Geology Reference
In-Depth Information
gence, commonly exhibit changes in shape and dimen-
sions due to solution, bioerosion and karstification.
Filling
In general, fissure infill occurs much less rapidly in
terrestrial than in marine environments. Marine fissure
fillings are primarily controlled by the relative rates of
autochthonous and allochthonous sedimentation, by the
size of the fissures and by the interconnection of the
fissure with the external environment. Laminations and
crossbedding of internal sediments within neptunian
dikes can indicate relatively high flow rates within fis-
sure systems (Pl. 24/2; Figs. 5.12, 5.13).
Neptunian dikes
are filled with sediment and fossils
derived from variable sources, and carbonate cements
formed at different times. Internal sediments comprise
allochthonous grains and mud as well as autochtho-
nous elements originating within the fissures.
Allochthonous constituents are predominantly fos-
sils (commonly pelagic fossils or organisms living on
the sea-bottom nearby) forming densely packed, strati-
graphically condensed accumulations of cephalopods,
thin-shelled bivalves (e.g.
Posidonia
,
Halobia
, 'fila-
ments') and gastropods ('Fossillagerstätten') as dem-
onstrated by Triassic and Jurassic S- and Q-fissures in
Austria and Sicily (Krystyn et al. 1971). Many of these
fossils have been swept into and trapped in the fissures
but some, characterized by significantly small growth
forms, represent probably cavity dwellers (Wendt
1976). Common cavity-dwelling organisms are ostra-
cods (Aubrecht 1997), foraminifera, and some brachio-
pods.
Autochthonous sediments consists of material
eroded from fissure walls and fossils formerly living at
the edges of or within neptunian dikes (Gonzalez-
Donoso et al. 1983; Playford 1984). Autochthonous
biota comprise microbial crusts as well as benthic fora-
minifera, ostracods, serpulids and brachiopods. Mi-
crobes are able to form distinctive morphological struc-
tures (fissure-dwelling 'endostromatolites': Monty
1982, see Sect. 9.1).
Fig. 5.12.
Early Jurassic neptunian dike (right) within a Late
Triassic reef limestone (left)
. The organic structures of the
white Upper Rhaetian reef limestones (
Lamellata wähneri
Flügel and Sy, a hydrozoan - L, and a cyanobacterial crust -
C) are cut by a vertical fissure (Q-fissure) filled with red Li-
assic echinoderm packstone. The planar boundary between
the host rock and the dike indicates a tectonic origin of the
fissure (related to initial rifting of a drowned platform). The
peripheral calcite tapestry consisting of radiaxial cement
points to a time break between the opening of the fissure and
the infilling of the Liassic sediment. Note the solution seams
on both sides of the calcite tapestry (arrows). Rofan, Sonn-
wend Mountains, Tyrol, Austria. Scale is 5 mm.
1995). Fissures can also be produced by differential
compaction of platform and basinal carbonates (Saller
1996). Rapid extensional tectonic activities and high
hydrostatic pressure can, perhaps, also cause injection
of sediments into voids and fissures (Castellarin 1982).
Hydrothermal effects related to thermochemical effects
(Sotak et al. 1993; Belka 1998) might also result in the
formation of neptunian dikes. Opening of fissures in
platform carbonates during rifting phases and infilling
of pelagic sediment during and/or after drowning of
the platforms have been repeatedly reported from the
Mesozoic of the Alpine-Mediterranean region. Many
dikes are associated with regional extensional features,
as shown by the coincidence of the preferred orienta-
tions of neptunian dikes and tectonic structures (Blen-
dinger 1986). Neptunian dikes are not restricted to plat-
forms and platform margins but also occur within pe-
lagic carbonates deposited on tilted fault blocks (Martire
1996). Opening of bedding planes can occur as a result
of both tectonic and synsedimentary induced fractur-
ing. Fracture orientation parallel or oblique to platform
margins can be indicative of synsedimentary or tec-
tonic origins of fissures, respectively.
Many neptunian dikes show little evidence of dis-
solutional and mechanical enlargement after the initial
opening. Conversely, fissures associated with emer-
Terrestrial infill
within subaerially formed fissures
differs from the internal sediments of neptunian dikes
in the lack of marine fossils (Pl. 24/5). Internal sedi-
ments of paleokarst fissures consist commonly of fine
limestone or dolomite lithoclasts eroded from fissure
walls, erosional relicts of overlying deposits, calcare-
ous silt, siliciclastics, and redeposited pedogenic sedi-
ment (MacDonald et al. 1994; Pl. 24/5, Pl. 129/1). Car-
bonate cements of paleokarst fissures are often of me-
teoric-phreatic origin (see Sect. 15.2.1).
