Week 9: Sequence Stratigraphy and Global Cycles

Tuesday: Sequence Stratigraphy

Reading: Boggs Chapter 13 (13.1-13.3), and the Online Guide to Sequence Stratigraphy by Steven Holland, University of Georgia (big thanks to him for that). And Sequence Stratigraphy II - at SEPM (Society for Sedimentary Geology).

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 will discuss 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: Global Cycles in the Stratigraphic Record

Reading: Boggs Ch. 12.4; and Handout

Here we discuss the broad topic of Global Cycles in stratigraphy, covering a wide range of different time scales: First-Order through Fifth-Order Cycles. Table 12.1. The term "cycle" is used here to denote regular changes in a stratigraphic section which can be attributed to regular fluctuations in global climate and sea level.

First-Order Cycles have 200-400 million year duration (Boggs Fig. 12.9; handout). The cause of these cycles may be: (1) creation and break-up of supercontinent Pangea, and related variations in age of ocean crust and volume of seafloor spreading ridges (Boggs); or (2) long term variations in volcanic activity due to "mantle overturn", which controls variations in both atmospheric CO2 and volume of spreading ridges (Prothero); or perhaps (likely?) both.

Second-Order Cycles have 10-100 million year duration and are widely known as "Sloss" cycles, named after the person who discovered them (see handout). These large cycles resulted in many episodes of complete flooding of the North American cointinent by ocean waters during Phanerozoic time. The base of the oldest cycle is preserved in the transgressive sequence in the Grand Canyon. These cycles are generally believed to result from long-term changes in the volume of mid-ocean spreading systems through time.

Third-Order Cycles have ~ 1-10 million year duration, and their cause is not as well known as other kinds of cycles. This may be because they have largely been interpreted from petroleum industry analysis of sequence stratigraphy, which is plagued with assumptions and controversy about global sea level. But, see Boggs Table 12.1 for possible causes of these cycles.

Fourth and Fifth-Order Cycles range from ~10 to 400 thousand years in duration. They are lumped together because it is generally agreed that they are produced by Milankovitch climate cycles in some way. These climate cycles are named after a Russian mathematician (Milankovitch) who discovered and quantified the variations in Earth's orbital parameters: eccentricity, obliquity and precession. Later, geologists discovered that stratigraphy preserves a record of past climate changes that have occurred on the same time scales as originally predicted by Milankovitch.

Check also: Milankovitch Theory and Climate for more information.

Cyclothems are cycles of fluvial and marine deposits, consider an example from the Appalachian basin (eastern U.S.). Their correlation between Europe and the U.S. supports the interpretation that they were produced by fluctuations in global sea level, not just local tectonic controls.

Fischer et al. (1991) (see handout) analyzed Cretaceous-age, rhythmically bedded pelagic limestone-shale couplets, and interpreted them as a record of varying orbital parameters (and climate) through time. This study provides a good example of how the different time scales (frequencies) of climate variation can be superimposed on each other to produce nested cycles in the resulting stratigraphy.

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