Macrostrat's lexicon is the shared vocabulary that lets columns, maps and outside
datasets describe rocks in comparable terms. Each part of it is a curated list with
its own structure. All of it can be browsed on Macrostrat's lexicon pages (/lex) and
queried through the API's definition routes (/defs/...).
Stratigraphic names
Stratigraphic names (the Navajo Sandstone, the St. Peter Sandstone, the Morrison Formation) are the most important link between datasets, and the hardest to get right.
Macrostrat distinguishes a name from a concept:
- A stratigraphic name is a name as used in a particular place, with a rank: supergroup, group, subgroup, formation, member or bed. Names are nested explicitly: the Apex Basalt is a formation within the Salgash Subgroup, which is in turn within the Warrawoona Group.
- A concept is the stratigraphic unit a name refers to, as defined in an authoritative lexicon, such as the USGS's Geolex or the lexicons of other national surveys. Concepts carry the lexicon's definition, age, geologic province and usage notes, and a link back to the source.
The distinction matters because names are ambiguous in both directions. Homonyms are common: both the United States and Australian lexicons recognize an "Admiral Formation", referring to unrelated rocks. And one unit can carry several names, or one name several meanings: Macrostrat tracks four distinct uses of "St. Peter" in the North American midcontinent. Linking each name to a concept, and keeping track of where each name is used, lets Macrostrat tell these apart. Names that are not yet tied to a lexicon concept can still be used, but linked names are preferred.
Lithologies
Lithologies are organized in a four-level hierarchy:
lithology → group → type → class
for example, basalt → mafic → volcanic → igneous
The classes are sedimentary, igneous and metamorphic; types include carbonate, siliciclastic, evaporite, organic, chemical, volcanic, plutonic, metasedimentary and others. A query at any level returns everything beneath it, so "all igneous rocks" or "all carbonates" can be selected as easily as a single rock type. Lithologies also carry nominal physical properties (for example, density and initial porosity) used in quantitative analyses.
Lithology attributes qualify a lithology: sedimentary structures (hummocky cross-stratified), bedforms, grain characteristics, colors and other descriptors.
Environments and economic resources
Depositional environments are classed as marine or non-marine, with types such as carbonate, siliciclastic, fluvial, lacustrine, glacial and eolian. Economic attributes record resources hosted by units, classed as energy, material, precious commodity or water, with types such as mineral, hydrocarbon, coal, nuclear, construction material and aquifer.
Time: intervals and timescales
Geologic time is represented by intervals: named spans with numeric top and bottom ages and a type (eon, era, period, epoch, age, and finer chronostratigraphic and biostratigraphic divisions such as zones). Intervals are grouped into timescales, including the international chronostratigraphic timescale and regional or specialized scales. Intervals anchor the Age model of every column, and map units are described against them as well.
How the lexicon is used
- Column units are described directly in lexicon terms when they are entered.
- Map legends, which arrive in each map author's own words, are matched to the lexicon during Map ingestion, so that a polygon described as "Ordovician dolomite of the Knox Group" becomes queryable by age, lithology and name (see Linking data).
- Outside records, such as fossil collections, samples and literature extractions, are tied to units through shared names.
Resources
- USGS Geolex search and the list of national stratigraphic lexicons
