Storm layer deposition on the Mississippi–Atchafalaya subaqueous delta generated by Hurricane Lili in 2002

Abstract

The Atchafalaya inner continental shelf, located along the north-central Gulf of Mexico offshore of Louisiana, is an area of rapid mud accumulation associated with the progradation of a subaqueous delta originating from this Mississippi River distributary. In September–October 2002, this region was impacted by two tropical cyclones (Tropical Storm Isidore and Hurricane Lili) separated by only 7 days. Water-column and hydrodynamic records from coastal observation platforms (WAVCIS network) are combined with seabed sampling 4–7 days after passage of Lili, to examine the impact of these events on the Atchafalaya inner shelf. Wind speeds at the CSI-3 platform on the delta (located in 4.5 m of water) peaked at 20 m/s during Isidore, and more than 30 m/s during the closer, and stronger, Lili event. Significant wave heights during Lili peaked at more than 2 m at the CSI-3 platform, coincident with a storm surge of about 2 m. Water-column flow structure during both storms was closely tied to the storm surge (coastal setup–setdown) cycle despite variations in wind direction with storm passage. Flow was onshore throughout the water column during the waxing phase (1.5 days in Lili, 4 days in Isidore), with a rapid (1–2 h) reversal to offshore flow after storm passage (∼12 h waning phase). Flow velocities remained above 1 m/s throughout the ADCP-measured water column (>65 cm above the bottom) for more than 2 days during the Lili event. Sediment cores reveal the presence of a basal erosional surface, hypothesized to represent seabed deflation from the combined resuspension attributable to both storms, overlain by a silty clay storm deposit 2–19 cm thick. Comparison with 7Be seabed profiles and X-radiographs taken at two delta stations (5 m water depth) prior to and following the storm suggests erosional deflation of 3–13 and 7–17 cm occurred at these stations. The overlying, physically stratified storm deposit contains radioisotopic inventories ( 7Be, 234Th, 137Cs, 210Pb) that are consistent with an origin primarily from re-deposition of particles resuspended in the waxing phase of the storm. X-radiography and granulometry suggest two-phase re-deposition: an initial, normally graded basal deposit 1–2 cm thick containing sand that likely was deposited from normal settling, and a slightly normally graded, sand-poor unit hypothesized to be deposited from consolidation of a fluid mud (>10 g/l), hindered settling suspension later in the waning phase. Macrofaunal burrows in the storm deposit suggest rapid (days) settlement of surviving fauna, likely due to high abundance in the sediments at this time of year when burial rates (from Atchafalaya River sediment supply) and energies sufficient for bottom resuspension are normally low.