ABSTRACT

Pleistocene alternations of ocean volumes (expressed as relative sea-level changes) are mainly related to accumulation (sea-level lowering) and melting (sea-level rising) of continental ice sheets; these sea-level changes are designated as eustatic cycles. Although eustatic cycles punctuate the geologic record throughout the Phanerozoic, the eight major events documented for the Pleistocene define a unique interval of earth history. A significant change in marine plankton faunas and carbonate composition caused in part by lowering of the oceans' surface temperature corresponds closely with widespread continental glaciation that seemingly is confined to the past 2.5 to 3.0 million years. Pleistocene climatic cycles were markedly more frequent, shorter in duration, more severe than those of the Tertiary and resulted in significant sea-level changes associated with notable expansion of middle-latitude continental ice sheets. Pre-Pleistocene sea-level changes probably resulted from different factors, including global tectonic forces. Little or no evidence exists for middle-latitude continental glaciations (tills) prior to about 3.0 million years before present (base of Pleistocene) although cooling and pene-contemporaneous sea-level lowering have been documented (e.g., in the Late Miocene). The Antarctic ice cap and associated sea ice probably originated in the Early Oligocene and attained relative invulnerability to climatic change by Late Miocene time. Bipolar sea-level ice sheets developed approximately 3.0 M.Y.B.P. (near the beginning of the Pleistocene) and probably continued to exist, even during warmer interglacial periods, much as they do now.

Seismic sequence analysis is based on the identification of stratigraphic units composed of a relatively conformable succession of genetically-related strata termed depositional sequences. Seismic sequence analyses provide a sound basis for applying the global system of geochronology to seismic data for improvement of stratigraphic and structural interpretations. Moreover, seismic sequence analyses in new exploration areas allow a rather accurate estimate of geologic age ahead of the drill and facilitate preliminary tectonostratigraphic reconstructions.

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