ABSTRACT

Pleistocene and Plio-Pleistocene sediments have been studied in northern Florida to detect tectonic displacement. Previous studies in southeastern Georgia have defined six prominent former coastlines which are essentially horizontal and have altitudes (oldest to youngest) of 95-100 feet, 70-75 feet, 40-45 feet, and approximately 24, 13 and 4.5 feet above present sea level. The heights of the former sea levels have been determined from the upper limit of littoral trace fossils and the altitude of salt marsh sediments associated with each coastline. Each coastline is marked by a series of barrier island deposits consisting of well-sorted, fine-grained, angular sand and by lagoonal-salt marsh sediments of fine-grained sand, silt and clay which accumulated landward of the barriers.

The former coastlines have been traced southward from Georgia into northern Florida where the lagoonal-salt marsh sediments of the Wicomico formation (sea level 95-100 feet in Georgia) are encountered at progressively higher altitudes. Maximum uplift is east of Starke along the northeast flank of the Ocala Uplift where salt marsh sediments have an altitude of approximately 166 feet or 65 to 70 feet higher than in Georgia. The Penholoway salt marshes sediments (sea level 70-75 feet in Georgia) reach an altitude of 95 feet in Florida. The Talbot salt marsh sediments (sea level 40-45 feet in Georgia) are not well preserved but appear to be displaced 5 to 10 feet in the area of maximum uplift in Florida. Pamlico salt marsh sediments (sea level approximately 24 feet in Georgia) are as high as 32 feet south of St. Augustine; thus, there is progressively less warping of the lower, younger coastlines. South of the Ocala Uplift the five lower Pleistocene coastlines return to the approximate altitudes they have in Georgia. The oldest coastline, which may be either Pliocene or Pleistocene in age, maintains an altitude of 140 to 150 feet for a distance of over 150 miles to the south of the area of maximum uplift.