Sequence stratigraphy

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The sequence stratigraphy is a geoscientific method of stratigraphic correlation of sedimentary rocks . The correlation is based on abrupt changes in the sedimentary facies or prominent strata surfaces, which reflect changes in the sedimentation dynamics within the compared sediment sequences. These are in turn the result of cyclical fluctuations in relative sea level (also known as accommodation space) over the course of geological time periods, which are controlled by both global sea level cycles and regional tectonics (“uplift” and subsidence ). Although fluctuations in the relative sea level are expressed, among other things, by the change in lithological features , with sequence stratigraphy, in contrast to lithostratigraphy , layers with the same lithological features are not correlated with one another, but only those that are likely to occur during the same phase of a cycle of relative sea level Deposit came. This also allows a correlation of layers that originate from relatively strongly different deposit environments (e.g. neritic and pelagic sediments).

Since the sequence stratigraphy was developed by North American geologists, it has an English-language terminology that is also widely used in the German-speaking area.

principle

Traditionally, the sequence stratigraphy mainly considers passive continental margins and epicontinental seas . When the relative sea level rises ( transgression ), such a deposit system changes in such a way that sediments with the same facial properties (e.g. grain size, sedimentary structures, fossil record - reflect the deposit environment ) further inland (i.e. closer to the place of origin or the Edge of the sediment basin) than before. When the relative sea level falls ( regression ), sediments of the same facies are deposited further seaward (i.e. closer to the center of the sedimentary basin) than previously and sediments that were previously deposited more landward are removed.

Since sediments or certain sedimentary facies that occur next to each other (in lateral sequence) at a certain point in time, also occur one above the other (in vertical sequence) in geological time periods (Walther's rule of the facies), sedimentation trends or movements can be in any vertical profile the relative sea level (regressive or transgressive) in the geological past can be determined. These trends exhibit cyclical patterns that form the backbone of sequence stratigraphy.

The smallest unit of sequence stratigraphy is the parasequence. It corresponds to a regressive small cycle (a so-called 4th or 5th order cycle), represented by a stack of layers with a regressive tendency. A parasequence is limited by so-called flooding surfaces. A flooding surface is defined as the layer surface above which there are always deposits that correspond to a more seaward (deposited in deeper water) facies than the deposits below, whereby Walther's rule of facies cannot be applied, i.e. H. the deposits above and below the flooding surface do not originate from any directly laterally adjacent deposit environment.

Sequences of parasequences form parasequence sets. The latter have a certain stacking pattern, a distinction being made between progressation (corresponds to a regressive trend), retrogradation (corresponds to a transgressive trend) and aggradation (corresponds to a constant relative sea level).

A sequence, the eponymous unit of sequence stratigraphy, corresponds to a larger cycle (a so-called 3rd order cycle) and consists of a sequence of parasequence sets, which are allocated to up to four so-called systems tracts, each of which consists of at least one parasequence set and for each of which a particular stacking pattern is characteristic. The systems tracts are limited by particularly striking flooding surfaces. The sequence itself is limited by so-called sequence boundaries.

The affiliation of a parasequence set to one of the four System Tracts depends on its stacking pattern, which reflects both the sedimentation trend (transgressive vs. regressive) and the rate of change in relative sea level during their deposition. Both factors have a direct influence on the sedimentation dynamics and are therefore of particular importance for sequence stratigraphy. The four systems tracts that (can) be differentiated within a sequence are characterized as follows:

  • Highstand Systems Tract (HST)
    • represents a generally high, but slowly rising or stagnating relative sea level
    • Stacking pattern: aggradational-progradational
    • Corresponding deposits occur throughout the basin.
    • The slope boundary is the sequence boundary, which is always characterized by a pronounced facies contrast with a significantly more seaward facies, whereby the overlying facies are either separated from the underlying facies by a layer gap or, in areas towards the basin, the boundary is designed as a so-called correlative conformity .
  • Falling Stage Systems Tract (FSST)
    • represents falling sea level
    • Stacking pattern: progradational ("downstepping")
    • Corresponding deposits occur exclusively in the pelvic areas or can be completely absent.
  • Lowstand Systems Tract (LST)
    • represents a generally low but slowly rising relative sea level
    • Stacking pattern: progradational-aggradational
    • Corresponding deposits occur predominantly in the basin areas.
    • The hanging boundary is the transgressive surface, characterized by the superposition of retrogradational parasequence sets.
  • Transgressive Systems Tract (TST)
    • represents a rapidly rising relative sea level
    • Stacking pattern: retrogradational
    • Corresponding deposits occur throughout the basin.
    • The hanging limit is the maximum flooding surface, a stratum surface characterized by the effects of deficient sedimentation.

All flooding surfaces are so-called isochronous surfaces, i.e. they were created at the same time and can therefore be used for stratigraphic correlation, even if the sediment sequence in which they occur is built up differently lithologically at different locations within the sedimentary basin.

literature

  • Octavian Catuneanu: Principles of Sequence Stratigraphy. Elsevier, Amsterdam et al. 2006, ISBN 0-444-51568-2 .
  • Angela L. Coe (Ed.): The Sedimentary Record of Sea-Level Change. Cambridge University Press, Cambridge 2003, ISBN 0-521-53842-4 (Reprint with corrections. Ibid 2005).
  • Andreas Schäfer: Clastic Sediments. Facies and sequence stratigraphy. Elsevier - Spektrum - Akademischer Verlag, Munich et al. 2004, ISBN 3-8274-1351-6 .

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