1.3 Categories in the Code
The categories of geological units included in the Code are those containing units that can be defined formally and be given separate names (Table 1). A few categories and units which NSK recommends be used only in informal contexts, are also included. When the individual categories are dealt with, rules may be given for one or more geological unit(s). Some units are divided into subtypes.
There are two fundamentally different kinds of categories. One comprises categories consisting of units defined on the basis of material content or other physical properties (Chapter 3). Common for these is that they denote distinctive, geological units that can therefore be given special names or proper names.
The other main group consists of units defined on the basis of time or age (Chapter 4). Such units are either a division of time, or material units that are formed within a specific time span. They are collective types and are given characterizing names that are to be looked upon as collective names.
Categories of material units and units defined on the basis of physical properties (dealt with in Chapter 3):
Lithostratigraphical units -- Lithostratigraphical units comprise stratified rocks and superficial deposits that are defined on the basis of special lithological properties and boundary relationships. Lithostratigraphical units are units deposited in compliance with the "Law of Superposition", i.e. younger beds are deposited above older ones (Section 3.2).
Lithodemic units -- Lithodemic units were introduced as a new category in the NACSN stratigraphical classification system (NACSN 1983). Lithodemic units are rocks that are defined on the basis of their lithological properties alone. These are generally non-stratified rocks such as plutonic or minor intrusive rocks, or strongly metamorphosed and deformed rocks (Section 3.3).
Magnetostratigraphical units -- These are units that are defined on the basis of their remanent-magnetic properties (Section 3.4).
Biostratigraphical units -- Biostratigraphical units are distinguished on the basis of fossil content, particularly from the viewpoint of biological evolution in the course of Earth history (Section 3.5).
Pedostratigraphical units -- A pedostratigraphical unit is a body of sediment that represents a former soil or zone of weathering (palaeosol) (Section 3.6).
Geological form units -- This category embraces present-day and buried landforms and form elements of structural origin; they need never have been topographical landscape features (Section 3.7).
Linear structural units -- This category only embraces linear elements that are classifiable as lineaments, i.e. marked linear features in the landscape of uncertain, unknown or composite origin (Section 3.8).
Planar structural units -- This category embodies fractures, joints and faults (Section 3.9). Other kinds of planar elements that have once been topographical surfaces, or that define structural form elements, are dealt with in Section 3.7.
Morphostratigraphical units -- Morphostratigraphical units are geomorphological elements that show a geological evolutionary sequence, such as a series of ice-marginal deposits (Section 3.10).
Tectonostratigraphical units -- Tectonostratigraphical units are bodies of rock that are distinguished as units because they are located above or between thrusts, i.e. nappes, thrust sheets and the like (Section 3.11).
Seismostratigraphy -- Seismostratigraphy is the study of stratigraphy and depositional facies as they can be interpreted from seismic data (Section 3.12). Terms used in seismostratigraphy are briefly mentioned, but no rules are proposed for seismostratigraphical units.
Categories of units defined by time or age (dealt with in Chapter 4):
Chronostratigraphical units -- Chronostratigraphical units are bodies of stratified rock that form reference units for all rocks formed during the same period of time; they have synchronous boundaries (Section 4.2).
Geochronological units -- Geochronological units are time units embracing divisions of time defined on the basis of boundaries of chronostratigraphical units (Section 4.3).
Geochronometrical units -- Geochronometrical units are time units, and are convenient direct divisions of geological time whose boundaries are expressed in years; they lack material referents (Section 4.4).
Polarity-chronostratigraphical units -- This category consists of lithological units that have preserved an "imprint" of the original magnetic polarity that existed at the time the unit was deposited or crystallized (Section 4.5).
Polarity-chronological units -- These are time units representing divisions of time when specific polarity-chronostratigraphical units were formed (Section 4.6).
Diachronous units -- Diachronous units were first defined by NACSN (1983). They are time units that comprise the unequal periods of time when stratigraphical or geomorphological units with non-synchronous boundaries were formed (Section 4.7). Climatostratigraphical units are included in this category (Section 4.7.6).
Deformational-diachronous units -- This category comprises diachronous time units delimited by deformational events, both within and outside orogenic belts (Section 4.8).
