Page 162 - Petrology of Sedimentary Rocks

PETROLOGY OF CARBONATE ROCKS

Carbonate rocks comprise some 25-35 percent of the stratigraphic section. They
are quite complex because of the varied constituents that form them and the amount of
replacement and recrystallization that they undergo. Books by Bathurst (19761,
Milliman (I 9741, Wilson (I 9751, and Scholle (I 978) are excellent references.

Carbonate rocks may be best studied by the following means: (I) etching, wherein
the specimen is immersed in dilute hydrochloric acid for say five minutes; this reveals
distribution of relatively insoluble constituents such as sand, silt, clay, glauconite,
phosphates, authigenic quartz, feldspar, chert, pyrite, and dolomite; and also reveals
the morphology of the calcite, whether ooze, spar, fossils, intraclasts, etc. (2) Peels,
wherein an acetate impression is made of the slightly etched surface; this can then be
examined under a petrographic microscope in transmitted light and a wealth of textural
detail is brought out in the calcite, in some ways superior to a thin section. (3)
Insoluble residue, wherein the rock is dissolved and the insoluble constituents described
under binocular and petrographic microscope. (4) Thin section, the best all-round
method and absolutely necessary. All methods should be used to study a specimen
adequately. Electron microscopy is also very useful (Longman & Mench 1978 Sed.
Geol.).

The following scheme for carbonate classification is presented in more detail in
January, 1959, A.A.P.G. Bulletin, and A.A.P.G. Memoir #I (I 962).

Classification of Limestones

Constituents. Disregarding admixture of terrigenous sand, silt, and clay, there
are three basic components of limestone: (I) microcrystalline calcite ooze, (2) sparry
calcite, usually a cement, and (3) allochems. These are, in most limestones, analogous
with the clay matrix, chemical cement and sand grains of a sandstone.

(I)MicrocrystalIine ooze, forming grains l-4 microns in diameter, usually sub-
translucent to almost opaque in thin sections. In modern sediments, it may consist of
calcite, magnesian-calci te, or aragonite; in pre-Pleistocene limestones, it is almost
always converted to calcite. Carbonate mud is polygenetic. Some is the result of
inorganic chemical precipitation in warm, shallow, saline water (probably the situation
in the Persian Gulf). Most mud today comes from disintegration of organisms, mainly
green algae, into constituent crystallites. Some mud also is windblown dust, can come
from abrasion of shells, or may be precipitated by algae, bacteria, etc. The origin can
sometimes be deciphered by study with electron microscope, isotopes, etc. Carbonate
mud today forms in broad, warm shallow protected shelves, bays or peritidal flats such
as in the Bahamas, Florida Bay, Trucial Oman, etc. Lithified ooze, or micrite, is the
main constituent of the “lithographic” limestones.

(2) Sparry calcite forms crystals generally over IO microns in diameter, and is
distinguished from micrite by its clarity as well as coarser crystal size, which may
range up to I mm or more. This type of calcite usually occurs as a pore-filling cement;
pores are often lined first by fibrous calcite, with equant mosaic calcite filling the final
spaces. In modern sediments, the first fibrous coating is usually aragonite or magne-
sium-calcite, but in older limestones all cement is converted to calcite. Some sparry
calcite is the result of recrystallization of micrite; this subject is not touched upon
here. For criteria see Folk (I 965 SEPM Spec. Publ. I3), and Bathurst (I 97 I).

156