Geology Reference
In-Depth Information
Preservation
• Test the preservation of the fossils.
• Consider the possibility of recrystallization, diage-
netic solution or replacement of the original microstruc-
ture.
• Are the fossils complete or fragmented?
• Consider the degree of fragmentation and the re-
sulting sizes.
• Are the fossils in place or transported?
• In place fossils may need several sections in order
to recognize the variability of internal structures.
• In-place fossils, particularly colonial reef organisms,
are often large and their study requires oversized thin
sections or peels (up to 10 and 20 cm in length).
• In-place fossils often have characteristic growth
forms which will influence the figures seen in thin sec-
tions.
criteria in reconstructing the shape and internal struc-
ture of the fossils.
Skeletal mineralogy and microstructure
• Look at the mineralogical composition of the fos-
sils (calcareous, siliceous, phosphatic, organic).
• Decide whether the original mineralogy of the skel-
etal grain was aragonitic or calcitic. Preservation of the
bioclast as a mold or cast indicates a primary arago-
nitic mineralogy that has been subsequently altered.
Well-preserved microstructures point to a primary cal-
citic mineralogy.
• For shells try to discriminate the original microstruc-
tures and structures produced by diagenesis.
• Describe the microstructure by looking at the shape,
size and orientation of the component crystals.
Advancing systematic determinations
• Bear in mind the age of the rock examined, and the
likely biota will be encountered.
• Browse among the thin-section microphotographs
in the microfacies monographs (see Sect. 10.3).
• If you think you have found a reliable assignment
to a specific group or on taxa, look for further informa-
tion in the relevant parts of Sect. 10.2.
Quality of the thin sections
• Compare the thin sections with the rock sample, and
if necessary, make additional and better oriented thin
sections.
• Differentiate between random sections and oriented
sections.
• Some fossil groups, particularly sessile benthos (e.g.
corals, coralline and stromatoporoid sponges) need at
least two sections (parallel and perpendicular to the
growth direction).
• Indicate the orientation of the section using defined
terms (e.g. vertical, tangential, oblique).
• Look especially for sections that contain defined
morphological criteria necessary for taxonomic deter-
minations (e.g. axial structures in larger foraminifera).
• Decide which of the taxonomically important crite-
ria are shown by the sections and try to combine these
10.2 Diagnostic Criteria of Fossils in
Thin Sections
Thin-section fossils comprise six informal groups:
• cyanobacteria (calcimicrobes) and calcareous algae,
• foraminifera and other protozoa (radiolarians, calpio-
nellids),
Box 10.1.
Fossil groups common in thin sections of marine limestones
formed in different settings. Ranking indicates
the approximate frequency. Note that this list is highly subjective and does not consider specific associations, e.g. in reef
limestones and slope sediments, as well as microfossils of uncertain systematic position present predominantly in inner
and outer platform environments and reefs.
Late Tertiary
Inner shallow platform:
Smaller benthic foraminifera, cor-
allinacean red algae, mollusks, bryozoans, barnacles, ser-
pulids, ostracods
Edge:
Reefs with corallinacean red algae, corals, bryozo-
ans
Slope:
Benthic and planktonic foraminifera
Basin:
Planktonic foraminifera
Early Tertiary
Inner shallow platform:
Smaller and larger benthic fora-
minifera, calcareous dasyclad algae and udoteacean green
algae, corallinacean and squamariacean red algae
Edge:
Larger foraminifera
Slope:
Planktonic and benthic foraminifera
Basin:
Planktonic foraminifera
Upper Late Cretaceous
Inner shallow platform:
Benthic foraminifera, mollusks,
bryozoans, echinoderms
Edge:
Rudists
Slope:
Planktonic and benthic foraminifera, 'calcispheres',
siliceous sponges
Basin:
Planktonic foraminifera, 'calcispheres'
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