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Fig. 20.10. Bioclastic layer (packstone) rich in fragments of trilobites, brachiopods, bryozoans, ostracods, crinoids, and
black pebbles (small black grains) sharply overlain by a micritic layer (mudstone). The bioclastic layer was not yet lithified
when the overlying sediment was deposited as the two layers have been slightly mixed by bioturbation. Some components
are completely recrystallized due to their originally aragonitic composition. The large intraclast with a similar lithology as
the overlying mudstone layer contains a sharply truncated trilobite fragment indicating lithification of the sediment before
reworking. The dark, pyrite-rich upper surface of this clast indicates that it represents a reworked piece of a hardground (cf.
Fig. 20.11). This sample represents a storm deposit where both fragments of various biogenic components and a hardground
have been reworked. Silurian (Ludlow): Gotland, Sweden.
BRACH - Brachiopod, CLAST - Two types of clasts: Originally aragonitic components and micritic clasts, BIOTURB -
Bioturbation, BRY - on top: Bryozoa ( Ptilodictya) , on bottom bryozoa (?Helopora) , TRIL - Trilobite
Fig. 20.11. Hardground on top of the Laframboise
Member (Late Ordovician: Anticosti Island, Canada).
The irregular surface and the impregnationwith iron
are typical features of hardgrounds. The lower layer
represents a floatstone with various components
surrounded by oncolithic crusts. The centers are
bryozoans, gastropods, and some recrystallized com-
ponents (probably remnants of algae). At the hard-
ground surface components are truncated. The over-
lying sediment shows grading with larger com-
ponents deposited in the depressions of the hard-
ground and finer-grained, quartz-rich material above.
Dark lense-like structures in the overlying sediment
represent filled burrows.
1 Truncated component, 2 - Truncated oncoid, 3-
Bryozoan- Wetheredella oncoid, 4 - Originally ara-
gonitic clast (alga?), 5 - Larger components (crinoids)
in a depression, 6 - Gastropod
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