Navajo National Monument Geologic Resource Evaluation Report

Graham, John

This report has been developed to accompany the digital geologic map produced by Geologic Resource Evaluation staff for Navajo National Monument in Arizona. It contains information relevant to resource management and scientific research. Navajo National Monument lies entirely within the Navajo Reservation in northeast Arizona. The monument preserves the most intact cliff dwellings of the Ancestral Puebloan people in three separate units: Betatakin Unit (160 acres), Keet Seel Unit (160 acres), and Inscription House Unit (40 acres). These Pueblo villages, built in natural alcoves eroded into the Navajo Sandstone, date to the late 13th Century. Navajo National Monument is located on the Shonto Plateau, part of the larger Colorado Plateau Physiographic Province. Uplift and erosion have carved deeply incised canyons into this part of the Colorado Plateau. The Organ Rock Monocline, an uplift underlying U.S. Highway 160 between Cow Springs and Kayenta, Arizona, forms the border between the Shonto Plateau and the Black Mesa region to the south. The Black Mesa region includes about 5,400 square miles (14,000 sq km) in northeastern Arizona and has a diverse topography of flat plains, mesas, and incised drainages. Navajo National Monument is in the Tsegi Canyon system, the primary drainage of the eastern part of the Shonto Plateau. Tsegi Canyon exposes nearly flat-lying Mesozoic sedimentary strata. Mesozoic strata at Navajo National Monument record marine, nearshore, and continental depositional environments. These environments formed in response to tectonic activity on the western margin of North America. The three main formations exposed in the canyon include, from youngest to oldest, the Navajo Sandstone, the Kayenta Formation, and the Wingate Sandstone. Isolated exposures of the Chinle Formation are found in the canyon bottom, with unconsolidated alluvial deposits overlying bedrock. Rock-fall and cliff collapse pose the greatest geologic issue for resource management at the monument. Because of the steep canyon walls, significant rock-fall hazards threaten cultural resources in alcoves as well as visitor safety. The Aspen Forest Trail leading to Betatakin Ruins was closed in 1982 due to rock-fall and currently remains closed. Flash flooding and erosion in the canyon bottoms, groundwater and surface-water quantity and quality, and regolith compaction are also management concerns. Flash flooding threatens cultural resources, erodes stream banks, and promotes vertical incision of the arroyos. Hikers and grazing livestock trample stream banks, compacting the regolith, and accelerating erosion. The Navajo Sandstone is the primary aquifer for the Black Mesa area. As visitation increases at the monument and coal mining activities increase on Black Mesa, groundwater withdrawal from the Navajo aquifer is an increasing concern at Navajo National Monument. The primary geologic features at Navajo National Monument are the alcoves in which the Ancestral Puebloans built their dwellings. Geologic factors such as the presence of springs and seeps, large sweeping cross-beds in the Navajo Sandstone, and vertical fractures that serve as conduits for water, all contribute to the formation of alcoves. Springs and seeps occur at the contact between the porous and permeable Navajo Sandstone and the less permeable Kayenta Formation. Groundwater flows laterally in the subsurface when it encounters the Kayenta, emerging as springs and seeps along the canyon walls. The discharged water dissolves the carbonate cement holding the sand grains of the Kayenta Formation together and subsequent erosion removes the grains. Undercutting of the Navajo Sandstone controlled by both cross-bedding and vertical fractures, leads to cliff collapse.

Navajo National Monument Geologic Resource Evaluation Report

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