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Navajo National Monument

Geologic History

This section highlights the map units (i.e., rocks and unconsolidated deposits) that occur in Navajo 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 3. Geologic time scale; adapted from the U.S. Geological Survey and International Commission on Stratigraphy.

The strata in Navajo National Monument record Upper Triassic through Lower Jurassic time, but rocks on the adjacent Black Mesa span the entire Mesozoic Era (Map Unit Properties Table; figure 3). The Mesozoic Era followed the most extensive mass extinction preserved in the geologic record - the end of the Permian Period when 96 percent of all species were eliminated (figure 3) (Raup 1991). At the beginning of the Mesozoic, shallow, marine water stretched from Utah to eastern Nevada over a relatively level continental shelf.

Triassic Period (251-200 Ma)
During the Triassic Period, the major land masses came together forming the supercontinent, Pangaea. On the western margin of Pangaea, a subduction zone formed that trended north- northwest to south- southeast with oceanic crust dipping eastward beneath the continental margin. In the Early Triassic, volcanic activity decreased on the western margin of Pangaea and igneous rocks were emplaced along this subduction zone (Saleeby et al. 1992; Christiansen et al. 1994).

Moenkopi Formation:
The reddish siltstone, shales, and sandstones of the Moenkopi Formation were deposited in fluvial, mudflat, sabkha, and shallow marine environments that formed as the shallow sea withdrew from northeastern Arizona. Ripple marks and low- angle cross- bedding in the Moenkopi formed as a result of fluvial processes that leveled the area into a relatively flat plain. The presence of fossilized plants (reeds and Equiseta), trackways of reptiles and amphibians, and fossils of warm water marine invertebrates in strata above gypsum zones indicate a shift from a cool, dry climate to a warm, tropical climate in the Early Triassic (Stewart et al. 1972A; Dubiel 1994; Huntoon et al. 2000).

In northern Arizona, red beds of the Holbrook Member of the Moenkopi Formation are lowermost Middle Triassic (235- 240 Ma) based on paleontologic evidence (Dubiel 1994). However, Middle Triassic rocks are generally absent in the Western Interior. Local deposits of lowermost Middle Triassic strata suggest that erosion, rather than nondeposition, is largely responsible for the absence of Middle Jurassic rocks. A regional unconformity separates the Lower Triassic from the Upper Triassic.

Paleogeography of the southern part of the Western Interior basin during deposition of the Late Triassic Chinle Formation.
Figure 10. Paleogeography of the southern part of the Western Interior basin during deposition of the Late Triassic Chinle Formation. A volcanic arc has formed along the western margin of North America by this time.

Chinle Formation:
Rocks of the Chinle Formation in the Western Interior of North America are a complex assemblage of fluvial, marsh, lacustrine, playa, and eolian deposits from the Late Triassic (208- 235 Ma) (figure 10) (Stewart et al. 1972B). This suggests the Chinle was deposited in a densely vegetated flood plain or mud flat that contained localized shallow ponds and small, shallow, sinuous streams (Scott et al. 2001).

Floodplain mudstones that completely encase fluvial sandstones in the Petrified Forest Member of the Chinle Formation signify deposition by high- sinuosity streams. Altered glass shards and bentonitic mudstones indicate that volcanic ash formed a significant component of the sediment (Dubiel 1994). Fossils of phytosaurs, lungfish, and lacustrine bivalves reflect river, lake, and marsh environments. The Petrified Forest Member grades upward into the Owl Rock Member.

The knobby texture in the Owl Rock Member is thought to result from extensive bioturbation, an interpretation supported by the numerous crayfish burrows found locally in the Owl Rock. Owl Rock sediments were deposited in an extensive lacustrine and marsh environment in response to continued subsidence and to a reduction in clastic and volcanic sediment input (Stewart et al. 1972B; Dubiel 1994). Major fluvial environments, evident in the underlying members of the Chinle, are lacking in the Owl Rock Member, suggesting that the paleoflow in the lower part of the Chinle was disrupted so that rivers and streams backed up and formed ponds during deposition of the Owl Rock. Subaerial exposure in dry periods when lakes and marshes dried up, allowed soil formation processes to modify primary textures (Dubiel 1994). In places, the Owl Rock strata fills valleys eroded into the underlying Petrified Forest Member.

Sandstones and mudstones of the overlying Church Rock Member were deposited by fluvial systems, on lacustrine or playa mudflats traversed by small fluvial systems, and as eolian sand sheets and dunes (Dubiel 1994). The large- scale, eolian cross- stratification and mudcracks found in the Church Rock Member in northern Arizona and the Four Corners area indicate that dry periods became more prevalent during deposition of the uppermost part of the Chinle.

The development of extensive fluvial, lake and marsh systems in the Triassic may be related to uplift associated with converging lithospheric plates along the west coast (Dubiel 1994). As the oceanic plate pushed beneath the overriding North American continent, magma was generated forming linear or arc- shaped belts of volcanoes on the overriding plate, parallel to the subduction zone. Mount St. Helens and the other volcanoes in the Cascade Range formed in a similar way and lie parallel to an active subduction zone that extends from northern California to Canada. Triassic subduction and the evolution of an arc- shaped belt of volcanoes probably influenced the lake systems of the Owl Rock Member, causing drainage reversal in the Church Rock.

Paleogeography of the southern part of the Western Interior basin during deposition of the Early Jurassic Navajo Sandstone.
Figure 11. Paleogeography of the southern part of the Western Interior basin during deposition of the Early Jurassic Navajo Sandstone.

Jurassic Period (200-146 Ma)
In the present Four Corners region of Arizona, New Mexico, Colorado, and Utah, the Jurassic Period was a time of extensive dune formation (figure 11). The region was located about 18 degrees north latitude at the beginning of the Jurassic Period and moved to 30- 35 degrees north latitude by the end of the Jurassic (Kocurek and Dott 1983; Peterson 1994). This is the latitude of the present day northeast trade wind belt where cool, dry air descends from the upper atmosphere and sweeps back to the equator in a northeast to southwest direction. The cool, dry air becomes warm, dry air causing intense evaporation. Most modern hot deserts of the world occur within the trade wind belt. The climate of the Colorado Plateau during the Jurassic appears similar to that of the modern Western Sahara of Africa.

The Jurassic deserts that existed for roughly 40 million years (not counting the time represented by erosion) contained sand dunes that may be the largest ever recorded (Kocurek and Dott 1983). Similar to the modern Sahara, these ergs formed on a coastal and inland dune field. These dunes extended from present day southern Montana south and east into eastern Utah, westernmost Colorado, southwestern Colorado, northeastern Arizona, and northwestern New Mexico (Kocurek and Dott 1983; Peterson 1994).

Volcanic islands formed an unknown distance west or southwest of the west coast of North America in the Middle Jurassic. During the Late Jurassic, these islands accreted to the North American plate (Busby- Spera 1988; 1990; Marzoff 1990). Major tectonic plate reorganization in the Late Jurassic followed the Cordilleran magmagenerating episode of the Middle Jurassic. At this time, the Gulf of Mexico opened and the North American lithospheric plate rotated counterclockwise. To accommodate the plate motion, a large transform fault zone called the Mojave- Sonora megashear, developed along what is now the Mexico- United States border and truncated the southwestern margin of North America. This northwest- southeast trending megashear zone accommodated approximately 500 to 600 miles (800- 1000 km) of left- lateral displacement (Kluth 1983; Stevens et al. 2005; Anderson and Silver 2005; Haenggi and Muehlberger 2005).

Wingate Sandstone:
During the Lower Jurassic, the northern sea in the Arctic region did not encroach onto the continent as it had in the past. Paleozoic sandstones exposed from as far north as Montana and Alberta provided abundant sand transported by wind to the Colorado Plateau (Kocurek and Dott 1983).

Westerly to southwesterly winds transporting sand from Alberta to Arizona may have been diverted to the south by a rising a volcanic arc off the western coast of North America (Kocurek and Dott 1983). Sediments from the volcanic arc to the west are missing from the dune sand on the Colorado Plateau. While the volcanic arc diverted the wind from Alberta, the trade winds probably swept the volcanic ash to the southwest, out to sea. Rivers flowing from the Ancestral Rockies may have provided an additional source of sand to the growing dune fields.

The regional depositional geometry of the Wingate Sandstone, the high- angle cross- bedding and the wellsorted frosted quartz grains indicate that the Wingate was eolian (Peterson 1994). Regionally, six major erg sequences have been mapped in the Wingate (Nation 1990; Blakey 1994). The six erg units vary in detail from one another, but generally, both the overall Wingate Sandstone succession and the individual erg sequences display an upward drying trend with small dunes and sandsheets of large cross- bedded dunes overlying sabkha and lacustrine deposits (Blakey 1994).

Kayenta Formation:
A change from eolian to fluvial deposition is recorded in the sandstones of the Kayenta Formation. In contrast to the sweeping eolian cross- beds of the underlying Wingate and overlying Navajo Sandstones, the crossbeds in the Kayenta are only a few feet thick. Interbedded sandstones, basal conglomerates, siltstones, and mudstones are typical channel and floodplain deposits. Paleocurrent studies show that during deposition of the Kayenta, rivers flowed in a general westward to southwestward direction (Morris et al. 2000). The rocks of the Kayenta Formation display an excellent example of the effects of a climate change resulting in ergs of the Wingate Sandstone being reworked by fluvial processes (Blakey 1994).

Navajo Sandstone:
The Navajo Sandstone records a return to arid conditions and the development of extensive ergs on the Colorado Plateau (figure 11). Sand dune deposits reaching 800 to 1,100 ft (240 to 340 m) high gradually overtook the fluvial systems of the Kayenta. The large- scale (18 m, 60 ft), high- angle, cross- beds of the Navajo attest to the presence of Sahara- like sand dunes during the Early Jurassic (Morris et al. 2000). The paleolatitude of Navajo National Monument during the deposition of the Navajo Sandstone was near 20 degrees north latitude (Parrish and Petersen 1988; Chan and Archer 2000). Paleo- wind directions shifted more northerly giving rise to subtropical and monsoonal circulation patterns in the region. Studies of the cyclicity in Navajo dune sets suggest that the region experienced alternating wetter and drier periods on a decade scale in the Early Jurassic (Chan and Archer 2000).

Lithospheric plate collisions intensified off the western coast in the Middle Jurassic causing rock layers on the continent side of the collision to bulge upward. Weathering and erosion stripped away the exposed rocks and a regional unconformity surface formed on the Navajo Sandstone.

Carmel Formation:
As plate tectonic activity increased, the sea lapped onto the continent from the north. The reddish siltstones and mudstones of the Carmel Formation (Middle Jurassic) were deposited on broad tidal flats marginal to a shallow sea that lay to the west.

Entrada Sandstone:
The Entrada Sandstone (Middle Jurassic) originally covered the entire Colorado Plateau. The Entrada is the most widespread of the preserved late Paleozoic and Mesozoic eolianites. The cross- bedded sandstone was deposited in an extensive dune field in a back- beach area (Kocurek and Dott 1983; Hintze 1988; Peterson 1994; Doelling 2000). Together, the Entrada Sandstone and Carmel Formation record three of the five transgressiveregressive episodes that deposited the Middle Jurassic strata on the Colorado Plateau.

Summerville Formation and Cow Springs Sandstone:
As lithospheric plate collision increased on the western margin, a major transgression of the inland seaway destroyed the vast eolian sand seas that once covered the Colorado Plateau. Tidal flats formed in the area as marine environments encroached from the north. Restricted marine and tidal flat deposits of the Summerville Formation (Middle Jurassic) mark the southern extent of the seaway. The Cow Springs Sandstone (now considered to be a member of the Entrada Sandstone) preserves the remnants of a once vast eolian sand sea (Kocurek and Dott 1983; Peterson 1994).

Morrison Formation:
The Morrison Formation (Upper Jurassic), known for dinosaur fossils and for uranium occurrence (Peterson 1994), was deposited across the western continental United States with the final regression of the Jurassic sea. The stratigraphy of the Morrison Formation reflects a mostly fluvial origin: mudflats, overbank and floodplain deposits, and stream channels, as well as small eolian sand fields, and scattered lakes and ponds. The Morrison with its banded pink, maroon, green, and gray shales is prominent and identifiable over much of the Colorado Plateau.

Paleogeographic map showing the extent of the Cretaceous Interior Sea.
Figure 12. Paleogeographic map showing the extent of the Cretaceous Interior Sea. Shaded areas indicate land above sea level.

Cretaceous Period (146-66 Ma)
Fast- flowing streams from highlands to the southwest eroded the softer shales and siltstones of the Morrison Formation, creating a regional unconformity in the rock record between the Jurassic and Cretaceous Periods. However, Lower Cretaceous fluvial, floodplain, and lacustrine deposits, present elsewhere on the Colorado Plateau, have been eroded from the Black Mesa area.

With plate collisions continuing on the western margin, the continental landscape experienced a dramatic change in the Upper Cretaceous. Mountains rose in the west and a north- south trough formed adjacent to the mountains. As the trough subsided, a shallow sea advanced onto the continent from both the Gulf of Mexico and the Arctic Ocean. The sea advanced and retreated many times during the Cretaceous until the most extensive interior seaway to cover the continent drowned much of western North America from about 95 to 64 Ma (figure 12). The advances and retreats of the Cretaceous Sea created a myriad of environments including incised river valley systems, estuaries, coal swamps, lagoons, delta systems, beaches, and shallow marine. These deposits are complex, and the rocks formed from these sediments include alternating and interfingering marine sandstones, shales, and coal beds forming the Dakota Sandstone, Mancos Shale, and the Mesaverde Group.

Dakota Sandstone:
In general, the Dakota Sandstone records shallow marine deposition with some intermittent mud flat and stream deposition. Coal swamps formed in the quiet backwaters of estuaries. Some of the sandstones may have been deposited in paleovalleys incised into the coastal plain during a regressive episode (Gardner and Cross 1994). With burial and increased temperature, the organic material of the Dakota Formation slowly transformed into coal and hydrocarbons. The coarse- grained sandstone layers are today reservoirs for oil and gas as well as and groundwater.

Mancos Shale:
The thick sequence of shale and siltstone with sandstone stringers and minor gypsum and limestone forming the Mancos Shale was deposited in the advancing Cretaceous seaway. For roughly ten million years, clay, silt, sand, and shell debris were deposited in the Cretaceous Interior Sea. At first glance, the formation appears to be 2000+ ft (700 m) of uniform, monotonous black and gray shales. Yet, the history of the Mancos on the Colorado Plateau reflects at least four major changes in depositional systems where shoreline and near- shore environments replaced with new environments created as sea level rose and fell (Aubrey 1991).

Fossil evidence suggests that ocean currents within the Cretaceous Interior Sea were variable. At times, the currents circulated oxygenated water throughout the water column allowing life to prosper at all levels, including within the muddy sea bottom. Conversely, at other times, the circulation in the Seaway was restricted to the upper water layer, and black, organic- rich muds accumulated in the oxygen- poor sea bottom. In those environments, the fossil material includes very few, if any, bottom fauna.

Torevo Formation and Wepo Formation:
The Torevo Formation and Wepo Formation are part of four major transgressive- regressive marine cycles in the Upper Cretaceous (Elder and Kirkland 1994). The formations were deposited from approximately 91- 84 Ma and are slightly younger than the Point Lookout Sandstone at Mesa Verde National Park. The rest of the Cretaceous record, from 84- 66.4 Ma, is missing in the Black Mesa area.

Aggrading fluvial deposits that formed the Toreva Formation record the early phases of transgression. The sediment sources for the Toreva were rifted uplands to the southwest and south (Elder and Kirkland 1994). The rivers flowed to a northwest- southeast trending shoreline that extended from southwest Utah, across northeastern Arizona, and into central New Mexico.

The Wepo Formation resulted from a regressive interval following the transgression that deposited the Toreva Formation. Organic matter accumulated in lagoons and marshes that formed in a coastal- plain depositional system. With burial and elevated temperatures, the organic matter was transformed into coal.

The Yale Point Sandstone is a marine sandstone and refelcts another transgression into the Western Interior Basin. Cretaceous strata deposited above the Yale Point Sandstone have been eroded from the Navajo National Monument area.

Late Cretaceous-Early Tertiary (70-35 Ma)
The North American lithospheric plate collided with the Farallon plate, producing the Laramide Orogeny (about 70- 35 Ma). This event transformed the extensive basin of the Cretaceous Interior Seaway into smaller faultbounded basins. Thrust faulting during the Laramide Orogeny brought Precambrian plutonic and metamorphic basement rocks to the surface.

Tectonic activity during this time warped the Colorado Plateau region into broad anticlinal and synclinal folds with very little large- scale (kms) faulting (Dickinson and Snyder 1978; Chapin and Cather 1983; Hamilton 1988; Erslev 1993). The Organ Rock Monocline that plunges the Navajo Sandstone beneath Black Mesa is a result of Colorado Plateau deformation during the Laramide Orogeny.

With continued uplift in the late Tertiary and Quaternary, most of the Upper Cretaceous strata was eroded from the Black Mesa area.

Tertiary Period (66-1.8 Ma)
Near the end of the Laramide Orogeny, from about 35- 26 million years ago, in mid- Tertiary time, volcanic activity erupted across the Colorado Plateau. The laccoliths that formed the Sleeping Ute Mountain, La Plata Mountains, Henry Mountains, La Sal Mountains, and Abajo Mountains were emplaced during the mid- Tertiary.

Today, Late Cretaceous shoreline deposits are found on the Colorado Plateau at elevations of several thousand feet. Since the end of the Cretaceous Period 66 million years ago, the Colorado Plateau has risen about 12,000 ft (3,660 m) (Fillmore 2000). Some of this uplift occurred quite rapidly in geologic time. As the rate of uplift increased, so did the rate of erosion. The Colorado River, for example, carved its present course within the last 6 million years.

Quaternary Period (1.8-0.01 Ma)
In the Pleistocene (1.8- 0.01 Ma) ice ages, streams carved deep valleys and river channels into the Colorado Plateau. In the wetter climate, groundwater flow through the permeable Navajo Sandstone was restricted at the contact with less permeable shale and siltstone layers in the Kayenta Formation. The groundwater flowed laterally along the contact to the canyon walls forming seeps and springs. During this time, deep alcoves began to form in the Navajo Sandstone.

The Colorado Plateau as a whole has been subjected to repeated minor uplifts and stream rejuvenation since the end of the last ice age. Today, the climate is drier than during the Pleistocene ice ages yet the intermittent streams in the canyons are still actively downcutting, only at a slower rate than in the past.

The Pleistocene Jeddito Formation and the Holocene Tsegi and Naha Formations are alluvial units in Tsegi Canyon in Navajo National Monument. The Jeddito Formation occurs as remnant high alluvial terraces below Holocene alluvial units. The Tsegi Formation rests on bedrock or locally on the Jeddito Formation (Clay- Poole 1989). The Naha Formation is the youngest unit, forming a terrace 40- 50 feet (12- 15 m) above the stream floor and 10- 15 feet (3- 5 m) below the Tsegi terrace. From carbon- 14 dating of wood fragments near the base of the Tsegi, deposition began around 5,389 years BP and ended about A.D. 1275- 1300. Erosion removed extensive areas of the Tsegi, often to bedrock. Naha deposition began after A.D. 1375 and continued until A.D. 1884. Fresh- water gastropods, pelecypods, and ostracods have been found in the Naha Formation (Briscoe 1974). A new cycle of erosion is in progress today (Clay- Poole 1989).


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