We will be sharing this report with our congressional
members to help them make informed decisions about the proposed Grand Canyon Watershed
National Monument.
A 1989 USGS study mapped 1294 breccia pipes in the region. Subsequent USGS maps and new investigations by AZGS identified at least 1,000 additional features in just two small study areas (outlined in red in map above). It appears from this work that the number of likely breccia pipes
is one to two orders of magnitude greater than previously recognized.
The study raises the possibility that the higher concentration of
breccia pipes extends across the entire region.
Mineralized breccia pipes—pipe-like masses of
broken rock—may contain high-grade uranium ore and variable amounts of
copper, gold, silver, vanadium and other mineral ore. More than 71
mineralized breccia pipes have been discovered in the region, and as of
2010, nine of these pipes yielded more than 10,500 metric tons of
uranium.
Breccia pipes are vertical formations, typically a
few tens to hundreds of feet across and hundreds to thousands of feet
in vertical extent. The pipes formed more than 200 million years ago
within Paleozoic and Triassic rocks over a broad area around Grand
Canyon. The pipes formed as groundwater, flowing through Redwall
Limestone dissolution breccias and along fracture zones, dissolved more
limestone, causing collapse of overlying rocks and possibly creating
sink holes.
This new map is accompanied by an Excel Workbook
database with three datasets. The datasets are drawn from geologic maps
produced by the U.S. Geological Survey and from mapping by geologic
consultant and co-author Karen Wenrich. The datasets include point
locations and comments on features identified as 1) breccia pipes, 2)
collapse structures that might be breccia pipes, and 3) circular
features that might be collapse features or breccia pipes.
Some
features occur in more than one dataset, so the total number of
features is less than the 3,286 features comprising the three datasets.
GIS data as ArcGIS shapefiles built from the three datasets are
included with this publication.
US Geological Survey geoscientists estimated that
roughly 8% of breccia pipes contain some mineralization (Wenrich and
Sutphin, 1988). A fraction of those are likely to host economic
concentrations of minerals.
In 2012, the U.S. Department of the Interior
withdrew from mining 1,006,545 acres of federal lands in northern
Arizona for a 20-year period to prevent further exploration or
development of uranium on those lands. Withdrawal curtails new
exploration of breccia pipes and limits production to those pipes with
valid existing mineral rights.
Citations:
Spencer J.E., Wenrich, K. and Cole, T., 2015,
Partial database for breccia pipes and collapse features on the Colorado Plateau, northwestern Arizona. Arizona Geological Survey Digital Information, DI-42, 5 p., 1 map plate, shapefiles, and Excel Workbook.
Wenrich, K.J. and Sutphin, H.B., 1988,
Recognition of Breccia Pipes in Northern Arizona. Arizona Bureau of Geology and Mineral Technology, Fieldnotes, v18, #1, p1-5.
In the 1930s a landslide in the center of the city of Jerome is credited with moving the jail house hundreds of feet downslope. In fact, the building was moved to get it on more stable ground. But this spring, part of the landslide started moving again. City officials have blocked off part of a parking lot where large cracks are continuing to grow. [Top, view to the northwest, with Sliding Jail in center background. My photo.] ADOT is monitoring the movement because of the potential for impacts on US 89A. Otherwise there does not seem to be much of a threat to property, in large part due to the city keeping development off the old landslide complex.
