Geology Reference
In-Depth Information
Plate 70 Cretaceous Larger Foraminifera: Orbitolinids
Orbitolinids (Textulariina, Orbitolinacea) are large agglutinated foraminifera. They comprise important bio-
stratigraphic markers and are used for biozonations, particularly for Barremian, Aptian, Albian and Cenomanian
platforms and ramps. These zonations are based on taxon ranges, assemblage composition and the abundance of
species. Identifying orbitolinids requires oriented thin sections. The tests consist of calcareous or non-calcare-
ous particles cemented with a mineral cement (-> 3). The conical forms have uniserial stacks of low, saucer-
shaped chambers following an early trochospiral stage. The chambers grow rapidly in diameter and are subdi-
vided by septula on the periphery, often exhibiting a thin marginal zone of minute cellules, and a reticulate zone
in which the radial chamberlets are further subdivided by vertical pillars (-> 8, 9). Of diagnostic importance are
the shape of the initial and subsequent embryonic chambers (-> 4, 7, 13; see figures at the bottom of this page).
1
Preservation:
Tests of orbitolinid foraminifera were frequently transported as shown by the various orientations (see ->
15). A common case is downslope transport from subtidal shelf environments into deeper ramp or basinal settings.
Cretaceous (Mishrif Formation, Cenomanian): Subsurface, Ras al Khaimah, United Arabian Emirates.
2
Recognition and composition:
Reworked or worn, finely agglutinated orbitolinid tests can be misinterpreted as micrite
clasts if the microcrystalline calcite cement is more conspicuous than the fine-grained particles building the wall. Middle
Cretaceous (Aptian): Subsurface, Saudi Arabia.
3
Composition of the test:
Agglutinated test composed of angular quartz grains (white) cemented by calcite. Albian: Bavar-
ian Alps, Germany.
4
Diagnostic criteria:
Taxonomic differentiation of orbitolinids requires sections showing the initial and embryonic cham-
bers that changed in shape and size from the Albian to the Cenomanian. The axial section of the macrospheric embryonic
stage of
Orbitolina (Orbitolina) duranddelgai
Schroeder.
Orbitolina (Orbitolina)
occurs from the Late Albian to Late
Cenomanian. The sample is from the Late Albian of the Sierra de Aulet, Huesca Province, Spain.
5
Orbitolinopsis capuensis
(de Castro). Axial section. Genus range: Barremian to Early Aptian. Early Barremian: Djebel
Debar, Constantinois, northeastern Algeria.
6
Palorbitolina lenticularis
(Blumenbach). Axial section exhibiting the initial chamber. Compare this figure with -> 7 and
note the difference in the shape of the initial chamber and the arrangement of the chambers following the initial cham-
bers. The increase in the size of the initial chamber through time can be used for dating (Gusic 1981). Genus and species
range: Barremian to Aptian. Early Aptian: Resita Zone, Apuseni Mountains, Romania.
7 Macrospheric embryonic stage of
Palorbitolina lenticularis
(Blumenbach). Axial section. Early Aptian: Bari, Puglia,
Italy.
8
Palaeodictyoconus arabicus
(Henson). Axial section. Genus range: Valanginian to Bedoulian. Axial section. Barremian-
Early Aptian: Resita Zone, Apuseni Mountains, Romania.
9
Palaeodictyoconus arabicus
(Henson). Transversal section. Late Barremian-Early Aptian: Grand Banks, Eastern Cana-
dian Shelf (DSDP leg 43).
10
Palaeodictyoconus arabicus
(Henson). Oblique section. Aptian: Subsurface, Ras al Khaimah, United Arabian Emirates.
11
Neoiraqia convexa
Danilova. Axial section of the macrosphere. Genus range: Cenomanian. Early Cenomanian: Caserta,
Campania, Italy.
12
Orbitolina (Mesorbitolina) texana
Roemer. Tangential
axial section. Range of
Mesorbitolina:
Late Aptian to
Early Cenomanian. Late Albian: Padurea Craiului,
Apuseni Mountains, Romania.
13
Orbitolina (Mesorbitolina) aperta
Schroeder. Oblique
section of the embryonic macrospheric stage. Lowermost
Cenomanian: Montagne de Tauch, Département Aude,
France.
14
Orbitolina (Orbitolina) duranddelgai
Schroeder. Tangential section of the macrospheric stage exhibiting the embryonic
chamber. Same locality as -> 4.
15
Sandy orbitolinid limestone.
Ramp environment. Foraminifera were reworked and eroded (compare -> 1 and 2). Late
Aptian: Calizas del Busco, Barranco de las Casillas, Requena, Valencia, Spain.
-> 4, 5, 9, 11: Courtesy of R. Schröder (Frankfurt).
Orbitolina (Mesorbitolina) texana
(Roemer). Axial and
transversal basal sections.
Left figure: Differences in the morphology of initial and em-
bryonic chambers of orbitolinid foraminifera. After Arnaud-
Vanneau (1980).
