Fluvial response to sea-level changes: a quantitative analogue, experimental approach

Quantitative evaluation of fluvial response to allogenic controls is crucia l for further progress in understanding the stratigraphic record in terms o f processes that control landscape evolution. For instance, without quantit ative insight into time lags that are known to exist between sea-level chan ge and fluvial response, there is no way to relate fluvial stratigraphy to the sea-level curve. It is difficult to put firm constraints on these time- lag relationships on the basis of empirical studies. Therefore, we have sta rted to quantify time-averaged erosion and deposition in the fluvial and of fshore realms in response to sea-level change by means of analogue modellin g in a 4 x 8-m flume tank. The rate of sea-level change was chosen as an in dependent variable, with other factors such as sediment supply, discharge a nd initial geometry kept constant over the course of 18 experiments. Our ex perimental results support the common view that neither fall nor rise in se a level affects the upstream fluvial system instantaneously. An important c ause for the delayed fluvial response is that a certain amount of time is r equired to connect initial incisions on the newly emergent shelf (canyons) with the fluvial valley. Lowering of the fluvial longitudinal profile start s only after the connection of an active shelf canyon with the fluvial vall ey; until that moment the profile remains steady. We quantified the process of connection and introduced the quantity 'connection rate'. It controlled , in conjunction with the rate of sea-level fall: (1) the amount of fluvial degradation during sea-level fall; (2) the total sediment volume that bypa sses the shelf edge; (3) the percentage of fluvial relative to shelf sedime nt in the lowstand delta; (4) the volume of the transgressive systems tract and (5) the amount of diachroneity along the sequence boundary. Our experi ments demonstrate also that the sequence-stratigraphic concept is difficult to apply to continental successions, even when these successions have been deposited within the influence of sea level.