Halimeda reefs in the upper Miocene strata (∼6 Ma) of the Sorbas basin, southeastern Spain, shed light on the internal structure of more extensive but less accessible Holocene counterparts, and challenge conventional reef concepts. Coarse discoid segments, released by Halimeda during life or immediately after death, dominate the lenslike mounds. Their chaotic, loose appearance disguises the reefal nature of the mounds. Segments, accumulating at or very close to sites of growth, were quickly stabilized by microbial and cement crusts that bound them into distinctive rigid gravel fabrics. This early lithification generated relief but inhibited off-mound export of sediment, although large blocks detached locally and moved downslope. Encrustation of parautochthonous Halimeda gravel created a unique reef type: segment reefs.