Species extinctions, often referred to as “tops,” are used as horizons of correlation. The first downhole occurrence (“+” in the illustration below) in a well section is the datum most commonly used. The inception (“*” in the illustration below), or lowest occurrence of a species or lineage, is a reliable datum only in core or outcrop samples because caving is virtually unavoidable in cutting samples; however, it can help refine the stratigraphy. The overlap of species extinctions and inceptions allows the development of range zones (see figure below), which can be correlated from site to site. | Species extinctions, often referred to as “tops,” are used as horizons of correlation. The first downhole occurrence (“+” in the illustration below) in a well section is the datum most commonly used. The inception (“*” in the illustration below), or lowest occurrence of a species or lineage, is a reliable datum only in core or outcrop samples because caving is virtually unavoidable in cutting samples; however, it can help refine the stratigraphy. The overlap of species extinctions and inceptions allows the development of range zones (see figure below), which can be correlated from site to site. |