STRONTIUM ISOTOPES AND MIOCENE SEQUENCE STRATIGRAPHY ACROSS THE NORTHEAST FLORIDA PLATFORM

A preliminary assessment of Miocene sea-level fluctuations is presente d based on the integration of Sr-derived ages and sequence-stratigraph ic concepts applied to subsamples of 12 cores that transect the Hawtho rn Group on the northeast Florida Platform. The Hawthorn Group section includes phosphorite nodules, peloids, and crusts as well as dolosilt and dolostone nodules and beds formed during early burial diagenesis of organic-rich sediments deposited during periods of intensified and persistent upwelling associated with rising and maximum sea level. The Sr-87/Sr-86 composition of phosphorite and dolomite is used to determ ine the age of in-place phosphorite crusts and dolostone beds (condens ed sections) and reworked phosphorite and dolostone sand and gravel (u nconformities and transgressive surfaces). Regional correlation of unc onformities and condensed sections provides the basis for a sequence-s tratigraphic framework from which the age and relative amplitude of se a-level fluctuations are constructed. Seven depositional sequences are identified from the data. Depositional sequences correspond to local sea-level fluctuations with maximum water depths at approximately 25-2 4, 21-20, 19-18, 17-15, 14-12, 11-9, and 8-6 Ma. Greatest water depth apparently occurred at 17-15 Ma.