MAPPING AND SIGNIFICANCE OF ALLUVIAL FANS IN NORTHWESTERN MISSISSIPPI

Alluvial fans lie at the edge of the broad floodplain of the Mississippi River in northwest Mississippi. The fans have recently been mapped by Mississippi's Office of Geology and Office of Land & Water Resources. The fans form a discontinuous chain at the foot of a low bluffline, from DeSoto County near Memphis to Wilkinson County near the Louisiana state line.

Alluvial fan lithologies vary along the bluffline, and reflect varying sources of sediment supply from outcropping formations in the adjacent uplands. Distinct types of morphologies are noted and are related to specific conditions of deposition: material available, size of mother stream, and other factors.

While some fans are fairly recent surficial features, drilling has revealed others have deep roots in the floodplain alluvial sequence and reflect multiple episodes of deposition and burial. While archaeologic data indicates that some have seen virtually no deposition for thousands of years, others have significant deposits which have accumulated since European settlement.

The fans are of great significance in allowing recharge into the Mississippi River Valley alluvial aquifer (MRVA) in Mississippi's "Delta" region, at far greater rates than in most of the Delta plain. Although no public drinking water supply wells are located in the fans or in most of the MRVA, there should be awareness of the fans as recharge source areas which may need protection from pollution.

Another practical concern is for engineering geology. Roads or other structures built upon the fans need to account for their special soil and lithologic properties, rather than using data from the floodplain or upland lithologies.

The fan wedges may serve as a modern analogue for stratigraphic trapping which can exist at the edge of a floodplain, where preserved. While high-angle fans formed in desert basins with adjacent high relief are well known in sedimentology, these are more subtle, formed in a temperate humid climate and easily overlooked.

Most valuable to archaeologists and stratigraphers, however, will be the detailed chronology which lies buried in these sediments. It is hoped that these new maps will spur studies in various disciplines which will shed light on the Quaternary history of Mississippi.