The stratification is a typical feature of sediments and sedimentary rocks . A layer is a three-dimensional sedimentary or sedimentary rock body that is delimited by two surfaces, the lower side of the layer (base, bottom) and the upper side (top, roof). The sequence of layers, as can be observed in an outcrop , usually comes about through the sedimentation of different material ( layer (s) sequence ). Such a change of material is usually due to a change in the deposit conditions. The differences between the neighboring layers can consist of the mineralogical composition , the grain size , the color and the texture . With regard to the mineral stock, the material contrast can subsequently have been significantly increased by diagenetic processes (“pseudo-stratification”).
If a stratification can be seen in metamorphic rock , which can be traced back to tectonic processes, this is not referred to as stratification but as foliation . If the individual layers also consist of different minerals, one speaks of a banding instead of the layer sequence . Both phenomena can also be summarized as foliation .
In lithostratigraphy , the layer is called the stratum . The investigation of geological and pedological stratifications and their temporal allocation is called stratigraphy . The constituents contained in the individual layers and their fossils enable a temporal and genetic correlation with deposits of the same age, often distant.
Sedimentary structures usually appear within a certain layer and characterize it. However, they can also emboss the upper or lower side of the layer or penetrate several layers, especially if they are of biogenic origin (see → Palichnology and Bioturbation ). They reflect the specific deposition conditions resist and enter, for example, information about the type of sedimentation (z. B. previous transport by wind or running water or paragraph from the still water), allow the reconstruction of the flow direction of the depositing medium or suggest the living world through . In folded rock they can serve as a top-bottom criterion.
Examples of different sediment structures are:
- hummocky cross stratification : symmetrical ripple structures formed by oscillating storm waves. They are therefore mainly found in shelf areas from 30 to 50 meters deep (only the convex parts of the ripples can be observed)
- swalley cross stratification : as above, only the ripple valleys have been preserved; they occur in the deeper shelf environment
- herring bone stratification : every few centimeters an opposite oblique stratification can be observed; this indicates a deposit in shallow marine area of high tidal influence towards
- flute marks : missing material that was torn away by a storm event
- casts load ( load brands ) are observed at the bottom of sediment layers; they arise from a sudden overlay with denser layers, which are pressed into the sediment
- flaser bedding : ripple structures filled with mud; they occur in the vicinity of changing flow conditions, e.g. B. in river mouths or under the influence of tides
- Graded stratification : vertical grain size differentiation in one layer