COATED PHOSPHATE GRAINS: THE GRANULAR EQUIVALENT OF CONDENSED BEDS

Phanerozoic phosphate grains deposited beneath highly productive surface waters in organic-rich paleoenvironments exhibit three recurring textures. (1) Unconformity-bounded grains (UB) contain internal discordances and erosional surfaces, attributable to iterative episodes of phosphogenesis and sedimentary reworking. Such grains form during periods of relative stratigraphic condensation and may aid, when coupled with other sedimentologic data, in the identification of transgressive and highstand systems tracts. (2) Redox-aggraded grains (RA) consist of concentric phosphate laminae that are intimately intercalated with circumgranular layers containing either chamosite, barite, or pyrite. These grains record in situ diagenetic mineralization accompanied by changes in pore water redox potential. Such changes are attributed to variations in biological oxygen demand within suboxic pore water environments that result from fluctuations in sedimentation rate of organic carbon. RA grains are thus sensitive indicators of variations in organic carbon export, and record changes in primary productivity and/or ecological dynamics of the surface ocean. (3) Hybrid grains (HY) contain characteristics of both UB and RA grains, and form when episodes of grain exhumation alternate with changes in the export of sedimentary organic matter.

This model of coated phosphate grain formation necessitates long residence time at the sediment-water interface. Low net sedimentation rates and/or repeated substrate reworking facilitate phosphogenesis by allowing buildup of pore water phosphate and fluoride. If sedimentation rate is too high grains are rapidly buried within the sediment column, and so removed from the zone of active phosphate precipitation. In this context coated phosphate grains are considered the granular equivalent of condensed beds. These concepts are equally applicable to the interpretation of other types of coated grains and concretions that contain Eh sensitive minerals, such as iron ooids and polymineralic concretions.