QUATERNARY LANDSCAPES - Fundamentals of Geomorphology - page 374

Table 14.5 Quasi-periodic sea-cycles revealed by seismic stratigraphy

Cycle order Duration (years) Eustatic expression Possible cause

First More than 50 million Major continental flooding cycle Change in ocean volume owing to sea-floor

spreading rates and continental dispersal

Second 3 million to 50 million Transgressive-regressive cycles Change in ocean volume owing to sea-floor

spreading rates

Third 500,000 to 3 million Sequence cycles Unclear

Fourth 80,000 to 500,000 Systems tracts Orbital forcing in the Croll-Milankovitch

frequency band

Fifth 30,000 to 80,000 Episodic parasequences Orbital forcing in the Croll-Milankovitch

frequency band

Sixth 10,000 to 30,000 Episodic parasequences Orbital forcing in the Croll-Milankovitch

frequency band

Source: Partly adapted from Vail et al . (1991)

has a distinct signature. First-order cycles reflect major
continental flooding. Second-order cycles register
facies changes associated with major transgression-
regression cycles. The lower-order cycles record strati-
graphic deposition sequences, systems tracts (sets of

linked contemporaneous depositional systems), and
parasequences (the building blocks of systems tracts). The
changes in sea level resulting from the first- and second-
order cycles can be as much as 250 m (Figure 14.9).
Such very long-term changes of sea level provide a useful

Cambrian Ordovician Silurian Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Cenozoic

200

100

0

500 400 300

Age (millions of years)

200 100 0

Figure 14.9 Proposed Phanerozoic eustatic curve.
Sources: The Palaeozoic section is adapted from an average estimated by Algeo and Seslavinsky (1995) and the Mesozoic
and Cenozoic portion is adapted from Haq et al . (1987)