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Sedimentary Rocks
Chert
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Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO2). It
occurs as nodules, concretionary masses and as layered deposits. Chert breaks with a conchoidal fracture, often
producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and
weapons. Dark varieties of chert are common and are often called "flint".
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How Does Chert Form? Diatoms are microscopic, single-celled algae that live in marine or fresh water. They
produce hard parts made of silicon dioxide. Most chert forms when microcrystals of silicon dioxide grow within soft
sediments that will become limestone or chalk. In these sediments, enormous numbers of silicon dioxide microcrystals
grow into irregularly-shaped nodules or concretions as dissolved silica is transported to the formation site by the
movement of ground water. If the nodules or concretions are numerous they can enlarge and merge with one another to
form a nearly continuous layer of chert within the sediment mass. Chert formed in this manner is a chemical sedimentary
rock.
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Some of the silicon dioxide in chert is thought to have a biological origin. In some oceans and shallow seas large
numbers of diatoms and radiolarians live in the water. These organisms have a glassy silica skeleton. Some sponges also
produce "spicules" that are composed of silica. When these organisms die their silica skeletons fall to the bottom,
dissolve, recrystallize and might become part of a chert nodule or chert layer. Chert formed in this way could be
considered a biological sedimentary rock.
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Chert occurs in a wide variety of colors. Continuous color gradients exist between white and black or between
cream and brown. Green, yellow and red cherts are also common. The darker colors can result from inclusions of
sediment or organic matter. The name "flint" is often used in reference to the darker colors of chert. Red to reddish-brown
cherts receive their color from included iron oxide. The name "jasper" is frequently used for these reddish cherts.
Conglomerate
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Conglomerate could be thought of as a giant sandstone, containing grains of pebble size (greater than 4 mm) and
cobble size (>64 mm). Conglomerate forms in a very energetic environment, where rocks are eroded and carried downhill
so swiftly that they aren't fully broken down into sand. Another name for conglomerate is puddingstone, especially if the
large clasts are well rounded and the matrix around them is very fine sand or clay. These specimens could be called
puddingstone. A conglomerate with jagged, broken clasts is usually called a breccia, and one that is poorly sorted and
without rounded clasts is called a diamictite.
Coquina
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Coquina ("co-KEEN-a") is a limestone composed chiefly of shell fragments. It's not common, but when you see it
you want to have the name handy. Coquina is the Spanish word for cockleshells or shellfish. Coquina forms near shore,
where wave action is vigorous and sorts the sediments well. Most limestones have some fossils in them, and many have
beds of shell hash, but coquina is the extreme version. A well-cemented, strong version of coquina is called coquinite. A
similar rock, composed chiefly of shelly fossils that lived where they sit, unbroken and unabraded, is called a coquinoid
limestone. That kind of rock is called autochthonous (aw-TOCK-thenus), meaning "arising from here." Coquina is made of
fragments that arose elsewhere, so it is allochthonous (al-LOCK-thenus). Those are handy words in geology.
Gypsum
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Gypsum is a common mineral, with thick and extensive evaporite beds in association with sedimentary rocks.
Deposits are known to occur in strata from as early as the Permian age.[8] Gypsum is deposited in lake and sea water, as
well as in hot springs, from volcanic vapors, and sulfate solutions in veins. Hydrothermal anhydrite in veins is commonly
hydrated to gypsum by groundwater in near surface exposures. It is often associated with the minerals halite and sulfur.
Because gypsum dissolves over time in water, gypsum is rarely found in the form of sand. However, the unique conditions
of the White Sands National Monument in the US state of New Mexico have created a 710 km2 (270 sq mi) expanse of
white gypsum sand, enough to supply the construction industry with drywall for 1,000 years.[9] Commercial exploitation of
the area, strongly opposed by area residents, was permanently prevented in 1933 when president Herbert Hoover
declared the gypsum dunes a protected national monument.
Limestone
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Limestone is usually made of the tiny calcite skeletons of microscopic organisms that once lived in shallow seas
like today's Bahamas. Limestone dissolves in rainwater more easily than other rocks. Rainwater picks up a small amount
of carbon dioxide during its passage through the air, and that turns it into a very weak acid. Calcite is vulnerable to acid.
That explains why underground caverns tend to form in limestone country, and why limestone buildings suffer from acid
rainfall. In dry regions, limestone is a resistant rock that forms some impressive mountains. Under pressure, limestone
metamorphoses into marble. Under gentler conditions that are still not completely understood, the calcite in limestone is
altered to dolomite, changing it to dolomite rock.
Sandstone
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andstone forms where sand is laid down and buried—beaches, dunes and seafloors. Usually sandstone is mostly
quartz. andstone is a category of rock made from sediment (a sedimentary rock). The sediment particles are clasts, or
pieces, of minerals and fragments of rock, thus sandstone is a clastic sedimentary rock. It is composed mostly of sand,
which means particles of a medium size, so sandstone is a medium-grained clastic sedimentary rock. More precisely,
sand is between 1/16 millimeter and 2 mm in size (silt is finer and gravel is coarser). Sandstone may include finer and
coarser material and still be called sandstone, but if it includes more than 30 percent grains of gravel, cobble or boulder
size it's classified instead as conglomerate or breccia (together these are called rudites).
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Sandstone has two different kinds of material in it besides the sediment particles: matrix and cement. Matrix is the
fine-grained stuff (silt and clay size) that was in the sediment along with the sand whereas cement is the mineral matter,
introduced later, that binds the sediment into rock.
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Sandstone with a lot of matrix is called poorly sorted. If matrix amounts to more than 10 percent of the rock, it is
called a wacke ("wacky"). A well-sorted sandstone (little matrix) with little cement is called an arenite. Another way to look
at it is that wacke is dirty and arenite is clean.
You may notice that none of this discussion mentions any particular minerals, just a certain particle size. But in fact,
minerals matter.
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Sandstone is formally defined strictly by particle size, but rocks made of carbonate minerals don't qualify as
sandstone. Carbonate rocks are called limestone and given a whole separate classification, so sandstone really signifies
a silicate-rich rock. (A medium-grained clastic carbonate rock, or "limestone sandstone," is called calcarenite.) This
division makes sense because limestone is made in clean ocean water, whereas silicate rocks are made from sediment
eroded off the continents.
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Mature continental sediment consists of a handful of surface minerals, and sandstone therefore is usually almost
all quartz. Other minerals—clays, hematite, ilmenite, feldspar, amphibole and mica— and small rock fragments (lithics) as
well as organic carbon (bitumen) add color and character to the clastic fraction or the matrix. A sandstone with at least 25
percent feldspar is called arkose. A sandstone made of volcanic particles is called tuff.
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The cement in sandstone is usually one of three materials: silica (chemically the same as quartz), calcium
carbonate or iron oxide. These may infiltrate the matrix and bind it together, or they may fill the spaces where there is no
matrix.
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Depending on the mix of matrix and cement, sandstone may have a wide range of color from nearly white to
nearly black, with gray, brown, red, pink and buff in between.
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Sandstone forms where sand is laid down and buried. Usually this happens offshore from river deltas, but desert
dunes and beaches can leave sandstone beds in the geologic record too. The famous red rocks of the Grand Canyon, for
instance, formed in a desert setting. Sandstone does not usually contain good fossils because the energetic environments
where sand beds form don't favor their preservation.
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When sand is deeply buried, the pressure of burial and slightly higher temperatures allow minerals to dissolve or
deform and become mobile. The grains become more tightly knit together, and the sediments are squeezed into a smaller
volume. This is the time when cementing material moves into the sediment, carried there by fluids charged with dissolved
minerals. Oxidizing conditions lead to red colors from iron oxides, while reducing conditions lead to darker and grayer
colors.
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The sand grains in sandstone give information about the past. The presence of feldspar and lithic grains means
that the sediment is close to the mountains where it arose. The degree to which the grains are rounded is a sign of how
far they were transported. A frosted surface is generally a sign that sand was transported by wind—that in turn means a
sandy desert setting.
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Various features in sandstone are signs of the past environment. Ripples can indicate the local water currents or
wind directions. Load structures, sole marks, rip-up clasts and similar features are fossil footprints of ancient currents.
Liesegang bands are signs of chemical action after burial of the sand.
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The layers, or bedding, in sandstone are also signs of the past environment. Turbidite sequences point to a
marine setting. Crossbedding (truncated, tilted sandstone layering) is a rich source of information on currents. Interbeds of
shale or conglomerate may indicate episodes of different climate.
Shale
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Shale is a claystone (or mudstone) that is fissile, splitting in layers. Shale is usually soft and does not crop out
unless harder rock protects it. Geologists are strict with their rules on sedimentary rocks. Sediment is divided by particle
size into gravel, sand, silt and clay. Claystone must have at least twice as much clay as silt and no more than 10 percent
sand. It can have more sand, up to 50 percent, but that is called a sandy claystone. What makes a claystone shale is the
presence of fissility—it splits in more or less thin layers whereas claystone is massive.
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Shale can be fairly hard if it has a silica cement, making it closer to chert, but usually it is soft and easily weathers
back into clay. Shale may be hard to find except in roadcuts, unless a harder stone on top of it protects it from erosion.
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When shale undergoes greater heat and pressure, it becomes the metamorphic rock slate. With still more
metamorphism, it becomes phyllite, then schist.
Siltstone
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Siltstone is made of sediment that is between sand and clay in the Wentworth grade scale; it's finer grained than
sandstone but coarser than shale. Silt is a size term used for material that's smaller than sand (generally 0.1 millimeter)
but larger than clay (around 0.004 mm). The silt in this siltstone is unusually pure, containing very little sand or clay. The
absence of clay matrix makes siltstone soft and crumbly, even though this specimen is many millions of years old.
Siltstone is defined as having twice as much silt as clay.
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The field test for siltstone is that you can't see the individual grains, but you can feel them. Many geologists rub
their teeth against the stone to detect the fine grit of silt. Siltstone is much less common than sandstone or shale.
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Siltstone usually forms offshore, in quieter environments than the places that make sandstone. Yet there are still
currents that carry off the finest clay-size particles. This rock is laminated. It's tempting to suppose that the fine lamination
represents daily tidal surges. If so, this stone might represent about a year of accumulation.
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Like sandstone, siltstone changes under heat and pressure into the metamorphic rocks gneiss or schist.
Travertine
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Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine often has a
fibrous or concentric appearance and exists in white, tan and cream-colored varieties. It is formed by a process of rapid
precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. In the latter it can form
stalactites, stalagmites and other speleothems. It is frequently used in Italy and elsewhere as a building material.
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Travertine is a terrestrial sedimentary rock, formed by the precipitation of carbonate minerals from solution in
ground and surface waters, and/or geothermally heated hot-springs. Similar (but softer and extremely porous) deposits
formed from ambient-temperature water are known as tufa.
Sedimentary Rock Kit from Wardʼs Natural Science contains the following rocks: chert, conglomerate, coquina,
gypsum, limestone (2), sandstone (2), shale (2), siltstone, travertine. You must determine which numbered specimens are
these rocks. Look at as many specimens of the same type of rock as possible. To get you started 14, 21, and 23 fizz in
dilute acid. Fill in the chart below.
Sample
Composition (6.2 & 6.3)
Textual & Other Properties (6.1 & 6.8)
Rock Name
How Formed (6.3-6.7)
Dolomite or silty
limestone. Variably
fizzes in dilute acid.
Gray, hard, silt to clay with
fractures filled with calcite of
dolomite crystals that fizz.
Dolomitic
limestone
Probably marine
environment, limey
mud.
13
14
15
16
17
Sample
18
19
20
21
22
23
24
Composition (6.2 & 6.3)
Textual & Other Properties (6.1 & 6.8)
Rock Name
How Formed (6.3-6.7)