HIGH-RESOLUTION MAPPING, DATING, AND SEQUENCING OF HOLOCENE MEANDER LOOPS IN THE MISSISSIPPI AND MISSOURI RIVER VALLEYS: GEOARCHAEOLOGICAL IMPLICATIONS OF LESSONS LEARNED

After considerable review of available archaeological and other dates during his compilation of the Lower Mississippi Valley alluvial map, Saucier (1994) questioned whether anyone would ever consistently date development of individual meander loops in the Holocene Mississippi River floodplain. These dating difficulties stem from the high degree of local reworking of both younger and older dateable organic materials, and the potential for archaeological materials here to underestimate the age of the valley landform on which they occur.

Through allostratigraphic mapping, we recorded over 65 discrete meander loops comprising the 100km of the Mississippi River Holocene floodplain spanning the New Madrid seismic zone. The relative sequence of development for these loops was assessed by cross-cutting relationships and relative soil development. Radiocarbon (RC) dating techniques were then employed to ascertain numerical dates. Most all fluvial lithofacies failed to yield RC dates consistent with relative-date sequences. The boundary between abandoned and active channel fill deposits within channel plugs was the only horizon which yielded consistently verifiable dates, and records the time for abandonment of the dated loop. OSL dates taken of point-bar materials for dated loops consistently reinforced RC and relative dates. This confirmed OSL as a reliable tool for dating meander development. Similar mapping techniques in the Missouri River Holocene floodplain reveal over 100 meander loops which are currently sequenced and dated successfully with OSL samples alone.

These results have two broad implications for geoarchaeology studies in large alluvial meandering river floodplains. First, accurate dating for development of fluvial land surfaces provides both a check on the maximum reasonable date of human occupation for that surface, and a guide for locating best prospects for sites older than currently reported. Second, detailed meander and landform sequencing here permits detailed reconstruction of river courses and landscape characteristics for specific times in the past. This information can be used to place archeological sites into a detailed environmental context consistent with the time of occupation, and is a powerful tool for likelihood analysis in archaeological site investigations.