Geology Reference
In-Depth Information
Plate 46 Folk's Classification of Carbonate Rocks
The classification proposed by Folk (1959) for non-reef carbonates is based on the ratio grains : micrite : sparite.
Required for using this classification is (a) separation of carbonates with and without a micrite matrix - see
figure on this page, (b) differentiation of fundamental grain types, and (c) frequency and proportion of grain
types. The names of grains of the rock name are arranged in a specific sequence (intraclasts, ooids, skeletal
grains/peloids). Although the classification is widely used, the names are often defined in a way which does not
follow the strict rules proposed by Folk starting with a prefix of the less frequent grain type and increasing to the
most abundant type or vice versa). The
extension of Folk's classification
by Strohmenger and Wirsing adds
oncoids and extraclasts as separate categories to the rock names, the change in frequency boundaries for grains,
and the incorporation of all grains in the rock name as long as they comprise > 20% (or more than 40 % for
skeletal grains/peloids). The modification is useful, because it results in reproducable rock names allowing
statistical treatment.
1
Micrite
. Corresponding names are mudstone and calcimudstone. Folk's original definition characterizes 'microcrystalline
calcite ooze' composed of 1-4
m-sized calcite crystals, 'generally subtranslucent with a faint brownish cast in thin
section' and dull and opaque in hand specimen. These criteria are typical for many lithographic limestones and also for the
type of these extremely fine-grained rocks, the Solnhofen limestone shown in this figure.
Lithographic limestones
represent a particular type of micritic carbonate. They are formed under a wide range of
environmental conditions, including lagoons (e.g. Solnhofen,), open shelf and also lacustrine settings (Oost et al. 1994;
Swinburne and Hemleben 1994). The Solnhofen limestone is famous for its unusually well-preserved whole body fossils
and the fact, that the fine- and even-grained rock has long been used in lithography, buildings, and in experimental work
on deformation fabrics of carbonates affected by high strain rates (Casey et al. 1998). Some of the Solnhofen micrite
consists of coccoliths, some of calciodinoflagellates embedded in minimicrite, and still others (this sample) are composed
of very fine bioclastic detritus (arrows point to radiolarians). Fourier spectral analysis of millimeter-scale lamina couplets
consisting of a carbonate-rich lamina and a relatively carbonate-poor lamina indicates a short-time cyclic climatic control
on the deposition of this carbonate mud (Park and Fürsich 2001). Late Jurassic (Tithonian): Solnhofen, Bavaria, Germany.
2
Dismicrite.
Corresponding name: fenestral bindstone (see Pl. 41/2). 'Disturbed micrite' is a sackname for micritic lime-
stones containing irregular patches of sparry calcite representing spar-filled openings of diverse origin (see 5.1.5). Note
geopetal infilling of some voids (arrows). Intertidal part of a loferite sequence (see Pl. 140). SMF 21. Late Triassic
(Norian): Gaissau, Northern Alps, Austria.
3
Oomicrite.
Corresponding name: oolitic lithoclast wackestone. Grains are ooids and intraclasts. Taking into account the
frequency of ooids (> 25% of the grains) and the micritic matrix, the rock is called oomicrite. Because this name excludes
the rather conspicuous intraclasts (I), some authors would prefer the name oo-intra-micrite or even, loosely, intra-oo-
micrite, emphasizing reworking of the sediment as indicated by the large intraclasts. The sample indicates transport and
redeposition of ooid shoals in protected ramp settings (see Pl. 136). Lowermost Cretaceous: Subsurface, Kinsau well,
southern Bavaria, Germany.
4
Biomicrite.
Corresponding name: Red-algae bearing echinoderm packstone
or
Komia
-bearing echinoderm packstone. The reference to
Komia
is impor-
tant because of the facies-diagnostic value of Late Paleozoic red algae (see
Pl. 108). All the grains are bioclasts, usually echinoderms associated with a
few shells (S) and a red alga (
Komia,
K; see Pl. 56/6). There are >25% sand-
sized skeletal grains associated with a micritic matrix. Hence, the rock is a
biomicrite. Packing is conspicuous, hence the name 'packed echinoderm
biomicrite' is more appropriate (see Fig. 8.6). Pennsylvanian (Honnacker
Trail Formation): Limestone Ridge, Goosenecks, San Juan, Utah.
5
Intrasparrudite.
Corresponding name: Poorly sorted lithoclast rudstone.
Grains are intraclasts consisting of pelmicrite, cemented by various genera-
tions of submarine calcite spar. Because the grain size (> 2 mm) is included
in the rock name the prefix 'rudite' is a part of the name. Slope environ-
ment. Early Jurassic: Adnet near Salzburg, Austria.
6
Oosparite.
Corresponding name: Poorly sorted ooid grainstone. Grains are
large and small ooids (O; 65%), bioclasts (25% bryozoans, B; foraminifera,
F; shells, S), and cortoids (10%; C). Strohmenger and Wirsing would clas-
sify this sample as oobiosparite (see Fig. 8.7). Early Cretaceous: Subsur-
face, Ostermünchen well, Bavaria, Germany.
7
Classification exercise
. Try to find names for this sample, following the
proposal of Folk and Dunham. For explanation see Appendix. Middle Tri-
assic (Anisian): Karawanken Mountains, Austria.
Basic concept of the Folk classification.
