Petrology of the Tertiary phosphorite system of Florida

Abstract

Four basic types of macroscopic phosphorite have been recognized within the Florida stratigraphic section. Orthochemical and allochemical phosphorite are primary marine sediments formed within the environments of deposition. The authigenic microcrystalline phosphorite mud (microsphorite) which precipitates in situ either biochemically or physicochemically is orthochemical phosphorite. If the microsphorite mud is subsequently modified into discrete clastic particles, then it is considered to be allochemical phosphorite. The muds may be torn up by biological or physical processes to produce the intraclastic allochems or ingested and excreted by organisms to form the pelletal phosphorites; if there is sufficient energy, the muds may also aggregate around a nucleus grain to form the oolites or pseudo-oolites. A fourth type of allochemical grain is the fossil skeletal material which rains into the sediment system. The bulk of the phosphate macrograins deposited during the Miocene phosphogenic system consisted of these orthochemical and allochemical phosphorites. Subsequent processes modified some of the primary phosphorites to produce the other two varieties of macroscopic grains. Any phosphorite which is later reworked into a younger sediment system is called lithochemical phosphorite. Subaerial weathering processes chemically and mineralogically change the phosphorite to produce the metachemical grains.Microscopically, most of the phosphate macrograins are true aggregates composed of a complex mixture of various mineralogical and biological components. The primary component is the cryptograined carbonate fluorapatite matrix with various types and amounts of disseminated cryptograined coloring matter. Also, there is a multitude of micrograined included material which represents a mixture of everything that was in the environment at the time of precipitation of the orthochemical mud. The inclusions consist primarily of bacteria-like rods and rod aggregates, microorganism fossil hash, dolomite rhombs, and terrigenous sand and clay. Consequently, each phosphate macrograin is a complex sedimentary rock, the chemical composition and physical characteristics of which are totally dependent upon the specific types of inclusions and their abundances.