Clastic sandstone with more than 10 percent matrix of indeterminate detrital or diagenetic
nature. Matrix is mud size silicate minerals (clay, feldspar, quartz, rock fragments,
and alteration products).

example

graywacke

note

Distinction from mudstone is based on inference that less that 50 percent of the mud
size fraction (matrix) is original mud size detrital particles. May also grade into
diamictite or conglomerate based on size distribution of discernible particles. If
more than 50 percent of rock is detrital particles of intrabasinal orgin and carbonate
composition, categorize as carbonate wackestone. Term is typically applied to diagenetically
altered volcanic-lithic clastic rocks in which the definition of the original clasts
has been obscured. Suggested boundaries between wacke and arenite range from 5 to
15 percent matrix. See Dickinson (1970) for discussion of interpretation of undiscernible
matrix in diagenetically altered lithic clastic rocks. Dickinson, W.R., 1970, Interpreting
detrital modes of graywacke and arkose: Journal of Sedimentary Petrology, v. 40, p.
695-707.

source

Pettijohn, Potter, Siever, 1972, Sand and Sandstone: New York, Springer Verlag, 681
p.