1. Unit 2: Introduction to Sequence Stratigraphy
UNAVCO

2. Accommodation space
Sequence stratigraphy is based on the idea that there is a given “accommodation space”—or, simply put, space for sediment to be deposited or water to fill—and this is what controls sediment deposition and preservation. Accommodation space, in turn, is influenced by a few factors shown in the figure above. Tectonic subsidence increases space, as does eustatic sea-level rise. Sedimentation rate and water depth reflect the filling of accommodation space.
Image source: All the images in this pptx have been provided by Steve Holland of University of Georgia. He has given permission for them to be distributed with this learning module. If you wish to use them in any way outside of this learning module, please seek separate permission.
Tectonic subsidence + eustatic sea level = sedimentation rate + water depth
strata.uga.edu

3. Cyclical deposition strata.uga.edu
As you might imagine, the interplay between the factors on the previous page means that the accommodation space is constantly changing. This happens in a cyclical fashion. Four “systems tracts” have been identified from sedimentary sections. Each of these represents a combination of rate of sea-level change and rate of sedimentation. Each systems tract—also known as a sequence—is made up of parasequences.
strata.uga.edu

4. Parasequences strata.uga.edu
These are examples of two parasequences from different depositional environments (left is siliciclastic, right is muddy siliciclastic shoreline). Parasequences are defined as “relatively conformable succession of genetically related beds bounded by marine flooding surfaces and their correlative surfaces.” Generally, parasequences shallow upwards and obey Walther’s Law.
strata.uga.edu

5. Review: Walter’s Law
“Facies adjacent to each other in a continuous vertical sequence also accumulated laterally”
This only applies within parasequences, because it cannot apply to a section with unconformities
Sandstone
Siltstone
Mudstone
To review, Walther’s Law means “Facies adjacent to each other in a continuous vertical sequence also accumulated laterally.” Parasequences are bounded by flooding surfaces, which result in unconformities. This law, therefore, only applies within parasequences.
Limestone
Shallowing upwards

6. Parasequences strata.uga.edu
Individual beds are often the result of “events” that reflect periodic rapid perturbations in conditions within the shallowing water column over time. These events are often found to have repeat times of 100–500 years.
strata.uga.edu

7. Parasequence analysis
Interpret your section: identify the parasequences and parts (lower, middle, upper) of one parasequence
Find bed thicknesses for the whole parasequence and for each part (lower, middle, upper)
Interpret your results: What is the sedimentation history?
[Read this slide. Specific questions about the sedimentation rate or deposition rate depend on the instructor.]

8. All the images in this presentation have been provided by Steve Holland of University of Georgia. He has given permission for them to be distributed with this learning module. If you wish to use them in any way outside of this learning module, please seek separate permission.