Coupled Transgressive - Regressive Systems

Introduction

Some other day, I thought that I need to post a small blogger note. It should describe the spatial extent of a sequence stratigraphic unit. In my experience, it has been an common misunderstanding in communication and interpretation. If one interprets a systems tract over 100s of kilometers with the same nature (e.g. regression), then other will sub-divide it into several strike oriented systems tracts. Well, I will not go that deep but will simply say, "all sequence stratigraphic units are site-specific just like lithostratigraphic units." Sequence stratigraphy has an advantage because it defines time-based units e.g. base level rise/fall.

The main objective of this post is to show my personal opinions on the present day example taken from Louisiana Holocene continental shelf (N. America).

See the figure below.

Supply vs. Accommodation vs. Shoreline Trajectories:

In the figure above, what do you learn?

Increasing sedimentation of Holocene Mississippi delta compared to the transgression rate. Sediments progrades on to the shelf and build outwards - a regression (HST).
There was an abandoned delta of Pleistocene epoch as it has been annotated in the figure. Nevertheless, that part of the delta has been transgressed and at present it is under strong wave dominated setting. There are several barrier shoals developed parallel to the coast giving hints of wave-dominated settings. Overall, the St. Bernard Shoals and the back shoal margin (Chandeleur region) are transgressive systems (TST).
These systems occur within 20Km of a distance along the coast.
If you try to compare the rate of aggradation in both (red and yellow circles), you will obviously notice that the delta systems aggrades more compared to the wave dominated settings. Note that the later ones are highly destructive systems.

Open Discussion

Along the basin strike, it is quite common to have coupled or coeval systems tracts. When a delta develops outwards, there are some of its components which will either be tide/wave dominated. In principle, most of the delatic systems could have both components at the same time as highlighted in the above system. I wonder if a delta (at present) has all three components (tide, wave, and river) dominating at the coast/shelf. If you know any example, please post here.

I look forward to your comments on my interpretation. I am open and excited to hear back from you either positive or negative.

Cheers,

Farrukh

Comments

Maximum Flooding Surface

Maximum Flooding Surface (MFS) By F. Qayyum This is a short post about the MFS 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 MFS in a stratigraphic unit and how to avoid pitfalls in its interpretation. Definition It is a surface that marks the end of shoreline transgression towards land or coastal plain. It suggests a base level rise and subsequent marine transgression onto the shelf or coastal plain. Recognition It can be treated as a downlap surface because overlying strata may prograde and downlap on it. This is a typical case for a normal regressive deltaic progradation or simply siliciclastic system reaching the shoreface and basin. The overlying system could either be a HST or LST depending on the location. On the well logs, it is marked by a deepening up-ward GR trend. This means that one is going to mostly define a massive shale as a MFS marker at a shelf lo...

Maximum Regressive Surface (MRS)

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 i...

Stratigraphic Base Level Revisited in Deep-water Settings

It has been 100 years since Barrell (1917) solidified the base level concept, which is widely used in sequence stratigraphy. The concept was first introduced by Powel (1875). It is still broadly used in non-marine, near shore and deep-water settings. The problem arrives when the base level concept is applied in deep-water settings. Most of us will consider the sea level/wave base as a base level. However, it is simply not! The magnitude at which sea level or wave base varies is much smaller than the changes that we observe on the modern or ancient sea floor. Recalling, below the base level deposition happens and above it erosion is possible. This original definition gets violated when sea level is chosen as a base level proxy. As annotated in the figure below, the sea level or wave base can not be considered as a proxy for the base level since erosion and deposition happen below this level. To solve such a conceptual problem, we always need to place the base level close to the sea ...

Sequence Stratigraphy

Search This Blog