IDENTIFICATION OF STRATIGRAPHY AND TEMPORAL SCALING OF MASS-WASTING SEDIMENT TRANSPORT REGIMES ACROSS THE MISSISSIPPI RIVER DELTA FRONT DELINEATED BY 210PB/137CS GEOCHRONOLOGY

The Mississippi River Delta Front (MRDF) is a subaqueous apron of rapidly depositing and weakly consolidated sediment extending from the subaerial portions of the Birdsfoot Delta of the Mississippi River, long known to be characterized by mass-wasting sediment transport. In order to better understand spatio-temporal scales of controlling processes within the MRDF, twenty-eight piston cores collected seaward of river channel outlets, up to 8.9 m in length, were analyzed for magnetic susceptibility, and bulk density with the addition of CHIRP sub-bottom seismic data. For this study, eight cores from the south and southwest passes were selected to explore sediment properties in specific depositional environments (undisturbed proximal apron, medial mudflow gully, mudflow lobe, and prodelta) and further analyzed for grain size, sediment fabric (X-radiography), and geochronology (210Pb/137Cs radionuclides) down to a depth of up to 724cm. Building on previous work (Keller et al 2017, Coleman et al 1980), data indicates mass wasting sediment transport processes can be detected and measured by diagnostic properties preserved in sediment cores. Within the mudflow gully and lobe core, homogeneous 210Pb/137Cs activities and corresponding stepped profiles of bulk density indicate the presence of dm/m-scale mass-failures with cm scale seasonal variations throughout. 137Cs (fallout from nuclear testing beginning in 1952, peaking in 1963) is present throughout the gully core indicating that medial gully sedimentary deposition since 1952 is greater than 580cm (≥ 8.4 cm/yr or 0.84 m/decade). Rate of sediment accumulation observed in the undisturbed proximal apron adjacent to the medial gully is 2.6-5.4cm/yr with 137Cs being present only to 144cm with excess 210Pb to 250cm and bulk density following a constant consolidation profile downcore indicating undisturbed, constant deposition. Lower rates of sedimentary deposition within identified gullies were observed in the S Pass, (3.75cm/yr or .375m/decade) indicate a lower sediment deposition/failure rate. Sedimentation in the adjacent undisturbed core considerably lower also following a consolidation profile down-core with 0.938cm/yr. Two elements of proximal deltaic sedimentation are evident: (1) mudflow structures are present in both the South West and South passes of the Mississippi River regardless or relative sediment discharge/depth (2) decadal deposition of mudflow deposits ≥ 0.8 m and ≥ 0.35m within gullies in the SW and S passes respectively.