The Sedimentology and Clay Mineralogy of Choctawhatchee Bay, Northwest Florida

JACKSON, R. BRADFORD, Jim Stidham and Associates, Inc., Tallahassee, FL, WAYNE C. ISPHORDING, University of South Alabama, Mobile, AL, MARIO V. CAPUTO, Mississippi State University, Mississippi State, MS, and TAMARA L. GIBBONS, University of South Alabama, Mobile, AL

Nearly 30 large bays and estuaries are located along the Gulf Coast of the United States in the area from Brownsville, Texas, to Key West, Florida. These estuaries are of special importance because they not only receive approximately 50% of the runoff from rivers that drain the United States but also house major port facilities and are the sites of a significant part of the nation's seafood industry.

Continued industrial and residential development in the watershed regions that drain into many of these estuaries has impacted them in a number of ways. Not only have increased rates of sedimentation been observed, but changes have also been noted

in some of the more basic sediment properties such as sediment texture, mineralogy, and heavy metal chemistry. One such estuary is Choctawhatchee Bay, located in the northwest Florida panhandle.

Results from a recently completed investigation of this estuary were compared with an earlier study carried out in 1966 and disclosed that a marked increase in clay-rich sediments has taken place at the expense of sediments that were previously dominated by silt-rich material. The cause of this can be traced to anthropogenic effects associated with shifting agriculture patterns in the watershed and improved erosion control as well as an increased trap efficiency of the river delta. An increase in the salinity of the bay and in the content of suspended organic material may have also influenced retention of clay-size sediments in the bay.

An apparent change in the overall clay mineralogy since 1966 was also observed. Whereas the bay had previously been described as dominated by kaolinite-rich sediments in its western part and more smectite-rich sediments in its western part along with minor amounts of chlorite and unknown "zeolite mineral," a far more uniform suite is now present. The clay mineral suite now consists of kaolinite, smectite, and vermiculite (with minor illite). This suite represents a transition from the kaolinite-rich sediments found in bays located farther to the east of Choctawhatchee Bay (St. Andrew Bay, Apalachicola Bay) vs. the smectite-dominated sediments associated with those farther to the west (Mobile Bay, Mississippi Sound). In each case, the clay mineral suites are simply a reflection of the ineralogy of the indigeneous sediments undergoing erosion in the different watershed areas, and little in the way of modification has apparently taken place following deposition.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)