marker bed (key bed, chronohorizon, chronostratigraphic horizon, datum) A thin bed of a distinctive character, often widely distributed and capable of being recognized and traced over a large geographical area. Marker beds are taken to be the results of either (geologically speaking) very short episodes of deposition (e.g. a thin coal seam), or of almost instantaneous events (e.g. turbidity deposits (see TURBIDITY CURRENT), or bentonite beds derived from volcanic ash falls). Such beds are thus very valuable as time markers in stratigraphic correlation.
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