Geology Reference
In-Depth Information
eralogy, and minimal early cementation and dolomiti-
zation. Grain-supported carbonates may be resistant to
compaction down to 700 m.
tion seams that are often associated with fracturing
structures (Logan 1984). Pressure solution is caused
by load and/or tectonic stress. In more strongly ce-
mented rocks solution takes place along extensive sur-
faces, forming stylolites. Dissolution often starts along
a bedding surface. Mechanisms by which pressure so-
lution occurs are dissolution in a thin solution film as
result of compressive stress inside grain-to-grain bound-
aries (Weyl 1959; Rutter 1983) or dissolution at or just
outside the rims of grain contacts resulting in an un-
dercutting (Bathurst 1975; Tada and Siever 1986).
7.5.2 Chemical Processes:
Pressure Solution and Stylolitization
Following mechanical compaction many sediments
are subject to chemical compaction, expressed by pres-
sure solution and the formation of stylolites and solu-
Plate 36 Mechanical Compaction and Deformation Structures
Mechanical compaction is a process caused by sediment overburden and resulting in a general reduction of
porosity and rock volume. The thickness of the overburden necessary to produce compaction structures is con-
troversial. Overburden results in the mechanical failure of grains. Compaction is commonly followed by pres-
sure solution recorded by stylolites and solution seams (see Pl. 37). The degree of mechanical compaction is
inhibited or reduced by early precompaction cementation (-> 2) and can be studied in grainstones by cement/
grain ratios (Meyers 1980).
1
Mechanical and chemical compaction
resulting in the formation of a diagenetic packstone exhibiting a
condensed fabric
after Logan and Semeniuk (1976). Overburden stress at grain contacts is accommodated by grain deformation in the form
of ductile squeezing and grain fracturing (black arrows). Lime mud within the foraminifera is protected from compaction.
Pressure solution is indicated by nonparallel multigrain sutured seams (microstylolites). The diagenetic packstone con-
sists of worn, redeposited, and strongly compacted fusulinid foraminifera (F) and platy algae (A). Some matrix has been
lost during compaction and a welded mass of skeletal grains remained. The sequential history of fusulinids includes:
(1) Infilling of the tests with gray micrite (GM) and red micrite (RM) prior to compaction. Globular tests of the fusulinids
suffered stronger breakage than spindle-shaped shells (top right). Deep burial environment. Early Permian (Rattendorf
Formation): Carnic Alps, Austria.
2
Differential compaction.
Ooid-peloid grainstone. The sample consists of alternating layers with ooids (center and upper-
most layer), and layers with peloids, crustacean coprolites (
Favreina
, F) and ooids (below and above the ooid layer).
Compaction is indicated by plastic bending (arrows) of peloids and
Favreina
grains. Compaction within the ooid layer
was inhibited by early cementation and is only indicated by the close packing of the grains. The interparticle cement
postdates compaction of grains, pointing to shallow subsurface burial cementation. Early Cretaceous (Purbeck facies,
Berriasian): Subsurface, Bavaria, Germany.
3 Crushed fusulinid test, the victim of physical compaction caused by bioclast-to-lithoclast (L) contact. The whorls are
broken and rearranged (arrows). Differences in the rigidity of lithoclasts and the fossil caused breakage and splittering of
whorls associated with burial fracturing. Note the absence of stylolitization. The limestone is a sedimentary microbreccia
generated by the erosion and deposition of platform-derived material. Early Permian (Tressdorf limestone): Carnic Alps,
Austria.
4
Tectonically deformed limestone
with branching scleractinian corals. Preservation stages of corals reflects several of the
diagenetic and deformation stages shown in Fig. 7.15: Corallites with well-preserved septa (1); calcite within partially or
totally dissolved corallites (2); infilling of red micritic sediment (RM) in hollow corallites (3). Corals were affected by
shearing (4) and fracturing of the rock. The diagenetic pattern of aragonitic corals is often related to growth types: The
open framework of branching corals favors dissolution, whereas massive corals are completely replaced by neomorphic
spar. Cretaceous: Building stone of the Hittite wall, Bogazköy, central Anatolia.
5
Strong compaction
. Ooid grainstone with distinctive compaction features: Flattened grains (FG) and parallel grain con-
tacts (PGC). The nuclei of the ooids are dissolved. Deformation (arrows) of partially dissolved ooids suggests that com-
paction took place during burial. Late Permian (Main Dolomite, Zechstein): Subsurface, Debki well, northern Poland.
6
Shallow-burial compaction.
Diagenetic pisoid grainstone with contorted grains. The concentration of calcite cement
below the grains (arrows) points to precipitation in the meteoric-vadose zone subsequent to burial compaction. Another
explanation is indicated by the laterally continuous cement garlands underneath the grains, which may mark stylolites
filled with burial cements. Late Permian (Main Dolomite, Zechstein): Subsurface, Miloszewo well, northern Poland.
-> 4: Flügel and Flügel-Kahler 1997; 5 and 6: Peryt 1986.
