 | Zion ( Utah) geology |
C http://www.nature.nps.gov/geology/parks/zion/geol_history.cfm
Key words: Kolob Canyons ,Taylor Creek thrust fault zone, Kanarra anticline, Taylor Creek fault zone, West Cougar Mountain faults, Grapevine Wash, Wildcat Canyon fault, Cougar Mountain faults, Lava Point flow, Bear Trap Canyon fault, Four Corners area
The tectonic history in Zion Natural Park includes two major orogenic (mountain- building) events: one that occurred in the Mesozoic and one that began in the Mesozoic and ended in the Cenozoic with uplift of the Colorado Plateau. The depositional history includes a wide variety of depositional environments.
Tectonic History
Folds and Faults
Folds and faults are not abundant in Zion; however, fault locations are important because faults are zones of weakness where earthquakes and mass- movements tend to reoccur. The Hurricane fault, created by Tertiary- age (Miocene) Basin- and- Range faulting, coincides with part of the older Sevier thrust fault. This coincidence suggests that the Sevier thrust fault created a zone of weakness that was reactivated by the Hurricane fault.
The folds and thrust faults in Zion are primarily associated with two Mesozoic to Tertiary orogenic events: the Sevier Orogeny and the Laramide Orogeny. Both orogenies are the result of lithospheric plate collisions and subsequent subduction along the western margin of North America.
Compressive forces during the Sevier Orogeny initiated thrust faulting and mountain building to the west during the Cretaceous. The Rocky Mountains were built during the Laramide Orogeny that extended from Late Cretaceous to Eocene. Figure 8 lists some of the important North American tectonic events and life forms that occurred throughout geologic Extending for nearly 64 km (40 mi) from near Toquerville to near Cedar City, the Kanarra anticline has its east limb exposed within Zion (Appendix A). Parts of the crest of the fold are also exposed at the mouths of Taylor Creek and Camp Creek. Strata on the east limb dip from 20 degrees to 35 degrees east (Biek et al. 2000). The dip of the beds flattens abruptly and is nearly horizontal under the great cliffs of Navajo Sandstone. The Hurricane fault zone has sheared off the western limb of the fold as well as parts of the crest and eastern limb along a line roughly parallel with the fold axis.
In the Kolob Canyons area, the Taylor Creek thrust fault zone, which has pushed older strata on top of younger, replicates Jurassic strata on the east limb of the Kanarra anticline (Biek et al. 2000). Taylor Creek thrust faults are back thrusts generated by the regional west to east compression during the Sevier Orogeny. The backthrusts are subparallel to bedding with fault planes dipping to the east (Appendix A) (Hamilton 1987; Biek et al. 2000). In Zion repetition of the resistant, cliffforming Springdale Sandstone member of the Moenave Formation best illustrates the Taylor Creek fault zone. The Kayenta, Chinle, Moenkopi, and Kaibab strata are also displaced by smaller back thrusts associated with the Taylor Creek fault zone. The strata were displaced about 610 m (2,000 ft) vertically and about 762 m (2,500 ft) horizontally. Cenozoic- age normal faulting has further disrupted the sedimentary rocks at Zion. While most of the Colorado Plateau was not greatly affected by Basin- and- Range normal faulting, extensional forces broke the western margin of the Colorado Plateau into a series of large blocks bounded by the north- south trending Hurricane, Sevier, Paunsaugunt, and other faults (figure 4) (Gregory 1950; Biek et al. 2000). These large fault systems that parallel the western margin of the plateau demonstrate that Zion, CEBR, and BRCA are in a transition zone between the Colorado Plateau Province and the Basin and Range Province. Zion lies on an intermediate fault block bounded by the Hurricane fault zone to the west and the Sevier fault zone to the east.
Faults of lesser linear extent are also present in Zion (Appendix A). A graben (fault bound valley) is formed by the offset along the East and West Cougar Mountain faults located in the southwest part of Zion. These faults are parallel, northwest- trending, steeply dipping normal faults probably related to Basin- and- Range extension although the timing of the faulting is poorly defined. The faults do not offset the 250,000 year- old Grapevine Wash basalt flows and the youngest rock displaced by the fault is Jurassic (Biek et al. 2000).
Another northwest- trending normal fault is the Wildcat Canyon fault that parallels the Cougar Mountain faults (Appendix A). Temple Cap and Carmel strata in Wildcat Canyon have been displaced about 55 m (180 ft) (Biek et al. 2000). Biek et al. (2000) in their text and Hamilton (1987) on his map interpret the movement along the fault to be down- to- the- east, but Biek et al. (2000) have the offset drawn as down- to- the- west. Determining the correct orientation of this fault may be important in order to predict direction of movement in the future.
The 1.0 million year old Lava Point flow to the north is not offset by the Wildcat Canyon fault. This fault is probably contemporaneous with the East and West Cougar Mountain faults.
The Bear Trap Canyon fault, a northeast- trending, highangle normal fault, with down- to- the- west movement, and more than 274 m (900 ft) of displacement (Hamilton 1992; Biek et al. 2000) merges with the East Cougar Mountain fault (Appendix A).
Minor folds of limited extent have been mapped in the Kolob area. One fold that is about 151- 172 m (495- 564 ft) long has the Jurassic Kayenta at the surface. Another fold 72 m (236 ft) long is located in Jurassic Moenave Formation through Quaternary deposits. A third fold also affects Moenave Formation strata and extends for about 106- 116 m (348- 380 ft) on the surface.
Joints
In contrast to the limited number of folds and faults, joints are ubiquitous throughout Zion. The joints are exceptionally well developed, and are instrumental in orienting today''s canyon network by channeling runoff (Biek et al. 2000). Joints are simply cracks in the bedrock without any significant offset. The most prominent joints in Zion trend north- northwest and are found in the Navajo Sandstone. These joints are nearly vertical and are spaced widely apart with some uniformity.
Crushed or sheared zones associated with the joints indicate two diametrically opposite types of crustal stresses: one set related to compression and one set related to tension (Biek et al. 2000). Rogers (2002) suggests that joints were initiated with tension related to Basin- and- Range extension, but that the joints did not propagate until surface erosion began cutting into the rock and preferentially following and facilitation joint development. The result is near parallel, regularly spaced, joint- controlled canyons.
Some joints near rock surfaces formed because of erosion. These joints are termed exfoliation joints and form roughly parallel to the rock face as overlying bedrock and sediments are eroded. Other joints, such as those at Checkerboard Mesa, are thought to form due to local expansion and contraction near the surface of the rock as it is subjected to constant, persistent temperature and moisture changes.
Depositional History
The strata of Zion represent layer upon layer of overlapping and interfingering marine and non- marine depositional environments (figure 9).
Permian Period
About 275 million years ago the Permian equator passed through what is now eastern Utah and Wyoming along the western margin of Pangaea, the supercontinent forming as the globe''s landmasses sutured together (Biek et al. 2000; Morris et al. 2000). A dry, high atmospheric pressure climatic belt prevailed in this western part of Pangaea and resulted in restricted marine evaporitic conditions over much of the cratonic shelf seaway (Peterson 1980). Warm, shallow seas and sabkhas (broad, very flat surfaces near sea level) covered the area. Farther to the west, a complex island arc assemblage formed above a subduction zone as lithospheric plates collided (Silberling and Roberts 1962). To the east, in western Colorado, the majestic, jagged peaks (similar to today''s Himalayas) of the Uncompahgre Mountains bordered the Utah lowland.
The Toroweap Formation contains evidence of four environments of deposition created by the advance and retreat of the shoreline across northern Arizona and southwestern Utah (Rawson et al. 1980). From west to east, these four environments include an open marine environment, restricted marine, sabkha, and eolian dune environments. As sea level continued to rise during the initial Toroweap transgression, normal marine organisms such as brachiopods, crinoids, corals, and bryozoans entered the Zion area, and their shell material is incorporated in the Toroweap limestones. The fossiliferous limestone, dolomite, and limy sandstone environments record three transgressive pulses (Rawson et al. 1980).
Following the last transgression, the sea withdrew to Nevada and coastal sabkha environments spread over Zion. Eolian dune fields formed east of the sabkhas. One last Toroweap transgressive pulse swept marine environments back into the area from the west.
The Kaibab Limestone records the last in a long series of shallow seas that transgressed over the Zion region throughout the Paleozoic Era. Oolites, disarticulated and broken marine fossil fragments, dolomite, siliceous sponge spicules, and gypsum, all found in the Kaibab, formed under shallow, near- shore, warm and arid climatic conditions (Hamilton 1992). Spherical, modern oolites, similar to those found in the Kaibab Limestone, are currently being formed in warm, shallow marine water where they slowly accrete carbonate mud to their round surfaces as waves gently roll them back and forth over the sea bottom.
The interfingering of the Kaibab with the White Rim Sandstone in the Capital Reef National Park area to the east suggests that the marine facies of the Kaibab migrated eastward in response to a relative sea- level rise, or transgression (Dubiel et al. 1996). The sea moved back and forth across Utah, but by the Middle Permian, the sea had withdrawn and the Kaibab Limestone was exposed to subaerial erosion (Morris et al. 2000). Dissolution of the Kaibab created karst topography and channels reaching 30 m (100 ft) in depth cut into the limestone surface (Morris et al. 2000).
The close of the Permian brought the third, and most severe, mass extinction of geologic time. Although not as famous as the extinction event that exterminated the Dinosaurs at the end of the Mesozoic, the Permian extinction was much more extensive.
Almost 96% of all species were extinct by the end of the Permian (Raup 1991). The most recent hypothesis regarding the Permian event suggests that a comet, about 6- 13 km (4- 8 mi) in diameter, slammed into Earth (Becker et al. 2001), triggering vast volcanic eruptions that spread lava over an area two- thirds the size of the United States.
Triassic Period
During the Triassic (250 to 206 million years ago), the supercontinent Pangaea reached its greatest size. All the continents had come together to form a single landmass that was located symmetrically about the equator (Dubiel 1994). To the west, explosive volcanoes arose from the sea and formed a north- south trending arc of islands along the border of what is now California and Nevada (Christiansen et al. 1994; Dubiel 1994; Lawton 1994).
Shallow, marine water stretched from eastern Utah to eastern Nevada over a beveled continental shelf. As the sea withdrew, fluvial, mudflat, sabkha, and shallow marine environments developed (Lower Triassic, Moenkopi Formation) (Stewart et al. 1972A; Christiansen et al. 1994; Doelling 2000; Huntoon et al. 2000). The Red Canyon Conglomerate, the basal member of the Moenkopi, fills broad east- flowing paleochannels carved into the Kaibab Limestone (Biek et al. 2000). Some of these channels are up to several tens of feet deep and may reach 61 m (200 ft) deep in the St. George area. A thin poorly developed soil or regolith formed over the paleotopographic high areas between the channels (Biek et al. 2000).
The fossilized plants and animals in the Moenkopi are evidence of a climate shift to a warm tropical setting that may have experienced monsoonal, wet- dry conditions (Stewart et al. 1972A; Dubiel 1994; Huntoon et al. 2000; Morris et al. 2000).
At Zion, the limestones and fossils of the Timpoweap, Virgin Limestone, and Shnabkaib members of the Moenkopi Formation document transgressive episodes. Unlike the Timpoweap and Virgin Limestone members, the Shnabkaib contains abundant gypsum and interbedded mudstone resulting from deposition in a restricted marine environment with complex watertable fluctuations (Biek et al. 2000).
Regressive, red- bed layers separate the transgressive strata. Ripple marks, mud cracks, and thinly laminated bedding suggest that these intervening red shale and siltstone units were deposited in tidal flat and coastalplain environments (Stewart et al. 1972A; Hamilton 1992; Biek et al. 2000).
The Early Triassic is separated from the Late Triassic by a regional unconformity (figure 5). This unconformity marks a change from the shallow marine environments of the Lower Triassic Moenkopi Formation to mostly continental sedimentation in the Upper Triassic Chinle Formation. The Middle Triassic remains a mystery. No rocks that span this time (from 242- 227 Ma) have been preserved in Utah. By the Late Triassic, Utah was part of a large interior basin drained by north- and northwestflowing rivers (Biek et al. 2000). Braided streams deposited coarse sediments (Shinarump Conglomerate member) in paleovalleys eroded into the underlying Moenkopi Formation (Dubiel 1994; Biek et al. 2000).
High- sinuosity stream, flood plain, and lake sediments (Petrified Forest member) overly the braided stream deposits in the Zion region (Stewart et al. 1972B; Dubiel 1994; Biek et al. 2000). Aquatic crocodile- like Phytosaurs, lungfish, and lacustrine bivalves inhabited a Utah that looked vastly different in the Upper Triassic than it does today. Rather than a semi- arid desert environment, the Zion area was a coastal lowland supporting amphibians, reptiles, freshwater clams, snails, ostracodes, and fish. The moist climate supported conifer trees, cycads, ferns, and horsetails (Stewart et al. 1972B; Dubiel 1994; Biek et al. 2000). Periodically, volcanic ash from the volcanic arc off the continental margin to the west drifted into the area and was subsequently altered to bentonitic clay that today is notoriously susceptible to landslides and for causing foundation problems in southwest Utah.
About ten million years is missing between the Chinle Formation and the Early Jurassic Moenave Formation. This basal Jurassic unconformity extends from central and western Wyoming, through Utah and the Four Corners area, and into northwest New Mexico and the San Juan Basin (Pipiringos and O''Sullivan 1978; Peterson 1994).