Geology Reference
In-Depth Information
Compaction and decompaction
The degree of compaction can be calculated accord-
ing to the
Carbonate Compaction Law
(Ricken 1987,
1992). The calculation requires quantitative data on the
CaCO
3
content and insoluble residue contents of the
sample, rock porosity measurements, and compaction
values derived from deformed burrows. An estimate of
compaction percentages is necessary for estimating the
precompaction thickness and the geometry of carbon-
ate units by decompaction (Ricken 1985).
Compaction values are used for
decompaction cal-
culations
that can restore the original thickness of the
limestone sequences. The evaluation of original thick-
ness is necessary in understanding paleorelief geom-
etries and the spatial relationships between reefs, plat-
forms and basins (e.g. Lang 1989). Low compaction is
common in reef boundstones, is medium in mudstones,
high in packstones and marls, and often high in grain-
stones. Decompaction is commonly measured by evalu-
ating of spheres deformed into ellipsoids. This tech-
nique uses deformed trace fossils that originally had a
circular cross section (Wetzel and Aigner 1989), ooids
(Coogan 1970) as well as originally spherical micro-
fossils (Martire and Clari 1994; Monaco et al. 1996).
Another common method for calculating thickness
changes during burial compaction is based on poros-
ity-depth curves (Perrier and Quiblier 1975; Doglioni
and Goldhammer 1990). Algorithms relating compac-
tion and decompaction of carbonates were developed
by Goldhammer (1997).
(e.g. bryozoans) are more susceptible to mechanical
and chemical diagenesis than single-crystal echino-
derm grains (Meyers 1980)
•
skeletal mineralogy: Apparently without major in-
fluence on inter- and intraparticle chemical compac-
tion
•
clay content: Clay-bearing rocks are often more sus-
ceptible to strong compaction than clay-free rocks.
In limestone/marl sequences limestones are not or
less compacted, but marls exhibit strong compaction
(Munnecke 1990). The precise effects of clays (e.g.
decreasing permeability to cementing pore waters,
enhanced diffusion of dissolved CaCO
3
) require
further studies
•
grain size and sorting
•
grain shape.
Inhibiting factors
•
preburial cementation; precompaction diagenesis of
interparticle cements may inhibit compaction
•
preburial dolomitization
•
clay content (see
inherited factors
above).
Compaction of carbonate muds and sands
Compaction data on sedimentary carbonates have
been gathered by Goldhammer (1997). In contrast to
muddy siliciclastic beds, which are commonly highly
compacted to fissile shales, muddy carbonates preserve
sedimentary structures much better, because carbonate
muds are far less compacted than clay-rich muds and
because precompaction cementation is widespread, at
least in shallow-marine carbonate.
Modern shallow-marine
lime mud
, consisting of
loosely arranged aggregates of micron-sized aragonite
needles, has porosities in the range of 70-80% (Enos
and Sawatsky 1981; Shinn and Robbin 1983). Squeez-
ing experiments prove that these muds can not be con-
densed by mechanical compaction below a porosity of
about 25%. Initial porosity loss is dominantly a result
of dewatering. Grain stabilization is reached under
burial conditions of 100 m or less. A fully compacted
lime mud may then have the porosity of a compacted
sand, but differs in the very low permeability. Mechani-
cal compaction is particularly important in mud-domi-
nated sediments as proved by studies of pelagic oozes
and chalks (Scholle 1971; Neugebauer 1973; Schlanger
and Douglas 1974; Scholle et al. 1983).
Compaction of
carbonate sands
is favored by deep
burial and high effective strength, a low geothermal
gradient, a low rate of loading, low pore pressure, long
burial under stress, and water-wet grain surfaces. Fur-
ther factors are small average grain size, moderately to
well sorted grains, close primary packing, stable min-
Controls on compaction
Many dynamic, inherited, and also inhibiting vari-
ables influence the degree of compaction:
Dynamic factors
• overburden
• subsurface temperature
• pressure
• duration of burial stress
• pore pressure
• pore-water chemistry: Pore water is of prime im-
portance in chemical compaction but not in mechani-
cal compaction (Weyl 1959). The greater the degree of
undersaturation with respect to CaCO
3
, the lesser the
amount of overburden needed to cause interparticle and
intraparticle pressure solution. The greater the degree
of oversaturation, the greater the amount of overbur-
den needed to cause pressure solution.
Inherited factors
•
sedimentary fabric
•
skeletal architecture: Microcrystalline skeletal grains
