Geol 370: Sedimentology and Stratigraphy Topic 20: Stratigraphic Code

1. 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.

2. 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

4. 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.

5. 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.
Stratotypes
A 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

6. 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 Units
Contacts 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.

7. 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

8. Lithostratigrahic units do not extend vertically across an
unconformity.
Unconformities

9. Formal Units

10. Kaiparowits Formation
Claron/Wasatch Formation

11. 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

12. 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.

13. 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.

15. Formal Units

16. 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.

17. 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

18. Formal Units

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

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

21. 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.

22. 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

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

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

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

29. Index Fossils
• Environmental
generalist (eurytope)
• Fast evolving
• Geographically
widespread
• Abundant
• Easily preservable
• Easily recognizable

32. 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.

33. Formal Units

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

36. Formal Units

37. “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)

38. 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.

39. Formal Units

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

41. Formal Units

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

43. Formal Units

44. 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.

45. Formal Units

46. 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.

47. Formal Units

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

50. Formal Units