Sequence Stratigraphy Essay Example

change driven by eustasy.

As indicated above the surfaces and encompassed strata are assumed to have time significance so provide a relative time framework for the sedimentary succession. This latter is the reason that sequence stratigraphy is considered to be a branch of allostratigraphy. The technique of subdividing sedimentary rock on the basis of surfaces leads to a better understanding the inter-relationship of the depositional settings and their lateral correlation.

Hence the definition that "sequence stratigraphy" is the study f 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). In conclusion sequence stratigraphy is more than a matter of "surface definitions", but a powerful strategy that, in conjunction with biota and sedimentary structures, enables the conceptual modeling of the different combinations of "sedimentary processes" including eustasy responsible for the hierarchies of geometry or elemental packaging exhibited by the lithofacies.

Allostratigraphy versus sequence stratigraphy In terms of the original definitions of “sequence stratigraphy” this methodology is applied when changes in the accommodation of sedimentary rocks are interpreted to have been the products of allocyclic change, i. e. base level change, caused by changing rates of eustatic and tectonic movement, and changing rates of sedimentation (Vail et al, 1977; Posamentier, et al, 1988; Van Wagoner et al, 1988; Van Wagoner, et al, 1990).

However as sequence stratigraphy has become more widely used so the order of these layers and the surfaces that bound them are also often tied to the sequential packaging of autocyclic processes that are the result of point migration and mass wasting events, rather

than base level change. This latter is because most petroleum and other stratigraphers, including academic geologists, analyze local fields, plays, or local geologic facies geometries and are unable to resolve “regional” versus “local” surfaces and sediment bodies.

In the interest of the simplification of this problem these same stratigraphers avoid the confusion of what “regional” versus “local” surfaces ("autostratigraphic", "allostratigraphic" versus "sequence stratigraphic") by considering these surfaces and bodies as all sequence stratigraphic. In spite of this simplification, or because of it, these same earth scientists working with local bodies of carbonate and clastic bodies are able to build accurate predictive models of the facies geometries for their locality by identifying repeated cycles of sediment bound by correlatable, but local, surfaces.

These same geologists then often maintain they are using sequence stratigraphy to make their analysis, basing their fallacious assertion on the fact that their predictions are "spot on". To the contrary the "pure" sequence stratigrapher argues that of most sedimentary stratigraphers though they have made accurate lithofacies predictions, have “blurred” the meaning of sequence stratigraphy and their approach has become more allostratigraphic and autostratigraphic than sequence stratigraphic.

This more general lumping of sequence stratigraphy with allostratigraphy, sedimentology and/or sedimentary stratigraphy means that a student’s understanding of sequence stratigraphy may become confused. Additionally this “fuzzy” application of sequence stratigraphy has probably aided the evolution of the different viewpoints of the various sequence stratigraphic models (Depositional, Genetic, or T-R Sequences) and leads to the major points of their disagreement. Some of these differences of opinion are expressed in the sequence stratigraphy terminology debate of this web site.

Other problems of confusion associated with sequence stratigraphy are tied to the origins

of the real and sometimes hypothetical surfaces associated with the stratigraphic column. Expressly are these surfaces confined to the area study or do they extend beyond it? If the answer to this question is unknown but assumed, this should be stated up front or emotional words are likely to be exchanged? So as you become more familiar with the literature or the geology of the terrains you study you will often find that only the most obvious low frequency sequence stratigraphic layers can be identified.

This means that sequence stratigraphy frequently only supplies a low frequency allocyclic template to the high frequency autocyclic processes within small-restricted areas, often only a few kilometers across at the most. These sequences may be identified from deep to shallow water settings, and even may include continental settings. The products of base level changes for continental settings are not quite the same as those of coastal settings, or a marine shelf or the deeper down slope.

Facies assemblages and subdividing surfaces will be found to change with setting, be they associated with aeolian, glacial, fluvial, alluvial fan, deltaic barrier, margin and slope and distal slope and basin settings? For instance the Stokes surfaces of aeolian dune fields are different to channel boundaries, or those associated with evaporite basin shape, etc. Some of the framework of surfaces and sedimentary bodies from these different settings can be tied to base level change and other sedimentary elements are tied to autocyclic change.