Introduction to sequence stratigraphy

Preamble

Stratigraphy is the study of the layered character of sedimentary rocks. Geologists use a variety of strategies to interpret the origin of these rocks and predict the extent of their lithofacies and rock character. Strategies mix sedimentological tools like: Steno's Law that sediment accumulation is captured by the superposition of its layers; vertical stacking and lateral associations of lithology coupled to Walther's Law; biofacies identification ( ichnofacies and fossils); sedimentary structures; sequence stratigraphy (a consideration of relationships to base level change and the production of erosional and depositional generated surfaces); chronostratigraphic markers (bio-stratigraphic, volcanic, magneto-stratigraphic, bio-markers, radioactive markers or storm layers or sequence stratigraphic). None of these tools are used in isolation from one another. In fact the differences between many of these sub-disciplines of stratigraphy are ‘fuzzy’. Sequence stratigraphy for instance carries not only the connotations related to the interpretation of the surfaces used to interpret the stratigraphic section but also a consideration of sedimentology and chronostratigraphy. However the bottom line is that most geologists want to determine the extent and origin of a lithofacies body, package or series of packages that concerns them and not how the tools they use are defined or fit preconceived classifications of the stratigraphic techniques being used. Just as an eater tends is not concerned by the number of prongs a “fork” should have so long as it conveys food to one’s mouth so most stratigraphers do not care what obscure stratigraphic surfaces are called!

The interpretation of sedimentary sections and prediction of their facies heterogeneity's involves the analysis and integration of geometrically related data. It also involves the mental process of iteratively and successively back stripping the sediment in reverse order of accumulation. The 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 limits to this analytical strategy are 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 on this web site 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 turbidite 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:

• Maximum Flooding Surfaces (mfs)
• Transgressive Surfaces (TS)
• Sequence 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 arrangment 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:

• Unconfined sheets that:
  • Prograde (step seaward)
  • Retrograde (step landward)
  • Aggrade (build vertically)
• Sheets and unconfined lobes containing
  • Non-amalgamated bodies
• Incised topography fill
  • Amalgamated, multi-storied confined bodies (e.g. incised valleys)
  • Within unconfined lobes

As is explained in the pages that follow, 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 (Wheeler, (1958); Middleton, (1973); Vail et al (1977); Galloway, (1989); Catuneanu, et al, (1998); Schwarzacher, (2000); Catuneanu, (2002); Embry, (2002); Cross, and Lessenger, (submitted)). 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.

The Figure displays a hierarchy of sedimentary structures: A. Flaser structures from an intertidal flat setting in which the individual components probably accumulated over minutes but the whole section may represent tidal cycles over months (Bar Scale - 2 cm); B. Cross bedded Ordovician carbonates from a beach or nearshore shallow shoal setting that probably represent accumulation and reworking over several years; and C. Flat bedded Mississippian downslope siliciclastic fan deposits in which each bed may have accumulated over a period of hours but whole section encompasses potentially hundreds of thousands of years (Bar Scale - 1 M).

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. If one thinks about this, these simplifications don't contravene logic (which is literally Fuzzy) and aid in the interpretation of the sedimentary section. The above discussion provides a general introduction to the subdivision of the sedimentary section by the surfaces listed above and their relationship to base level change. For for a more complete and thorough discussion of this topic you should read Catuneanu (2002).

Introduction to the Web Site

This Web Site explains:

1. How to make sequence stratigraphic interpretations of sedimentary sections:

• Subdivide of these sections into sequences, parasequences &/or their associated system tracts
• Determine their depositional setting
• Characterize and predict of the extent of their lithofacies, particularly when associated with hydrocarbon reservoirs, and aquifers.
  

2. The use of:

• 2-D and 3-D seismic sections
  • Well log data
  • Outcrops

  to identify and correlate surfaces of:

• Erosion and non-deposition (sequence boundaries [SB])
• Transgressive surfaces [TS]
• Maximum flooding surfaces [mfs]

3. How the above surfaces have time significance and establish:

• A relative time framework for the sedimentary succession
• The inter-relationship of the depositional settings and their lateral correlation
• A compartmentalization of hydrocarbon reservoirs

In summary this web site explains how "sequence stratigraphy" can be used to study sedimentary rock relationships within a time-stratigraphic framework of repetitive, genetically related strata bounded by surfaces of erosion or non-deposition, or their correlative conformities (Posamentier et al., 1988; Van Wagoner et al., 1988).

Using the pulldown menu above you can select topics in sequence stratigraphy and access exercises related to this. You should be able to learn how to subdivide the sedimentary section into packages defined by bounding unconformities and internal surfaces. You will be able to see how sequences and their associated system tracts are the products of changes in relative sea level and rates of sedimentation. The various forms of sequence stratigraphic analyses outlined include the use of seismic cross-sections, well logs and outcrop studies of sedimentary rocks to infer changes of relative sea level and rates of sedimentation. You will be shown how to construct chronostratigrapic diagrams and also be show how to predict facies geometries and build depositional models using a variety of techniques!

Later in the section on the Basics of Sequence Stratigraphy you will be introduced to the details of how system tracts respond to changing base level. However as a preview you can trace Clastic Systems Tract Evolution through time in the linked movie!