Geology Reference
In-Depth Information
Fig. 8.7.
Modified Folk classification
after Strohmenger and Wirsing (1991).
(Elf-Aquitaine 1975; Flügel 1982). In reality, Folk's
classification has often been applied in a rather loose
manner without following the grain ranking concept
(resulting in rather monstrous names like 'pelonco-
biosparrudite'!) or by creating names, not provided for
within the original classification, by including several
grain types in order of
increasing
importance within
the rock name (Friedman and Sanders 1978). 'Intrabio-
oncopelmicrite', for example would describe a micritic
carbonate rock whose grains are predominantly peloids,
followed by oncoids, bioclasts and fewer intraclasts.
In order to restore the reproducibility of Folk's clas-
sification and to strengthen the value of rock names for
paleoenvironmental interpretations, an expanded and
modified textural classification has been proposed by
Strohmenger and Wirsing (1991); see Fig. 8.7.
special depositional conditions, e.g. subaerial exposure)
as two new grain groups,
• adjustment of the percentage of individual grains
from 25% to 20% for intraclasts, ooids and oncoids
and 40% for fossils and peloids,
• Incorporation of all the grains into the rock name as
long as they comprise >20% (for intraclasts, ooids or
oncoids) or >40% for fossils and peloids).
Owing to the introduction of oncoids as a fifth grain
type - they are in third position in the ranking list (in-
traclasts, ooids, oncoids, fossils and peloids) - the volu-
metric grain proportions have been changed, except for
the fossil/peloid group. For Strohmenger and Wirsing
the ranking list and the grain percentage must be re-
spected, even if, as a consequence, the most frequent
grain type is not indicated by the first prefix of the rock
name.
Major modifications
are:
• the introduction of new grain categories: (1) oncoids
(indicating special environmental conditions, e.g. dif-
ferences in sedimentation rates, salinity, or substrate
types; see Sect. 4.2.4.1) and (2) extraclasts (indicating
A strict use of the modified classification scheme
results in the differentiation of a maximum of 31 pos-
sible rock names. The practical steps involved in ap-
plying the classification are shown in Fig. 8.7.
