SPENCER G. LUCAS
New Mexico Museum of Natural History, 1801 Mountain Road NW, Albuquerque, NM 87104
Abstract Using the wealth of research data on the stratigraphy, sedimentology and paleontology of the Upper Triassic strata exposed at Petrified Forest National Park, Arizona, artist Mary Sundstrom undertook three landscape restorations of the Park during the Late Triassic, representing approximately 225, 220 and 215 million years ago. They are intended to be used in exhibition and other interpretive information for visitors to the Park.
Upper Triassic rocks exposed in Petrified Forest National Park (PEFO) preserve a record of riverine, channel and floodplain environments and the extinct biotas that inhabited them between approximately 225 and 215 million years ago. For decades, scientific researchers have studied the diverse aspects of the geology and paleontology of the Upper Triassic strata exposed at PEFO. It is thus possible to reconstruct with some accuracy the depositional environments and biota of PEFO during 15 million years of Late Triassic history. To do so, artist Mary Sundstrom, under my direction, drew three restorations, each corresponding to a time slice during this 15 million-year-long history. This article presents those drawings and briefly reviews their basis.
The three drawings (Figs. 1-3) are full-color restorations executed as hand-drawn originals that are now at PEFO. The drawings show the restored landscape, and, in cross section, the buried layers of Upper Triassic sediment (scaled to actual thicknesses). Each drawing thus is a cut-away, three-dimensional figure. The stratigraphic perspective of the drawings is cumulative, so that the Painted Desert cross section (Fig. 3) encompasses the entire section of rock deposited at the PEFO during the interval 215-225 million years ago.
The oldest Triassic rocks exposed at PEFO are color-banded (gray, purple, blue and red) mudstones and thin lenses of gray and tan sandstone. These strata are approximately 225 million years old and are best exposed at the Teepees area in the southern part of PEFO (Lucas, 1993; Heckert and Lucas, 1995b). These rocks represent part of a vast, muddy tropical floodplain (Fig. 1) (e.g., Dubiel, 1989). Small rivers meandered across this landscape, heading northwestward to a seashore that lay near the present Utah-Nevada border. Large plant eaters, such as the cow-sized mammal-like reptile Placerias, browsed along the river banks.
BLUE MESA TIME
About 220 million years ago, when the rocks that cap Blue Mesa (Sonsela Sandstone) in the southern part of the PEFO were deposited, the muddy floodplain was replaced by a wide braidplain crisscrossed by large, gravelly-bedded rivers (Fig. 2) (e.g., Dubiel, 1989). This change probably was due to a slowing of the rate at which the crust was subsiding in western North America during the Late Triassic (Blakey and Gubitosa, 1984). Large trees grew along the river courses, and their fossilized stumps and logs became the Park's famous petrified forests (Heckert and Lucas, 1995a). Plant-eating reptiles, such as the heavily armored Desmatosuchus, foraged on the forest floors.
PAINTED DESERT TIME
About 215 million years ago, when the red-bed mudstones and sandstones in the Painted Desert formed, the Triassic river system had again changed. Now, it was again a vast muddy floodplain (E.g., Dubiel, 1989). But, unlike Teepees time, some 10 million years earlier, huge rivers meandered across this floodplain, and tall trees lined the watercourses (Fig. 3). These trees became the logs of the Black Forest seen near Kachina Point. Crocodile-like phytosaurs, some as much as 7 meters long, hunted along the river banks.
The drawings of restored Late Triassic landscapes in PEFO depict three times during the evolution of the Late Triassic river system and biota. They are intended to be used in exhibition and other interpretive information for visitors to PEFO.
PEFO financially supported the production of the drawings. Mary Sundstrom executed the artwork, and Daniel Weismann provided computer support. Three anonymous reviewers provided helpful comments on an earlier version of the manuscript.
Figure 1. Restoration of the Late Triassic landscape at Petrified Forest National Park during "Teepees Time," about 225 million years ago.
|Figure 2. Restoration of the Late Triassic landscape at Petrified Forest National Park during "Blue Mesa Time," about 220 million years ago.|
|Figure 3. Restoration of the Late Triassic landscape at Petrified Forest National Park during "Painted Desert Time," about 215 million years ago.|
Blakey, R.C., and R. Gubitosa, 1984. Controls on sandstone body geometry and architecture in the Chinle Formation (Upper Triassic), Colorado Plateau. Sedimentary Geology, 38: 51-86.
Dubiel, R.F., 1989. Depositional and climatic setting of the Upper Triassic Chinle Formation, Colorado Plateau, 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. 171-187.
Heckert, A.B., and Lucas, S.G., 1995a. Stratigraphic distribution and age of petrified wood in Petrified Forest National Park, Arizona, in Santucci, V. L., and McClelland, L., eds., National Park paleontological research: Geologic Resources Division Technical Report NPS/NRGRD/GRDTR-98/01, p. 125-129.
_____, and _____, 1995b. The oldest Triassic strata exposed in the Petrified Forest National Park, Arizona, in Santucci, V. L., and McClelland, L., eds., National Park paleontological research: Geologic Resources Division Technical Report NPS/NRGRD/GRDTR-98/01, p. 129-134.
Lucas, S.G., 1993. The Chinle Group: Revised stratigraphy and biochronology of Upper Triassic nonmarine strata in the western United States. Museum of Northern Arizona Bulletin, 59:27-50.