Subdivisions of Stratigraphy
There are three major subdivisions of stratigraphy that approach the problem of creating a geologic column in distinct ways with differing nomenclature. The most commonly referred to, with respect to a geologic timescale, would be Geochronology. This discipline uses absolute time and names for absolute time intervals to describe sets of rocks. Geochronology uses familiar terms such as eons, eras, periods and epochs to describe the divisions of time involved.
Chronostratigraphy uses analogous terms to geochronology. Geologists using chronostratigraphic terms speak of groups of rocks determined by the time period in which they were deposited. Rocks of a certain time are named as that time period. While geochronology marks the dates between 445 Ma to 415 Ma as the Silurian Period, chronostratigraphy labels the rocks deposited in that time as the Silurian System. One scheme names the interval while the other names the material created during that same interval. Chronostratigraphy’s nomenclature compares exactly to that of geochronology. The rocks found in the Silurian Period compose the Silurian System. Other terms used would correlate similarly, eon to eonothem, era to eranothem, period to system, and epoch to series.
Lithostratigraphy uses strictly the physical characteristics of the rocks being described. The time of deposition has no bearing on the lithostratigraphy of the rock beyond, perhaps, relating to the environment of deposition. Sandstones, limestones, basalts and marbles would all be in different lithostratigraphic units because their physical characteristics differ, even though they may all have been created in the same geochronological time frame. Lithostratigraphy uses terms such as supergroup, group, formation, member, or bed to describe strata. Formation – or a unique, mappable stratum of rock – is a commonly used lithostratigraphic term.