Maximum Regressive Surface (MRS)
Other Synonyms: Transgressive Surface (TS)
This is a short post about the MRS that is considered as an easy to identify on outcrops and well data because of some reasons. This post would covers the fundamentals of placing a MRS in a stratigraphic unit and how to avoid pitfalls in its interpretation.
Definition
It is a surface that marks the start of shoreline regression towards basin. It suggests a base level fall and subsequent marine regression.
Recognition
In the outcrop and wireline logs it is mostly recognized as a surface separating a coarsening upward succession from the overlying fining upward succession. This suggests that the underlying succession is a regressive unit and the overlying sequence is a transgressive unit.
In the above seismic data, you may notice a distinct MRS separating the underlying regressive unit (RST) from the overlying transgressive unit (TST). This surface forms a healing phase wedge above that is dominated by the transgressive fills. This healing phase is formed soon after the river dominated stages when the delta prograded maximum into the basin. The basinal i.e. deepwater part of the MRS are not within this survey range.
Note that the slope segment of this surface can also be treated as a slope onlap surface (SOS) as defined by Embry (2009).
Same as above but with an emphasis of Embry (2009) concept of SOS.
If a part of this surface is later on being modified by marine transgression, you may also highlight that segment as TS.
Better to colour code this surface as a distinct blue colour.
For the carbonates, similar things should be identified. Note that in carbonates things could be very complex as when maximum regression occur, the platform may be exposed and karstified. In such a case, the MRS may adjoin up-dip with a SU. Other cases could be that during max. regression the platform may contain shallow water environment and the carbonates are happily flourishing. In such a case, MRS would mostly be resolved on wireline logs or cores. Seismic data would mainly show an adjoining surface.
An example from Bover-Arnal et al. (2009) is shown below:
Another point to note that the MRS is a time-based surface (Embry 2009) and it may have associated segments such as SOS, RSME and even SU.
In the above seismic data, you may notice a distinct MRS separating the underlying regressive unit (RST) from the overlying transgressive unit (TST). This surface forms a healing phase wedge above that is dominated by the transgressive fills. This healing phase is formed soon after the river dominated stages when the delta prograded maximum into the basin. The basinal i.e. deepwater part of the MRS are not within this survey range.
Note that the slope segment of this surface can also be treated as a slope onlap surface (SOS) as defined by Embry (2009).
If a part of this surface is later on being modified by marine transgression, you may also highlight that segment as TS.
Better to colour code this surface as a distinct blue colour.
For the carbonates, similar things should be identified. Note that in carbonates things could be very complex as when maximum regression occur, the platform may be exposed and karstified. In such a case, the MRS may adjoin up-dip with a SU. Other cases could be that during max. regression the platform may contain shallow water environment and the carbonates are happily flourishing. In such a case, MRS would mostly be resolved on wireline logs or cores. Seismic data would mainly show an adjoining surface.
An example from Bover-Arnal et al. (2009) is shown below:
Another point to note that the MRS is a time-based surface (Embry 2009) and it may have associated segments such as SOS, RSME and even SU.
Advantage
The benefit of MRS is that one clearly discriminates between transgressive strata and regressive strata. It marks the boundary or transition from regression to transgression and could be used to predict reservoir (and its quality). In some cases, MRS could also form a seal (e.g. mud rich delta) and in other cases could destroy the sealing potential (e.g. wave-storm dominated settings). In the carbonates it is an indication that the tidal - supratidal facies may seal the previously build-ups reservoirs. In mixed settings, it can trace the regressive delta and its spatial distributions to identify the downslope or basinal reservoirs.Common Pitfalls
Quite similar context as in the MFS.References
Catuneanu, O. 2006. Principles of Sequence Stratigraphy, Elsevier.
See also:http://www.sepmstrata.org/Terminology.aspx?id=maximum%20regressive%20surface
Bover-Arnal, T., Salas, R., Moreno-Bedmar, J.A., Bitzer, K., 2009. Sequence stratigraphy and architecture of a late Early-
Middle Aptian carbonate platform succession from the western Maestrat Basin (Iberian Chain, Spain). Sedimentary Geology,
219, 280-301.
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