Class Information by Ellin Beltz

Outline of Sedimentary Rocks


The four classes of sedimentary rocks are:
  1. Shales
  2. Sandstones
  3. Carbonates
  4. Other sedimentary rocks
    • Conglomerates
    • Evaporites
    • Chert
    • Iron rocks
    • Phosphorites -- and
    • Carbonaceous sedimentary rocks
In a pie chart of modern rock distribution, shales comprise about half. The other three classes of sedimentary rocks would split the other half into thirds.


Shales are a study all to themselves. Studying them reveals ancient environments, often times in breathtaking detail. In the mineralogy of the silt and clay sized particles, the rocks are called by their particle size first, so "silt-stone" and "silty-shale" define the particle size as does "claystone" and "clayshale." The difference in the second half of the name refers to bedding thickness. Thus claystone has bedding greater than 10 mm while in clayshale the bedding is less than 10 mm. Oil can be incorporated in shales; others form caps below and above artesian formations.

Once exposed to erosion, shales weather quickly back to their clay mineral producing often-economic clay deposits and hillsides which slip when wet. Certain clay layers have distinctive colors: ex. the "Blue Goo" of Humboldt County. Others are used in pottery. Tonstein clays are also called fire clays. Clay minerals form sheets or layers held apart by very weak bonds. The space within the layers may contain accessory minerals or water. Kaolinite is an example of a 2 layer clay, while Illite, Chlorite and Smectite (Montmorillionite) are 3 layer clays. The more layers, the more water they hold (on average) and the easier they slide when saturated!


Sandstones are composed primarily of silicate minerals derived from

  1. Ferromagnesian minerals: Olivine, pyroxene, amphibole and biotite
  2. Feldspar minerals: Ca and Na feldspars are collectively known as plagioclase feldspars; while Na and K feldspars are known as the alkali group. Globally, K and Na are greater than Ca and Na, but in volcanic sediments Ca and Na outnumber K. Feldspars weather to micas and clays.
  3. Quartz minerals form from silica tetrahedrons loosened by physical weathering and trasported. If by water, they may be rounded, have chatter marks and show conchoidal fracture. If by air, grains are usually frosted.
  4. Accessory minerals like zircon and rutile which can be used for age dating.

Sandstone cements include from most to least common: silica (SiO2), siderite (FeCO3), calcite (CaCO3) and others.

Silica is often more common in the sediment than it was in the parent material because of concentration of silica during weathering, transport, deposition and lithification as well as the less durable nature of many other minerals.

Using Gilbert's Classification of Sandstones (particles + matrix + cement) if


Whole books can and have been written on carbonates, compounds containing (C03) in their structure. Carbonates include:
  1. CaCO3 -- Calcite // Aragonite // Limestone. Living organisms build aragonite and deep sea oozes contain calcite. Whether this relationship is stable over all of geologic time is an open question. Limestones older than the Cretaceous show little aragonite.
  2. (Mg,Ca)CO3 -- Dolomite // Dolostone. Modern examples are forming on the Bahama Banks.
  3. (Fe,Ca)CO3 -- Siderite // Ironstone. Include the Mazon Creek fossils and are discussed under "other sedimentary rocks" below.
While fossils are not limited to carbonates, limestones, dolomites and ironstones preserve some of the limited record of Earth's past. The study of forams and other small planctonic creatures is key in the oil industry. Carbonate rocks are used for building materials, facing stones, crushed rock and shoreline revetments worldwide.

Other sedimentary rocks

The Significance of the Mineral Composition in Clastic Rocks

Minerals show where sediments came from:
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Ellin Beltz /
January 10, 2008