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Geology - Sedimentary Petrology


Author:  Scott Robertson

Geology 101 - Gale Martin - Class Notes

Any rock that is exposed to the earth's surface will result in the production of sediments. Given sufficient time, these sediments will lithify and become sedimentary rock. There are four steps in the formation of sedimentary rock: weathering, erosion, deposition and lithification. Indirectly these processes are driven by plate tectonics: Masses of rock are deformed into high mountains. Their instability with surface temperatures breaks them down into new minerals that are stable on the earth's surface. Gravity will move the sediment to low lying regions where it accumulates. As more sediment piles up, weight from burial will lithify underlying sediment into rock.

Weathering

All rocks weather. If a rock is moved from where it's formed the minerals that it contains will become unstable and alter. Weathering is alteration of minerals by chemical or physical means when they are exposed to surface conditions. Weathering is divided into two groups: mechanical weathering and chemical weathering. The two sets of processes work together and are influenced by such factors as climate, rock type, gradient of the slope and amount of time exposed.

Mechanical Weathering

Physical conditions that cause a rock to be broken into smaller pieces without altering the original mineral composition are referred to as mechanical weathering or disintegration. Several processes can result in disintegration of rock including frost wedging, thermal expansion, biological activity and exfoliation.

Frost wedging is caused the expansion of ice as it freezes. Water easily penetrates into small cracks in rock outcrops. If the temperature drops below freezing at night, the expanding ice will "wedge" the rock apart, creating a larger opening in the crack. When the ice melts, more water seeps into the crack. In areas where there is sufficient water and cycles of freezing/thawing (ex.: temperate climates, "up north") this processes will occur on a daily cycle. The result is that rock can easily be "wedged" from the side of a rock outcrop. (Notice how "potholes" in roads are more abundant during some seasons (winter in this climate; spring/fall in temperate climates).

In climates where daily fluctuations in temperatures are extreme (ex.: deserts), the effects of thermal expansion are common. When exposed to heat, every mineral will expand by different amounts. (Think of black cars vs. white cars in the parking lot.) Rocks are often mixtures of several minerals. (Granite, for example, contains quartz, feldspars and mafic minerals.) When exposed to heat, the rock is stressed by the various crystals pushing at each other as they expand. Given enough time, the rock weakens and separates into individual mineral fragments.

Exfoliation processes are common in mountainous regions. Here the rock is put under large pressures during orogenic events and deep burial. When the rock is exposed through erosional processes, the pressure is released and the rock expands; fracturing and "popping" as the overlying weight is removed. This produces a "stepped" appearance as the rock "peals off" like layers in an onion (Ex.: Yosemite, CA).

Plants and animals also produce mechanical weathering in rocks. Animals grind and abrade rock as they walk across the surface. Paths are created in areas of mass migrations from multiple animals wearing down the rock. (Every major campus that has multiple buildings has a "diag" between buildings. This originates by "migrations" of students between classes! Think of the wagon trains and the still visible Trails out west!) Plant roots wedge rocks apart as they enlarge during plant growth. (Ex.: buckled sidewalks because of tree roots.)

These processes, and others, act to break rock into small pieces, exposing more surface to the external environment. This "increase in surface area" allows weathering to proceed at a faster rate and exposes the minerals inside the rock to the chemical processes that result in alteration of the mineral compositions.

Chemical Weathering

Once exposed to surface environments, minerals become unstable and "decompose". (Note: Mineral stability of the common igneous silicates is the exact opposite of Bowen's Reaction Series. High temperature minerals (Olivine, pyroxene, Ca-plagioclase) are unstable at the earth's surface. Low temperature minerals (Feldspars and muscovite) are more stable at the earth's surface. Quartz is relatively stable.) Chemical reactions occur that break down the unstable minerals and release their chemical elements. The reactions are known as chemical weathering, and are commonly driven by the presence of water. Ions that are released recombine to form new stable minerals. Three common processes are involved: dissolution, hydrolysis and oxidation.

When a mineral breaks down into the original ions and remains "in solution" (i.e. in a water base) the process is called dissolution. Minerals belonging to the chemical groups halides, sulfates and carbonates often dissolve easily in water or mild acids. (Water and carbon dioxide, from air or soils, will naturally combine to form a mild acid.)

Ex.: Halite

  • NaCl + H2O=Na+ + Cl- + H2O
Ex.: Calcite
  • CaCO3 + H2CO3=Ca +2 + ( 2 CO2 + 2 H2O )

Hydrolysis occurs when the water that is present during the chemical reaction, bonds with the newly forming mineral and become part of the crystal structure. Feldspars, common silicate minerals, weather through hydrolysis.

Ex.: K-spar

  • KAlSi3O8 + acid + H2O=Al2Si2O5(OH)4 + H4SiO4 + K+

Notice how the "K" (Na or Ca, depending on the feldspar) is released into solution. The remaining elements recombine into a "clay" (with water bonded into the crystal) and "silica" (H4SiO4). The silica can crystallize as cryptocrystalline quartz, known as chert, or form cement in sedimentary rocks.

Minerals that are mafic or contain metals in their composition (mafic silicates, elementals or sulfides) will commonly weather through the process of oxidation. When the metals (iron, especially) are released by weathering, they recombine with oxygen in the water/air. This produces an oxide. In the case of iron, the common term is "rust".

Ex.: Olivine

  • Fe2SiO4 + acid=Fe+2 + silica; then

    4 Fe+2 + 3 O2=2 Fe2O3

The material that remains after both mechanical and chemical weathering is referred to as sediment. It consists of rock fragments, clays, oxides, ions in solution and quartz. This material accumulates to form soil. There are many types of soil, each characteristic of the climate and type of rock it developed from.

Erosion

After weathering, the sediment is transported to a new region through a process known as erosion. Here the material is picked up by water, wind, ice, waves or any "erosional agent" and it is moved to a new location. The sediment can be carried in solution or as solid grains (suspended, rolled or bounced along). The shape of the grains is altered as it's transported: rounding, "frosting" and sorting occur. Each type of erosional agent will influence the way a grain will look, but the further a grain is moved, the rounder and smaller it becomes.

Deposition

Deposition of the sediment will occur when the erosional agent loses energy (wind stops, rivers slow, ice melts, water evaporates, etc.). The sediment load becomes too great and the transporting agent "drops" it. (Deposition includes both "settling" out of loose sediment and crystallization of minerals that become to concentrated in a solution to stay dissolved.) Many of the transporting agents produce distinctive "patterns" as sediment settles and deposits. These characteristics are known as sedimentary structures and can be used to determine the depositional environment of ancient sedimentary rocks.

Examples of Sedimentary Structures

Strata or bedding - Most sedimentary rocks are deposited in horizontal layers known as strata or beds. Each layer is believed to be one depositional event (a flood, a windy day, etc.); no time frame is inferred for a single layer.

Lenticular beds - Filled abandoned river channels are commonly preserved in the rock record. After burial they form "lens" shaped beds that "pinch-out" on both sides where the river banks end.

Crossbedding - With in some layers of sediment there are inclined surfaces. They are produced by large scale currents (wind and water) that move large amounts of sediment as "piles" (ex.: dunes). The inclined surface is produced when sediment is pushed along the top of the "pile" and cascades down the front (slip-face). As the "pile" shifts, the inclined surface is buried and migrates in the direction of the current. Smaller ripples (undulating waves of sediment) are usually present along the surface of any sediment moved within a current.

Graded Bedding - Sediment that is being deposited in a single layer may become sorted by density or size. Each layer will contain a gradational change of sizes/densities with the coarsest or heaviest material at the base; becoming finer as the top of the bed is reached.

Mudcracks - In arid regions, clays dry to form layers with mudcracks. The clay shrinks and cracks, curling up into "chips". If wind blown sediment fills the cracks, the hexagonal matrix of "chips" will be preserved. Occasionally, raindrop impressions (small, round impact depressions) will be preserved on the surface of the mudcracks.

Fossils are commonly used to determine sedimentary depositional environments. Remains of plants and animals are usually preserved near the environment in which they lived and can be of great assistance in understanding the climates in areas of deposition.

Lithification

Sediment will continue to accumulate at the site of deposition - each depositional event producing an additional layer piled on top of the previous event. As the sediment becomes buried, lithification occurs. Overlying weight results in compaction of the layers; driving out air and water from between the grains. Cementation results when the ions in the water precipitate minerals that cement the loose grains into solid rock.

Classification of Sedimentary Rock

Sedimentary rocks, like all rocks, are classified by their texture and compositions. Sedimentary textures (for this class) can be divided into three groups based on how they are formed. These include clastic (or detrital), chemical precipitates and biochemical (or bioclastic) textures.

Clastic Texture

Clastic or detrital textures include sedimentary rocks that are developed through the accumulation of loose sediment weathered from pre-existing rock. They are classified according to the grain sizes contained in the rock. (See text for Wentworth Scale)

Grain Size

Name of Sediment

Rock Name

> 256 to 2 mm "gravels" - Conglomerate (if round grains)
   
  • or Breccia (if angular grains)
1/16 - 2 mm sand - Sandstone
1/256 - 1/16 mm silt - Siltstone
< 1/256 mm clay - Shale (if fissile)

The sediments are commonly composed of rock fragments, quartz, and clays, cemented with silica, calcite and/or oxides. The rock names can be modified with compositions that give the rock distinctive characteristics. (Ex.: Hematitic, limonitic, fossiliferous and calcareous)

Chemical Precipitate Texture

Chemical precipitate textures are developed by the growth of minerals from solutions that become "saturated" with ions. The solution can no longer support that mineral in the liquid and solid crystals form (Ex.: evaporites). The crystals are randomly oriented and commonly remain microscopic in size but may grow larger given sufficient time. Chemical precipitates are commonly composed on one mineral and are classified as follows:

Composition

Rock Name

calcite limestone
dolomite dolomite
gypsum rock gypsum
halite rock salt
cryptocrystalline quartz chert

Biochemical Textures

Biochemical or bioclastic textures are also composed of minerals that grow from solutions but with one major difference: they are produced through the activity of plants or animals. These rocks are usually composed of fossils, either visible or microscopic, and are classified by the composition of the fossils and matrix. The most common types of biochemical rocks are:

Composition

Rock Name

calcite fossiliferous limestone
dolomite fossiliferous dolomite
plant fragments peat or coal

Further burial of sedimentary rock will cause the minerals in the rock to become unstable in character. This may result in recrystallization of the minerals and eventually lead to metamorphic environments.


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