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Carbonate Rocks
• Carbonate rocks - limestones composed of
calcite (CaCO3) or dolostones composed
of dolomite (CaMgCO3).
• Form through biological and biochemical
processes and through inorganic
precipitation from seawater
• Carbonate rocks widespread and in every
geological period beginning with Cambrian
Environment
• Most carbonate sediments form in
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•
•
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warm waters
E.g., corals and algae common in
carbonate rocks, and mostly found
tropical to subtropical latitudes or
from about 30o N and S latitudes
Most carbonate sediments
generated in photic zone
Unlike sandstones, most
carbonate grains formed in
environment in which they are
deposited
Detrital grains in sandstones
usually derived from outside the
environment in which they were
deposited
Environment Continued
• Carbonate sediments
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do not usually occur
below depths of about
3500 -4000 meters in
the oceans
Below these depths
carbonate sediments
will dissolve
The depths at which
carbonate sediments
dissolve in the ocean
is called the carbonate
compensation depth
or CCD
Classification Scheme
• Two principal classification schemes used for limestones.
– Folk's classification scheme, named after Robert Folk,
• Dunham's classification scheme, named after Robert Dunham
• Folk's scheme based on presence or absence of
allochems (carbonate particles or grains), matrix
(limestone clay called micrite), and type of cement (spar
is a cement composed of coarse xls)
• Dunham's scheme based on texture of rock. i.e. whether
rock contains matrix, whether carbonate grains float in
matrix or whether they are in contact with each other,
and whether rock has carbonate grains
Dunham’s Classification
• Based on whether have
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more or less that 10%
grains
Whether rock is mud or
grain supported.
Also if original
components were bound
together like coral.
Tells about transport
history.
Folk Classification
• Carbonate rocks consits
of:
– Allochems—grains
– Intersitial material—micrite
or spar cement
• Micrite is "lime mud", the
•
dense, dull-looking
sediment made of clay
sized crystals of CaCO3.
Spar—clear to translucent
carbonate cement.
Micrite and Spar
Folk Continued
•
Name is built up by stringing together all the
allochem names in order from least to most
abundant, and then adding the interstitial material
name ("matrix" below for short). For example, a
rock like this:
Oolites + Fossils + Spar matrix = Oo bio
sparite
The name is written as one word, Oobiosparite.
Another example (again allochems from least to
most abundant):
Pellets + Oolites + Fossils + Micrite matrix =
pel oo bio micrite
The name is written as one word,
Peloobiomicrite.
But what if there is both micrite and spar
matrix? The system is the same; just list them from
least to most abundant.
Fossils + Spar matrix + Micrite matrix = bio
spar micrite
Carbonate Grains
• ooids--Ooids are
spherical grains formed
by calcite precipitation
around a nucleus such as
a shell fragment, a quartz
grain, etc. They are less
than 2mm in diameter
and typically form in
shallow, warm, agitated,
and carbonate-saturated
waters such as those
near the Bahamas.
• pisolites--Pisolites
•
are ooids greater than
2mm in size
Similar environments
as ooids
• stromatolites--Stromatolites are
laminated carbonate sediments
composed of mats of blue-green algae
and layers of sediment. The algae,
which forms the mats, is a plant and
plants require sunlight to survive.
Therefore, stromatolites generally form
in warm shallow waters. The algae is
"sticky" and it grows filaments. This
sticky and filamentous algae traps
sediment brought in by the currents.
Consequently, a layer of algae forms
then a layer of sediment is deposited
on top of the algae. Subsequently, the
algae grows through the sediment to
form another mat and the cycle begins
again. Ultimately, a layered rock
composed of alternating algal mats and
sediments is produced. Stromatolites
form in quiet, hypersaline waters with
little animal life around to destroy the
mats.
• oncolites--
Oncolites are
stromatolites
rolled in a ball.
• corals--Corals have a
symbiotic relationship with
dinoflagellate algae called
zooanthellae. Algae are
plants. Therefore, in order
for corals to survive they
generally have to be within
the photic zone (the zone
of maximum light
penetration in the ocean).
Plants require sunlight for
photosynthesis. In the
process of photosynthesis,
plants produce oxygen and
consume CO2. Corals are
major reef formers today
and in the past. However,
keep in mind that some
corals do occur in deep
waters.
• Intraclasts--Semiconsolidated
carbonate material
ripped-up and
incorporated in the
rock.
• dolostone--A
dolostone is a rock
compose of 90100% dolomite.
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