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How Geological Age Is Calculated

A Timely Perspective

The geologic time periods that are important to the oil industry in the Lloydminster area are the Devonian (408-360 million years ago), the Mississippian (360-320 m.y.a.), and the Cretaceous (144-66 m.y.a.). 408 million years is an unfathomable amount of time and is difficult to understand within our finite human experience. But how are these dates arrived at and why are they so widely accepted by geologists.

The age of the earth is believed to be 4.5 billion years because the oldest rocks ever found have been radiometrically dated to be that old. Those rocks were found in Australia, while closer to home some rocks from the North West Territories were dated at 3.8 billion years, as well, moon rocks are about 4.6 billion years old.

Radiometric dating is accomplished through measuring the amount of radioactive decay the isotopes in the rock have experienced. Isotopes of a given element have the same number of protons, but a different number of neutrons. For example: U-238 and U-235 are two common uranium isotopes, U-238 has 92 protons and 146 neutrons while U-235 has 92 protons and 143 neutrons. These isotopes are unstable and decay at a spontaneous, but predictable rate. This radioactive decay is measured in half-lives, a half-life is the amount of time it takes 1/2 of the original isotopes to decay into a stable or daughter element, and is different for each isotope. K-40/Ar-40 dating is an exception as it is not the result decay, but a process by which an electron is captured in the K nucleus; the negatively charged electron cancels a positively charge proton changing the element’s atomic number from 19 to 18, this changes the atom from K(potassium) to Ar(argon). The scientist compares to ratio of potassium to argon gas within the rock. The K40/Ar40 method cannot be used if the argon gas has had an opportunity to escape from the rock.

Isotope 

Half-Life

Daughter Product

K(potassium)-40 1.3 billion years Ar(argon)-40
U(uranium)-238  4.5 billion years Pb(lead)-206
U(uranium)-235 713 million years Pb(lead)-205
Th(thorium)-232 14.1 billion years Pb(lead)-208
Rb(rubidium)-87 49 billion years Sr(serium)-87
C(arbon)-14 5,730 years N(nitrogen)-14

These forms of dating are used to find the amount of elapsed time from when the rock crystallized to the present, therefore, rock from the origin of the earth will have a different ratio of parent to daughter elements than a rock that was created by recent volcanism or sedimentation. When a scientist dates a rock he/she uses as many processes as possible and correlates the results to find the rock’s age. This is called absolute dating. For rocks whose position within the strata can be ascertained absolute dating is combined with relative dating.

Relative dating does not give the rock an age, it is simply a means to determine which rock came first. Relative dating includes certain assumptions:

Original Horizontality: when sediments are deposited they create horizontal or nearly horizontal layers.

Superposition: in a sequence of undisturbed sedimentary rock the layers get younger as you go from bottom to top. An expansion of this idea includes the use of fossils, depositional environments, and an understanding of facie changes to correlate strata of different areas. For example the change in the rocks that occurs during an on-lap sequence, sandstone/shale/limestone, indicates a facie change; the sediments were deposited at the same time and within a naturally occurring sequence of environments, shore to marine. As well, sediments occurring a distance apart, but containing the same fossils or fossils known to be of the same age would indicate rock of the same age.

Cross-cutting Relations: states that disturbed rock is older than the rock which caused the disturbance. For example when volcanic rock intrudes into previously existing sedimentary rock, the volcanic rock is obviously younger. There is also the assumption of conformity, this supposes that events happen in a predictable sequence, when this sequence is disturbed an unconformity is observed.

Below Lloydminster the economically important strata include those from the Devonian, Mississippian, and Cretaceous. An over simplification of events shows the deposition of Devonian and Mississippian rocks as having occurred in a predictable manner, however, between the Mississippian and the Cretaceous there is an obvious unconformity representing approximately 200 million years. Rocks missing below Lloydminster are present to the west.

The rules original horizontality and superposition must be carefully observed as there are many examples where these rules cannot be followed. In regard to original horizontality, viewing the bent, tilted and twisted rock beds in the Rockies indicates that this rule cannot be taken as an absolute. As well, there are instances where tectonic forces have pushed older chunks of the earth on top of younger and at times the rock has also been folded over onto itself. When this folding occurs not only is older rock above the younger but the sequence is also reversed.

When dating rocks the scientist must take great care. The only rule that is universally accepted is to NEVER SAY ALWAYS!!! Relative dating rules are not always true, not all absolute dating methods can be used in every instance, and there could be some anomaly that distorts the results. Consequently, scientists use as many of the methods as they can and hope that the results of the tests correspond.