Young Brunette Girl Shows Off Outside On Sedimentary Rocks

• Fossils
Sedimentary rocks are the only type of rock that can contain fossils, the remains or imprints of dead organisms. In nature, dead organisms are usually quickly removed by scavengers, bacteria, rotting and erosion. In some exceptional circumstances a carcass is fossilized because these natural processes are unable to work. The chance of fossilisation is higher when the sedimentation rate is high (so that a carcass is quickly buried), in anoxic environments (where little bacterial activity exists) or when the organism had a particularly hard skeleton. Larger, well-preserved fossils are relatively rare. Most sedimentary rocks contains fossils, though with many the fact only becomes apparent when studied under a microscope (microfossils) or with a loupe.
Fossils can both be the direct remains or imprints of organisms and their skeletons. Most commonly preserved are the harder parts of organisms such as bones, shells, woody tissue of plants. Soft tissue has a much smaller chance of being preserved and fossilized and soft tissue of animals older than 40 million years is very rare. Imprints of organisms made while still alive are called trace fossils. Examples are burrows, foot prints, etc.
Being part of a sedimentary rock, fossils undergo the same diagenetic processes as the rock. A shell consisting of calcite can for example dissolve, while a cement of silica then fills the cavity. In the same way, precipitating minerals can fill cavities formerly occupied by blood vessels, vascular tissue or other soft tissues. This preserves the form of the organism but changes the chemical composition, a process called permineralisation. The most common minerals in permineralisation cements are carbonates (especially calcite), forms of amorphous silica (chalcedony, flint, chert) and pyrite. In the case of silica cements, the process is called lithification.