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Homegrown Quartz Muddies the Water
30 August 2000 7:00 pm
Fine-grained rocks called mudstones hold clues to the Earth's history--but they must be read correctly. Now a team of geologists has found that researchers may have been using a faulty key to decipher mudstones' codes, throwing into question conclusions about everything from ancient climate to paleogeography. However, no one knows yet what proportion of mudstones may have been misinterpreted.
Mudstones, now commonly exposed in cliffs and roadcuts, were formed in the sea by clay that was washed from land. The clay contains fine grains of quartz. The size and distribution of these grains, it's believed, can reveal how far the sediments traveled from shore or even whether they took an airborne journey from a desert. Such inferences assume that quartz silt, like mudstone clay, started out on land. However, sedimentary geologist Jürgen Schieber of the University of Texas, Arlington, and his colleagues didn't think interpreting mudstone quartz was this simple. Several years ago, they found that quartz sand grains in 370-million-year-old mudstones of the eastern United States formed inside sand-sized, hollow algal cysts.
To see whether smaller, silt-sized quartz grains might have a similar origin, Schieber's team took a closer look at those same mudstones. The quartz grains showed distinctive concentric rings, as though they'd been deposited over time, sort of like the coating on a pearl, and were bordered by amber-colored rims that resembled the walls of algal cysts. The team confirmed the diagnosis by looking at oxygen isotope ratios: The mudstone quartz grains were more similar to quartz precipitated under the sea than quartz from mountains. The local quartz was surprisingly common, too. In some samples, Schieber found that all the silt had grown in place rather than eroding from land.
The presence of so much homegrown silt may have skewed geological interpretations of mudstones, Schieber says. Mistaking local quartz silt for windborne silt, for example, might lead one to erroneously postulate desertlike conditions on land. The finding "makes life more complicated," says Kitty Milliken, a geologist at the University of Texas, Austin, who studies mudstones, "but it gives us the tools to be clear and figure it out."