Occurrence and Distribution of Phosphatic Miocene Hawthorne Group on Georgia Inner Continental Shelf: ABSTRACT

Abstract

Miocene-age strata contain economically significant quantities of phosphate in Florida and North Carolina. Previously, units of similar age and lithology offshore Georgia were poorly defined geographically and stratigraphically. Through interpretation of high-resolution seismic data and correlation with well logs, it is possible to delineate middle Miocene units on the Georgia shelf. The middle Miocene Coosawhatchie formation consists primarily of phosphatic, shallow marine, terrigenous sands and clays, with clays (Berryville clay member) dominating offshore and grading shoreward into the predominantly sandy Ebenezer member. The Tybee phosphoritic member, the basal unit of the middle Miocene in the coastal area, averages 20 ft thick in eastern Chatham County and is present in varying thickness along coastal Georgia. The Tybee phosphorite member is of interest as its phosphorite concentration is roughly comparable to the phosphorites currently being mined in Florida and North Carolina. It is not always possible to identify the Tybee phosphorite member as a unique reflector on seismic records. As the basal member of the Coosawhatchie formation, however, its geographic extent is approximated by the structure-contour of the base of the middle Miocene. The base of the middle Miocene deepens southward from about 82 ft below surface, adjacent to Tybee Island, to more » more than 395 ft, seaward of Cumberland Island. Middle Miocene sediments on the Georgia shelf range in thickness from 0 to greater than 175 ft, thickening to the east and south. « less