Page 165 - Petrology of Sedimentary Rocks

Allochems represent the framework of the rock: the shells, oolites, carbonate
pebbles or pellets that make up the bulk of most limestones, analogous to the quartz
sand of a sandstone or the pebbles of a conglomerate. Microcrystalline ooze represents
a clay-size “matrix” whose presence signifies lack of vigorous currents, just as the
presence of a clay mineral matrix in a sandstone indicates poor washing. Sparry calcite
cement simply fills up pore spaces in the rock where microcrystalline ooze has been
washed out, just as porous, non-clayey sandstones frequently become cemented with
chemical precipitates. Thus the relative proportions of microcrystalline ooze and
sparry calcite cement are an important feature of the rock, inasmuch as they show the
degree of “sorting” or current strength of the environment, analogous to textural
maturity in sandstones. If we plot these two constituents and the allochemical
“framework” as three poles of a triangular diagram (see figure), the field in which
limestones occur is shown by the shaded area; divisions between the three major
textural types of limestone are also shown on this figure. A similar field appears if one
plots terrigenous rocks on a triangle with the three poles of sand and silt, clay and
orthochemical cement.

This classification is predicated on the assumption that the sparry calcite has not
been formed by aggrading recrystallization of a fine calcite ooze, and that microcrys-
talline calcite has not formed by degrading recrystallization of coarser calcite. In most
carbonates the writer has examined, this assumption is believed to be very largely true.
Nevertheless, the writer agrees that recrystallization is a very important process in
some limestone formations, and the classification proposed here does not apply to
recrystallized rocks. It provides a starting point for study of recrystallized rocks,
though, because on original deposition these rocks must have belonged to one of the
groups here proposed.

Type I limestones (designated as Sparry Allochemical rocks) consist chiefly of
allochemical constituents cemented by sparry calcite cement. These rocks are
equivalent to the well-sorted terrigenous conglomerates or sandstones in that solid
particles (here intraclasts, oolites, fossils or pellets) have been heaped together by
currents powerful or presistent enough to winnow away any microcrystalline ooze that
otherwise might have accumulated as a matrix, and the interstitial pores have been
filled by directly precipitated sparry calcite cement. The relative proportions of sparry
calcite cement and allochems varies within rather restricted limits because of the
I imi tations of packing:

(I) There is a limit to the tightness with which allochems may be packed, thus
there will always be some pore space available for cement to fill, and

(2) There must be a certain minimum amount of allochems present in order to
support the structure-sparry calcite cement grows only in poor spaces and in general
cannot form a rock on its own right, unless recrystallization occurs. Similarly
sandstones require a minimum amount of sand grains (say 60 percent) to support the
rock structure. It may be noted that carbonate rocks on deposition often have porosity
much greater than sandstones or conglomerates of equivalent size because of the
irregular shapes of fossils (see Dunham 1962).

Type II limestones (designated as Microcrystalline Allochemical rocks) consist also
of a considerable proportion of allochems, but here currents were not strong enough or
persistent enough to winnow away the microcrystalline ooze, which remains as a
matrix: sparry calcite is very subordinate or lacking simply because no pore space was
available for it to form in. These rocks are equivalent texturally to the clayey
sandstone or conglomerates, which also tend to have little cement. In these rocks the

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