Marine Geology and Estuarine History of Mobile Bay, Alabama Part 1. Contemporary Sediments
John J. Ryan, H. G. Goodell, 1972. "Marine Geology and Estuarine History of Mobile Bay, Alabama Part 1. Contemporary Sediments", Environmental Framework of Coastal Plain Estuaries, Bruce W. Nelson
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Mobile Bay is the terminus of the fourth largest river system in terms of discharge in the United States. Currently, an annual average of 4.3 million metric tons of suspended sediment and an unknown quantity of bed load are being transported into the estuary. The dominant terrigenous clastics are quartz, kaolinite, and montmorillonite.
Circulation patterns within t he estuary are controlled primarily by river discharge, tides, and geometry of the bay. Meteorological tides cause important short-term variations. Depending upon the season and (or) year, the estuary may be classified as highly stratified, moderately stratified, or, with the exclusion of the ship channel, vertically homogeneous.
Comparisons of the bathymetry of 1847 to 1851 and 1960 to 1962 indicate an average shoaling of 56 centimeters per century. Considerable variation in sedimentation rates exists in various areas within the estuary, ranging from 0 to more than 3 meters per century. The majority of anomalously high rates results from changes in circulation and bathymetry due to dredging operations. On the basis of computed sediment accumulation over the 110-year period, approximately 1.3 million metric tons of suspended sediment is currently by-passing the estuary and being deposited to the south and west of the tidal inlet.
Clean quartz sands occur around the periphery of the bay generally in water depths less than 2 meters. Most of t he bay bottom sediments consist of silty clays and clays. Multiple linear regression analysis reveals that grain size decreases and sorting increases downbay and to the southeast. Higher montmorillonite content near the bay-head delta and increasing kaolinite content toward the southeast in Bon Secour Bay are attributed to sediment dispersal patterns, differences in sedimentation rates, and to increase in kaolinite content in the suspended river sediments over the historic period (the result of the development of agriculture within the drainage basin).
The distribution and ages of buried oyster reefs indicate a progressive downbay migration in response to progradation of the bay-head delta and changing circulation pattern within the estuary over the past five to six thousand years.