Page 163 - Petrology of Sedimentary Rocks

Allochemical constituents (allos: differentiation from the normal) are those
components that have formed by chemical precipitation within the basin of deposition,
but which for the most part have suffered some later transport; or, if they have not
been transported, they include such organized aggregates as sedentary fossils or fecal
pellets differentiated from “normal” chemical precipitates as one usually thinks of
them. Only four types of allochems are of importance: intraclasts, oolites, fossils, and
pel I ets.

( I) lntraclasts represent pieces of penecontemporaneous, usually weakly consol-
idated carbonate sediment that have been torn up and redeposited by currents (hence
the term intraclast, signifying that they have been broken from within the formation).
lntraclasts vary from fine sand size up to pebble or boulder size, as in the familiar
limestone conglomerates. lntraclasts themselves may consist of any kind of limestone,
micrite, biomicrite, intrasparite, pelsparite, etc. (see later). They indicate a tearing-up
of the bottom by an increase in current velocity (such as in storms), lowering of wave
base by partial emergence, or possible tectonic instability of the basin of deposition.
Rare intraclasts form by accretion, analogous to lumps in a sugar bowl, and are then
transported. Of such nature are the “grapestone” aggregates of fecal (?) pellets in the
Bahamas (III ing). They may also form in the wave-attacked margins of broad areas
where ooze is being precipitated, or upon tidal and supratidal flats.

(2) Oolites range from 0.1 - 1.0 mm in diameter, and show radial or concentric
structure. They usually form around a nucleus, often a small fossil fragment, fecal
pellet, or quartz grain. They form in loci of vigorous and continuous current or wave
action, in places where aragonite is being rather rapidly precipitated. An individual
oolite may go through several cycles of abrasion alternating with growth. In the
Bahamas (Illing) oolites form in current-swept tidal channels or form submarine dune
ridges near shore where currents are powerful. Modern oolites are commonly aragonite,
rarely Mg-calcite. Ancient ones may have been calcite (Sandberg).

(3) Fossils are irnportant constituents of many limestones. The most common
rock-formers are algae, foraminifera, sponge spicules, corals, bryozoans, brachiopods,
gastropods, pelecypods, ostracods, trilobites, echinoids. See Horowitz and Potter for
details.

(4) Pellets are homogeneous aggregates of microcrystalline calcite, well
rounded and sorted, averaging .03 to .20 mm. They probably represent fecal pellets of
worms or other invertebrates. It is possible that some may form in place by a form of
recrystallization; vaguely defined “pellets” in ooze are sometimes termed grumeleuse.

Rock Classification. Almost all carbonate rocks contain more than one type of
material; one may be a mixture of oolites, fossils and sparry calcite cement while
another may consist of quartz silt, pellets, and microcrystalline ooze partially replaced
by dolomite and chert. Thus the problem of classification becomes one in systematizing
these variations of composition and drawing significant limits between types. Carbon-
ate rocks are so complex that it is usually necessary to make a thin section study in
order to pigeon-hole a specimen properly, although good estimates can be made in the
field or with an etched specimen.

Disregarding for a moment the content of terrigenous material, it is possible to
base a practical limestone classification on the relative proportions of three end-
members (I) allochems, (2) microcrystalline ooze, and (3) sparry calcite cement.

I57