Geologic unit classification

Geologic unit classification The purpose of this classification is grouping kinds of geologic units according to the content model that would apply to descriptions of the unit. The resulting hierarchy is designed to facilitate implementation of an XML schema, and while it is conceptually consistent with NADM, it is organized along different lines (XML implementation vs. science concepts) and so looks different.

This page summarizes discussions by SteveRichard, JohnLaxton from GeologicUnitProperties page.

Abbreviations used: Salvador 1994 is International Stratigraphic Guide, NACSN is North American Code of Stratigraphic nomenclature, NADM is NADM C1 model (

Scope of classification: Domain: Geologic units, and geologic unit description instances Range: a vocabulary of geologic unit types, each associated with a GML content model.

Geologic unit types:

Units that identify a particular body of rock

Note that rock is used here in the generic sense of RockMaterial in the GeoSciML model to include consolidated and non-consolidated materials.

Lithologic unit

Defined by lithology independent of subdividing external and internal boundaries. May be stratified or non-stratified. Use of ‘facies’ is ambiguous—sometimes refers to particular body of rock (a NADM geologic unit), sometimes refers to a ‘kind’ of rock body characterized by some internal sequence of rock types and the presence of various geologic (typically sedimentary…) structures

Lithostratigraphic unit (NADM)

Stratified, defined by lithology

Lithodemic unit (NADM)

Not stratified, defined by lithology

Allostratigraphic (or surface-defined) Unit

Geologic unit defined by bounding surfaces. Not necessarily stratified. Donovon (2004, IUGS abstract Florence) makes good case for use of a noncommittal term for the bounding surface; he proposes surface-bounded, but this presents difficulties because all geologic units are surface bounded, thus, I favor surface-defined. Discontinuity-bounded is problematic because of arguments that would ensue about defining ‘discontinuity’. To quote Donovon – “While there may be no agreement that a given stratal boundary is a discontinuity, there is consensus that all the identified boundaries are stratal surfaces.”

Unconformity bounded units (Salvador 1994)

Defined by bounding stratigraphic discontinuities (‘significant unconformities’; unconformity defined as surface of erosion in Salvador 1994). “… an individual unconformity-bounded unit exists only where its two bounding unconformities are present. The incontrovertible requirement for the presence of both bounding unconformities is the reason why Sloss/Wheeler sequences or unconformity-bounded units by any other name (e.g. synthem, alloformation) have very limited utility and are basically impractical stratigraphic units. The vast majority of unconformities within a sedimentary basin, be they of subaerial or submarine origin, have a limited areal extent. Thus, as unconformities appear and disappear within a sedimentary succession, an incredibly complex set of unconformity-bounded units, many of very limited extent, can be delineated and named (informally or formally).” (Embry, 2004, IUGS abstract)

Sequence stratigraphic unit

An allostratigraphic unit that is used to interpret the depositional origin of sedimentary strata and assumes, though this is not always stated, an implicit connection to base level change. It does this by establishing how the sequence of strata accumulated in order in the sedimentary section over a subdividing framework of surfaces. The major bounding and subdividing surfaces of this template are commonly represented by: Maximum Flooding Surfaces; Transgressive Surfaces; Sequence Boundaries.

Biostratigraphic unit (NADM)

defined based on fossil content. Five kinds of biozones are recognized by the revised NACSN (Lenz et al., 2000, Note 64, a recommended complete replacement of Articles 48 through 54 of the North American Stratigraphic Code (NACSN, 1983) accepted for publication 2000.): range biozone, interval biozone, lineage biozone, assemblage biozone, and abundance biozone. These five kinds of biozones are not hierarchically interrelated. The words “range,” “interval,” “lineage,” “assemblage,” and “abundance” are merely descriptive terms. They represent different approaches in the process of setting up, and in the recognition of, a biozone. The kind of biozone chosen will depend on the nature of the biota, the approaches and preferences of the individual scientist, and the specific problem being investigated. The most common choice of biozone is one in which both the lower boundary and the upper boundary are based on the lowest occurrences of individual taxa; the two taxa may or may not have a direct phylogenetic link. The ranges of the taxa whose lowest or highest occurrences or maximum abundances define the boundaries of the biozone are not necessarily restricted to the biozone, nor is it necessary that they range through the entire biozone.

Range biozone

A range biozone is a body of rock representing the known stratigraphic and geographic range of occurrence of any selected element or elements of the chosen fossil taxon, or taxa, present in the rock record. There are two kinds of range biozones: taxon-range biozone and concurrent-range biozone. Taxon-range biozone is a body of rock representing the known stratigraphic and geographic range of a single taxon. Concurrent-range biozone is a body of rock including the concurrent, coincident, or overlapping part of the ranges of two specified taxa.

Interval biozone

An interval biozone is a body of rock between two specified biostratigraphic surfaces (biohorizons of the International Stratigraphic Guide, p. 56). The features on which biohorizons are commonly based include lowest occurrences, highest occurrences, distinctive occurrences, and changes in the character of individual taxa (e.g., changes in the direction of coiling in foraminifers or in number of septa in corals).

Lineage biozone

A lineage biozone is a body of rock containing species representing a specific segment of an evolutionary lineage.

Assemblage biozone

An assemblage biozone is a body of rock characterized by a unique association of three or more taxa, the association of which distinguishes it in biostratigraphic character from adjacent strata. An assemblage biozone may be based on a single taxonomic group, for example, trilobites, or on more than one group, such as acritarchs and chitinozoans.

Abundance biozone

An abundance biozone is a body of rock in which the abundance of a particular taxon or specified group of taxa is significantly greater than that in adjacent parts of the section. Abundance zones may be of limited, local utility because abundances of taxa in the geologic record are largely controlled by paleoecology, taphonomy, and diagenesis. The only unequivocal way to identify a particular abundance zone is to trace it laterally.

POSC includes three subtypes of biostratigraphic unit in their implementation ( These may be sufficient to include the more detailed typing of biozones of NACSN (in terms of content model):

Biostratigraphic abundance zone : A biostratigraphic unit characterized by the abundant representation of a fossil taxon.

Biostratigraphic assemblage zone : A biostratigraphic unit characterized by the presence of a distinctive assemblage or community of fossil taxa.

Biostratigraphic interval zone : A biostratigraphic unit characterized by the presence of a fossil taxon.

Lithogenetic unit

Defined by genesis. These suggest the kinds of material that compose the unit, but the material is not the defining property.

Soil unit

defined by genesis (soil development degree and process..). Soil units not treated in Salvador (1994)
Geologic unit defined based on soil type classification; defines parts of earth surface based on soil development and character. Pedoderm is not a surface classification unit because soil classification requires knowledge of the soil profile, which always extends some distance beneath the surface.
Pedostratigraphic unit
Geologic unit that represents a single pedologic horizon in a sequence of strata (consolidated or non-consolidated). The presence of an overlying geologic unit is required, but locally the soil horizon may be at the Earth surface (in which case is may be coincident with a Pedoderm…) (see PedostratigraphicUnit)

Artificial ground unit

defined by genesis involving direct human action to deposit or modify material

Excavation unit

Defined by human-made genesis involving excavation. Not necessarily defined by landform (a hole...), as they could have been subsequently filled/landscaped etc. If the excavation is filled becomes an excavation with artificial ground wholly or partly superimposed on it. This sort of thing can become quite important in urban geology where an excavation can be filled, landscaped and then someone proposes to build houses on top of it!

Deformation unit (NADM lithotectonic unit?)

defined by genesis (deformation history) or geologic structure

Alteration unit

defined by alteration process (BGS—‘refers to a process and not lithology’)

Mass movement unit

defined by Movement type the type of movement giving rise to a landslide deposit, and Movement style describes how the individual movement types present in a landslide are related in time and space

Chronostratigraphic Unit

Defined by age of formation (deposition of sedimentary rock, crystallization of igneous rock)

Surface classification units

Map units that classify patches on the map horizon:

Surface classification unit

units defined by inspection of a surface; e.g. using air photos. Some implication of underlying material may be present, but is not a defining property. These do not inherently describe a volumetric part of the earth.

Geomorphological unit (NADM)

defined by surface landform(?), eg hummocky moraine

Vegetation unit

defined by assemblage of plants or some characteristic plant; may be related to underlying lithology, or to geologic surficial processes (flooding, mass wasting…).

Lithofacies classification unit

Classifier for assigning patches on the map horizon to a class according to the lithology of rocks cropping out in the polygon.

The next step is defining the content model. For each of these unit types, the GML schema will need to specify a content model, both for 'defining descriptions' and for 'instance descriptions'. A defining description specifies necessary or identifying properties. Instance descriptions are based on some defining description (by a 'described concept' property), and may narrow the value specification for one or more defining or necessary properties, and add value specifications for 'optional' properties

-- SteveRichard - 18 Jul 2005

This looks a good classification and a good basis for defining the content model. I have just one issue with it - I don't see why you have Lithofacies Unit as a type of Classification Unit rather than as a type of Lithologic Unit? A Lithofacies Unit is classified on the basis of the lithology of the rocks and this lithology is not just a surface property but extends to depth in the same way as for any other Lithologic Unit.

-- JohnLaxton - 09 Aug 2005

Lithofacies unit should probably change name to 'Surface Materials' unit. The idea is units that simply classify the kind of material at the surface. Lithofacies and Lithogenetic units are probably the same thing by most reckoning. These are not lithologic units 1) because they are defined based on genesis and material, e.g. 'alluvial fan deposits', 'point bar deposit', 'glacial outwash deposit'; and 2) because the designation refers to a kind of body of material, not a particular body of rock. A lithologic unit may have some lithofacies ('the overbank facies') scattered in lenses horizontally and vertically within the unit.

-- SteveRichard - 02 Oct 2008
Topic revision: r6 - 15 Oct 2010, UnknownUser

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