Week 9: Global Cycles; Sedimentation and Tectonics


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.


Thusday: Sedimentation and Tectonics

Sedimentary basins are areas of crustal subsidence where sediments accumulate by deposition in environments such as rivers, lakes, alluvial plains, coastal areas, deltas, continental shelves, and deep oceans. Whenever we see a thick (> ~100 meters) succession of sedimentary rocks in outcrop or in the subsurface, the basin analyst will ask: why did this pile of sediment accumulate here, how fast did the basin subside through time, where did the sediment come from, what types of environments and climate existed here during deposition, and what were the driving structural, tectonic and geophysical forces that created the basin? Through integrative analysis that includes stratigraphy, sedimentology, paleocurrents, structure, regional tectonics, and physical modeling, we often are able to answer these questions and thereby gain a good understanding of the geologic, climatic, and tectonic evolution of a region.

We can understand how basins form by considering different tectonic settings, the main geologic processes active in those regions, and the related physical mechanisms that cause subsidence. The following table provides a summary of the main processes that create sedimentary basins, provided in the context of common tectonic settings found on Earth today and in the past. Of the following tectonic settings, we discussed (1) continental rift zones, (2) how they evolve to become passive continental margins, and (3) foreland basins produced by lithospheric flexure due to crustal loading in a thrust belt. Thrust loading and foreland basins can be found in both "Andean-type" margins, and zones of contintental collision. Note that the term "collision" in geology refers to a geometry of plate interaction that can (and often does) last for millions or tens of millons of years, not like a car accident on the highway.

Note: in this lecture we focus on divergent and convergent plate boundaries and related sedimentary basins, may not have much time to discuss strike-slip basins.

Plate-Tectonic Setting

Geologic Process

Subsidence Mechanisms

Basin Name

Continental Rift Zones

Extension, Crustal Thinning

Isostatic Subsidence

Rift Basin

Passive Continental Margins

Lithospheric Cooling

Thermal Subsidence

Miogeocline

Convergent Margins: Orogenic Fold-Thrust Belts, Continental Collision Zones

Crustal Thickening, Loading

Flexural Subsidence

Foreland Basin

Subduction Zones, Volc. Arcs

Possible Lithosph. Cooling

Possible Thermal Subs.

Forearc Basin; Trench, Trench-Slope Basins

Strike-Slip Fault Zones:

Transtensional

Oblique Extension

Isostatic Subsidence

Pull-Apart Basin

Transpressional

Oblique Contraction

Flexural Subsidence

Foreland-type

Stable Plate Interiors

Slow Cooling

Slow Thermal Subsidence

Intracratonic Basin


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