New Chronostratigraphic Model for Mississippi River Chenier Plain

Shea Penland, John Suter

The chenier plain of the Mississippi River is a shore-parallel zone of alternating transgressive clastic ridges separated by progradational mud flats. The term "chenier" is derived from the French/Cajun word chene for live oak, the tree species that colonizes the crests of the higher ridges. The Mississippi River chenier plain stretches 200 km from Sabine Pass, Texas, to Southwest Point, Louisiana, and ranges between 20 and 30 km wide, with elevations of 2-6 m. The chenier plain evolved during the Holocene as a sequence of prograding mud flats intermittently reworked into ridges.

The timing and the process of formation could be reevaluated in light of new chronostratigraphic findings in the Mississippi River delta plain. The stratigraphic relationship between the Teche and Lafourche delta complexes and Ship Shoal offshore indicates that these delta complexes belong to different delta plains that developed at different sea levels. It appears that the Teche delta complex is associated with the late Holocene delta plain which developed 7,000-3,000 years ago when sea level stood 5-6 m lower than present. A regional transgression occurred between approximately 3,000 and 2,500 years ago, leading to the transgressive submergence of the late Holocene delta plain and producing the regional Teche shoreline. The timing of this transgression conforms to the age of the lan wardmost ridge in the chenier plain, the Little Chenier-Little Pecan Island trend, which dates about 2,500 years ago. This ridge trend was originally interpreted as representing the Teche delta-complex switching event, with the landward Holocene/Pleistocene contact representing the highstand shoreline. The implication of this new interpretation is that the Little Chenier-Little Pecan Island trend represents the highstand shoreline, a continuation of the Teche shoreline separating the late and modern delta plains, and that the Holocene/Pleistocene contact represents the leading edge of the marshes transgressing onto the Prairie terrace. Significant mud-flat progradation seems to require a westerly position of the Mississippi River, but the numerous forms and ages of cheniers do not corres ond well to the timing of major delta-complex switching. Progradation of the chenier plain appears to be associated with building of the modern delta plain and not the Teche complex of the late Holocene delta plain. Occurrence of individual ridges appears primarily tied to delta-lobe switching within the Lafourche complex and variations in sediment supply from local rivers. The recent development of the Atchafalaya delta complex to the west is the closest position of an active distributary to the chenier plain since sea level stabilization; as a result, a new episode of rapid mud-flat progradation is taking place.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.