ZION A Geologic and Geographic Sketch of Zion National Park

A GEOLOGIC AND GEOGRAPHIC SKETCH OF ZION NATIONAL PARK

JURASSIC (?) FORMATIONS

The Jurassic (?) formations (Middle Mesozoic) are of exceptional scenic interest. They make up the highest and most continuous cliffs of southern Utah and in them are cut the deepest canyons. Displayed as the Great White Cliffs, between the Paria and the Parunuweap, as the ??? Block Mesas overlooking the Moccasin Terrace (north of Pipe Spring), and as the temples, domes, and lofty buttes of Zion National Park, they dominate the landscape from almost any viewpoint. Zion Canyon itself is essentially a slot sunk into Jurassic sandstone (Fig. 3.) Within the park three formations of probable Jurassic age are represented: the Wingate, overlain in turn by the Kayenta and the Navajo, and one of known Jurassic age, the Carmel. The higher beds in the Jurassic of Utah appear in fragmentary form along the northern park border and are fully developed along the Zion-Mt. Carmel highway. As displayed in the park, the Wingate, Kayenta, and Navajo formations (classed by geologists as the Glen Canyon group) differ in thickness and topographic importance from equivalent beds in eastern Utah and Arizona. Thus the Wingate elsewhere generally thick and prominent in cliffs, is here thin and hence inconspicuous. Likewise, particularly in distant views, the red Kayenta, rarely more than 200 feet thick, holds no well-marked place in the series of red beds of 3000 feet. On the other hand, the Navajo attains its fullest development. With a maximum thickness exceeding 2000 feet it forms the canyon walls of the Virgin, the Parunuweap, Pine Creek, North Creek, La Verkin Creek, the towers of the park and such outlying masses as Eagle Crags and Smithsonion Butte. The Carmel formation, though relatively thin, is distinctive in form and composition and a topographic feature of wide extent. It constitutes the floor of Kolob terrace into which the great canyons of the park are incised and is the cap of the loftiest mesas.

WINGATE (LOWER JURRASIC ?).—In the park as elsewhere the Wingate is a strongly cross-bedded sandstone, white, tan, or red in color, and consists predominantly of clear rounded quartz grains weakly cemented with lime, mingled with iron. In large part it is an accumulation of wind blown sand. On cliffs and steep slopes the formation appears as a step, some 80 feet thick in middle Parunuweap Canyon and with decreasing thickness westward where it loses its individuality. Its characteristic features are shown in Kanab Canyon along the Zion-Grand Canyon highway.

KAYENTA (LOWER JURASSIC ?).—Above the Wingate and without any great lapse of time the sediments that comprise the Kayenta (Kay-en-ta) were deposited by ephemeral and intermittent streams. The formation is chiefly a series of dark red and purple, thin and thick-bedded, coarse and fine-grained sandstones that overlap, tail out, or end abruptly within short distances. Quartz in rounded, well-sorted grains is the predominant mineral; the cement is chiefly calcite in intergrain spaces and iron in films tightly wrapped about the grains. Compared with the formations above and below, the Kayenta is impervious. Throughout the park its top marks the position of springs.

NAVAJO (MIDDLE JURASSIC ?).—The Navajo is essentially a huge mass of remarkably homogeneous, fine-grained, friable, quartz sandstone held together by calcareous cement. Everywhere most of it, and in places all of it, is a single, massive, elaborately cross-bedded stratum. In the Zion National Park region, particularly where the Wingate and Kayenta are absent, its basal portion, 100 to 300 feet thick, consists of somewhat regularly bedded strata. The top beds are also distinctive. They consist of sandy shale and massive sandstone that on weathering form secondary cliffs and mesa-like tops of isolated masses of the Navajo. Fully 98 per cent of the Navajo formation consists of rounded grains of translucent quartz, many of them frosted and some wind-etched. The grains are held together chiefly by lime, iron oxides, and clay-like substances, but the cement is weak and the rock therefore remarkably porous and friable. Big slabs on falling to the talus may crumble into dust, and seemingly solid boulders may crush under foot. Projecting ledges are precarious footholds. The iron is the coloring matter in the rock; hematite is chiefly responsible for the red colors, limonite for the yellow, and iron-bearing clay for the rare green tones. Where the iron cement is largely absent the rock may be nearly white. See Fig. 11.)

Figure 11. Checkerboard Mesa, one of many similar headlands along the Zion-Mount Carmel Highway. Shows the cross-bedding and adjoining characteristic of the Navajo which forms the White Cliffs.

The outstanding feature of the Navajo is the bedding. Unlike most sedimentary rocks that over wide areas consist of horizontal layers superposed in orderly fashion, this sandstone is built of laminae disposed in all sorts of positions. Some laminae appear as groups of parallel straight lines, vertical, horizontal or oblique, and are sharply truncated along inclined or horizontal planes. Others are curved, sweep through long arcs, and gradually decrease in curvature until they become tangent to those beneath. Series of parallel curves overlap or merge with series of different radii. Some are grouped as wedges. In places the laminae are wrinkled and squeezed into loops. Many sets of laminae retain their individuality for a few hundred feet, though commonly they extend tens of feet, some are measured in inches and are recognizable only on weathered surfaces. The remarkable bedding (cross bedding) of the Navajo, its composition, its fossil land shells and dinosaur tracks indicate that the sands that compose it were deposited on a land surface in an arid region and that the dominant agent of deposition was wind.

Figure 12. The towering West Temple as seen approaching Zion National Park from the southwest. Immediate foreground the pioneer village of Grafton, Utah, or the Virgin River. Center foreground Moenkopi formation, capped by Shinarump that forms bench (center), which is overlain by the lower Chinle. The heavy ledge in upper part of middle third of picture is the "big ledge" member of the Chinle. Talus slope at base of upper straight wall Kayenta and Wingate. Upper exposures all Navajo except round cap on top of West Temple which is Carmel (Photo by Zion Picture Shop).

The unevenly worn surface at the top of the Navajo is a feature full of meaning; it marks the time when land in southwestern Utah was replaced by the sea; when desert sands were covered by silts laid down by waves and currents. As expressed by geologists, the wind and stream-made sediments constituting the Navajo are uncomfortably overlaid by the marine Carmel.

CARMEL (UPPER JURASSIC).—The Carmel formation, 200 to 300 feet thick, is characteristically hard compace limestone in groups of beds 1 to 4 feet thick, separated by groups of thinner, more sandy, limy beds. Its bedding and composition reveal its origin as silts deposited in a shallow sea. Abundant fossils fix its age. Within the park the Carmel is not readily accessible. It is exposed in road cuts near the east entrance,