Page 26 - Carbonate Facies in Geologic History

Grain Size, Rounding and Coating 13

1962) and pellet micritic limestone (Leighton and Pendexter, 1962). Folk prefers
to construct compound abbreviations or acronyms in combining grain kind with
textural nouns: pelmicrite and oosparite.

4. Grain Size, Rounding and Coating

From Amadeus Grabau carbonate petrographers inherited some cumbersome
but useful words describing grain size in limestone: calcilutite, calcisiltite, calcar-
enite, and calcirudite. Since most limestones are mixtures of all four of these sizes,
and since the names alone do not indicate whether or not lime mud (micrite) is
present in the coarse varieties, these terms are usually relegated to a secondary
role in modern textural classifications. An exception occurs where such criteria as
good sorting, sparry cement between grains, rounding and coating of grains and
certain sedimentary structures indicate an origin as a grainy, well-washed lime
sand. In such cases, grain size becomes an important attribute, just as it is in a
quartz sandstone, although even in clean lime grainstone mixtures of sand and
gravel size particles are common. Recently, Embry and Klovan (1971) (Fig. 1-6)
added a grain size modification to the Dunham classification, based on work on
very coarse reef-derived sediments. A framework of coarse gravel with little or no
matrix is termed a rudstone by these writers. Cobbles floating in a finer-grained
matrix (sand to mud sizes) constitute a floatstone. The matrix of the floatstone
may be described separately using the accepted Dunham terms. The well-known

Allochthonous limestones Autochthonous limestones
original components not organically original components organi-
bound during deposition cally bound during deposition
Greater than
Less than 10% > 2 mm components 10%>2mm By By By
components organ- organ- organ-
isms isms isms
No which which which
Contains lime mud lime act encrust build
« .03mm) mud as and a rigid
baffies bind frame-
Mud supported >2mm work
Grain Matrix com-
Less than Greater supported sup- ponent
10% than ported sup-
grains 10% ported
(>.03mm grains
<2mm)
Mud- Wacke- Pack- Grain- Float- Rud- Baffie- Bind- Frame-
stone stone stone stone stone stone stone stone stone

Fig.I-6. Amplification of original Dunham (1962) classification of limestones according to
depositional texture by Embry and Klovan (1971, Fig.2), courtesy of Canadian Society of
Petroleum Geologists