Orbital forcing of low‐frequency glacioeustasy

Abstract

The role of orbital forcing in glacioeustasy has long presented a major problem regarding the reconciliation of data‐oriented seismic sequence stratigraphy with process‐oriented dynamic stratigraphy. Dynamic stratigraphy commonly cites orbital forcing periods of approximately 20,000 years (precession), 40,000 years (tilt), and 100,000 years (eccentricity) which are too fine scale to be resolved consistently by seismic sequence stratigraphy. Seismic sequence stratigraphy consistently identifies seemingly eustatic events on a scale of millions of years which have no heretofore easily identifiable causal mechanisms. In this paper, we propose a geologic mechanism and realistic quantitative construct to explain how Antarctic ice volume varies as a function of orbital forcing in the Tertiary. We demonstrate that this mechanism, which has a nonlinear response to long‐period modulation of the orbital forcing time series, can produce major glacioeustatic events with quasi‐periodicities of the order of 2 m.y. We use a FORTRAN program, STRATA‐various, to construct a two‐dimensional forward model demonstrating that this proposed mechanism can produce a synthetic sequence stratigraphy which bears strong resemblance to the generalizations of seismic sequence stratigraphy. We acknowledge important achievements of seismic sequence stratigraphy. Nevertheless, we propose that the seismic sequence stratigraphy concepts of long‐term and short‐term eustatic curves be replaced by independent estimates of tectonoeustasy and glacioeustasy based upon data sets which are, wherever possible, independent of seismic sequence stratigraphy. Likewise, we propose that qualitative generalizations be replaced with explicit forward models as the targets for model/data convergence.