In the 1997 field season a locality containing abundant vertebrate fossils was located within the Wahweap Formation within Bryce Canyon National Park. The locality has yielded fossils of mammals, dinosaurs, turtles, crocodilians, fish, and herptiles as well as invertebrates such as gastropods and ostracods. The mammals are more primitive than those recovered from the Kaiparowits Formation, and sharks and rays are abundant as they are in the type area of the Wahweap Formation. We have also significantly increased sampling of the uppermost Cretaceous rocks on the Paunsaugunt Plateau and the additional material confirms the distinct differences between this fauna and that of the Wahweap Formation. This data strongly suggests that the uppermost Cretaceous rocks along the southern margin of the Paunsaugunt Plateau and Bryce Canyon National Park are equivalent to the Kaiparowits Formation.
Figure 1Map of the Paunsaugunt Plateau area showing the major faults, distribution of undifferentiated Cretaceous formations (Ku), the Claron Formation (Cl), Tertiary basalts and other volcanics (Tb), Quaternary alluvium (Qa), and the area from which most of the Cretaceous vertebrates have been recovered from the uppermost Cretaceous rocks (indicated by the "1") and from the Wahweap Formation (indicated by the "2") (modified from Eaton, 1993a). Outline of Bryce Canyon National Park is approximate.
Bryce Canyon National Park is situated along the eastern margin of the Paunsaugunt Plateau, southwestern Utah. The plateau is bounded to the east by the Paunsaugunt fault system and to the west by the Sand Pass and Sevier fault systems (Figure 1) (Eaton et al., 1993). The southern margin of the plateau is an erosional scarp formed above the Skutumpah Terrace. The top of the plateau is formed by the pink and white resistant caprocks of the Claron Formation of Early Tertiary age.
Below the colorful cliff-forming rocks of the Claron Formation is a problematic series of Upper Cretaceous rocks. The Paunsaugunt Plateau contains less than half the thickness (approximately 900 m, Figure 2) of Cretaceous rocks that are present immediately to the east on the Kaiparowits Plateau (approximately 2000 m) (Eaton et al., 1993). Critical to interpretation of regional history and tectonics is why the Cretaceous sequence on the Paunsaugunt Plateau is so much thinner than that of the Kaiparowits Plateau.
The presence of the Dakota Formation, Tropic Shale, and the Tibbet Canyon, Smoky Hollow, and John Henry Members of the Straight Cliffs Formation has been documented on the Paunsaugunt Plateau (Gregory, 1951; Eaton, 1993a; 1993b; Eaton et al., 1993). Difficulties arise in interpreting the equivalency of the uppermost Cretaceous rocks on the Paunsaugunt Plateau. Gregory (1951) was unable to distinguish the Straight Cliffs Formation from the Wahweap Formation and mapped them as a single unit (Ksw). Gregory (1951) placed a conglomerate high in the Cretaceous section at the base of the Kaiparowits Formation and considered it to lie unconformably on top of the combined Straight Cliffs-Wahweap formations (Ksw). Overlying the conglomerate are gray and variegated mudstones and sandstones that Gregory included, along with the basal conglomerate, in the Kaiparowits Formation. Doelling and Davis (1989) also maintained that the Kaiparowits Formation is present on the Paunsaugunt Plateau (and to the west) and suggested that the Straight Cliffs Formation is very thin (30-80 m) on the plateau but provided no justification for this hypothesis.
Bowers (1990) and Tilton (1991) suggested that the conglomerate did not represent an unconformity at the base of the Kaiparowits Formation, but rather the uppermost member of the Straight Cliffs Formation, the Drip Tank Member, which can be conglomeratic on the Kaiparowits Plateau (Eaton, 1991). Both Bowers (1990) and Tilton (1991) suggested that the beds immediately overlying the conglomerate belong to the lower part of the Wahweap Formation rather than to the Kaiparowits Formation. This is certainly the most parsimonious explanation as it requires only a single erosional unconformity prior to deposition of Tertiary rocks.
Goldstrand (1994), Goldstrand et al. (1993), and Eaton et al. (1993) suggested that the Kaiparowits Formation may be present on the plateau based on comparisons of sandstone petrology between the uppermost rocks of the Paunsaugunt and Kaiparowits plateaus. Eaton et al. (1993) suggested several possible scenarios that might preserve a remnant of the Kaiparowits Formation on the Paunsaugunt Plateau. All of these scenarios require a major unconformity within the Cretaceous sequence on that plateau that is either absent or undetected on the adjacent Kaiparowits Plateau.
An attempt to compare the mammalian fauna recovered from the uppermost Cretaceous rocks of the Paunsaugunt Plateau to faunas from the Wahweap and Kaiparowits formations on the Kaiparowits Plateau yielded equivocal results. Eaton (1993a) tentatively suggested that the fauna recovered from the uppermost Cretaceous rocks of the Paunsagunt Plateau correlated more closely to the fauna of the Kaiparowits Formation rather than that of the Wahweap Formation. There was some concern at the time as to whether the fauna might represent a more upland facies of the Wahweap Formation than had been sampled on the Kaiparowits Plateau.
Figure 2A preliminary interpretation of a generalized stratigraphic section on the Paunsaugunt Plateau (see Eaton et al., 1993, and Eaton, 1993a, for other interpretations) compared to the section on the adjacent Kaiparowits Plateau (based on Eaton, 1991). Only the stratigraphic position of localities within the Kaiparowits(?) and Wahweap formations of the Paunsaugunt Plateau are indicated.
Vertebrate localities are common throughout the Cretaceous sequence on the Paunsaugunt Plateau; however, access is extremely limited due to localities occurring on cliff faces far from roads. As a result, few localities have been screen-washed for vertebrates.
Vertebrates have been recovered from the Dakota Formation along the western margin of the plateau near the town of Alton and the mammals recovered from that locality (Museum of Northern Arizona Locality 939) have been described in Eaton (1993b, 1995).
No vertebrates have been recovered from the Smoky Hollow Member of the Straight Cliffs Formation around the margins of the plateau, but vertebrates have been recovered on the nearby Kaiparowits Plateau (Cifelli, 1990a; Eaton, 1995). Fossils are relatively common from stratigraphically higher rocks variously interpreted to represent the Straight Cliffs or Wahweap formations, particularly in the area of Bryce Canyon National Park (Eaton, 1994). There is no road access to any of these localities as most are high on roadless cliff faces. As such, surface collection of crocodilian, dinosaur, and turtle scrap are common, but small, biostratigraphically useful materials have not been recovered until recently. A single test screen-washing of a small sample from a locality within Bryce Canyon National Park yielded microvertebrate fossils including a partial upper molar of a marsupial (Eaton, 1994). During the 1997 field season, a locality was discovered in the Wahweap Formation within Bryce Canyon National Park (Utah Museum of Natural History Vertebrate Paleontology (=UMNH VP) Locality 77). This locality is considered to represent the Wahweap Formation rather than the Straight Cliffs Formation because there is no evidence of the brackish water or coal deposits that characterize the Straight Cliffs Formation, the mudstone sequences (floodplain deposits) are proportionally thicker relative to sandstones as in the Wahweap Formation in its type area on the Kaiparowits Plateau, a thick conglomerate that appears to be equivalent to the Drip Tank Member of the Straight Cliffs Formation is present at the base of the section, and the area of the locality (Campbell Canyon) was mapped as the Wahweap Formation by Bowers (1990).
Eighteen sacks of matrix were hauled by backpack from this locality and processed. At the time of this writing most of the concentrate has been picked, but there has been relatively little taxonomic study. The material recovered to date includes the teeth of mammals (a pediomyid marsupial and a multituberculate with teeth smaller than, but morphologically similar to, those of Cimolodon similis), dinosaurs, sharks, rays, fragmentary jaws and postcrania of herptiles, and shell material of turtles (Table 1). These fossils will be accessioned into the collections at the Utah Museum of Natural History.
The vertebrate fauna described by Eaton (1993a) is from the top of the plateau in an erosional window cut through the Claron Formation into the uppermost Cretaceous rocks (Figure 1). Approximately 2,000 kg of matrix were processed for microvertebrates during the 1988-1989 field seasons. The fauna includes material of dinosaurs, herptiles, and mammals, which are housed at the Museum of Northern Arizona, Flagstaff (mostly from MNA localities 1073 and 1074). An additional 4,000 kg was processed during the 1996-1997 field seasons and will be accessioned into the collections at the Utah Museum of Natural History in Salt Lake City (mostly from UMNH VP Locality 83, the same locality as MNA 1073, but also from two new localities UMNH VP Locs. 61 and 84). Much of this new material remains to be picked and studied.
The material discovered to date from this uppermost Cretaceous unit does not include a single shark or ray tooth. The turtle Compsemys is relatively abundant as are ceratopsian teeth. Only the mammals have been studied in detail and the list presented in Table 2 is essentially the same as that presented in Eaton (1993a). Differing from the original faunal list is the question mark preceding Cimexomys gregoryi. Eaton (1995) noted the similarity of this taxon to Bryceomys described from the Smoky Hollow Member of the Straight Cliffs Formation. More material of the Paunsaugunt Plateau taxon would be required to confirm this synonymy. An M2 recently recovered strongly suggests the presence of Bryceomys in the fauna and compares closely to MNA (Museum of Northern Arizona) 7042 from the Kaiparowits Formation.
Faunal Age and Correlation
The material from the new Wahweap locality is
consistent with previous known localities from the Wahweap
Formation of the Kaiparowits Plateau (see Eaton, Cifelli et al.,
in press). Sharks and rays are abundant. The rays are similar
to the species of Myledaphus from the Kaiparowits
Formation but appear to be more primitive. There is another taxon of
ray informally referred to as "smile button" which is known
from the Wahweap Formation, but not the Kaiparowits. The
turtle Compsemys is present, but rare relative to other turtles,
whereas Compsemys is a common element of Kaiparowits faunas.
The mammals are similar to those known from the Wahweap,
particularly the specimen of Cimolodon. The single
pediomyid tooth is a deciduous fourth premolar and compares almost
identically to a tooth recovered from the uppermost
Cretaceous beds on the Markagunt Plateau interpreted to be either
latest Santonian or earliest Campanian in age (Eaton, Diem et
al., in press). Nothing like this odd pediomyid tooth is
known from the Kaiparowits Formation.
Table 1Vertebrates from the Wahweap Formation (UMNH VP Loc. 77), Bryce Canyon National Park.
cf. Myledaphus sp.
cf. Ischyriza sp.
Lepisosteus or Astracosteus sp.
Suborder Theropoda indet.
Family Hadrosauridae indet.
cf. Mesodma sp.
Family Pediomyidae indet.
cf. Alphadon sp.
Amia or Melvius sp.
Mesodma sp., cf. M.
Mesodma sp., cf. M.
Cimolodon sp., cf. C.
Suborder and Family
Alphadon sp., cf. A.
Alphadon sp., cf. A.
Alphadon sp., cf. A.
Turgidodon sp. indet.
cf. Turgidodon sp.
Eaton (1993a) considered the fauna from the uppermost Cretaceous rocks of the Paunsaugunt Plateau to be more closely equivalent to Kaiparowits (most closely correlative to the Judithian Land-Mammal "Age") than to Wahweap (most closely correlative to the Aquilan Land-Mammal "Age") faunas. The species of Alphadon and Mesodma appear to be even younger than those known from the Kaiparowits Formation. Unfortunately, many of these species are based primarily on size and it is now clear (see Eaton, Diem et al., in press) that species of Alphadon and Mesodma appear to have had a wide range of sizes throughout the Late Cretaceous and that size alone is not a reliable taxonomic guide. Two new specimens (M1s) of Mesodma (UMNH VP 6789, 6794) compare closely to specimens from the Kaiparowits Plateau specimens (MNA V5291 and V7525 respectively) and are similar to M. hensleighi, also supporting correlation to the Kaiparowits Formation. Turgidodon was described from the Kaiparowits Formation by Cifelli (1990b) and is unknown from the Wahweap Formation. Its presence strongly supports correlation of the fauna to that of the Kaiparowits Formation. A recently recovered upper molar that is Turgidodon-like (but lacks stylar cusp C; B and D are subequal) compares closely to an undescribed Kaiparowits Formation molar (Oklahoma Museum of Natural History (=OMNH) 23320). This indicates a diversity of Turgidodon-like taxa in the uppermost Cretaceous of the Paunsaugunt Plateau which would not be expected if this unit represented the Wahweap Formation.
The presence of Symmetrodontoides foxi originally argued strongly against correlation to Kaiparowits faunas as symmetrodonts were completely unknown from the Kaiparowits Formation and the type of this species was recovered from the Wahweap Formation. However, a single symmetrodont tooth (probably Symmetrodontoides) has now been reported from the Kaiparowits Formation (Eaton, Cifelli et al., in press).
The complete absence of sharks and rays suggests the
seaway was well to the east. The sea was at its greatest
distance from the area (during the time represented by Cretaceous
strata on the Kaiparowits Plateau) during deposition of
the Kaiparowits Formation and sharks are unknown from
that formation except from near the base. Teeth of ceratopsian
dinosaurs are relatively common in the uppermost
Cretaceous beds of the Paunsaugunt Plateau. Although ceratopsians
have an older record (Wolfe et al., 1997), they are not found commonly in this region until after deposition of the Wahweap Formation. Also the occurrence of the salamander Habrosaurus from the uppermost beds on the Paunsaugunt Plateau suggests a Late Campanian age (Kaiparowits equivalence) as this salamander is not known from older rocks.
So in the years that have followed the original description of the fauna from the uppermost Cretaceous beds of the Paunsaugunt Plateau (Eaton, 1993a), the age of this fauna still remains somewhat equivocal but the specimens recently recovered more strongly indicate an equivalence of this fauna to that of the Kaiparowits Formation .
In the 1997 field season we resampled the uppermost Cretaceous localities on the Paunsaugunt Plateau and plan to increase our sampling of the Wahweap Formation in Bryce Canyon National Park over the next several field seasons. It is hoped that this continued work will provide final resolution to the question of the presence or absence of the Kaiparowits Formation on the Paunsaugunt Plateau.
The thinning of the Cretaceous section across the Paunsaugunt Plateau remains somewhat of a mystery. The thinning occurs 130-150 km from the thrust belt and may reflect a short wavelength forebulge associated with a zone of crustal weakness (Eaton et al., 1997); however, this is only one of many possible hypotheses and further work is needed.
A new locality from the Wahweap Formation of Bryce Canyon National Park has produced a fauna typical of the Wahweap to the east on the Kaiparowits Plateau. The fauna contains abundant sharks and rays, mammals, and other taxa consistent with an interpretation of an Early Campanian age (approximately correlative to the Aquilan Land Mammal "Age"). Most of the vertebrates previously recovered from the Paunsaugunt Plateau were from the stratigraphically highest Cretaceous rocks on the plateau. This fauna appears to be correlative to those of the Kaiparowits Formation (approximately correlative to Judithian Land Mammal "Age" ) based on comparison of the mammalian and herptile taxa to those from the Kaiparowits Formation, the occurrence of abundant ceratopsian teeth along with the common occurrence of the turtle Compsemys, and the absence of sharks or rays. Continued work on these sequences should produce final resolution of this problem over the next few years.
The National Park Service, particularly Richard Bryant, and the Bryce Canyon Natural History Association under the guidance of Gayle Pollock, have aided our research within Bryce Canyon National Park. The U.S.D.A. Forest Service, particularly Marion Jacklin and Carlton Guillette, have been very helpful in all aspects of our research on the Paunsaugunt Plateau. Jared Morrow, Patrick Goldstrand, Terry Tilton, William Bowers, and Ed Sable have all made contributions to my understanding of the plateau. The Petroleum Research Fund of the American Chemical Society (30989-GB8) and the Bryce Canyon Natural History Association are funding our current work on the plateau.
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