INITIAL REPORT OF NORMAL MARINE, SHALLOW SUBTIDAL STROMATOLITES IN THE LOWER MIOCENE CHIPOLA FORMATION, ALUM BLUFF, LIBERTY COUNTY, FLORIDA

Exceptionally-low water levels of the Apalachicola River during the fall of 2012 exposed approximately ten, moderately large, in situstromatolites within the Lower Miocene Chipola Formation at the classic Alum Bluff section in Liberty County, Florida.

Exposed here are approximately 140 feet of strata, including (in ascending order) the Chipola, Alum Bluff Group (undifferentiated), Jackson Bluff, Citronelle, and Quaternary (undifferentiated) formations. The stromatolites lie uniformly beneath the well-known, molluscan-rich calcareous sands of the Chipola, and are surrounded by medium- to coarse-grained, calcareous quartz sands containing Ophiomorpha trace fossils. All colonies are subglobular in shape and most approach one meter in diameter, with the largest specimen reaching nearly two meters in diameter. A smaller colony, about 40 centimeters in diameter, was harvested for study. Preliminary sectioning and examination reveal that the colonies are laminated and well-calcified, with abundant calcified tubules or sheaths, and some are bored by macro-invertebrates. A variety of sediments are entrapped and bound in the colonies, including very course-grained- to gravel-sized quartz sand.

Non-lacustrine stromatolites are rare in Cenozoic strata worldwide, so the recognition of marine stromatolites in the otherwise well-studied, fossiliferous Chipola Formation was surprising, and offers promise for future discoveries within similar facies in the Cenozoic section of Florida.