Geopetal structure
Geopetal structures are structural features that serve to determine the chronological deposition sequence of sedimentary rocks . Taking into account the stratigraphic principle , they are important criteria for differentiating between normally stored and overturned structures.
The following main types are distinguished:
- Internal structure - layer characteristics that can be found within a layer
- External structure - structural features on the layer surfaces
- Tectonic fabric - relationships between stratification and foliation
Internal structure
Within a rock layer, the following can serve as characteristics of the storage sequence:
- graded stratification - vertical grain size differentiation
- sessile animals in the sediment in living position - e.g. B. corals
- Worm burrows
- partially filled cavities, especially in fossils, the filling of which points downwards in the direction of gravity. This sometimes even allows subsequent tilt angles to be determined ("fossil spirit levels")
- Oblique stratification within the layer
External structure
At the layer boundaries and on the layer surfaces, the geopetal structures serve:
- asymmetrical flow ripples ( ripple marks )
- Oscillation ripples
- Regulated mussel shells, the convex side of which shows "up", as they are tilted by the current
- Dry cracks - pointed wedges point downwards
- Rock salt pseudomorphs - prints point upwards
- Raindrop impressions in the sediment
- Animal traces of life in general, as far as identifiable
- Root soils ( coal )
- Washout marks
- Debit marks
- Grinding marks
- Erosion and transgression discordances
Tectonic structure
The relationship between stratification and foliation applies:
- when stratification and foliation collapse in the same direction
- Storage normal if the stratification is shallower than the foliation
- Storage tips over when the stratification is steeper than the foliation
- if stratification and foliation collapse in opposite directions, storage is always normal
The positional relationship of "drag-folds" to a large fold can also serve as a geopetal structure.
literature
- Bruno Sander : Contributions to the knowledge of the accumulation structure (rhythmic limestone and dolomite from the Triassic) . Mineralogisch-Petrologische Mitteilungen, Volume 48, 1936, pp. 28-209
- Robert R. Shrock : Sequence in layered rocks - a study of features and structures useful for determining top and bottom or order of succession in bedded and tabular rock bodies . New York 1948, 507 pp.
- B. Hückel & Volker Jacobshagen : Geopetal sedimentary structures in the main dolomite and their significance for the tectonic analysis of the Northern Limestone Alps. Journal of the German Geological Society, Volume 113, 1961, pp. 305-310
- Adolf Seilacher : Preservational history of Ceratite shells . Palaeontology, Vol. 14, 1971, pp. 16-21