THE SEQUENCE STRATIGRAPHY REVOLUTION: WHERE NOW?

The sequence stratigraphy revolution grew in the 50s with the discovery of unconformity-bounded stratigraphic units. Further fuel to the fire came in the 1960s and 70s from process sedimentology, particularly the work on modern coastal processes and on Mississippi river channels, and especially from the use of subsurface data. Sloss’s students at Exxon gained access to global seismic data sets but were significantly influenced by their graduate work on Icehouse cyclothems, so moving away from Sloss's time scales and tectonic mechanisms. They had also spotted the onlap-downlap motif on global seismic data, that became the sequence model of the late 70s. Weaknesses of the concept were debated in the 1980s and 90s, particularly a heavy reliance on glacio-eustasy and inbuilt relationship between systems tracts and sea-level curve. However, the lowstand model for shelf-edge incision and the development of basin-floor fans worked, and was replicated in outcrops in the 1990s on Spitsbergen. A key contribution of the Exxon model was to link the shelf with the deepwater system, something not done by sedimentologists earlier.

Where is sequence stratigraphy going now? The main value of the concept remains its predictive power and its systematic methodology. But new directions emphasize the value in having multiple explanations for observed stratal stacking, for example that sand can equally well be delivered across wide shelves to deepwater areas during rising and highstand of sea level, that transgressions and regressions occur at the same time along different reaches of coastlines or that transgressions can be autogenic, self-organizational responses with constant forcing of the main drivers. Much of this is now driven by physical and numerical modeling. The same experiments suggesting that more than half the medium and large sized river deltas today can reach their shelf edges at sea-level highstand also suggest that the basic regressive-transgressive shelf transit that generates the fundamental stratigraphic sequence requires no more than 1-200Ky, an order of magnitude shorter time than the original seismic sequences. Current research on source-to-sink sediment-budget partitioning and on autogenic responses in stratigraphy are among the new exciting paths in dynamic stratigraphy.