Driving mechanisms for globally correlatable Phanerozoic eustatic sequences?

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• Mike Simmons (Netflex Petroleum, Oxford)
• Tuesday 28 October 2008, 16:30-17:30
• Tilley Lecture Theatre, Department of Earth Sciences.

If you have a question about this talk, please contact Francis Rowland.

Large-scale changes in stratigraphic architecture and facies that are brought about by changes in relative sea-level have been the focus of much academic and industry study over the last few decades. Within industry in particular, the prediction of hydrocarbon play elements using sequence stratigraphic techniques is now well established. Six years ago we demonstrated the veracity of a sequence stratigraphic model for the Arabian Plate based on the recognition of 63 major stratal surfaces (maximum flooding surfaces (MFS) and sequence boundaries (SB)). Ongoing work, incorporating the stratigraphy of nearly all sedimentary basins on Earth, can now demonstrate the occurrence of >200 MFS and SB that are both synchronous and global in extent. These sequences are thus eustatic in origin.

This then raises the question – what causes globally synchronous eustatic sea-level change? To answer this question requires an understanding of the both the pace and amplitude of the observed eustatic sea-level change. In successions where duration can be deduced from orbital forcing cycles, our observed sea-level changes appear to be relatively rapid – less than 500,000 years, for example, for sea-level rises in the Late Jurassic. The amplitude of such rises is in the order of tens of metres. Such rates and amplitudes as inferred from our global model preclude tectonism as a primary driver and implicate glacio-eustacy as a key driving mechanism, even in supposed “greenhouse times”.

Given the clear economic importance of understanding the underlying mechanisms driving this eustatic change we have compiled records of key isotopic proxies through the entire Phanerozoic in an effort to explore the relationship between global sea-level and palaeoclimate. Our research reveals a clear link between the timing of some of our large-scale MFS events with known episodes of palaeoclimatic warming, as well as a similar link between our SB and climatic cooling.

Further, in addition to isotopic proxy evidence for polar ice presence, we have compiled direct evidence for the occurrence of polar ice sheets in the Phanerozoic (tillites, glendonites, etc). Both isotopic and direct evidence suggest the presence of polar ice sheets for periods previously supposed as ice free and that glacio-eustacy can be suggested as the major driver of Phanerozoic eustatic sea-level change.

This talk is part of the Department of Earth Sciences Seminars (downtown) series.

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