Geology Reference
In-Depth Information
5.3.1 Sediment-Filled Fissures: Neptunian
Dikes and Fissure Fills
Box 5.6.
Glossary of terms used in the context of the
study of sediment-filled fissures.
Based on Bates and
Jackson (1980), Visser (1980), Smart et al. (1994).
Many limestones yield fissures filled with carbonate
or other sediment (sedimentary dikes) and with car-
bonate cements.
Fissure walls are commonly subparallel, planar or
irregularly undulated. The fissures cut the host rock
obliquely or vertically across bedding planes, or paral-
lel to the bedding. The width of vertical or obliques
fissures varies between a few centimeters and several
meters, they may be several hundreds of meters up.
Horizontal sediment-filled fissures can be followed a
few tens of centimeters up to several hundreds of
meters. Many fissures occur beneath erosional or strati-
graphic unconformities. Fissures may extend several
tens of meters below unconformity surfaces. The for-
mation and infilling of these fissures can take place
both in submarine and subaerial environments. Sub-
marine fissures are common in platform carbonates,
particularly at platform margins, but also occur in slope
and basinal settings.
Clastic dike
: A sedimentary dike, transecting bedded
sediments at various angles, and consisting of clas-
tic material derived from underlying or overlying
beds.
Cavity fill
: Subaqueous fills deposited in brackish or
phreatic freshwater environments (Swart et al. 1988).
Dike
(British spelling: dyke): A discordant or concor-
dant fissure filling.
Fissure
: A fracture or a crack in rocks along which
there is a distinct separation of walls.
Fissure fill
:
Terrestrial deposit infilling voids and fis-
sures in older rocks created, e.g. by karstification.
The term is the terrestrial equivalent to neptunian
dikes referring to submarine infillings.
Injection dike
: A sedimentary dike cutting vertically
through massive or normally bedded strata filled by
material through hydrostatically controlled pressure
of injection from below or above or from the side.
Neptunian dike
:
A sedimentary dike formed by infill-
ing of submarine sediment in fissures and cavities
of rocks exposed on the sea floor. The term was origi-
nally coined by Pavlov (1896) for dikes, occurring
in Early Cretaceous clays and filled with Oligocene
sandstone. The British term 'neptunian', now com-
monly used for carbonate rocks, refers to the sub-
marine origin of the fissures and their submarine
infillings.
Neptunian sill
:
Fissures that essentially follow bed-
ding planes and are filled with submarine sediments
(Playford et al. 1984).
Sedimentary dike
: A tabular mass of sedimentary ma-
terial that cuts across the structure of bedding of pre-
existing rock. It is formed by the filling of a crack or
fissure from below, above or laterally.
Terminology
Several terms describe submarine and subaerial
voids and fissures filled with sediment (see Box 5.6).
Definitions of these terms are not consistent, because
of differences in the descriptive and/or genetic aspects.
Common terms are fissure and dike.
Fissure stresses the existence of fractures within
rocks, whereas dike relies on the filling with sedimen-
tary material and chemically precipitated cements. The
conditions of sediment deposition are conventionally
used to define the major types of sediment-filled fis-
sures.
Neptunian dikes
are defined as fissures within rocks
exposed on the sea bottom that have been filled with
submarine sediments. Most definitions include discor-
dant cutting of bedding, infill of younger (but also strati-
graphically coeval) sediments and infilling from above.
If we accept these criteria, then neptunian dikes repre-
sent a specific type of sedimentary dike.
The most workable definition of neptunian dikes is
probably that of Fischer (1964): 'Discordant dikes of
limestones within limestones' parallel to the bedding
planes. This definition coincides with that of 'S-fis-
sures'. The names S- and Q-fissures (German: 'S- und
Q-Spalten', Q = quer, oblique) describe the geometri-
cal relationship of fissures with the host rock (Wendt
1971). Q-fissures cut the bedding (Pl. 24/1, Fig. 5.12),
and S-fissures run parallel to it (Pl. 24/2, Fig. 5.13).
The use of the term
fissure fill
for continental sedi-
ments filling subaerial cavities and fissures (e.g. paleo-
karst, see Sect. 15.2.1) may be meaningful but some-
times troublesome when mixed infills of terrestrial and
marine sediment occur.
5.3.1.1 Origin, Development and Filling of
Sedimentary Fissures
The initiation, development and sedimentary filling of
fissures can take place in a variety of environments
and is influenced by various processes such as karst
solution in subaerial and vadose settings, solution in
phreatic and mixing zones between freshwater and sa-
line waters, and processes occurring in shallow and
deep marine settings.
Initiation and development of fissures
Initial fissures can be depositional, synsedimentary
or tectonic in origin. Primary
depositional fissures
are
