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Natural Bridges National Monument, Utah�s first national monument, was established to protect three large natural bridges and ancient masonry structures constructed by ancestral Puebloan people. Unlike an arch, like those at Arches National Park, a natural bridge forms through the process of flowing water. The landscape of the monument stands as an elegant testimony to the power and splendor of geologic processes and the dynamic change that operate throughout geologic time (see Appendix 6). Although carved into Permian age Cedar Mesa Sandstone that was deposited about 270 million years ago, the bridges are probably less than 30,000 years old.
Being part of the geological region known as the Colorado Plateau, the strata in the Monument region record the growth of the North American continent. In the continent�s infancy, an ocean bordered the Colorado Plateau. As land accreted to the western margin of the North American continent during the Paleozoic era, the region became part of a Western Interior Basin. Towards the end of the Paleozoic, as the crustal landmasses on the globe sutured together into one big supercontinent, Pangaea, the ancestral Rocky Mountains were uplifted and supplied sediment to the Natural Bridges area.
The Uncompahgre Uplift was part of the Ancestral Rocky Mountains that formed as the last land masses sutured together to form the supercontinent, Pangaea, beginning in the Pennsylvanian Period. South America collided with the southern part of North America near Texas and Oklahoma, generating the Ouachita Orogeny. The Marathon-Ouachita-southern Appalachian mountain chain resulted from this collision. The effects were felt in the interior of the continent, as well, where jagged peaks split the skyline as the Ancestral Rocky Mountains were thrust from the plain. Two principal mountain ranges formed along northwest-southeast trending high-angle reverse faults: the Uncompahgre and Front Range uplifts (figure 6).
Boulder conglomerates eroded from the fault-bounded Uncompahgre Highlands were deposited in the Paradox Basin and eventually became the Cutler Formation. Like today in the southwestern desert of the United States, Pennsylvanian � Permian age rivers flowing from the canyons cut in the Uncompahgre Mountains lost their momentum when they debouched from the mouth of the canyon onto the plain, and the coarse material was distributed in fan-shaped deposits called alluvial fans. The sediments became progressively finer-grained away from the fault so that feldspar-rich sandstone and silty sandstones were deposited in the distal portions of the fan. Fluvial systems transported sediment throughout the Paradox Basin.
Isopach (thickness) maps showing the thickness of the Cutler Group or Formation and the lower Cutler beds illustrate the effect of the Uncompahgre Highland on the depositional patterns in the Paradox Basin during Pennsylvanian time (Condon, 1997). Within 40 km (25 mi) of the fault that defines the southwestern border of the Uncompahgre Highland, over 2000 m (6500 ft) of Cutler Group or Formation was deposited in a trough that parallels the mountain front. A similar pattern is found in the lower Cutler beds although the trough is not as well defined. Rather, the lower Cutler beds form three, fan-shaped deposits over 300 m (1000 ft) thick next to the Uncompahgre front (Condon, 1997). The Monument Upwarp doesn�t appear to have influenced the depositional patterns, and thus, the upwarp had not yet developed.
The Monument Upwarp formed on the southwestern edge of the Paradox basin during the Early Permian, however, and contributed sediments to the basin possibly into the Early Triassic (Huntoon et al., 2000). Extending from Monument Valley to about the confluence of the Green and Colorado Rivers, the Monument Upwarp was a broad, elongate, topographic high during the Early Permian (figure 7). Blakey (1996) refers to the upwarp as the Monument �Bench�, suggesting a subtle feature in the Permian with subdued relief compared to the Uncompahgre Highlands.
Natural Bridges National Monument lies near the crest of the Monument Upwarp, a broad, structural feature called the Monument Upwarp that extends from southern Utah into northern Arizona, so that the Lower Permian Cedar Mesa Sandstone and Organ Rock Formation are relatively flat-lying strata with only a gentle dip to the southwest. During the Early Permian Period, three major paleotectonic elements influenced deposition in southeastern Utah (Huntoon et al., 2000). The Uncompahgre Mountains lie to the northeast (figure 6). The Monument Upwarp was a north-south trending, positive feature crossing the Arizona/Utah border, and the Paradox Basin lay between the two highlands. Following burial and lithification, the Permian strata were deformed and molded into the present landscape by the Late Cretaceous Period to Tertiary Period Laramide Orogeny and subsequent Cenozoic uplift.
Bordered by the Monument Upwarp and the Uncompahgre Highland, the Cedar Mesa Sandstone thins from the northwest, where it is over 365 m (1200 ft) thick, to the southeast, where it is about 122 m (400 ft) thick (Condon, 1997). Facies changes in the Cedar Mesa Sandstone seem to follow this thickness trend.
To the northwest, the Cedar Mesa is a thick sequence of cross-bedded sandstone, but southeast of the Monument Upwarp, the formation thins and becomes dominated by fine-grained sandstone, mudstone, and evaporite deposits (Blakey, 1996; Condon, 1997).
Depositional trends and isopach maps further document the effect of the Monument Upwarp on the Organ Rock Formation (Stanesco et al., 2000). In the Lower Permian, the upward diverted fluvial channels in the Organ Rock to the northwest. Eolian deposits onlapped the structure from the west and thinned over the crest of the upwarp. During the Lower Permian, Natural Bridges National Monument was on the southeastern edge of the Monument so that the Organ Rock facies are different from those on the northwestern margin of the structure (Stanesco et al., 2000).
Permian Period strata, including the Cedar Mesa Sandstone, were deposited along the western margin of North America in a variety of terrestrial and marine environments. As the Mesozoic dawned, these nearshore to marine environments changed as winds blew across the region, pushing sand into great dune fields that surpass today�s Sahara Desert. These dunes and other later deposits buried the Permian strata to a depth of 1,500-3,000 m (5,000 to 10,000 ft). Now exposed at the surface, the Cedar Mesa Sandstone, the dominant formation in Natural Bridges National Monument, is surrounded by the younger Triassic Period Moenkopi and Chinle Formations and the Jurassic Period Wingate Sandstone.
The rocks exposed at the surface today are the result of a complex relationship between tectonics and sedimentation rates. As Pangaea began to break apart and the landmasses began to drift to their present positions, the climate affecting Natural Bridges became more humid. Sand dunes transformed into river systems, swamps, beaches, and broad level plains. Dinosaurs roamed the region, and periodically, ash drifted into the area from volcanoes far to the west. Rippling effects of lithospheric plate collisions on the western margin of North America caused the Western Interior of North America to be flexed into a shallow basin in the Cretaceous. This Western Interior Basin was flooded by seawater from the Arctic region and from the ancestral Gulf of Mexico as Africa and South America rifted away from North America.
By Late Cretaceous time, about 97 million years ago, the Western Interior Seaway had drowned the previously continental deposits and the Natural Bridges region was an ocean basin, collecting fine-grained sediments that drifted far from the shoreline to the west and southwest. Oscillations in the shoreline, either from increased sedimentation coming off the highlands to the west or from tectonic response to collisions on the western continental margin, caused the shoreline to prograde and recede several times during the next 13 million years. Thick sequences of shale, siltstone, and thin limestone beds accumulated on the margin.
As the Cretaceous Period came to a close, compressive forces outside the borders of the Colorado Plateau, an extensive physiographic province covering parts of Utah, Colorado, New Mexico, and Arizona, caused the region to bow upward as a relatively coherent unit during the Late Cretaceous to Mid-Tertiary Laramide Orogeny. The Monument Upwarp was a minor topographic feature in the late Paleozoic, but formed a broad, north-south trending, regional anticline during the Laramide Orogeny in late Cretaceous to Tertiary time. Erosion stripped the Tertiary and Mesozoic strata from the area of Natural Bridges and left the older Cedar Mesa Sandstone exposed at the surface. Most of the erosion occurred during the last 6 million years when the Colorado Plateau began to rise. The sea retreated from the continent and horizontal forces thrust the Rocky Mountains skyward. As the plateau rose, the Colorado River and its tributaries cut down through the relatively soft sedimentary rocks and effectively entrenched their meandering patterns into the underlying bedrock.
River channels incised into the underlying sediments and filled with Tertiary gravels. Violent volcanic eruptions soon followed as the San Juan Mountains exploded in the mid-Tertiary. Extensional tectonics resulted in the opening of the Rio Grande Rift near the southeast margin of the Colorado Plateau. As the mountains rose, the processes of weathering and erosion began to bevel the mountain front into a relatively flat landscape (peneplain) gently sloping to the southwest. A combination of glaciation, increased runoff, a rising Colorado Plateau, and a subsequent lowering of the Colorado River�s baselevel, the carved the present-day topography.
During the Pleistocene Epoch of the Quaternary Period (1.64 million � 10,000 years before present) the climate of southeastern Utah was wetter and cooler and runoff from nearby glaciers caused massive, catastrophic flooding in the canyons. The hydraulic force of the rivers, coupled with other erosion processes such as frost wedging, root growth, and groundwater seepage, caused the canyon walls to thin on the upstream and downstream portions of the meander loops.
Today, chunks of canyon walls spall off during flash floods as swirling, turbulent water pounds both sides of the narrow necks, making them even thinner, and this process probably occurred in the Pleistocene, as well. Percolation of water through the wall during times of low water would have weakened the base of the cliff even more. When the canyon walls were breached, natural bridges formed. Three large natural bridges, Sipapu Bridge, Kachina Bridge, and Owachomo Bridge, formed within White and Armstrong canyons in the Monument. These bridges are among the ten largest natural bridges in the world.
The stratigraphic relationships, hydrology, and tectonics offer research projects that might benefit the park. They also create some potential geological issues that need to be addressed. Large oil and gas fields have been developed in southeastern Utah as have uranium mines that may all pose potential environmental impacts to the Monument region. The effects of alternative energy development have not been addressed for the area. The bridges of Natural Bridges National Monument stand as monuments to the grand expression of deep time, that time that surpasses our understanding but reminds us that Earth is not static but is subject to change.
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