As a rule of thumb, grain size – the diameter of individual grains of sediment in ‘clastic’ or sedimentary rocks – reflects the energy of the environment a rock forms in.
Because of this association between grain size and the energy of a sedimentary environment, the proportion of different grain sizes in a fossil-baring rock can help palaeontologists piece together a picture of the environment their fossils formed in. Was it a quiet burial by a lakeside, or a tumultuous end in the pits of a mountainous river channel?
Generally speaking, higher energy water or wind currents are required to move larger grain sizes.
What’s more, the degree of size-sorting (a.k.a. how uniform the size of the grains is) can also help indicate the energy of the depositional environment. Well-sorted sediments tend to indicate higher energy depositional settings. Think about what happens if you vigorously shake a bowl of popcorn (no, not that much).
Picture the kinds of environments you might see grains of these sizes laying around, and you’ll be on your way to thinking like a sedimentary geologist:
- boulders, >256 mm
- cobbles, 64-256 mm
- pebbles, 4-64 mm
- coarse sand, 2-4 mm
- sand, 1/8-2 mm
- silt, 1/256-1/8 mm
- clay <1/256 mm
In my research, I was able to use grain size differences to decipher layers of rock that formed from the high-energy heart of river channels to the low-energy, distant floodplains of a forested landscape that persisted in the South Polar circle roughly 100 million years ago. From looking at these characteristics of the grain sizes (along with other clues), I reconstructed the ecology of different plants fossilised within these ancient sediments.
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