--> ABSTRACT: Aging and Sediment Characteristics of Northern Gulf of Mexico Estuaries, by Wayne C. Isphording, F. Dewayne Imsand, and George C. Flowers; #91029 (2010)

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Aging and Sediment Characteristics of Northern Gulf of Mexico Estuaries

Wayne C. Isphording, F. Dewayne Imsand, George C. Flowers

Eight major estuarine systems present along the northern margin of the Gulf of Mexico serve as primary depositional basins for all rivers draining into the Gulf from central Louisiana eastward to the Florida peninsula. These estuaries consist of Apalachicola Bay, St. Andrews Bay, Choctawhatchee Bay, Pensacola Bay, Perdido Bay, Mobile Bay, Mississippi Sound, and Lake Pontchartrain. Because each receives sediment from a different river system (or systems), each estuary is characterized by sediments that are both physically and mineralogically distinct. Estuaries in the eastern Gulf, for example, possess a clay mineral suite dominated by kaolinite (derived from deeply weathered piedmont rocks), whereas those from the western Gulf are rich in smectite and mixed layer clays (r flecting a Western Interior or provenance from Paleozoic or older coastal plain sources). Similarly, weathering of rocks in the southern piedmont has provided eastern Gulf estuarine sediments with a suite of largely metamorphic rock-derived heavy minerals, whereas those in the western Gulf contain a mixed suite of both igneous- and metamorphic-derived minerals. Equally distinctive, however, are the textures of the bottom sediments themselves for each estuary when plotted on standard sand-silt-clay ternary diagrams. The relative percentages of these components are uniquely different for most of the estuaries and reflect both natural and anthropogenic conditions that exist in the watershed areas that drain into each estuary.

Sedimentological studies carried out in recent years that have duplicated older investigations have shown that the evolution of any single estuary is highly complex and, at least in the short term, follows no set rules. While the theoretical fate of all estuaries is ultimately to become filled in by sediment (and for the sediment texture gradually to become finer in nature because of reduction in stream gradients of the contributory river or rivers), these trends are generally not observed over periods ranging from a few tens of years to several thousand years. Although some estuaries (e.g., Lake Pontchartrain, Choctawhatchee Bay) have experienced the classical shallowing and shift toward finer grained sediments, others (Apalachicola Bay) have actually become deeper and display a comp ex change in sediment texture over the years. Still other estuaries, however (Mobile Bay, Pensacola Bay, Mississippi Sound), show only slight (or local) changes in sediment.

The reasons for the observed differences are varied and can be traced to factors such as cataclysmic natural events (hurricanes), industrialization and urban development within the various watershed regions, and various long-term natural phenomena that tend to "overprint" the textures of all estuaries (stream gradients, channel complexity, weathering history and petrology of source rocks, etc). Each of these has acted to a greater or lesser extent to influence the physical and mineralogical characteristics of the estuaries of the northern Gulf of Mexico and, in combination, to provide a distinctive nature to the sediments of each estuary.

AAPG Search and Discovery Article #91029©1989 AAPG GCAGS and GC Section of SEPM Meeting, October 25-27, 1989, Corpus Christi, Texas.