Abstract

Carbonate rocks consist of a closely related family of consolidated sediments, normally quite simple in mineralogical composition and generally marine in depositional environment. Various mixtures of only two mineral species — calcite and dolomite — make up the bulk of limestones and dolomites in this interesting and economically valuable group.

In addition to their mineralogical composition, the carbonate rocks possess three distinctive features which must be recognized in classification. First, most carbonate sediment is derived from sea water, the particles remaining within the basin where they originated. They are admixed with land-derived clastics along the margins of epicontinental basins, but offshore, and particularly in isolated banks of the Bahama type, the sediment is nearly pure carbonate of local origin. Second, there is strong dependence on organic activity. Secreted skeletal elements of both animals and plants, conspicuous in so many limestones that the possibility of chance occurrence is absolutely ruled out, demonstrate how calcium and magnesium carbonates are abstracted from sea water and fixed as sediment. Even the more enigmatic lime muds are known to be at least partly derived through organic processes. Finally, as a result of their peculiar chemical composition, the carbonate rocks are especially susceptible to solution and recrystallization. In all these respects the carbonate rocks are set distinctly apart from their nearest sedimentary relatives — sandstones and shales — and because of these distinctions they have unique problems of classification and interpretation.

Recrystallization in particular destroys original features, locally to the extent that meaningful classification becomes impossible. Pervasively re-crystallized carbonates are thus set aside as a special group.

Another specialized group consists of reefs and beds that are dominated by in-place organic remains. Carbonate rocks of this group are unique and must be set apart in any classification scheme.

Carbonate rocks that are strongly dolomitized may be classified in the same way as ordinary limestones, provided only that relicts of the original calcium carbonate sediment are preserved. Other dolomites, originating as wave-swept grains that do not replace limestone, are classified according to grain size into dololutites and dolosiltites.

The major classification scheme of normal limestones is based on the concept that the rock consists of grains that have been transported away from the site at which the carbonate particles were originally fixed from the marine water. In moving away from their original depositional site, the grains have an opportunity to become sorted by size, shape, and density, with the resulting formation of calcilutites, calcisiltites, calcarenites, and calcirudites. Only four compositional features of these grains are quantitatively important — skeletal grains; lithoclasts, derived by mechanical erosion of consolidated or semi-consolidated carbonate sediment; fecal pellets and other composite grains that have originated by agglutinization on the sea floor, these two types being genetically dissimilar but extremely difficult to distinguish in consolidated rocks; and oolites, pisolites, and other similar grains that are coated through organic, mostly algal, activity.

In addition to the composition of the grains and a quantitative measurement of their size, the classification of normal limestones must also consider whether the interstitial space between grains is an open pore space or is filled with lime mud or with clear calcite cement. A consideration of just three paramaters — grain composition, graith size, and interstitial pore space, matrix, or cement — is enough to provide a working classification for basic investigations of most limestones for petroleum research.