Geology Reference
In-Depth Information
radiolarians consist exclusively of chalcedony or micro-
quartz. The mineralogical composition seems to depend
on the time involved in the long-term transformation
processes, and is influenced by the lithology of the ra-
diolarian-bearing sediments. Burial transformation of
radiolarians in limestones is said to be much faster than
in argillaceous sediments. In cherts radiolarian tests are
commonly better preserved than in carbonate rocks, but
exhibit a wide spectrum in regard to the preservation
of morphological structures. This spectrum can be used
to estimate recrystallization, corrosion and metasomatic
alterations (Kiessling 1996).
The preservation of radiolarian tests in limestones
is generally poor as compared with radiolarians in cherts
or radiolarites, and in concretions. The siliceous radi-
olarian tests are often replaced by calcite, pyrite or other
ore minerals.
Calcitization is common in limestones, but also oc-
curs in radiolarites. It leads to poor preservation, which
is evident both in SEM studies of extracted specimens
and in thin sections. The replacement of the siliceous
skeletons starts as an early diagenetic process but con-
tinues under shallow burial conditions (Faupl and Beran
1983). Pyritization is an early diagenetic process that
takes place in reducing diagenetic environments. It af-
fects siliceous skeletal grains in different intensities,
siliceous sponge spicules more frequently than radi-
olarians, probably because of their higher organic con-
tent, which in turn controls microbial sulphate reduc-
tion and the accumulation of iron sulfides at organics.
Microfacies of radiolarianbearing limestones
Radiolarians occur predominantly in micritic car-
bonates and in marls. They are found in lime mudstones,
wackestones and packstones. Common Jurassic micro-
facies types are (Kiessling 1996):
• Pelagic and hemipelagic marly mudstones with ra-
diolarians and sponge spicules.
Plate 76 Radiolarians and Calpionellids
The plate displays Late Jurassic planktonic organisms represented by radiolarians and calpionellids, a group
of the ciliate Protista.
1 Pelagic radiolaria packstone. Note the differences in size and preservation of radiolarians. Calcitized spumellarian radi-
olarians are recorded by circular sections of different size. Sponge spicules (arrows) exhibit parallel orientation, indicat-
ing the influence of bottom currents. The dark micritic matrix yields abundant coccolithophorids, as proved by SEM
studies. Using the abundance of radiolarians and sponge spicules and the radiolaria/spicule ratio within a standard field of
the thin section as a measure for the depositional setting (Kiessling 1996), sedimentation in the distal parts of a deep basin
influenced by bottom currents appears to be likely. The sample comes from the Oberalm limestone, a representative of
widely distributed pelagic carbonates, better known as
Aptychus
limestone in the Alpine region, and Biancone limestone
in the Mediterranean area. These limestones are commonly associated with radiolarites. The limestone beds are often
separated by marls and contain some cherts and intercalations of allochthonous limestones consisting of material derived
from the nearby carbonate platforms. Radiolarians, sponge spicules, thin shell fragments (filaments) and tiny bioclasts
form wackestone and packstone fabrics. SMF 3-R
AD
. Late Jurassic (Tithonian): Hallein, Salzburg, Austria.
2 Spumellarian radiolaria. The original opaline silica of the spherical test has been replaced by calcite, obscuring structural
details. The pores on the surface of the test were partially infilled by carbonate mud, giving the margin a toothed appear-
ance. This feature differentiates circular sections of radiolarian casts from sections of calcispheres or globular foramin-
ifera. Same locality as -> 1.
3
Acanthocircus
sp., a member of associations which occur in relatively shallow depths. Late Jurassic: Kraiburg, Bavaria,
Germany. The sample is a white limestone, used in Roman mosaics of southern Bavaria, Germany.
4 Radiolaria packstone. Limestone concretion in mudstones. Concretions are protected with regard to aggressive pore
waters and compaction and can, therefore, yield well-preserved microfossils. The densely packed radiolarians are
multicyrtoid nassellarians. The sample comes from an oxygen-deficient basin in a high paleolatitudinal position. Late
Jurassic (Nordenskjöld Formation): Longing Gap, Graham Land, Antarctic Peninsula.
5
Tritrabs ewingi
Pessagno. The radiolarian skeleton is replaced by sparry calcite but has still preserved its morphology.
Same locality as -> 3.
6
Podobursa
sp. The nassellarian skeleton is preserved as microcrystalline calcite. Same locality as -> 3.
7 Calpionellid wackestone. Arrows point to the few longitudinal sections showing the criteria necessary for the taxonomic
determination of calpionellids
(Calpionella
sp.). Well-bedded micritic limestones with abundant remains of extinct cili-
ates are common in Late Jurassic and Early Cretaceous open-marine basinal settings. SMF 3-C
ALP
. Late Jurassic (Titho-
nian): Seebühel, Switzerland.
8 Calpionellid-radiolaria wackestone. C: Transversal and longitudinal sections of calpionellids; R: Recrystallized radiolaria.
Note the difference in size and preservation between radiolarians and calpionellids. Locality as -> 7.
-> 3, 5, 6: E. Flügel and Ch. Flügel 1997, 4: Kiessling 1996
