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Recent and Ongoing Bureau of Geology Research Projects:
Geochronologist studies missing rocks

The Great Unconformity that J.W. Powell recognized in the Grand Canyon is easily seen in the Sandia Mountians where the 1.4 Ga (billion years old) Sandia granite is overlain by Pennsylvanian sedimentary rocks. The gap in the rock record between the ancient granite and the limestones on Sandia Crest represents nearly a quarter of the earth's geological history.

Mark Nohl photo, courtesy of New Mexico Magazine

Dr. Matthew Heizler (geochronologist) has just been awarded a three year grant from the NSF tectonics division to study the "Great Unconformity" exposed in western North America. An unconformity is a span of time for which no rock record is represented because it has been eroded away or because sediment was never deposited. The Great Unconformity was coined by John Wesley Powell during his epic run of the Colorado River through the Grand Canyon, AZ in 1876. Here he noticed that deformed ancient metamorphic rocks were covered by much younger undeformed sedimentary rocks. New Mexico has some of the best exposures of the contact between these very old Precambrian rocks (1.7 billion years) and younger sediments (300 million years) of anywhere in North America.

The question(s) to be answered are what happened during the more than 1 billion years that is no longer recorded by any rock record? To provide answers, Matt collects samples of the old rocks just below the unconformity and uses a technique known as 40Ar/39Ar thermochronology to measure the temperature history locked within various minerals. At times when the old rocks were being heated, it can inferred that they were being buried, and when they where cooling, it can be inferred that rock overlying the sample site was being removed. This information eventually leads to geological reconstruction despite the lack of any remaining rock. So, along with collaborators at the University of New Mexico and graduate students at Tech, Matt will need to climb the highest peaks of New Mexico and Colorado and raft the Colorado River through the Grand Canyon to collect samples beneath the Great Unconformity. These samples will be analyzed at the NM Geochronology Research Laboratory and will hopefully tell the story of the billion years of missing geological history.