Page 47 - Sedimentology and Stratigraphy

34 Biogenic, Chemical and Volcanogenic Sediments

carbonate-forming environments and can be the under quite different conditions to a foraminiferal
main constituent of limestone. wackestone.

3.1.6 Classification of limestones 3.1.7 Petrographic analysis
of carbonate rocks
The Dunham Classification is the most widely
used scheme for the description of limestone in the Thin-section analysis of limestones and dolostones
field, in hand specimen and in thin-section. The pri- can reveal a great deal of information about the en-
mary criterion used in this classification scheme is the vironment in which the sediment was deposited.
texture, which is described in terms of the proportion Assessment of the proportions of carbonate mud and
of carbonate mud present and the framework of the larger fragments provides an indication of the environ-
rock (Fig. 3.6). The first stage in using the Dunham ment of deposition: a high proportion of fine-grained
classification is to determine whether the fabric is carbonate material suggests a relatively low-energy
matrix- or clast-supported. Matrix-supported lime- setting, whereas an absence of mud characterises
stone is divided into carbonate mudstone (less than higher-energy environments. The mud to fragmental
10% clasts) and wackestone (with more than 10% component ratio is also the basis for classification
clasts). If the limestone is clast-supported it is termed a using the Dunham scheme of carbonate mud-
packstone if there is mud present or a grainstone if stones, wackestones, packstones and grainstones. If it
there is little or no matrix. A boundstone has an isnotalreadyevidentfromhandspecimen, thin-sections
organic framework such as a coral colony. The origi- will also reveal the presence of framework organ-
nal scheme (Dunham 1962) did not include the sub- isms such as corals and algae that form a bound-
division of boundstone into bafflestone, bindstone stone fabric.
and framestone, which describes the type of organ- The nature of the fragmental material provides
isms that build up the framework. These categories, further evidence of the conditions under which the
along with the addition of rudstone (which are clast- sediment was deposited: for example, high concentra-
supported limestone conglomerate) and floatstone tions of ooids indicate shallow, wave-dominated
(matrix-supported limestone conglomerate) were coastal settings (15.3.1) whereas a rock composed of
added by Embry & Klovan (1971) and James & biogenic material that is all from the same group of
Bourque (1992). Note that the terms rudstone and organisms, such as bivalves or gastropods, is an indi-
floatstone are used for carbonate intraformational cator of a lagoonal setting (15.2.2). The degree to
conglomerate made up of material deposited in an which the shelly material is broken up also reflects
adjacent part of the same environment and then the energy of the setting or the amount of transport
redeposited (e.g. at the front of a reef: 15.3.2). These and reworking of the sediment. It is usually possible to
should be distinguished from conglomerate made up determine the fossil group to which larger bioclasts
of clasts of limestone eroded from older bedrock and belong from their overall shape and the internal struc-
deposited in a quite different setting, for example on ture (Fig. 3.1). Additional clues may also come from
an alluvial fan (7.5). the mineral that the original bioclast was made of
The nature of the grains or framework material (Fig. 3.7): shells originally composed of aragonite
forms the secondary part of the classification. A rock tend to recrystallise and the primary fabric is lost;
consisting entirely of ooids with no matrix would similarly, high-magnesium calcite commonly recrys-
be an oolitic grainstone, one composed of about tallises and also results in bioclasts with a recrystal-
75% broken shelly fragments in a matrix of carbo- lised fabric. Organisms such as many brachiopods and
nate mud is a bioclastic packstone, and rock com- bivalves that were formed of low-magnesium calcite
posed mainly of large oyster shells termed a tend to retain their primary structure.
bioclastic rudstone. Naming a limestone using the It should be noted, however, that all carbonate
combination of textural and compositional criteria rocks are susceptible to diagenetic alteration (18.4)
in the Dunham scheme provides information about that can change both the mineralogy and the struc-
the likely conditions under which the sediment ture of the fragments and the carbonate mud. Diage-
formed: for example, a coral boundstone forms netic alteration can vary from a simple cementation of