The purpose of this task is to provide surficial geologic map coverage of the Mesa Verde National Park area in southwestern Colorado (fig. 1), at a 1:24,000 scale. This task will focus on identifying, mapping, and developing unit descriptions and a chronology for surficial deposits over an area of approximately 140 sq mi. The surficial 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 geologic map of Mesa Verde National Park, a highly visible (500,000 visitors/year) park unit and one of the crown jewels of the National Park system. This task is one 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. Photograph of Cliff Canyon, a typical canyon in Mesa Verde National Park, showing the broad mesa top and deep canyon entrenched by an ephemeral stream.
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Figure 3. Photograph from Point Lookout looking west showing northern escarpment of the upland surface, which drops off steeply into Montezuma Valley.
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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," while 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, on the World Wide Web at URL 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).
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Four geologic formations (fig. 4), all Cretaceous in age, are exposed in Mesa Verde National Park (Wanek, 1959; Griffitts, 1990). 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 periphery 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 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 failure on steep slopes and forms large landslides in many of the canyons of the Park. Above the Menefee Formation is the Cliff House Sandstone, 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. Photograph of Long House cliff dwelling built in an alcove of the upper sandstone unit of the Cliff House Sandstone.
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The 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, as mentioned above, are usually associated with the Cliff House Sandstone. In addition, the ancient people farmed the thick, reddish-colored 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" allowed 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 it's 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. Over the years, millions of dollars have been spent keeping the Park's road system open (G. San Miguel, written communication, March 17, 2005). Hence, the surficial mapping will also include a hazard assessment of the Park's road system.
Figure 6. Photograph of kiva at Far View Community on Chapin Mesa. Note the reddish-colored loess, characteristic of the Mesa Verde area, at the bottom (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.
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Figure 7. Photograph of entrance road into Mesa Verde National Park. Road traverses several landslides and is a source of almost continuous maintenance.
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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 are either (1) 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.
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.
Although work on this task began only recently (October 1, 2005), preliminary analysis of aerial photographs has revealed numerous landslides within the Park's canyons. Many of these landslides head in the Menefee Formation; future studies will focus on the age of these landslides.
Other possible studies related to the surficial mapping may focus on the (1) age and origin of the loess capping the upland mesas, (2) alluvial stratigraphy in the canyons, and (3) age of igneous dikes present in the southern area of the Park.
None as yet - Task was begun October 1, 2005
Colton, R.B., Anderson, L.W., Holligan, J.A., Patterson, P.E., and Shaver, K.C., 1975, Preliminary map of landslide deposits, Cortez 1° X 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.
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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