Correlation by electric logs, lithology, and fossils indicates that the Bucatunna Clay Member of the Byram Formation (middle Oligocene) extends eastward from its type locality in Mississippi into westernmost Florida. The Bucatunna dips southwest about 30 feet per mile and thickens from a featheredge 8 miles east of the Okaloosa-Walton County line, Florida, to a maximum of 215 feet near Pensacola. In the Florida panhandle the Bucatunna rests unconformably upon limestones of late Eocene age and is overlain conformably by limestones of late Oligocene age.
Within the area the Bucatunna is a gray, silty to sandy clay consisting chiefly of illite and containing foraminifers, ostracods, mollusks, and other organisms of Oligocene age.
The Bucatunna Clay Member is a hydrologically important aquiclude below which ground water is under artesian pressure. In places the clay prevents the salt water below from rising and contaminating the fresh water above. In the northern half of the area the Bucatunna is probably underlain by several hundred feet of fresh water which as yet is virtually untapped by wells. Rapid settlement and industrialization of the area suggest that this supply of fresh water may become important; for to the south, the Bucatunna is underlain only by salt water.