Cyclic sediments (also known as rhythmic sediments) are repeating sequences of different layered sedimentary rocks. (e.g. rock-A, rock-B, rock-C, rock-D, A,B,C,D, A,B,C,D &etc)

They are found at many locations globally - e.g. at the Musandam Peninsula (Oman), the High Atlas, Morocco and Campo Formoso, Brazil.

The process(es) which created the repeating sequences has puzzled geologists for more than a century.

There are several proposed models - most of which assume some kind of cyclic sea-level changes, or cyclic changes in rainfall patterns - but there is still no general agreement amongst geologists.

Example reference paper Sedimentary Geology Volume 172, Issues 1–2, 15 , Pages 187-209