Geology Reference
In-Depth Information
production organ (oogonium) is elliptical in shape, 0.2-
1.0 mm in size, and exhibits a wall composed of
variously arranged tubular cells. The spiral or vertical
arrangement of the enveloping cells is recorded in the
'gyrogonites' (Fig. 10.20) corresponding to the calci-
fied parts of the oogonium and representing the essen-
tial criteria for the classification of fossil charophytes.
Thin sections:
Taxonomic studies are predominantly
based on isolated gyrogonites. Differentiating genera
in thin sections is difficult and would need longitudi-
nal sections displaying the apical and the basal part of
the gyrogonites. The calcified spiral cells of the gyro-
gonites exhibit distinct microstructures (Schudack 1987,
1993). The calcified vegetative internodes of the charo-
phycean stems correspond to cylindrical segments
making up a hollow central channel. Cross sections
exhibit an axial cavity surrounded by a peripheral zone
of small, densely arranged circular structures corre-
sponding to sections of external longitudinal grooves.
Environment:
Today the group is restricted to fresh
and brackish waters (lakes, rivers and brackish mar-
ginal seas). Some species also occur in salt lakes.
Charophytes are found in shallow lakes, predominantly
in water depths of a few meters only, often exhibiting
distinct shore-parallel distributional patterns. Modern
charophytes are abundant in warm temperate climates
but have a wide geographical range, and also occur in
tropical and arctic settings. Major controlling factors
on the distribution of charophyta are salinity, the oxy-
gen content and chemistry of the waters as well as sub-
strate type, salinity, nutrient supply and substrate type.
Because oogonia can be transported by water and wind,
the occurrence of abundant fossil oogonia, therefore,
must not necessarily reflect biocoenoses. Many ancient
charophytes are found in association with nonmarine
or brackish water biota. Oogonia found in marine sedi-
ments are usually interpreted as allochthonous. How-
ever, Devonian forms (e.g.
Sycidium
, Pl. 65/8) are
known from shallow-marine open shelf environments.
Distribution:
Modern charophytes represent a relict
group comprising only six genera. Charophytes have
been known since the Silurian. Important radiations
took place during the Devonian, Late Jurassic and Early
Cretaceous ('Purbeck' and 'Wealden' facies of West-
ern Europe: Schudack 1993). Gyrogonites are useful
in correlations between marine and nonmarine sedi-
ments; they are important zone fossils for the Early Cre-
taceous and Early Tertiary time interval.
Paleoenvironmental significance
: Charophyta are
used in evaluating paleoclimate, paleolatitudes and
paleoenvironmental constraints. Fossil gyrogonites are
important paleosalinity indicators. They are often as-
sociated with nonmarine and brackish-water faunas (e.g.
Plate 66 Paleozoic and Mesozoic Calcispheres
Calcispheres is a term referring to spherical or egg-shaped microfossils which have been regarded for a long
time as microproblematica with respect to their systematic position. Most authors now agree that most of these
fossils are algal cysts, some perhaps also planktonic algae. Devonian and Carboniferous calcispheres are abun-
dant in restricted and semi-restricted lagoonal environments and in back-reef settings. Most of the Jurassic and
Cretaceous calcispheres occurring in pelagic limestones, and often described in thin sections as pithonellids or
cadosinids represent calcareous dinoflagellate algae.
1
Paleozoic calcispheres. Peloidal calcisphere-foraminiferal grainstone. The sediment consists of calcispheres, peloids,
small intraclasts and some endothyrid foraminifera. The calcispheres exhibit different shapes and different wall struc-
tures. These differences are used for taxonomic differentiations. This microfacies is common in Early Carboniferous
inner ramp settings. SMF 16-N
ON
-
LAMINATED
. Early Carboniferous (Viséan): Basin of Dinant, Belgium.
2
Paleozoic calcispheres. Variously sized thin-walled calcispheres. Note the distinct difference in the size of calcispheres
and peloids. Early Carboniferous (Late Viséan): Nassfeld, Carnic Alps, Austria.
3
Paleozoic calcispheres.
Radiosphera
characterized by a radially structured wall. Back-reef facies. Late Devonian (Fras-
nian): Holy Cross Mountains, Poland.
4
Mesozoic calcispheres.
Pithonella
, a calcareous dinoflagellate. The SEM photograph shows the regular pattern of the
calcite crystals forming the shell surface and the circular exterior pore. Late Cretaceous (Turonian): Switzerland.
5
Mesozoic calcispheres. Pelagic
Pithonella
packstone. Open-marine deep shelf environment. SMF 3. Late Cretaceous
(Seewen limestone, Turonian): Switzerland.
6
Mesozoic calcispheres. Association of calcispheres and pelagic foraminifera (
Globotruncana
). SMF 3. Same locality
as -> 5.
7
Mesozoic calcispheres. Strongly burrowed calcisphere packstone. Late Cretaceous (Pläner limestone, Cenomanian):
Teutoburger Wald, Westphalia, Germany.
