Geology Reference
In-Depth Information
tion is necessary because the original shape of the grains
may determine the type of grain contacts and not com-
paction effects.
Box 7.9.
Compaction criteria seen in thin sections.
Mud-supported micritic limestones (mudstones, wacke-
stones)
• elongated and merged peloids forming a clotted struc-
ture,
• closely compressed shells,
• shell breakage. Note that shells in experimentally
compacted lime muds may not show breakage (Shinn
et al. 1977; Shinn and Robbin 1983). Compaction
effects on allochems decrease as the matrix percent
increases (Fruth et al. 1966),
• enrichment of skeletal grains in defined patches,
• degree of burrow deformation (Ricken 1986),
• distorted fenestral voids and desiccation structures,
• the abundance of microfenestrae (Lasemi et al. 1990)
decreased in experiments with increasing pressure.
They describe the porosity evolution in micritic lime-
stones and can be studied only in SEM. Micro-
fenestrae have a maximum diameter of 1.5-15
m.
• thinned and wispy laminations,
• irregular stringers of organic matter draping over rigid
grains, forming wispy seams,
• deformation of thin-walled organic microfossils
within the sediment (Westphal and Munnecke 1997).
Early cemented limestones contain spherical to
slightly deformed microfossils, mechanically com-
pacted carbonates exhibit flattened microfossils.
Grain-supported limestones (grainstones, packstones)
•
Several methods have been proposed for quantifying
the degree of compaction:
•
Measuring the increase in grain volume percentage
.
This can be measured directly in thin sections of grain-
stones using point counter methods and expressed by
Packing Density
(Kahn 1956; Coogan 1970). Packing
Density is the grain volume percentage as measured in
thin section. A
Compaction Index
based on Packing
Density was proposed by Coogan (1970) for oolitic
grainstones, grainstones with aggregate grains, and skel-
etal grainstones. The Compaction Index is a measure
of the degree of grain compaction, given as a percent-
age. It is derived from the grain volume measurement
and adjusted for the original volume percentage of the
uncompacted sediment according to grain type.
•
Measuring the increase in grain closeness.
This is
reflected by the increased number of grains that touch
each other as compared to the original uncompacted
sediment. It is based on grain boundary measurements,
requiring measurement of grain-to-grain, grain-to-ce-
ment, or grain-to-pore contacts along traverse lines and
is expressed by the
Packing Index
(Masson 1951;
Coogan 1970).
The Packing Index is determined from the ratio of
the number of grains touching each other to the total
number of grains in thin sections. The rationale behind
the Packing Index is the idea that compacted grainstones
should have more grains touching each other than
uncompacted grainstones. This index is more quickly
determined than packing density, but is less useful be-
cause it depends on the total number of grains counted,
and is affected by grain shape and interparticle cemen-
tation, which may change grain contact configurations.
•
Measuring porosity and cements.
In thin sections of
grainstones, the degree of compaction is estimated by
measuring the percentage of interparticle porosity and
interparticle cements and comparing it with modern
sediments of similar composition (Nicolaides and
Wallace 1996).
•
Measuring the diameters of deformed burrows
and
comparing it with non-deformed circular burrow di-
ameters (Ricken 1987; Gaillard and Jautee 1987). Open
burrows of organisms living within the sediment will
be deformed, whereas burrows filled with early sedi-
ment or cement will not be. Open burrows are deformed
similarly to the neighboring sediment.
•
Measuring deformed fossils
with originally circular
cross sections, e.g. nautilid cephalopods.
plastic deformation of peloids (Pl. 36/2) and cortoids,
•
collapsing and telescoping of grains,
•
mechanical rearrangement of grains (Pl. 36/1, 3),
•
grain rotation indicated by rotated geopetal fabrics,
•
overpacking,
•
truncation of grains by adjacent grains,
•
spalled ooids (Pl. 36/5) and shells, e.g. foraminifera
(Pl. 36/1),
•
broken and welded fossils (Pl. 36/1, 3),
•
broken micrite envelopes,
•
grain flattening (Pl. 36/5, 6),
•
curviplanar parallel grain contacts (Pl. 36/5),
•
stylolites.
Compaction criteria in thin sections
Box 7.9 lists common compaction criteria for lime-
stones. The intensity of the compaction of carbonate
muds and carbonate sands is different as is the possi-
bility of recognizing all the effects in thin sections.
Description and measurement of compaction
Measurements of compaction in thin sections require
point-counter determinations of grain volume percent-
age, or the minus-cement porosity expressed by the fre-
quency of cement or grain contact types. The resulting
numbers are compared with inferred average porosi-
ties of modern carbonate sediments of similar compo-
sition. Grain contacts should be described following
precise definitions (Taylor 1950; Fig. 7.16). Some cau-
