Geology Reference
In-Depth Information
10 Fossils in Thin Section: It is Not That Difficult
Many students of carbonate rocks are bewildered and
sometimes frustrated by the morphological and micro-
structural diversity of skeletal grains and are satisfied
by distinguishing major fossil groups. The present chap-
ter will hopefully demonstrate that more detailed iden-
tifications of thin-section fossils are not so difficult and
that identifications at lower systematic levels can pro-
vide fresh insights into the environmental controls on
depositional processes. However, keep in mind, that
thin-section fossils present only a part of the biota
present in the rock and that thin-section diversity is not
equivalent to biotic diversity (see Sect. 6.2.1.4).
The chapter consists of four parts. The first part dis-
cusses specific problems related to the study of fossils
in thin sections. The second part documents the princi-
pal criteria used to distinguish fossil groups commonly
or abundantly occurring in thin sections of limestones,
their distribution in time and their significance with
regard to carbonate production, the recognition of pa-
leoenvironments and depositional settings. The fourth
part deals with the potential of thin-sections in recog-
nizing assemblage zones
Because a good picture may be worth more than a
thousand words, the last chapter includes a list of mono-
graphs and papers showing useful thin-section micro-
photographs.
Many fossils are restricted to specific time inter-
vals. Knowledge of distinct time ranges is necessary
for estimating their value as guide fossils or in recog-
nizing time-dependent facies types. Age and time ranges
of the fossils discussed in this chapter and throughout
the topic are noted within the frame of the chronostratig-
raphic time scale shown in Fig. 9.17, see left page!
Determining fossils in thin sections used in micro-
facies analysis may be difficult, because:
• randomly cut sections of microfossils may exhibit
very different shapes and outlines, resulting in an often
confusing taxonomic situation of 'thin-section fossils'
(Racki and Sobon-Podgorska 1992),
• diagenetic alterations may produce 'diagenetic taxa'
as, e.g. exemplified by calcimicrobes (e.g.
Renalcis
:
Pratt 1984), or some foraminifera whose tubular struc-
tures are difficult to distinguish from micritized shells
or agglutinated worm burrows,
• the lack of modern counterparts of many extinct fos-
sils may cause very different opinions varying from
author to author regarding the systematic position and
classification of fossils known only from thin sections.
Despite their uncertain position, these 'microproblem-
atica' are often excellent facies indicators and can be
used as guide fossils,
• very small-sized skeletal grains may present prob-
lems in attributing the grains to specific systematic
groups as demonstrated by the investigation of silt-sized
fragmented material from known organisms (Feray et
al. 1962; Matthews 1966; Force 1969). In general, skel-
etal identity is lost when the particle size lies below
62
m. Additional SEM studies significantly increase
systematic assignment, particularly of mollusc shells.
Some fossils (e.g. larger foraminifera) common in
thin sections require oriented sections to follow defined
planes. These sections may be rare in randomly cut
microfacies samples, contrary to the opinion of many
geologists who believe that thin sections yielding abun-
dant microfossils
must
be useful in generic or specific
determinations.
10.1 Specifics of Thin-Section
Fossils
The problem is demonstrated by an Early Permian
fusulinid limestone from Oman (Fig. 10.1). The sample
was studied in six over-sized thin sections cut in vari-
ous directions. These thin sections exhibit more than
2000 sections through the spindle-shaped foraminifers.
Only a very small number of the sections can be used
in systematic assignments as shown by a thin section
with 457 sections of fusulinids. 25.5% of the fossils
represent broken shells and 10.5% of the samples are
10.1.1 How to Determine Fossils in
Thin Sections?
Several well-illustrated topics assist in a first iden-
tification of skeletal grains: Horowitz and Potter (1971),
Majewske (1974), Lucas et al. (1976), Scholle (1978),
and Adams and MacKenzie (1998).
