debris, but since then they have been most common in
relatively deep waters (>100 m). In contrast, other
echinoderms have commonly inhabited shallow
marine environments throughout their geological
time range.
Most echinoderms are benthonic and a few are
pelagic. Echinoids are usually vagrant, whereas
crinoids are generally sessile.
5.5.6.4 Mineralogy
The skeleton is composed of high-Mg calcite
(5–15 mol% MgCO 3 ). Each plate, spine or sclerite
has a single-crystal microstructure behaving optically
as a single calcite crystal when viewed under the
polarising microscope. However, the optically
uniform crystals are really a mosaic of submicroscopic
crystals whose c-axes are aligned almost perfectly
parallel.
5.5.6.5 Geological Range
Echinoderms range from the Lower Cambrian to
Recent, but were not common before the Ordovician.
Their greatest abundance was during the Carboniferous.
Crinoids appeared for the first time in Lower Ordovician sediments, but did not become abundant before the
Silurian. They remained numerous in the Devonian and
Carboniferous, sometimes forming small bioherms or
beds dominated by crinoid debris. Their abundance
declined in the Permian, but revived in the Mesozoic,
although they never equalled their Palaeozoic peak.
Crinoids remain prolific today.
Echinoids appeared for the first time in the Upper
Ordovician, but are most common in Mesozoic and
Cenozoic sediments. They remain a vital part of the
invertebrate marine realm and are probably as abundant now as at any time in the past.
5.5.6.6 Significance for Reservoir Quality
Echinoderms are commonly overgrown by syntaxial
calcite cement which completely obliterates both
intragranular and intergranular porosity in
echinoderm-rich rocks. This, along with the common
neomorphic replacement of the echinoderm skeleton,
gives these rocks low reservoir potential. Dolomitised
crinoidal sediments may occasionally be oil producing,
as for example in the Silurian crinoid-rich skeletal
build-ups in the northeastern Anadarko Basin of
Oklahoma, USA (Morgan 1985).
5.5.7 Post-mortem Destruction
As shown in the preceding review, carbonate grains
differ widely in size with diameters ranging from a
b
1 cm
c
a
Ambulacrum
Tooth
In te ra m b u la c ru m
Fig. 5.28 (a) Adoral surface of the regular Archaeocidaris sp.
with terminology. (b) Part of the surface showing ambulacra and
interambulacra with tubercles. (c) Interambulacra with spines
preserved. (d) Examples of different growth forms of spines
from regular echinoids. (e) Part of Archaeocidaris sp. showing
a few ambulacral plates with spines preserved. (modified from
Rasmussen 1969, Clarkson 1979)
5 Carbonate Sediments
175
relatively deep waters (>100 m). In contrast, other
echinoderms have commonly inhabited shallow
marine environments throughout their geological
time range.
Most echinoderms are benthonic and a few are
pelagic. Echinoids are usually vagrant, whereas
crinoids are generally sessile.
5.5.6.4 Mineralogy
The skeleton is composed of high-Mg calcite
(5–15 mol% MgCO 3 ). Each plate, spine or sclerite
has a single-crystal microstructure behaving optically
as a single calcite crystal when viewed under the
polarising microscope. However, the optically
uniform crystals are really a mosaic of submicroscopic
crystals whose c-axes are aligned almost perfectly
parallel.
5.5.6.5 Geological Range
Echinoderms range from the Lower Cambrian to
Recent, but were not common before the Ordovician.
Their greatest abundance was during the Carboniferous.
Crinoids appeared for the first time in Lower Ordovician sediments, but did not become abundant before the
Silurian. They remained numerous in the Devonian and
Carboniferous, sometimes forming small bioherms or
beds dominated by crinoid debris. Their abundance
declined in the Permian, but revived in the Mesozoic,
although they never equalled their Palaeozoic peak.
Crinoids remain prolific today.
Echinoids appeared for the first time in the Upper
Ordovician, but are most common in Mesozoic and
Cenozoic sediments. They remain a vital part of the
invertebrate marine realm and are probably as abundant now as at any time in the past.
5.5.6.6 Significance for Reservoir Quality
Echinoderms are commonly overgrown by syntaxial
calcite cement which completely obliterates both
intragranular and intergranular porosity in
echinoderm-rich rocks. This, along with the common
neomorphic replacement of the echinoderm skeleton,
gives these rocks low reservoir potential. Dolomitised
crinoidal sediments may occasionally be oil producing,
as for example in the Silurian crinoid-rich skeletal
build-ups in the northeastern Anadarko Basin of
Oklahoma, USA (Morgan 1985).
5.5.7 Post-mortem Destruction
As shown in the preceding review, carbonate grains
differ widely in size with diameters ranging from a
b
1 cm
c
a
Ambulacrum
Tooth
In te ra m b u la c ru m
Fig. 5.28 (a) Adoral surface of the regular Archaeocidaris sp.
with terminology. (b) Part of the surface showing ambulacra and
interambulacra with tubercles. (c) Interambulacra with spines
preserved. (d) Examples of different growth forms of spines
from regular echinoids. (e) Part of Archaeocidaris sp. showing
a few ambulacral plates with spines preserved. (modified from
Rasmussen 1969, Clarkson 1979)
5 Carbonate Sediments
175