What is Stratigraphy | Uses Of Sequence Stratigraphy

By: Stig Kristoffersen

The study of the layered character of sedimentary rocks is what we call stratigraphy within Geology.

Geoscientists can use a variety of techniques in order to be able interpret the origin of rocks and predict the extent of their lithofacies and rock character. Techniques are;

1.Stenos law that says sediment accumulation is captured by the superposition of its layers
2.Vertical stacking and lateral associations of lithology coupled to
3.Walther's Law; biofacies identification(ichnofacies and fossils
4.Sedimentary structures; sequence stratigraphy (a consideration of relationships to base level change and the production of erosional and depositional generated surfaces)
5.Chronostratigraphic markers (bio-stratigraphic, volcanic, magneto-stratigraphic, bio markers, radioactive markers or storm layers or sequence stratigraphic).

All these methodologies in fact overlap each order and are used simultaneously in order to reveal the origin of the rocks.

Sequence stratigraphy does not only involve the interpretation of the surfaces that are used to interpret the stratigraphic section but also takes into consideration sedimentology and chronostratigraphy.

Geologists want to determine the extent and origin of a lithofacies body, package or series of packages that concerns.

Just as a carpenter is not concerned by the length of the hammer as long as it makes the nail into the wall. The same goes for a geoscientist as he do not care what obscure stratigraphic surfaces are called as long as he can reveal the relationship between layers and give its origin.

The interpretation of sedimentary sections and prediction of their facies heterogeneity's involves the analysis and integration of geometrically related data and involves the mental process of iteratively and successively back strip the sediment in reverse order of accumulation. This reassembly tracks the evolution of the sedimentary system, its hydrodynamic setting, and accommodation. The genetic character of the sedimentary sequences, cycles, parasequences, and/or beds is determined by assuming that they are the products of changes in accommodation as the sediment is reassembled.

The limitation of this analytical strategy is tied to knowledge of the inferred depositional setting while the advantage is that it formulates new questions that lead to more realistic interpretations and enhanced predictions of lithofacies heterogeneity's.

Sequence Stratigraphy
Stratigraphic interpretations explain how sedimentary rocks acquire their layered character, lithology, texture, faunal associations and other properties. The analysis of these properties can be used to explain how the mechanisms of sediment accumulation, erosion and inter-related processes produced the current configuration of these rocks. The sequence stratigraphic approach recommended for the interpretation of sedimentary rocks contrasts with Lithostratigraphic analysis which maps lithofacies independent of subdividing external and internal boundaries or Allostratigraphic analysis that uses bounding discontinuities including erosion surfaces, marine flooding surfaces, tuffs, tempestite, and/or turbidities boundaries etc. as time markers independent of any model of base level change.
Sequence Stratigraphy analysis applies allostratigraphic models to interpret the depositional origin of these sedimentary strata and while assuming, though this is not always stated, an implicit connection to base level change. It does this by establishing how the sequence of strata accumulated in order in the sedimentary section over a subdividing framework of surfaces. The major bounding and subdividing surfaces of this template are commonly represented by:
&bull Maximum Flooding Surfaces (MFS)
&bull Transgressive Surfaces (TS)
&bullSequence Boundaries (SB)

This subdivision of the sedimentary section provides the order in which the sediments were laid down (the law of superposition of Steno), and so their relative age. The arrangement of the vertical succession of facies of the sediment geometries bounded by the surfaces, stacking patterns, forms a major element to the interpretation of the depositional settings of stratigraphic section. These stacking patterns vary between:
&bull Unconfined sheets that:
o Prograde (step seaward)
o Retrograde (step landward)
o Aggrade (build vertically)
&bull Sheets and unconfined lobes containing
o Non-amalgamated bodies
&bull Incised topography fill
o Amalgamated, multi-storied confined bodies (e.g. incised valleys)
o Within unconfined lobes

Using the above approach geologists infer the processes responsible for that sedimentary rock and so interpret its origin.

Sequence Stratigraphy & Over Simplifications Related to Time.
The sedimentary layering of a stratigraphic section has a vast array of dimensional hierarchies. These range from units millimeters thick that might be formed over seconds to thousands of feet thick and formed of millions of years. As much of the literature related to these surfaces indicates, it should be recognized that whatever the dimension of a layer is and whatever the time involved in its deposition, each layer is bounded by surfaces that transgress time
This means an interpretation of the depositional setting for a section cut by these diachronous surfaces contravenes Walther's Law. Most interpretators accept and take into consideration that the layered units bounded by these surfaces formed at different times, and recognize that the subdividing surfaces are of a higher order frequency than the time envelope of the parasequence being considered. In other words the situation is simplified when the surfaces are taken to represent instances in time between which sediments continuously accumulated. Thus the surfaces of the layers transgress time and the sediments filling between these surfaces also transgress time while being continuously reworked through a series of geological events.
Thus it should be recognized that in sedimentary interpretation the application of Steno's principles and Walther's Law provide powerful and useful simplifications that assume the sediments packaged by surfaces accumulated within discrete moments of time.

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