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Two close associations : Si-P-Fe and Si-P-C in the Upper Campanian and Lower Maastrichtian sediments of the Negev, Israel. Deux paragenèses associées : Si-P-Fe et Si-P-C dans les sédiments du Campanien supérieur et du Maastrichtien inférieur du Néguev, Israël

1989; Volume: 42; Issue: 3 Linguagem: Inglês

10.3406/sgeol.1989.1820

2613-277X

Yaacov Shahar, Y. Nathan, Yair Shiloni,

Clay minerals and soil interactions

Two geochemical-mineralogica ! associations, Si-P-Fe and Si-P-C, are found in the Upper Cretaceous strata of the Negev, southern Israel. The Si-P-Fe association occurs only within a very short time span, represented by the uppermost part of Mishash Formation and the lower part of the Ghareb Formation (Upper Campanian-Lower Maastrichtian). The Si-P-C association occurs in the same time interval, and also lower in the section within the Mishash Formation. There appear to be a structural control (water depth) on the occurrence of these two associations with Si-P-Fe occurring in higher structural positions than Si-P-C. The two associations occur in close proximity, in places within less than 500 m of each other, but they are not genetically related since the Si-P-C association has a depositional origin while the Si-P-Fe is diagenetic. In the Si-P-C association organic carbon was preserved along the synclinal axes together with carbonate, silica and phosphate accumulations. The thickness and P2O5 content of the «main phosphorite » bed increases downslope, and in places this trend is enhanced by the presence of Campanian faults, with the down-faulted margins serving as traps for the accumulation of phosphate. In the Si-P-Fe association, iron oxide, mostly goethite, associated with apatite nodules, is located on the upper slopes, occurring as concretions, grains and as ooliths consisting of an intergrowth of goethite and apatite. Iron and phosphate grains are in places replaced by silica ; the replacement probably took place before the final burial, since the phosphate host rock is in most cases not affected.