Fill Patterns of Evaporitic Half-Graben Basins, Late Triassic Mohilla Formation, Southern Israel

Sediment fill in half-graben basins on rift-edge faults were found to record signals from repeated structural movements, from sea level change, and from a climate signal, often rhythmic. High sensitivity carbonate-evaporite successions strongly record each of these signals as the basin evolves.

The Late Triassic (Carnian) Mohilla Fm was deposited within small (10-20 km wide, 20-60 km in length) half-graben basins situated in a chain along the Levant shelf-edge of the Triassic. Patterns of sedimentation were found that were common to the Ramon outcrops and to the Kurnub, and Qanaim basins penetrated by boreholes. Basin fill is characterized by two main end member facies: an evaporite-rich succession ca. 200m thick found in the basin center, and a carbonate succession of half this thickness on the barrier. These end members are sometimes juxtaposed near basin-edge faults, with the evaporite facies on the hanging wall side and the carbonate facies on the footwall side. The evaporite succession commences with a lower dolomite, followed by a middle evaporite unit, and terminated by an upper limestone; the carbonate facies is more monotonous. The upper and lower carbonates are mainly microbialites, interspersed with subaerial exposure horizons and marine intercalations.

Control on fill patterns of multiple orders as the Mohilla Fm small basins evolved, may be traced to eustatic, climatic and local tectonic rhythms affecting the flux of fresh and marine waters into these basins. Three orders of basin fill processes were recorded in the Mohilla Fm - long term eustatically-related restriction and reflooding; intermediate scale fill patterns, with carbonate/evaporite ratios controlled by climate; and short term fill events attributed to movement on basin edge faults. When the barrier is submerged, carbonates dominate from the barrier edge to the basin center; when it is exposed, evaporites are deposited in the basin center.

Climate variability governs intrabasinal sedimentation dynamics. Runoff episodes yield increased siliciclastics, especially shales, and increased proportion of carbonates vs. evaporites within the basin. Changes in carbon isotope ratios in the carbonates were found to track these episodes.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California