Geology Reference
In-Depth Information
zones may be deduced from calcite-filled cracks in lime-
stones. The spatial distribution of geological fractures
is of particular interest with regard to reservoir poten-
tial, hydrology or slope stability. Microfractures are only
a part of largescaled fracture systems, and can there-
fore, be genetically understood only in the context of
regional structural analysis. However, small-scaled frac-
tures should not be overlooked in microfacies studies
because of their importance in deciphering tectonic
activities, fluid migrations, diagenetic history, reservoir
potential and the mechanical properties of carbonate
rocks.
Box 5.8.
Terms refering to tectonically induced fissures.
Fractures
: Discrete breaks in a rock mass where cohe-
sion was lost due to brittle failure by stress. Fractures
include faults, across which shear displacements oc-
cur, joints which have an aperture but show no sig-
nificant shear displacements at hand specimen scale,
and filled fractures, such as veins and dikes (Ramsey
and Huber 1987, Gillespie et al. 1993).
Microcracks
: An opening that occurs in rocks and has
one or two dimensions smaller than the third dimen-
sion (Simmons and Richter 1976). Generally milli-
meters to centimeter in size.
Microfractures
: mm- to cm-sized structures developed
by brittle failure.
Veinlets
: Small veins.
Veins
:
Fractures filled with coarse crystalline calcite or
various epigenetic minerals. Vein widths range from
millimeters to over a meter, vein walls are subparallel,
planar or irregular undulating. Contacts with host rock
are usually sharp but embayed margins due to solu-
tion and/or replacement of host rock are also com-
mon (Logan and Semeniuk 1976).
5.3.2.1 Origin and Classification of Calcite
Veins
Fracture systems in carbonate rocks form in various
stages of sedimentation (e.g. oversteepening of semi-
lithified sediments, network of thin veinlets represent-
ing water escape structures: Macdonald et al. 1994) and
diagenesis (e.g. compaction load, freshwater dissolu-
tion, deep-burial diagenesis: Geiser and Sansone 1981),
but most calcite veins are due to brittle failure and tec-
tonic fracturing of lithified carbonate rocks. Tectonic
fracturing is caused by
• stress and shear displacement (Narr and Burruss
1984; Logan 1984; Ramsay and Huber 1987; Roehl
and Weinbrandt 1988; Vrolijk and Sheppard 1991),
• extensional movements, or
• natural hydraulic fracturing (Sibson et al. 1975;
Gretener 1976).
rate, fluid pressure, temperature and lithology (Pater-
son 1978).
Crack pathes observed in carbonate rocks appear to
be controlled by texture and composition (e.g. irregu-
lar cracks preferentially within the matrix, not through
the grains: Hugman and Friedman 1979; Olsson 1974).
Fracture abundance often shows an inverse correlation
to the amount of argillaceous material. Relations also
exist between the carbonate texture type and the fre-
quency of calcite veins. Lime mudstones and wacke-
stones seem to be more strongly affected by shear-in-
duced fracturing than grainstones.
Abnormally high natural pore fluid pressures form
during (a) rapid sediment burial in basins of high sedi-
mentation rate, (b) tectonic deformation causing clo-
sure of pore spaces at one site and high pore pressure
due to migrating fluids at some other site, or (c) aqua-
thermal pressure or mineral-phase changes as a result
of diagenesis or low-grade regional metamorphosis and
contact metamorphism (e.g. smectite-illite transforma-
tion). These fractures are characterized by the domi-
nant vertical orientation and parallel nature of the frac-
tures (Longman 1985).
Vertical orientation, parallel nature and close verti-
cal spacing of fractures, and the absence of shear off-
sets can be indicative of hydrofracture origin. Indica-
tions of fractures formed by extension are the lack of
shear offsets on the fractures and the presence of only
a single fracture set. The history and timing of shear-
induced fractures in carbonate rocks is elucidated by
laboratory experiments providing information on the
controls of fracture development by stress, strain, strain
Classification of calcite veins
Attempts to classify calcite veins rely on fracture
types, geometry and morphology of veinlets, mineral-
ogy and cement texture of filling as well as the timing
and age relationships of veins. Four types of tensional
fracture veins in outcrop and hand specimens have been
distinguished (Logan and Semeniuk 1976): (1) paral-
lel sheet (a single subparallel vein set), (2) rhomboidal
and (3) rectangular (conjugate veins cutting the host
rock idens appearing as rhomboid to rectangular in cross
section), and (4) breccioid (caused by irregular and nu-
merous conjugate veins). Misik (1966) distinguished
separate veinlets formed by tension from those formed
by compression using filling and relationships with
microstylolites. He drew attention to a specific veinlet
type characterized by a set of thin parallel veinlets sepa-
rated by micrite and believed to have been formed by
repeated cracking and recrystallization (Misik 1998).
