1. What is the special feature of sedimentary rocks?
Answers
Sedimentary rocks fall into two broad categories, and they do not share a specific set of features. You really should pick up a textbook because pictures (and actual hand specimens and a hand lens) do help hugely.
Detrital (clastic) sedimentary rocks are made of particles that are arranged in patterns expected from settling in water or through the atmosphere.
However, the particles can be very diverse in shape, size (from microscopic shells in chalk, to huge blocks in conglomerates) and origin (fossilized remains of organisms are reliable indicators of a sedimentary origin, while grains or chunks that were weathered or fragmented from pre-existing rocks could have just about any composition).
Texture is highly variable: the grains can all be of similar size (well sorted) or vary widely (poorly sorted). The grains could be well rounded (because of their original shape or because any sharp edges were lost after weathering, during their transport to where they settled) or have remained angular (because of a specific physical weathering process followed by little or no transport) — or anywhere in between. Nature produces all kinds of gradations.
Parallel layers are common but not a give away. Geologists recognise a layering as produced by sedimentation rather than by igneous crystallization or by metamorphism by the distinct nature of the grains, their contacts (do they look cemented only at points of contact, and which minerals acts as cement) and their composition.
Many environments will produce more complex layering patterns, including ripple marks and cross-bedding (in sand dunes, tidal channels and river channel deposits). We can find their modern equivalent digging shallow trenches at a beach, or into dry river beds and deltas. Some layers can also preserve the burrowing traces of animals that used to live in the mud. Again, the best way to learn about those is to be an observer of modern environments or read textbooks that show modern examples and how they are preserved in rocks. Wind-blown dunes can be preserved as rock if they are buried deep enough and the grains become cemented: they have their own characteristic cross-bedding patterns, and a very well sorted, well-rounded grain size.
Storm deposits, glacial deposits (till) and have their own distinctive textures, but the context (what rock types are around them, vertically and laterally) is important to the interpretation (which is not always without ambiguities).
Chemical sedimentary rocks can be fine-grained or coarsely crystalline.
Many form by the evaporation of seawater, and only a few minerals are common in them (mostly calcite, aragonite, dolomite, gypsum, anhydrite, halite and sylvite). They can have some layering.
Siliceous chemical sediments accumulate in some areas of the deep sea, but siliceous (“chert’) concretions can also form during the compaction and (hardening) of impure clay-rich or calcium carbonate-rich muds. The silica typically comes from the hard parts of some organisms (sponges, diatoms).
Algal activity can lead to the chemical precipitation of calcium carbonate in some environments.
deposits (tuffs, ash beds) are an interesting category. The material is igneous, ejected fragments of glass and crystals (often broken) from explosive eruptions, that settled through the air or in a water body. The textures are unlike those of any other sedimentary rocks. We can recogniSed them because it is possible to compare ancient examples to younger ones.