ZION A Geologic and Geographic Sketch of Zion National Park

A GEOLOGIC AND GEOGRAPHIC SKETCH OF ZION NATIONAL PARK

TRIASSIC FORMATIONS

As displayed in the Zion National Park region, the Triassic comprises two intricately carved brightly-colored steep slopes of shales, sandstones, limestones, and gypsum sharply separated by a gray cliff of resistant conglomerate. Beginning at the mouth of the Parunuweap, the lower slope (Moenkopi formation) broken midway up by a cliff and capped by a bench-making conglomerate (Shinarump), extends along Virgin River westward to the Hurricane escarpment. Above the Shinarump a precipitous slope of even more brilliantly colored rock (Chinle formation) rises to the walls of Kinesava Mountain and Smithsonian Butte, and continues along Virgin River past Springdale and into Zion Canyon beyond the Lodge as the lower pediment of such towers as West Temple, Bridge Mountain, the Patriarchs, Lady Mountain, and Castle Dome. Each of these three formations has characteristic features, (See Fig. 8.)

MOENKOPI (LOWER TRIASSIC).—The Moenkopi formation comprises many thousands of thin layers of sandstone and shale, also beds of limestone and gypsum, most of them deposited in the sea, but some on coastal plains, and others along inland streams. The lower third of the formation consists of oil-bearing sandy limestones and shales overlain by several hundred feet of red, brown, and pink gypsum-bearing sandy shales capped by a few beds of fossiliferous limestone (Virgin limestone member) which forms a prominent cliff. The upper two thirds consists of strongly colored red sandy shales that include as a middle-division the remarkable group of gray, white, pink, and red soft gypsiferous beds known as the Shnabkaib (Schnab-kaib) shale member. Along the highway, the eroded edges of successively higher-lying beds are displayed as bright bands, a brilliant landscape in which the color bands outline clearly the minor erosion forms characteristic of the Moenkopi—flights of tiny steps in thin sandstone, slopes and mounds in soft gypsiferous shales, and low cliffs in the hardest beds—features prominent in themselves, but somewhat inconspicuous in this region of enormous steep-walled mesas. Marine fossils serve to date the beginning of Moenkopi deposition as about 175,000,000 years ago. (See Fig. 9.)

Figure 9. View of the Moenkopi as exposed along the highway north of Virgin City. Conspicuous parts of the formation are the cliff-making Virgin limestone member (Mv) and the white-banded Shnabkaib shale member (Mu). The cap rock (s) is Shinarump conglomerate.

After the Moenkopi sediments had been deposited in the sea and along its borders, parts of them were removed by erosion producing an uneven surface on which the sands and gravels of the succeeding Shinarump were laid down. The irregular contact between the two formations (an unconformity) is well known near Rockville and Grafton.

SHINARUMP (UPPER (?) TRIASSIC).—The Shinarump conglomerate consists chiefly of white, gray, and brown conglomerate and coarse sandstone; thin, short lenses of fine-grained brown and red sandstone and mudstone; vari-colored sandy shale; and fossil wood. The dominant pebbles are well rounded, clouded and translucent white, yellow and gray quartz; red, gray, brown, white quartzite rock composed of firmly cemented quartz grains; and blue-gray limestone. About 10 per cent of them exceed 1 inch in diameter. Locally cavities in the fossil wood contain yellow carnotite or other uranium minerals and the spaces between pebbles are filled with iron and manganese oxides and copper sulphide in quantities sufficient to lure inexperienced prospectors. In the cliffs at Rockville the formation is typically displayed.

CHINLE (UPPER TRIASSIC).—Viewed as a whole the Chinle formation is a huge pile of coarse and fine sandstones, shales, gypsum, and limestone conglomerates, and is remarkable for its vividness and variety of color, for its unusual kinds of rocks, and for the abruptness of change in color, composition, texture, and bedding. The materials that constitute the formation were originally muds, silts, oozes, and volcanic ash deposited by streams and in shallow bodies of fresh water. Incorporated in them are fossil bones, tracks, fish, shells, and large amounts of petrified wood. In general the bedding is lenticular on both large and small scale. Some beds in the upper third of the formation may be traced for a mile, but most of them lose their individuality within short distances. Some are little more than thin, narrow lenses that appear and disappear within a few feet. Within the Chinle four zones, each with its characteristic bedding, color, and topographic expression, are readily distinguished. Two of them are especially conspicuous; the "big ledge" (Fig. 6) of resistant sandstone 80 to 100 feet thick that along the highway through Springdale divides the Chinle slopes into two parts; and below in the series of softer beds in which are buried forests of petrified trees. The beds that contain the petrified wood are marvelous assemblages of shales, soft sandstones, weathered volcanic ash, and many kinds of calcareous rocks colored with bands, streaks, and irregular blotches of yellow, lavender, purple, pink, lilac, ash-gray, and various shades of red, blue, and brown. They are the most richly colored beds in Utah. Their wide expanse, their brilliant coloring, and fantastic weathering make them an outstanding scenic feature, even in a region where most rocks are highly colored and eroded into picturesque forms. Some beds remain intact for several hundred feet but generally along the strike finely laminated sands or silts are replaced within a few feet by short thick lenses of sandstone conglomerate, gypsum, limestone conglomerate, or sheets of hard limestone, thin as paper. The colors likewise come and go along beds and across beds in a seemingly capricious manner, (See Fig. 10.)

Figure 10. View of the lower part of the Chinle along the highway between Rockville and Springdale. Beds in this part of the formation contain much petrified wood,