Which statement best describes graywacke?

Which statement best describes graywacke?

• A. Impure and poorly sorted with a clay matrix
• B. Very well sorted with quartz cement
• C. Light colored, fine grained
• D. Pure quartz grains with little matrix

Answer: (A)

Graywacke is a matrix-rich sandstone formed from rapid deposition, so its grains are poorly sorted and a clay-rich matrix binds together a mix of angular grains and rock fragments. That combination—impure composition with a clay matrix and poor sorting—is the defining feature, making the description of being impure and poorly sorted with a clay matrix the best fit. The other descriptions point to rock types with different textures: very well sorted with quartz cement describes a clean, quartz-rich arenite; light colored and fine grained suggests a finer, less muddy rock like siltstone or mudstone; and pure quartz grains with little matrix describes quartz sandstone or quartz arenite, not graywacke.

Graywacke is a matrix-rich sandstone formed from rapid deposition, so its grains are poorly sorted and a clay-rich matrix binds together a mix of angular grains and rock fragments. That combination—impure composition with a clay matrix and poor sorting—is the defining feature, making the description of being impure and poorly sorted with a clay matrix the best fit. The other descriptions point to rock types with different textures: very well sorted with quartz cement describes a clean, quartz-rich arenite; light colored and fine grained suggests a finer, less muddy rock like siltstone or mudstone; and pure quartz grains with little matrix describes quartz sandstone or quartz arenite, not graywacke.