Mar. 6, 1998 A new technique to determine the age of sedimentary rocks could refine the geologic time scale and provide new insight into global sea level variations, according to a study in the March 6 issue of Science.
A team of researchers at the University of Toronto and the Université P. et M. Curie in Paris has developed a method to directly date individual grains of a group of clay minerals, called glaucony, that commonly form within sediments while they are being deposited in water.
Using a method developed at U of T-- laser probe argon-argon dating-- the study analysed individual glaucony grains from three bulk samples previously used to construct the geologic time scale. The individual grains yielded ages scattered over millions of years and almost all of the ages were younger than the true age. Only the oldest glaucony grains gave the correct ages, which were known before by comparison to dates from igneous minerals.
Until now, scientists have calculated the ages of sedimentary rocks either by inferring from the ages of surrounding igneous rocks or using potassium-argon dating to obtain average ages on large glaucony samples within the sediment. The glaucony technique has been considered unreliable since the ages arrived at are often a few million years younger than those found in the surrounding igneous rocks, and for this reason some geologists have ignored glauconies in constructing their time scales.
"The ability to look in detail at a sample grain by grain is what proved to be crucial in discovering why glaucony dates come out too young," says Norman Evensen of the U of T's department of physics. "We hope other scientists will now reintegrate glauconies to revise dates and ultimately produce a better geologic time scale."
The study says the different states of evolution among grains in a sample may also indicate variable sea levels. Glaucony forms in shallow sea water and this process slows or stops if the ocean is too shallow or too deep. Consequently some grains may form in a sediment millions of years after it was deposited when sea level conditions above it are appropriate.
"We think we have a way of dating the times at which glaucony formation is going on, which allows us to follow the ups and downs of sea level," says Patrick Smith, another investigator from the department of physics at U of T. "Each grain is a different clock." This new knowledge has applications in the oil exploration industry since the sea level conditions for forming glaucony are similar to those required for the growth of the organisms that eventually turn to oil.
Evensen and Smith worked on the study with lead investigator Derek York of U of T's physics department and Gilles Odin of the Université P. et M. Curie. Funding was provided by the Natural Sciences and Engineering Research Council.
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