Abstract: Stratigraphic Concepts and Terminology
Warren L. Calvert
Geologists deal with rock units, time-rock units, chronologic-time units, and actual-time units. Actual time is a continuum measured in regular units of thousands and millions of years. Chronologic time (geologic time) is a continuum measured by irregular units based upon the relative occurrence of all known physical and organic events in earth history as recorded in the rocks of the earth's crust. Time-rock units are bodies of rock strata deposited during intervals of geochronologic time. Rock units are bodies of rock strata representing physical events, which contain evidence of organic events, and from which time-rock units and divisions of geochronologic time are derived.
Physical events, such as deposition, erosion, diastrophism, metamorphism, vulcanism, glaciation, eustacy, and climatic change, are most important in geochronology. Organic events (paleomutations) do not necessarily coincide with physical events, but commonly are influenced by them. Neither physical nor organic events occurred worldwide at exactly the same time. Lithostratigraphy is the study of the products of physical events. Biostratigraphy is the study of the products of organic events. Chronostratigraphy is the study of the placement of rock strata according to divisions of geochronologic time.
Transfer of the terms "system" and "series," meaning subdivisions of rock strata larger than group in a given area, from lithostratigraphic terminology to chronostratigraphic terminology, to mean all rocks formed anywhere during a specific interval of geochronologic time, such as during a period of epoch, leads to confusion. It is recommended that "system" and "series" be retained as lithostratigraphic subdivisions greater than group, and that when a chronostratigraphic connotation is intended, the words "chronosystem" and "chronoseries" be used to modify system and series names. It also is suggested that the terms "division" and "chronodivision" be used for bodies of rock strata greater than system and chronosystem.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma