Use of alloformations for definition of Holocene meander belts in the middle Amite River, southeastern Louisiana

Abstract

Geomorphological studies in drainage basins of the northern Gulf of Mexico coastal plain have indicated similarities in flood-plain character and comparable influences on basin evolution. Alluvial morphology and stratigraphy can be linked to flood-plain evolution through the use of alloformations, unconformity-bounded stratigraphic units. This procedure extends classical terrace mapping concepts to conform with formally defined stratigraphic procedures. A meander belt segment of the middle Amite River valley in southeastern Louisiana was selected to test the applicability of alloformation mapping in part of a regional-scale (5,000 km 2 ) drainage basin. Three alloformations, identified as Magnolia Bridge (MAG), Denham Springs (DS), and Watson (WAT), can be differentiated by unconformable boundaries, landscape morphology, and relative pedogenic development. The valley fill consists of a lower sandy and upper silty facies. Preserved deposits consist of channel lag, point bar and scroll bar, channel fill, and overbank sequences. Lithofacies and sedimentary environments are dependent on fluvial process, and a suite of meander belt environments can be identified within each alloformation. Surface horizons of alluvial soils are all similar (ochric epipedons), but subsoil horizons are chiefly stratified parent material on the MAG (Entisols), cambic horizons on the DS (Inceptisols), and minimally developed argillic horizons on the WAT (Alfisols). Allostratigraphy provides an objective method for definition of geologic units that contain genetically related, but heterogeneous sedimentary deposits. Alluvial sediments are differentiated in a manner that successfully integrates geomorphic, sedimentologic, and pedologic data into significant stratigraphic models. This approach is applicable to a wide variety of Quaternary settings where prior stratigraphic techniques have resulted in ambiguity.