POSTDEPOSITIONAL PROCESSES ON A BURIED CAMBRIAN SEQUENCE IN SOUTHERN ISRAEL, NORTH ARABIAN MASSIF - EVIDENCE FROM NEW K-AR DATING OF MN-NODULES

The Cambrian sedimentary sequence in Israel and adjacent countries mar ks the beginning of the Phanerozoic sedimentation on the Arabian-Nubia n craton. The maximal burial of this sequence, in the southernmost par t of Israel, was approximaly 2.5 km. Manganese nodules hosted by shale s of the marine Cambrian Timna Formation, Timna Valley, were subjected to K-Ar analysis in order to date their Mn-mineral formation. In addi tion, the < 2 mu m clay fraction in the host rock was dated by K-Ar an d Rb-Sr methods. The K-Ar ages (average 365 +/- 4 Ma) and Rb-Sr isochr on (381 +/- 10 Ma) of the illitic clay fraction yielded a Middle/Late Devonian age. The results imply that K-Ar and Rb-Sr systems of < 2 mu m illites in the Cambrian host rocks, as well as those enclosed in the Mn nodule insoluble residues, were completely reset in a Middle/Late Devonian thermo-tectonic event, coeval with the beginning of a stratig raphically recorded epeirogenic uplift. The Mn-nodules which were stud ied fall into two types: (1) nodules constituted by massive, well-crys tallized hollandite and pyrolusite; and (2) younger nodules of poorly crystallized massive hollandite and coronadite solid-solutions. Type-1 nodules yielded a calculated Early Cretaceous age of 112 +/- 11 Ma, w hereas type-:! nodules yielded calculated apparent dates of 20 and 49 Ma. The first age suggests a first stage of manganese nodule formation within the Timna Formation, in Early Cretaceous time, possibly geneti cally connected with the shallow basic intrusions and volcanic explosi ve activity in the area. The much younger K-Ar dates of type-2 Mn nodu les may be due to late manganese remobilization and mineralization pro cesses. This activity is interpreted as being related to the nearby Te rtiary Dead Sea rifting, which was accompanied by low temperature hydr othermal processes.