Abstract

Long known to be characterized by mass-wasting sediment transport, 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. In order to better understand spatio-temporal scales of controlling processes within the MRDF, twenty-eight piston cores up to 8.9 m in length were collected seaward of river channel outlets, then analyzed for magnetic susceptibility and gamma density, in the context of CHIRP sub-bottom seismic data collected by the U.S. Geological Survey. For this study, three cores were selected for study of sediment properties in three depositional environments (undisturbed proximal apron, medial mudflow gully, and mudflow lobe) and further analyzed for grain size, sediment fabric (X-radiography), and geochronology (²¹⁰Pb/¹³⁷Cs radionuclides). Building on previous work (Keller et al., 2017; Coleman et al., 1980), data indicate mass wasting sediment transport processes can be detected anmeasured by diagnostic properties preserved in sediment cores. Within the mudflow gully and lobe core, homogenized ²¹⁰Pb/¹³⁷Cs activities within deposited sediment and corresponding stepped profiles of gamma density indicate the presence of dm- to 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 580 cm (≥ 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 2.6–5.4 cm/yr with ¹³⁷Cs being present only to 144cm an excess ²¹⁰Pb to 250cm and gamma density following a constant consolidation profile down core, indicating undisturbed, constant deposition. Two temporal scales of proximal deltaic sedimentation are evident: (1) seasonal deposition by river flood events and related storm/cold front reworking of annual flood deposits; and (2) decadal deposition of mudflow deposits ≥0.8 m within gullies.