In addition to large amounts of phosphorite, the Miocene Hawthorn Group of Florida contains abundant dolomite. Dolomite is present as disseminated 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 adjacent, 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 Group increases from north to south; the dominant carbonate mineral in north Florida is dolomite, whereas dolomite and calcite are both abundant in south Florida. The delta 13 C values of the dolomite, from +1.82 to -6.21 per thousand PDB, suggest that metastable biogenic carbonate (aragonite and high-Mg calcite) and seawater were the predominant sources of carbonate. However, negative delta 13 C 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 organic matter enhanced dolomitization by removing sulfate ion and increasing the carbonate alkalinity of the pore waters. The oxygen and strontium isotopic values along with moderate Na contents indicate a marine origin. Evaporation of seawater or mixing of seawater and meteoric water were apparently not major factors in dolomite formation. The 87 Sr/ 86 Sr ratios of the dolomite range from 0.708129 to 0.708820 and correspond to Sr-derived ages of 26 to 12 Ma. The similarity of associated dolomite and phosphorite Sr-derived ages (r 2 = 0.79) combined with geochemical and textural evidence suggest that dolomite and phosphorite formed 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, oxidizes organic matter, and exhumes buried dolostone beds to boring and encrusting organisms on the seafloor. Therefore, the presence of dolomite, along with phosphorite, in reworked sequences can indicate deposition of organic-rich sediments from which most of the organic matter has since been removed.