Week 9: Global Cycles in Stratigraphy


Tuesday: Sequence Stratigraphy

Reading: Boggs Chapter 13 (section 13.3), and the Online Guide to Sequence Stratigraphy by Steven Holland, University of Georgia (big thanks to him for that). And, Sequence Stratigraphy at the University of South Carolina, an in-depth treatment.

A Depositional Sequence is a relatively conformable succession of genetically related strata bounded by unconformities or their correlative conformities. So, sequence boundaries are unconformities, which are defined here (narrow definition) as surfaces produced by subaerial exposure and erosion. Thus, every sequence records one cycle of relative sea-level fall, rise, and fall.

Sequences are commonly divided into different "systems tracts", including low-stand, transgressive, and highstand systems tracts. They occur in distinctive parts of each sequence and, together, record cycles of relative sea-level rise and fall.

Parasequences. Sequences are made up of smaller, individual shallowing- and coarsening-up packages called parasequences, which are internally conformable and are bounded by marine flooding surfaces. We can say that Walther's Law applies within parasequences but not across marine flooding surfaces that bound them. Most parasequences probably record gradual progradation of sediment from a shoreline (often deltaic), followed by abandonment and submergence when sediment input stops due to channel or delta-lobe switching.

Groups, or sets of parasequences may be arranged in a varity of different geometries that record the evolution of relative sea level and sediment influx during the cycle through which a sequence evolves. These different geometries are referred to as parasequence stacking patterns, which may be progradational, aggradational, or retrogradational, depending on what's happening with the long-term balance between relative sea level and sediment influx over the course of multiple parasequence cycles.

Use of Sequence Stratigraphy in Sea-Level Analysis. Boggs Figure 13.18 shows 3-step analysis:

Although it has been successful in many ways, and is a powerful approach to interpreting stratigraphy, there has been quite a lot of controversy over this type of analysis. The main problem was that the Exxon group claimed their curves record rises and falls in global eustatic sea level through time (Figs. 13.20, 13.21), which assumes that all changes in relative sea level were caused by changes in global eustatic sea level. This assumption ignores the possible (and very real) effects of variable subsidence or uplift due to tectonic forces.

We also briefly discussed forced regressions and resulting lowstand features, and watched a movie from the Univ. South Carolina web page to help understand the geometries and controls on sequence stratigraphy.


Thursday: no class (Thanksgiving)


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