Geology Reference
In-Depth Information
16.5 Testing for Ancient Vent and
Seep Carbonates
sedimentary faults, fractures, joints, diapirs; preserved
fluid-flow conduits, e.g. carbonate chimneys, pipes,
doughnuts).
• Fluid related structures: Micritic breccia (explained
by the explosive release of methane), pock mark struc-
tures, nodular structures, micritic doughnut structures
in calcite or aragonite veins explained by bacterial ac-
tivity.
Carbonate beds, lenses, mounds (chemoherms) or nod-
ules, occurring within a strongly differing facies (e.g.
shales) and yielding specific benthic faunas (predomi-
nantly bivalves, tube-worms) and conspicuous amounts
of carbonate cement may represent authigenic 'seep-
and vent-carbonates' (see Sect. 2.4.6). There is still
much to learn about these limestones, but the evidence
for carbonates bearing products of chemosynthesis re-
lated to submarine fluid and gas expulsions is growing
in the Phanerozoic record. Isolated and areally limited
limestones within deep-marine siliciclastic sediments,
which are often considered as resedimented blocks,
should be tested with regard to this interpretation.
Authigenic carbonates and benthic invertebrate com-
munities that cluster around gas, cold brine, petroleum
seeps and hot hydrothermal vents are increasingly be-
ing reported from the Phanerozoic rocks (Box 16.5).
Seep and vent carbonates offer the opportunity to
study the effects of bacterial transformation of chemi-
cal compounds from the Earth's interior into energy in
shallow and deep sea environments.
The recognition of the vent origin of ancient car-
bonate mounds is important for understanding hydro-
carbon formation and mineralizations (Bitter et al.
1992). Pb-Zn-Fe-Ba-Sr mineralizations in Early Car-
boniferous mounds of southwestern Newfoundland,
formerly interpreted as being of late diagenetic origin,
have been reinterpreted as sulfides and sulfates intro-
duced simultaneously with mound construction via
vents, and fed through fractures in the basement.
Fabric and carbonate cements
• Layered micritic microbial fabrics.
• Nodular fabrics associated with cylindrical to en-
circling concretions.
• Laminated, crustose, or void-filling carbonates, in
places extensive and thick brecciated carbonate crusts.
• Brecciation within centimeter ranges, resulting in
autobreccia.
• Microcrystalline, radial-fibrous, splayed fibrous,
botryoidal cements and irregular yellow calcite ce-
ments. Mg-calcite and aragonite cements.
• Hydrocarbon inclusions in carbonate cements.
• Cements may be volumetrically far more important
than fossils. Rocks consisting only of authigenic car-
bonate (cementstone) are common (Pl. 148/1).
Biota
• Stratigraphically limited occurrence of extraordinary
(eventually chemoautotrophically-based) benthic biota
consisting predominantly of tube-worms, large bi-
valves, gastropods as well as brachiopods (Campbell
and Bottjer 1993). Modern cold seeps are character-
ized by abundant clams ( Calyptogena, Solenomya,
lucinids) and mussels ( Bathymodiolus ), all of them
linked to symbiotic bacteria; and the 'tube worm' host-
ing chemolithoautotrophic bacteria.
• Predominance of sessile, filter-feeding organisms,
with a symbiotic relationship to chemosynthetic bac-
teria or feeding directly on bacteria.
• Anomalous low diversity/high abundance faunas in
comparison with faunas of the surrounding sediments.
• Commonly high density, but also low density fau-
nas.
• Strong endemic character of the fauna.
• Strong microbial micritic encrustations on fossils.
• Size frequency of shells bimodal, but dominated by
large individuals.
• Shell beds consisting of in-situ assemblages of
strongly dissolved, corroded and variously broken,
sometimes pyrite-coated shells. Fragmentation and ar-
ticulation frequency are major distinguishing criteria
of cold seep-related autochthonous shell beds and shell
accumulations formed in non-seep environments
(Powell et al. 1992).
16.5.1 Diagnostic Criteria of Ancient Seep
and Vent Carbonates
Data used to demonstrate an ancient vent-seep origin
of carbonate rocks include regional stratigraphic, sedi-
mentologic, paleontological and geochemical criteria.
Microfacies criteria are of major importance.
Occurrence, geometry and structural features
• Anomalous occurrence of autochthonous limestones
in deep-water or high-latitude environments.
• Isolated occurrence of limestone bodies within non-
carbonate sequences (e.g. siltstones, shales, black
shales), or carbonate-poor lithologies (marls).
• Mound-shaped geometries with dimensions of a few
meters to several tenths of meters.
• Localized limestones, related to structural features
along which methane or other fluids could migrate (syn-
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