Stratigraphic column

Earlier in this activity you learned about stratigraphy and Steno’s four laws of stratigraphy. Now you are going to learn how geologists document stratigraphic layers so they can be applied to research in an organised manner.

To keep track of all the units and formations present in an area, we stack them on top of each other in a diagram. This creates a visualisation of the order of rock types present in the area, just like we do in the geological timescale. We call this diagram a stratigraphic column. Every rock unit and formation are given a specific name and colour to make them easier to discriminate in a stratigraphic column and for visualisation on maps. A detailed stratigraphic column is not straight but varies in width grain size between rock types and is important in many studies.

Stratigraphic columns help geologists to better comprehend the area they are researching and to put relative ages to units in the field. This can be useful if you are looking for a certain occurrence. For example, if you are looking for an intrusion that is known to contain certain metals, you would like to know what stratigraphic layers are older than the intrusion since the intrusion might be present in those layers but not in the younger ones according to the law of cross-cutting relationships. Knowing which layers are interesting is hard to visualise with only the names of the formations, but if you take a glance at the stratigraphic column, you will instantly see in what layers to look for the metal hosting intrusion. In the stratigraphic column shown below for example, you would look at the lowest four units since those are cross-cut by the intrusion.

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Figure: Detailed, hand-drawn stratigraphic column that contains formations, units, package thickness, lithology and grain size.

In short, a stratigraphic column can be used as a timescale for the formation of rocks, but sadly not all times are represented in the current stratigraphy everywhere on Earth. At some point, deposited rocks will likely encounter an uplifting event and get subjected to erosion, or possibly a subduction event that may convert the rock into an igneous rock as we can learn from the rock cycle. In both cases, a disruption or a break in the ongoing deposition occurs. This creates a time gap in the rock record.

To exemplify, if we were making a rainbow cake, this can be seen as the loss of a colour in the rainbow. If you forget to add the yellow layer to your rainbow cake, the cake will not be complete and will miss a crucial component. Such a gap is called a hiatus. After the hiatus, new sediments are deposited on top of the older ones, and we call the contact between the old and new rock layers an unconformity. Unconformities are very common in stratigraphy and almost always act as the borders between formations since they tend to represent a notable geological event in the history of that particular area.