Geology Reference
In-Depth Information
structions de Tratau et de Nizhni-Irginsk (Permien inférieur
de l'Oural, Russie). - Geobios,
30
, 635-649
Further reading
: K138 (general), K139 (Paleozoic), K140
(Mesozoic), K141 (Cenozoic)
stratigraphic ranges of benthic shallow-water organisms
must be calibrated with global pelagic biozones (based,
e.g., on cephalopods, calpionellids, radiolarians). Iso-
lated skeletal grains, reworked on platforms, transported
to basinal environments and deposited within calcitur-
bidites may aid in this calibration. Care must be taken
in using microfossils enclosed in reworked platform
lithoclasts because this material might represent older,
non-synchronous sediment. Using the cenozone con-
cept must consider the following points:
10.3 Biozonation of Platforms with
Thin-Section Fossils
The section discusses the potential of microfacies in
subdividing platforms sections that are generally poor
in guide fossils into biozones. The example used is from
the Late Mesozoic, but similar problems exist for Pa-
leozoic, Early Mesozoic and Tertiary platforms.
Starting in the 60s of the last century many attempts
have been made in subdividing the thick Jurassic and
Cretaceous carbonate platform sequences in the circum-
Mediterranean and Alpine regions and in stratig-
raphically using thin-section fossils, predominantly cal-
careous algae and benthic foraminifera. Sartoni and
Crescenti (1962) studying Mesozoic platforms in the
southern Apennines introduced the concept of 'ceno-
zones'. The authors defined a cenozone as strata con-
taining a characteristic association of microfossils. Later
authors combined characteristic association with con-
spicuous abundance of specific taxa. Doing this, the
definition of cenozones and the practical use of these
units include both characteristics of assemblage zones
as well as abundance zones. Characteristics of assem-
blage zones and abundance zones are enclosed in the
modern definition of biozones which are determined
by the stratigraphic and geographic distribution of (1)
a single taxon, (2) an diagnostic association, or (3) the
relative abundance of taxa (Salvador 1994).
• Environmental control. Dasyclad algae and benthic
foraminifera are strongly controlled by environmental
conditions. Biostratigraphic interpretations based on
dasyclads generally suffer from an apparently slow
evolution rate, facies-dependence, and poor knowledge
of the stratigraphic range of many taxa.
• Environmental conditions (e.g. substrate, nutrients,
water energy) are different in different parts of carbon-
ate platforms or ramps, and different zonation patterns
may be developed in restricted inner platform settings
(e.g. shelf lagoons), open-marine parts of platforms and
at platform margins (Crescenti 1971; De Castro 1987;
Dya 1992).
• Mesozoic and Cenozoic dasyclads and also benthic
foraminifera exhibit strong provincialism. Biozonations
established in one paleogeographical province may not
be found again in another province.
Future developments.
Many current microfacies
studies of platform carbonates devoid establishing
biozones within a stratigraphic framework and use the
range of single taxa in deliminating stratigraphic units.
However, biozones based on occurrence, association
and abundance of algae and foraminifera provide still
an important tool in microfacies analysis of carbonate
platforms if the definition of formal biozones considers:
• Depositional environment controls. Sections used in
studying changes in the composition of algal and fora-
miniferal assemblages must be differentiated with re-
gard to depositional setting (restricted or open platform,
platform margin).
• Provincialism of the fossil used in establishing bio-
zones demands rigorous statistical tests indicating
which geographical areas can be included within one
paleogeographical site and which not. The Jaccard simi-
larity index has been found very useful in comparing
Late Jurassic dasyclads from various localities in the
Northern Calcareous Alps, and recognizing high or low
similarities (Rasser and Fenninger 2002).
• Only paleogeographical sites having high similarity
indices should be used in correlating biozones within a
chronostratigraphic framework. In the case of the Late
Jurassic platforms of the Northern Calcareous Alps, the
In earlier studies, zones have been set up for Juras-
sic and Early Cretaceous platforms, predominantly
based on dasyclad algae and associated benthic fora-
minifers: Plate 62 and Pl. 69 display dasyclad and fora-
miniferal taxa used in defining Jurassic biozones. Pl. 63,
Pl. 70 and Pl. 71 show taxa characterizing Early Creta-
ceous zones. The zonations developed in the Apennines
were compared with biozones recognized in the North-
ern Calcareous Alps, the Dinarides, eastern and south-
ern Carpathians, Switzerland, Spain, South France and
Greece (see basics and references in K142 and K144).
Jaffrezo (1980) tried to correlate the local biozones and
suggested a general biozonation of Jurassic and Early
Cretaceous platforms in the western Tethys.
Pitfalls.
The cenozone approach was widely used
until about 1990 despite serious pitfalls causing diffi-
culties in correlating and determining the chrono-
stratigraphic age. The age of the biozones and the bio-
