MISSISSIPPI RIVER SAND PETROLOGY: COMPARISON AMONG MODERN, MID-HOLOCENE, AND PLEISTOCENE DEPOSITS IN THE NORTHERN MISSISSIPPI EMBAYMENT

Has there been a measurable change in the source and/or dynamics of the Mississippi River since rapid deglaciation began about 20,000 years ago? Archived sand samples from two previous studies (from modern Mississippi River midchannel bars and from cores through nearby Pleistocene channel and Holocene point bar deposits) near Memphis, Tennessee provided an opportunity to compare and contrast sand mineralogy and grain size through the past 20,000 years of Mississippi River evolution. Modern samples, hand excavated to a depth of 2.5 m, came from Densford Bar, a midchannel non vegetated sand bar. Pleistocene (11 to 20 ka) and mid-Holocene (4 to 5 ka) samples came from two 40 m cores through the modern floodplain in Arkansas. Grain size was estimated from samples sieved into half-Phi intervals. Data were summarized in histograms, cumulative curves, and cross plots of statistics. Grain composition was determined by counts of 200 grains in thin sections prepared from samples in the 1.0 to 0.5 Phi size (0.50 to 0.35 mm). Grain composition was divided into quartz (monocrystalline, polycrystalline), feldspar (K-spar, Na-spar) and rock fragments (igneous, metamorphic, sedimentary, chert). The grain size of modern sand and that of both Pleistocene and Holocene sand is similar (median ~ 0.5 mm), however their particle size distribution is markedly different. Grain size distribution for modern sand follows a normal distribution whereas both Pleistocene and Holocene sands are decidedly non-normal (positive skew). The composition of all three samples in this study is very similar (Sublithic arenite) but they differ from composition reported by Potter (1978, 1988, lithic arenite) and Russell (1937, Subarkose). We attribute the differences to be due to the different grain size of samples studied coupled with operator bias. We conclude that the source(s) of Mississippi River sand did not change appreciably in the past 20,000 years. The contrast in grain size distribution suggests either the sediment dynamics of the modern river differs from pre-modern conditions or (more likely) the sediment dynamics of mid channel sand bars differs from that of point bars.