The Wingate / Moenave Formation
(LATE TRIASSIC OR EARLY JURASSIC)
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Between the upper and lower members is the thin Whitmore Point Member, consisting of gray mudstone and shale, which was almost certainly deposited in an environment of ponds and lakes. Peterson has found palynomorphs (pollens and spores) in the Whitmore Point Member that have Jurassic affinities. In 0ak Creek canyon near the old Visitor Center, there is a large lens-shaped deposit of clay clasts (actually clay gravel) in the Springdale Sandstone that shows that the sediments of a nearby Whitmore Point lake were ripped away in a flood and redeposted nearby during Springdale time.
The Moenave streams and ponds were inhabited by fish, as shown by fossilized remains of Semionotus found at the base of the Springdale cliff near Pine Creek. Semionotus's modern relatives are the gar and sturgeon.
The continued abundance of water, though gypsum indicates occasional aridity, suggests that an equatorial climate still prevailed in Zion. At the end of Moenave time stream discharge diminished and the fine sands and silt of the Kayenta Formation were laid down.
SOURCE
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory
The Wingate is exposed in the walls of San Juan Canyon at Comb Ridge and below Clay Hill Crossing. Complete or partial sections appear in upper Cottonwood Wash and its western tributaries that cut deeply into tilted beds along the flanks of Elk Ridge. Castle Wash, Moki Canyon, Cedar Creek, and other gorges that carry the drainage of Red Rock Plateau to the San Juan and the Colorado reveal the Wingate, and many mesas on the divide between Red Canyon and White Canyon, likewise the Bears Ears, Woodenshoe Buttes, and other picturesque landmarks on Elk Ridge, are capped with this rock.
SOURCE
-from The San Juan Country - A Geographic and Reconnaissance of Southeastern Utah by Herbert E. Gregory
CHINLE-WINGATE CONTACT.
Wherever the contact between the red or orange-red shales of the upper Chinle and the massive Wingate sandstone was observed care was taken to search for evidence of unconformity. The strata are well displayed for such an examination, as along cliff bases and in canyon walls the contact may be traced without interruption for distances of 1 to 5 miles. No channeling or other conspicuous evidence of a break in sedimentation was observed, but at many places the contact is marked by a thin band of leached material and a disturbance of the otherwise regular stratification. In lower Piute Canyon 5 to 15 feet of irregularly curved, highly varied strata resting unevenly on shales and limestone conglomerates mark the base of the Wingate. East of Tanner Crossing Mr. Heald found at the top of the Chinle a 2-foot bed of "conglomerate" consisting of well-rounded pebbles of quartz with rare limestone, gray shale, and red quartzite fragments arranged as stringers, cross-bedded, and with ripple-marked and mud-cracked surfaces. A slight change in direction of strike and dip was also noted. Six miles southwest of Tuba a similar stratum forms the top of the Chinle formation. In Laguna Canyon, at Azansosi Mesa, and at the east base of Carrizo Mountain the Wingate sandstone rests on a slightly irregular knobby surface of calcareous shales on which are spread sheets of gray sandstone containing white o and black quartz and chunks of gray shale. In Piute and Copper canyons similar relations were noted. In Todilto Park an irregular bleached band at the base of the Wingate sandstone rests on wavy, knobby, imperfectly bedded layers of Chinle shale into which project strings of white sandstone like the filling of mud cracks.
Although such features are proofs only of local unconformity, I am inclined to consider them of greater significance, in view of the fact that an erosion interval at this horizon has been noted beyond the borders of the Navajo Reservation. Thus Gilbert 5 states:
In the region of the Virgin River and Kanab Creek the change from the variegated shales of the Upper Shinarump to the homogeneous sandstone of the Vermilion Cliff is gradual, but in one locality, at least, there is direct evidence that the surface of the clay was exposed to the air before it was covered by the sand. On the northern flank of Mount Ellsworth are the vestiges of a system of mud cracks, such as form where wet clays are dried in the sun. Where the under surface of the Vermilion sandstone is exposed to view it is seen to be marked by a network of ridges, which once occupied the sun cracks of the Shinarump clay; and where the clay is seen in juxtaposition, tapering fillets of sand can be traced from the ridges downward 10 feet into the clay.
Dutton remarks: "The contact [of the Triassic Vermilion Cliff sandstone] with the shales [Shinarump] below is usually conformable, but in the vicinity of Hurricane fault the junction is often unconformable." Cross indicates an unconformity between the Dolores and the La Plata sandstone.
The abundance of fossil wood both north and ' south of Colorado River is almost incredible, and its presence has made a profound impression on the native tribes. To the Navajos the logs are yeitsobitsin, the bones .of yeitso, and a monster who was destroyed by the sun and whose blood was congealed in lava flows. In the Piute mythology the broken trunks are the spent weapons of Shináray, the great Wolf God; the accumulated masses mark the sites of battlefields.
In the Navajo country fossil wood constitutes a characteristic feature of the Triassic sedimentary beds; it is found wherever the Shinarump conglomerate or Chinle formation is exposed by erosion. On Lithodendron Creek, in Beautiful Valley, at Round Rock, and at Willow Springs petrified logs and chips are sufficiently abundant to justify the term fossil forests. At those localities solid logs exceeding 50 feet in length may be counted by the dozen, blocks 3 to 10 feet long occur in hundreds, and the scattered chips are innumerable. At other localities the wood is only slightly less abundant. In the North Forest, on Lithodendron Creek, where the trees are best displayed within an area of about 1,200 acres, a number of logs have lengths of 30 to 40 feet, and diameters of 3 to 4 feet; the largest seen is about 70 feet long and measures 61/2 feet at its flattened butt. The Beautiful Valley Forest, covering about 3 square miles,' contains 10 logs between 50 and 80 feet in length and averaging about 3 feet in diameter, in addition to hundreds of smaller dimensions. The floor of the valley in places is literally paved with blocks of fossil wood. In the Round Rock or Senakahn Forest the trees are as abundant as at any other place known. Trunks 30 to 60 feet in length, with diameters of 1 foot to 5 feet, were measured. The Willow Springs Forest, about 5 square miles in area, includes dozens of silicified trees in the midst of chips so abundant as to conceal the strata beneath. At this locality Mr. Pogue measured a trunk 4 feet in diameter at the butt and 150 feet long, the largest tree so far reported from the Navajo Reservation. At Lees Ferry logs 60 feet long are not uncommon.
The tree trunks are very unevenly distributed. They usually occur in widely spread groups of unassorted large and small trees, all lying flat and trending in parallel or diverse directions, or overlying one another, like fallen timber in the path of a tornado. In Nokai Canyon a nicely laid pile of eight logs, 7 to 15 feet long and 3 to 4t feet in diameter, occupies an isolated position, and at certain localities only a single log is to be found within an area of several acres. No complete trees were seen; most of the logs terminate abruptly, with worn surfaces at both ends. A few trees are still attached to their upturned stumps, and at several places stumps with root bases attached were noted. There is a singular scarcity of small branches and twigs, and a somewhat careful search for cones and needles resulted in finding none.
The logs are not confined to a single horizon. Their lowest stratigraphic position is the base of the Shinarump conglomerate, from which they protrude in places like projecting roof beams of houses. Some of the largest logs seen are lying directly on the old erosion surface separating the Permian (?) from the Triassic. Throughout the Shinarump conglomerate the wood occurs in the coarser materials usually as angular pebbles, in the sandstone lenses as blocks and logs. Tree trunks are common in the uppermost part of the Shinarump conglomerate and in the immediately overlying Chinle shales a horizon that includes the Willow Springs Forest and the largest logs at Lees Ferry, Nokai Canyon, and Copper Canyon and in Middle Moonlight Valley. To judge from the descriptions by Ward 1 and by Merrill 2, the trees in the Petrified Forest National Monument occur at this horizon. The Beautiful Valley Forest and the Senakahn (Round Rock) Forest are in division C of the Chinle formation, and the trees in the forest on Lithodendron Creek lie among the limestone conglomerates of division C or the lower part of division B.
The conditions under which the large amount of fossil wood was accumulated in the Triassic sediments are not clearly understood. That the trees grew in the spots where they are now found is highly improbable. Of the standing trees reported one has its roots in the air, the others so far as I have observed are wedged among other logs in a manner common to driftwood. The few stumps noted are in proper position, and Ward3 is of the opinion that a group of stumps near Tanner Crossing is in place. Mr. Heald, who studied these stumps, considers this conclusion to be open to doubt. No roots extending downward have been found attached to stumps.
It is believed that the tree trunks now turned to stone were carried by streams during floods. Many of them have worn ends and battered sides, and most of them are without bark. Trees of various sizes and ages are huddled together; the blunt end of one log abuts against the side of its neighbor; and collections of trunks are wedged tightly together with different angles of inclination. The sandstone in which most of them occur is cross-bedded and lenticular, is laterally unconformable, and has other features suggestive of fluviatile deposition. The accumulations of trunks in the fossil forests are closely similar to piles of driftwood now seen along Colorado, Little Colorado, and San Juan rivers-piles of trunks and branches, some much worn, some still retaining the bark, crowded together and overriding one another; stumps attached to trees or separate in various positions, some upright, some lying on the surface, others buried in alluvium or wind-blown sand. The logs now stranded on the surface of the lava at Black Falls are about equal in number to the fossil trees in Beautiful Valley. Most of the wood, particularly the logs, must have become silicified in its present location, the process being favored by rapid burial, a water table fluctuating through short periods, and the presence of alkaline solutions.
As seen in the field, the logs appear to represent a number of different kinds of trees, and Knowlton 4 reports that several species are present in the fossil forests at Adamana. The species so far described, Araucarioxylon arizonicum and Woodworthia arizonica, were obtained in the valley of Lithodendron Wash, along the southern border of the Navajo country.
SOURCE
-from Geology of the Navajo Country, A Reconnaissance of Parts of Arizona, New Mexico and Utah by Herbert E. Gregory
The Moenave Formation is a cliff-forming unit which forms part of the bold escarpment of the cliffs surrounding Lee's Ferry. It overlies the soft colorful Chinle Shale and is characterized by various shades of red, orange, and reddish brown. It consists of lenticular beds of sandstone and is the lowest of three formations in the surrounding prominent cliffs. The Moenave Formation is approximately 350 feet thick near Lee's Ferry and represents sediments deposited by stream systems over a low-lying land.
SOURCE
Geology Studies Vol. 15- Part 5-1968
