Step 1: Diagenesis - Deposition
Sedimentary Structures
Sediments are layed down horizontally in layers; thus, sedimentary rocks are layered or “stratified.” A bed is a fundamental unit for rocks. Beds contain rocks that are similar in composition, color, and texture, and are often laterally continuous for long distances. Individual beds are caused by changing conditions during deposition. These can be changes in energy conditions, and hence, grain size, in addition to disturbance by organisms (worm tubes, bioturbation. Bedding may also reflect episodes of non-deposition or erosion.
Strata = A series of beds
Formation = sequence of strata that is sufficiently unique to be recognized on a regional scale Fundamental geologic mapping unit.
Water flowing over loose sediment creates bedforms. Bedforms are linked to flow velocity and sediment size. Ripples, cm-scale ridges, and troughs indicate flow and often preserve information about flow conditions. Asymmetric ripples indicate a unidirectional flow of water; they are characterized by a short, steep downcurrent slip face and a long, gentle upcurrent ramp. Symmetric ripples form as a result of wave oscillation. They are characterized by sharp ridges and concave-up troughs. Dunes are similar to ripples, except much larger.
Other bedforms include graded beds. These are bedding layers that contain sediments that fine upward (ex: from coarse particles at the bottom that gradually get smaller towards the top of the bed). Some bedforms form after deposition, while sediment is still soft. Mudcracks form as a result of polygonal desiccation (drying out) of mud. Troughs eroded in soft mud by the current flowing above it forms scour marks.
Fossils can also be found in sediments and sedimentary rocks. Fossils are traces or remains of prehistoric life and are the most important inclusions. They can help determine past environments, can be used as time indicators or for matching rocks from different places.
Types of fossils
- Petrified – cavities and pores are filled with precipitated mineral matter
- Formed by replacement – cell material is removed and replaced with mineral matter
- Mold – shell or other structure is buried and then dissolved by underground water
- Cast – hollow space of a mold is filled with mineral matter
- Carbonization – organic matter becomes a thin residue of carbon
- Impression – replica of the fossil's surface preserved in fine-grained sediment
- Preservation in amber – hardened resin of ancient trees surrounds an organism
- Indirect evidence - Tracks, Burrows, Coprolites (fossilized poop), Gastroliths (stones that some animals, like cows, use to help break up food in their stomachs)