 | Zion geology |
eastern rim, see http://www.zionnational-park.com/zion-national-park-pictures.htm |
C http://www.gorp.com/parks-guide/travel-ta-zion-national-park-geology-sidwcmdev_067619.html#ixzz2ELkfYVIA
key words Eocene Wasatch formation at Cedar Breaks National Monument, Petrified Forest National Park and the Painted Desert, Dinosaur Canyon, Navajo Desert, Uinkaret Plateau
The geologic story of Zion National Park area begins with sedimentation and ends with uplift and erosion. The oldest formation exposed in the area is the Permian Kaibab limestone, which is the youngest formation along the rim of the Grand Canyon. Thus, where the Grand Canyon story ends, the geologic history exposed at Zion begins. The story continues through younger and younger strata to the north and Eocene Wasatch formation at Cedar Breaks National Monument and Bryce Canyon National Park.
Environmental conditions that created these formations ranged from oceans to coastal floodplains to river floodplains and channels. The scene changed to lakes, swamps, and desert dunes for great lengths of time. Volcanoes deposited ash. Sediment washed down from adjacent highlands. Deposition was not continuous. There are numerous breaks, called unconformities, in the stratigraphic sequence due to nondeposition or deposition followed by erosion.
Within the confines of Zion National Park, the oldest rock with significant exposure is the Triassic Moenkopi formation, which totals approximately 1,800 feet in thickness and consists of sandstone, shale, limestone (the Virgin limestone), and gypsum (the Schnabkaib member), deposited in the sea and on coastal and river floodplains. Unconformably overlying the Moenkopi is the Chinle formation, which is composed of the lower Shinarump Conglomerate member and the upper Petrified Forest member. The Shinarump averages about 100 feet in thickness and consists of coarse sand and pebbles of hard quartz rocks transported by rivers from eroded highlands.
About 120 m thick, the Petrified Forest member is composed of shales, gypsum, and volcanic ash deposited by rivers and in lakes. This is the bright, multi-colored "rock" of Petrified Forest National Park and the Painted Desert in Arizona. Much petrified wood is found in the Chinle. Unconformably overlying the Chinle formation is the red Moenave formation, which is divided into two members. The lower member, or Dinosaur Canyon sandstone, is 140 to 375 feet thick in the park and is a river and lake deposit containing fossil fish.
The upper member, or Springdale sandstone, is a river deposit 25 to 50 m thick in Zion. Overlying the Moenave is the red Kayenta formation, consisting of 70-200m of river sandstone and shale. Dinosaur footprints preserved within the Kayenta formation have been found up the Left Fork of North Creek in Zion Park. Going up in stratigraphic sequence the next formation is the most famous of all and forms the spectacular cliffs within Zion, namely, the Navajo Sandstone. This geographically widespread formation, which reaches its maximum thickness of 2,200 feet in the park, consists almost entirely of desert dunes. Ancient dunes truncated one another resulting in cross-bedding and the fantastic patterns on the sandstone surfaces now exposed. Approximately 98 percent of the formation consists of quartz grains blown from the highlands to the west in what is now central Nevada. Cementing materials are calcium carbonate and iron oxide, which gives the red color to the lower part of the formation (the upper part as now exposed is white from lack of iron oxide, which was either never deposited or has been leached out). Deposition of the sand occurred during the Jurassic period and lasted for perhaps ten million years.
Streams carrying red mud flooded the Navajo Desert for a brief time flattening the dunes and depositing a few yards of silt and clay. This thin deposit is known as the Temple Cap formation. Desert conditions resumed for a short time, then seas again encroached over the land and 200 to 300 feet of the Carmel limestone was laid down. This was followed by the deposition of 2,800 feet of more dunes sands, river deposits, swamp accumulations, and marine beds through the remainder of the Jurassic and Cretaceous. These younger post-Carmel Mesozoic formations, and the 500 feet of Eocene Wasatch formation at Cedar Breaks and Bryce Canyon, have now been eroded off the area within Zion (except for a small exposure of Dakota formation on Horse Ranch Mountain). Younger basalt (lava) flows are quite common in the area and many were extruded along the Hurricane Fault, which lies just west of the park paralleling Interstate 15. Volcanic cones within the borders of the park retain their form and thus are fairly recent.
Uplift and Erosion
Beginning about 13 million years ago, uplift of the entire Colorado Plateau resumed. Accompanying the uplift was profound erosion. Initial uplift had begun perhaps 25 million years ago at the close of the Oligocene. Rivers found their gradients increased; greater loads could be carried and greater downcutting and widening resulted. Gradually the younger formations were eroded away, giving the spectacular series of step-like cliffs and terraces and plateaus that are encountered northward into Utah from the Grand Canyon. First are the Belted (or Chocolate) Cliffs of the Moenkopi formation; then the Vermilion cliffs of the Moenave and Kayenta formations; then the White Cliffs of the upper part of the Navajo; then the Gray (or Straight) Cliffs of the Cretaceous; and finally, the Pink Cliffs of the Eocene Wasatch formation at Bryce Canyon National Park.
Offsetting the cliffs and terraces are large NNE-SSW-striking normal faults with thousands of feet of vertical displacement that have broken the Colorado Plateau into separate blocks. These faults are the Grand Wash Fault on the west followed progressively eastward by the Hurricane Fault, then the Sevier Fault, then the Paunsagunt Fault, and finally, a fold instead of a fault, the East Kaibab monocline. Zion National Park lies within the block bounded by the Hurricane and Sevier faults. The Hurricane Fault is the dividing line between the Basin and Range Province to the west and the Colorado Plateau stretching east to the Rocky Mountains.
On the south within this fault block is the Uinkaret Plateau floored by the Kaibab limestone, and rising step-like to the north, are the Little Creek Terrace (floored by the Shinarump conglomerate), the Moccasin Terrace (floored by the Springdale sandstone), the Kolob Terrace (floored by the Navajo sandstone), and finally, the Markagunt Plateau, farthest to the north at Cedar Breaks (floored by the Wasatch marls). Elevations range from 4,000 feet above sea level on the Uinkaret Plateau to 10,000 feet on the Markagunt Plateau. Zion Canyon has been cut into the White and Vermilion Cliffs along the southern edge of the Kolob Terrace.
Erosion of Zion Canyon has been principally by the Virgin River, which rises on the rim of the Markagunt Plateau at an elevation of 9,000 feet and flows southwestward for a distance of 200 miles to where it enters Lake Mead, at an elevation of less than 1,000 feet, giving the river an average gradient of 40 feet per mile. In places, through Zion Canyon, the gradient is twice as steep. After heavy summer thunderstorms or winter rains, the Virgin River becomes a raging torrent, cutting down through the soft Navajo sandstone with its tools of harder rock picked up from above. Uplift, which permitted the downcutting, did not take place at a regular rate but was interrupted by still stands when the Virgin River ceased to cut downwards and instead cut sidewards allowing tributaries to enter the main stream at grade.
Resumption of uplift caused the Virgin River to cut down but the tributaries couldn''t keep up and their valleys have been left hanging above the main canyon floor. At present, these hanging valleys stand at 1,100 to 1,300 feet above the Virgin River. It is interesting to note that this is also the general level of the division between the lower red part of the Navajo and the upper white part. It has been suggested that this level represents the ground water table at the time, and that the red iron oxide was then leached out of the overlying beds.
Widening of Zion Canyon has been influenced primarily by the presence of soft shale beds, especially at the top of the Kayenta formation below the Navajo sandstone. Groundwater percolates through the sandstone until it reaches the impervious shale horizons and then flows out laterally to form a line of springs. It also washes the shale away and undermines the overlying sandstone that then caves in. Surface runoff after heavy rains also washes the shale away and undermines the overlying rock. The Narrows of the Virgin River begin at the Temple of Sinawava where the contact between the Navajo and Kayenta formations is at river level. Upstream the river has not reached these soft shales and the canyon is narrow. Downstream the Kayenta shales are exposed and are washed away, and the canyon is widened. White streaks of various salts (sodium bicarbonate, calcium carbonate, etc.) and travertine deposits mark the spring lines. Red and black streaks paint the canyon walls from above where rain water has washed down iron oxide and organic matter. Desert varnish composed of iron oxides (such as hematite and magnetite) and manganese dioxide is common on fracture planes.
Arches
Rock, constantly under tensional stresses set up by the pull of gravity, creates arch formations. Rock falls and slides are quite common: Notable ones being the fall of 5,000 tons of rock at the end of the Narrows Trail on August 1, 1968; the 60,000 tons of rock that fell in 1958 over one of the windows of the Zion-Mount Carmel Tunnel; and the large slide from Bridge Mountain, directly across from the visitor center, that occurred in December 1990. A big slide occurred thousands of years ago near the Court of the Patriarchs and formed a lake 50 feet deep and perhaps three miles in length by damming the Virgin River. Movement still occurs along this slide.