Patterns of sediment deposition in the middle Aucilla River
By Dr. S. David Webb
After investing considerable time and energy in locating late Pleistocene sedi- ment deposits in the Middle Aucilla River, the ARPP might be expected to have discerned some patterns of how such sediments accumulated and how one might locate further bodies of similar nature. Besides recording the kinds of sediments and the geometry of their deposition, the ARPP also has a substantial inventory of their ages based on about 90 carbon-dated samples from more than a dozen sites in the river bottom.
The ancient sediments of the Aucilla River consist of two unequal sets of very different genetic origin. The prevailing “bedrock” is the Suwannee Limestone of Oligocene age, about 30 million years old. Along the river bottom this limestone is often exposed as shallow hard bottom. But it alternates with deeper, subcircular exposures of highly organic peats, clays, silts and fine sands of late Pleistocene age, less than a million years old. ARPP’s work shows clearly that these sediments represent sinkhole fillings.
The late Pleistocene sediment accumulations are related to water tables which in turn are related to sea level cycles. When sea levels are down due to glaciers tying up much of the world’s water budget, the sinkholes tend to open up and drain by subterranean systems far out into the Gulf of Mexico. On the other hand, when sea levels rise during interglacial intervals, the water tables in coastal Florida rise substantially and sinkholes tend to backfill with sediments. The Aucilla River’s record of late Pleistocene deposits must reflect these global sea level cycles.
The substantial number of carbon dates obtained by the ARPP from late Pleistocene deposits in the Aucilla River provides a general schedule of sinkhole filling sediments in the Wakulla Springs Karst Plain. The essential data are summarized in the accompanying figure. The dates are grouped by river segments, the Half-Mile Rise, farthest inland, and West Run, nearest the mouth. A major cluster of dates in the last deglacial hemicycle, from about 15,000 to about 9,000 radiocarbon years before present is to be expected. The older clusters, however, are somewhat surprising. The explanation for an older cluster of carbon dates between 24,000 and 32,000 years, best shown in Little River, is that this represents the penultimate glacial rise. The oldest cluster of dates between 36,000 and more than 42,000 corresponds to still another meltdown. It is interesting to note that as one moves down river the accumulations of late Pleistocene back-filled sediments appear to become more equally recorded. In Half-Mile Rise, farther inland, there is no record, insofar as we know, of older interglacial or interstadial records.
Editor's Note: In October 1998, Floridians celebrated Earth Day along with many others in the United States and Canada. Governor Lawton Chiles issued a proclamation underlining the importance of Earth Day and of appreciating geology. One of the events in recognition of Earth Day in Florida was the Wakulla Springs Karst Plain Symposium held at the Turnbull Conference Center on the campus of Florida State University (see "Wakulla Springs Karst Plain Symposium"). This article is a synopsis of the paper presented on behalf of the ARPP by project director S. David Webb. A longer version is in press in the symposium volume to be issued by the Florida Geological Survey, edited by Dr. Walt Schmidt, Survey Director.