Abstract: Eustatic Cycles from Seismic Data for Global Stratigraphic Analysis

Peter R. Vail

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 eustatic cycles are determined best on seismic sections with age and environmental control. Charts show age, duration, and amplitude of the eustatic cycles. The basic stratigraphic units for determining eustatic cycles are sequences of coastal onlap. Significant applications of eustatic cycles include: (1) building a regional stratigraphic framework, (2) providing a global stratigraphic zonation, (3) predicting the geologic age of units prior to drilling, and (4) predicting shelf versus slope depocenters.

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. Eustatic cycles are grouped into 13 supercycles showing the same asymmetry, but on a large scale. Stratigraphic units corresponding to eustatic cycles are called sequences of coastal onlap and stratigraphic units corresponding to supercycles are called megasequences. These megasequences approximately correspond to the original "sequences" of Sloss.

The overall shape of the eustatic cycle curve for the Phanerozoic shows relatively small sea-level fluctuations in the Paleozoic, followed by a large rise culminating in the Late Cretaceous with an irregular drop during the Tertiary. Times of low sea levels are characterized by erosional unconformities on the land and widespread nondepositional unconformities in the ocean basins. Examples of major pre-Pleistocene lowstands are late Pliocene, late Miocene, middle Oligocene, late early Eocene, middle Paleocene, Valanginian, post-Hettangian (Early Jurassic), latest Permian-Early Triassic, late Wolfcampian, earliest Pennsylvanian, late Ulsterian, Chazyan, and Early Cambrian times. Times of high sea levels are characterized by widespread coastal and shallow-marine deposits on the continent . Examples of major pre-Pleistocene highstands are early to middle Pliocene, middle Miocene, late Eocene-early Oligocene, late Paleocene-earliest Eocene, Late Cretaceous, Late Jurassic (Berriasian), Late Triassic, Late Permian, Late Pennsylvanian-early Wolfcampian, Mississippian, Middle Silurian, Late Cambrian-Early Ordovician times.

The fundamental seismic stratigraphic unit is the sequence which is a stratigraphic unit bounded by unconformities or their correlative conformities at its top and base. The time interval represented by strata of a given sequence may differ from place to place, but the difference is confined to synchronous limits marked by ages of the sequence boundaries where they become conformities. Eustatic cycles are defined from sequences with coastal onlap which 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 aggradatio or encroachment) provided that adjustments are made for divergence and/or slope angle.

Coastal onlap is a better criterion of relative change of sea level than are cycles of transgression-regression or bathymetric changes. Transgressions occur during relative rises of sea level, but in our experience regressions also occur during periods of relative rise of sea level and stillstands. We have not documented with seismic data stratigraphic units deposited during a fall of sea level.

Causes of eustatic cycles are not understood fully. Plate tectonics may be responsible for long-term volume changes of ocean basins affecting the overall shape of the curve. Short-term fluctuations commonly are related to glacial events. Most eustatic cycles are of intermediate duration. Their rates of change commonly are similar to those of glacial events. However, there is evidence for some tectonic control.

Ages and durations of eustatic cycles are essentially documented. Although their amplitudes still are 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 geologic history.

AAPG Search and Discovery Article #90977©1975-1976 Distinguished Lectures