| Popis: |
40 Ar/ 39 Ar dating. Isr. J. Earth Sci. 52: 77‐95. Vents (diatremes) filled with volcaniclastic breccia cut across Lower Cretaceous formations and the underlying Cambrian strata in the northern Timna Valley. The vents are the result of an ascending gabbroic intrusion interacting with groundwater, breaching the surface atop the Avrona Formation. A small plug of microgabbro, which intrudes the Cambrian dolomites near one of the volcaniclastic remnants, belongs to the same event. The volcaniclastic fill consists of hetrogeneous fall-back tephra and epiclastic debris of partly altered volcanic and sedimentary clasts of various sizes, and dispersed xenoliths and xenocrysts. 40 Ar/ 39 Ar and K/Ar analyses of three biotite crystals and one hornblende crystal from a single vent (Tamar) yield a weighted average age of 108.4 ± 1.7 Ma (late Early Albian). This age also marks the boundary between the Avrona and Samar formations, constraining their ages. Contemporaneous exposures of volcanic rocks, crossing or overlying strata equivalent to the Avrona Formation and terminating below the Samar equivalent, occur also in Makhtesh Ramon 90 km to the north. These volcanic and volcaniclastic occurrences constitute an important marker for correlation of the Lower Cretaceous clastic sequence in the Negev. |
