RADIAL PROGRADATION AND RAPID ABANDONMENT OF A GULF COAST SUBDELTA

Much of our knowledge of quantitative delta growth comes from either historical records that are limited in duration or modeling that is difficult to scale in time. While it is well-accepted that deltas follow a lifecycle, the rates and timescales of land change within this cycle are not well-known. Here, we address these issues using stratigraphic and geochronologic data from the ~10,000 km² Lafourche subdelta, Mississippi Delta over its ~1 kyr lifecycle.

The pre-Lafourche coastline is constrained with borehole data; south of this boundary ~6,000 km² of new land formed between 1.6 and 0.7 ka as Lafourche prograded into a shallow bay. Optically stimulated luminescence (OSL) ages of its mouthbar sands reveal that Lafourche prograded at an average rate of ~150 m/yr and created new land at ~6 km²/yr, sustained for centuries. The progradation occurred in three phases: 1) rapid progradation at the onset of sedimentation circa 1.55 ka, 2) steady progradation between 1.5-1.0 ka, and 3) rapid progradation as the subdelta approached abandonment from 1.0-0.7 ka. The rapid progradation at 1.55 ka indicates Lafourche mobilized and deposited a great quantity of sediment when it initiated, and that it was filling a relatively small volume basin. Growth then stabilized for an interval in middle of the subdelta’s lifecycle. The rapid progradation at 1.0-0.7 ka indicates Lafourche did not exhibit autoretreat; that is, a shift to aggradation and a loss of land area in late-stage deltas as sediment supply becomes insufficient to feed the delta front. Our data suggest instead that deltas may maintain the ability to build new land through progradation late in their lifecycle, while concurrently aggrading to keep pace with base-level rise. This is supported by the occurrence of multiple high-energy crevasse deposits in the upstream reaches that date to this late stage. We observe that mouthbars adjacent to Lafourche distributaries grew in a radial fashion throughout its ~1 kyr lifespan; mouthbars of major and lesser distributaries at similar river km from the paleo-coastline have matching OSL ages. Progradation and land creation rates of Lafourche have valuable implications for coastal restoration through river diversions, and may give insight into the future growth of nascent systems like the Wax Lake Delta.