A surface separating younger from older rocks along which somewhere there has been subareal erosional truncation with a significant hiatus indicated (Van Wagoner, 1995). Erosion which is only local is generally excluded but sometimes surfaces of submarine erosion are included. Generally evidenced by a basinward shift in facies, onlap, and/or truncation, and generally involves an appreciable time of nondeposition. The date assigned to an unconformity is that of its correlative conformity; see sequence boundary. An unconformity is often a good seismic reflector and can be recognized even where the layers above and below the unconformity are parallel. At an angular unconformity the older strata dip at a different angle (usually steeper) than the younger strata. At a disconformity the beds above and below are generally parallel despite a significant hiatus. At a nonconformity stratified beds lie on igneous or metamorphic rocks.