Diamictite is a poorly sorted or non-sorted terrigenous non-calcareous sedimentary rock that contains variously sized clasts from clay to boulders in a muddy matrix. Diamicton (or diamict) is a non-lithified diamictite (sediment).

The definition above was proposed by Flint et al. in 19601 and it was meant to be and still is purely descriptive without any genetic connotations.

There is a need for a descriptive rock definition in this case because the rocks that superficially look alike may form in different ways. In many cases diamictite is a glaciogenic rock tillite. Sometimes diamictite is erroneously considered to be a synonym of tillite. But diamictite may also be a lithified lahar (volcanic mudflow), volcanic flank collapse breccia, it may form underwater as a part of a turbidite flow, it may be composed of dropstones in marine sediments, etc. These are all lumped together under an umbrella term diamictite.

This rock is sedimentary, it contains larger clasts in a fine-grained matrix-supported groundmass. The material is terrigenous and non-calcareous. It clearly fulfils every criteria of diamictite and it almost certainly is of glaciogenic origin. It is a tillite from the Varangian glaciation. Finnmark, Norway.

That, however, creates a problem because modern geology strives to explain how rocks and sediments form(ed). It is not enough if we simply describe them. From that point of view “diamictite” does not fit well into contemporary science. Every time someone uses the term, the question of genesis immediately arises. “Diamictite” has its role in the field, though. It continues to be useful as a preliminary term to describe rocks with a certain appearance.

Continue reading Diamictite