Lab 5: Sedimentary Rocks - Classification

 

Sedimentary rocks may be classified generally as either detrital or chemical.

 

Detrital

 

Detrital sedimentary rocks are composed of sediment derived from weathering across the landscape. Detritus (or sediment) is any such material weathered out of rocks. All types of rocks may be weathered to produce sediment. Rock fragments are simply the crumbled material from rocks. Individual mineral grains are simply crumbled rock fragments. Erosion is the process of movement of this material from the landscape, and following that we see sediment being transported by water in rivers and streams, by wind in arid areas, and by ice where glaciers move. All of this is largely an inorganic process, although tree roots and the roots of other vegetation can work their way into the cracks and crannies in bedrock and help to break the material apart.

 

From the first part of the lab, you should appreciate that the material in detrital sedimentary rocks is mostly quartz, feldspar (if close to the source of weathering rocks), rock fragments, and clay minerals.

 

We classify detrital sedimentary rocks on the basis of grain size (size definitions are given in the first part of this lab):

 

Grain Size Comments Name
gravel grains are rounded conglomerate
gravel grains are angular breccia
sand mostly quartz quartz sandstone
sand mostly feldspar arkose
silt nonfissile (compact) siltstone
clay nonfissile (compact) claystone
clay fissile (splits easily) shale

A mixture of silt- and clay-size particles can be called mudstone.

 

Discussion

 

Gravel size material is found in deposits called conglomerate and breccia. The grains in conglomerate are more rounded, indicating that the gravel pieces have been moved by water a sizable distance. The rounding happens when gravel grains hit one another as water moves the material along a riverbed. Breccia has angular grains, which indicates something simple and immediate. The sedimentary particles in breccia must not have been carried too far down a river system, otherwise they would have had the angular edges knocked off by collision with other particles. So, conglomerate deposits form farther away from the source materials (perhaps uplifted mountains) than breccia deposits, which accumulate near the source.

 

Sand size grains are dominantly quartz for the reasons mentioned in the first part of the lab: quartz is hard and has a simple chemical structure that is resistant, so it tends to simply break into smaller and smaller grains, mostly in the sand size range. Other minerals are found in the sand size range in sedimentary rocks. Arkose is a notable type of sandstone. Arkose is often pink, because of the abundant feldspar mineral grains (recall orthoclase). The feldspar grains in arkose could not have been transported a great distance from the source (from granitic mountains usually), because the feldspar is still feldspar, it has not yet been transformed to clay minerals.

 

Recall that silt is in the size range tinier than sand, such that you can barely see the grains, if you can. Silt grains are most often quartz. Because of the size of the grains and the way the grains pack together, siltstone tends to have a "blocky," or compact, appearance. Where it says that siltstone is nonfissile, that means the same thing, that it does not split into thin partings easily.

 

Claystone is similar to siltstone, in that the overall appearance is more "blocky" than for shale, which appears in rather flat pieces usually. Shale has the property of fissility mentioned above. Fissility is the tendency to split in the same direction as the tiny flat clay minerals are aligned from compaction.

 

Chemical

 

Chemical sedimentary rocks are found in two subtypes, inorganic-chemical and biochemical. Inorganic-chemical sedimentary material forms as evaporite deposits (mainly gypsum and halite), or as significant mineral growth and transformation in rocks (especially chert). Biochemical material is found in various types of limestone (micrite, coquina, chalk, fossiliferous limestone) and in coal. In each of these rock types, the sediment originated by the action of organisms, especially large invertebrates such as clams, snails, and corals, who build their skeletons of calcite, and also tiny microscopic organisms, who also have little calcite skeletons. Dolostone, made of the mineral dolomite (similar to calcite, but with magnesium added to the chemical structure), is somewhere in the gray area between biochemical and inorganic-chemical, because it is often original limestone that was modified by the chemical addition of magnesium from groundwater rich in this element. Oolitic limestone is also special. It contains ooids, which are sand-size calcium carbonate (calcite, aragonite) particles that are concentrically laminated like an onion skin. The lamination happens as a result of repeated coating by calcium carbonate of a tiny piece of shell or other starting fragment. The coating happens in clear warm water, agitated by wave action, in certain places like the Bahamas.

 

These are some common biochemical sedimentary rocks (these are types of limestone):

 

Composition Comments Name
Mainly calcite very fine, clay-size grains micrite
Mainly calcite coarse hash of shell fragments coquina
Mainly calcite silt- to clay-size tiny skeletons chalk
Mainly calcite shells, shell fragments fossiliferous limestone

These are some common inorganic-chemical sedimentary rocks:

 

Composition Comments Name
Calcite often thinly laminated; in caves travertine
Calcite, aragonite concentrically laminated ooids oolitic limestone
Mainly dolomite orignally limestone, w/ Mg added dolostone
Varieties of quartz (flint, opal, chert, jasper) microcyrstalline (very dense) chert, generally
Halite (NaCl) intergrown crystals rock salt (halite)
Gypsum (CaSO 4 ) intergrown crystals rock gypsum (gypsum)
Fossilized plant fragments all sizes of plant material coal

Note that some of these, e.g., oolitic limestone, dolostone, coal, and in some instances, even chert, can be thought of as combination inorganic - biochemical, because remains of organisms are involved, or mixed in with inorganically derived sediment. For example, oolitic limestone can have shell fragments mixed in; dolostone is often chemically modified original limestone, of any type; some chert was originally formed as siliceous ooze on the seafloor (from siliceous microfossils), and coal is, well, originally fossil plant material, even if you can't see much of the original plant structure.

 

Coal comes in several varieties. Buried organic material starts out as peat, which is found in bogs at the surface. With more burial and heat and compaction, peat is transformed to lignite, the lowest grade of actual coal. Lignite is usually brown and dull. With even more burial and heat and compaction, lignite transforms to bituminous coal, which is darker black and more shiny than lignite. Anthracite is the highest grade of coal, and is harder, denser, deeper black, and more shiney than bituminous coal. Anthracite forms when coal is metamorphosed during mountain building.