Episodic overbank deposition as a dominant mechanism of floodplain and delta-plain aggradation

Abstract

The common view that frequent overbank flooding leads to gradual aggradation of alluvial strata on floodplains and delta plains has been challenged by a variety of studies that suggest that overbank aggradation occurs in a strongly episodic fashion. However, this remains a largely untested hypothesis due to the difficulty in establishing age models with sufficiently high resolution. Here we use 39 optically stimulated luminescence (OSL) ages from proximal overbank deposits in the Mississippi Delta to demonstrate for the first time that alluvial aggradation over centennial to millennial time scales is predominantly episodic, with aggradation rates of 1-4 cm/yr that can persist for centuries. OSL ages from three separate study areas produce age clusters that are distinctly different yet complement each other. These findings suggest that a substantial portion of the continental stratigraphic record consists of patchworks of relatively discrete, centennial-to millennial-scale sediment bodies assembled by autogenic processes.

Crevasse Splays Versus Avulsions: A Recipe for Land Building With Levee Breaches

“…10 8 m 3 and was formed by a roughly 4 m water level head during floods. Optically stimulated luminescence dating determined that aggradation rates were approximately 0.01-0.10 m yr À1 (Shen et al, 2015). If we consider a simulation forced with a 4-m water level head that resulted in a similar splay volume (experiment #6), we find maximum aggradation rates of about 0.5 m yr…”

“…The splay studied by Shen et al (2015) and Esposito et al (2017; Figure 1) has a volume of 1.6 . 10 8 m 3 and was formed by a roughly 4 m water level head during floods.…”

“…Inspired by recent, detailed studies of crevasse splays in the Mississippi River Delta (Esposito et al, 2017;Shen et al, 2015;Törnqvist et al, 2008; Figure S1 in the supporting information), we developed MATLAB soil consolidation (note that we use the term "soil" here broadly, that is, in a geotechnical rather than a pedogenic sense) and vegetation routines that feed back into Delft3D to study their effects on floodplain morphology. From our model experiments we develop an understanding of floodplain response to levee breaching and derive a simple predictive metric of erosional versus depositional floodplain response.…”

Efficient retention of mud drives land building on the Mississippi Delta plain

“…DR2 in Shen et al, 2015). We chose this particular crevasse splay because of the well-established stratigraphy based on 132 cores and a chronology based on extensive 14 C and optically stimulated luminescence (OSL) dating in the region (Törnqvist et al, 1996;Shen et al, 2015) and because its size and timescale of activity are in line with those of planned diversion projects in the MD (Coastal Protection and Restoration Authority of Louisiana, 2017).…”

“…We set this study in the Attakapas Crevasse Splay (ACS), a ∼ 60 km 2 landform that was constructed from 1.2 to 0.6 ka (Shen et al, 2015) and initially discharged into a mature cypress swamp that is now preserved as a regionally continuous wood peat bed underlying overbank strata (Törnqvist et al, 2008) in the Lafourche subdelta of the MD (Fig. 1).…”