3.2 Lithostratigraphical units
3.2.1 General properties and rules
Lithostratigraphy is concerned with the description and organization of sediments and sedimentary and volcanic rocks on the basis of their lithological properties and superposition relationships.
A lithostratigraphical unit is a body of sedimentary, volcanic, metamorphosed sedimentary or metamorphosed volcanic beds delineated on the basis of characteristic lithological properties and stratigraphical position. Lithostratigraphical units conform to the "Law of Superposition", i.e. younger beds are deposited on older ones.
Lithostratigraphical units are defined independently of inferred geological history, mode of genesis or biological development. Their boundaries are in principle independent of time horizons, and most are time-transgressive. The lower boundary of a unit formed by a sudden, brief catastrophic event may, nonetheless, be nearly synchronous throughout the area in which the unit is found, e.g. a volcanic ash deposit or a bed deposited by a major flood or storm.
Boundaries between lithostratigraphical units may be lithologically sharp and mark an unconformity surface (Section 3.7.2), but gradual transitions may also occur. These are particularly common where lateral facies changes take place. Lithostratigraphical units may wedge into each other in this manner.
Lithostratigraphical units often have a three-dimensional shape such as a sheet, slice, lens, wedge, ridge, finger or tongue. Various lithostratigraphical boundary relationships are shown in figures 2, 3 and 4. In addition to having lithostratigraphical boundaries, lithostratigraphical units may be delimited by secondary boundary surfaces of intrusive, tectonic or metamorphic origin (Fig. 1).
Lithostratigraphical units serve as units when geological mapping is being carried out. They are used in the field and when writing descriptions, elucidating the geological history and assessing economically exploitable deposits (oil, ores, minerals, gravel, groundwater, etc.).
Lithostratigraphical units are, in decreasing order of rank, supergroup, group, formation, member and bed/flow. The formation is the fundamental unit.
