Accuracy of Eustatic Amplitude Estimates: Challenges in Flexurally Backstripping Continental Margins

Garry Karner
ExxonMobil Upstream Research Company, Houston, Texas ([email protected])

Measuring the amplitude of eustatic sea-level fluctuations has proven to be a difficult problem, whose resolution is essential for the establishment of an accurate eustatic sea-level curve that can be used to infer ice volume, rapid and long-term climate variations, and for the accurate prediction of sediment facies of depositional sedimentary systems. Amplitudes and rates of glacial-eustatic sea-level change obtained by backstripping studies and from marine δ¹⁸O records are markedly lower than those originally interpreted from seismic stratigraphic studies. During Leg 104, the Ocean Drilling Program drilled a sequence of wells on thecarbonate platforms of the Marion Plateau, Northeast Australia. In particular, an early middle Miocene highstand carbonate platform was drilled while in the adjacent carbonate clastic apron, an interval of shallow-water carbonates with hardgrounds overlain by deeper-water sediments was recovered. The geometric relationship between the exposed Miocene carbonate platform relative to the adjacent shallow water clastic apron offers an unparalleled opportunity to estimate the late middle Miocene eustatic fall (13.6-11.4 Ma).
The relatively small horizontal distance between the drilling sites (~20 km), the undisturbed and consistently dipping pre–carbonate platform strata observed on seismic reflection profiles, and the general lack of any major faulting suggests that no significant differential subsidence (i.e., faulting) has occurred between the drilling sites. In an attempt to maximize the accuracy of the late middle Miocene estimate of eustasy, corrections for thermal subsidence induced by Late Cretaceous rifting, flexural sediment loading, and sediment compaction were taken into account. The response of the lithosphere to sediment and water loads was considered for a range of effective elastic thicknesses (10 < Te < 40 km).
This presentation summarizes the most recent estimates for the amplitude of the late middle Miocene eustatic change based on flexural backstripping of the Marion Plateau stratigraphic sequences and discusses the main contribution to errors and error ranges.

AAPG Search and Discovery Article #90073 © 2007 AAPG Foundation Distinguished Lecturer Series 2007-2008