Geology Reference
In-Depth Information
Basics: Microfractures and 'calcite veins' in carbonate
rocks
Al-Aasm, I.S., Coniglio, M., Desrochers, A. (1995): Forma-
tion of complex fibrous calcite veins in Upper Triassic
strata of the Wrangellia Terrain, British Columbia, Canada.
- Sed. Geol.,
100
, 81-95
Antonellini, M., Mollema, P.N. (2000): A natural analog for
a fractured and faulted reservoir in dolomite: Triassic
Sella Group, Northern Italy. - Amer. Ass. Petrol. Geol.
Bull.,
84
, 314-344
Boukadi, N., El Ayeb, S., Kjarbachi, S. (2000): Analyse quan-
titative de la fractuation des calcaires yprésiens en Tunisie:
l'exemple de Jebel Ousselat. - Bull. Soc. géol. France,
171
, 309-317
• Details of small-scale fracturing assist in understand-
ing relationships between tectonic structures and frac-
ture distribution and their origin within a regional geo-
logical frame (Flügel and Kirchmayer 1962; Logan and
Semeniuk 1976; McQuillan 1985).
• Mineralogical and geochemical data of microfissure
fillings associated with larger joints and major faults
add to the explanation of the course of mineralization
and ore formation (Fairnbairn and Ferguson 1992).
• Abundance of calcite-filled veins influences tech-
nological properties (hardness, strength, breakage, slake
durability and weathering) of carbonate rocks.
Fractures can raise the permeability of hydrocarbon reservoir rocks. Mineralogical and geochemical data of
microfractures assist in explaining ore formation. The abundance and type of calcite-filled veins influences the
technological properties of carbonate rocks (e.g. strength, breakage, weathering). Many limestones exhibit mil-
limeter- to decimeter-sized calcite- or sediment-filled microfractures, commonly called 'calcite veins'. These
structures are part of large-scaled fracture systems affecting the rock together with joints and faults. Although
microfractures can be understood genetically only in the context of a regional structural analysis, the relation-
ships between fractures, faults and folds, 'calcite veins' should not be overlooked in microfacies studies because
of their importance in deciphering the postsedimentary history of the rocks, including the tectonic activities,
burial diagenesis, fluid migrations, reservoir potential and mechanical properties of carbonate rocks.
Useful descriptive criteria of microfractures are (a) spatial relations of fractures and grains (-> 1), (b) occur-
rence of isolated microfractures or fracture sets, (c) morphology (e.g. straight, arched, branching, -> 6), (d) offsets
(-> 3), (e) shape of terminations (-> 4, 6), (f) filling and crack-sealing (e.g. mineralogy, cement type, inclusions,
growth pattern, dynamic structures) and sealing, (g) orientation with regard to bedding planes, (h) crosscutting
(-> 1, 6), (i) distribution, spacing and density, width and length, volumetric importance, association with stylo-
lites (-> 4), and (j) the relation of vein abundance and bed thickness.
Fracture systems in carbonate rocks can form during various stages of sedimentation (e.g. oversteepening of
semilithified sediment, water escape structures, -> 5) and diagenesis (compaction load, freshwater dissolution),
but most calcite veins are due to brittle failure and tectonic fracturing of lithified carbonates caused by stress and
shear displacement (-> 2, 3), extensional movements or natural hydraulic fracturing. Attempts to classify calcite
veins rely on fracture types, geometry and morphology of veinlets, mineralogy and cement texture of filling (->
2) as well as the timing and age relationships of the microcracks.
1
Microfracture sets.
Radiolaria wackestone. Most larger radiolarians are crossed by calcite-filled cracks. Early Cretaceous
(
Aptychus
beds): Schrecksee, Allgäu, Germany.
2
Shear fractures.
Fusulinid foraminifera (
Neoschwagerina
), displaced along calcite cement-filled shear systems. Note the
difference in cement fillings. Middle Permian: Straza quarry, Bled, Slovenia.
3
Microcracks with offsets.
Bioclastic mudstone. Late Triassic (Kössen beds): Schrecksee, Allgäu, Germany.
4
Burrowed lime mudstone
, crossed by crosscutting calcite veins associated with stylolite seams (arrows). Note the inter-
connected burrow network (dark appearance) created by sediment-inhabiting organisms. Burrowing took place in aerobe
environment. Stylolites are often associated with small-scale calcite-filled fracture sets, which can be used as paleostress
indicators. Early Jurassic (Liasbasiskalk): Schrecksee, Allgäu, Germany.
5
Early diagenetic fractures.
Vertical microcracks caused by dewatering of the sediment. The rock is characterized by wavy
to crinkled fine laminae composed of couplets of mudstone layers (dark) and layers with quartz grains (light). These
couplets are known from peritidal settings and muddy lagoonal environments. Late Triassic (Kössen beds): Schrecksee,
Germany.
6
Bedded lagoonal fenestral bindstone.
Note the differences in shape, thickness and terminations of calcite veins. Middle
Triassic (Wetterstein limestone): Schrecksee, Allgäu, Germany.
