Sequence Stratigraphy

A short note for beginners/students One of fundamental problems in our stratigraphic understanding is that we consider a stratigraphic unit as layers that may continue over a part of basin and may also correlate within and across the basin. This leads us to a troublesome procedure of correlation to build a sequence stratigraphic framework. This post is a brief review on the correlation concept within a sequence stratigraphic perspective. I would like to cover briefly one point here that a sequence stratigraphic unit is spatially a limited unit. A stratigraphic unit is always formed within a specific space (or accommodation space; do not want to complicate with terms). Within such a space, it may be altered or modified by post-depositional sedimentary and structural processes. Despite the fact of its origin; the unit would remain within the same space and may be reduced in volume (if eroded). Such a space would always be geographically restricted suggesting that a unit woul...

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 ...

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 sedimentat...

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...

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...

H.E. Wheeler's Innovation A short note for those who are innovative and do not bother about making patents. By F. Qayyum Wheeler spent his time with Sloss and finally got succeeded in presenting a stratigraphic unit within a spatio-temporal framework. When he proposed the idea to the scientific community, he probably did not think that his innovation would be commercialize in such a way that at present days people will earn money. He proposed an idea  voluntarily just like other geologists such as Steno, Lyell, William Smith etc. No one would have ever thought that their principles (non patented) would   be used everyday. They actually benefited the entire mankind and thanks to them that we are using them to explore for the energy resources. In any case, we started using those concepts and put patents on top of each others. If you scroll through the historical development of Wheeler diagrams, you would be fully surprised that why are we doing this. We inven...