Rapid calcite seafloor cementation in cystoid tests Ordovician, Estonia: A test of the calcite sea m

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Rapid calcite seafloor cementation in cystoid
tests (Ordovician, Estonia) A test of the
calcite sea model?

• Andrew Milligan
• Fall 2007

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Northeastern Estonia Kohtla oil shale mine
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(No Transcript)
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Close-up of oil shale?
Trilobite
Trace fossil
1 cm
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What are calcite seas?

• In calcite seas low-magnesium calcite was
  precipitated on sea floors.

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What are calcite seas?

• In calcite seas low-magnesium calcite was
  precipitated on sea floors.
• An important source of that calcite was
  dissolved aragonite.

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What are calcite seas?

• In calcite seas low-magnesium calcite was
  precipitated on sea floors.
• An important source of that calcite was
  dissolved aragonite.
• These seas were common in the Jurassic and
  Ordovician

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• The Paleozoic calcite sea was happening at a
  time when many major invertebrate groups were
  establishing themselves, and may have had an
  effect on their skeletal mineralogy.

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Support for this theory?

• The cements in Ordovician hardgrounds generally
  retain their low magnesium calcite mineralogy.

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Support for this theory?

• The cements in Ordovician hardgrounds generally
  retain their low magnesium calcite mineralogy.
• Some vertical sequences in the Ordovician contain
  many closely-spaced hardgrounds thus giving the
  impression that sea floor cementation was taking
  place continuously with deposition of sediment
  which immediately started to lithify.

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?
PreC
Cam
Ord
Sil
Dev
Perm
Carb
Ceno
Jur
Tr
Cret
ICEHOUSE
ICEHOUSE
GREENHOUSE
GREENHOUSE
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High-Mg Calcite and, less abundantly, Aragonite
Calcite Mg content generally lower
After Sandberg (1983)
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What is a cystoid?

• Cystoidea is a class of animal and have a body
  composed of calcitic plates.

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What is a cystoid?

• Cystoidea is a class of animal and have a body
  composed of calcitic plates.
• They belong to the Phylum Echinodermata, which
  also includes starfish. They are spherical and
  were connected with a stem and holdfast to the
  seafloor.

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What is a cystoid?

• Cystoidea is a class of animal and have a body
  composed of calcitic plates.
• They belong to the Phylum Echinodermata, which
  also includes starfish. They are spherical and
  were connected with a stem to the seafloor.
• They first occurred in the Ordovician and
  disappeared in the Devonian

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Graptolite
1 cm
Cystoids are very well preserved in oil shale
clearly showing encrusters.
Brachiopod
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Bryozoan directly encrusting the molds outer
surface
Bryozoan base
Calcite cement crystal
0.5 mm
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The bases of the calcite crystals reflect the
shape and size of the original thecal plates.
These crystals thus grew directly from the
undersides of the original thecal plates toward
the center of the theca.
2 mm
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Remnant calcitic thecal plates
1 cm
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• Cross section of Calcite crystals radiating
  inward from each individual thecal plate.

1 cm
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My Hypothesis
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Living Cystoid
Brachioles
Thecae
Stem
Holdfast
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My Hypothesis
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Thecal cross-section
Calcite crystals radiating inward from thecal
plates
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My Hypothesis
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Rapid Disarticulation
Calcitic thecal plates
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1
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4
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Encrusters on internal mold
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Graptolite
1 cm

• Cystoids are very well preserved in oil shale
  clearly showing encrusters.

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Graptolites on Echinosphaerites
aurantium internal mold
5mm
Echinosphaerites aurantium reconstruction by Öpik
(1928)
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Encrusted internal molds are not uncommon during
calcite seas
Nautiloid
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Some samples are filled with crystals, while
some are filled with matrix, the reasoning for
this is not clear.
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The Periproctal plates
2 mm
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Hydropore
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Results
Because no stem fregments were found in oil shale
collected from site, it is possible the stem was
reabsorbed as proposed by Parsley (1998)
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Other remnant thecal plates ?
1 mm
1 mm
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Remaining work

• Identify possible thecal plates
• Determine possible cause of crystals Vs matrix
  interior
• Take more pictures (I hear the words film is
  cheap in my dreams)
• Make thin sections

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Remaining work

• Oh yes and write

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Acknowledgements

• Mark Wilson
• Olev Vinn
• Mare Isakar
• The Henry J. Copeland Fund
• Wengerd Fund