ABSTRACT: Sedimentary Signatures--Tectonics Vs. Eustasy

Peter R. Vail

Neogene stratigraphic successions from all continents are characterized by very similar stratal geometries. These stratal geometries can be packaged into globally correlative sequences and systems tracts for both tectonically active and passive basins and siliciclastic or carbonate lithofacies. In areas where the rate of subsidence changes slowly with sufficient sediment supply to prograde into deep water, the Neogene stratal patterns visible on seismic sections or in correlated well log and outcrop sections are very similar. This pattern is termed the "global stratigraphic signature" of the Neogene and is characterized by the following stages: (1) lower Oligocene landward thickening, (2) upper Oligocene basinward onlap wedge, (3) basal lower Miocene flooding, (4) lower M ocene (Aquitanian) aggradation commonly ending with lowstand deposits, (5) lower Miocene (Burdigalian) aggradation commonly ending with major lowstand deposits, (6) middle Miocene (Langhian and lowermost Serravallian) flooding, (7) middle Miocene (Serravallian) major progradation, (8) end of middle Miocene lowstand deposits, (9) upper Miocene aggradation commonly ending with lowstand deposits, (10) lowermost Pliocene flooding, (11) Pliocene-lower Pleistocene aggradation with multiple lowstand deposits, and (12) upper Pleistocene high-frequency sequences.

In areas of tectonic uplift or abrupt increases in subsidence rates, the characteristic Neogene stratal patterns are distorted and the sequence and systems tract boundaries are either enhanced or obscured. However, the age of these discontinuities is not changed when dated at their conformity.

Sequence stratigraphic and geohistory/backstripping analysis of data from the western Atlantic, offshore New Jersey; Gulf of Mexico, offshore Alabama and Texas; Maracaibo basin, Venezuela; Canterbury basin, New Zealand; northeast Java, Indonesia; Sabah; Bahamas; and Ross Sea, Antarctica demonstrate that the stratigraphic signature of tectonics and eustasy can be identified and separated. Tectonics (1) creates the major space for the sediments, (2) controls, in conjunction with climate, the sediment supply and the major transgressive and regressive facies cycles, and (3) enhances or obscures the sequence and systems tract boundaries. Eustasy causes the sequence and systems tract boundaries.

Evidence of eustatic control of the sequence and system tract boundaries is demonstrated in two ways. First, the global Neogene stratigraphic signature is similar to the Neogene stratal patterns in the Ross Sea, Antarctica, where the sequence and systems tracts are controlled by the waxing and waning of marine-based ice sheets. The stratal patterns in the Ross Sea suggest that large fluctuations in ice volume occurred during the Neogene. The global Neogene stratigraphic patterns are thought to have been produced by the eustatic changes that accompanied the Antarctic ice-volume fluctuations. Second, a computer simulation of the Neogene global stratigraphic signature demonstrates quantitatively how eustasy caused the Neogene stratal patterns.

Characteristic stratigraphic signatures also exist for the Paleogene, Upper-middle Cretaceous, and Lower Cretaceous-Upper Jurassic. These signatures are believed to have been caused by a similar combination of eustatic and tectonic controls.

AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990