Sequence Stratigraphy

 

Sequence stratigraphy is the science of subdividing a sedimentary basin fill into individual sequences of deposition (hence the name), which can then be linked to changes in the two fundamental parameters of sediment supply and accommodation (the amount of space available for deposition). The main aim of this is to reconstruct how sediments filled a basin and thereby, how the stratigraphy was formed through space and time. This can help scientists and exploration geologists to figure out many important things like how sea level changed and where coarse- and fine-grained sediments are located.

In sequence stratigraphic models such as the one shown above, sediment supply is often assumed to be constant while accommodation changes through time as a result of sea level fluctuations. This is a simplified, yet common model since changes in sea level can be the dominant control on the amount of accommodation, although tectonics, subsidence and compaction also play a major role.

Sedimentary sequences in this context refer to an amount of stratigraphy that was deposited during an episode of sea level fall and subsequent rise. Commonly, such depositional sequences are further subdivided into so-called systems tracts (meaning: a portion of stratigraphy linked to a position in sea level). For instance, when sea level falls during a glacial period and then subsequently rises during an interglacial, one sequence is deposited consisting of: a lowstand systems tract, followed by a transgressive systems tract and finally a highstand systems tract. The amount and type of systems tracts considered can differ among researchers and projects. Systems tracts are often district in their shape, size and type of sedimentary deposits. For instance, basin floor fans, usually form as part of the lowstand systems tract (sediments that were deposited when sea level was low or falling). The boundaries between systems tracts are often marked by significant surfaces like sequence boundaries or maximum flooding surfaces.

Like most things in earth science, sequence stratigraphic models are always a simplification of reality and sequence stratigraphy as a whole is infamous for being complicated and fraught with difficult terminology, numerous exceptions and additional complicating factors, many of which are unknown or poorly understood. Nevertheless, sequence stratigraphy has become an important branch of sedimentology and has greatly helped the sedimentological community as well as economic geologists in better understanding sedimentary basins and their stratigraphy.

This page introduces the methodology of sequence stratigraphy as applied to the interpretation of sediment accumulation in continental, marginal marine, basin margin, and down slope settings. Associated pages demonstrate how sequence stratigraphy involves subdividing the sedimentary section into a framework of erosional, depositional and non-depositional surfaces and characterizes the sediments enclosed by these surfaces. Pages on the site explain how these surfaces are interpreted to have been generated by changes in the relative position of the sea and enclose genetically related sedimentary facies formed into successions of stratal geometries. These facies and geometries are used to interpret and the predict the extent of these sedimentary facies geometries, lithologic character, grain size, sorting and quality of porosity and permeability away from control points. Reconstruction of depositional setting and paleogeography are facilitated by the framework of sequence stratigraphy and the successions of stratal geometries they enclose.

Modules: sequence stratigraphy course notes & exercises for students, professors and those who teach professional short courses in this topic.

Movies: Guided explanations of the generation of sedimentary sequences and their interpretation,

Sequence stratigraphic movie (to view click on image below)

This Sedpak movie captures a sedimentary section subdivided into geometric packages defined by bounding unconformities and internal surfaces that result from varying positions of relative sea level and rates of sedimentation. These geometries demonstrate why sequence stratigraphic analyses of seismic cross-sections, well logs and outcrop studies of sedimentary rock are used to predict the thickness and extent of sediment lithology from inferences built from this understanding of how sediment geometry changes with relative sea level and rates of sedimentation. Click here to Download Sedpak to make your own movies!

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Tuesday, June 25, 2019
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