ABSTRACT: The Link between Deep Sea Erosion and Relative Sea Level Changes: Implications for the Oligocene Eustatic Event

KARNER, G. D., J. P. MCGINNIS,* and N. W. DRISCOLL, Lamont-Doherty Geological Observatory, Palisades, NY, and W. D. BRUMBAUGH and N. CAMERON, Conoco, Worldwide Exploration Services, Houston, TX

Seismic reflection and drilling data from many passive margins document a prominent erosional event near the early/late Oligocene boundary (~29 Ma) in the form of canyon cutting and incision into the pre-existing strata. This Oligocene erosional event is independent of the tectonic and sedimentologic history of the margins and has been proposed as undeniable evidence for the existence of eustatic sea level changes. The early/late Oligocene sea level fall is regarded as one of the largest eustatic events during the Tertiary. A variety of studies based on oxygen isotope records, coral growth, and stratal geometries estimate the magnitude of this Oligocene eustatic fall to range between 30 and 150 m. Seismic stratigraphic evidence along many margins exists for a late Eocene/early Oligoce e (~38 Ma) deep sea erosional event that is best developed along the slope and rise. A synchronous increase in both the planktonic and benthic oxygen isotopic records (Carbon 18/Carbon 16) occurred at this time and is generally interpreted to indicate the onset of ice growth and the establishment of pole to equator climatic gradients. These gradients and the ensuing formation of cold bottom water are often invoked to explain the prevalent deep sea erosion associated with the late Eocene/early Oligocene event. We here present data from the West African and the U.S. east coast margins that suggest a possible link between the late Eocene/early Oligocene deep sea erosional event and the early/late Oligocene shallow water erosional event. To investigate this link, we have used ODP and DSDP dr lling results, and academic and industry seismic reflection and refraction data to model the thermal and chronostratigraphic development of these margins. Our modeling of these margins links the early/late Oligocene relative sea level changes with the isostatic rebound of the margins in response to late Eocene/early Oligocene submarine erosion of the continental slope and rise. The form of this rebound is critically governed by two factors: (1) the amount and distribution of the sediment load removed from the margin and (2) the flexural strength of the margin at the time of erosion. Our modeling indicates that the isostatic rebound caused by unloading is sufficient to uplift these margins to at least wavebase and perhaps expose them subaerially. Our results, while not precluding the exis ence of an early/late Oligocene eustatic sea level fall, suggests that its magnitude has been significantly overestimated for passive margins with late Eocene/early Oligocene submarine erosion.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)