Causes and Effects of Late Holocene Subsidence on the Pierre Part distributary complex Ascension and Assumption parishes, Louisiana

Paul V. Heinrich Louisiana Geological Survey, Louisiana State University,

Baton Rouge, Louisiana 70803

Within Assumption and Ascension parishes, Louisiana, the Late Holocene Pierre Part Distributary System exhibits anomalous anastomosing distributaries that originate from a formally active area of crevassing along Bayou Lafourche. These distributaries converge downstream back into a single distributary channel. First noticed 1995, its anastomosing pattern has been interpreted alternately to represent either its progradation into a shallow water lake within the backswamp of the Mississippi Alluvial Valley or the effects of neotectonics associated with the underlying Napoleonville Salt Dome. Interpretation of aerial imagery proved ambiguous in the determination of which of these hypotheses best explains this anomalous distributary pattern. An analysis of LiDAR digital elevation models strongly indicates that significant, noncatastrophic subsidence of the channels and natural levees occurred contemporaneously with and after the active discharge within the Pierre Part Distributary System. Such subsidence might explain the anastomosing pattern of this distributary system and topographic sags within the crest of its natural levees. Bayou Bleu in Iberville Parish is another distributary system that exhibits an anomalous drainage pattern associated with a salt dome.