ORIGIN OF DOLOMITE IN THE PHOSPHATIC MIOCENE HAWTHORN GROUP OF FLORIDA

In addition to large amounts of phosphorite, the Miocene Hawthorn Grou p of Florids contains abundant dolomite. Dolomite is present as dissem inated silt-size rhombs, as friable dolosilt beds, and as pore-filling cement in dolostone beds and clasts. The dolomite formed during early burial diagenesis both in the sulfate-reduction zone, overlapping and extending below sediment depths of phosphorite formation, and in adja cent, nonphosphatic, shallow-water lagoonal environments. Much of the dolomite is closely associated with the fibrous, Mg-rich clay minerals palygorskite and sepiolite. The percent carbonate in the Hawthorn Gro up increases from north to south; the dominant carbonate mineral in no rth Florida is dolomite, whereas dolomite and calcite are both abundan t in south Florida. The deltaC-13 values of the dolomite, from +1.82 t o -6.21 parts per thousand PDB, suggest that metastable biogenic carbo nate (aragonite and high-Mg calcite) and seawater were the predominant sources of carbonate. However, negative deltaC-13 values of dolomite from northeast Florida suggest that as much as 30-40% of the carbonate was derived from degradation of organic matter. Degradation of organi c matter enhanced dolomitization by removing sulfate ion and increasin g the carbonate alkalinity of the pore waters. The oxygen and strontiu m isotopic values along with moderate Na contents indicate a marine or igin. Evaporation of seawater or mixing of seawater and meteoric water were apparently not major factors in dolomite formation. The Sr-87/Sr -86 ratios of the dolomite range from 0.708129 to 0.708820 and corresp ond to Sr-derived ages of 26 to 12 Ma. The similarity of associated do lomite and phosphorite Sr-derived ages (r2 = 0.79) combined with geoch emical and textural evidence suggest that dolomite and phosphorite for med at approximately the same time during rising or maximum sea level and were reworked during marine regressions. Reworking of the sediment concentrates disseminated dolomite rhombs and phosphorite grains, oxi dizes organic matter, and exhumes buried dolostone beds to boring and encrusting organisms on the seafloor. Therefore, the presence of dolom ite, along with phosphorite, in reworked sequences can indicate deposi tion of organic-rich sediments from which most of the organic matter h as since been removed.