Craig Alan Cox1,
(1) Southeast Missouri State University, Cape Girardeau, MO
Abstract: Testing the Relationships Between Channel-Fill Type, Sinuosity, and Active Tectonics in Mississippi River Holocene Strata, New Madrid Seismic Zone
Traditional models of Mississippi River channel fill process (Alan 1965 and Fisk 1944) predict that channels cut off by chutes are typically filled by active processes (sand/silt dominant), while channels cut off at the neck of meanders are typically passive filled (mud dominant). The assumption is as sinuosity increases, so do the chances of a channel being filled by passive processes. The purpose of this study was to assess the implicit relationship between sinuosity and channel fill types, as well as examine tectonic control on filling processes, in a reach of river subject to active tectonics and wide variations in sinuosity.
Seventeen meander loops were mapped and drilled and their fills were classified into categories of passive (90% clay in upper 7m), active fills (no more than 2m of consecutive passive fill in upper 7m), and intermittent (mixed active and passive fills). Minimum sinuosity was measured in complete loops and estimated in partially preserved loops by extending the curvature to a point perpendicular to the valley trend.
Loops with sinuosity 2 (12) were consistently dominated by passive fill, whereas loops with sinuosity <2 (5) had active or intermittent fills. Active filled channels were found only on the low-gradient upstream side of the Lake County uplift, reflecting historically reduced sinuosity by tectonic processes. Three other pertinent observations were made. Towheads commonly split the channel flow yielding shallow, narrow, active filled channels, which resume large size and passive fill processes where the main channel rejoins. Near loop cutoff all channels were actively filled as predicted by previous models. One unpredicted fill type was also noticed. The upper 2 m of fill on three loops was dominated by sandy, splay-delta deposits. This reflects the impingement and spill over of avulsing channels into partially filled neighboring passive loops.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana