Today we discussed 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. 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 Cycles, and Milankovitch Theory and Climate. Good web pages.
We briefly mentioned Cyclothems (cycles of fluvial and marine deposits) with an example from the Appalachian basin (eastern U.S.), and noted that their correlation between Europe and the U.S. supports the interpretation that they were produced by fluctuations in global sea level.
We did not have time to discuss Fischer et al. (1991; see handout). They 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.