Physical Geology

Sedimentary Rocks

Sedimentary Rocks: rocks composed of particles of weathered mineral matter that are deposited as layers in low-lying areas on the surface of the Earth.

The most distinctive feature of sedimentary rocks is their layered appearence that reflects how they formed.

An individual layer is called a stratum, multiple layers are called strata. The layered arrangement of sedimentary rocks is often called bedding, with each layer being a bed of rock..

Why study sedimentary rocks?

Igneous rocks give us information on the nature of the interior of the earth and the processes that form the crust, ocean floor, continents, etc.

Sedimentary rocks give us different information. Sedimentary rocks are formed primarily by processes that occur at the surface of the earth. The surface of the earth is varied, with many different environments which collect characteristic sediments and sediment layers. Thus for any particular layer of sedimentary rock, we can ask the question: What was its environment of deposition?

If we understand how sedimentary rocks form, we can look at a group of sedimentary rock layers and figure out what kinds of environments produced them. This is important because of the fact that the surface of the earth is constantly changing. We know what it looks like today because we can see it, and we know what it looked like back to a few thousand years ago because people observed and kept records. But the earth is 4.6 billion years old! How do we know anything about what the surface of the earth was like before people kept records, for most of its history? We know because of rock layers from past times that preserve information about the surface environments in which they formed. This includes any plants or animals that were trapped in the sediments to form fossils. Our whole knowledge of the history of life on earth, from the origin of life, to the age of dinosaurs, to the evolution of human beings, comes from layers of sedimentary rock.

Of course, there are the less esoteric reasons for studying sedimentary rocks. All of our economic deposits of fossil fuel occur in sedimentary rocks. Oil, coal, and natural gas are all formed from organic materials trapped in sedimentary rocks.

There are four general steps in the formation of sedimentary rocks.

Sediment production: there must be a source of sedimentary particles, the ‘raw material’ of sedimentary rock. For example, a body of granite on the side of a mountain will weather to form sand and clay sediments. These sediments do not stay on the side of a mountain, rather they become the victims of...

Transport: Sediments are moved by wind, water, and glaciers from the place where they form to another place, sometimes over distances of only a few feet, sometimes over thousands of miles. Sediments move, come to rest, move again, etc. until they reach a place where they can no longer be removed.

Transport media: The majority of sediments are probably carried by water in rivers and streams that drain eroding landscapes. However, a lot of smaller sediments are carried by wind, and in areas and climates where glaciers exist, ice can carry large quantities of sediment many miles. Additionally, independent of a transport medium, gravity can cause sediment to move.

So what happens to sediment as it is moved along? For one thing, it continues to mature, that is, to weather mechanically and chemically.

The larger particles that are rolling and saltating are constantly colliding with each other. This results in rounding. Similar to rounding is the frosting that occurs when sand is wind transported.

Sediments also can become sorted by size as they are transported. This occurs due to changes in the speed of flow and turbulence of the transporting medium.

Depostion: Deposition happens when the medium that is moving sediments stops carrying them. This usually occurs in some kind of basin. Typical examples of large scale depositional basins are lakes, valleys and floodplains, grabens, swamps, and, most importantly, oceans. These are the places where sediments can collect and remain for long periods of time.

.A given fluid can move larger particles as its speed and turbulence increases. Conversely, as a fluid slows down and its depositional energy decreases, it lose its ability to move large, followed by increasingly smaller particles.

The general rules:

Shallow water = high energy = large particle sizes

Deep water = low energy = small particle sizes

Long transport = well rounded, well sorted = mature sediments

Short transport = angular, poorly sorted = immature sediments

Burial / compaction / cementation: We lump these three together because they all occur simultaneously. In a large depositional basin, as sediments accumulate in layers, the layers at the bottom of the pile get buried under an increasingly deeper pile of overlying sediments.

As the layers of sediments are buried they come under increasing pressure. This causes the sediments to be compressed - squeezes out all of the air and water trapped with them and moves the individual sediment grains into closer contact with each other.

As the gains come into contact with one-another under pressure they become cemented, usually by dissolution and re-precipitation of whatever mineral they are composed of. Generally, the older and more deeply buried the sediments have been, the more tightly they are cemented together.

Different kinds of sedimentary rocks

There are two general types of sediment that represent two general processes for making sediment.

Clastic sediment ? (from the Gr. klastos, meaning ‘broken’). These are the broken fragments of eroded rock and mineral that result directly from the weathering process.

Most clastic sediments are composed of either quartz or clay minerals, because these are the stable breakdown products left from the weathering of most other rocks and minerals.

Pebble to boulder size grains

Conglomerate - rounded quartz pebbles, matrix of smaller pebbles or sand

Breccia - poorly sorted, angular rock fragments, matrix of smaller fragments

Sand size grains

Quartz sandstone (sandstone) - sand-sized grains of quartz

Arkose - sand sized grains of both quartz and orthoclase feldspar

Greywacke - Like sandstone, but with sand-sized rock fragments mixed with quartz sand and silt.

Silt to clay size grains

Siltstone - rock composed of tiny, microscopic quartz grains

Claystone and Shale - rock composed of clay minerals. If fissile, has splitting capability,we call the claystone a shale.

Chemical sediment ? Chemical sediments are made up of minerals that precipitate from dissolved ions in solution. Chemical sediments, once precipitated, can be transported, or they can sometimes come to rest right where they were formed.

Carbonates: A major group of chemicals sediments are those that are composed of calcium carbonate (calcite). These form the limestones and related rocks, called carbonates. Limestones are never formed from eroded clasts of pre-existing limestone, because limestone erodes away completely by dissolving in the naturally slightly acidic rain and groundwater.

Although the precipitation of calcite can occur spontaneously from saturated solutions in seawater and lakewater, the vast majority of calcium carbonate is biogenic in origin.

Calcium carbonate sediments form Limestones.

Limestones are differentiated based on grain size and type.

Coquina - a limestone composed exclusively of large shell fragments.

Grainstone - a limestone composed of sand-size particles of carbonate.

Oolitic limestone - a limestone composed of small, concentrically accreted, spherical carbonate particles.

Chalk - a limestone composed of the shells of tiny, planktonic one-celled organisms.

Evaporites: Other chemical sediments precipitate when their component ions become concentrated by evaporation, usually in hot, dry, climates. Sedimentary deposits of salt, gypsum, and carnallite (used to make potassium fertilizer) result from evaporative precipitation.

Coal ? forms from the accumulated remains of plants that have not decayed. As the plant material is buried and put under increasing heat and pressure the non-carbon elements are driven off to produce a sequence of substances increasingly concentrated in carbon.