Post-depositional processes on a buried Cambrian sequence in southern Israel, north Arabian Massif: evidence from new K–Ar dating of Mn-nodules

Abstract

The Cambrian sedimentary sequence in Israel and adjacent countries marks the beginning of the Phanerozoic sedimentation on the Arabian-Nubian craton. The maximal burial of this sequence, in the southernmost part of Israel, was approximaly 2.5 km. Manganese nodules hosted by shales of the marine Cambrian Timna Formation, Timna Valley, were subjected to K-Ar analysis in order to date their Mn-mineral formation. In addition, the < 2 um clay fraction in the host rock was dated by K-Ar and Rb-Sr methods. The K-Ar ages (average 365 ± 4 Ma) and Rb-Sr isochron (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 um 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 stratigraphically recorded epeirogenic uplift. The Mn-nodules which were studied fall into two types: (1) nodules constituted by massive, well-crystallized 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, whereas type-2 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 genetically connected with the shallow basic intrusions and volcanic explosive activity in the area. The much younger K-Ar dates of type-2 Mn nodules may be due to late manganese remobilization and mineralization processes. This activity is interpreted as being related to the nearby Tertiary Dead Sea rifting, which was accompanied by low temperature hydrothermal processes.

500 m.y. of thermal history elucidated by multi-method detrital thermochronology of North Gondwana Cambrian sandstone (Eilat area, Israel)

“…Rb-Sr ages of <2 μm clays within shales and dolostones of the Early Cambrian Timna Formation are identical (365)(366)(367)(368)(369)(370)(371)(372)(373)(374)(375)(376)(377)(378)(379)(380)(381) to the ZFT data, as are K-Ar and Rb-Sr ages of the insoluble residue of manganese nodules that are found in the same formation (Segev et al, 1995). Identical ages were obtained by Harlavan (1992) on clay minerals from the Cambrian section in Israel using 40 Ar/ 39 Ar geochronology, and Heimann et al (1995) obtained 40 Ar/ 39 Ar ages of ca.…”

“…2 of Weissbrod and Nachmias, 1986). Jointly considered, these observations imply that major erosion occurred at or near the Devonian-Carboniferous boundary, when deepest burial also occurred (Segev et al, 1995).…”

“…The middle Cambrian Shehoret Formation of southern Israel was covered by ~2.5 km of younger sediments by the middle and late Devonian, more than at any other time (Segev et al, 1995). At ca.…”

“…380 Ma, a thermotectonic event increased the ambient temperature and heated the Shehoret Formation to ~200 °C, resetting the ZFT system but leaving the K-feldspar 40 Ar/ 39 Ar system relatively unaffected. Uranium-rich fl uids circulated through the sandstone and formed authigenic clays (Segev et al, 1995) and apatite overgrowths (Weissbrod and Nachmias, 1986), which complicate the alpha-ejection correction applied to our apatite (U-Th)/He data. After this fl uid circulation event, the Shehoret Formation was repeatedly buried and exhumed in the upper crust (<200 °C), as several regionally important stratigraphic unconformities were created.…”

“…The weathered basement was progressively blanketed (from north to south; e.g., Garfunkel, 1999) and the entire Arabian-Nubian Shield as far south as Ethiopia and Yemen was covered by a siliciclastic veneer by Ordovician time (Garfunkel, 2002;Kolodner et al, 2006). Continuous sedimentation in the Levant continued until Late Devonian time, when the sedimentary cover reached a thickness of ~2.5 km (Garfunkel and Derin, 1985;Gvirtzman and Weissbrod, 1985;Segev et al, 1995). Hercynian doming interrupted Paleozoic sedimentation, and a regional unconformity marks the base of the Carboniferous (Gvirtzman and Weissbrod, 1985).…”