Fossils in Thin Section: It is Not That Difficult - Microfacies of Carbonate Rocks - page 455

Wall and microstructure Appearance Criteria Group Time range

Organic, proteinaceous Poor preservation potential Tubular, singlechambered Allogromiina Cambrian to

material round or flaskshaped forms Holocene

Various foreign grains Dark in transmitted light Simple agglutinated (arenaceous) Textulariina Early

glued together by organic, walls. Advanced Mesozoic forms Cambrian to

calcareous, ferrugineous with outer layer covering complex Holocene

or siliceous cement inner layer forming chambers,

alveolar and labyrinthic structures

Microgranular calcite, Dark in transmitted, Advanced forms show Fusulinina (include Early Silurian

randomly or normally opaque in reflected light differentiation of the wall into Carboniferous and to Late

oriented crystals of two or more layers Permian larger foram Permian

unequal size inifera, Fusulinacea)

Calcite crystals oriented Dark or ambercolored in Porcelaneous. Imperforate wall Miliolina Carboniferous

randomly or in a bricklike transmitted light, milky white with inner and outer surface layers to Holocene

pattern, LowMg and and brilliant in reflected

HighMg calcite light. Under crossed nicols

calcite crystals appear as
tiny multicolored specks

Crystals arranged in prisms Lightcolored, clear or gray in Hyaline, finely perforate wall. Rotaliina, including Triassic to

with caxes normal to test transmitted light, glossy and Nonlamellar, mono or multi benthic Rotaliacea (many Holocene

surface radial hyaline or dark in reflected light lamellar multilamellar and Cenozoic

randomly granular hyaline. larger foraminifera) and

Low or HighMg calcite. planktonic Globeriginacea

Few groups with aragonite (predominantly

Cretaceous and Cenozoic)

Fig. 10.22. Microstructure, wall composition and major systematic categories of foraminifera. Note that the suprageneric
high-level classification of the foraminifera is much more complicated than shown in this simple graph.

of the benthic Rotaliacea with multichambered spiral
shells ( Orbitoides , Pl. 72/7-10; Nummulites , Pl. 75/2)
and the planktonic Globigerinacea with bubble-shaped
chambers arranged in expanding spirals (see Globigerin-
oides , Pl. 73/9).

distinguished. Cold-water foraminiferal associations in-
habiting the shallow waters of high latitudes or the
cooler waters of the deep ocean are fairly uniform.
Because of the lower solubility of calcium carbonate
in warmer than in cool waters, accumulations of fora-
minifera and foraminiferal sediments are more frequent
in carbonate platforms of low latitudes.
The increasing solubility of carbonate with depth
results in a distinct and final drop of calcareous fora-
minifera at the lysocline and the CCD (see Box 2.6)
and results in a dominance of agglutinated foramin-
ifera in deep-water environments.
Oxygen and nutrients: Most benthic foraminifera
occur in areas of normal oxygen concentration, although
deep-water forms may tolerate very low oxygen lev-
els. The composition of slope and outer shelf foramin-
iferal faunas is considerably influenced by upwelling
oxygen-poor and nutrient-rich waters. Specific faunas
occur in the oxygen-minimum zone of the continental
slope.
Substrate: Various types of sea bottom contain dis-
tinctive associations differing in shell morphology and
taxonomic composition. Microenvironments around the
sediment-water interface provide distinct substrate-con-
trolled microhabitats for benthic foraminifera living free
or attached to sedimentary grains, algae and sea grass,
hard substrates or as infauna at depths between 0.5 to
about 10 cm (Langer 1988; Yamano et al. 2000).
Water depth: Many foraminifera are limited to the
photic zone because of their endosymbiotic associa-

Environmental constraints of foraminifera
Foraminiferal environments include shelves, plat-
forms, ramps, reefs, slopes and basins. Most modern
foraminifera are marine and benthic and live from the
intertidal zone down to the deep sea. Some benthic fora-
minifera inhabit fresh and brackish waters. Planktonic
foraminifera live in the upper part of the open oceans.

(A) Benthic foraminifera are restricted to a narrow range
of depths and substrates, and have characteristic toler-
ances to salinity and temperature:
Salinity: Most foraminifera are found in water with
a normal marine salinity of 35‰. Diversity decreases
with decreasing salinity. Low salinities (e.g. in coastal
lagoons) favor low-diversity assemblages, rich in ag-
glutinated forms. High carbonate concentrations in hy-
persaline marine waters favor the occurrence of por-
celaneous Miliolina. Triangular plots of the relative pro-
portions of Textulariina, Miliolina and Rotaliina have
proved to be valuable indicators of paleosalinity and
paleotemperatures and can also be derived from thin-
section analysis of foraminiferal limestones.
Temperature: Benthic foraminifera enable warm-
water, temperate and cool-water environments to be