A rock described as a well sorted sandstone, with less than 10% argillaceous matrix, and a relatively simple composition is called which?

• A. Siltstone
• B. Wacke
• C. Arenite
• D. Mudstone

Answer: (C)

Grain size and how much fine clay-rich material is present determine how a sandstone is named. A sandstone that is well sorted and contains only a small amount of argillaceous (clay) matrix—typically less than about 10%—is called an arenite. The rock is dominated by sand-sized grains with a straightforward mineral makeup, usually quartz, and lacks the substantial clay or silica-rich matrix that would give it a more complex or finer character.

If there were more matrix and the grains were mixed or poorly sorted, the rock would be called a wacke. Rocks with clay- or silt-dominated fabric and lacking sand-sized grains are mudstones or siltstones, not sandstones. So the described rock fits arenite.