A carbonate mud mound is a sediment body consisting
of structureless or crudely bedded fine crystalline
carbonate. Modern examples of carbonate mud
mounds are rare, so much less is known about the
controls on their formation than is the case for reefs.
From studies of mounds of fine-grained carbonate in
the rock record (e.g. Monty et al. 1995) it appears
that there are two, possibly three types. Many
mounds are made of the remains of microbes that
had calcareous structures and these microbes grew
in place to build up the body of sediment. Others have
a large component of detrital material, again mainly
the remains of algae and bacteria, which have been
piled up into a mound of loose material. It is also
possible that some skeletal organisms such as calcareous
sponges and bryozoans are responsible for building
carbonate mud mounds. They appear to form in
deeper parts of the shelf than reefs, but within the
photic zone. Cementation of the mud requires circulation
of large amounts of water rich in calcium carbonate,
a process that is not well understood.