Bedding can form in almost any depositional environment. Partings may represent periods of non-deposition that could range from a few decades to a few centuries. Bedding is an indication of changes in depositional processes that may be related to seasonal differences, changes in climate, changes in locations of rivers or deltas, or tectonic changes. By understanding the origins of these features, we can make some very useful inferences about the processes that led to deposition the rocks that we are studying.īedding, for example, is the separation of sediments into layers that either differ from one another in textures, composition, colour, or weathering characteristics, or are separated by partings - narrow gaps between adjacent beds (Figure 6.19). In addition to these principles that apply to all sedimentary rocks, a number of other important characteristics of sedimentary processes lead to the development of distinctive sedimentary features in specific sedimentary environments.

The principle of faunal succession states that there is a well-defined order in which organisms have evolved through geological time, and therefore the identification of specific fossils in a rock can be used to determine its age.

For example, the cobbles in a conglomerate must have been formed before the conglomerate.

The principle of inclusions states that any rock fragments in a sedimentary layer must be older than the layer.The principle of superposition states that sedimentary layers are deposited in sequence, and that unless the entire sequence has been turned over by tectonic processes, the layers at the bottom are older than those at the top.The implication is that tilted sedimentary layers observed to day must have been subjected to tectonic forces. The principle of original horizontality states that sediments accumulate in essentially horizontal layers.Through careful observation over the past few centuries, geologists have discovered that the accumulation of sediments and sedimentary rocks takes place according to some important geological principles, as follows: Available online.Chapter 6 Sediments and Sedimentary Rocks In: M Hall, ed., 52 Things You Should Know About Geology.

Review: Stratigraphy, Meta-Stratigraphy, and Chaos. And you just have to look at some seismic reflection data from the continental shelf to appreciate that this principle applies to even an outwardly rather continuous environment (excerpt of a line in the Virtual Seismic Atlas):īailey, R (1998).

In it, he refers to Bailey (1998), who described sedimentary bodies as 'lenticles' with complex 3D geometries.

Geology (edited by me, 2014) - see the link below. Brian Romans wrote a nice essay on this subject in 52 Things. Why is the POOH dodgy? On small and large scales, the concept of 'layer-cake' stratigraphy does not hold up. with some caveats due to resedimentation and other processes. That law is, in a general sense, a foundation stone (ha!) of stratigraphy. It is not a useful theory.ĭon't confuse it with the law of superposition (also due to Steno), which is useful. No, Steno's principle of original horizontality (I'll call it the POOH) is not 'safe'.

Principle of original horizontality