Redwall Limestone

The Redwall Limestone of the Grand Canyon

 

Redwall 2

The 350-million year old Redwall Limestone is one of the most prominent features of the Grand Canyon. Its features:

• Diverse and long history over the last 350 million years.
• Magnificent cliffs and red walls.
• Composed of ~99.5% pure limestone, ~95% of which is biologically formed in the presence of organisms.
• Forms very chemically resistant cliffs, yet it is a very soft rock (slightly harder than a
finger nail).
• Has 1,000’s of miles of interconnected caverns spread out over the Colorado Plateau.
• Has many caverns, some with ancient and modern speleothems.
• Is the source of carbonate for the growth of abundant travertine deposits.
• Provides precious minerals, trace metals and uranium in breccia pipes
• Is a major source of high quality groundwater to numerous and voluminous springs in
the canyon and region, consumed by most visitors to the canyon.
• “Living in the Past” implies the past of the Redwall Limestone is living with us today.

Brian Gootee of the Arizona Geological Survey has designed a 27-slide history of the Redwall Limestone intended for guides and educators. It can be downloaded here:
http://repository.azgs.az.gov/uri_gin/azgs/dlio/1572

It has great graphics and an interesting story.

See also:

Origin of the Grand Canyon
Origin of the Lower Colorado River – a geological detective story

 

 

Sedona’s Sinkholes

The City of Sedona is surrounded by seven sinkholes. That is the subject of a new report by geologist Paul Lindberg published by the Arizona Geological Survey.  The spectacular red rock country near Sedona, Arizona, hides an unusual and potentially hazardous geologic feature.

DevilsKitchenSinkhole

Sinkholes are collapse features that form when surface, and near-surface rocks subside into the cavernous Redwall Limestone, which lies more than 600 feet below the surface. Gradual collapse of the roof of the cave results in a breccia pipe that extends upward from the Redwall Limestone to the floor of the sinkhole, situated up to 100 feet, below the exposed rim of the sinkhole. See cross-section below.

The Devils Kitchen sinkhole is the most active of the seven, with historic collapses in the 1880s, 1989, and 1995. The other sinkholes are in various stages of collapse, some beginning as early as the end of the last ice age, about 10,000-years ago. Structures like this on the north rim of the Grand Canyon host uranium deposits.

Sinkhole-Xsection

The following graphic (from the Devil’s Canyon report, cited below) shows how sinkholes develop.

Sinkholestoping

The Sedona sinkholes range in size from about 225 feet in diameter by 100 feet deep at Red Canyon, to Sinkhole 4543, which is 13 feet in diameter and about 3 feet deep. Devils Kitchen, the most well known of the sinkholes, has an opening 150 feet by 90 feet, with the floor situated 35 to 70 feet below the rim. Lindberg estimates that caverns in the Redwall Limestone, could have volumes on the order of 1.3 million cubic feet (a cave roughly 100 feet high and 130 feet in diameter).

According to the report, the groundwater of the Middle Verde watershed, which fills the Redwall caverns, began as precipitation on the Colorado Plateau, near the western flank of the San Francisco Peaks above 6,900 feet. Groundwater passes beneath Sedona at a flow rate of approximately 15 million gallons per day.

Sinkhole-arcuate

Lindberg notes, “While the danger of future collapse is minimal to humans, unregulated septic leakage into hidden sinkhole breccias within the town limits could contaminate groundwater being tapped for municipal use ….” A second hazard, which could threaten hikers and sightseers, are the presence of sandstone overhangs at several sinkholes that could collapse without warning in the near future.

The arcuate patterns as shown in the photo above represent a place where the rock has been stressed enough to break, but the rock has not yet collapsed. This may be the site of a future sinkhole.

(I wonder if arcuate patterns such as shown above might have given rise to the legend of vortexes near Sedona?)

Lindberg’s reports include pictures, maps, and schematic, to-scale drawings of each sinkhole. A pdf copy of the full report is available at the Geological Survey’s publications page in the Contributed Reports section: http://www.azgs.az.gov/publications_online.shtml

The general report on Sedona sinkholes (21 Mb) can be downloaded from:

http://www.azgs.az.gov/publications_online/contributed_reports/cr10c.pdf

A report specific to the Devil’s Canyon sinkhole (24 Mb) can be downloaded from :

http://www.azgs.az.gov/publications_online/contributed_reports/cr10b.pdf