If Earth’s geological history were a book, sedimentary rocks would make up most of the
pages.
Much of what we know about the evolution of life on Earth is based on observations of
the fossil record from sedimentary rocks.
For the siliciclastic rocks we discussed in the previous tutorial, living organisms are
analogous to the book’s editor.
They aren’t responsible for the rock’s formation, but modify them in several ways,
such as digging burrows or leaving behind fossils.
However, moving on to biogenic rocks, with these, living organisms assume the role of
author, publisher, and editor.
That is not to say that other depositional processes, such as rounding and sorting, aren’t
in operation, but simply that biological processes assume leading roles.
Most of the grains in biogenic sedimentary rocks are ultimately derived from the skeletal
material of organisms, which may or may not be modified by transport.
This tutorial will focus on carbonate rocks, since they are by far the most abundant type
of biochemical rocks.
You may know carbonate rocks by another name, limestone, which is a general term used for
rocks composed mainly of calcium carbonate.
Calcium carbonate crystallizes as two polymorphs, or minerals with the same chemistry but different
structure, those being calcite and aragonite.
Over much of geologic history, calcite has been the main carbonate mineral to form in
Earth’s oceans, though there have been times where aragonite crystallized instead.
In fact, we are currently living in a time of “aragonite seas”, where aragonite is
the main carbonate mineral that precipitates from seawater.
Whether marine sediments are dominated by aragonite or calcite depends on the magnesium
to calcium ratio of seawater.
This ratio varies throughout time and is mainly controlled by the magnitude of mid-ocean ridge
volcanism, sometimes described as the rate of seafloor spreading.
Hydrothermal reactions that take place near mid-ocean ridges enrich seawater in calcium
and deplete it in magnesium.
For example, plagioclase exchanges its calcium for sodium in seawater, becoming more sodium-rich
in a process called albitization.
The more volcanism at the ridge, the smaller the Mg/Ca ratio will be.
We currently have aragonite seas because the rate of seafloor spreading is relatively slow,
which has been the case for about the last 25 million years.
When seawater is more concentrated in magnesium, it favors the crystallization of aragonite,
which forms metastably; while calcite is actually the more thermodynamically stable phase.
The progressive addition of magnesium to seawater decreases the nucleation rate of calcite,
and when the molar ratio of Mg/Ca surpasses about 5, calcite nucleation is so restricted,
that aragonite forms metastably instead.
This has important implications.
For example, some marine taxa build tissues from aragonite, others calcite, and as Earth’s
oceans fluctuate between calcite and aragonite seas, it exerts a selective pressure on organisms.
Times of aragonite seas favor organisms that build tissues from aragonite, calcite seas
favor organisms that build tissues from calcite.
Additionally, aragonite is unstable both at surficial and burial conditions, and readily
recrystallizes to calcite during diagenesis.
So, sediment that was initially deposited as aragonite almost always displays replacement
textures.
The carbonate minerals are relatively soluble and are prone to recrystallization during
burial, which sometimes destroys valuable depositional textures that geologists use
to interpret a rock’s history.
Though sometimes original textures can be preserved as “ghosts”, where the outlines
of original grains are visible.
In addition to calcite and aragonite, another common carbonate mineral is dolomite, which
has the chemical formula CaCO3∙MgCO3.
It is essentially calcite that had half of its calcium replaced with magnesium.
Dolomite is prevalent in the geologic record but has never been observed to precipitate
in the modern oceans, even though seawater is saturated with respect to dolomite.
This is known as the “Dolomite Problem”.
Therefore, geologists believe that most dolomite was probably formed by the reaction of calcite
with magnesium-rich basinal fluids during diagenesis.
Calcite, aragonite, and dolomite make up about 90% of the minerals in biogenic rocks, with
the majority being calcite.
Because of this, biogenic rocks are mainly classified based on their texture, which we
will cover next.
Most carbonate sediment is ultimately derived from the skeletal material of organisms.
Any organism that builds body parts from calcium carbonate contributes to the pool of biogenic
sediment.
Consequently, carbonate grains come in all shapes and sizes, from the microscopic exoskeletons
of coccolithophores, to the gravel-sized shells of bivalves.
Additionally, skeletal materials are often abraded to some degree during transport, where
they are broken-down and rounded, just like any other sediment.
Furthermore, carbonate sediment can be reworked by benthic, or bottom-dwelling organisms,
creating a wide assortment of grain shapes and textures.
Let’s now discuss the different types of carbonate grains and how they’re formed.
The first and most abundant type are skeletal fragments.
In this context, skeletal fragments refer to identifiable pieces of organisms, including
entire fossils and broken fragments of them.
Fossils representing all the major phyla of calcareous marine invertebrates are present
in carbonate rocks.
The types of skeletal fragments found in a limestone is dependent on both the age and
the environment in which it was deposited.
Obviously, you wouldn’t expect to find trilobites in a limestone that formed either before they
evolved or after they went extinct.
Since the carbonates are the most fossiliferous of the rocks, geologists often use limestones
to interpret the depositional environment in which a rock was formed.
For example, a limestone containing coral fossils would be interpreted to have formed
in a shallow marine environment, since they rely on a symbiotic relationship with photosynthetic
algae to survive.
Next, there is microcrystalline calcite, also called micrite or carbonate mud.
It is texturally analogous to siliciclastic mud, making up the matrix fraction of the
rock, and encompassing all carbonate grains smaller than silt.
Most carbonate mud in the geologic record is probably biogenic, mainly being composed
of broken-down calcareous algae such as Halimeda, and protist exoskeletons such as Coccolithophores.
However, carbonate mud has been observed to inorganically precipitate in modern marine
environments in the form of aragonite needles.
Geologists are uncertain as to exactly how much of the carbonate mud in the rock record
was inorganically derived.
Carbonate mud can take on different morphologies, making the basis for several other types of
carbonate grains.
For example, ooids form when carbonate mud precipitates, often concentrically, around
a nucleus of some sort, such as a skeletal fragment or grain of quartz sand.
Ooids are observed to form in wave-agitated shallow waters that are enriched in dissolved
calcium carbonate.
When ooids are larger than two millimeters, they are called pisoids.
Peloid is a term used to describe aggregates of carbonate mud, typically forming silt to
sand sized particles.
The most common type of peloids are fecal pellets, which are produced by organisms that
ingest carbonate mud on the seafloor.
They digest the organic matter in the mud and excrete the inorganic materials as fecal
pellets.
They tend to be oval-shaped and uniform in size.
Next, there are aggregate grains, which are like peloids, but are instead composed of
larger grains held together by carbonate mud.
That covers the main types of primary biogenic carbonate.
There are two more types of carbonate grains found in limestone.
The first are lithoclasts, which are similar to lithic fragments, which we covered in the
tutorial on siliciclastic rocks.
Lithoclasts are composed of lithified clasts of carbonate rock, that can either be derived
from outside the depositional basin, called extraclasts, or from inside the basin, called
intraclasts.
The final type of carbonate grain isn’t sediment at all, but rather describes a crystalline
texture, known as sparry calcite.
Sparry calcite often forms as cement within the pore space of carbonate sediment, but
can also form from the recrystallization of primary carbonate grains during diagenesis.
Now that we have an understanding of what makes up carbonate rocks, we can start classifying
them.
The classification of carbonate rocks can be a bit confusing.
Not only does it use a completely different scheme than siliciclastic rocks, but there
are two completely different systems used, both being based on the rock’s texture.
We will focus on the classification system of Folk from 1962.
It is based on the relative abundances of carbonate grains, carbonate mud, and sparry
calcite cement.
For example, if a limestone contains more carbonate mud than sparry calcite cement,
it is classified as micrite, if instead it has more sparry calcite, it is classified
as sparite.
Next, you determine the dominant type of carbonate grain present, if any.
A rock composed entirely of carbonate mud is simply called micrite.
If the carbonate grains are dominated by ooids, add the prefix -oo to the first term.
So a rock composed of ooids with abundant sparry calcite cement would be classified
as an oosparite.
Other modifiers based on the type of carbonate grain are -bio, for skeletal fragments, -pel,
for peloids, and -intra, for intraclasts.
So, a biosparite refers to a rock dominated by skeletal fragments and sparry calcite cement,
a pelmicrite refers to a rock dominated by peloids and carbonate mud, and an intrasparite
refers to a rock dominated by intraclasts and sparry calcite cement.
These terms are all used to describe carbonate rocks whose sediment was transported prior
to deposition.
These transported grains are called “allochems”.
Sometimes carbonate rocks are deposited in place, as is the case for coral reefs.
They are classified as biolithites.
It is not uncommon for sedimentary rocks to consist of a mix of carbonate and siliciclastic
grains.
In this scenario, the modifiers “calcareous” or “siliceous” are added to its classification.
For example, a sandstone containing subordinate carbonate grains would be classified as a
calcareous sandstone; a limestone with subordinate grains of quartz sand would be called a siliceous
limestone.
Furthermore, additional modifiers can be added to further describe the rock’s texture,
such as the ratio of matrix to carbonate grains.
A carbonate rock that is grain-supported is said to be “packed”.
If it has between 10 and 50 percent matrix, it is said to be sparse.
Other terms such as ‘sorted’ and “rounded” can also be applied.
Carbonate rocks are sometimes given informal names, such as chalk or coquina.
Chalk is a type of fine-grained carbonate rock composed almost entirely of nanofossils,
such as foraminifera.
Coquina refers to an uncompacted carbonate rock composed mainly of poorly sorted skeletal
fragments.
Like the siliciclastic rocks, inferences can be made about the depositional environment
based on textural properties, such as grain size and the degree of rounding and sorting.
Rocks consisting of carbonate mud are analogous to siliciclastic mudstones, requiring calm
waters to be deposited.
A limestone consisting of rounded and sorted skeletal fragments would be interpreted to
have formed in a high energy environment, such as a wave agitated shoreline.
Furthermore, the rounded and sorted nature of the grains indicates that the sediment
has been transported a moderate distance.
Before moving on, there is one more type of biogenic rock to discuss, which is called
chert.
Chert is a general term used to describe ultra fine-grained siliceous sedimentary rocks that
are mainly composed of microcrystalline quartz.
In modern environments, most chert is formed on the seafloor, though it also occurs in
some lake deposits, especially those that are hydrothermally active.
The formation of chert begins with the deposition of a type of sediment that geologists call
siliceous ooze.
Oozes are gel-like sediments that contain a sizeable fraction of the microscopic exoskeletons
of planktonic protists.
Being siliceous oozes, these sediments are composed of organisms that build skeletal
materials from silica, rather than calcium carbonate as in the White Cliffs of Dover.
For the most part, siliceous skeletal materials are produced by two main groups of organisms,
diatoms, and radiolarians.
Diatoms comprise a large portion of Earth’s biomass and are responsible for 20-50% of
global annual oxygen production.
Additionally, some types of sponges build structural components, or spicules from silica,
also contributing to the pool of siliceous ooze.
It is important to note that the silica these organisms build is not crystalline, it is
a hydrated, amorphous form of silica called opal-A. Most probably know of opal as a type
of gemstone.
Geologists currently think that most, if not all siliceous ooze, is biogenic.
This is because on average, modern oceans are grossly undersaturated with respect to
both quartz and amorphous silica.
Thus, biologic assistance is the only viable way to precipitate large amounts of silica
in Earth’s oceans.
If you think about it, it is impressive that organisms can build skeletal tissues using
a material that should dissolve in seawater.
They escape dissolution by using various active protective systems, which obviously shut off
after death.
So, why don’t the skeletal fragments dissolve when the organism dies?
Well, they often do, but not always.
Siliceous oozes are most abundant in today’s oceans in areas where there is upwelling.
Upwelling transports deep, nutrient-rich waters toward the ocean surface, increasing biological
productivity and inflating radiolarian and diatom populations.
If there are enough organisms dying and sinking to the bottom, then the accumulation rate
of siliceous skeletal material can overcome the rate of dissolution, leading to the formation
of a siliceous ooze deposit.
Siliceous oozes are later converted to chert during diagenesis, owing to the increased
pressures and temperatures associated with burial, which drives the crystallization of
opal-A into quartz.
Chert has a cryptocrystalline texture, meaning that it is so fine-grained, that its crystalline
nature is only vaguely revealed in thin section, or a microscopic slide made of rock.
Chert sometimes occurs as nodules in other sedimentary rocks, especially limestone.
Nodular chert is thought to form via diagenetic replacement of the original rock.
This would occur in areas of the basin that develop silica-rich pore waters, supplied
by the dissolution of siliceous oozes at the seafloor, which can later seep into the sediment
pile.
Chert is classified based on the whether fossils are present, the types of fossils present,
and whether it is bedded or nodular.
Jasper is a type of red chert, which gets its color from the mineral hematite.
Flint is a name given to chert that forms as nodules in limestone and can be a variety
of colors.
Chert has historically been used for stone tools, especially those that require a sharp
edge.
Its ultra-fine-grained nature and high hardness allow it to be formed into a sharp, durable
blade.
And with chert covered, that wraps up biogenic sedimentary rocks.
Let’s move on to the final category of sedimentary rocks, chemogenic rocks.