The Kayenta Formation
(EARLY JURASSIC)
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In the southeastern part of Zion Park a stratum of cross-bedded sandstone is found roughly halfway between the top and bottom of the Kayenta Formation. It is a "tongue" of sandstone that merges with the Navajo formation east of Kanab, and it shows that desert conditions occurred briefly in this area during Kayenta time. This tongue is the ledge that shades the lower portion of the Emerald Pool Trail, and it is properly called Navajo, not Kayenta.
Fossil mud cracks attest to occasional seasonal climate, and thin limestones and fossilized trails of aquatic snails or worms mark the existence of ponds and lakes. The most interesting fossils, however, are the dinosaur tracks that are relatively common in Kayenta mudstone.
These vary in size, but all seem to be the tracks of three-toed reptiles that walked upright, leaving their tracks in the muds on the flood plains. Unfortunately, so far no bone materials have been found in the County that would enable more specific identification.
Apparently during Kayenta time Zion was situated in a climatic belt like that of Senegal with rainy summers and dry winters at the southern edge of a great desert. The influence of the desert was about to predominate, however, as North America drifted northward into the arid desert belt.
SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory
In most sections that include all three formations of the Glen Canyon group the Kayenta is easily recognized. Even at a distance it appears as a dark-red, maroon, or lavender band of thin-bedded material between two thick, massive, cross-bedded strata of buff, tan, or light-red color. Its position is also generally marked by a topographic break. Its weak beds form a bench or platform developed by stripping the Navajo sandstone back from the face of the Wingate cliffs. The Kayenta is made up of beds of sandstone, shale, and limestone, all lenticular, uneven at their tops, and discontinuous within short distances. They suggest deposits made by shifting streams of fluctuating volume. The sandstone beds, from less than I inch to more than 10 feet thick, are composed of relatively coarse, well-rounded quartz grains cemented by lime and iron. The thicker beds are indefinitely cross-bedded. The shales are essentially fine-grained, very thin sandstones that include lime concretions and balls of consolidated mud. The limestone appears as solid gray-blue beds, a few inches to a few feet thick, and as lenses of limestone conglomerate. Most of the limestone lenses are less than 25 feet long, but two were traced for nearly 500 feet and one for 1,650 feet. On the Hammond trail, which crosses Comb Ridge opposite the mouth of Arch Canyon, a conglomerate of lime mud balls that weather into a spongy mass is a distinctive, feature. In Moki Canyon interbedded limestone lenses and shales that contain fragments and concretionary masses of limestone make up a calcareous zone as much as 50 feet thick. In places along Castle Wash- the Kayenta is obscure; the upper two-thirds of a continuous wall is marked off from the lower third by thin beds and f at, short lenses of blue, green, and white limestone. On lower Cedar Creek the Kayenta is exceptionally well displayed as the floor of an esplanade that 100 feet above the stream extends back from the canyon rim nearly a mile. Here the basal beds are sandstones and limestone conglomerates in long, thin lenses and. thick, short, stub-ended masses that are replaced along the strike by dark-red mudstone decorated with - green bands that mark joints and incipient faults. The middle part is a series of 10 massive, friable, cross-bedded layers 2 to 5 feet thick marked on bedding surfaces by mud cracks. Less than a mile distant the limestones occur only at the top.
In different sections the limestone beds range from 5 to 18 in number and constitute 9 to 25 percent of the material. The formation is 200 feet thick at the head of Comb Wash, 140 feet thick on the Hammond trail, 160 feet in Castle Wash , and 220 feet in Cedar and Knowles Canyons. Immediately west of the Colorado and along the lower San Juan thicknesses ranging from 125 to 249 feet are recorded.
Viewed as a whole the Kayenta, is readily distinguished from the formations above and below it. It is unlike them in composition, color, manner of bedding, and sedimentary history. Obviously the conditions of sedimentation changed in passing from the Wingate to the Kayenta and from the Kayenta to the Navajo, but the nature and regional significance of the changes have not been determined. In some measured sections the transition from Wingate to Kayenta is gradual; the material in the basal Kayenta, beds seems to have been derived from the Wingate immediately below and redeposited with only the discordance characteristic of fluviatile sediments. But in many sections the contact between the two formations is unconformable; the basal Kayenta consists of conglomerate and lenticular sandstone that fills depressions eroded in the underlying beds. In Moki Canyon near Red Cone Spring nearly 10 feet of Kayenta limestone conglomerate rests in a long meandering valley cut in Wingate. Likewise, the contact between the Kayenta and the Navajo in places seems to be gradational, but generally a thin jumbled mass of sandstone and shales, chunks of shale and limestone, mud balls, and concretions of lime a iron lies at the base of the fine-grained, cross bedded Navajo, Mud cracks, a few ripple marks, and incipient drainage channels were observed in the topmost bed of the Kayenta on Red Rock Plateau; and west Glen Canyon wide sand-filled cracks appear at the horizon. These features indicate that, in places least, the Wingate and Kayenta were exposed to erosion before their overlying formations were deposited, are it may be that the range in thickness of the Kayen thus in part accounted for. However, unconformity ties within and between formations are so common in the Mesozoic of the plateau province that the filed geologist hesitates to give regional significance to break in sedimentation that have not been correlated wit changes in faunas and large-scale tectonic movement.
SOURCE
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory
The Kayenta Formation is approximately 400 feet thick and consists of a fine-grained sandstone interbedded with layers of siltstone. The alternation of these units generally produces a series of ledges and slopes between the cliffs of the Navajo and Moenave formations. Dinosaur tracks are fairly common in the siltstone, and fresh-water mussels and snails occur but are rare. The Kayenta Formation is colored pale red and adds to the splendor of the Vermillion Cliffs. It accumulated as deposits of rivers.
SOURCE
-Geology Studies Vol. 15 Part 5 1968
