Paper No. 2
Presentation Time: 3:45 PM
EXHUMED LIGNITE BLOCKS, PALEOCURRENTS, & HIGH ENERGY SEDIMENTARY FEATURES ON A MISSISSIPPI RIVER POINT BAR, LOWER HATCHIE NATIONAL WILDLIFE REFUGE, WEST TENNESSEE
Historic 2012 low-water levels of the Lower Hatchie National Wildlife Refuge Mississippi River section exposed a sandbar preserving an unusual accumulation of lignite blocks, large-scale scour features, modern and fossilized current-oriented trees, large-scale depositional bedforms, and associated sediment. This point bar developed just downstream of, and in the normally lower-energy shadow zone of, a tight meander loop on the river. It is close to a well-documented geologic stratigraphic section that was exposed by earlier low-water levels with extensive cutbank erosion that provides excellent local stratigraphic control. The exposed sandbar is aerially subdivided into sedimentologically distinct northern and southern halves. The northern "scoured" half consists of a relatively flat muddy-sand to gravel & coarse-sand substrate, characterized by randomly-oriented large lignite blocks (sizes up to 2m) surrounded by decimeter-deep current scours, as well as sub-modern to modern tree trunks interspersed with geologically older bituminous coal (up to 0.5m in size). The trees are stripped of foliage and have root masses without sediment; paleocurrent rose plots of 74 trees show a bidirectional trend of 195o, consistent with modern river channel orientation. The lignite blocks do not occur on any sandbar upriver or downriver. Size and limited distribution of the blocks could represent reworking from an underlying localized unit; however, no contributing unit has been identified in the field or nearby reference section. Cross-stratified gravel deposits occur immediately below the sand-covered scoured surface. The exotic bituminous coal blocks, presumably dropped from barges, and occasional modern-era artifacts demonstrate "legacy sediment" influence to the deposit. Additionally, several current-oriented, meter-scale, permineralized/coalified fossil logs occur, suggesting an exotic or reworked fossil component. The southern half of the point bar is a thick sandy sheet deposit dominated by depositional features, especially large-amplitude transverse dunes and ripple bedding. This southern blanket is interpreted to have been reworked largely from the northern portion of the sandbar, accumulating downstream in a single large-energy event prior to the low-water exposure event.