THE INFLUENCE OF GRAIN SIZE ON FLUVIO-DELTAIC RESPONSE TO SEA-LEVEL SIGNALS

Fluvio-deltaic deposits are formed by the interplay of upstream and downstream environmental variables, such as upstream grain-size input and downstream sea-level conditions. Both grain size and sea-level signals have been shown to separately influence fluvio-deltaic sedimentary processes and ultimate stratigraphic architecture. The interpretation of these environmental signals in the stratigraphic record is an essential step in stratigraphic techniques, such as sequence stratigraphic analysis. However, previous work suggests that environmental signals may interact with each other, as well as with autogenic fluvio-deltaic processes, resulting in signal modification that makes their interpretation from the sedimentary record difficult or impossible. It remains unclear how grain size and sea-level signals interact within the fluvio-deltaic realm and the extent to which the signals are preserved in the deposits. To explore these questions, we numerically simulate fluvio-deltaic growth under a set of varying grain-size conditions and sea-level signals using a one-dimensional source-to-sink model of sediment transfer. Results suggest that both grain size and sea-level rise rate have positive relationships with the system’s autogenic cycles of deposition and erosion: systems composed of coarser grains and subjected to faster sea-level rise rates have more frequent avulsions. These relationships are likely caused by higher aggradation rates in such systems, which in turn cause increased occurrence of superelevation-induced avulsions. Additionally, grain size influences the maximum upstream propagation extent of sea-level signals from the shoreline. Coarser-grained systems have decreased upstream propagation distances, whereas finer-grained systems record sea-level signals farther upstream of the shoreline. These results may prove valuable in determining the extent to which we expect to see sea-level signals preserved in different depositional systems and suggest that it may be challenging to form sequence stratigraphic interpretations in coarser-grained, steep fluvio-deltaic systems due to the less extensive sea-level signal preservation.