Processes of Sedimentary Rock Formation

Sedimentary rocks form from materials that first existed in older rocks. Whatever those earlier rocks were, and we can often determine a lot about

them from what’s in the sediment, are called the “source rocks” of the sediment.

This short presentation serves to introduce the various steps in taking material

from a parent and making a sedimentary rock of it. Those steps are:

1) Weathering

2) Erosion

3) Transport

4) Deposition

5) Lithification

1) Weathering

You should recognize the igneous rock on the left. It is one possible source rock for the rock on the right, and the only possible igneous parent. The pink in the two rocks results from different minerals. On the left the obvious large pink crystals are something you have already seen. On the right the pink is more diffuse: not large crystals but microscopic ones that coat the grains of sand in the rock.

K Feldspar (pink) Na Plagioclase (white) Biotite (black) Quartz (light gray)

The source rock is granite because of the phaneritic texture and the minerals present, all of which are low temperature igneous minerals according to Bowen’s reaction series.

The obvious crystals in the sandstone are all quartz. The pink color results from a thin rind of the mineral hematite on each of the sand-sized quartz grains. Hematite is chemically the same as rust – iron oxide.

This rock is made of sand-sized grains tightly held together. The name of such a rock is sandstone. “Sand” is the name of the material if it is not tightly held together in rock. “Gravel” comprises bigger grains than sand. “Mud” comprises smaller grains.

In making the sandstone from the granite a couple of obvious changes must occur. First, everything but the quartz must be separated out and have a different fate than the quartz. Second, the grain size of the quartz must be reduced from around 6mm (the coin is 2.5cm) to around 1/2mm. (NOTE: There are source rocks with smaller quartz crystals, and gravels can have bigger quartz grains than this sand, but reduction in size is the rule and not the exception in going from source to sediment. Source crystal sizes are virtually never preserved.)

The alterations to the source material that happen in the process of forming sediment are collectively called weathering. It is called that because exposure to weather at or near the Earth’s surface is required to accomplish any weathering. There are numerous ways that weathering can occur, but they fall naturally into one of two categories. We will not explore detailed ways that each of these two can be accomplished but if you are interested you can easily find more material on the topic. First, any change that breaks the source rock into smaller pieces without altering the minerals present, or the chemistry of the minerals present, is called mechanical weathering or physical weathering. Second, any change that does affect the mineral or chemical content of the source is called chemical weathering. The results of these two things are very different. In the first case you end up with pieces of the source rock and/or of its constituent minerals. In the second you end up destroying minerals and creating new ones from the residue. Both types of weathering generally occur hand-in-hand on a rock but certain conditions favor one and different conditions favor the other. Usually one or the other predominates in a particular place. Chemical processes all require water and relatively warm temperatures so chemical weathering tends to predominate where there is plenty of rain and warmth, at least in summer. Mechanical weathering predominates in very cold or very dry climates – glaciers and deserts, for example.

There are some sandstones (less than ~10% of all we know of) that do not show the degree of mineral loss that the quartz sandstone in the previous examples shows. What might allow this?

Arkose has quartz and feldspar.

Lithic sandstone has quartz, feldspar, and other minerals in it.

Qtz

Qtz

K spar

K spar

Plag

Biotite

If mechanical weathering is strongly predominant in the parent’s region then all the minerals might survive and produce a lithic sandstone. (Some of the minerals in lithic sandstone are highly susceptible to chemical weathering.) If mechanical weathering predominates but there is some chemical weathering then maybe only a couple of the minerals survive and arkose results. (Quartz and K feldspar resist chemical weathering better than the others.) For a quartz sandstone to result, chemical weathering has to predominate. (Quartz is the only one of the common minerals in granite that resists chemical weathering.) Because quartz sandstone is far more abundant in the world than the other types it stands to reason that chemical weathering is more common than purely mechanical weathering.

So what happens to the other minerals in granite if quartz is the only one that survives chemical weathering?

The other minerals all contain aluminum in their structure. They are aluminosilicates. In

part they all behave the same way, but if there is iron as well then something else occurs too.

The feldspars do not contain iron. They are weathered by a process that strips ions from their structure leaving behind only aluminum and silicon oxides/hydroxides – clay minerals. Clay minerals are tiny sheet silicates (like mica). Because feldspars (and other aluminosilicates) are the predominant minerals in crustal rocks that serve as parents for sediment, clay is the most common mineral in sediments and sedimentary rocks. The ions that are removed are the cations (K+, Na+, and perhaps a little Ca++) and SiO4

-4 as Si-O tetrahedra. These ions are dissolved in the water that does the weathering and removed to wherever that water goes next. The biotite is a ferromagnesian aluminosilicate – it does contain iron. The iron ions are oxidized (rusted, essentially) to make the stable iron oxide hematite. The aluminosilicate part of the mineral becomes clay. Si-O tetrahedra, and any K ions that might have been present are removed in solution. The quartz survives. (Unless the weathering is ridiculously severe.)

CHEMICAL SEDIMENT

Dissolved ions from chemical weathering (feldspars, biotite)

DETRITAL SEDIMENT

Clay minerals (feldspars, biotite)

Hematite (biotite)

Resistant minerals (quartz

2) Erosion &

3) Transport

Erosion and transport are conceptually related but they should be treated as separate things. Erosion is the initial movement that begins the transport

process, but that transport process can be a very lengthy and complex thing. When we think about erosion we are generally interested in the effects on

what’s left behind – the eroded landscape, but sedimentologists are interested in it as the start of the process or transport.

Both erosion and transport are accomplished by one (or more, in the case of

transport) of four things:

1) Gravity, acting alone,

2) Ice (glaciers), moving in response to gravity,

3) Wind, moving in response to gravity, or

4) Water, moving in response to gravity, wind, or tidal forces.

Weathered material that has not been moved from the site of weathering is called “regolith”. As soon as it begins moving (at the moment it is eroded) we begin to call it sediment. That sediment is of two main types. Detrital sediment is particles of weathered rock – mud, sand, or gravel. It includes the clay minerals and hematite formed by chemical weathering, the particles that resist chemical weathering, like quartz, and the residue of mechanical weathering. Once eroded these materials can be transported by any of the transport mechanisms – gravity, ice, water, or wind or by some combination of them. Chemical sediment is the dissolved ions produced by chemical weathering. These can include essentially any ion from any weatherable mineral, but K, Na, Ca, Mg and SiO4 are the most common ones. Because they are dissolved they can only be moved by water.

CHEMICAL SEDIMENT

Dissolved ions from chemical weathering (feldspars, biotite)

DETRITAL SEDIMENT

Clay minerals (feldspars, biotite)

Hematite (biotite)

Resistant minerals (quartz

An earlier slide gives us a good idea about the types of materials that can be eroded and transported away from a granite source. Let’s look at it again.

Remember that this rock is sandstone with two minerals in it. Assuming a granite source rock, 1) Where did the quartz come

from?

2) Where did the hematite come from?

CHEMICAL SEDIMENT

Dissolved ions from chemical weathering (feldspars, biotite)

DETRITAL SEDIMENT

Clay minerals (feldspars, biotite)

Hematite (biotite)

Resistant minerals (quartz)

An earlier slide gives us a good idea about the types of materials that can be eroded and transported away from a granite source. Let’s look at it again. In this sandstone we see only quartz (a resistant mineral) and hematite (an oxidation product of a ferromagnesian mineral). What happened to the other weathered materials?

???

4) Deposition

At some point whatever is transporting sedimentary material loses the ability to transport it any farther. A falling or rolling stone reaches the bottom of a hill. Ice melts from the downhill end of a glacier. A mountain stream flows out onto a flatland and slows down. The wind quits blowing. Additionally, sediment may build up and cover older sediment making it impossible to move the older sediment even if the strength of the transport system returns. All of these things mean that the sediment load of the transport system must be dropped or deposited.

The sandstone in our examples so far was deposited on a beach in what is now the western Appalachian Mountains (the Valley and Ridge) during the Silurian period. The sand washed off an older mountain chain in rivers and travelled in those rivers to the shore. There the sand was moved along the shore by waves. Eventually enough sand was delivered to the shore by the streams that the waves abandoned this sand in order to move other sand, this was covered up, and eventually it was hardened into this rock.

The streams that were able to move sand all the way to the coast were not strong enough to move gravel that far. This rock is mostly quartz gravel with quartz sand grains between the pebbles. Hematite (and a similar yellow iron oxide) are obvious as well. This rock was deposited on the bed of a fast-flowing stream. (Probably not a Silurian Stream – I’m not sure where this rock is from.) The vigorous motion has rounded the originally sharp edges of the hard quartz pebbles because of their impacts with each other. Gravels like this are generally abandoned (deposited) far upstream of where finer sands accumulate. The stream sorts the grains by size, depositing them in different places.

Conglomerate – a detrital rock made of rounded pebbles.

If coarser detrital sediment (like gravel) is dropped earlier from a transport system than finer sediment (like sand) then you can probably guess that even finer sediment (like mud, including clay minerals) will be transported even farther. Mud is clay and silt combined. These fine materials are quite easy to transport and so do not wind up on beaches. Instead they tend to be carried out onto the continental shelf (or even beyond, to the deep ocean) and finally settle there. Rocks that are made of mud are called mudstone unless they display very thin bedding. In that case they are called shale. The difference is the degree to which burial pressure has compacted and re-aligned the clay minerals, which are like tiny mica flakes.

Mudstone has no thin layering. This one is made of the clay mineral kaolinite and is mined in Macon County near Andersonville.

Shale does have thin layering. This one is black because it contains a great deal of un-oxidized organic matter from the organisms (mostly plankton, not dinosaurs) that lived in the sea above where it was deposited. The rocks that are being “fracked” to produce natural gas are black shales and look like this.

CHEMICAL SEDIMENT

Dissolved ions from chemical weathering (feldspars, biotite)

DETRITAL SEDIMENT

Clay minerals (feldspars, biotite)

Hematite (biotite)

Resistant minerals (quartz)

???

The chemical sediment is, of course, not deposited in the same way that detrital sediment is. Instead of settling as particles these ions must recombine to form new minerals. They can only do this when the chemical environment is right. The energy that moves and deposits detrital sediment is not as important as the water chemistry that allows precipitation of the mew minerals. There are two ways that precipitation can occur: with and without the influence of organisms. Rocks made from biologically precipitated material are called “biochemical”. Rocks made from minerals that precipitate directly from the water are called simply “chemical” or “orthochemical”.

The most common biochemical rock is limestone. The color is variable, ranging from pure white to pink to black. What is usually evident (though only microscopically in most cases) is that the rock is a hardened accumulation of fossils. The large disk-shaped things in the rock on the left and the obvious shells in the rock on the right are all shells or other skeletal material of marine organisms. The mineral in limestone is calcite, which you have seen. Its formula is CaCO3. Most limestone is marine because that is where it is easiest to get Ca ions to make the calcite. The carbonate is easy – it forms from atmospheric CO2 dissolved in the water.

This is another common biochemical “rock”. Like obsidian it is not truly a rock because it contains no minerals. All that is here is organic carbon. Microscopic slides generally reveal the organic material to be mostly or entirely plant material and this rock typically occurs with shales that are loaded with plant fossils. It there is mud coming in the plants are preserved as fossils. If there is no mud then the plants are preserved as coal.

Anthracite coal is a higher temperature more compressed version of coal that is often treated as a metamorphic rock. I don’t agree because the rocks found with it are typical sedimentary rocks (shale and sandstone).

Bituminous coal is the normal, clearly sedimentary version of the “rock”. The fossils are generally much more easily found in this lower T&P version.

In some cases orthochemical rocks can precipitate directly from water as brand new material where there was previously none. This is most common in places where the concentration of ions can be driven high enough to force precipitation. As a general rule ions cannot build up to this level so the concentration is generally accomplished by decreasing the water in the solution, not increasing the number of ions. Think about the beach for a minute. If you’ve ever been in the water at a sea beach you have almost certainly accidentally gotten some in your mouth. What’s in the water? What you taste is, of course, dissolved Na and Cl ions – salt. There are other “salts” as well – other dissolved ions, including Ca, Mg, K, SiO4, etc., etc. Indeed, all the ions that weather out of source rocks of any kind are found in seawater in varying amounts. The Na and Cl predominate so they are most noticeable in the taste. Why do you suppose the “Great Salt Lake” has that name? Desert lakes like that also contain a lot of dissolved ions, often far more than seawater does.

This is rock salt. It is from a sedimentary deposit that accumulated in a coastal bay with little connection to the open sea. Seawater could get into the bay through narrow inlets, but it was difficult to get it back out. Once in the bay the desert climate that prevailed during deposition evaporated enough of the water (only pure water evaporates) leaving a high concentration of Na and Cl ions in the water to the point of precipitation. This happens today in shallow bays around the Persian Gulf and Arabian Sea, and seasonally in Laguna Madre on the Texas coast.

Other evaporite minerals include gypsum (plaster of Paris), borax, Epsom salts, and sylvite (0 sodium salt). The evaporation required to form these minerals can occur in coastal bays or desert lakes.

Other orthochemical rocks form as the water they are in interacts with some pre-existing rock and replaces its original minerals with new ones. The rock below is called chert. It is made of microcrystalline quartz. (It has conchoidal fracture and scratches glass, etc.) It is very common in this part of the state where it is often found as a replacement of limestone. The original calcite shells and surrounding matter are often still obvious but they are now quartz instead of calcite. Native Americans found this hard rock ideal for making spear and bird points, scrapers, and so on. Non-native Americans found it useful to replace the flints in their flintlock rifles. Indeed, flint is simply the name for a black chert, common in England, where “flintlock” “flints” were named well before they were brought to America. None of the chert in this area is black, so it is technically not flint. Settlers in the region did not draw that distinction though when they named our river the “Flint River”.