Marker horizons or chronohorizons or marker beds are stratigraphic units of the same age and of such distinctive composition and appearance, that, despite their presence in separate geographic locations, there is no doubt about their being of equivalent age (isochronous) and of common origin. A stratified unit with distinctive characteristics making it an easily recognized geologic horizon. A rock layer which accounts for a characteristic portion of a seismic refraction time-distance curve. Englishedit. Nounedit. marker bed (plural marker beds). (geology) A rock unit with distinctive stratigraphic features which can be traced over a wide area.
A geologist may observe a sequence of rocks in one road cut: mudstones, sandstones, limestones, etc. These marker beds are not only distinctive, but are formed practically instantaneously, so that they are precise time markers. Lithostratigraphic units are the basic units of geologic mapping. Customarily only distinctive beds (key beds, marker beds) particularly useful for stratigraphic purposes are given proper names and considered formal lithostratigraphic units. Hanging the line on the marker-bed stratigraphic datum allows the petroleum geologist to see the relationship of the formations more clearly, and proceed to the important work of connecting them properly.
Sedimentary geology focuses on sedimentary rocks (e.g., limestone, shale, sandstone, coal) and the processes by which they were formed.