Geochronology

© Craven & Pendle Geological Society

Principle of Uniformitarianism
Present day geologic processes have operated throughout geologic time.

"The present is the key to the past."

The laws of physics and chemistry that governed geologic processes in the past are the same as those that govern processes now and in the future.

The geologic timescale is a chronology (calendar) of events on Earth based on obtaining ages of past events. These ages have been derived from relative dating and absolute dating (radiometric dating) of rock layers and fossils.

(a) Relative Dating
This technique uses principles of stratigraphy (rock strata) and the study of fossils (palaeontology) to determine the relative ages of rocks and sediments. (relative geologic timescale)

(b) Absolute Dating
Following the discovery of radioactivity in 1895, radiometric dating techniques were developed to determine the absolute ages, i.e. dates in years, of rocks and sediments. (absolute geologic timescale).

Field geologists' rely on a number of simple techniques for dating rocks and constructing geological successions.  Underpinning this are four key concepts:

1. Principal of Original Horizontality i.e. sediments are deposited as horizontal beds.

2. Law of Superposition i.e. each layer of sedimentary rock in a tectonically undisturbed sequence is younger than the one beneath it. Things get a little more complicated especially when the layers of strata are folded in such a way as to be completely overturned!

3. Principle of Cross-cutting Relationships i.e an igneous intrusion or fault must be younger than the rocks it intrudes or cuts.

4. Law of Strata Identified by Fossils

The Law of Strata Identified by Fossils is a little bit more complex.  In the succession of strata, each layer represents the geographical conditions that occurred over that area at the time the layer was deposited.  In each period of earth's history different fauna and flora evolved  with similar faunas being characteristic of similar beds.

The diagram right shows two distinct faunal assemblages.  What can be deduced from our two distinct faunas in terms of their usefulness in the relative dating of rocks?  The table below summarises key features:

Fauna A
E.g. goniatite: Gastrioceras listeri

1 Evolved
2 Rapidly spread
3 Dies out quickly
4 Valuable Zone Fossil

Fauna B
E.g. brachiopod: Lingula

1 Evolved
2 Spreads slowly
3 Does not die out quickly
4 Not much use for zoning or identification of rocks

Gastrioceras listeri is a particularly good example of a ZONE fossil.  As it is free swimming it could have travelled a considerable distance.  When found in rocks as far away as Australia you could reasonably expect those rocks to be the same age as those in Cliviger Valley!

A third fact emerges from the study of stratigraphy - the UNCONFORMITY. If the geological record was 100% complete and there were no breaks in sedimentation, the stratigrapher's job would be an easy one.  However, breaks in sedimentation do occur and when this happens, geologists' refer to this missing strata as an unconformity.  Techniques of accurately dating rocks is crucial in establishing the time-gap that exists.

The classic type is the angular unconformity of which Thornton Force in Ingleton and Combs Quarry, Foredale are perhaps the best known local examples.  This is shown below:

The Angular Unconformity

The strata A to E was deposited first, tilted, subjected to erosion (peneplained) and submerged.

Strata X to Z were laid down horizontally on top of the older beds.  The plane or junction, marked by the thick green line, between both sets of strata is the unconformity.

At Thornton Force the junction is marked by a basal conglomerate.

It can get more complicated!