Now that we better understand the nature of sedimentary rocks, we are ready to
start classifying them. Sedimentary rocks are grouped into three primary categories:
Terrigenous or siliciclastic, biogenic, and chemogenic. Terrigenous, which literally
means “originating from Earth” from the Latin terra, refers to materials derived from the
erosion of preexisting rocks. Siliciclastic refers to materials composed of clasts,
or fragments of silicate minerals. Terrigenous and siliciclastic are used synonymously since most
terrigenous sediments are derived from the erosion of silicate rocks. Biogenic, or biochemical rocks,
are derived from living organisms, both directly and indirectly. Chemogenic,
or chemical sedimentary rocks are derived from the precipitation of minerals from natural waters,
usually involving evaporation in arid climates. Let’s start with the siliciclastic sedimentary
rocks, since they are the most abundant, accounting for roughly three-fourths of
all sedimentary rocks. In our study of igneous rocks, we learned that one of the first steps in
analyzing a hand sample is to determine the grain size, and the same holds true for sedimentary
rocks. In fact, the main way that geologists classify siliciclastic rocks is based on grain
size. Recall from earlier that the grain size of sediments is broken down into gravel, sand, silt,
and clay. So, a sandstone is a stone composed of mainly sand-sized particles, while a claystone is
a stone composed mainly of clay-sized particles. What about a rock that consists of a mix of sand,
silt, and clay? Geologists often use ternary diagrams to systematically classify
rocks. Ternary diagrams represent the relative proportions of three components, in this case,
different grain sizes. For example, this ternary diagram has the components sand,
silt, and clay, with each corner of the triangle representing 100% of one component. For example,
if you were to determine that a sedimentary rock contains 60% sand, 20% silt, and 20% clay,
it would be classified as a muddy sandstone. It is important to choose the appropriate ternary
diagram when classifying a rock. For example, you would not use a ternary diagram with gravel as
a component for the aforementioned rock, since it does not contain gravel. Furthermore, it is
important to recognize that the classification of a rock as a sandstone, siltstone, or otherwise,
says nothing about its mineralogy, though it is implied to be siliciclastic, mainly
because the biochemical and chemical rocks follow a different classification scheme.
Before we explore the main types of siliciclastic rocks, we must cover some terminology so we
can better describe them. Let’s use a typical sandstone as an example. Sandstones consist mainly
of sand-sized grains of silicate minerals, often quartz, which make up the framework portion of the
rock. Imagine you are building a stone wall, using only the randomly shaped rocks found in a stream.
How would they fit together? Obviously, there will be a good amount of empty space between the rocks,
owing to the fact that random shapes do not perfectly fit together. The same applies to
the sand in a sandstone, there is an amount of empty space between the sand grains due to the
way they fit together. This is known as a rock’s pore space, and it is often filled with materials
to an extent. In some cases, it’s filled with fine-grained muds, which is referred to as the
matrix portion of a rock. The matrix is commonly composed of clay minerals, such as illite and
kaolinite. Other times, pores get filled in with minerals that precipitate out of groundwater,
called cement. The cement helps glue the rock together, making it harder and more cohesive.
The most common cement-forming minerals are quartz and calcite, with lesser occurrences of
the iron oxide minerals and gypsum or anhydrite. Cementation along with compaction are together
known as lithification, which is the process of converting sediments into sedimentary rock.
We now have sufficient knowledge to start looking at siliciclastic rocks in more detail. Let’s start
with sandstones. We already know that sandstone is a rock composed mainly of sand-sized grains,
which make up the rock’s framework. The most common framework minerals are quartz, followed
by feldspar. Another common framework constituent are pieces of weathered and eroded rock, called
lithic fragments. For example, the sedimentary rocks of the Appalachian Basin contain abundant
fragments of metamorphic rock derived from the erosion of uplifted metamorphic terrane during the
Acadian Orogeny. Geologists classify sandstones by combining two properties: framework mineralogy,
and the proportion of framework to matrix. A quaternary diagram can be used to illustrate
the different types of sandstone. Let’s start by assuming the amount of matrix is zero; these
are known as “arenites”. Sandstones that are very rich in quartz are called quartz arenites. If the
framework is composed mainly of quartz and lithic fragments, it is called a lithic arenite, if it
is instead quartz and feldspar, it would be an arkosic, or feldspathic, arenite. Sandstones with
more than 15% matrix are called wackes (wack-ees), so if the framework was dominated by quartz and
lithic fragments, it would be classified as a lithic wacke, and so forth. Sedimentary rocks with
more than 50% matrix are considered mudstones. Recall from the previous tutorial that feldspar
is unstable in weathering environments and alters to clay over time. The relative concentration
of stable materials such as quartz, to unstable materials such as feldspar, is used in conjunction
with the degree of rounding, sorting, and amount of matrix to determine a sandstone’s “maturity”.
More specifically, maturity is broken down into two categories, textural and compositional.
Compositionally mature sandstones are depleted in materials that are thermodynamically unstable in
weathering environments, including most igneous and metamorphic minerals except for quartz.
Consequently, they tend to be quartz arenites and quartz wackes, though they can occasionally be
lithic arenites or lithic wackes if the lithic fragments are quartz-rich, such as chert or
quartzite. Compositional maturity indicates that a rock’s sediment has spent a great deal of time in
a weathering environment prior to deposition and burial, sufficient to chemically alter unstable
sediments. We will talk more about chemical weathering in a later tutorial. Texturally mature
sandstones have well-rounded and well-sorted grains with little to no matrix mud, indicating
great transport distance from source to burial. Let’s move on to the coarsest-grained sedimentary
rocks, conglomerates. Conglomerates are composed of at least 30% gravel-sized
particles. True conglomerates have well to sub-rounded clasts, whereas the
term “breccia” describes a conglomeritic rock that is composed of angular clasts.
Since it is relatively uncommon for rocks to contain mineral grains larger than 2 millimeters,
most conglomerate gravels are composed of lithic fragments, rather than monocrystalline grains,
as is typically the case for quartz sandstones. Conglomerates are classified based on the
proportion of framework to matrix, similar to sandstones. True conglomerates contain very
little matrix and are framework-supported, meaning that gravel-sized clasts are touching. When a
conglomerate is matrix-supported, it is called a diamictite, with diamictite meaning “mixed
rock”. Diamictite is a general term for extremely poorly sorted geologic material in a muddy matrix
and is sometimes used to describe glacial till. To summarize, conglomerate typically refers to a
clast-supported rock composed of rounded gravel. Breccia also refers to a clast-supported rock,
but composed of angular gravel. And diamictite refers to any matrix-supported rock that
contains gravel-sized clasts. The compositional classification of conglomerates can get a bit
complicated but is generally based on the type of lithic fragments that comprise the framework
fraction, mainly distinguishing sedimentary, metamorphic, and igneous clasts. Additionally,
they can be classified based on clast stability, similar to sandstone maturity. Conglomerates with
greater than 90% stable clasts are said to be “quartzose”, while those with less than
90% stable clasts are said to be “petromict”. Last but not least are the mudstones and shales,
which are the finest-grained variety of siliciclastic rocks. More than half of
all sedimentary rocks are mudstones, compared to about 20% for sandstones and only 1% for
conglomerates. Recall that mud is a general term for silt and clay-sized grains. Hence,
mudstones are composed of more than 50% clay-sized grains, with silt-sized grains
making up most of the rest. Conglomerates, with their very coarse grains, are deposited in the
highest energy depositional environments, like rocky beaches and braided streams. At the other
end of the spectrum are mudstones, which can only be deposited in low energy environments,
where fine-grained sediments are able to settle out of suspension. The largest mudstone deposits
form in the deep, calm waters of lakes and oceans. It is not uncommon for clay, silt, and fine sand
to be simultaneously present in a mudstone, elevating the importance of proper grain size
classification. This can be done accurately with a petrographic microscope, though the clay-sized
grains will be too small to resolve. Field geologists have a few tricks for determining
grain size. Mud can be easily distinguished from sand based on whether grains are visible to the
naked eye, since anything smaller than fine sand is too small to be seen. So, if we have
a sedimentary rock without visible grains, it would be classified as a mudstone. Furthermore,
the relative proportions of clay to silt can be determined by rubbing a small sample over your
teeth to determine the amount of grit present. If little to no grit is detected, it is probably
nearly pure clay and would be classified in the field as a claystone. If moderate grit is present,
that will suffice to classify it as a general mudstone. If a mudstone contains a fraction
of visible fine-grained material, it would be classified as sandy mudstone.
Siltstone is a more specific type of mudstone, being mainly composed of silt-sized particles.
Compositionally, mudstones are primarily composed of clay minerals, with moderate amounts of
silt-sized quartz and feldspar. There are several types of clay minerals including illite, chlorite,
and kaolinite. They are a product of the chemical weathering of primary silicate minerals, such as
feldspar, amphibole, and pyroxene. Mudstones can be quite complex mineralogically, containing minor
amounts of several other minerals, especially the carbonate, sulfide, and oxide/hydroxide
groups. They can also be quite rich in organic carbon, making up an enormous fraction of global
source rocks for oil and gas extraction. The classification of mudstones isn’t as
straightforward as that of sandstones, which can be easily classified by plotting compositional
and textural properties on ternary diagrams. Classifying mudstones involves stringing together
descriptive terms. For example, a mudstone that is rich in organic carbon with abundant fossils would
be classified as a fossiliferous carbonaceous mudstone. Mudstones rich in organic carbon
tend to be black in color and often contain pyrite, or FeS2. If a mudstone is fissile,
meaning it easily breaks into planar sheets, it is called a shale, the term most nongeologists
probably know rather than mudstone. If a mudstone contains very thin layers,
called laminae, it is said to be laminated. Another interesting property of mudstone is
that it’s the most radioactive of the sedimentary rocks. There are a few reasons for this. First,
some clay minerals are rich in potassium, and 0.01% of potassium is the radioactive
isotope potassium-40. Second, several clay minerals contain trace amounts of thorium,
which is also radioactive. And three, organic materials help concentrate uranium.
The association between organic-rich mudstones and radioactivity is exploited by oil and gas
prospectors, who measure the gamma ray intensity throughout boreholes to identify likely targets.
Homes built in areas with mudstone bedrock are often afflicted with high levels of radon gas,
which is part of the decay chain of uranium and thorium. That covers the classification
of siliciclastic rocks. Let’s move forward and examine another type of sedimentary rock next.