Page 161 - Petrology of Sedimentary Rocks

5. The Clan designation (arkose, quartzose phyllarenite, etc. analogous to
granite or quartz diorite in igneous rocks) is the most important as it indicates the bulk
mineral composition. Mineralogy depends chiefly on source area Iithology, a factor
which stays relatively constant for long periods of time and consequently is the most
characteristic feature of a sandstone format ion. It is a more dependable property than
the other parts of the name, which tend to fluctuate more rapidly. The clan name is
determined as shown previously by using the composition triangles.

Order of terms
No order is inherently more sacred than any other; however, it facilitates
comparison of samples if the same order is followed by everyone. As in igneous or
metamorphic rocks the most important part of the name, the clan designation, should
come last. Grain size, the one sedimentary rock characteristic that is used by all
geologist, should come first. By convention, the following order is suggested for the
fivefold name (asterisks denote sections of the name that can be omitted if the
constituents are absent):

(Grain Size): (Cements*) (Textural Maturity) (Miscellaneous
Transported Constituents*) (Clan Designation). Following are some examples:

Fine sandstone: siliceous submature glauconitic quartzarenite.

Pebbly medium sandstone: hematitic bimodal supermature chert-bearing subphyl-
lareni te.

Silty very fine sandstone: gypsif erous mature arkose.

Sandy granule conglomerate: calcitic submature calclithite.

Muddy fine sandstone: immature phyllarenite.

Slightly granular medium sandstone: calcitic, siliceous mature micaceous phyl-
lareni te.

Clayey very fine sandstone: immature fossiliferous plagioclase arkose.

Fine sandstone: supermature quartzarenite.

Silty coarse sandstone: chert-cemented submature quartzose phyllareni te.

Very fine sandstone: opaline mature chert-bearing and calcarenitic quartzarenite.

Mudrocks
For siltstones and silt-shales, the same terminology is used as above (e.g.
siltstone: siliceous submature glauconitic phyllarenite). For mudstones, the order is
inverted with the term mudstone coming last (e.g. immature chert-bearing subarkose
mudstone, or calcitic immature orthoquartzite mudstone). Both of the above can
usually be easily related to one of the terrigenous rock clans. For claystones, omit the
textural maturity and clan names and follow the pattern (chemical cements) (odd
elastic constituents) (chief clay mineral or minerals) (grain size). For example, calcitic
fossiliferous illite-chlorite clay shale; chert-cemented cellophane-bearing illite clay-
stone. A classification has also been proposed by Picard (1971 JSPL

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