| The purpose of this investigation is to describe and analyze the geomorphic development of alluvial fans in the humid mid-latitude climate region of the Yazoo Basin of northwestern Mississippi. The investigation is based on the hypothesis that alluvial fan development in the Yazoo Basin is the product of geomorphic processes, whose frequency, magnitude, and rate are controlled in part by meteorological events of the local climate, which differ considerably from those active in arid or semiarid regions, where most alluvial fan studies have occurred.; Four objectives comprise the investigation, including: (1) to describe and analyze the factors which have controlled alluvial fan development; (2) determine and characterize the geomorphic processes active in fan growth; (3) construct a general stratigraphic model for fan development in the Yazoo Basin; and (4) construct a morphometric model for alluvial fan systems in the Yazoo Basin. Examination of the factors which have controlled alluvial fan development revealed that the copius supply of easily erodible upland lithologies, the frequent occurrence of intense precipitation events (especially during winter and early spring), and the existence of a relatively stable surface upon which the alluvial fans have prograded throughout most of the Holocene, are the dominant factors which are responsible for alluvial fan growth in the Yazoo Basin. Four geomorphic/sedimentologic processes were identified and characterized which are active in fan growth in the Yazoo Basin, including channel, intersection point lobe, sheetflood, and splay processes. Stream channel deposits consist of three facies--lateral sandbars, lateral gravel bars, and channel bed, and collectively comprise less than 10 percent of total fan sediments. Intersection point lobes exhibit five facies--channels, gravel bars, proximal apron, medial apron, and distal apron, which, combined make up 10 to 15 percent of fan deposits. The three types of sheetfloods, fine-grained, coarse-grained, and channeled, are responsible for most of the fan sediments (up to 85 percent). Splay deposits comprise less than three percent of most fans. Morphometric analysis of the Yazoo Basin alluvial fan systems indicates that the fans are well adjusted to a state of dynamic equilibrium with their drainage basins. |
