Identification of Ancient Storm Events in Buried Gulf Coast Sediments

ISPHORDING, WAYNE C., University of South Alabama, Mobile, AL, and GREGORY W. ISPHORDING, Thompson Engineering Testing, Inc., Mobile, AL

Within the past 100 years, 25 hurricanes have struck the coast of the Gulf of Mexico in the area from the mouth of the Mississippi River to Apalachicola, Florida. During the passage of the storms, marginal coastal lowlands were often flooded, passes from coastal estuaries to the sea were heavily eroded, and the size and position of offshore barrier islands suffered significant modification.

Not generally observed, however, were changes that took place in the submarine environment. These changes, from a geologic standpoint, were equally impressive. Investigations have now confirmed that the passage of Hurricane Elena near Apalachicola Bay, Florida, in 1985 caused severe scouring of the bottom of the bay and also the resuspension and removal of over 80 million tons of sediment from the bay in a period of some 8 to 10 hours. Mobile Bay, similarly, was impacted by Hurricane Frederic in 1979. The passage of this storm caused an overall deepening of the bay of approximately 1.5 ft and the removal of nearly 290 million tons of sediment in a period of 7 hours. This is especially striking when it is realized that this amount of sediment is nearly equivalent to that delivered by t e Mississippi River to the Gulf of Mexico during one entire year!

Obviously such events also must have taken place in the geological past. While no unequivocal means probably exists to identify positively storm events in ancient sediments, detailed examination of sediment cores from Apalachicola Bay, Florida, strongly suggests that passage of major storms does leave a discernible imprint. Abrupt changes in particle median diameters, sorting coefficients, and percentages of sand-size sediment, when viewed simultaneously, may offer such a possibility. When these parameters are determined and plotted for closely spaced samples from cores, and the cores are laid side by side, the simultaneous occurrence of abrupt changes in median diameter, sorting, and sand percent is strongly suspicious of a storm event. While such changes also might be explained by o her causes, the likelihood of these is significantly lower than changes caused by major hurricanes, given the frequency of occurrence of storms in the Gulf's historical past.

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