
an upper (Norian) shale member of the Chinle Formation of the Dockum Group, but it is
uncertain to which shale member Ash (1972a, 1975) refers. Therefore, the Chinle Shale
locality cannot unequivocally be considered Norian.
Figure 4—Topographic map of Gatesy's Plunge area in Jasper Forest with locations of stratigraphic
sections indicated. Specimen found in NE1/4 NW1/4 NW1/4 Section 20 T17N R24E. Contour interval 10
feet. Taken from USGS, Arizona, Agate House 7.5 min. quadrangle.
With respect to the Trujillo Sandstone, recent stratigraphic work (Ash, 1980,
Dubiel, 1994) correlates this unit with sediments just under the Sonsela Sandstone. These
studies suggest a late Carnian age for Ash's (1972a, 1975) Trujillo Sandstone locality.
As for the third possible Norian occurrence of Z. powellii, the Walker's Stump
locality in PEFO, more work needs to be done to clarify the stratigraphic position of these
fossils. The Walker's Stump locality is discussed by Ash (1972b) who only states that
"Otozamites powellii" is "in or above Sonsela Sandstone" (p. 27, table 1). This locality
has not been discussed since Ash's (1975) reassignment of the fossils found there from
Otozamites to Zamites. Ash maintains that the fossils are correctly assigned to Zamites
(S. Ash, personal communication, May 1999).
We were fortunate to find the PEFO locality at Gatesy's Plunge, because we can
place the PEFO Z. powellii locality within the context of an unambiguous local
stratigraphy. Based upon that stratigraphic position, Z. powellii from Gatesy's Plunge in
PEFO is undoubtedly of Norian age.
ACKNOWLEDGMENTS
We thank M. Hellickson, P. Quinn, W. Grether, P. Thompson and all the staff of
PEFO for their unstinting support of this work. Likewise we thank Dr. Sidney Ash for his
discussions, and willingness to look at the fossil and the site. M. F. Hilfinger, M. M.
Jones, and F. Therrien contributed to the work presented here. We are particularly
indebted to the Petrified Forest Museum Association for generously funding the field
work reported here. The work has also been partially supported by the URI Department
of Geosciences.
REFERENCES
Ash, S. R., 1967. The Chinle (Upper Triassic) megaflora of the Zuni Mountains, New
Mexico. New Mexico Geological Society Guidebook, 18th Annual Field
Conference. p. 125-131.
_____, 1970. Pagiophyllum simpsonii, a new conifer from the Chinle Formation (Upper
Triassic) of Arizona. Journal of Paleontology, 44:945-952.
_____, 1972a. Upper Triassic Dockum flora of eastern New Mexico and Texas. New
Mexico Geological Society Guidebook, 23rd Annual Field Conference. p. 124128.
_____, 1972b. Plant megafossils of the Chinle Formation. in Breed, C.S. and W.J. Breed
(eds.), Investigations in the Triassic Chinle Formation. Museum of Northern
Arizona Bulletin 47, p.23-43.
_____, (ed.), 1974. Guidebook to Devonian, Permian and Triassic plant localities, eastcentral Arizona. Paleobotanical Section of the Botanical Society of America 25th
Annual AIBS Meeting. 69p.
_____, 1975. Zamites powellii and its distribution in the Upper Triassic of North
America. Paleontographica, 149B:139-52.
_____, 1978. Plant megafossils. in S. R. Ash, (ed.), Geology, Paleontology, and
Paleoecology Of A Late Triassic Lake in Western NM. Brigham Young University
Geological Studies, vol. 25: 23-43.
_____, 1980. Upper Triassic floral zones of North America. in D. L. Dilcher, and T. N.
Taylor, Biostratigraphy of Fossil Plants Successional and Paleoecological
Analyses. Dowden, Hutchinson, and Ross, Inc, Stroudsburg, PA., p. 153-170.
_____, 1988. Fossil plants from the mudstone member of the Santa Rosa Formation of
Middle and Late Triassic age, Guadalupe County, New Mexico. in Finch, W. I.,
(ed.), Principal Reference Section for the Santa Rosa Formation of Middle and
Late Triassic Age, Guadalupe County, New Mexico, Geological Survey Bulletin,
p. 21-25.
_____, and R.H. Hevly, 1974. Road Log. in Ash, S. R. (ed.), Guidebook to Devonian,
Permian and Triassic plant localities, east-central Arizona. Paleobotanical Section
of the Botanical Society of America 25th Annual AIBS Meeting, p. 1-25.
Billingsley, G.H., 1985. General stratigraphy of the Petrified Forest National Park,
Arizona. Museum of Northern Arizona Bulletin 54: 3-8.
Chatterjee, S., 1986. The Late Triassic Dockum vertebrates: their stratigraphic and
paleobiogeographic significance. in Padian, K. ed., The Beginning of the Age of
Dinosaurs: Faunal Change Across the Triassic-Jurassic Boundary. Cambridge
University Press, p. 139-150.
Daugherty, L.H., 1941. The Upper Triassic flora of Arizona. Carnegie Institution of
Washington Publication 526, Washington, D.C. p. 108.
Demko, T.M., R.F. Dubiel,and J.T. Parrish, 1998. Plant taphonomy in incised valleys:
implications for interpreting paleoclimate from fossil plants. Geology 26:11191122.
Dott, R.H. Jr., 1964. Wacke, graywacke and matrix -what approach to immature
sandstone classification? Journal of Sedimentary Petrology, 34: 625-632.
Elzea, J., 1983. A petrographic and stratigraphic analysis of the Petrified Forest Member
(Triassic Chinle Formation) sandstones, Petrified Forest National Park, Arizona.
Unpublished B.S. thesis, University of California, Berkeley.
Herrick, A.S., (in prep). Telling time in the Triassic: aetosaur biochronology and Chinle
stratigraphy in Petrified Forest National Park, Arizona. M.S. thesis, University of
Rhode Island.
Litwin, R. J., 1986. The palynostratigraphy and age of the Chinle and Moenave
formations, southwestern United States. Unpublished Ph.D. dissertation,
Pennsylvania State University, 266p.
______, A. Traverse, and S.R. Ash. Preliminary palynological zonation of the Chinle
Formation southwestern U.S.A., and its correlation to the Newark Supergroup
(eastern U.S.A.). Review of Palaeobotany and Palynology 68: 269-287.
Lucas, S.P., and A.P. Hunt, 1989. Revised Triassic stratigraphy in the Tucumcari Basin,
East-Central New Mexico. in Lucas, S. G.and A.P. Hunt, (eds.), Dawn of the Age
of Dinosaurs in the American Southwest. New Mexico Museum of Natural
History, p. 140-148.
Murry, P.A., and R.L. Long, 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 S.G. Lucas, and A. P. Hunt (eds.),
Dawn of the Age of Dinosaurs in the American Southwest. New Mexico Museum
of Natural History, p. 29-64.
Reeside, J.B., et al., 1957. Correlation of the Triassic formations of North America
exclusive of Canada. Bulletin of the Geological Society of America, 68: 14511514.
Walker, M.V., 1974. Early scientific work in the Petrified Forest. in Ash, S. R. (ed.),
Guidebook to Devonian, Permian and Triassic plant localities, east-central
Arizona. Paleobotanical Section of the Botanical Society of America 25th Annual
AIBS Meeting, p. 51-56.