Geology Reference
In-Depth Information
Impressive examples of long-ranging graduated fis-
sure fillings have been reported from Devonian, Trias-
sic and Jurassic platform and reef carbonates as well as
from slope and basinal carbonates (Box 5.7).
The 'Blue Holes' of the Bahamas: a modern example
of neptunian dikes
Underwater caves formed on synsedimentary frac-
tures paralleling the margin of the Bahama Bank and
enlarged by dissolution activity in a mixing zone of
fresh and saline waters may represent a modern ana-
logue of neptunian dikes (Smart et al. 1988; Mylroie et
al. 1995). These so-called Blue Holes got their name
from the seemingly blue water of the surface water.
The underwater cavities were formed as a result of
erosional and dissolution processes controlled by Pleis-
tocene high and low stand phases of the sea level. Some
of the fractures of the Bahama Bank are associated with
large submerged cave systems. Steep-sided planar
walls, synchronous deposition of erosional and encrust-
ing biota within the cavities, as well as the size and
geometry of the cavities indicate that the Blue Holes
are a good present-day example of active neptunian
dikes, fissures and cavern infill.
Neptunian dikes in Devonian reef and platform car-
bonates, Western Australia
Neptunian dikes are frequent along the Frasnian plat-
form margins of the Canning Basin (Playford et al.
1984). They become rarer towards the platform inte-
rior in the reef-flat facies and down marginal slopes in
reef slope facies. Interestingly, dikes are especially well
developed in Famennian back-reef limestones formed
subparallel to the Frasnian reef front.
Individual dikes may become 6 km long, 80 m deep
and 20 m wide. But most of them are less than 100 m
long, 20 m deep and about 5 cm wide. S-parallel dikes
('neptunian sills') are very abundant in marginal-slope
deposits. Both fissure types show a complex history of
fissure development, filling, lithification and refractur-
ing. Early fillings include encrusting organisms, fossil
debris, ooids, lime sand and mud, terrigenous sand, cav-
ity peloids and 'spar balls', radiaxial calcite and inter-
nal breccias.
Fig. 5.13. Horizontal Liassic fissure (S-fissure) in Late Tri-
assic peloidal grainstones (bottom, PG). Note that the pic-
ture does not show the entire width of the fissure. The Upper
Rhaetian grainstone originated in a back-reef setting. The nep-
tunian dike exhibits two-fold fillings: (1) alternating infill-
ings of fine-grained crinoidal debris and carbonate mud, fol-
lowed by (2) occlusion of a younger fissure within the lithi-
fied echinoderm packstone by several generations of calcite
cement. The lack of calcite tapestries at the boundary (indi-
cated by two short black lines) between the host rock and the
fissure and current-induced textures within the fissure (align-
ment of shells - black arrows, laminations in micrite layers -
white arrow) points to a rapid infilling of the dike. Both, the
Triassic host rock and the neptunian dike were transected by
a network of thin microcracks (3), filled by gray calcareous
silt possibly of meteoric origin and related to late tectonics.
The Early Jurassic age of the echinoderm packstone is indi-
cated by diagnostic foraminifera (In - Involutina liassica
Jones). Rofan, Sonnwend Mountains, Tyrol, Austria. Scale is
5 mm.
Synsedimentary dikes in the Late Permian reef com-
plex of Texas and New Mexico
Vertical dikes near and parallel to the shelf-edge,
composed of encrusting carbonate laminae, as well as
dikes with abundant marine biota (algae and/or inver-
tebrates) are known from the Capitan limestone
(Stanton and Pray 1993). Fossil-rich dikes are usually
0.3-0.5 m wide and exhibit progressive opening, fill-
ing by intact and fragmented skeletal grains and/or ce-
ment, and reopening which creates vertical layering.
Most bioclasts in the dikes were loose seafloor particles,
but some (whole and paired shells and large Tubiphytes )
probably were fissure dwellers. The scarcity of micritic
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