Geology Reference
In-Depth Information
• The enclosure of fossils within sedimentary precipi-
tates derived from fluid seepage (e.g. sulfide/sulfate
minerals, isotopically distinctive authigenic carbon-
ates).
• Evidence for the presence of sulfide and methane:
Fibrous cements derived from mixing of methane-rich
fluids with seawater, micritic microbial fabrics, pyrite-
coated corrosion surfaces, pyrite framboids, pyrite crys-
tals, irregular yellow calcite cements.
• Penecontemporaneous sulfide and sulfate mineral-
izations may be common.
• Biomarkers indicating anaerobic oxidation of meth-
ane by bacteria (Peckmann et al. 2001).
Microfacies
• Common microfacies types are bivalve-dominated
wackestones, floatstones and packstones; intraclastic
packstones; worm tube-dominated boundstones, rud-
stones and cementstones; peloidal grainstones. Some
caution is necessary in using these rock names because
most of the textures seen in seep and vent carbonates
are diagenetic and not depositional in origin. Depend-
ing on the volumetric importance of the microbial con-
tribution, an abundant rock type, composed predomi-
nantly of calcite cement, micrite crusts and shells might
be designated as cementstone or microbial bindstone.
• Micritic coatings and crusts. Micritic coatings and
micrite crusts (representing microcrystalline cement,
possibly microbially triggered) are abundant. Coatings
alternate with crusts consisting of calcite cement.
•
Micrite. Non-ferroan calcite, occurring (a) as inter-
nal micrite precipitate in burrows and shells, (b) mi-
crocrystalline cement between carbonate or siliciclas-
tic grains, (c) pelmicrite. Often containing clay- and
sand-sized grains, pyrite and organic matter. Sometimes
recrystallized to microspar.
•
Micritic peloids are common. Some millimeter- to
centimeter-sized peloids are fecal pellets (Pl. 148/2).
•
Clasts and breccia
.
Micrite and pelmicrite lithoclasts
in autobreccia may be caused by burrowing or fluid
circulation in the sediment (Fig. 16.17).
• Lamination. Laminated fabrics composed of irregu-
larly arranged layers of micrite and/or calcite cement
are common.
• Burrows and bioturbation. Burrows filled and lined
with fine-grained carbonate containing grains similar
in lithology and texture with those of the surrounding
strata are common. Pyrite coatings.
•
Cement. Volumetric contribution of cement high to
medium. Carbonate, sulfate and sulfide cements. Com-
mon carbonate cements are botryoidal calcite (rolling
extinction between crossed nicols) forming botryoids
up to 2 cm; fibrous calcite; irregular non-isopachous
cement layers.
• Organic matter. Dispersed debris, organic inclusions
in calcite or black bands.
• Pyrite. Common, occurring as linings, coatings, iso-
lated and framboidal crystals.
16.5.2 Case Studies
Isolated occurrence of limestone bodies within non-
carbonate sequences or carbonate-poor lithologies as
well as carbonate mound-like structures characterized
by low-diversity but abundant specific benthic faunas
(predominantly bivalves, tube-worms as well as bra-
chiopods) and conspicuous amounts of δ
13
C-depleted
carbonate cements may represent authigenic 'cold-seep-
carbonates' and not sedimentary structures. Some mol-
lusks and worms are able to directly exploit gas seep-
age and fluid expulsions by hosting chemolithoauto-
trophic bacteria. Diagnostic criteria of ancient seep and
vent carbonates are irregular textures, low-diversity but
abundant mollusks, abundance of carbonate cements
as well as of pyrite and organic matter.
Deposits related to the venting of cold fluids (mainly
hydrocarbons, carbon dioxide and water) are known
from different geological settings (Box 16.5)
.
Meth-
ane-derived and hydrocarbon-derived chemosynthetic
carbonates produced at fossil cold-seeps and vents are
commonly developed as carbonate masses, mounds and
lenses within shales and siltstones. These 'chemoherms'
form conspicuous structures in outer marine, fault-
bounded forearc, rift and accretionary prism settings,
facilitating conduits for seeping hydrocarbons. Many
ancient cold-seep assemblages occurred in water as
shallow as the mid-shelf and outer shelf. In contrast,
most modern cold-seep communities characterized by
large biomass aggregations of animals with chemoauto-
trophic bacterial symbionts are restricted to bathyal and
abyssal depths (Callender and Powell 1999).
Seep mounds are associated with specific structures
(fractures, diapirs, vents, methanogenic zones) rather
than with specific depositional settings. The growth of
structural highs may have been an effective mechanism
for concentrating gas hydrates by gradually upwardly
removing the base of their stability zone. Fluid release
may result in sediment instability and large-scaled slid-
ing structures (Conti and Fontana 2002). The internal
lithology of seep mounds has little in common with
Geochemistry
•
Authigenic carbonate cements and carbonate shells
extremely depleted in δ
13
C, indicating that the carbon-
ates were derived from bacterial oxidation of methane.
