Lithodemic stratigraphy

Bodies of rock which do not conform to the Law of Superposition are described as lithodemic.  They are generally composed of intrusive, highly deformed or metamorphic rocks, determined and delimited on the basis of rock characteristics.  Their boundaries may be sedimentary, intrusive, extrusive, tectonic or metamorphic.  A formal classification of lithodemic units was presented in the 1983 North American Stratigraphic Code, comprising a lithodeme, which is comparable to a formation, a suite, which is roughly equivalent to a group, and a supersuite, comparable with a supergroup.

Lithodemic units are not included in the 1994 International Stratigraphic Guide, which instead regards intrusive igneous bodies and non-layered metamorphic rocks of undetermined origin as special cases within lithostratigraphy.  The guide advises against using the term suite. The term complex (see below) is used loosely, a term that is less likely to be of use in Quaternary geology.  Here it is recommended that the procedures advocated in the North American Stratigraphic Code are followed, with the modifications noted below.

Definition of lithodemic units

A two-fold hierarchy is recommended:

Supersuite             Supercomplex
Suite                      Complex

lithodeme is the fundamental unit.  No lower rank units have been defined.  It should possess distinctive lithological features and be internally consistent by comprising either a single rock type or a mixture of two or more types that serve to distinguish the unit from those bounding it.  Normally a lithodemic unit should be given a geographical name combined with either a rock name or some term descriptive of its form.

suite consists of two or more lithodemes of the same genetic class (igneous, metamorphic or sedimentary). Whitten (1991) gives a full account of the range of uses of the term and concludes that a set of plutons can be divided into different suites according to which petrographical or chemical characteristics are used to define the suite.  It would seem, from this, to be undesirable to attempt any more precise a definition of suite than that provided by the North American Stratigraphic Code.

In the North American Stratigraphic Code, the use of the term complex is confined to an assemblage or mixture of rocks of two or more genetic classes. The rank of the term was not defined, but was said to be comparable to a suite or supersuite.  In practice this can lead to problems, such as nested complexes;  for example the Laxfordian Complex within the Lewisian Complex of NW Scotland.  Where recent, practical attempts to apply lithodemic nomenclature have been made in North America the tendency has been to confine the term complex to a unit that has the same rank as a suite and consists of lithodemes of more than one genetic class. For higher rank units, the term supersuite is available. A supersuite has been defined as consisting of two or more suites or complexes, having a degree of natural relationship with each other, but not necessarily of the same genetic class.  By implication a supersuite could consist entirely of complexes. However, there appears to be a resistance in the UK to the latter usage. As a pragmatic solution we recommend the introduction of the term supercomplex for such as the Lewisian, thus limiting supersuite to a grouping of two or more suites.

In Quaternary stratigraphy these units best refer to volcanic complexes which are characterised by a diverse assemblage of extrusive rocks, related intrusions and their weathering and erosional products.  Because a mixture of genetic classes is required in a complex there may be components which can be divided lithostratigraphically as well as lithodemically.  A typical volcanic complex, for example, may consist of rocks of all three classes.  Extrusive volcanic and intercalated sedimentary rocks may be named following lithostratigraphical procedures, while the associated dykes, sills and subvolcanic plutons, vent intrusives, metamorphic and metasomatic rocks are lithodemic.

Problems of usage

There are many examples of inappropriate use of the term complex in geology, even based on the North American Stratigraphic Code definition.  Thus it is recommended that the term ‘complex’ is used only for units equivalent in rank to suite.  Some problem cases reflect historical precedent.  In general, it is better to confine the terms supersuite and supercomplex to units equivalent in scale to supergroup and use informal terms at intermediate levels where detailed work justifies it.

A large number of informal terms have been used to describe lithodemic units. Those, such as series, which have other defined and formal uses, should not be used.  Others, such as zone, are acceptable at the rank of lithodeme, with appropriate descriptors. In general, informal terms which describe rock units that could otherwise be described as suites, complexes, supersuites and supercomplexes should be avoided.

*This guide is based on that produced by Rawson et al. (2002) for the Stratigraphy Commission of the Geological Society of London.