A tale of two deltas: seismic mapping of near surface sediments on the Mississippi–Alabama outer shelf and implications for recent sea level fluctuations

High-frequency seismic reflection profiles showing near-surface sediments of the Mississippi–Alabama outer shelf define two shelf-margin deltas and three unconformities. The deepest unconformity lies below the westernmost delta, the middle unconformity separates the two deltas, and the shallowest unconformity is atop the easternmost delta. The deeper two unconformities are extensive and erosional to the shelf edge, implying Type-1 sequence boundaries. The shallowest is restricted in its depth range and extent, suggesting a Type-2 boundary. Both delta wedges contain sigmoid reflectors, representing foreset beds, and have apparent transgressive layers at their bases. Thus, these two sediment wedges and their bounding unconformities represent parts of three sea level cycles. Although we lack good age constraints, the middle unconformity appears to represent the last glacial maximum, so the western delta must have been deposited before and the eastern delta, afterwards. Internal reflector geometries suggest the eastern delta was deposited during an episode of falling sea level, implying a relative sea level regression within the overall deglacial transgression.