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The Effects of Relative Base Level Fall on River Deltas: A Quantitative Assessment


Correct sequence stratigraphic interpretation of falling stage deltas depends upon their downward-stepping geometry, but the dependency of this geometry on basin floor characteristics, sediment supply, and rate of base level fall is poorly known. Here we document and quantify the morphologic and stratigraphic character of forced deltaic regressions using Delft3D numerical simulations tested against observations from the Goose River delta, Labrador, Canada. For a fixed sediment flux and caliber and basin bathymetry, the numerical simulations predict that the number of active distributaries decreases with an increase in the rate of base level fall and that the shoreline becomes increasingly irregular, whereas mean clinoform dips near the rollover and clinoform concavity show no such dependence. These results are compared to the Goose River delta, a coarse-grained, fan delta prograding into a 30m deep fjord. The Goose River delta has experienced 5 mm per year of relative base-level fall during the past 7,500 years due to post-glacial rebound. The delta consists of four down-stepping lobes, whose topsets now sit at 15, 4.5, 1, and 0 m asl and were each fed by 15 to 20 distributary channels, and whose shorelines are relatively smooth and arcuate. Foreset dips of the modern delta at the clinoform rollover average 17°, similar to preserved clinoforms of the older lobes which average 18°. These results are used to reinterpret the Cretaceous Panther Tongue Mbr. of the Starr Point Fm, central Utah, which is conjectured to comprise a falling stage systems tract.