Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion

Abstract

Tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale, making use of improved chronostratigraphic control and additional Oligocene isotope data. Synchronous changes in both benthic and planktonic δ 18 O values which occurred in the Oligocene to Miocene (36–5.2 Ma) are interpreted, in part, to represent ice growth and decay. The inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records. This association is consistent with a link between Oligocene to Miocene erosional events and rapid (>15 m/m.y.) glacioeustatic lowerings of about 50 m. High benthic foraminiferal δ 18 O values suggest the presence of continental ice sheets during much of the Oligocene to Recent (36–0 Ma). Substantially ice‐free conditions probably existed throughout the Paleocene and Eocene (66–36 Ma). The mechanisms and rates of sea level change apparently were different between the early and late Tertiary, with glacioeustatic changes restricted to the past 36 m.y. Pre‐Oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes. We apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands, stimulating slope failure and submarine erosion. The different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late Tertiary on passive continental margins. This speculation needs to be confirmed by examination of data from several passive margins.