Buell Park Pipe, Arizona - July 2004 Issue

Buell Park Pipe, Arizona

by Edgar B. Heylmun, PhD

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One of the world’s largest kimberlite pipes is located on the Navajo Indian Reservation in northern Arizona. The pipe is larger than any diamond-bearing pipe in South Africa. It is 16 miles north of Window Rock, the capital of the Navajo Nation. Permission to enter Buell Park should be obtained at tribal offices in Window Rock, though the writer received permission from one of the residents in Buell Park. The floor of the park is covered with grass that, after rains, gives it a park-like appearance. Even though surrounding lands are covered with junipers and pinyon pines, scarcely a single shrub or tree grows on the floor of the structure. This same phenomenon can be seen in the Stateline kimberlite district of Colorado and Wyoming.

Buell Park is in the Navajo volcanic field in northeastern Arizona. There are over 80 volcanic necks, dikes, and pipes in the field, all of which are composed of unusual rock types. Buell Park is circular and 5 square miles in area, the floor of which lies at an average elevation of 7,250 feet above sea level. Surrounding sandstone cliffs of Permian age rise as high as 400 feet above the grassy park. Buell Mountain, in the northwest part of the park, rises to 8,171 feet in elevation.

Geology

Buell Park lays near the axis of the Defiance Uplift, probably just a coincidence. The dominant rock type is a serpentinized kimberlite. It is well exposed at a reddish weathered knob, 120 feet high, in the northeastern part of the park. A minette ring dike is present in the eastern half of the structure. The dike rises to an elevation of 7,520 feet at Peridot Ridge (see map). It is believed that the kimberlite pipe was emplaced at the close of the Oligocene epoch, some 25 million years ago. The alkalic rocks that form Buell Mountain intruded the kimberlite at that time and erupted as a volcano. Most geologists believe that Buell Park was fanned by a massive phreatic explosion, the explanation given for the origin of the maars in Germany. That explanation is far more bizarre than to suggest that the pipe was the result of a comet or asteroidal impact, which created fractures that extended downward and trapped ultramafic magma in the Earth’s upper mantle. Erosion has removed hundreds of feet of sedimentary rock, including megabreccias and shatter cones that would have confirmed an impact origin. In 2002, an asteroid passed within 75,000 miles of the Earth, considered a “near-miss” by astronomers.

The kimberlite at Garnet Ridge, near the Utah line, is not circular, but very irregular in outline. This could represent the roots of what was, 25 million years ago, another impact site caused by a fragment of the same comet or asteroid that hit Buell Park. It no doubt broke up in the Earth’s atmosphere. The Shoemaker-Levy comet created several impact sites, hundreds of miles apart, when it broke up upon entering Jupiter’s atmosphere. It crashed into Jupiter in 1994, and the event was recorded on film.

The kimberlite at Buell Park was gas-charged, and rose explosively as it was emplaced. The writer believes that the kimberlite at the Green Knobs is rootless, and represents ejecta from the violent eruption in Buell Park. The fracture pattern at the Green Knobs is like a stack of saucers, which one might expect when ejecta lands on a firm surface. The harmonic oscillations in the Earth’s crust, caused by the impact, opened avenues for gas-charged magma to rise and form other igneous features in the region.

The kimberlite was highly gas-charged but was not as hot as one might think, because there is virtually no contact metamorphism nor alteration of the sedimentary host rock. Many of the alkalic basalt intrusions in the region had the consistency of putty and as they were intruded, fluted borders were formed. There are inclusions (xenoliths) of mantle rock, such as eclogite, in many of the diatremes in the region, and they are not altered.

The map shows Buell Park and Green Knobs. At Buell Park, the ring dike rises to an elevation of 7,520 feet at Peridot Ridge.

view large map in separate window: 649K

As the kimberlite magma lost its volatiles, the whole mass collapsed, causing the sandstone walls encircling Buell Park to slide inward. The rimrock slumped, and there are circular slump faults on the west side of the pipe. Alkalic basalt erupted and formed Buell Mountain in the northwest part of the pipe.

Indicator Minerals

Kimberlite indicator minerals are abundant at Buell Park. The most obvious are olivine and pyrope garnet. At the aforementioned reddish knob in the park, composed of serpentinized kimberlite, numerous green anthills are composed of rounded olivine crystals. The largest crystals that the ants can excavate are 7mm in length, but larger crystals are no doubt present in the area. Some are of gem-quality (peridot). Red pyrope garnet is also abundant. One of the best collecting sites is about 1,000 feet southeast of the knob. Collecting can also be done on Peridot Ridge, part of the minette ring dike. Chrome diopside is a spectacular mineral when it is large enough to cut.

The kimberlite contains olivine, pyrope, enstatite, chrome diopside, and titan-clinohumite, whereas the minette contains chrome diopside, gem-quality olivine (peridot), and phlogopite. Diamond is a key indicator mineral, if present. There is a tendency for smaller pipes to contain more diamonds than larger pipes. Diamond, if present at Buell Park, is microscopic.

Summary

Gem-quality minerals are present at Buell Park. Navajo Indians collect gems at Buell Park periodically and sell them to tourists. Indians now work at sawmills and wood product plants, and rarely spend time collecting. Weathering and erosion have probably brought many new specimens into view. Remember that these are Indian lands, not public lands, and permission to enter and collect must be obtained from the proper authorities.

Considerable literature has been published on Buell Park. A good place to start is US Geological Survey GQ 1649, published in 1990.