In geology, a horizon refers to either a bedding surface where there is marked change in the lithology within a sequence of sedimentary or volcanic rocks, or a distinctive layer or thin bed with a characteristic lithology or fossil content within a sequence. In the interpretation of seismic reflection data, horizons are the reflectors (or seismic events) picked on individual profiles. These reflectors represent a change in rock properties across a boundary between two layers of rock, particularly seismic velocity and density.
Horizon (geology) Wikipedia
Marker horizons are stratigraphic units of distinctive lithology (different from the bulk of the sequence) with a wide geographical extent that are used in stratigraphic correlation. Layers of tuff (lithified volcanic ash) are often used for this purpose.
An event horizon is a bed that marks a geological event, such as an earthquake or a meteorite impact. It is the basic unit used in event stratigraphy.