Patterns and Rates of Deltaic Aggradation and Progradation: Insights from the Mississippi Delta
Abstract:The aggradation and progradation of deltaic plains over stratigraphically relevant (centennial to millennial) timescales is not well constrained, largely due to the difficulty to directly date clastic strata. Here we address this problem by means of ~50 new OSL ages from the late Holocene Mississippi Delta. We focus on the Lafourche subdelta, a precursor of the modern Mississippi River that was active from ~1500 to 600 years ago and built a land area of ~10,000 km2. The upstream reach of this system (~50 km in length) exhibits widespread aggradation by means of crevasse splays that is highly episodic in nature. The fluviodeltaic stratigraphic record at any given locality within this reach consists of a patch work of discrete, autogenic sediment bodies, with century-scale aggradation rates typically ~1 to 4 cm/yr. Sand accounts for only ~5% within these overbank strata. Hence, inland crevasse splays are extremely efficient at trapping muddy overbank sediment, with a retention rate conservatively estimated at >60%. We attribute this to the fact that aggradation occurred within a swamp, where vegetation-induced roughness enhanced the settling of the fine grain-size fractions.
In contrast, the lowermost ~100 km of the Lafourche subdelta formed by progradation into shallow open waters of the Gulf of Mexico. The stratigraphy within this reach is substantially different and features mouth-bar sheet sands ~2-4 m in thickness, similar to the modern Wax Lake Delta. These mouth bars are draped by mud-dominated overbank deposits that thin in a seaward direction, likely due to the shorter time span of aggradation in the lowermost reaches. OSL dating of the mouth-bar sands enables us to quantify the progradation rate of the shoreline. Collectively, the new chronological constraints on aggradation and progradation rates have implications for the potential rates of wetland creation by proposed river diversions along the modern Mississippi River.