Abstract

Some phosphatic sediments occur in many places in the geologic column and form in many environments on the sea floor in response to "normal" conditions and processes of sedimentation. However, that portion of the Miocene which contains the extensive phosphorites and associated anomalous mineralogies of the Hawthorn Group represents a very "abnormal" period of sedimentation. This phosphogenic system was characterized by a specific tectonic setting, structural framework, and an abnormal set of environmental conditions; the consequences represent one of the most extensive and important phosphate deposits in the world.The structural framework which controlled the formation and deposition of the phosphorites of the Florida Miocene was a series of arches or topographic highs associated with the major Ocala Upland and Sanford High. Extensive coastal, shallow nearshore shelf, and platform environments occurred around the highs and were the sites of major phosphorite sedimentation--the phosphate machines and associated entrapment basins. The perimeter phosphogenic belt is situated around the Ocala Upland and was dominated by microsphorite-intraclast sedimentation. The outer phosphogenic belt occurs downdip from the Ocala and in the offshore areas around the Sanford High; this area was characterized by pelletal phosphorite sedimentation. Phosphorite precipitation took place as the cold, chemically supercharged and somewhat toxic upwellings moved across the shallow platforms and into the coastal environments. The biologically stressed shallow-water environments received the bacterially precipitated microcrystalline phosphorite mud, or microsphorite, as well as the other biologically produced phosphate grains. This orthochemical microsphorite mud, along with all of the included microorganism hash, dolomite mud, and fine terrigenous sediment, then responded to the local environmental energy conditions and biological processes. The muds, populated by a benthic community characterized by high environmental tolerances and a low diversity index, injested and excreted the muds as fecal pellets. Under low-energy conditions the muds settled out, became indurated, and were subsequently broken up by biological and physical processes producing intraclasts. Very locally, under high-energy conditions, some of the mud was aggregated to produce pseudo-oolites. The resulting phosphorite allochems (phosphorite gravel, sand, and clay) were then transported as clastic particles along and off the shoals by periodic high-energy conditions. They were diluted by the associated terrigenous and carbonate sediment systems and were deposited and accumulated in the adjacent entrapment basins and on the flanks of the structural highs. Thus, wherever the phosphorus sources were adequate, the physical current and the geochemical systems were appropriate, and the shallow marine environments had the proper geometry, then the "phosphate machine" produced and supplied clastic phosphorites to the associated "entrapment basins." The ultimate magnitude of phosphorite deposition was then dependent upon the size and extent of the structural system, the duration of the phosphogenic system through geologic time, and the volume and rate of terrigenous or carbonate diluent sedimentation. Subsequent fluvial erosion and subaerial weathering severely modified the most updip portions of the Hawthorn phosphorites following emergence.

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