Geochronology of Mudflow Deposits on the Mississippi River Delta Front, Louisiana, USA

Short multicores (<50cm) and longer gravity cores (up to 3m) were collected seaward of the Southwest Pass of the Mississippi River Delta (Gulf of Mexico) and were analyzed to assess the frequency, extent, and potential causes of submarine mass wasting events. Cores were analyzed for radionuclide activity, grain size, and density at 2cm resolution, with x-radiography for the whole core. Short-term sedimentation rates calculated from ⁷Be are 2-12cm/y, while longer-term accumulation from ²¹⁰Pb are only 1.3-5.5cm/y. In most cores, ²¹⁰Pb activity steadily decreases downcore without displaying a “stairstep” nature. However, six cores have layers of low ²¹⁰Pb activity stratigraphically above layers with higher activity. In one long core from a mudflow gully, ²¹⁰Pb steadily decreases for the upper 90cm before stabilizing for the remaining 130cm. Clay content generally ranges between 25-40% and sand ranges between 5-15% with silt making up the rest of each sample. Sedimentation rates derived from ²¹⁰Pb in the short cores indicate that proximity to the river mouth has stronger influence than depositional environment (mudflow gully, depositional lobe, prodelta). This finding may be explained by rapid sedimentation rates coupled with a reduced tropical cyclone activity over the delta in the last seven years (2006-2013). The regions of decreased ²¹⁰Pb activity may be evidence of scavenging effects of plume sedimentation because they do not correspond with decreases in clay fraction. The zone of homogenized activity below 90cm in the gully core occurs at a depth equivalent to 18 years, indicating mixing on a decadal scale, potentially from mudflows. These results may be explained by a lack of recent mass failures corresponding with lulls in tropical cyclone activity over the delta, preceded by a period of more active hurricane-driven mudflow activity.