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Resting above the unconformity marking the top of the Moenkopi Formation throughout most of southeastern Utah is a layer of conglomerate and coarse sandstone up to several hundred feet thick. This gravelly material at the base of Chinle is known as the Shinarump Conglomerate. Most geologists consider it to be the first subdivision of the Chinle Formation (or Group). The Shinarump Conglomer- ate was deposited by swift rivers flowing toward the northeast across southern Utah. As the rivers con- stantly shifted their courses back and forth across the lowlands of southern and eastern Utah, a broad sheet of coarse deposits was formed. The sand and gravel of the Shinarump is usually well cemented and commonly forms a hard “cap” that protects the softer sediment of the underlying Moenkopi For- mation from the ravages of erosion. Many of the mesas and benches of southern Utah have lower slopes of soft red rock that ascend to a horizon- tal caprock of Shinarump Conglomerate. Hurricane Mesa, near Zion National Park, and the Mummy Cliffs near Torrey (fig. 1.1) are good examples of this phenomenon.

Above the Shinarump Conglomerate are other units that record different conditions of sediment deposition. The Petrified Forest Member (or For- mation), for example, consists of variegated red, green, and purple siltstone and mudstone that con- tains abundant bentonite. Most of this fine-grained material accumulated in swamps and lakes that developed on the low and poorly drained floor of the interior basin in Utah and adjacent states. The fine grains of silt were primarily derived from the south, but the volcanic ash represented by the ben- tonite probably drifted in from the west, where there is good evidence for volcanic activity during the late Triassic. Some units within the Chinle For- mation (Group) consist of reddish-brown to pur- ple siltstone and fine sandstone that lacks bentonite and contains small grains of mica and quartz. These red beds commonly exhibit sedimentary structures produced by flowing water, such as ripple marks and cross-bedding. Most of the Chinle red beds

represent sediment that accumulated on the flood- plains adjacent to the larger stream courses. Units deposited primarily on floodplains include the Church Rock Member or Rock Point Formation of the Chinle Group. Other units within the Chinle sequence consist of a mixture of lake-bed clays, river-channel sands and gravel, and floodplain silt. The Monitor Butte Member/Formation and Owl Rock Member/Formation are examples of such het- erogeneous units. Finally, other subdivisions of the Chinle (such as the Mossback and Cameron Mem- bers/Formations) represent relatively coarse-grained sands and gravel that accumulated in the channel of the larger rivers, much like the Shinarump Mem- ber. The Chinle Formation is thus a complex assem- blage of sediments deposited in a variety of specific setting across the lower interior basin of late Trias- sic time.

nondinosaur fossils of the Chinle formation: a rich ecosystem revealed

In contrast to the sparse assemblage of fossils pre- served in the underlying Moenkopi Formation, fossils occur in amazing abundance in the Chinle sediments. The list of fossils from the Chinle beds is a long one indeed, including plants, invertebrates, footprints, and vertebrate material. The various dep- ositional environments in which Chinle sediments accumulated all had relatively high potential for preserving fossils of the organisms harbored there.

The fossil plants from the Chinle Formation are so spectacularly abundant in places that in 1962 the Petrified Forest National Park in northeast Arizona was established to protect the great number of enor- mous fossil logs weathering out of the soft pastel mudstones. Elsewhere in the Colorado Plateau fos- sil plant material is commonly found in exposures of the Chinle Formation, though nowhere else do the abundance and preservation rival the national park. Fossil plants from Chinle sediments in Utah occur in notable abundance in areas around the Cir- cle Cliffs near Capitol Reef National Park (fig. 2.10)

and in the San Rafael Swell. More than fifty different kinds of plants have been identified from the Chinle Formation, including fungi, lycopods (relatives of the club mosses), ferns, conifers, ginkgoes, and cycads. The large trees preserved at Petrified Forest National Park and elsewhere are mostly the remains of Araucarioxylon, a large conifer. Preserved logs of Araucarioxylon up to 7 feet (2.3 meters) in diameter and over 120 feet (40 meters) long have been found. Even such large fossil specimens are incomplete; the living tree must have been even larger, likely exceed- ing 200 feet (about 65 meters) in height. The giant conifers might have been the most impressive ele- ment of the late Triassic flora in the Colorado Pla- teau, but many other plants of lesser stature were growing in their shadows. Ferns and horsetails car- peted the forest floor, while cycads similar to the modern “sego palms” and ginkgoes stood a bit higher as shrubs.

One major group of plants that is not represented in the Chinle flora is the flowering plants (angio- sperms). These plants, with their flowers and seed- bearing fruit, dominate the modern global flora but did not appear until the late Cretaceous. Thus the forests of the Chinle basin were lush and dense but consisted of an aggregation of relatively prim- itive types of plants. Researchers have concluded from the character of the flora and other factors that the climate nurturing these forests was warm and humid, at least during the early part of the time rep- resented by the Chinle Formation. A stroll through the forest in late Triassic Utah might have been similar to hiking through today’s rain forests in the Amazon Basin, although the prehistoric forest had no flowers or fruit. Near the end of the Trias- sic period, during the time represented by the upper Petrified Forest Member, the Chinle forests appear to have became a bit more open. A general trend 2.10. Fossilized trunks of ancient trees belonging to the genus Araucarioxylon weathering out of the Chinle Formation

toward a drier and more strongly seasonal climate seems to have somewhat limited the density of the plant growth, producing a more open woodland.

For most of the late Triassic the lowland basin of southeast Utah was without doubt a well-watered terrain. The basin was laced by rivers and dotted with lakes and ponds. Not surprisingly, then, the Chinle Formation produces a diverse fossil fauna dominated by aquatic forms. The fossils of freshwa- ter bivalves and the burrows of crustaceans (cray- fish) are fairly common in some portions of the Chinle Formation. In addition, many kinds of fossil fish have been discovered from localities throughout the Colorado Plateau. Around Zion National Park and in the Lisbon Valley area of San Juan County, the Chinle sediments have produced specimens of Cionichthys, Hemicalypterus, Ceratodus, and several other species of fish (fig. 2.11). Among the fish, Cer- atodus is particularly interesting because it belongs to the order Dipnoi, which includes a close rela- tive: the living Australian lungfish. Lungfish are spe- cifically adapted to life in ephemeral streams that periodically dry up. During droughts the lungfish burrow into the drying mud, seal the cavity with mucous, and wait for the waters to flow again. The presence of lungfish in the Chinle Formation sug- gests that the climate alternated between humid and

dry conditions, perhaps on a seasonal basis. The late Triassic fish fed on plants, molluscs, and other fish populating the sluggish rivers of the Chinle basin.

In addition to the fossil fish, a diverse array of semiaquatic vertebrates has also been recovered from the Chinle Formation in the Colorado Pla- teau region. Several different amphibians are known from the Chinle strata, of which the most common and best known is Metoposaurus. This amphibian had an almost comical appearance, with a huge flat- tened spadelike head and laughably small legs on a body some 6 feet (2 meters) long (fig. 2.3A). Lin- ing the jaws of Metoposaurus were many sharp- pointed teeth, clearly indicating the predatory habits of this amphibian. Metoposaurus and other similar amphibians probably spent most of their lives in the rivers, lakes, and ponds of the Chinle basin, lying in wait on the bottom for any unwary prey that ven- tured within reach of their “fish-trap” jaws. Met- oposaurus and its kin (the metoposaurids) would have been extremely clumsy on dry land and prob- ably never traveled far from the water’s edge. Less common amphibians from the Chinle include Buettneria and Apachesaurus.

In addition to the fish and amphibians, other ver- tebrates swam in the Chinle streams and lakes. In many localities where fossils have been found in