Progradation Rates of Marine Deltas under Influence of Longshore Current - a Numerical Modeling
Stable to falling sea-level, or increasing sediment supply shift shorelines basinwards and tend to force deltaic depocentres to the shelf-edge where they become an important source for deep-water sands. As a result, shelf-edge deltas may develop not only during periods of forced regression and lowstand, but also during a time of slow relative sea-level rise and highstand when sediment supply to deltas is sufficiently high to exceed rising accommodation.
Recent numerical modeling for stable sediment supply and the present-day relative sea-level rise of 2.1 ¬¬m/ka rate has indeed shown that many modern deltas appear to be able to cross their entire shelf width within time range as short as 2.5 - 70 ka. Factors involved in these simulations include rate of sediment supply, rate of relative sea-level rise, shelf and delta gradients, and sediment compaction, whilst the role of marine erosion and sediment redistribution have remained unaccounted for. In particular, nearshore circulation may result in at least in temporary removal of part of a delta sediment, thus significantly slowing delta cross-shelf progradation on a 4th-order cycle scale. The non-cohesive sediment loss for a delta budget due to alongshore flow activity and the impact of this on shelf transit times were modeled here for five modern river deltas (Ebro, Kizil Irmak, Nile, Rio Grande, Brazos, Colorado, Krishna, Orange) based on CERC formula. Results have shown that sediment redistribution by longshore drift alone may lengthen the time of delta progradation to the shelf-edge by 4-51 % for 2.1 ¬¬m/ka relative sea-level rise. The strongest increase of a potential cross-shelf transit time was found for the wave-dominated Orange Delta, which enters open ocean. For deltas facing basins of smaller fetches, storm-intensified coastal erosion can also be a critical factor. For example, inclusion of storm data into simulations for the Ebro Delta resulted in an increase of its initial cross-shelf transit time by 9%.
These preliminary results suggest that 4th-order highstands may be too short in duration for the development of deltas at the shelf edge of broad, passive margin shelves, especially during Icehouse times.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009