Shelf genesis revisited
|Location||International Geological Congress,oslo 2008|
|Holding Date||08 October 2008|
Whereas several classification systems exist for modern shelves, ancient shelves are, except for those that are well-imaged on seismic data, dominantly classified according to processes, with little or no focus on morphology and gradient-profiles and without addressing whether a shelf-edge was present or not. This is because large-scale geometric information often is absent in outcrop or well based data sets that include supposed shelfal sediments. In this presentation, ancient and modern shelves are reviewed and their genesis discussed. The focus has been on geometrical aspects rather then present and past processes of deposition. In particular, shelf-genesis with emphasis on the impact of underlying structure relative to sedimentation, and the use of shelf, platform, slope and ramp terminology will be debated.
Porêbski and Steel’s (2003) morphological definition of a shelf as "a shallow-marine platform located around the margin of a deeper basin irrespective of the basin’s tectonic setting" is followed. Joint genetic features of shelves are: 1) deep frontal waters; 2) a hinterland that can deliver sediments in significantly large amounts to prograde a margin; and 3) transgressions that flood low-gradient coastal and alluvial plains. Shelves can be categorized according to the relative influence of structure and sedimentation in the margin-progradation in general and the development of a break-in-slope (shelf-edge after transgression) in particular. A three stage division of shelves according this criterion has been proposed:
Type 1): For this type the prograded margin owes its relief to stratigraphic climb and/ or a basinward dipping depositional foundation in combination with nucleation, propagation and amplification of the break-in-slope at the shoreline through successive shoreline transits. The only contribution from tectonics is long term, evenly distributed or sagging style basin subsidence. The maximum basinward level of margin progradation is dictated by overstepening or overextension.
Type 2): This type has all the characteristics of the Type 1, except that the control for the maximum basinward level of margin progradation is being dictated by underlying structure. If underlying structure has, at some level, a marked increase in its basinwards directed dip, the prograding margin is likely to achieve its maximum extent of progradation at this point owing to oversteepening.
Type 3): A Type 3 prograded margin has a clear relation to the underlying structure in the sense that sedimentation has inherited topography from structure, but that this topography, in form of a break-in-slope, has been propagated and displaced relative to its nucleation point.
Additionally, a shelf type dictated by structure and not related to prograded margins can be defined. Here, the only role of sediment is the draping of smaller scale topography. The shelf-edge is entirely of structural origin (e.g. fault escarpment), and has not been prograded.