Page 184 - Petrology of Sedimentary Rocks

Aggrading neomorphism may be either porphyroid or coalescive. In porphyroid
neomorphism (or porphyroid recrystallization, porphyroid inversion) a few large crystals
grow at the expense of a static matrix, thus a porphyroid fabric with large and small
crystals is in existence during intermediate stages of the process. In coalescive
neomorphism, most of the crystals are either growing or being consumed, i.e. the whole
mass is changing like the growth of bubbles in a soap foam; a uniform crystal size is
maintained throughout the mass, though the average crystal size gradually grows larger.
Porphyroid neomorphism appears to be a much more common process, but coalescive
neomorphism may be the mechanism by which microspar is formed; it is usually very
equigrai ned. The importance of calcite-to-calcite recrystallization is an unsolved
point. In Folk’s opinion this occurs importantly in some localities and at some
stratigraphic horizons, but is overall volumetric importance is minor. Recrystallization
of micrite to 5-15 micron microspar is rather common, however.

Recrystallized allochems can be recognized if they consist almost entirely of
mosaic spar; some fossils and intraclasts are thus affected. Recrystallization of
micrite matrix should yield allochems floating far apart in a “sea” of spar, like plums in
a pudding, far too loosely packed to fit the requirements of packing; it should also
produce isolated patches of recrystallization spar in homogeneous micrite beds. Such
rocks are quite rare. Usually, where allochems are closely packed they have a sparry
calcite cement (like close-packed, well-sorted sandstones); where allochems are loosely
packed, they lie in a micrite matrix (like sand grains in a shale). The overwhelming
dominance of these two types argues that most spar is a pore-filling cement, not the
result of recrystallization. The abundance of well-sorted, now-porous calcarenites in
recent sediments indicates that well-sorted, calcite-cemented calcarenites should be
abundant in the geologic column.

Recrystallization can be proven if spar can be shown to transgress allochems and
matrix more or less indiscriminately. The criteria for proving recrystallization are the
same as those for replacement. Bathurst (I 958) has pot-posed some fabric criteria for
recognition of recrystallization, and Folk (I 965) has a few comments. Longman and
Mench (1978 Sed. Geol.) have a brilliant SEM study on fresh-water neomorphism of
carbonates.

Dolomi tes
Dolomites are classified as shown in the preceding pages. Most dolomite is of
replacement origin, and replacement usually seems to have occured while the limy
sediment was fairly soft or not buried very deeply. Replacement origin can be shown by
transaction of allochem structures by mosaic dolomite. Most definite replacement
dolomite is coarser than .Ol5 mm: it can be rather safely stated that if the massive
dolomite is coarser than .03 mm it is certainly a replacement. Replacement of
limestone may be complete or partial; in partially dolomitized limestones, the dolomite
may occur as isolated grains, as winding tubules or irregular patches following zones of
greater permeability.

Dolomites finer than ,010 are believed to be of quasi-primary origin, accumu-
lated directly as a dolomite ooze (dolomicrite). A directly precipitated origin is not
proved, however, and it is possible that these beds were dolomitized very rapidly either
at the sea bottom or during the first few cm of burial. These beds sometimes show
small-scale cross-bedding and are frequently reworked into pebbles; this indicates that
they were in existence as dolomite at a time when they were still close enough to the
depositional interface to be eroded by stronger-than-usual waves.

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