GMEG - Geology and Geophysics
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TASK 4 - Surficial geology of the Mesa Verde National Park area, Colorado
Statement of Problem:
|Mesa Verde National Park is an upland surface sloping gently to the south deeply entrenched by ephemeral canyons. Elevations range from about 8,500 ft along the northern escarpment to about 6,200 ft along the Mancos River in the southeast corner of the Park. The Park includes parts of six 1:24,000-scale quadrangles (Cortez, Mancos, Moccasin Mesa, Point Lookout, Trail Canyon, and Wetherill Mesa – see accompanying figure). The Park was established in 1906 to preserve and protect the Native American archeological sites, including the famous cliff dwellings, of the Ancestral Puebloans, who lived in the area from about 550 AD to 1300 AD. 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 commonly associated with the Cliff House Formation of late Cretaceous age, which weathers to form deep alcoves. In addition, the ancient people farmed the thick loess deposits on the mesa tops, which because of its clay content has 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.||
Cliff Palace Ruin at Mesa Verde National Park
Today geology is still an important concern within Mesa Verde National Park as the park is plagued by various forms o 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 buildings). Because the Park has only one entrance road, which crosses several miles of active landslides, a major slump during the peak of the tourist season has the potential to trap thousands of visitors in the Park. In April 1979 such a landslide occurred and closed the Park for about one month (Smith, 2002). Over the years the “Federal Highways Commission has sunk millions of dollars into keeping the Park’s road system open for business” (G. San Miguel, written communication, 3/17/05).
Landslide at Point Lookout on the main entrance road to Mesa Verde National Park
|Several years ago 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, rock fall, and flood erosion and deposition. Erosion is common after wildfires, which cause hydrophobic conditions and hence, increased runoff (G. San Miguel, written communication, 3/17/05). Many of these hazards endanger roads, buildings, archeological 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, 2) under represent 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 the surficial deposits and associated hazards, is needed in order to help the Park better manage its resources.
As 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.
The main objective of this task is to provide surficial geologic map coverage of the Mesa Verde National Park area 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 300 sq km (120 sq mi). The surficial map will focus on the various physical properties of the map units and associated geologic hazards (a "recently published "Preliminary landslide" map has identified more than 200 landslide in the park area - see products). 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. In 1978 the Park became the first U.S. location designated by UNESCO as a world World Heritage Cultural Site.
As stated above, the objective of this task is to produce a surficial geologic map of Mesa Verde National Park area. To adequately map the Park area will require; 1) about 30 days in the field each year, 2) stereo coverage of the Park with high resolution aerial photos, 3) inspection and review of previous literature and geologic maps in and near the park, 4) use of a PG-2 stereo plotter and VR-1 digital system in the USGS's Denver Photogrametric Lab in order to accurately map the various deposits, and 5) the work of the Earth Surface Processes Team GIS Lab, also in Denver, to prepare the digital geologic map of the Park.
In addition, this mapping task will require enough OE to purchase radiocarbon ages from within the USGS's in-house lab (currently $375/age). The radiocarbon ages are necessary to date alluvium in the deep canyons and loess on the mesa tops. Currently, there are no published radiocarbon ages associated with surficial deposits within the Park. Charcoal fragments have been found in several alluvial fills within the Park and radiocarbon dated. Ages obtained from these charcoal fragments indicate recent (< 500 yrs) down cutting of the alluvial fills. Other related studies have focused on; 1) the age and location of previously undated minette dikes in the Park, (in collaboration with David Gonzales, Fort Lewis College)– (Ar/Ar ages of 25.65±0.08 and 24.97+0.12 Ma have been obtained from dikes in Navajo and Mancos Canyons). 2) Alluvial stratigraphy in the canyons and 3) age and origin of the loess capping the upland mesas – (in collaboration with Dan Muhs, ESP Team, Denver).
Carrara, P. E. and O’Neill, J. M., 2010, Tree-ring dated landslide movements and seismic events in southwestern Montana, USA, in Stoffel, M., Bollschweiler, M., Butler, D. R., and Luckman, B. H., editors, Tree rings and natural hazards: a state-of-the-art. Springer, Berlin, Heidelberg, New York, p. 421–436.
Ager, T. A., Carrara, P. E., Smith, J. L., Anne, V., and Johnson, J., 2010, Postglacial vegetation history of Midkof Island, Alexander Archipelago, southeastern Alaska. Quaternary Research, v. 73, p. 259-268
Carrara, P. E., 2009, Preliminary map of landslide deposits in the Mesa Verde National Park area, U.S. Geological Survey Scientific Investigations Map 3090, scale 1:50,000. http://pubs.usgs.gov/sim/3090/.
Map, Planned: P. E. Carrara, 2011, Surficial geologic map of the Mesa Verde National Park area, southwestern Colorado, USGS, Scientific Investigations Map
Carrara, P. E., 2010, Treeline changes during the Holocene in the upper Colorado River Basin [abs.]. U.S. Geological Survey Climate Change Conference March 9–11, 2010, Denver, CO. p. 20.