Sedimentary configuration and Cyclicity in The Late Triassic Mohilla formation, Southern Israel

The Mohilla Formation of Carnian age (Late Triassic)  formed in the course of a long-term climatic transition from humid to arid, and coincided with relative sea level fall and reflooding. It was investigated in five outcrop sections at Ramon, and in seven boreholes across southern Israel.

Material was investigated macroscopically in the field, as well as by optical and cathodoluminescence petrography in all sections. The carbonate fraction was analyzed for carbon and oxygen stable isotopes analysis. Grain size analysis of siliciclastics was on the shale lithology.

The succession at Makhtesh Ramon consists of a pre-evaporite carbonate member, a main evaporite member, and a post-evaporite carbonate member , a subdivision that could be applied to all the studied sections. It is spatially quite variable, and six facies belts were identified:  mud flats, evaporite lagoons (dominating the Ramon outcrop), proximal shoals, evaporitic seaways, distal shoals and reef-like carbonate buildups. A facies model based primarily on the carbonate phases but including the evaporites and siliciclastics was constructed to track lateral changes at various scales. The morphology of the facies belts was attributed to early Mesozoic block faulting which generated a basin-and barrier paleogeography/paleobathymetry that controlled sedimentary patterns in the course of sea level rise and fall under episodes of emergence vs. drowning, restriction vs. opening of seaways.

The Negev sections studied represent proximal shoals and evaporite lagoons of two localized sedimentary basins, the Ramon and Kurnub basins. Lateral and vertical types of facies changes were used to track the vertical/temporal evolution of the succession in both basins, and their subdivision into systems tracts; these included the initial regressive phase, a transgressive phase and a final regressive phase. Bounding surfaces of these systems tracts were used independently of the lithostratigraphy, and in the absence of reliable biostratigraphic constraints, to extend correlation into northern Israel and to the Arabian platform to equivalent sequences Tr₆₀ and Tr₇₀ of the Arabian Plate sequence stratigraphy framework,.

Chemostratigraphic correlation was possible within the transgressive phase using stable isotopes of carbon, in the evaporite lagoon localities. This correlation shows that regional phases of water body stratification occurred in both basins that were likely to be connected to events of climate change.

Data from this study and from archives (e.g. wirelogs) were used to discern cyclic patterns in the Mohilla Formation.

Four orders of cyclicity below the sequence level were clearly identified:

lithological cycles, facies cycles, exposure-to-exposure cycles and water body stratification cycles. Deconvolution of these factors results in a complex of periodic and aperiodic frequencies that could be attributed to different control mechanisms. The lithological and water body stratification cycles appear to have been controlled by climatic mechanisms, possibly orbitally tuned, governing runoff into the basin and evaporation from it. These climatic fluctuations resulted in unstable salinity of the upper water body, thus disrupting the carbonate factory while, in short term events introduced shales into the basins. As these fluctuations were local manifestations of global changes, they apparently reflect coeval events manifesting in the entire region.

Facies - and exposure-to-exposure cycles were affected also by relative sea level and tectonic elements which governed the availability of water through the marine inlet as well as water depth. These changes dictate the rate of replenishment of evaporated water, the ratio between govern salinity.