Marker horizon

Such clear markers facilitate the correlation of strata, and used in conjunction with fossil floral and faunal assemblages and paleomagnetism, permit the mapping of land masses and bodies of water throughout the history of the earth.

[1] They usually consist of a relatively thin layer of sedimentary rock that is readily recognized on the basis of either its distinct physical characteristics or fossil content and can be mapped over a very large geographic area.

Typically, key beds were created as the result of either instantaneous events or (geologically speaking) very short episodes of the widespread deposition of a specific types of sediment.

[3] One particular bolide impact 66 million years ago, which formed the Chicxulub crater, produced an iridium anomaly that occurs in a thin, global layer of clay marking the Cretaceous–Paleogene boundary.

[4] Iridium layers are associated with bolide impacts and are not unique, but when occurring in conjunction with the extinction of specialised tropical planktic foraminifera and the appearance of the first Danian species, signal a reliable marker horizon for the Cretaceous–Paleogene boundary.