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):

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, silstone 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.