Retrograde

Above, retrogradation: shift of the facies belts inland during a transgression; below Progradation: Shift of the facies belt towards the pelvic center during a regression.

In sedimentology, retrogradation or retrogradation ( Latin : retro = backwards; gradus = step; step back ) is the shift of the horizontal facies sequence in the direction of the edges or the threshold regions of a sedimentary basin over time. This usually takes place in connection with a (marine) transgression . In contrast, there is progression , in which the facies belts move towards the center or away from the threshold regions of a sedimentary basin .

By retrogradation, deposits of finer, pelitic sediments are often introduced into the former sedimentation space of more psammitic deposits. Within a vertical sequence of layers, a retrograding system is therefore often characterized by a decrease in grain size ( fining upward ) and a decrease in the thickness of individual layers ( thinning upward ) from the lying (below) to the hanging wall (above) , and in marine carbonatic systems by (more) a decrease in the calcium carbonate content. Coarsening and fining upward can be accompanied by a change in the fossil fauna, indicating an increase in marine influence or the distance from the coast. The conclusion (roof, top) of a retrogradational cycle can show signs of deficient sedimentation (e.g. mass occurrence of fossils from open water inhabitants , in carbonatic systems also hard grounds ).

The identification of sedimentary cycles that indicate retrogradation and the underlying causes is carried out in sequence stratigraphy for the reconstruction of the sedimentation process and the paleogeographical development as well as the parallelization of geological profiles in a sedimentary basin.

For example, in the course of the Flanders Transgression in the early Holocene, the coastline migrated inland (southwards) in the transition area between the North Sea Basin and the Northwest German Basin. This is assignable to the fact that in the Lower Saxony Watt sea solid sediment cores show a sequence in which terrestrial sediments or palaeosols of Pleistocene be overlaid by a peat horizon, in turn, of brackish water - Schlick Watt - and finally Sandwatt sediments is superimposed.

literature

Octavian Catuneanu: Principles of Sequence Stratigraphy. Elsevier, Amsterdam 2006, pp. 148-149, ISBN 978-0-444-51568-1 .

Web links

Retrogradation - SEPM Strata, stratigraphy pages of the Society for Sedimentary Geology (SEPM) (English)

Individual evidence

↑ See e.g. B. Frank Wiese, Ulrich Kaplan: The middle / upper Turon border area in the Lengerich area . In: Geology and Paleontology in Westphalia. Issue 62, 2004, pp. 37–70 ( PDF 4.37 MB; complete issue).
↑ Note: With retrogradation, tidal sediments show no fining upward, but coarsening upward, which is why sand follows clay (silt), see Friederike Bungenstock: The Holocene sea level rise south of the East Frisian island of Langeoog, southern North Sea - high-frequency sea level movements over the last 6000 years. Dissertation to obtain the doctoral degree (Dr. rer. Nat.) Of the mathematical and natural science faculty of the Rheinische Friedrich-Wilhelms-Universität Bonn. 2005, online , p. 13 f., 52.

[1] See e.g. B. Frank Wiese, Ulrich Kaplan: The middle / upper Turon border area in the Lengerich area . In: Geology and Paleontology in Westphalia. Issue 62, 2004, pp. 37–70 ( PDF 4.37 MB; complete issue).

[2] Note: With retrogradation, tidal sediments show no fining upward, but coarsening upward, which is why sand follows clay (silt), see Friederike Bungenstock: The Holocene sea level rise south of the East Frisian island of Langeoog, southern North Sea - high-frequency sea level movements over the last 6000 years. Dissertation to obtain the doctoral degree (Dr. rer. Nat.) Of the mathematical and natural science faculty of the Rheinische Friedrich-Wilhelms-Universität Bonn. 2005, online , p. 13 f., 52.