Geology Reference
In-Depth Information
Pressure solution can increase the dissolution of cal-
cite and is believed to be a significant source for the
formation of porosity-occluding subsurface cements in
deep burial settings. On the other hand, pressure solu-
tion may create conduits for fluids and open migration
paths. Fracturing and pressure solution are prime fac-
tors for reservoir rocks in the Middle East.
Stylolitization contributes to bulk volume reduction,
resulting in a marked drop in the original thickness of
carbonate units. Stylolitization modifies the primary
sedimentary fabric of limestones dramatically and their
description, therefore, requires a specific terminology
focussing on the fabric and structure of the altered rocks.
Plate 37 Chemical Compaction: Pressure Solution (Stylolites and Seams)
Chemical compaction is exemplified by pressure solution resulting in stylolites and solution seams formed
under burial conditions. Stylolites are irregular, suture-like contacts produced by differential vertical movement
under pressure accompanied by solution. They are marked by irregular and interlocking penetration on two
sides: Columns, pits and tooth-like projections on one side fit into their counterparts on the other side (-> 2).
Concentrations of insoluble residues along stylolite surfaces are common (-> 5). Solution seams are isolated or
swarm-like partings characterized by thin seams, often with accumulations of insoluble residues. Pressure solu-
tion processes involve a strong reduction in rock bulk volume with a resultant loss in porosity caused by the
occlusion of pores by late diagenetic subsurface cements (Wong and Oldershaw 1981). Volume loss caused by
chemical compaction is facies-dependent.
Stylolitization changes original depositional textures and produces new diagenetic fabrics. A specific termi-
nology is necessary for describing the type of interfaces between adjacent particles, the structures produced by
pressure solution and shear fracture, the fabrics developed by pressure solution, and the resulting rock type
(Fig. 7.18).
1
High amplitude columnar stylolites in a peloidal bioclastic wackestone. Stylolite interfaces are partly covered by reactate
calcite (arrow). SEM . Late Cretaceous (Mishrif Formation): Middle East.
2
Peaked high amplitude stylolites along a bedding plane separating laminated fine-grained peloid grainstone (LPG) and
non-laminated medium-grained peloidal grainstone (PL). Both microfacies types originated in intertidal environments.
The lower one is a microbial mat with abundant bacterial peloids (see Pl. 8/5). The upper microfacies corresponds to
another mat formed by organically bound peloids and oncoids (O). The limestone was not mechanically compacted prior
to stylolitization owing to early cementation by calcite. Stylolitization has dissolved a high amount of the limestone. Early
Cretaceous (Purbeck facies, Berriasian): Subsurface of the Molasse zone, Bavaria, Germany.
3
Bedding-parallel irregular anastomosing sutured seams (microstylolite set) in lime mudstone. Note dark insoluble residue
(stylocumulate, SC) along pressure-solution surfaces and calcite, representing styloreactate (material that has crystallized
at stylolite interfaces, SR). Pressure solution took place subsequent to leaching of evaporite minerals (black arrows). Late
Permian (Zechstein): Harz, Germany.
4
Pressure solution postdates compaction. Grain-to-grain pressure solution of conspicuously large, tangentially structured
fractured ooids (TSO) exhibiting jagged surfaces. Nonparallel reticulate grain contacts. Note dark stylocumulate (clay
residue, calcite fragments and dolomite). Dolomitization occurs within the stylocumulate, but also in ooids as selective
replacement. Arrows point to dolomite crystals. Cambrian: Takkanawa Fall, Yoho Valley, British Columbia, Canada.
5
Widely spaced high-amplitude columnar stylolites in a burrowed ostracod wackestone. The dark stylocumulate consists
of clay and pyrite. The limestone has suffered no compaction prior to deep-burial pressure solution. Silurian: Valentintörl,
Central Carnic Alps, Austria.
6
Non-sutured wispy parallel sets of dissolution seams producing a stylolaminated fabric. This fabric tends to develop when
the clay content is high, larger than at least 10%. Arrows point to individual seam sets. Note that very similar features have
been produced by mechanical experimental compaction (Shinn and Robbin 1983). Lime mudstone. Jurassic: Peloponnesus,
Greece.
7
Stylobreccia. Sutured reticulate multi-grain fabric (Wanless) or condensed fabric (Logan and Semeniuk). Idens (bodies
that behave as homogeneous entities under physical or chemical conditions) are densely packed and bound by irregular,
anastomosing microstylolites (white arrows). The black arrow points to dissolution of a miliolid foraminiferal shell. RC:
Reactate calcite formed by pressure solution. Subsurface of the Molasse zone, Bavaria, Germany.
8
Horse tail structure characterized by lateral splitting of seams (arrows). Bedded non-sutured wispy seams (Wanless).
Irregular anastomosing sets (Logan and Semeniuk). Estimates of the minimum local extent of chemical compaction can
be made by determining the difference in thickness between a solution seam and the laterally equivalent wedge of diage-
netically laminate carbonate (horsetails: Kendall 2000). Cretaceous: Egypt.
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