New dating technique may revise geologic scale — Age test developed by U of T researchers expected to have practical applications

A new technique used to determine the age of sedimentary rocks could refine the geological time scale and provide new insight into global sea-level variations, according to a recent study.

A team of researchers from the University of Toronto and the Universite P.

et M. Curie in Paris has developed a method of directly dating individual grains of a group of clay minerals (known collectively as glaucony), which commonly form as sediments deposited in water.

Using a method developed at U of T, laser probe argon-argon dating, the study analyzed individual glaucony grains from three bulk samples previously used to construct the geological 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 with dates from igneous minerals.

Traditionally, sedimentary rocks have been dated by inferring their age from the ages of surrounding igneous rocks, or by using potassium-argon dating to obtain average ages on large glaucony samples within the sediment. The ages arrived at using the latter technique, however, are often a few million years less than those calculated for the surrounding igneous rocks, so the technique has been considered unreliable.

“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

U of T’s physics department. “We hope other scientists will now re-integrate glauconies to revise dates and ultimately produce a better geological time scale.”

The study found that different states of evolution among grains in a sample may also indicate variations in sea level. 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 were 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 of the U of T researchers.

The results of the study are expected to have practical application in the oil exploration industry, because the sea-level conditions for forming glaucony are similar to those required for the growth of the organisms that eventually turn into oil.

Evensen and Smith worked on the study with lead investigator Derek York and Gilles Odin of the Universite P. et M. Curie. The study was paid for by the Natural Sciences and Engineering Research Council.