The history of stratigraphy traces our understanding of rock units and time. Early on, geologists placed emphasis on the description of rock units and did not consider geologic time. As geologists would name new rock sequences, they would assign them different times. Later, geologists realized that different rock sequences could occur during the same time period and the principle of faunal succession showed that rocks of similar lithology could occur during different time periods. Rock units are not unique to specific times and have no time connotation.
Rock units are the differentiation of strata on the basis of lithologic unity. Basically, similar types of rock are classified together. The similarities can be based on the lithology, or on similar fossil assemblages. There are five different rock units: supergroup, group, formation, member, and bed.
The basic rock unit is the
formation. Formations are characterized by two properties. They must have identifiable and distinctive lithic characteristics and must be mappable on the surface or traceable in the subsurface. Formations are ranked into larger classifications called groups and supergroups and can be divided into small classifications called members and beds. A rock sequences ranking depends on the geologist who names the rocks. A decision to name a formation is based upon qualitative properties of the unit, its distinctiveness, lateral continuity, and ease of recognition, but not on quantitative qualities like the units’ thickness. Sometimes members can be as thick or thicker than other formations, and some formations can be as thick as groups or as thin as beds. In rock units, its position in the hierarchy doesn’t denote the size of the unit, but is a reflection of the units distinctiveness compared to other rock units.
Geologic time can be measured by several different methods. Changes in fossil faunas, measurement of radioactive decay, identifying magnetic reversals, or changes in oxygen isotopes are just some of the ways to attempt to qualify geologic time. With so many methods to measure time, geologic time can be an abstract concept. Because of this, geologists identify separate time units to differentiate the different divisions of time. The different time units are: eon, era, period, epoch, and age. I will cover the geologic time scale in more detail along with the age of the earth in a later article.
Time units are hierarchal, with each unit in the series composed of one or more units beneath it. So, eons, the longest time unit, are composed or eras that are composed of periods that are composed of epochs that are composed of ages. Using the geologic time scale we can place names with the concept. The Phanerozoic eon is composed of the Paleozoic, Mesozoic, and Cenozoic eras. The Mesozoic era is composed of the Triassic, Jurassic, and Cretaceous periods. The Cretaceous period is divided into Early and Late epochs. The Early Cretaceous epoch is divided into the Berriasian, Valangian, Hauterivian, Barremian, Aptian, and Albian ages.
