Stratigraphic Guide

Chapter 4. Stratotypes and Type Localities

A. Stratotypes in the Definition and Characterization of Stratigraphic Units

1. Standard definitions.

Named stratigraphic units must be defined or characterized at a specified locality where they are well exposed and developed in order that there will be a common, material standard of reference for their identification.

2. Reference to a specific rock section

The particular sequence of strata chosen as a standard of reference of a layered stratigraphic unit is called a stratotype. In the case of nonlayered rocks the standard of reference is a type locality. It may be an area of exposure (or well or mine) and is an essential part of the establishment of a formal stratigraphic unit. In cases where the written description and the stratotype are not the same, the data from the stratotype take precedence.

For some stratigraphic units, such as biostratigraphic range zones, the standard of the unit cannot be tied to a specific stratigraphic section or area because the stratigraphic scope of the unit may vary with increasing information.

However, the characterization and description of these and other biostratigraphic units can be enhanced by the designation of one or more specific reference sections.

B. Definitions

1. Stratotype (type section)

The designated exposure of a named layered stratigraphic unit or of a stratigraphic boundary that serves as the standard of reference.A stratotype is the specific stratal sequence used for the definition and/or characterization of the stratigraphic unit or boundary being defined.

2. Unit-stratotype

The type section of a layered stratigraphic unit that serves as the standard of reference for the definition and characterization of the unit.

3. Boundary-Stratotype

The specified sequence of strata that contains the specific point that defines a boundary between two stratigraphic units.

4. Composite-Stratotype

A unit-stratotype formed by the combination of several specified intervals of strata combined to make a composite standard of reference.

5. Type Locality

The specific geographic locality was the stratotype of a layered stratigraphic unit is situated. The name also refers to the locality where the unit was originally described and/or named.In the case of units composed of nonlayered igneous or metamorphic rocks, the type locality is the specific geographic locality where the unit was originally defined.

6. Type Area or Type Region

The geographic area or region that encompasses the stratotype or type locality of a stratigraphic unit or boundary.

7. Holo-, para-, neo-, lecto-, and hypostratotypes

C. Requirements for Stratotypes (Type Sections)

The following requirements apply to stratotypes:

1. Expression of concept

The most important requisite of a stratotype is that it adequately represents the concept for which it is the material type.

2. Description

The description of a stratotype is both geographic and geologic. The geographic description includes a detailed location map and/or aerial photographs and indication of the means of access to the type locality and the distribution of the unit in the area. The geologic description covers the geologic, paleontologic, geophysical, and geomorphic features of the unit at the type section. The description contains two parts:a part that deals with the boundaries and a part that deals with the content of the unit.

3. Identification and marking

An important requirement of a stratotype is that it should be clearly marked. A boundary-stratotype is marked at a point, preferably by a permanent artificial marker.

Unit boundaries should be clearly designated by reference to permanent geologic and geographic features at the type locality.

4. Accessibility and assurance of preservation

Stratotypes must be accessible to all who are interested in their study, regardless of political or other circumstances, and there should be reasonable assurance of their long-term preservation.

5. Subsurface stratotypes

Subsurface stratotypes are acceptable if adequate surface sections are lacking and if adequate subsurface samples and logs are available.

6. Acceptability

The usefulness of the stratotypes for stratigraphic units of international extent is directly related to the extent to which they are generally accepted or acknowledged as the standard of reference for the units.It is, therefore, desirable that the designation of a stratotype be submitted for approval to the geologic body having the highest standing in any particular case.

The IUGS International Commission on Stratigraphy is the body to which proposals for the designation of stratotypes of units of worldwide application are submitted. Stratotypes of local units require the approval from local or national surveys or stratigraphic commissions.

D. Requirements for Type Localities of Nonlayered Igneous or Metamorphic Rock Bodies

Type localities and type areas for nonlayered igneous or metamorphic rock bodies should represent the material concept of the unit and have other attributes of description, definition, location, and accessibility that apply to layered stratigraphic units.