Mudstone
fine grained sedimentary rock whose original constituents were clays or muds / From Wikipedia, the free encyclopedia
A mudstone is a siliciclastic sedimentary rock which has a mixture of silt- and clay-sized particles (at least 1/3 of each).[1] So, a mudstone is a type of mudrock with a more varied grain size than either siltstones or claystones.
Alternatively, some sources say it is a synonym for mudrock.[2]