Which rock name refers to a non-clastic carbonate rock formed in part from shell fragments and clay?

• A. Marl
• B. Chalk
• C. Dolostone
• D. Sandstone

Answer: (A)

This item is about recognizing a carbonate rock that isn’t built from clastic grains but includes both clay and calcareous material. Marl is a non-clastic carbonate mud that forms from a mixture of fine clay minerals and calcium carbonate from shell fragments and other calcareous debris. The presence of clay alongside shell-derived carbonate distinguishes marl from rocks formed purely by microfossil shells or by dolomite replacement.

Chalk, while also non-clastic and carbonate, is dominated by microfossil shells with little clay content, so it doesn’t emphasize shell fragments and clay in the same way. Dolostone is a carbonate rock dominated by dolomite mineral rather than by shell fragments combined with clay. Sandstone is clastic, made of sand-sized grains cemented together, not a carbonate rock formed from shell fragments and clay.

So, marl best matches the description of a non-clastic carbonate rock formed in part from shell fragments and clay.