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The St. Clair River Delta: Sedimentary Characteristics and Depositional Environments
J. M. Pezzetta
Progradation of the northeastern shoreline of Lake St. Clair by the St. Clair River has created a modern, freshwater delta with a birdfoot configuration similar to that of the Mississippi deltaic plain.
Textural parameters based on moment measures for surface sediments collected at 231 stations located on 78 square miles of the exposed and submerged parts of the active distributary system indicate that the recent sediments are moderately sorted (standard deviation = 1.03 phi), very fine sand to very coarse silt with a mean grain-size of 3.35 phi (97.9 microns). Grain-size frequency distributions are slightly asymmetric (skewness = +0.39) and moderately leptokurtic (kurtosis = 5.5). At the 90% confidence level and absolute values of r 0.15, significant linear correlations were obtained between the first two moments (mean and standard deviation) and current speed for the coarse (sand) fractions. In other words, coarser textured and som what poorly sorted sediments are associated with the occurrence of higher current speeds. Winnowing action by currents may not be as dominant a process in some areas as the introduction of coarser fractions into essentially finer grained sedimentary environments. Mean and standard deviation also show high correlations with wind speed (hence, wind-induced currents and wave action) for the fine (silt-clay) sediments. Only kurtosis and wind speed are linearly related for the composite (sand-silt-clay) samples.
Primary distributive processes are the river and channel currents which supply the sediments to the delta, and wind-driven currents and wave action along the delta front where the finer components are dispersed to the west and south into the deeper parts of the lake basin. Five depositional subenvironments distinguished on the basis of factor analysis of the textural and environmental parameters are:
1. an environmental complex comprising the open lake and interdistributary troughs where high energy processes are dominant,
2. the interdistributary bay margins including the levees, backswamps and marshes,
3. shallow areas of sparse aquatic vegetation and medium energy sedimentary processes,
4. low energy environments with thick overgrowths of aquatic vegetation, particularly near the headward parts of the interdistributary bays, and,
5. the deep lake basin beyond the delta front.
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