Primary bedded phosphorites in the Campanian Mishash Formation, Negev, southern Israel
Sedimentary Geology 80(1-2): 77-88
ISSN/ISBN: 0037-0738 DOI: 10.1016/0037-0738(92)90033-n
Thin, lentiform beds (up to 1 cm thick) of primary (non-pelletal) phosphate have been found sporadically in the highly condensed "anticlinal" sections of the late Campanian Mishash Formation of the central Negev (southern Israel). These primary phosphate beds are typically associated with iron-rich montmorillonites of probable authigenic origin, and occur at the top of upward-fining phosphate-dominated sequences. The clay-rich and primary phosphate levels are commonly truncated by amalgamated beds of high-energy coarse-grained phosphorites which form the basal part of these sequences, with ripped-up clasts of the primary phosphate commonly re-incorporated in these rocks. SEM-EDS study of both the primary phosphate levels and the reworked phosphate clasts shows that they are built by a crudely stratified network of closely packed and gently compressed, capsule-like phosphate cells 2.5 m in external diameter and with walls of 0.5 m thick. These structures are interpreted as being the synsedimentarily phosphatized remnants of former mat-forming coccoid prokaryotes (now represented as extracellular envelopes), which intermittently colonized the Mishash sea bottoms at the end of the high-energy episodes. Differential degradation of the periphytic mat-community, possibly coupled with Fe- P redox cycling across the surficial mat (iron was trapped as iron-rich montmorillonites in the anoxic zone below the living mat), probably resulted in an excess of dissolved phosphate immobilized as solid phases on decay-resistant microbial remains which escaped total degradation. Development of the mat-forming microbes on the Mishash bottoms, and their mineralization to form primary phosphorite beds, occurred during periods of halted deposition, which resulted in a high residence time of the mat on the seafloor and hence, in intensified interface-linked biogeochemical reactions ending in phosphate precipitation. The restriction of primary phosphate occurrences to the condensed Mishash sections of marginal (palaeohigh) positions, strengthens previous assumptions that these zones were probably the main locus of Mishash phosphate production. It also points to a phosphate grain supply from marginal to basinal sites, leading to the accumulation of pelletal phosphorites in the latter areas.