A Detailed Look inside a Complex Channel Belt: Processes, Rates, and Architecture for an 8K-Duration Mississippi River Meander Belt

Holbrook, John¹, Whitney Autin², Ronald J. Goble³, Tammy M. Rittenour⁴, Stephen Marshak⁵ (1) University of Texas at Arlington, Arlington, TX (2) State University of New York, Brockport, Brockport, NY (3) University of Nebraska, Lincoln, Lincoln, NE (4) Utah State University, Logan, UT (5) University of Illinois, Urbana, Urbana, IL

The Lower Mississippi River has long stood as a type model for meandering-river deposition, and is well understood at both the regional floodplain and individual meander-loop scales. We have taken the next step, and produced the first detailed study of a single and entire channel belt (30 X 100 km) in order to better understand belt-scale architectural elements, facies, formative processes, and depositional rates. The channel belt studied here records a complex and long-standing (>8 ka) trunk belt from which no less than six contemporary belts disperse downstream.

Meander amplitude overall is variable, producing amalgamated and reworked lateral-accretion elements/point bars of 1 km to 19 km with 5-6 km as typical. Channel-fill elements constitute <25% of the belt deposits. While the more familiar chute and neck cutoff channel fill process are common, additional process of splay fill, avulsion fill, and re-occupation fill are also commonly recognized. Splays are common, but generally small and thin, except where filling tectonic and depositional lows on the floodplain surface. Overbank fines are generally minimal within the belt, but thick on the belt flanks. Belt architecture is hierarchal in the middle 30 km, and includes three subbelts that record tectonically initiated in-belt avulsion not observed elsewhere in the belt. Meander growth rates are approximately 5m/year, tectonic response rates are on the scale of a few centauries, tectonic recovery rates are on the scale of one millennium, and channels require several hundred years to a few millennia to fill depending upon process.