GEOL 102 Historical Geology

Spring Semester 2017
Index Fossils, Correlation & Radiometric Dating

Search for some means by which strata in one region could be correlated with those at another.

Needed something that had a particular non-repeating, unique, global pattern.

William "Strata" Smith, creating first geologic maps of southern England (and expanded out to include the Continent) observed that the pattern of fossils through the strata was consistent from location to location. Developed this into a new stratigraphic principle:

In order to be an index (or guide) fossil, the organism used must have certain desirable features:

Here is an excellent summary of biostratigraphic correlation.

Combining physical stratigraphy and biostratigraphy, 19th Century geologists were able to correlate and tie together strata, igneous events, and structures across western Europe. And biostratigraphy allowed correlation with rocks even on other continents.

In combination, the principles of stratigraphy were useful for determining a global relative time scale, but questions of numerical time were still unresolved.

Discovery of radioactive decay at the dawn of the 20th Century gave the key:

Radiometric Dating: the single most important method of determining numerical rock ages.

Need some special conditions, however:

When possible, radiometric dates of different isotopes with different decay rates are calculated for same sample. If these converge, good support for that age.

Isochron Dating techniques: a way to get around the problems of standard radiometric dating.

The following is an animated demonstration of the isochron method, from

Animated Isochron

(Incidentally, while the two main geological radiometric systems (40Ar/39Ar and 238U/206Pb) are now highly precise, the numbers they yielded were slightly different based on traditional calibrations. There was already reason to suspect that Ar-dating was slightly miscalibrated on geochemical arguments. Recalibrating the Ar-clock based on astrochronology (see below) suggests that this technique as typically used yield numbers about 1% too young. As a consequence, dates from before the 2010s often don't quite match up with our current understanding...)

Other methods of numerical dating:

Some other methods of relative dating:

To Lecture Notes.

Last modified: 18 January 2017