Chapter 14: Sediment and Sedimentary Rocks
Sedimentary rocks are those rocks that form from sediment that has been mechanically
deposited by a fluid (e.g., wind, water, or ice) or chemically deposited (precipitated) from
solution. As I said at the end of Chapter 12 (weathering), the end products of mechanical and
chemical weathering are the raw material needed to create a sedimentary rock.
Sedimentary rocks fall into two broad categories, detrital rocks and chemical rocks. Detrital
sedimentary rocks consist of cemented sediment grains that represent fragments of pre-existing
rocks. Chemical rocks are deposited by the chemical process of precipitation from solution.
Sedimentary rocks form in the following way: weathering of preexisting rocks creates rock
fragments, clay minerals, iron oxides, and dissolved compounds in solution. These weathering
products are then eroded and transported to somewhere else. Then, rock fragments are
deposited to create a detrital sediment, or chemical compounds are precipitated to create a
chemical sediment. The sediment (whether detrital or chemical) is then lithified (i.e., turned into
a sedimentary rock) via two processes, including compaction and cementation. To summarize,
sedimentary rocks are created via the following processes:
weathering
erosion/transport
deposition/precipitation
lithification.
Let’s start by making some comments about detrital rocks. As shown in Figure 14.2, rock
fragments change as they are eroded and transported. In general, with increasing transport
distance, they become smaller, rounder, and better sorted, as shown here:
Non-marine
Marine
Alluvial Fan
River
Nearshore
marine
Delta
Offshore
marine
Sea level
Gravel/sand
silt/clay
Sand
silt/clay
Silt/clay
GRAIN SIZE DECREASES
ROUNDING INCREASES
SORTING INCREASES
Change in sediment characteristics with transport.
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Clay/CaCO3/SiO2
Think of sorting as representing grain size similarity. A well-sorted sediment or sedimentary
rock contains grains that are all similar in size. In contrast, a poorly sorted sediment or
sedimentary rock contains a wide range of grain sizes, as shown here:
Poorly sorted
Well sorted
Poorly sorted vs. well sorted sediment.
We can get a general sense of how far a river sediment has been transported by making some
basic observations about grain size, degree of rounding, and degree of sorting. This gives us a
basic point of comparison between modern sediments and ancient sedimentary rocks.
To understand the environment in which an ancient sedimentary rock formed, we compare the
characteristics of this ancient rock with the characteristics of modern sediments.
Stated another way, we use present depositional environments as a standard of comparison in
understanding ancient depositional environments. The whole game with sedimentary rocks is to
figure out as much as we can about the ancient depositional environment in which the
sedimentary rock formed. Fundamentally, the only way to do this is to compare ancient
sedimentary rock characteristics with those of modern sediments forming in present-day
depositional environments.
Hey, did you notice that we’re invoking one of geology’s big ideas here? For an honorary A, do
remember the name of the principle that the present is the key to the past?
That would be….uniformitarianism! Simply put, we understand ancient depositional
environments in terms of modern ones.
Once we get far enough offshore, the only detrital sediments we find are very fine-grained clay
deposits. Under the right conditions, chemicals can precipitate directly from sea water or with
the help of small organisms (plankton) living in the water column. In both cases, chemical
deposition (precipitation) can lead to the accumulation of biological sediments and ultimately
biochemical sedimentary rocks.
Alluvial fans and rivers are two common non-marine depositional environments, whereas marine
depositional environments include nearshore (e.g., continental shelf) and offshore (e.g., abyssal
plain) environments.
Here’s an image of an alluvial fan in Death Valley:
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2,000 ft approx.
Alluvial Fan, Death Valley, CA
(image courtesy Google Earth)
Note the road for scale along the bottom of the fan. High up, near the head of the fan, the
sediments are quite gravelly. Further downstream on the fan surface, near the road, the
sediments become more sandy.
Heres’ a photo of some ancient alluvial fan deposits in the Mecca Hills, southeast of Palm
Springs:
Alluvial fan deposits, Mecca Hills, CA.
I didn’t have a hand lens or car keys with me when I took the above picture, so I improvised in
terms of showing an object for scale . Note the large fragment sizes, angular shapes, and
poor sorting of the fragments in the above photo. Having been lithified, this deposit has been
converted into sedimentary rock (conglomerate). However, we can correlate the characteristics
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of this rock (e.g., grain size, rounding, and sorting) with similar ones seen in modern alluvial fan
environments…that’s how geologists know this rock formed in an ancient alluvial fan
environment.
It should be clear to you by now that gravelly detrital sedimentary rocks like the one above
typically form in an upstream environment, near the base of mountains, because that’s where
we’re likely to find, large, poorly sorted, angular fragments that haven’t travelled very far from
their source (and so haven’t become very well rounded or sorted).
In contrast, here’s a photo of some shale from the Borrego Formation, near Borrego Springs,
CA (note my ring for scale).
Gypsiferous shale, Borrego Formation, near
Borrego Springs, CA.
The above rock is very, very fine-grained, well sorted, and mostly consists mostly of clay. You’d
need a high-powered microscope to see individual sediment grains.
The above rock is very thin-bedded, which is why we call it a shale rather than claystone (shale
= thin-bedded claystone). The white, needle-like crystals growing in between bedding planes
(sedimentary layers) are gypsum crystals.
Judging from the fine grain size, this rock formed in a downstream environment, where currents
were weak.
Once again, we can determine the ancient depositional environment of this rock by correlating
its characteristics with modern depositional environments.
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In this case, the shale in the above photo was deposited in an ancient lake that last occupied
the Borrego Valley about a million years ago. Around that time, the climate became more arid
(dry), and evaporative conditions resulted in the precipitation of calcium sulfate (i.e., the mineral,
gypsum) between bedding planes. The presence of abundant gypsum tells us that this rock
formed under evaporative conditions.
Once a sediment is deposited, it becomes hardened, or lithified, by two processes, including
compaction and cementation (Figure 14.6). Common sedimentary cements include calcium
carbonate (calcite, CaCO3), iron oxide (hematite, Fe2O3), and silica (quartz; SiO2). Remember
that iron oxide, silica, and the chemical building blocks of calcium carbonate (HCO3- and Ca++)
are common chemical weathering products, so here’s a great example of the end products of
weathering serving as the raw material for sedimentary rocks.
Your book goes into quite a bit of detail describing various detrital sedimentary rocks, including
conglomerate, breccia, sandstone, and shale. You should familiarize yourself with the overall
properties of these rocks.
Study Table 14.1 (and the summary rock table, below), but don’t worry about knowing particle
diameters. Just remember that conglomerate and breccia are coarse-grained, gravelly
sedimentary rocks. Sandstone is obviously composed of sand-sized fragments. Mudrocks
include mudstone, siltstone, and shale. “Mud” in this context just refers to a fine-grained detrital
sediment that represents a mixture of silt- and clay-sized particles. “Clay” in this context refers
to very, very fine-grained particles (many of which also happen to be clay minerals, but it’s also
possible to have quartz and feldspar grains in the clay-sized range). Shale is simply mudstone
or claystone that is finely layered (laminated) such that it tends to break apart in thin sheets.
Here’s a link to an absolutely amazing scanning electron microscope image of a detrital
sandstone (magnified millions of times!)—basically some rounded quartz grains cemented
together with calcite: SEM Sandstone Image.
As you might imagine, many detrital sedimentary rocks consist of a mixture of gravel, sand, silt,
and clay. Gravel-rich rocks are easy to identify in the field because the grains are clearly visible.
But what about finer-grained rocks that may be mixtures of sand, silt, and clay?
Over hundreds of years, geologists have developed a very sophisticated field test for judging the
relative amounts of sand-, silt-, and clay-sized particles in detrital sedimentary rocks….the taste
test!
Basically, it works like this: say you walk up to an outcrop and want to get an approximate
sense of how much sand vs. silt vs. clay comprises the detrital rock you’re studying. Just break
off a small piece, pop it into your mouth, and chew on it a bit! If your rock is “crunchy”, it’s
mostly made of sand. If it’s “gritty”, it’s mostly silt, and if it has a wonderfully “creamy” texture,
it’s mostly clay. So when a geologist says she’s hungry for knowledge about detrital
sedimentary rocks, you can take her at her literally .
Here’s a simple classification chart for detrital sedimentary rocks:
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DETRITAL SEDIMENTARY ROCKS
Particle Size
Coarse (> 2 mm)
Gravel
Medium
(1/16 - 2 mm)
Sand
Fine
(1/16 - 1/256 mm)
Silt
Fine to Very Fine
(1/16 to less than 1/256 mm)
Silt and Clay (Mud)
Fine to Very Fine
(1/16 to < 1/256 mm)
Description
Rounded rock or mineral fragments,
typically poorly sorted.
Rock Name (link)
Conglomerate
Angular rock or mineral fragments, typically
poorly sorted.
Breccia
Quartz grains & minor accessory minerals;
typically well-sorted, rounded grains;
cements of all types common
Quartz Sandstone
At least 25% feldspar
Arkose
at least 20% clay minerals, some rock
fragments, quartz, and feldspar; poorly
sorted
Greywacke
Composed of mostly silt-sized particles;
gritty texture.
Siltstone
Composed of a mixture of silt- and clay-sized
particles (mud) or clay-sized particles only;
laminated (thin-bedded).
Shale
Composed of a mixture of silt- and clay-sized
particles (mud) or clay-sized particles only;
non-laminated (not thin-bedded).
Mudstone/Claystone
You won’t be required to distinguish various types of sandstone; just know that sandstone is a
detrital sedimentary rock composed of sand-sized fragments.
Okay, on to the chemical rocks. Common chemical sedimentary rocks include limestone, chert,
and evaporates like rock salt and rock gypsum. We distinguish these various chemical rocks
mainly on the basis of chemical composition. Limestone is perhaps the most common chemical
sedimentary rock. It consists mostly of the non-silicate mineral, calcite (calcium carbonate),
which can form either organically or inorganically.
Although limestone composed of shell material is biochemical, such limestone still mostly
consists of fragments that range in size from relatively large to microscopic. Coarse-grained,
shell-hash limestone is called coquina (Figure 14.20), whereas fine-grained limestone
composed of the shells of marine microorganisms is called chalk.
Here’s a classification chart for chemical sedimentary rocks:
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CHEMICAL SEDIMENTARY ROCKS
Origin
Inorganic
Chemical
Composition
Texture/Description
Rock Name (link)
Calcium Carbonate
CaCO3
Crystalline
(interlocking crystals)
Limestone
Calcium Magnesium
Carbonate
CaMg(CO3)2
Crystalline
(interlocking crystals)
Dolostone
Origin
Precipitated directly
from sea water,
surface water (lakes)
or groundwater
(caves/springs).
Alteration of
limestone by
magnesium-rich
solutions.
Sodium Chloride
NaCl
Microcrystalline
(interlocking
microscopic crystals);
well-cemented
Crystalline
(interlocking crystals)
Calcium Sulfate
CaSO4-2H20
Crystalline
(interlocking crystals)
Rock Gypsum
Evaporation of saline
lakes or sea water.
Visible shells/shell
fragments, loosely
cemented
Coquina
(limestone)
Cementation of
shells, shell
fragments.
Microscopic marine
organisms, loosely
cemented
Chalk (limestone)
Cementation of
skeletal material.
Shell fragments,
cemented with calcite
cement
Fossiliferous
limestone
Cementation of
shells, shell
fragments.
Silica
SiO2
Microscopic marine
organisms, well
cemented
Chert
Cementation of
skeletal material.
Organic Matter
Fine-grained
Coal
Compressed plant
matter.
Silica
SiO2
Calcium Carbonate
CaCO3
Biochemical
Chert
Precipitation from
silica-rich fluids.
Rock Salt
Evaporation of saline
lakes or sea water.
Most limestones are biochemical and form in warm, shallow marine waters by invertebrates that
construct their shells out of calcium carbonate. You and I do the same thing except that we use
calcium phosphate to build our bones and teeth.
Note that limestone and chert can form both inorganically and biochemically, although
biochemical chert and limestone are more common than their inorganic counterparts.
Chalk, for example, is a biochemical form of limestone composed of billions of microscopic
organisms (plankton) that float freely in sea water. When these critters die, their shells sink to
the sea bottom, and in some environments accumulate to form chalk.
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Here’s a link to a website where you can see scanning electronic microscope (SEM) images of
one example of such a critter—a calcarious planktonic organism called a coccolith: SEM Image
of Coccoliths in Chalk. Check it out!
Dolomite (or more correctly, dolostone) is just magnesium-rich limestone.
In Florida, there’s an amazing military fortress made entirely out of coquina (shell hash)
limestone. Although coquina is about the last rock one would normally choose in making a
military fortress (because it’s very soft), the one in Florida has held up quite well over the
centuries. It’s so soft that cannon balls just get lodged inside the soft, coquina walls! Here’s a
link if you’re curious: Coquina Fortress (scroll down the page to see a close-up photo of
coquina).
Chert is a fine-grained chemical sedimentary rock formed mostly of pure silica (microcrystalline
quartz).
Like limestone, chert can form both inorganically and biochemically. It’s probably fair to say that
most chert forms in deep marine waters.
When planktonic (free floating) organisms that make their shells out of silica die, their bodies
sink to the sea floor and accumulate as siliceous ooze. With lithification, a hard, silica-rich
biochemical sedimentary rock is produced called chert.
Chert is a common rock type found in ancient subduction zones, because it often gets scraped
off the subducting plate and plastered onto the non-subducting plate as subduction proceeds.
Here’s an SEM image of a common microscopic organism from which chert is made—a silicarich, shelled animal (zooplankton) called a radiolarian: SEM Image of Radiolarian. Chert is
typically packed full of these critters!
Evaporites (mostly rock salt and rock gypsum) obviously form in drying, evaporative
environments such as playa lakes (Figure 14.24). As such, evaporates are good indicators of
dry environments.
A modern example of inland, playa (evaporative) lake is Searles Dry Lake, in the Mojave Desert,
near Ridgecrest, California.
Every year, thousands of folks descend on Searles Dry Lake for the annual, Gem-O-Rama
mineral show and also for the chance to collect rare, pink halite (salt) crystals from the lake bed.
I had the opportunity to experience Gem-O-Rama a few years back.
Here’s what the lake bed looks like:
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Searles Dry Lake, Trona, CA
It’s a veritable moonscape consisting of a halite crust and hundreds of salty brine pools. The
best halite crystals are found within the brine pools, underneath overhanging ledges…yes, you
have to wade in, but it’s worth getting hot, wet, and salty. Here’s a photo of some spectacular
pink halite crystals I found in one of the brine pools:
Pink halite crystals, Searles Dry Lake, Trona CA.
The pink color comes from millions of tiny, salt-loving microorganisms that become incorporated
into the growing halite crystals as they precipitate.
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Here again, we can understand the past in terms of the present. Anywhere in the geologic
record we find rock salt deposits, we know that the area was once a playa lake, or perhaps an
evaporating sea, because we know from direct observation that playa lakes and evaporating
seas are where extensive salt deposits form in current geologic time.
Coal (lithified plant material) is another very important biochemical sedimentary rock for
economic reasons. In contrast to evaporates, coal beds form is wet, soggy, swampy
environments. Here’s a photo of a large coal mine in Wyoming:
Wyoming coal mine
(courtesy Wikipedia; http://en.wikipedia.org/wiki/Coal).
Make sure you’re familiar with all of the common sedimentary rock types mentioned above.
Basically, you’ll want to become pretty familiar with Tables 14.1 and 14.2 and the summary
charts above. On the exam, I’ll probably ask you to name various examples of both detrital and
chemical sedimentary rocks. Or, I’ll give you a description of the rock and ask you to name it.
In general, sedimentary rocks contain clues about the depositional environments in which they
formed. What clues? Things like grain size, degree of rounding, degree of sorting, and even
rock composition, not to mention occasional fossils.
Sometimes the sequence of a group of sedimentary rocks tells us something about ancient
depositional environments and how they’ve changed with time.
Try the following link for a simple animation that shows how ancient sea level changes can be
inferred from sedimentary rock sequences: Transgression and Regression (scroll down to
5.2).
Sedimentary rocks also contain other clues about their depositional environments—clues that
geologists call sedimentary structures.
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Think of a sedimentary structure as any feature within a sediment or sedimentary rock that
developed during or after deposition or precipitation. Here’s a partial list of common
sedimentary structures and what they tell us about depositional environment:
Bedding—horizontal layering. This is the most common sedimentary structure of all, and its
presence is usually what alerts us to the fact that we’re dealing with a sedimentary rock (Figure
14.26).
Interbedded sandstone and siltstone.
Crossbedding—inclined layering; a common structure found in sandstones; indicative of a
marine or nonmarine dune environment (Figure 14.28). Crossbeds are inclined downward in a
downstream direction. For an animated version of the process that produces crossbedding, try
this Crossbedding Animation (5.1).
Downstream paleocurrent direction
Crossbedded sandstone, Torrey Pines State Park.
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Ripple Marks—indicative of fairly rapid current velocities (Figure 14.29); typically form in river,
tidal, desert, and delta environments. Here’s a good online photo of some ripple marks.
Ripple marks, Anza-Borrego State Park.
Mud Cracks—polygonal cracks sometimes preserved in fine-grained sediments as they dry;
indicative of a drying environment (e.g., a playa lake or floodplain). Try this link for some online
photos of mudcracks.
Mudcracks, Anza-Borrego State Park.
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Raindrop Imprints—small depressions created by the force of falling raindrops on muddy
sediment.
Raindrop imprints, Anza-Borrego State Park.
Imbrication—a domino-style stacking of gravel fragments with long axes inclined upward in a
downstream direction.
Downstream transport direction
Imbricated conglomerate, Anza-Borrego State Park.
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Fossils (evidence of ancient life) are also an important component of sedimentary rocks, in part
because sedimentary rocks are generally the only type of rock that contains fossils. Think of the
fossils in a sedimentary rock as a sort of sedimentary structure. Fossils tell us a lot about the
characteristics of ancient life, but they also tell us a lot about ancient depositional environments.
Most often, fossils can be used to distinguish marine from nonmarine depositional environments
and can also tell us something about ancient climates.
Scallop fossils from the San Diego Formation, Tourmaline Beach.
One last section to read carefully is the one entitled, Plate Tectonics and Sedimentary Rocks.
Like almost every other aspect of geology, we can relate sedimentary rock types in a general
way to plate tectonic environment. Let me summarize for you:
Thick accumulations of detrital sedimentary rocks can often be correlated with nearby uplift and
mountain building processes occurring at convergent plate boundaries.
Detrital sedimentary rocks containing abundant volcanic rock fragments (greywacke
sandstones, for example) are often associated with subduction zones, where nearby island arc
and continental arc volcanoes emit volcanic debris.
Fault-bounded basins with thick sedimentary rock accumulations are also associated with
transform plate boundaries. As your text points out, many of the petroleum reserves in
California formed in sedimentary basins that developed as the San Andreas transform fault
system developed.
Divergent boundaries often produce diverse groups of sedimentary rock, with coarse-grained,
gravel-rich detrital sedimentary rocks and basaltic volcanic rocks developing along the sides of
a rift valley in the early stages of plate divergence. Evaporites may form out in the middle of the
widening rift valley (Figure 4.42).
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