Larger foraminifera is the term applied to those benthonic foraminifera which in general are relatively large and of which only the internal structure can be used for determination. Thus tests must be sectioned.
The rapid evolution of most taxa makes larger foraminifera a valuable group for relative age determination. However, for a variety of reasons they probably are the most difficult to use for biostratigraphy :
- All larger foraminifera have a very limited facies range, and thus first appearance and last appearance more often reflect facies changes than real first appearance and extinction levels.
- All foraminiferal taxa have a certain variability. Being rather complicated the larger foraminifera have many characters in which they can vary. This has lead to unwarranted splitting into numerous species most of which can not be recognized and have no stratigraphic value.
- Existence of homeomorph. There are several independent evolutionary series which lead to virtually identical forms. These may be identical developments at different times (e.g. the evolution from Heterostegina to Spiroclypeus happened at least twice, once in the Upper Oligocene and once in the Upper Eocene) or similar developments from deferent ancestors leading to similar forms (e.g. Lepidocyclina/Lepidorbitoides).
Larger foraminifera are mostly found in carbonate deposits and often cannot be separated from the rock. They are therefore studied in thin section which permits identification to genus level only. Section rich in larger foraminifera are dated using the Indo-Pacific Letter Stage System of Classification.
This system was introduced as a simple but effective way of dividing up the tertiary.
The Indonesian Letter Stages were originally proposed by Van der Vlerk and Umbgrove (1927) to replace the European Tertiary Stages, which could not be applied in Indonesia. However, although, this zonation originated in Indonesia, Adams (1970) makes it clear that the zonation is an applicable to much larger area.
Many authors refer to the units of the letter classification as stages and use the letter units as chronostratigraphic units. However, the concept involved is wholly biostratigraphical. According to Article 46 of the Stratigraphic Code of Indonesia, the Letter Stages Classification is "a geochronologic concepts, essentially derived from biostratigraphic classification based upon a number of concurrent range zones of larger foraminifera". However, the letter stages are really only very broad stratigraphic units. Larger foraminifera are useful in correlation of shallow water marine carbonate sediments of Eocene - Late Miocene Age. The Letter Stage originally consisted of eight major units, designated "Tertiary a" to "Tertiary h". These are usually written Ta, Tb etc. Ta is the oldest division, Th the youngest. Some of these units are further subdivided into numbered divisions e.g. Tf1, Tf2, Tf3.