The Stratigraphic Code is the law when it comes to naming and defining stratigraphic units. There are two similar, but not identical, forms of the code as authored by the North American and International commissions on stratigraphic nomenclature, respectively.

The Stratigraphic Code

The purpose of a code is to facilitate communication by adhering as a science to a prescribed set of accepted and understood rules.

There are twelve categories of official stratigraphic units based on a variety of physical (material) characteristics and/or time. The boundaries of one type of unit do not necessarily coincide with those of another type. Boundaries of any type of unit must be chosen so as strata above and below can be distinguished by others.

Official Code Categories

Naming or modification of a stratigraphic unit requires:

• Publication in a recognized scientific medium• Indication of intent• Indication of category and rank• Specification of name (two parts)• Specification of stratotype (where applicable)• Definition of boundaries• Discussion of nomenclatural background• Description• Age• Correlations• Genesis (where applicable)

Naming Procedure

Naming of a new stratigraphic unit should serve a clear purpose and is only valid if accepted by the scientific community.

Unit names are binomial and include both a defining characteristic (lithology, species, etc.) and a geographic locality (type area) or rank as part of that name. Each formal unit should include a type (measured) section.

StratotypesA stratotype should include recognizable upper and lower (usually depositional) contacts, should be highly representative of the unit’s diagnostic characteric, and should be complete (no fault or unconformity).

•Type section•Lectosection•Reference section•Principle reference section

Lithostratigraphic units are bodies of rock named on the basis of physical content and stratigraphic position. These units conform to the Law of Superposition. Time, biological sequence, and depositional environment do not play part in the definition. Formal units are preferably thick enough to be mappable at a 1:24K scale.

Lithostratigraphic UnitsContacts between lithostratigraphic units can be sharp, gradational or interbedded and are both vertical and lateral in nature.

A lithostratigraphic unit can be defined on the basis of a single lithology or by a characterisitic heterogenity.

Common lithologic characteristics include composition, texture, fabric, structures, and color.

Lithostratigraphic units continue laterally until there is a significant enough change in lithology that the original definition no longer applies. Thickness can be highly variable.

Lateral Continuity

Lithostratigrahic units do not extend vertically across an unconformity.

Unconformities

Formal Units

Kaiparowits Formation

Claron/Wasatch Formation

Lithodemic units are non-stratoform bodies of rock named on the basis of physical content and cross-cutting relationships. Time does not play a play a part in the definition of these units.

Lithodemic Units

Lithodemic unit names are binomial and include both a lithology and a geographic locality (type area) as part of that name. As they are not stratiform, there is no accompanying measured section.

Type Area

A type are description should include a discussion of contacts with surrounding unit and should be highly representative of the unit’s lithological character.

Magnetostratigraphic Units

Magnetostratigraphic units are bodies of rock named on the basis of magnetic properties (polarity reversals, dipole position, secular variation, intensity) and stratigraphic position. Time does not play a play a part in the definition of these units.

Formal Units

Biostratigraphic units are bodies of rock defined on the basis of fossil content. Neither time nor lithology play a role in unit definition.

Biostratigraphic Units

Units can be named for the first and/or last occurrence of a species or multiple species, for the range of one or more species, for the overlap of multiple species, or for the relative abundance of one or more species.

Biostratigraphy is a useful tool for relative dating because organisms have changed through time. Each species has a definite range (origin to extinction) and each division of earth’s history is characterized by a recognizable assemblage of organisms.

Evolution

Formal Units

Range biozones are based on the first and last occurrences of one (taxon) or two (concurrent) taxa.

Range Biozones

Taxon Range Zones

The boundaries of a taxon range zones are based upon the first and last occurrence of a single species.

Concurrent Range Zones

The boundaries of a concurrent range zones are based upon the first and/or last occurrences of two species, one for each boundary and may require other species that extend beyond both boundaries. Defining species must be present within the zone.

Interval biozones are based on the first or last occurrences of two separate taxa. The defining taxa do not need to be present within the zone.

Interval Biozones

Lineage biozones are based on the first and last occurrences in a succession of related taxa.

Lineage Biozones

Assemblage zones are based on the common occurrence of three or more taxa.

Assemblage Biozones

Abundance zones are based on the relative abundance of one or more taxa.

Abundance Biozones

Index Fossils

• Environmental generalist (eurytope)

• Fast evolving• Geographically

widespread• Abundant• Easily preservable• Easily recognizable

Pedostratigraphic Units

Pedostratigraphic units are bodies of rock defined on the basis of soil development and stratigraphic position. Definition can include only a part of a recognized soil profile. Time does not play a role in unit definition.

Formal Units

Allostratigraphic Units

Allostratigraphic units are bodies of rock defined on the basis of position between unconformities.

Formal Units

“Time Stratigraphy”

“Time stratigraphy” divides earth history into recognizable intervals and includes both material (rock) and non-material (pure time) units.

Isochronous:of equal duration (have synchronous boundaries)

Synchronous:sychronous (at the same time or over the same period of time)

Diachronous:time transgressive (non-synchronous)

Chronostratigraphic Units

Chronostratigraphic units are isochronous bodies of rock deposited during a specified interval of time. Boundaries are synchronous and form referents for geochronologic units.

Formal Units

Polarity Chronostratigraphic Units

Polarity chronostratigraphic units are isochronous bodies of rock preserving the magnetic record for a specified interval of time. Boundaries are synchronous.

Formal Units

Geochronologic Units

Geochronologic units are conceptual (have no rock stratotypes) and are based purely on time. Boundaries are synchronous.

Formal Units

Polarity Chronologic Units

Polarity chronologic units are conceptual (have no rock stratotypes) and are based on magnetic field properties for a specified interval of time. Boundaries are synchronous.

Formal Units

Diachronic Units

Diachronic units are conceptual units representing unequal spans of time as represented by one or more other stratigraphic units, such as lithostratigraphic, biostratigraphic, allostratigraphic, etc.

Formal Units

Geochronometric Units

Geochronometric units are isochronous conceptual units based on time as expressed in a specific number of years.

Formal Units