by Paul Carrara, Geologist (email: Paul Carrara)
The purpose of this task is to provide surficial geologic map coverage of Mesa Verde National Park in southwestern Colorado (fig. 1) at a scale of 1:24,000. This task will focus on identifying, mapping, and developing unit descriptions and a chronology for surficial deposits over an area of approximately 82 mi2. The surficial geologic map will focus on the various physical properties of the map units and associated geologic hazards, which include landslides, flash floods, and swelling soils. The map will provide the National Park Service with a digital surficial geologic map of Mesa Verde National Park, a highly visible (500,000 visitors/year) park and one of the crown jewels of the National Park system. This task is part of a larger project focusing on the geology of various National Parks and federal lands in the southwestern United States and is funded by the U.S. Geological Survey's National Cooperative Geologic Mapping Program.
Figure 1. General location map of the Mesa Verde National Park area showing selected place names mentioned in the text and figures.
Mesa Verde is essentially a broad, flat, upland surface sloping gently to the south and dissected by deep canyons (fig. 2) containing ephemeral streams. At its northern edge, elevations of the upland surface range from about 8,000 to 8,570 ft and drop off steeply into Montezuma Valley (fig. 3), at elevations of 6,000 to 6,200 ft, which contains the towns of Cortez and Mancos (fig. 1). At the southern edge of the upland surface, approximately 9-10 miles distance from its northern edge, elevations range from about 6,700 to 6,900 ft and the canyons that have been entrenched into it are as much as 1,000 ft deep. The ephemeral streams draining Mesa Verde National Park are tributary to the Mancos River to the east and south of the park (fig. 1).
Figure 2. Cliff Canyon, a typical canyon in Mesa Verde National Park, showing the broad mesa top and deep canyon entrenched by an ephemeral stream. The Cliff House Sandstone forms the two prominent cliffs below the mesa surface, while the steep slope below the lower cliff is formed in the underlying Menefee Formation.
Figure 3. Photograph from Point Lookout looking west showing the steep northern escarpment, which drops off into Montezuma Valley.
Mesa Verde National Park has a semiarid climate. Climatic data, gathered near the park's headquarters at an elevation of 7,110 ft, indicate that the average annual precipitation is 17.86 inches. The minimum precipitation occurs in June (0.6 in.) before the start of the "Arizona Monsoon," and the maximum precipitation occurs in July (2.00 in) as the monsoonal precipitation arrives. The average annual temperature is 49.4°F, with January being the coldest month (29.2°F) and July the warmest (71.6°F) (unpublished data accessed December 8, 2005, at http://www.wrcc.dri.edu/index.html).
The vegetation in the park reflects the semiarid climate of the region. Pinyon-juniper woodland covers much of the mesa tops and canyon slopes within the park. With increasing elevation these trees become larger and are spaced closer together. In the higher reaches of the park, as well as in the well-shaded canyons, ponderosa pine, Douglas fir, and Gambel oak are also present.
Figure 4. Stratigraphic column of formations present in Mesa Verde National Park (after Wanek, 1959, and Griffitts, 1990).
Four geologic formations (fig. 4), all Cretaceous in age, are exposed in Mesa Verde National Park (Wanek, 1959; Griffitts, 1990; Condon, 1991). The lowest formation is the Mancos Shale, a thick sequence of gray to black marine shale containing minor tan siltstone and fine sandstone beds. On steep slopes, such as those near the northern and eastern boundaries of the park, this formation is prone to landslides and debris flows. In the Point Lookout area, near the park's entrance, the Mancos Shale is about 2,000 ft thick (Wanek, 1959). Overlying and grading into the Mancos Shale is the Point Lookout Sandstone (fig. 3) of the Mesaverde Group, a predominantly yellowish-gray or pale-orange, fine- to medium-grained marine sandstone, approximately 300-400 ft thick (Wanek, 1959). The Point Lookout Sandstone forms much of the cap rock in the northern park area. The Menefee Formation conformably overlies the Point Lookout sandstone and consists of lenticular sandstone beds with interbeds of siltstone, shale, and coal. In the Mesa Verde area, the Menefee Formation is about 350-400 ft thick (Wanek, 1959) and forms broad slopes within many of the park's canyons. This formation is prone to slope failure and forms large landslides in many of the canyons. Above the Menefee Formation is the Cliff House Sandstone (fig. 2), consisting of a grayish-orange to pale-yellow, fine-grained, thick-bedded sandstone. The formation commonly consists of two massive, cliff-forming sandstone beds, each over 100 ft thick, separated by a thin shale unit (Griffitts, 1990). The upper sandstone bed weathers to form deep alcoves in which many of the cliff dwellings have been built (fig. 5) (Wanek, 1959; Griffitts, 1990).
Figure 5. Long House cliff dwelling built in a large alcove formed in the upper sandstone unit of the Cliff House Sandstone.
Mesa Verde National Park was established in 1906 to preserve and protect the dwelling sites, including the famous cliff dwellings of the Ancestral Puebloans, who lived in the area from about 550 A.D. to 1300 A.D. The geology of the park played a key role in the lives of these ancient people. For example, the numerous (approximately 600) cliff dwellings are usually associated with the Cliff House Sandstone. In addition, the ancient people farmed the thick, reddish loess deposits on the mesa tops (fig. 6), which because of their clay content, have good moisture retention properties. The soil on this loess cover and the seasonal rains associated with the "Arizona Monsoon" enabled these people to grow their crops (corn, beans, squash) on the broad mesa tops.
Today, geology is still an important concern within Mesa Verde National Park because the park is plagued by various forms of mass movement (landslides, debris flows, rock falls), swelling soils, and flash floods that affect the park's archeological sites as well as its infrastructure (roads, septic systems, utilities, and building sites). Because the park has only one entrance road (fig. 7), a major slump or erosion during an intense thunderstorm has the potential to trap thousands of visitors in the park. After an exceedingly wet fall in 1978 a series of landslide movements, along the east side of Point Lookout occurred during the spring of 1979 and closed the entrance road for over a month (Smith, 2002). The first landslide occurred on April 28th, when 100,000 ft3 of material slumped onto a 200-ft section of the road. A second landslide on April 30th, deposited about 600,000 ft3 of material onto the road north of the first slide, and on May 27th, a 250-ft section of one lane of the road moved downslope, further delaying the reopening of Mesa Verde National Park (Smith, 2002). Over the years, millions of dollars have been spent keeping the park's road system open (G. San Miguel, written communication, March 17, 2005).
Figure 6. Photograph of a kiva at Far View Community on Chapin Mesa. Note the reddish-colored loess, characteristic of the Mesa Verde area, on the floor of the kiva. Depth of kiva floor estimated to be about 6 ft below ground surface, giving a minimum thickness for the loess at this location.
Figure 7. Photograph of the entrance road into Mesa Verde National Park. The road traverses a steep slope of Mancos Shale, along the eastern side of Point Lookout, which is prone to slope failure.
In 1998, a geologic resources inventory workshop was held concerning Mesa Verde National Park. Much of the workshop focused on the geologic hazards plaguing the park and specifically mentioned landslides, debris flows, rockfall, swelling soils, and flood erosion and deposition. Erosion is common after wildfires, which cause hydrophobic conditions and increased runoff (G. San Miguel, written communication, March 17, 2005). Many of these hazards endanger roads, buildings, archaeological sites, and utilities. One of the major conclusions of the workshop report was that "a detailed surficial geology map is a high priority for the park and would provide insight for both modern land use as well as ancient land use related to the cultural resources."
Although there are several existing geologic maps that encompass or contain parts of the Mesa Verde National Park area (Wanek, 1954, 1959; Hayes and others, 1972; Colton and others, 1975; Condon, 1991; Griffitts, unpublished), these maps (1) are at a small scale (1:250,000), (2) underrepresent the surficial geology, or (3) both. Hence, there is a lack of adequate surficial information for the park at a scale more appropriate for planning (1:24,000). Geologic information, including surficial geology with a careful consideration of the engineering characteristics of surficial deposits and associated hazards, is needed in order to help the park better manage its resources.
Because the distribution of surficial deposits is strongly related to archaeological sites, plant communities, animal habitats, and soils, the information produced by this task will lead to an increased understanding of the park's ecosystem and prehistoric land-use patterns. The task will also benefit visitors by providing information to update or expand current interpretive materials related to the park's geology and ecology.
Mapping of the surficial deposits in the study area will be accomplished by a variety of methods including (1) compilation from existing geologic maps, (2) stereoscopic analysis of 1:12,000-scale color and 1:40,000-scale black-and-white photographs, and (3) fieldwork. Where accessed in the field, detailed information will be obtained on the surficial deposits. Based on previous mapping in nearby areas, surficial units present in Mesa Verde National Park should include (1) alluvium of ephemeral streams, (2) alluvium of the Mancos River, (3) terrace alluvium of the Mancos River, (4) ancient gravels on the upland surface, (5) debris-flow deposits on pediment surfaces, (6) loess, (7) landslides, and (8) undifferentiated colluvial deposits.
Fieldwork has confirmed the presence of numerous landslides in the Mancos Shale and Menefee Formations within the park. Below the escarpment the major forms of landsliding appear to be large rockfalls, rock avalanches, and debris flows. Evidence of this is indicated by slopes and isolated hills capped by Point Lookout Sandstone rubble overlying and protecting the underlying Mancos Shale from erosion (fig. 8). One such hill, near the entrance station, is about 1.5 miles from the nearest outcrop of Point Lookout Formation and suggests an age of several hundred thousand years. Within the canyons of the park, the Menefee Formation commonly yields large rotational landslides. Many of these larger landslides probably date from the late Pleistocene (about 128,000 to 11,000 years ago) when the climate was cooler and effectively wetter.
Figure 8. Isolated hills of Mancos Shale capped by Point Lookout Sandstone rubble along the entrance road. The sandstone rubble was originally deposited by large rockfalls, rock avalanches, or debris flows on a Mancos Shale surface. The sandstone rubble then acts as a protective cap, while surrounding areas of Mancos Shale without caps were lowered by erosion.
A radiocarbon age of 2,235±30 yr B.P. (before present) was obtained from charcoal fragments recovered from a well-bedded clayey silt deposit behind a landslide dam in Navajo Canyon. This age provides a minimum (probably a very minimum) age for the landslide.
Charcoal fragments recovered from exposures of sandy alluvial fills at shallow depths in Whites and Morefield Canyons yielded radiocarbon ages of 350±30 yr B.P. and 415±30 yr B.P. These ages indicate that the alluvial fills are quite young and their incision even younger.
Charcoal fragments recovered near the base of a large (15 ft) exposure of colluvium near the head of Prater Canyon yielded a radiocarbon age of 8,170±35 yr B.P., which provides a rate of colluvial deposition of 1.8 ft/1,000 years.
Lag gravels, probably deposited by an ancestral Mancos River, were known to exist on Big Mesa (fig. 1) near the southeast corner of the park (Wanek, 1954, 1959). This study has found another area of lag gravels on Whites Mesa, about 1 mile west of Big Mesa. These lag gravels are scattered across the mesa surfaces and consist almost exclusively of quartzite pebbles and cobbles and a few small boulders. The present-day gravels of the Mancos River include a number of rock types (volcanics, sandstone, granites) including quartzite clasts similar to those found in the lag gravels. The mesas on which these lag gravels are located are about 1,500 to 1,600 ft above the present-day Mancos River. The fact that the lag gravels are almost exclusively quartzite (other rock types have weathered away) indicates their old age (pre-Pleistocene?).
Mesa Verde National Park contains several minette dikes trending southwest to northeast. Minette is a potassic (K) mica lamprophyre, greenish gray to black in color, characterized by a fine-grained groundmass of alkali (K) feldspar (typically sanidine), diopside, biotite-phlogopite, and apatite, with phenocrysts of biotite-phlogopite, clinopyroxene, and less commonly olivine. More than 80 Oligocene and Miocene (ca. 28-19 Ma) volcanoes and intrusive features are distributed across the Colorado Plateau (Semken, 2003). Previous to this study no radiometric ages had been determined on any of the minette dikes in Mesa Verde. An 40Ar/39Ar age of 25.65±0.08 million years was obtained on biotite from a sample of the minette dike exposed in Navajo Canyon (fig. 9) and is interpreted as the intrusion age of the dike (L. Peters, written communication, 2007). In addition, in June 2008 a sample from a minette dike in Mancos Canyon was submitted for 40Ar/39Ar dating, but the results are not yet available.
Figure 9. The minette dike in Navajo Canyon from which an 40Ar/39Ar age of 25.65±0.08 million years was obtained.
I am working with a team of researchers from Northern Arizona University who are studying the vegetation and fire history of Mesa Verde through pollen analyses and alluvial stratigraphy. Radiocarbon ages obtained by this research team will be incorporated into my studies of the surficial deposits within the park.
The task is funded by the National Cooperative Geologic Mapping Program.
The task staff consists of Paul Carrara (USGS Earth Surfaces Processes Team, Denver) who is assigned to this task on a half-time basis.
Carrara, P.E., 2011, Surficial geologic map of Mesa Verde National Park, Montezuma County, Colorado. U.S. Geological Survey Scientific Investigations Map, scale 1:24,000.
Colton, R.B., Anderson, L.W., Holligan, J.A., Patterson, P.E., and Shaver, K.C., 1975, Preliminary map of landslide deposits, Cortez 1° × 2° quadrangle, Colorado and Utah: U.S. Geological Survey Miscellaneous Field Studies Map MF-699, scale 1:250,000.
Condon, S.M., 1991, Geologic and structure contour map of the Ute Mountain Ute Indian Reservation and adjacent areas, southwest Colorado and northwest New Mexico: U.S. Geological Survey Miscellaneous Investigations Series Map I-2083, scale 1:100,000.
Griffitts, M.O., 1990, Guide to the geology of Mesa Verde National Park: Mesa Verde Museum Association, 88 p.
Griffitts, M.O., unpublished geologic map of Mesa Verde National Park and vicinity, scale 1:42,000.
Hayes, D.D., Vogel, J.D., and Wyant, D.G., 1972, Geology, structure, and uranium deposits of the Cortez quadrangle, Colorado and Utah: U.S. Geological Survey Miscellaneous Investigations Map I-629, scale 1:250,000.
Semken, S., 2003, Black rocks protruding up: the Navajo volcanic field. New Mexico Geological Society Guidebook, 54th field conference, Geology of the Zuni Plateau, p. 133-138.
Smith, D.A., 2002, Mesa Verde National Park; shadows of the centuries. University Press of Colorado, revised edition, 275 p.
Wanek, A.A., 1954, Geologic map of the Mesa Verde area, Montezuma County, Colorado: U.S. Geological Survey Oil and Gas Investigations Map OM-152, scale 1:63,360.
Wanek, A.A., 1959, Geology and fuel resources of the Mesa Verde area, Montezuma and La Plata Counties, Colorado: U.S. Geological Survey Bulletin 1072-M, p. 667-721, plate 49, scale 1:63,360.
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