Synonym: Stratigraphic gap
A lacuna, which is sometimes called a ‘gap’, is a large and obvious discontinuity in a rock formed by a disconformity or unconformity. If a lacuna forms due to an unconformity, when deposition stops, the erosion of the old strata also stops. This implies the end of deposition of new rock layers and the partial denudation of the old rock layers. For example, the parallel unconformity between the North China Middle Ordovician Series and the Middle Carboniferous Series lacks the Upper Ordovician Series, the Silurian System, the Devonian System and the Lower Carboniferous Series. This lacuna shows not only that the deposition of several systems did not occur and that some of the Ordovician strata had been eroded.