Page 167 - Petrology of Sedimentary Rocks

restrictions of packing impose a certain maximum on the amount of allochems; yet
there is no minimum, and Microcrystalline Allochemical rocks are found with percent-
ages of allochems (intraclasts, oolites, fossils, or pellets) varying continuously from
about 80 percent down to almost nothing. The reason for this is that microcrystalline
ooze can form a rock in its own right (comparable to a claystone in the terrigenous
series), and can accept any amount of allochem material that becomes mixed with it.
Thus the boundary line between Microcrystalline Allochemical rocks and Microcrys-
talline rocks is entirely arbitrary, and has been set at IO percent allochems.

Type I limestones indicate strong or persistent currents; type II limestones
indicate weak, short-lived currents or a rapid rate of formation of microcrystalline
ooze; and most limestones can be assigned readily to one or the other of these two
classes because usually either sparry calcite or microcrystalline calcite is clearly
dominant. In some rocks there are transitions, however, either because (I) washing is
incomplete and the ooze is only partially removed, or (2) in some very fine grained
pellet calcilutites, the pore spaces between pellets are so tiny that sparry calcite
crystals are very minute, and can only with great difficulty be told from microcrystal-
line ooze. Transitional types can be designated by symbol I - II. Some of these types
may also represent rocks in which the matrix has partially recrystallized.

Type III limestones (the Microcrystalline rocks) represent the opposite extreme
from Type I, inasmuch as they consist almost entirely of microcrystalline ooze with
little or no allochem material and no sparry calcite. This implies both a rapid rate of
formation of microcrystalline ooze together with lack of strong currents. Texturally,
they correspond with the claystones among the terrigenous rocks.

Some microcrystalline rocks have been disturbed either by boring organisms or by
soft-sediment deformation, and the resulting openings are filled with irregular “eyes” of
sparry calcite. Other beds of microcrystalline ooze have been partially torn up by
bottom currents and rapidly redeposited but without the production of distinct
i ntraclasts. These are considered as Disturbed Microcrystalline rocks, and a special
symbol and rock term (“dismicrite”) is used for them (see classification table).
Bioherm rocks (Cummings and Shrock, 1928), made up of organic structures
growing in situ, are unique and place in a special class, “biolithite” Type IV. The
dominant%gansm should be specified, e.g., coral biolithite, blue-greek algal biolithite,
etc. Of course, if these bioherms are broken up and redeposited the resulting rock is
considered to be made up of intraclasts and falls in Type I or Type II depending on the
interstitial material.

After the main division of limestones into Types I, II, or III it is most essential to
distinguish whether the allochemical portion consists of intraclasts, oolites, fossils or
pellets. In terrigenous sandstones, one wishes to know not only whether the rock has
clay or not, but what the composition of the sand is; hence geologists recognize arkoses,
phyllarenites and orthoquartzites, all of which types may or may not contain clay
matrix. It is just as important to recognize the radically different allochem types in
limestones, and the scheme for classification is presented in the table. The division
lines between the groups are set at levels believed to reflect the significance of the
constituent; for example, intraclasts are so important genetically, indicating as they do
a tearing up of previously-deposited limestone and possibly indicating tectonic uplift,
that a rock is called an intraclastic rock if it contains only 25 percent intraclasts,
although it may have 60 to 70 percent fossils. Whether a rock is intraclastic, oolitic,
biogenic, or pelletiferous is indicated by adding “i”, “o”,.“b”, or “p” to the symbol I or II
or III, as in Sparry lntraclastic rocks (Ii), or Microcrystalline biogenic rocks (Ilb).

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