Geology Reference
In-Depth Information
skeletal grains may be less soluble than micrite ma-
trix),
• the less soluble parts of three-dimensional fingers,
extending into the more soluble parts,
• amplitudes of stylolites may range from < 1mm to
> 1 m. The amplitudes indicate the minimum thickness
of the dissolved material,
• insoluble material (often clay, hydrocarbons) be-
comes concentrated along the stylolite surface. This ma-
terial does not appear to reprecipitate in the immediate
vicinity of its origin, implying transport by fluids,
• pressure solution-derived cements,
• inter- or intragranular dissolution at grain contacts
causing sutured boundaries.
Fig. 7.17.
Stylolites on a microfacies scale.
Stylolites are ser-
rated boundaries between units. The boundary usually has an
accumulation of clay, oxides, and/or organic matter. Lower-
most Cretaceous carbonate ramp: Subsurface, southern Ba-
varia, Germany. Scale is 1 mm.
Criteria of pressure solution in carbonate rocks
Chemical compaction (Lloyd 1977) is recorded by
• dissolution seams (horse tails, wispy stylolites,
microstylolites) characterized by surfaces that can be
followed across several grains; common in fine-grained
and clay-rich limestones,
• stylolites characterized by greater amplitudes than
in dissolution seams; common in more coarse-grained
limestones or at lithologic boundaries,
• stylolites often cross the whole rock, intersecting
grains, matrix and cement,
• stylolites often have a zigzag form, due to differ-
ences in the solubility of different components (e.g.
Terminology and classification of carbonate rocks af-
fected by chemical compaction
Diagenetic features of carbonate rocks, even if they
are striking, are only rarely recorded in names of lime-
stone types. Exceptions are the modification of the Dun-
ham classification proposed by Wright (1992; Sect.
8.4.1) and the consideration of pressure solution struc-
tures in the classification of Logan and Semeniuk
(1976). Fig. 7.18 summarizes the main criteria used by
Logan and Semeniuk and by Wanless (1979).
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Fig. 7.18.
Terminology of pressure solution features. Most of these categories can be derived from thin-section studies (see
Pl. 37).
A
-
Styles of
pressure-solution features
after Wanless (1979) modified by Choquette and James (1987). The
non-sutured type
is characterized by microstylolites, microstylolite swarms, clay seams and grain contact sutures; it forms in limestones that
contain a significant amount of insoluble material (clay, silt, organic matter). The
sutured type
is typified by stylolites and
grain-contact sutures and forms in limestones with very low amounts of insoluble residual material. Bedding-parallel sutured
seams correspond to stylolites of different amplitudes, which may occur as single structures or in swarms. Reticulate systems
of anastomosing microstylolites (fitted fabrics, see B) and non-sutured seams occur in limestones made up of rigid entities in
grain-to-grain contact or surrounded by more soluble sediment.
B
-
Classification of pressure solution features
by Logan and Semeniuk (1976)
.
The classification concentrates on the
morphology of stylolites (columnar, Pl. 37/1, 5; high amplitude - Pl. 37/2 and low amplitude - Pl. 37/3 - types, irregular,
hummocky or smooth) and differences in stylolite sets (parallel, irregular anastomosing, Pl. 37/3; conjugate - Pl. 37/6), and
focuses on fabric types caused by pressure solution and structural types that can be recognized on micro- to macroscales.
Note that in rare cases stylolites can also be formed by mechanical compaction without pressure solution (Braithwaite 1986).
Fabric types
are differentiated according to the behavior of idens (bodies that behave as statistically homogeneous enti-
ties under physical and chemical changes).
Iden-supported fabric:
Idens form a self-supporting frame and are in contact at
grain points.
Condensed fabric:
Idens are very densely packed, grains contact over a surface.
Fitted fabric:
Idens produce a
puzzle-like fabric. They are surrounded by solution seams and are in contact with their neighbors all along their margins.
Stylocumulate/reaction support:
Larger idens are supported by fine-grained stylocumulate (insoluble residue accumulated
along a pressure-solution surface).
Diagenetic structures:
Stylobedding
: Pseudo-bedding caused by parallel pressure solution that may be parallel to the
original horizontal layering.
Stylolaminated:
Laminated appearance due to swarms of parallel solution seams or low-ampli-
tude stylolites.
Stylonodular:
Nodules and lenses of limestone idens separated by broad stylolite swarms. The original mi-
critic matrix is more or less dissolved.
Stylomottled
: Characterized by patchy enrichment of insoluble stylocumulate and
reactate (material which has grown at solution interfaces, e.g. calcite, dolomite, or quartz), may be also caused by patchy
enrichment of marls due to inhomogeneous pressure solution.
Stylobreccoid:
Originates from selective pressure solution (Pl.
37/7).
