What is the maximum grain size of a sediment that can be point counted using a standard thin section?

Question

Point counting is an extremely useful tool for determining the composition of a clastic sedimentary rock. It is also very useful in provenance studies to determine the sediment source of a sedimentary rock (see, for example, the classic paper by Dickenson and Suczek, 1979). The theory behind point counting is straight forward. For me, putting it into practice has been challenging because I cannot find the "definitive guide to point counting". I know the general rule is 300-500 points per slide and that the step size should be 1.5 x grain size (although this company says otherwise). But what is the maximum grain size one can analyse on thin section before the counting becomes statistically unrepresentative?

Answer 1

you might have guessed: There is no such an easy answer as a precise, always valid maximum grain size.

Point counting is an upscaling technique. It works when the sample area you inspect is >= to the so-called representative elementary area (REA) of the rock facies. The larger your grains, the larger needs the REA to be. Same holds true for increasing heterogeniety and increasing number of mineral phases. So check if your point count analysis is changing when you increase the inspection area. Once the analysis comes to a stable ratio of analysed items, you found the REA of the rock facies. This REA is a material property and not depending on the step size you choosed (as long as you make sure to obey the law of large numbers for each area).

-> look for "REA"

Best, Ben