Lithofacies and cyclicity of Mohilla evaporite basins on the rifted margin of the Levant in the Late Triassic, Makhtesh Ramon, southern Israel



Marine-connected basins with evaporites occur beneath most extensional continental margins that originated at low-latitudes and often are of major economic significance. Cyclicity in the evaporite lithofacies reflects the degree of restriction of the basin, overprinted by sea-level changes, and caused by structural movements in the barrier region, whether by fault-block rotation, footwall uplift or hanging wall subsidence, in both extensional and compressional basins. The Upper Triassic evaporites of the Ramon section in southern Israel model cyclic sedimentation in such environments. The Mohilla Formation is a carbonate–evaporate–siliciclastic succession of Carnian age that fills a chain of basins extending along the Levant margin from southern Israel to Jordan and Syria. The basins developed in half-grabens adjacent to normal faults that formed during a period of regional extension. Evaporites of this formation are well-exposed in outcrops at Makhtesh Ramon, the southernmost of these basins. The M2 Member of the Mohilla Formation is composed of 42 sub-metre cycles of alternating dolostone, gypsum and calcareous shales. Field and microfacies analysis showed these cycles to conform mostly to restricted shallow and marginal marine environments, spatially limited by the uplifted shoulders of the half-graben systems. A total of 10 facies types belonging to six depositional environments have been identified. From stacking patterns and analysis of bed to bed change, cycles can be categorized into three groupings: (i) low frequency exposure to exposure cycles that developed under eustatic or climate control; (ii) high frequency deepening/shallowing-upward cycles, characterized by gradual transitions due to short-term sea-level or runoff-event oscillations possibly referable to orbital forcing; and (iii) high frequency shallowing-upward cycles, characterized by abrupt transitions, attributable to sporadic tectonic events affecting accommodation space or barrier effectiveness. The way facies and cycling of the sedimentary environments was deciphered in the Mohilla evaporite basin can be used to unravel the genesis of many other evaporite basins with barriers of tectonic origin.