Modelling Falling Stage Topset Aggradation and Shoreline Trajectories: Implications for Distinguishing Forced and Unforced Regressions in the Ancient Record
Prince, Guy; Burgess, Peter
Distinguishing between forced and unforced regressive strata is important for prediction of sediment bypass and reconstruction of relative sea-level histories. Conventional sequence stratigraphic models distinguish between forced and unforced regressive strata through presence of aggradational topset and style of shoreline trajectory. However, because present models contain implicit assumptions regarding sediment supply and the response of coastal plain and fluvial depo-systems to falling and rising relative sea-level, it is possible that these two scenarios are an over simplification.
This work uses a diffusional stratigraphic forward model to investigate how (1) topset aggradation might develop during relative sea-level fall and (2) how shoreline trajectories might vary in response to different sediment supply and relative sea-level histories. Multiple 2-D model runs suggest that sediment transport rate can be a key control on topset aggradation. Modelling a range of sediment transport rates for amplitudes of relative sea-level fall from 0 to 100 m shows that high sediment transport with no relative sea-level fall leads to topset aggradation very similar to that resulting from low sediment transport and high amplitude relative sea-level fall. This is an example of non-uniqueness, demonstrating that topset aggradation can occur during falling relative sea-level as well as steady to rising relative sea-level, depending on rates of sediment transport. This result has been tested and verified with different rates of relative sea-level fall and with 3-D model runs, as well as with an alternative stratigraphic forward model. Shoreline trajectories generated in models with different sediment supply and relative sea-level histories also show a degree of non-uniqueness. This is because shoreline trajectories created with a complex relative sea-level curve and steady sediment supply can look similar to shoreline trajectories created with steady relative sea-level rise and a complex sediment supply history.
These results imply that shoreline trajectories are sensitive to both accommodation and supply controls, and that sediment transport rate is an important control on topset aggradation. Consequently, reconstruction of relative sea-level histories from shoreline trajectories may be unreliable, and interpretation and prediction of sediment bypass history may be more complicated than current sequence stratigraphic models suggest.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013