ABSTRACT

Approximately two dozen large estuaries are present along the coastline of the Gulf of Mexico between the Mexican border and the Florida keys. These estuaries are of particular economic importance because they house not only some of the Nation's major port facilities but, in addition, are also the locations of extensive commercial fishing operations. Hence, the environmental health of these sites is of particular importance to Federal, State, and local governments, and to private industry as well.

Further, the estuaries and bays that dot the Gulf shoreline are of especial geologic significance. Over one-half of all stream discharge that takes place to the oceans bordering the United States is discharged into the Gulf of Mexico. A substantial portion of this first enters the estuaries, carrying with it large quantities of suspended sediment. This sediment is derived from many different sources; consequently each of the estuaries possesses its own distinctive characteristics, both in terms of the mineralogical and physical properties of the sediments. Analysis of the mineral component reveals that distinct mineral zones exist in the Gulf coast which are a reflection of the type of the source rocks and weathering conditions in the various watershed areas. The size parameters of the bottom sediments in the estuaries, similarly, are a reflection of not only the source rocks, but also of a complex number of variables. These variables include channel characteristics of the streams emptying into the basins, the morphology and tidal current characteristics of the depositional basins, anthropogenic factors that have accelerated or modified natural erosion within the watershed, and natural events that, over the space of centuries, decades, or even hours, have worked major changes on the distribution and type of sediments that now exist in the estuaries. Descriptions are provided of the general mineralogical changes that occur in the estuarine bottom sediments in the northern Gulf of Mexico and examples are offered to show that standard sand-silt-clay ternary diagrams can be used to interpret changes that have taken place in the estuaries as a result of natural and man-related causes.