Stratigraphic Guide

Chapter 2. Principles of Stratigraphic Classification

A. General

Stratigraphic classification encompasses all rocks of the crust of the Earth. Rocks have many tangible and measurable properties and may be classified according to any of them. Rocks may also be classified by their time of origin or interpreted attributes, such as environment or genesis.

The stratigraphic position of change for any property or attribute does not necessarily coincide with that for any other. Consequently, units based on one property commonly do not coincide with those based on a different property. Therefore, it is not possible to express the distributions in the rocks of all of the different properties with a single set of stratigraphic units. Different sets of units are needed.

However, all the different classifications are closely related because they express different aspects of the same rock bodies and they are used to achieve the same goals of stratigraphy: to improve our knowledge and understanding of the Earth’s rock bodies and their history.

B. Categories of Stratigraphic Classification

Rock bodies may be classified according to many different inherent properties. Each classification needs its own distinctive nomenclature. The following kinds of formal units are best known and most widely used:

Lithostratigraphic units - units based on the lithologic properties of the rock bodies.
Unconformity-bounded units - bodies of rock bounded above and below by significant discontinuities in the stratigraphic succession.
Biostratigraphic units - units based on the fossil content of the rock bodies.
Magnetostratigraphic polarity units - units based on changes in the orientation of the remanent magnetization of the rock bodies.
Chronostratigraphic units - bodies of rocks defined between specified stratigraphic horizons which represent specified intervals of geologic time.

Many other properties and attributes may be used to classify rock bodies and the way is open to use any that show promise. Whenever this is the case, the unit-terms being used should be defined.

Though each kind of stratigraphic unit may be particularly useful in stratigraphic classification under certain conditions or in certain areas or for certain purposes, chronostratigraphic units offer the greatest promise for formally-named units of worldwide application. Lithostratigraphic, biostratigraphic, and unconformity-bounded units are all of limited areal extent, and thus unsatisfactory for global synthesis.

Magnetostratigraphic polarity units, though potentially worldwide in extent, require extrinsic data from the other units for their recognition, and dating. For these reasons, chronostratigraphic units have been chosen for international communication among stratigraphers with respect to position in the stratigraphic column.

C. Distinguishing Terminologies for each Category

Appropriate distinguishing terms are needed for each of the various categories of stratigraphic units. Some of the classifications are best suited to a hierarchical classification whereas in others all categories are of equal rank (Table 1).

Table 1: Summary of Categories and Unit-Terms in Stratigraphic Classification

D. Chronostratigraphic and Geochronologic Units

Chronostratigraphic units are tangible stratigraphic units because they encompass all rocks comprised between two bounding chronostratigraphic horizons, i.e., horizons believed to be readily correlatable worldwide.

Geochronologic units are units of time-an intangible property-and thus intangible units, not in themselves stratigraphic units.

E. Incompleteness of the Rock Record

The rock record of any one area is far from continuous or complete. It is commonly interrupted by innumerable diastems, discontinuities, and unconformities.

Short interruptions of the record, in fact, exist in layered rocks at every bedding plane. The evidence which the rocks carry of these missing intervals is in itself a part of stratigraphy and a very important contribution to the understanding of Earth history.