We found that (1) a new channel-levee unit developed during the last stage of the Rhône Neofan evolution and (2) the last phase of up-building of this channel/levee system dates back to 15.1 ka BP (14C).
The crevasse splays and river levees units were predominantly characterized by herbaceous bushy marshy vegetation, with low mean NDVI values, while SB-C and BI-MCH units showed two types of forests.
The average tree-unit density (based on the number of tree crowns, either of a single tree or of cohesive trees, appearing in satellite images) was 6.27 units/ha, and was correlated with both density on the levee (5.30 units/ha on average) and levee length per unit paddy area (475.25 m/ha on average).
A ring levee may surround a whole parish.
Table 1 Geomorphic units of the study area Major geomorphic units Geomorphic features Fluvial or upper deltaic plain Natural levee Backswamps Meandering scrolls Ox-bow lake Channel bar River flood plain Other wetland Fluvio-tidal or lower deltaic plain Mangrove swamp Tidal flood plain.
Older units display fewer but greater-reservoir-quality channel and proximal levee facies as compared to thinner, more continuous, and unconfined sheet-like sands that characterize the younger units, suggesting a decrease in depositional gradient with time in the basin.
Units 3 and 4, which consist of wet levee deposits and stiff floodplain clays, respectively, formed during the subsequent phases of alluvial plain construction started around the Roman age (from ca. 2000 yr BP).
Three sand-rich units are described from the Walker Ridge site, including multiple ponded sand-bodies representing turbidite channel and associated levee and terminal lobe environments within the Terrebonne basin on the lower slope of the GoM.
