Stratigraphy means literally the descriptive science of strata. It deals with the composition, form, arrangement, distribution, succession, and classification of rock strata and it also involves the interpretation of these features in terms of mode of origin, environment, age, history, relation to organic evolution, and relation to other geologic concepts. Stratigraphy concerns itself with the complete picture of the rocks of the earth's crust as strata of various kinds and the significance of these strata in the earth's geological development.
There are many branches of stratigraphy, depending on the particular features of rock strata under consideration. One of the most important is chronostratigraphy which deals with the age determination and age classification of strata. Its basic purpose is to interpret the history of the earth through the chronologic sequence of its rock strata.
The principal means used to work out chronostratigraphy are (1) the physical interrelations of strata, (2) the relation of strata to sequence of organic evolution, and (3) radioactivity age determinations. Valuable supplementary evidence of age or chronostratigraphic position can be supplied by other features of rock strata and other geologic phenomena such as lithology, mineralogy, ore deposits, chemical composition, paleomagnetism, paleoclimatology, changes in sea level, orogeny, igneous activity, and unconformities. However, few of these can be proved to have had effects which were distinctly recognizable, identical in character, and synchronous over the whole world. Coordinated utilization of all possible lines of relative and absolute age determination and time correlation offers the best promise for continued progress in chronostratigraphy.
Joined to the problem of the dating of strata and the establishment of their sequence with respect to earth history is the task of chronostratigraphic classification. The record of 4 thousand million years, written in millions of cubic miles of strata, is too vast to be comprehended as a whole and it is necessary to break it down into smaller more practicable units. The only adequate reference standards for the scope of these chronostratigraphic units are specifically designated intervals of rock strata—stratotypes.
The fundamental unit of world-wide chronostratigraphic classification is the system. The systems, largely established in Western Europe during the first half of the last century, were originally thought to constitute “natural” units with respect to earth history. In view of the local and rather haphazard manner in which most of them originated and the primitive state of world geological knowledge at that time, it is difficult now to see them as “natural” divisions of world-wide extent. Nevertheless the belief is still supported by many, including the USSR Stratigraphic Commission, that the systems are marked off by a concurrence of major events in geologic history and major changes in the course of organic evolution. They assume that all lines of Stratigraphic evidence converge to form “natural” divisions of strata with respect to time and that hence separate kinds of Stratigraphic classification—lithostratigraphic, biostratigraphic, etc.—are unnecessary.
On the other hand, investigations by other competent workers of the evidence for world-wide “natural breaks” in either the diastrophic record or the record of organic evolution have resulted in strong judgments to the contrary. The conclusion seems reasonable, regardless of what may be proved eventually, that it has not yet been demonstrated that world-wide “natural breaks” in the general character and continuity of strata exist at the scale of the presently accepted geologic systems nor that the evidence at the boundaries of the present systems is such as to allow them to be considered as the “natural” world-wide division points of the chronostratigraphic scale. Rather, the evidence suggests that our geologic systems are only arbitrary chronostratigraphic units in a continuum characterized by intricately overlapping and not necessarily coincident changes in the many and various properties and attributes of rock strata and that their principal significance is that of standard units of chronostratigraphic reference, independent of other kinds of Stratigraphic classification. Regardless of opinion on the relation of these units to events of earth history, the critically important point is that the systems and their major subdivisions should be tied down by international agreement to specifically designated and delimited sequences of rock strata—stratotypes—so as to provide a uniform basis of definition for everyone.