THE FUTURE OF LITHODEMIC UNITS AND THEIR ROLE IN PRECAMBRIAN STRATIGRAPHY

The 1983 North American Stratigraphic Code introduced the concept of lithodemic units — lithostratigraphic units applicable to igneous and metamorphic rocks where the law of superposition does not apply. The concept should be well-suited to mapping Precambrian terrains, but apart from a few examples from North America, Britain and Fennoscandia, the geoscience community has been slow to adopt formal lithodemic units. Nonetheless in 2008 the British Geological Survey (BGS) introduced a lithodemic framework for igneous rocks in the United Kingdom, and are working on a similar proposal for metamorphic rocks. The BGS proposal suggests broadening the hierarchy of lithodemic units from the 3 defined in the Code (e.g., lithodeme, suite, supersuite) to 6 (e.g., dike, dike-swarm, cluster, subsuite, suite, supersuite). Like the Code, the BGS proposal applies “complex” to 2 classes of rocks (e.g., sedimentary + intrusive) rather than a heterogeneous mixture of one rock class (e.g., solely intrusive). Furthermore, in North America, several informal lithodemic-based chronostratigraphic units have gained local acceptance (e.g., gneiss associations, tectonic assemblages). Also, there are several proposals to use regional short-duration geologic events, such as emplacement of mafic dike swarms, as global chronostratigraphic correlation tools. One reason for the slow adoption of lithodemic units in Precambrian mapping may be that the reliance on high-precision geochronology has meant that researchers think in chronostratigraphic, rather than lithostratigraphic units, especially when it comes to correlation of rock units.

What does the future hold for lithodemic units? First, an evaluation is needed with respect to an expanded hierarchy of units as well as considering the definition of the term complex given its entrenched use in describing certain igneous rocks (e.g., alkalic complexes). Second, education of the geoscience community in adapting lithodemic units to their chronostratigraphic equivalents may be needed, given the importance of isotopic dating in Precambrian mapping and stratigraphy. As a first step, recommendations with respect to defining mafic dike swarms as lithodemic and chronostratigraphic entities should be priority given their potential importance in global stratigraphic correlation.