Class Information by Ellin Beltz
Outline of Sedimentary Rocks
The four classes of sedimentary rocks are:
- Other sedimentary rocks
- 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
- Ferromagnesian minerals: Olivine, pyroxene, amphibole and biotite
- 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.
- 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.
- 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
- less than 5 percent matrix:
- Quartz arenites: such as the Ordovician St. Peter S.S. which are more than 90 percent quartz.
- Feldspathic arenites: such as arkose which has more than 25 percent feldspars
- Lithic arenites which are high in rock fragments
- greater than or equal to 5 percent matrix
- Feldspathic-wacke including greywacke
Whole books can and have been written on carbonates, compounds containing (C03
) in their structure. Carbonates include:
- 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.
- (Mg,Ca)CO3 -- Dolomite // Dolostone. Modern examples are forming on the Bahama Banks.
- (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.
Key to the formation of sedimentary rocks is the partial pressure of carbon dioxide in the atmosphere and in the oceans. It is shown in writing as "Rho"CO2. The Greek character "Rho" looks like a Roman P.
- The ΡCO2 is linked to acidity (pH). As ΡCO2 rises, acidity rises (percent Hydrogen ions drops). As ΡCO2 rises, therefore, carbonates will dissolve more readily.
- The ΡCO2 at any given time is one control on the height of the "calcium compensation depth" [CCD] in the ocean. Below the CCD, shells will dissolve due to higher effective ΡCO2 due to overlying water pressure. As ΡCO2 increases, the CCD rises into shallower and shallower water. This may account for shell dissolution as seen in Thornton Quarry brachiopods and other organisms.
- As sea levels rise, the ΡCO2 usually goes up because former carbonates are uplifted and exposed to weathering much as is happening in the Italian Alps and Dolomites Mountains as well as in the Himalayas and other former sea floors. Exposing coal deposits or burning hydrocarbons or coals also raises ΡCO2.
- The ΡCO2 now is given as 1, a dimensionless number. However, numbers above one show a higher ΡCO2 and numbers below one show a lower ΡCO2 making comparison easy.
- CaCO3 secreting or depositing organisms
- Diatoms look like kaleidoscope flakes and exhibit wonderful geometry and symmetry. If you've never seen one, look them up on the Internet.
- Foraminifera are used as time indicators of the present, because they change shapes constantly; possibly in response to changing environmental conditions. "Forams" secrete rings as they grow larger.
- Bivalves like clams, oysters and other "shellfish" are common today. Pelecypod mollusks of past times are bivalves.
- Brachiopods. Sadly all sessile branches are extinct except for Lingula, formerly very common and found in Chicago area bedrock.
- Lacy Bryozoa are filter feeders common in Chicago area bedrock. Their remains look like wide woven fabric (like sheeting) in the limestone.
- Crinoids and other Echinoderms are composed of geometric body parts. Stems look like separated beads; heads are base five, like a modern starfish.
- Older corals, like Chain Corals and Horn Corals, grow five-sided or less chambers. Modern corals, Hexagonella, have six-sided structure.
- Gastropods like modern snails, conches and slugs. The twirled shells are very distinctive and this group extends far back in time.
- Ammonoids, like the modern chambered Nautilus, are relatives of squid, octopus and cuttlefish. The older forms occurred with straight shells or spiral shells and they are very distinctive until the end of the Paleozoic, when they disappear from the rock record.
- and others. Take Historical Geology and make billions of new friends.
Other plants and animals occurred with CaCO3 secreting organisms including trilobites and worms (and more). But their shells don't contain CaCO3, so they do not count as limestone builders; perhaps "limestone accessory fossils" would be appropriate.
- Limestone Textures
- Carbonate Grains
- Allochems, broken pieces of shells or forams
- Ooids (Illinois' Neda Oolite is a metal oolite.)
- Sponge spicules and other SiO2 particles in a limestone matrix. Spicular limestone can be observed on the Michigan Avenue Bridge and in the monument to Marquette at Damen and the south branch of the Chicago River, Chicago, Illinois.
- Microcrystalline Calcite, called "Micrite," formed of fine-grained calcite crystals
- Sparry Calcite, called "Sparite" and containing coarse-grained calcite crystals.
- Types of Limestone
- Coquina - made of broken shells
- Chalk - dead forams
- Diatomaceous earth or "fullers' earth" - dead diatoms
- Marl - gooey lime mud - lime ooze
- Fossiliferous limestone - a catch-all phrase to denote dead stuff, can be divided into subcategories "crinoidal limestone," "bryozoal limestone," and so on if one organism out numbers the rest substantially.
- Lithographic limestone - former marl of a shallow lake or basin, very good for preserving fossils, ex. Archeopteryx and other dinosaur era beasts.
- Lightly metamorphosed limestone may preserve fossils (ex. Pittsfield Building, Chicago) or they may be chemically or pressure changed beyond all recognition (Connemarah Marble, Ireland).
- Carbonate Reactions
- Water falling through our atmosphere is in the presence of CO2 and reacts with it to form carbonic acid (H20 + CO2 === H2CO3). Therefore, all rain is slightly acid. Marble tombstones and limestone buildings in urban areas are dissolving due to more potent acids produced by the burning of fossil fuels (H2SO4 and others) as well as by the increase in carbonic acid due to rising CO2 levels in the atmosphere.
- Plants and other creatures are photosynthetic, taking up water and carbon dioxide and releasing carbohydrates and oxygen during the daylight. (6H2) + 6CO2 === C6H12O6 + 6O2) In the dark, the plants take in O2 and release CO2.
- Some plants and animals taken in CO2 and produce CaCO3 shells, now mostly aragonite shells. If the dead material does not fall through the CCD, it may be preserved as a fossil; but if it falls through the CCD, it "melts" and forms crystaline and cryptocrystalline calcites at depth.
- Recall saturation of liquids by compounds results in precipitation of those compounds as solids. Some limestones and marls may form by spontaneous precipitation from oversaturated sea water.
- Dolomite Textures
Total confusion abounds here, with some geologists following Folk's classification of the carbonates offered above under "Limestone Textures." Others, prefer to classify as on the Fence Diagram. A third system exists based on cements and the amount of parts and pieces of various creatures. Some prefer to name this "The Dolomite Problem."
Other sedimentary rocks
- Conglomerates are divided into two major classes
- Clast supported conglomerates, where gravel-size grains touch each other or
- Matrix supported conglomerates, also called "diamictites" where gravel-sized grains are separated by smaller grains
Either class can be:
- oligomict conglomerate, containing a single type of clast
- polymict conglomerate, containing many types of clasts or
- petromict conglomerate, containing mostly metamorphic clasts
- Evaporites remain when original water has left by evaporation.
- Marine forms:
- Halite, ex. Great Salt Lake and Bonneville Salt Flats
- Anhydrite/gypsum/alabaster, ex. Arkansas Gypsum mine. Anhydrites can form in:
- Nodules in clayey sediments, displace other materials and are found forming in sabkha environments
- Laminated anhydrites where each lamina is less than 10 mm are believed to represent annual varves and show changes in water chemistry and temperature. Summer erosion/deposition yields light colored varves while winter erosion and deposition is responsible for the dark colored varves.
- Freshwater forms
- Nahcolite, Sodium Bicarbonate - NaHCO3
- Trona, Hydrated Sodium Bicarbonate Carbonate - Na3(HCO3)(CO3) + 2H2O
- Borax, Hydrated sodium borate - Na2B4O7 + 10H2O
- Epsomite, "Epsom Salts," Hydrated Magnesium Sulfate - MgSO4 + 7H2O
- Selenite, Satin Spar, Alabaster and Gypsum, Hydrated Calcium Sulfate - CaSO4+2(H2O)
- Chert - Si02 The solubility of silica changes in the presence of various organisms.
Some cherts form from silica rich layers laid down by organisms including:
- Sponges which use silica to build spicules. They are conservative, and haven't changed much since the Cambrian.
- Algae: blue-green, red, brown and green algae.
- Grasses use silica in structure to deter herbivory and first found in Cretaceous age rocks.
- Diatoms are unicellular aquatic algae, first found in Cretaceous age rocks. Opals are gemstone derived from diatomaceous ooze.
- Radiolarians are associated with oceanic deposits and are first found in Cambrian age sediments. They are still found in the oceans today and tend to survive diagenesis better than diatoms.
Chert is found as:
- original structures including:
- nodules, like in White Cliffs of Dover, England
- layers, like Flint Hills, Kansas
- diagenetic structures including:
- silica replacements, like the silicified corals, "petrified wood" and other fossils
- geodes, sometimes accompanied by calcite and other minerals.
- banded with iron including iron hematite, such as found in upper Michigan. The Banded Iron Formations are the source of much industrial iron. The BIF is reduced to the mineral Taconite and rounded for shipping. This is the source of the "iron balls" so often found on Lake Michigan beaches; they rolled off an ore ship.
- Iron rocks form in oxic and anoxic environments
- Bog-iron, found in fresh water lakes probably as a result of hydrogen sulfide bacterial anaerobic decomposition
- Laterites form below rain forest soils. They need an iron source to form an iron caliche (iron calcite) or a siderite layer such as is found at Ankor Wat in Cambodia where the indigenous people built a temple complex from dried Laterite. It is so extensive that it can be seen by satellites. Some other laterites include aluminum ore "bauxite" which forms in areas with a high aluminum source.
- Placer iron forms with other placer deposits on the point bars of
streams and in the intertidal areas of beaches as seen in the "black sand" of Lake Michigan.
- Framboidal iron is common in the St. Peter S.S. at Buffalo Rock State Park and in the same layer down in the Shawnee Hills. It seems to have been an iron rich groundwater percolating though the sandstone which formed these 3-dimensional structures of iron stone. The iron may have concentrated along an ionic gradient as happened in the Red River Gorge in Kentucky.
- Metal ooids like those found in the Neda Oolite which form layer upon layer as they roll back and forth in a warm shallow sea (as described by Dr. Robert Doehler).
- Siderite nodules like those in which Mazon Creek Fossils are found form when a iron, calcite and clay were all available during a bad day in the Pennsylvanian. A bunch of plants and animals died together and were buried by a fine silt. The positive nature of the dead material attracted the negatively charged clay particles; the iron and the calcite provided the cement. Waving a UV light over Mazon concretions often reveals soft structure details due to extra calcite in these areas.
- Pyrite and marcasite form from iron sulfide (FeS2) as
- stars like those found in Sparta, Illinois, where they form on top of coal layers and below the next stratum.
- crystalline pyrites are cubic and are often called "fools' gold"
- crystalline marcasites are not cubic and tend to be silvery
- pyritized fossils occur when the original material has been replaced by pyrite. They are common in Ohio State, but occur in localized areas in Illinois such as along the LaSalle Anticline near Lowell.
- pyrite crystals growing out of strip mine piles and other areas of high sulfide can be found at Buffalo Rock State Park and other former strip mines.
Phosphates are forming now in areas of upwelling of rich, bottom currents along the Pacific margin of North and South America, near California, Chile and Peru. See figure 8.21.We use modern phosphates as fertilizer. When preserved as rocks, they occur as
- layers, either massive or bedded
- as bioclastics including guano from birds or bats
- Carbonaceous sedimentary rocks
All carbonaceous sediments and sedimentary rocks contain carbon and are sometimes called "organic" rocks in other books.
- Humus from soil organics
- Peat from organics deposited in fresh water
- Sapropel from organic debris in water
- Kerogen from altered organics
- Oil shale
- aromatic hydrocarbons
- paraffin hydrocarbons
- Black shale when mud is more common than organics
- Coal from shallow, near marine environments, classified as organics being more common than mud and derived from plant material. Four types of coal: Anthracite, Bituminous, Subbituminous, and Lignite.
The Significance of the Mineral Composition in Clastic Rocks
Minerals show where sediments came from:
- Continental block: felsic outnumber mafic
- Magmatic arc, subduction zone ex. Caribbean Islands: volcaniclastic outnumber all others and plagioclase feldspars are more common than alkali feldspars.
- Recycled orogen in a continent-to-continent collision, ex. Himalaya Mountains: metamorphic rock fragments are most common.
- Rift zones
- Mid-ocean ridges, mafic more common than felsic
- Continental rifts, mafic and felsic more equal
- Hot spots
- In ocean, ex. Hawai'i: mafic
- Under continents, ex. Yellowstone: early mafics overlain by intermediates and felsics.