Lab 5: Sedimentary Rocks - Sediment

 

Sediment is loose material found at the Earth's surface, the product of weathering of rocks. The character of sediment, the grain size, the minerals present, and the way the grains are layered reflects the source of the material. Any rock type will be weathered by rain, wind, ice freezing and melting (or grinding, in the case of glaciers), and other manners of physical and chemical weathering. Granite is a good rock to use as an example.

 

Granite weathering on a mountainside will yield sedimentary particles of minerals contained in the rock, as large pieces that physically break into smaller pieces (physical weathering). A block of typical granite would mechanically break down into rock fragments, as gravity brings material down slope, sometimes in dramatic fashion, but usually as a steady, slow crumbling. Eventually the following single minerals will be set free from a typical granite:

 

  • a sizable amount of plagioclase, orthoclase, and quartz
  •  
  • and a little bit of biotite, or hornblende.

 

Physical weathering is always at play, for all rock fragments and for all individual mineral particles. The feldspar (plagioclase and orthoclase), however, is especially susceptible to chemical modification, as water runs across the surface of particles and gets down into the cracks in the minerals. Feldspar particles don't just break into smaller and smaller pieces by physical breakage, they undergo this chemical breakdown, changing into minute particles of clay minerals. Water molecules, with their positive and negative ends, tug on each other and at places where they have contact with the surface of minerals, helping to disassemble the mineral structure. What you know as mud is largely made up of clay minerals, and since feldspars are very common in igneous rocks, much of this fine-grained sedimentary material can be traced back to granite and similar rocks. Minerals, such as biotite, hornblende, and other silicate minerals also chemically weather to clay minerals. Quartz, on the other hand, is just silicon and oxygen bonded together with strong covalent bonds, and there aren't so many locations within the crystal structure for chemical breakdown to work. So, quartz just physically weathers to smaller and smaller pieces of quartz.

 

We could tell a similar story for any rock. List the mineral constituents in the rock and consider the particles resulting from physical and chemical weathering. Quartz will stand out for its simple chemical structure, which combined with its hardness, results in quartz persisting as you trace weathering from source rock to sediment. Clay minerals, primarily resulting from weathering of feldspars, forms the bulk of sediment, because so many source minerals, other than quartz, are found across the landscape.

 

When you visit a riverbed, you will find sand-size and smaller particles of quartz and much more volume of finer, clay mineral particles. The clay mineral particles form the mud dominating in quiet water areas of the river system away from the channel. Depending on the location of the river, you may also see gravel size particles.

 

When you visit a beach, you see sand-size quartz particles dominating the beach itself, where wave action concentrates these larger particles. Clay mineral particles are dominant in volume though, and are found in offshore muds and marshes and bay systems.

 

Quartz isn't the only mineral to be found in abundance with clay mineral particles, of course, because there are several other hard minerals resistant to chemical weathering. But these other hard minerals aren't very common in rocks, they are found as low percentage accessory minerals with quartz.

 

There are several other ways that sediment can form:

 

Evaporites

 

When water evaporates in shallow lakes and coastal lagoons, it can become highly concentrated in salts (the salinity goes up). When the concentration is high enough, calcium binds with the sulfate ion (SO 4 ) and water to form gypsum, growing as crystals directly in the water, where the bottom offers a substrate for minerals to grow. Salt (NaCl) will precipitate in this manner when the water becomes even more saline, followed by potassium salt (KCl) and other evaporite minerals.

 

Skeletal Debris

 

Shells of clams and snails are familiar sights on beaches, and in places like the Bahamas or on tropical reefs, they, along with the skeletal remains of other organisms, such as corals, dominate the sediment. The mineral composition of most shells and skeletons is calcite (CaCO 3 ), but silica is also found as the skeletal mineral in some microscopic organisms living near the ocean surface. Sediment derived from shells and skeletons in the nearshore environment usually get broken up into pieces, even very tiny ones. Sediment far out away from land, accumulating in deep water, tends to be composed of the tiny skeletal debris from siliceous and calcareous skeletons of microscopic organisms.

 

Grain Size (or particle size)

 

Geologists follow the categorization of grain size made by C. K. Wentworth in 1922. The Wentworth grain size classes are probably familiar words to you:

 

gravel

includes grains larger than 2 mm diameter. In order of descending size, gravel includes:

 

boulders

difficult or impossible to pick up

 

cobbles

around fist-size

 

pebbles

around pea size

 

granules

greater than 2 mm, so about BB size

 

sand

includes grains from 1/16 mm to 2 mm diameter. This is what you know as sand that you played with in your sandbox as a kid, and that you see on river sand bars and at the beach. Sand is mostly quartz, with accessory minerals. But

 

silt

includes grains from 1/256 mm to 1/16 mm diameter. This grain size class is probably unfamiliar. The grains are so tiny that they are difficult to see, but if you get silt in you mouth, as happens in a dust storm, you will notice the grittiness between your teeth, if the material is made of quartz, which it often is.

 

clay

includes grains less than 1/256 mm diameter

 

Sorting

 

The mixture of grain sizes in sediment is called sorting. A well-sorted sediment has grains all within a narrow size range. A poorly-sorted sediment has large grains mixed in with small grains. Sorting reflects the kind of wave or wind action present at the time of deposition.

 

The best sorting happens on beaches, because steady wave action keeps only the sand-size grains on and near the beach, and finer material is either washed offshore (bottom muds) or it is washed into bay systems and marshes.

 

Wind-blown sediment tends to show very good sorting also, because wind is powerful enough to move certain grain sizes, and winds tend to be fairly steady in desert environments where there are sand dunes.

 

River sand bars also have fairly well sorted sediment. In a river system there is good current to separate the coarse from finer sediment, but it isn't as steady as the wave action at a beach. Sediment sorting varies in a predictable way, as you sample across a river channel and onto a sandbar.

 

The poorest sorting is found in glacial deposits called till, which contains a mixture of large boulder-size grains, along with other grain sizes, all the way down to fine powdery material. Glaciers are thick ice accumulations formed from snowfall high in mountains or as gigantic ice sheets covering the landscape (Greenland, Antarctica today, but more coverage at previous times in the Ice Age). It doesn't matter the size of the glacier, they all tend to scratch and scour the rock they move across (slowly, but steadily), and larger rock fragments can be incorporated into the ice as the force of ice movement "plucks" rocks from the ice/rock interface. Larger rock fragments can also tumble down onto the surface of glaciers from adjacent mountainsides. This results in glaciers being mostly ice, but with large and small grains of sediment mixed in. When the ice melts, sedimentary grains of all sizes are released from the ice and form a poorly sorted deposit called till.

 

We mainly speak of sorting in the materials mentioned above, but the same principle can be applied to any sediment. For instance, consider two places along a coastline, one where wave action is high and shells are broken into small fragments that have been sorted by size, and in another area there isn't as much wave action, and the shell fragments are larger, and not as well sorted.

 

Rounding and Sphericity

 

The more sedimentary grains have been carried by water or wind, the more the edges of the grains get knocked off by collision with other grains. This results in rounding. Rounding refers to the fact that the edges have been rounded off, not that the shape of the particle is necessarily "round" or spherical. So, you could have a fairly triangular looking rock fragment, but if there aren't any sharp edges remaining, we can say that the particle has been rounded.

 

Sphericity does refer to the degree to which a particle's shape approaches that of a perfect sphere. Wind-blown sediment, and that found on a beach, in both cases mostly quartz, tends to be more spherical. As more and more transport by water and wind happens, the more the edges of grains get knocked down and rounded off, and the more the grains take on a spherical shape.

 

So, the more rounded, and the more spherical, the grains, the more transportation of the sediment has happened.

 

Again, most of the time we look at rounding and sphericity in quartz-dominated sediment, but the same principle applies to any sedimentary grains, even shells found on a beach. Usually shells break up into angular fragments, but there are situations in the oceanic environment where these concepts apply to deposits of shells and other skeletons of organisms. These concepts are especially applicable to the sediment found in different parts of a reef.

 

Clastic vs. Nonclastic

 

You will see the term clastic applied to sediment that consists of clasts. Clasts are just particles, or grains. Sediment consisting of rock fragments, sand grains, and silt and clay grains is by nature clastic. But other types of sediment can be clastic as well. Fossiliferous limestone is just an accumulation of shells and shell fragments. Reef rocks tend to have much associated crumbed and broken reef material, and so on.

 

Nonclastic sediment tends not to have many grains, and looks more solid, as a mass of intergrown crystals, or material so tiny that it looks smooth. Chert, rock gypsum, and rock salt are good examples that show a nonclastic texture (you'll learn about these in other sections).

 

Sediment of the Seafloor

 

Sediment found on the sea bottom around continents and islands is a mixture of the types of sediment described above, depending on the nature of the bedrock nearby, the size of rivers and the areas drained, the temperature of the water and so on. For instance, a nearshore sea bottom off of Alaska would contain sediment brought in by rivers and by glaciers, but it would not contain a large amount of shells and other material derived from the breakdown of skeletons of organisms. In the central Gulf of Mexico, near the mouth of the Mississippi River, the sea bottom is dominated by silt- and clay-size detrital sediment (mud), with shell debris thrown into the mix, but not dominant. At the tip of Florida, well away from the Mississippi River delta, the water is clear and warm, and the sediment is dominated by fine-grained shelly material from the organisms that are so common there. Around oceanic islands that lie near the equator, debris eroded from reefs, and the calcite skeletons of the reef organisms in place, dominate, as there are no large land areas nearby, and no source of material brought in by rivers.

 

Out in the deep ocean the type of sediment on the seafloor depends on the location. Along the equator and in polar areas there are belts of siliceous ooze (thick mud) composed of siliceous skeletons of microscopic organisms. Across the breadth of the ocean floor, calcareous ooze, composed of calcareous skeletons of microscopic organisms, dominates. Far out in the centers of the ocean basins, in areas where there isn't much surface life, the only thing reaching the bottom, very slowly, is very fine, clay-size material (dust) blown by the wind. In some places on the deep sea bottom, minerals grow in the cold, high pressure water to form hydrogenous sediment, including manganese nodules.