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The Mammal bed PAC (between 20-75 cm on meter scale); high-stand carbonate overlain by low-stand, dark organic-rich shale facies of the mammal bed, Lower Cretaceous, Durlston Bay, Dorset.
The PAC Hypothesis
(Goodwin and Anderson, 1985)
The PAC is the fundamental stratigraphic unit (Anderson and Goodwin, 1990). A PAC is a meter-scale lithic facies sequence bounded by surfaces of abrupt facies change from shallower to deeper facies (Fig. 5 PACs and Precession.ppt). The facies internal to a PAC typically shallow upward; however some PACs may be represented by a single facies. Sea-level fall surfaces are sometimes preserved internal to PACs. PACs are a product of the Earth's cycle of precession and in the genetic hierarchy of cycles and sequences are called 6th order.
The 5th Order Sequence is a stratigraphic bundle of up to five or six PACs produced by modulation of the precessional signal by the 100 ka variation in eccentricity of the Earth's orbit (Fig. 7 Milankovitch Stacking Patterns.ppt). Fifth order sequences are recognized by patterns of facies asymmetry that result from changes in magnitude of sea-level rises at successive PAC boundaries in the 5th order sequence. Larger facies changes mark boundaries lower in the sequence followed by progressively smaller facies changes at PAC boundaries higher in the sequence (Goodwin and Anderson, 1997).
The 4th Order Sequence is a bundle of four 5th order sequences produced by changing magnitudes of maximum eccentricity (over 400 ka) in a sequence of four eccentricity cycles . The 4th order sequence is again defined and recognized in the field by facies asymmetry. Larger facies changes occur at cycle boundaries earlier in the sequence smaller changes later. Fourth order sequences may in turn be bundled into sets of five with an analogous asymmetry of facies. These bundles are defined as 3rd order sequences and represent 2 ma of stratigraphic accumulation.
Note: It is typical for 5th order sequences to be the most obvious rock cycles in field exposures. Nevertheless PACs (6th order cycles) are the fundamental cycle because bundles at all scales are ultimately comprised of them and because the surfaces at all cycle or sequence boundaries are (at least in part) a product of precessional sea-level rises.
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