QUATERNARY GEOLOGY OF THE GULF INTRACOASTAL WATERWAY, WALTON AND BAY COUNTIES, NORTHWEST FLORIDA
The section consists of a basal, ~7.5 meter-thick, humatic, medium quartz sand with common Ophiomorpha traces. The sand is overlain by a thin peat of variable thickness, with abundant sub-fossil logs. Above the peat is ~9 meters of medium to coarse, frosted, quartz sand, with large-scale cross stratification, abundant logs and stumps, and paleosols. Radiocarbon dating of two peat samples, and one humate sample, all yielded dates exceeding 43,500 BP (i.e., beyond 14C resolution). Pollen analysis of the peat shows an association of oak, wax myrtle, sedges, holly, pine (Pinus, 29%), cypress (Taxodium, 19%), white water lily (Nymphaea, 5.3%), and an unusual abundance of alder (Alnus, 16%, generally indicative of more temperate climates). 14C analysis of wood from the upper, dune facies yielded a date of 3,930 + 30 BP.
We interpret the humatic, Ophiomorpha-rich quartz sands as a shoreface facies from the Sangamonian Interglacial (~130-118 ka), when sea level was between 4-6 meters above present. The peat layer formed in a freshwater wetland with nearby pine-oak woodlands. The wetland must have developed sometime between the Sangamonian Interglacial and Wisconsin Glacial Maximum (18,000 BP), perhaps between ~100-50 ka, when sea level was variably between -40 to -90 meters below present. The humate is a powdery precipitate from the mixing of coastal, organic-rich surface water and groundwater (“blackwater”) with brackish or marine waters. The top layer is an eolian sand sheet with buried trees and paleosols of Holocene age (~ 4,000 years to present).