An Evolutionary Model of a Retrograding Subdeltaic Distributary of a River-dominated System

Robert Clark, Duncan FitzGerald, and Ioannis Georgiou
[email protected]

The Mississippi River delta is experiencing some of the highest rates of relative sea-level rise in the world. Confinement of its river course has resulted in drastic reductions in sediment discharge since the mid-20th century leading to dramatic land loss in the delta. During the past 20 years, dredging at Head of Passes and deepening of Southwest Pass has caused shoaling and decreased discharge in South Pass and Pass a Loutre. Reduced flow down South Pass has resulted in wave reworking of distributary mouth bars. As this sand is moved onshore subtidal bars and then barrier spits form, lengthen, and extend along shore. Tidal inlets develop as barriers are breached during major storms and backbarrier tidal prisms are exchanged through these openings. Gradually, the sand supply is diminished and the barriers become highly transgressive dominated by washovers, spit extension and updrift barrier erosion. Eventually, overwash fans and tidal inlet generation lead to the development of subtidal sand sheets. South Pass exhibits many different stages of this evolutionary trend.

AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63^rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013