Breaching the levee of a channel on the Mississippi Fan

Abstract

GLORIA images of the youngest channel on the Mississippi Fan indicate that it has not been a stable feature, but instead has shifted its course several times. A detailed study of a site of channel shifting found a complex stratigraphy that resulted from one episode of channel avulsion. The channel avulsion appears to have been initiated by a large mass flow that choked the channel below the point where the levee was breached and additionally spilled a large volume of material through the breach in the levee onto the adjacent fan. Subsequent flows were redirected through this breach in the levee and built a channel-levee complex over the mass movement deposits. A second phase of mass movement resulted from another large mass flow that came down the channel and triggered the collapse of part of the newly developed levee. In this case, locally derived levee sediment was mixed with allochthonous sediment from farther up the fan and was spread northward from the levee. This localized study suggests that fan stratigraphy is complex and variable at several scales, not just at the scale resolved in seismic stratigraphic studies (Weimer 1989) and that large mass flows capable of choking the channel system have been an important mechanism in redirecting sedimentation on the Mississippi Fan.