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Houston Geological Society Bulletin


Houston Geological Society Bulletin, Volume 18, No. 2, October 1975. Pages 16-16.

Abstract: Eustatic Cycles from Seismic Data for Global Stratigraphic Analysis


Previous HitPeterTop R. Vail, R. M. Mitchell, Jr., J. D. Sangree, and S. Thompson, III

Eustatic cycles are evident throughout Phanerozoic time because many relative changes of sea level determined from the stratigraphic record of different continents are simultaneous, and because the magnitudes of the relative changes generally are similar. These regional cycles are determined from sequences with coastal onlap, seen best on seismic sections with age and environmental control.

Coastal onlap is the progressive landward onlap of littoral and/or nonmarine coastal deposits. Coastal onlap indicates a relative rise of sea level; a seaward shift of coastal onlap for the next sequence indicates a relative fall. Sea level changes can be measured using either the vertical or horizontal components of coastal onlap (coastal aggradation or encroachment) provided that adjustments are made for divergence and/or slope-angle.

A eustatic cycle is defined as a relative rise, stillstand, and fall of sea level on a global scale. Single cycles are asymmetrical with a gradual rise to stillstand and an abrupt fall, but the causes for such asymmetry are not fully understood. Eustatic cycles are grouped into thirteen eustatic supercycles showing the same asymmetry, but on a larger scale. Megasequences (stratigraphic units corresponding to supercycles) correspond approximately to the original "sequences" of Sloss.

Ages and durations of eustatic cycles are essentially documented. Although their amplitudes are still being determined, examples from nearly all continents have been analyzed. The eustatic cycle "clock" is an excellent worldwide time scale for dating significant events in geological history.

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