Ultra-fine structures and genesis of the Campanian Negev high-grade phosphorites (southern Israel)



Ultra-fine structures of the Negev high-grade phosphorites provide valuable clues to unravelling the genesis of these rocks, the question of their differential areal distribution, and the biosedimentary mechanisms involved in their considerable enrichment. Peculiar to these phosphorites is an intergranular phosphate matrix, for the most part constituted by a variety of phosphatic microbial tubules displaying a range of spatial micro-organizations. The phosphate particles fixed by this filamentous meshwork also consists of internally organized and non-organized packed microbial remains of different types.

Analogies between the fabrics of the matrix and the corpuscles lead to conception of a two-stage depositional scheme for these phosphorites, based on rhythmical repetition of two sedimentary mini-events, in a slightly oscillating, very shallow marine system—(1) a low-energy event of microbial colonization of the Mishash bottoms, followed by early phosphatization of the organic structures mainly in marginal situations; and (2) a higher-energy event which broke up the phosphatic mats into debris, redepositing them as clastic layers in nearby basinward sites, while becoming bound by a new meshwork of filamentous microphytes. The differential phosphatization of the intergranular microbial binder, again occurring mostly in marginal localities, produced highly enriched phosphorites. Minor truncations and redepositions leading to amalgamation of the successive layers account for the massive fabric now displayed by most of these rocks. Examining the structural and textural features, the validity of the sedimentary mechanisms of Recent phosphorite formation for the Campanian Negev rocks is discussed.