Geology Reference
In-Depth Information
Significance of foraminifera in microfacies analysis
Biozonations:
Benthic foraminifera, particularly
larger foraminifera, are reliable tools in the stratigraphic
subdivision and correlation of Late Paleozoic, Meso-
zoic and Tertiary shelf carbonates. Zonations have been
established both for low and high latitude deposits.
Biozones of different latitudes have different taxonomic
spectra. Planktonic foraminifera are important in the
interregional correlation of Cretaceous and Cenozoic
strata (Bolli et al. 1985). A useful summary of the tim-
ing of significant bioevents and changes in platform
and open-marine pelagic Mesozoic and Cenozoic bio-
fazies types characterized by the appearance, disappear-
ance or abundance of thin-section foraminifera was
given by Sartorio and Venturini (1988).
ate production of larger foraminifera in the Cretaceous
and Tertiary (Hohenegger 1999).
Reef-adapted encrusting foraminifera are known
from Permian, Jurassic and Early Cretaceous reef lime-
stones. Monospecific biostromes built by arenaceous
foraminifera, and surrounded by bioclastic grain- and
wackestones, occur in the Barremian of Haute Savoie,
France. These up to one meter high buildups were
formed in shallow water on a carbonate platform
(Schulte et al. 1993). Other foraminiferal reefs have
been reported from the Eocene (
Solenomeris
reefs:
Plaziat and Perrin 1992) and the Miocene (Jell et al.
1965; Ghose 1977; Hallock 1985; Hallock and Glenn
1986).
Larger foraminifera have been common constituents
of shallow-marine shelf carbonates formed in warm-
water environments since the Late Paleozoic. Exten-
sive Cretaceous and Cenozoic shelf carbonates were
produced by large, benthic foraminifera. Larger fora-
minifera (e.g.
Amphistegina
and
Calcarina
) contribute
significantly to the formation of sand-sized carbonate
sediments in reefs and other shallow, carbonate envi-
ronments due to the high turnover rates of the popula-
tions (Hallock 1981).
Paleoenvironment:
Abundance, diversity and com-
position of benthic foraminifera differ in modern sub-
tropical, warm temperate and cool temperate shelf seas.
Composition patterns of shelf sediments (e.g. 'foramol
facies') as well as taxonomic differences in foramin-
iferal assemblages provide guidelines for distinguish-
ing ancient shelf and ramp carbonates deposited in
warm or cold waters and in low or high paleolatitudes
(see Sect. 12.2). Frequency distribution, species asso-
ciation and diversity of larger foraminifera are strongly
correlated with environmental factors changing along
depth gradients. The resulting patterns can be success-
fully used in assessing paleodepths, if the coenocline
concept is applied (Hohenegger 2000).
Microfacies implications of rock-building abundance
of larger foraminifera:
Rock-building concentrations
of foraminifera (packstones, grainstones) are caused
by
• hydrodyamic processes: wave action hindering the
deposition of fines between the tests or winnowing fines
and smaller tests; current transport accumulating tests
in protected or deeper environments; storms inducing
deposition near beaches or in shoals on the shelf; tur-
bidity currents leading to deposition on slopes or in
basins,
• high production rates, e.g. in sea grass or sea weed
areas, associated with rapid deposition of epiphytic fora-
minifera forming washover deposits or in-place deposits
(Davaud and Septfontaine 1995), and
•
Depositional setting and facies zones:
The distribu-
tional patterns of benthic foraminiferal associations as-
sist in recognizing and subdividing depositional set-
tings and environments of ancient shelves, if depth
transport from shelf to basinal environments is consid-
ered (Hohenegger and Yordanova 2001).
Important papers are included in the list of Box 10.4.
Fig. 10.28 gives a very generalized idea of the occur-
rence and abundance of common foraminiferal groups
in Late Jurassic and Eocene carbonate environments.
See Sartorio and Venturini (1988) for other Mesozoic
and Tertiary distribution patterns of thin-section fos-
sils.
bioturbation, e.g. by burrowing echinoids.
In addition, the possibility of 'diagenetic packstone'
or 'condensed grainstone' fabrics must be considered.
Carbonate production:
Many tropical beaches are
composed almost entirely of tests from shallow-water
benthic foraminifera. Benthic foraminifera contribute
to the formation of carbonate grains (e.g. oncoids,
macroids), reefs and shelf carbonates. The proportion
of modern benthic foraminifera is approximately 5%
of the global carbonate reef budget (Langer et al. 1997).
This is low as compared with the high calcium carbon-
Biofabric analysis of foraminiferal accumulations
allows the hydrodynamic and sedimentologic history
of foraminiferal limestones to be reconstructed (Aigner
1983, 1985; Fig. 5.3). Important criteria for the descrip-
tion of biofabrics are packing, sorting and size of fora-
miniferal tests, imbrication or bedding structures, and
the amount of carbonate mud preserved in interparticle
voids. Worn tests can indicate continued transport, as-
