A geologic unconformity is a surface within the rock record that represents a significant break or gap in time, where deposition of sediment was interrupted by erosion or non-deposition before new sediments were laid down. This means that the rock layers above and below the unconformity are of different ages, and the older rock was exposed and eroded or simply not deposited on for a period before the younger rock formed above it
. Unconformities indicate missing intervals in the geologic record, called hiatuses, and are important for understanding Earth's history because they show that sedimentation was not continuous
. There are several main types of unconformities:
- Angular unconformity: Older rock layers were tilted or folded and then eroded before younger, horizontal layers were deposited on top, creating an angular discordance between the two sets of layers
- Disconformity: The layers above and below the unconformity are parallel, but there is a gap in time due to erosion or a pause in deposition, often marked by erosion features like channels or paleosols
- Nonconformity: Sedimentary rocks lie above older igneous or metamorphic rocks that were exposed and eroded before sediment deposition resumed
Unconformities are recognized as boundaries or contacts between rock units that represent these breaks in deposition or periods of erosion and are fundamental for interpreting geologic history and relative dating
