Biostratigraphic units

Abstract

Changes in fossil faunas and floras are the essential markers in biostratigraphy. Irreversible changes were caused by evolution, extinction, and large-scale migration; they approximate to time planes over large areas, and are considered to define the boundaries of zones, subzones, and superzones. Selected evolutionary forms of gradational bioseries like the Orbulina bioseries are considered to define the boundaries of gradation zones. Reversible faunal changes were caused by temporary changes in environment.Abrupt reversible changes that resulted from episodic or periodic events probably mark time planes over appreciable areas and are considered to define the boundaries of casuzones. Gradual reversible changes that resulted from secular lateral shift of facies belts are intrinsically time-transgressive and are considered to define the boundaries of ecozones.By original definition a zonule is distinguished by the occurrence of a single animal (or plant) community and it may form a part or the whole of an ecozone.