QUATERNARY GEOLOGY OF THE GULF INTRACOASTAL WATERWAY, WALTON AND BAY COUNTIES, NORTHWEST FLORIDA

The Choctawhatchee Bay to West Bay section of the Gulf Intracoastal Waterway in Walton and Bay counties of northwest Florida contains an exceptional sequence of up to 16 meters (52 feet) of Late Pleistocene and early Holocene strata for most of its length of ~24 kilometers (15 miles). The shore-parallel to oblique-downdip orientation of the canal exposes a variety of coastal facies that archive late Quaternary sea level and environmental change. Currently mapped as undifferentiated Quaternary (Qu) on the 2001 state geologic map, it is hoped that the study of this extended outcrop will result in the designation of formal lithostratigraphic units, and also contribute to the revised surficial geologic map of Florida now in progress.

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).