Geology Reference
In-Depth Information
• The terms have been proposed to characterize trans-
ported and resedimented fossils as illustrated by frag-
ments of stromatoporoids.
The original definition used was not intended for
classifying limestones with large components that owe
their size to biological growth processes (e.g. oncoids
or colonial fossils). A distinction in the nomenclature
of limestones with double-valved bivalves which are
either smaller or larger than 2 mm, will separate differ-
ent biological growth stages whose size differences are
not controlled by transport. This problem is commonly
overlooked in the current use of the terms floatstone
and rudstone. This is not of great importance as long as
the texture types are not interpreted with respect to pos-
sible water energy levels or other environmental fac-
tors.
• Coarse-grained bioclastic particles such as phylloid
algal plates or
Halimeda
blades can accumulate, pro-
ducing a rudstone texture. This term, however, is inap-
propriate, because it infers transport and allochthonous
sedimentation. The algal elements, however, are essen-
tially in-place accumulations. Therefore, the term
platestone
(Davies and Nassichuk 1990) is more ap-
propriate.
Folk (1959) distinguished three end members:
(1) discrete carbonate grains ('allochems'), (2) micro-
crystalline calcite matrix, and (3) sparry calcite, re-
garded as pore-filling cement. Using the relative pro-
portions of these three end members, the author distin-
guished 'sparry allochemical rocks' (consisting chiefly
of grains cemented by sparry calcite cement), 'microc-
rystalline allochemical rocks' (consisting of microcrys-
talline matrix and various amounts of grains) and 'mi-
crocrystalline rocks' (consisting of microcrystalline cal-
cite and few or no grains). Further subdivision of these
major groups is based on the ratio of grains to sparite
to micrite as well as the type of grains. Quantitative
boundaries include grain bulk percentages (> or < 10%
grains with differentiation between 1-10% and < 1%)
as well as proportions of special grain types related to
the total of all grains.
The Folk classification was designed to cover lime-
stones, partly dolomitized limestones, 'primary' dolo-
mites and replacement dolomites. Limestones were sub-
divided into eleven lithotypes, eight of them compris-
ing allochemical rocks, two orthochemical rocks and
one autochthonous reef rock. The kind of grains (only
intraclasts, ooids, fossils and pellets were differenti-
ated) and the ratio of grains, sparite and micrite are
used in classifying and naming rock types.
8.3.2 Original and Expanded Folk Classifica-
tion
A strict use of Folk's classification system requires
consideration of several points when establishing rock
names:
Non-genetic meaning of grain names:
Despite its
genetic definition, Folk (1962) regarded the term
in-
traclast
for purposes of nomenclature 'as a broad class
term without specifying the precise origin'. Using the
term in a rock name, says nothing
per se
about the ori-
gin of the grain or about the environment. The term
may refer to mud clasts reworked and transported by
tidal currents or hurricanes as well as synsedimentary
cemented grain aggregates (
grapestones
), or merely
fragments of weakly lithified sediment that have been
retectured by burrowing. Similar to intraclasts, the use
of the term
pellet
has no genetic implications. The dis-
tinction of pellets (i.e.
peloids
, see Sect. 4.2.2) and in-
traclasts is conventional and purely descriptive.
Strict ranking of grains (allochems):
Basically,
Folk's rock names consist of two prefixes. The first
(abbreviated by the prefixes intra-, oo-, bio- or pel-)
refers to the grain type, the second to the groundmass
(sparite or micrite), see inset on Pl. 46). But many lime-
stones exhibit various amounts of different grain types.
Folk introduced a ranking system and regarded intrac-
lasts as the most important carbonate grains 'because
of their implication of shallow water, lowered wave
Folk's classification is a useful and practical method
for describing carbonate rocks, particularly in thin sec-
tions and peels.
8.3.2.1 Concepts
The basic philosophy of Folk's classification is that
carbonate rocks are similar to siliciclastic rocks in their
mode of deposition, because their textures are both con-
trolled largely by the water energy in the depositional
area. In calm water with only sluggish currents, car-
bonate mud (represented as micrite) with or without
grains is predominantly deposited. By contrast, vigor-
ous water energy hinders deposition of fine-grained
material, thus favoring the sedimentation of winnowed
sands with large amounts of pore space that is later filled
with sparry calcite (sparite). The most important envi-
ronmental break is between limestones with a lime-mud
matrix and those with calcite cement, because this
should reflect the point where water energy becomes
turbulent enough to wash out (winnow) the lime mud,
keep it in suspension and carry it into lower energy
zones (Fig. 8.6).
