Sequence Stratigraphy

Describe the sequences as described by Sloss (1963) and discuss the possible controls on their deposition.

Sequence stratigraphy is a system of linking unconformity-bounded sediment packages to global events to temporally correlate sedimentary units across lithographic boundaries. This is important due to the imperfect stratigraphic record, and the difficulty of correlating time across wide areas. A sequence is a conformable succession of related tracts. Sequences can be divided into parasequences — packages of strata divided by abrupt changes in sea level. By picking the peak of transgression deposition, it is possible to temporally correlate sequences across stratigraphy. It may also be possible to match unconformity cycles in a similar manner to matching magnetic reversal sequences. Sloss and others have hypothesized about repeating, global cycles that can be identified in the sediment record, to varying degrees of confidence.

1st order cycles: Supercycles (200-400 million years)
These long-term cycles are related major tectonic activity, particularly the formation and breakup of super-continents, and the continental distributions leading to icehouse and greenhouse climate conditions. Severe climate change and increased volcanic activity result in mass extinction events. The change in tectonics and climate leads to transgressive or regressive cycles, periods of high and low sea level and changes in the accommodation space for sediment accumulation.

2nd order cycles: Sequence/Synthem (10-100 million years)
These medium-term cycles are related to change in activity in mid-ocean ridge systems, with more active volcanism increasing crust volume (young, hot rocks are more voluminous than old, cold rocks). These are also times of increased intensity of the magnetic field, possibly related to changes in mantle convection or polar wander.

3rd order cycles: Mesotherm (1-10 million years)
Evidence for these shorter term cycles is more hazy; the temporal resolution is smaller than can be determined by biostratigraphy, so these cycles may not have global distribution. If they do, they may be due to crustal flexure and changes in the geoid.

4th order cycles: Cyclotherm (0.2-0.4 million years)
These extremely short-term cycles are likely related to the growth and decay of continental ice sheets and growth and abandonment of deltas, possibly driven by the Milankovich cycle. The observational evidence to support these cycles is uncertain — on such short timeperiods, the cycle may actually be smaller than gaps in the stratigraphic record.

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