Sedimentary Rocks

Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay

What Rocks Tell Us

How Classified What it Tells Us
Igneous Mineral Composition Tectonic Setting
Texture Cooling History
Sedimentary Grain Size Energy Level of Environment
Chemical Composition Surface Environment
Metamorphic Chemical Composition Original Rock Type
Mineral Composition Temperature, Pressure Conditions
Texture Degree of Change

Sedimentary Rocks

Deposited on or Near Surface of Earth by Mechanical or Chemical Processes

Clastic Rocks

Biohemical Sedimentary Rocks

Environmental Clues in Sedimentary Rocks

Bedding or Stratification

Clastic Rocks

Classified on

Sediment Sizes and Clastic Rock Types

Sedimentary rocks made of silt- and clay-sized particles are collectively called mudrocks, and are the most abundant sedimentary rocks.

Some Special Clastic Rock Types


Removal of Unstable Ingredients - Mechanical Working






Chemical Sediments

Evaporites -Water Soluble


Example: Ca(sol'n) + SO4 (Sol'n) = CaSO4

Alteration After Deposition

Biogenic Sediments

Organic Remains

Fossil Fuels


Coal is a slam-dunk. It's carbonized wood. We know that because the actual wood fragments are easily visible in low-grade varieties of coal, fossilized wood is often found in adjacent rocks, the overall environment is typical of coastal swamp or delta settings, and ancient soils are sometimes found beneath the coal beds. Organic matter goes through a variety of changes as it becomes coal:

Compacted and partially decomposed organic matter. About 50% carbon.
Brown or gray brittle coal with lots of impurities, and often with easily visible plant fragments. About 80% carbon.
Black with banding. Some bands are dull, others shiny. These bands reflect different types of processed plant matter, which are still visible under the microscope. About 90% carbon.
Black or dark gray, metallic luster and conchoidal fracture. A true metamorphic rock, since it's heated beyond the temperatures found in normal sedimentary burial. About 95% carbon
Dark gray and metallic, 100% carbon but unburnable in normal flames.
Contrary to popular misconception, diamond is NOT the final stage in coal metamorphism! Coal is never buried deeply enough to reach the pressures needed to form diamond. Diamond form in the earth's mantle from carbon that was always in the earth's interior.

Anthracite is the purest and best form of coal. Unfortunately, the temperature difference between bituminous coal and graphite is small, so anthracite is uncommon. Also, unlike flat-lying bituminous, anthracite often occurs in folded rocks, making it hard to mine. Finally, since it burns so hot, it requires special furnaces. So despite its desirable properties, anthracite use has declined substantially.


The problem with petroleum is that it's a fluid and moves, so it may migrate far from its source. A typical petroleum molecule looks like this:

Octane molecule

The above molecule has eight carbon atoms in a chain and is called octane. Molecules with 1-4 carbons are called methane, ethane, propane and butane, respectively. From then on, they are named for the number of carbons in the chain: pentane, hexane, heptane, octane, etc.

Octane makes a good motor fuel, so fuel that burns as well as pure octane is termed 100 octane. Back when Detroit was turning out mammoth Klingon cruisers, some fuels were over 100 octane (outperformed pure octane). Nowadays fuels are 85-90 octane. So if petroleum is the remains of living things, what sorts of organisms make these molecules?

Answer: NONE. If it were that easy, we wouldn't have to look for oil, we'd just toss our garbage into a vat of the right microbes and skim off the petroleum. But lots of organisms make molecules that look like this:

Fatty acid molecule

This is called a fatty acid (octanoic acid to be exact). Most petroleum occurs in marine sedimentary rocks, so we want organisms rich in fatty acids that live in the sea, in huge quantities. And we have them. They're called plankton. Marine plankton, not dinosaurs, are the precursors of petroleum.

Being fluid, petroleum moves, and since it's lighter than water, it floats upward. Left unconfined, it will reach the surface and evaporate or be oxidized. So it has to be confined somehow. Contrary to the popular term "oil pool," oil does not collect in pockets in the rock. It floats upward on water until it either reaches the surface or is trapped from above. The most important economic application of an understanding of rocks in three dimensions is the search for petroleum traps.

Petroleum Traps In the diagram, light blue represents water-soaked porous rock, dark gray represents petroleum and light gray represents natural gas. Like water in an aquifer, the petroleum and natural gas fill pore spaces in the rocks. All other colors represent impervious rocks.

Structural Traps (top)
Traps that result from deformation of the rocks by outside forces.
Startigraphic traps (bottom)
Traps formed by variations within the sedimentary rocks themselves.

Since oil weighs a lot less than rock, the oil in a well weighs far less than the same volume of rock next to the well. Thus in many cases oil is still under pressure when it reaches the surface. In old-time movies, it was common to see the climax come when oil drillers on the verge of quitting hit oil, got a "gusher" and celebrated in the resulting rain of oil.

Fact: the absolute last thing anyone in the oil business ever wants is a gusher. They are incredibly dangerous to get under control. In fact, when PBS did a series on oil, they could not locate a sound recording of a gusher anywhere and had to interview a few surviving old-timers who could remember what one sounded like. It's been that long since one happened.

However, that pressure is extremely valuable because not only does it get the oil to the surface, but it helps move oil through the rocks to the well. Get greedy and drill too many wells, and you bleed off the pressure, and in the long run you get less oil, not more.

Gas Hydrates

Natural gas hydrates are a curious kind of chemical compound where two dissimilar molecules are mechanically intermingled but not truly chemically bonded. Instead one molecule forms a framework that traps the other molecule. Natural gas hydrates can be considered modified ice structures enclosing methane and other hydrocarbons, but they can melt at temperatures well above normal ice.

At 30 atmospheres pressure, methane hydrate begins to be stable at temperatures above 0 C and at 100 atmospheres it is stable at 15 C. This behavior has two important practical implications. First, it's a nuisance to the gas company. They have to dehydrate natural gas thoroughly to prevent methane hydrates from forming in high pressure gas lines. Second, methane hydrates will be stable on the sea floor at depths below a few hundred meters and will be solid within sea floor sediments. Masses of methane hydrate "yellow ice" have been photographed on the sea floor. Chunks occasionally break loose and float to the surface, where they are unstable and effervesce as they decompose.

The stability of methane hydrates on the sea floor has a whole raft of implications. First, they may constitute a huge energy resource. Second, natural disturbances (and man-made ones, if we exploit gas hydrates and aren't careful) might suddenly destabilize sea floor methane hydrates, triggering submarine landslides and huge releases of methane. Finally, methane is a ferociously effective greenhouse gas, and large methane releases may explain sudden episodes of climatic warming in the geologic past. The methane would oxidize fairly quickly in the atmosphere, but could cause enough warming that other mechanisms (for example, release of carbon dioxide from carbonate rocks and decaying biomass) could keep the temperatures elevated.

In the diagram at left, each vertex is occupied by an oxygen atom and the midpoint of each edge is a hydrogen atom. This atom is attached to one oxygen as part of a water molecule and hydrogen bonded to the other. In the diagram at left one cage is shown with oxygen atoms in blue and hydrogen in red. A methane molecule is shown inside one of the cage skeletons.

Facies Changes

Sedimentary rocks change laterally. These changes reflect the different environments where the rocks formed.

Environment During Deposition

sedimentary facies

Rock Types After Burial

Landforms Associated with Sedimentary Rocks

Sedimentary landforms
Flat-topped hill capped with hard hock
Gently-tilted layer of hard rock: Door Peninsula. The gentle upper slope, on top of the layer is called the dip slope
A sharp ridge of hard rock, edge of a steeply-dipping layer

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Created February 26, 1997, Last Update May 28, 2003