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Sunset Crater Volcano National Monument

Geologic History

This section highlights the map units (i.e., rocks and unconsolidated deposits) that occur in Sunset Crater Volcano National Monument and puts them in a geologic context in terms of the environment in which they were deposited and the timing of geologic events that created the present landscape.

Geologic time scale
Figure 7: Geologic time scale; adapted from the U.S. Geological Survey. Red lines indicate major unconformities between eras.
Included are major events in life history and tectonic events occurring on the North American continent. Absolute ages shown are in millions of years.

Striking geologic features including stark black lava flows, eolian sand dunes, and cinder cones dominate the landscape of Sunset Crater Volcano National Monument. In geologic time, the eruption responsible for the present day landscape was practically instantaneous - a snapshot in time. The story of Sunset Crater Volcano National Monument begins much earlier. The sedimentary strata in the monument region record the growth of the North American continent as part of the geological region known as the Colorado Plateau.

The geologic history of the area began in the Precambrian Era when sediments and volcanic rocks were deposited in oceans and scattered island arcs (Figure 7). These deposits, once buried to great depths, were changed by heat and pressure. The deposits became metamorphosed and were further intruded by large bodies of molten magma solidifying into granite. Rocks of this age are exposed in the deepest depths of the Grand Canyon (Reynolds, 1982).

Shallow seas covered the region about 1.1 billion years ago. The sedimentation responsible for the Grand Canyon Supergroup occurred at this time along with intrusions and eruptions of basaltic lava. The region became part of a Western Interior Basin as land accreted to the western margin of the North American continent during the Paleozoic faulting and deformation.. A shallow sea covered the area leading to vast deposits of mudstones, sandstones, limestones, and siltstones (Reynolds, 1982). The 240 million- year- old Kaibab Formation is made of Permian limestone deposited in the large inland seas. The ancestral Rocky Mountains were uplifted and supplied sediment to the Sunset Crater Volcano area near the end of the Paleozoic as the crustal landmasses on the globe sutured together into one big supercontinent, Pangaea.

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 joined 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 event.

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, the Uncompahgre and the Front Range uplifts (Figure 8), formed along northwestsoutheast trending high- angle reverse faults.

Map of Major uplifts and basins present during the Pennsylvanian age in the southwestern United States.
Figure 8: Major uplifts and basins present during the Pennsylvanian age in the southwestern United States. Sediment eroded from the uplifts was deposited in the adjacent basins. Modified from Rigby (1977).

The broad coastal plain present in the area during the early part of the Mesozoic Era was the site of the deposition of the Triassic age Moenkopi Formation, present at Sunset Crater Volcano National Monument. Well- preserved ripple patterns, abundant mud cracks, and fossilized burrow traces are seen in the layered sandstone and shale of the formation. This deep red rock layer reflects a period of meandering rivers, tidal flats, and other near- shore deposits laid down on top of the Kaibab after the ocean regressed westward.

Following Moenkopi deposition, the area to the south experienced a mountain- building event otherwise known as an orogeny. This orogeny created an erosional surface, and the stream- carried deposits resulting from that erosion form the Chinle Formation.

As Pangaea began to break apart in the late Triassic Period and the landmasses began to drift to their present positions, the climate affecting the monument area became more humid. Sand dunes transformed into river systems, swamps, beaches, and broad level plains. Dinosaurs and other reptiles inhabited 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.

By Late Cretaceous time, about 97 million years ago, the Western Interior Seaway or Basin had drowned the previously continental deposits, and the monument region was an ocean basin, collecting fine- grained sediments that drifted far from the shoreline to the west and southwest. This Western Interior Basin or Seaway 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. 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 deposited by streams, wind, and ocean currents. These make up the widespread San Rafael Group, Dakota Sandstone, and Mancos Shale. These units are useful in correlating depositional environments across the Four Corners region. Mesaverde Group deposits of siltstone and shale were next deposited by the shallow seas pervasive during the late Mesozoic.

Near the end of the Mesozoic Era as the Cretaceous Period came to a close, compressive forces outside the borders of the Colorado Plateau caused the region to bow upward as a relatively coherent unit during the Late Cretaceous to Mid- Tertiary Laramide Orogeny. The seas retreated from Arizona at this time. The Rocky Mountains rose to the east. Great folds and faults formed in the orogeny; the Doney fault and Black Point monocline in nearby Wupatki National Monument are examples of such features (Figure 9). The Black Point Monocline was not present in the late Paleozoic, but formed a broad, north- south trending, regional anticline during the Laramide Orogeny in late Cretaceous to Tertiary time.

Regional uplift started the intense erosion whose resulting landforms define the Colorado Plateau. The magnificent canyons and sculptured rock formations began to develop with the plateau's mass of marine and coastal rocks now exposed to weathering by rain, river, and wind. Orogenic compression ceased about 35 - 40 million years ago.

Cross section of Sunset Crater Volcano National Monument.
Figure 9: Cross section of Sunset Crater Volcano National Monument. Graphic shows a cross sectional view of the structures beneath
Sunset Crater Volcano and Wupatki national monuments. Note regional Black Point Monocline and Doney Fault. Sedimentary layers
dip towards Little Colorado River Basin.

Erosion stripped the Tertiary and Mesozoic strata from the area of Sunset Crater and left the older sedimentary rocks 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 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 followed by hot spot volcanism in the San Francisco Volcanic Field of northern Arizona. Extensional tectonics resulted in the opening of the Rio Grande Rift near the southeastern 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 carved the present- day topography.

During the Pleistocene Epoch of the Quaternary Period (1.64 million – 10,000 years before present) the climate of northern Arizona 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 extreme erosion of the landscape.

The Little Colorado River left terrace gravel deposits behind as it cut down through the sedimentary layers over the last two million years. These deposits form flattopped mesa- like caps of grayish gravel over many of the red Moenkopi hills.

Geologic processes and resulting landforms continue to shape the ecosystem today. Volcanic mountain building continues with the formation of Sunset Crater Volcano only 900 years ago, a snapshot in geologic time (Reynolds, 1982). The slow, sometimes imperceptible erosional processes are still at work wearing down mesa tops and deepening the canyons. Uplift and buckling of the Colorado Plateau resulted in fracturing of the sedimentary rock layers.

limestone, dissolving the carbonate rock to form long, interconnected, vertical fractures, similar to how a cave forms. Small surface openings to this fracture system, called blowholes, breathe as the underground air responds to changes in the temperature and pressure. One such blowhole may be seen along the Wupatki Pueblo trail.

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. The cinder cone and surrounding landscape at Sunset Crater Volcano National Monument stand as a monument 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.


References:

Reynolds, S.J., 1982, Geologic features of northeastern Arizona. Fieldnotes from the State of Arizona, Bureau of Geology and Mineral Technology, vol.12, no.1, p.1- 8.

updated on 08/08/2007  I   http://www.nature.nps.gov/geology/parks/sucr/geol_history.cfm   I  Email: Webmaster
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