Naming Sedimentary Rocks

Clastic sedimentation is the type we just talked about, where there is weathering, erosion, transportation, deposition, and finally lithification.

Clastic sediment has a wide range of sizes. The different sizes go from clay, which is 1/256th of a millimeter, to boulders, which are larger than 256 mm or 10 inches in diameter. In decreasing size, you have boulders, cobble, pebble, sand, silt, and then clay.

If you consolidate the different kinds of sediments, you'll get various kinds of rocks out of them. Conglomerate or breccia comes from things that are pebbles or bigger. Sand makes sandstone and silt makes siltstone. Clay makes mudstone, shale, or claystone.

NAMING SEDIMENTARY ROCKS

As we learn to name sedimentary rocks in this section, keep in mind that the size matters and the way the sediment came together matters. Starting with clastic rocks, you get these:

• Shale – this is dark-gray rock made from clay. You can't see these without a strong microscope.

• Siltstone – the pieces here are easier to see with a microscope. Notice the erosion of these rocks as siltstone in figure 37:

Figure 37.

• Mudrocks are a lot of silt and clay that have layered out over time. Shale is a finer type of mudrock compared to siltstone; shale has the finest sediment, which is why it is so fragmental. Mudstone is a type of shale that does not break up. If shale is very organic, this is where petroleum products come from.

• Sandstone – this has visible grains at up to 2 millimeters in size. There are fine, very fine, coarse, medium, and very coarse sand grains. A sandstone called quartz arenite is about 100% quartz, while arkose is high in feldspar. Lithic sandstone is

high in rock fragments, while Wacke is a type of sandstone with at least 15% mud in it. Arkose is easily recognized as the pink sandstone seen in figure 38:

Figure 38.

• Conglomerates —these are very poorly sorted and can have stone and sand in the same conglomerate rock. If you see that the gravel pieces are not rounded, you'd call it breccia.

• Carbonate rocks —these are all based on the CO3 molecule. Some directly precipitate out, while others are made from corals and mollusks. As calcite is precipitated out along with other calcium salts, you will get limestone. After burial, calcite will become dolomite, which is a combination of magnesium, calcium, and carbonate. Dolomite doesn't fizz when mixed with hydrochloric acid, while calcite will do this.